t e sou rakes | was Structural Geology and Volcanism of Owens Valley Region, California- A Geophysical Study bl :3 > it f GEOLOGICAL SURVEY/PROFESSIONAL PAPER 438 Prepared partly in cooperation with the California Division of Mines and Geology ~r"~> £ EARTH _. sCiENCES LIBRARY Structural Geology and Volcanism of Owens Valley Region, California- A Geophysical Study By L. C. PAKISER, M. F. KANE, and W. H. JACKSON 6EOLOGICAL._.-SURVEY PROFESSIONAL PAPER 433 Prepared partly in cooperation with -the California Division of Mines and Geo/0g y UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library has cataloged this publication as follows : Pakiser, Louis Charles 1919- Structural geology and volcanism of Owens Valley region, California ; a geophysical study, by L. C. Pakiser, M. F. Kane, and W. H. Jackson. Washington, U.S. Govt. Print. Off., 1964. iv, 67 p. maps (2 col.) 'diagrs., profiles, table. ° (U.S. Geological Survey. Professional paper 438) _ Part of illustrative matter fold. in pocket. ons ~ Prepared partly in cooperation with the California Division of Mines. Bibliography : p. 63-65. (Continued on next card) Pakiser, Louis Charles 1919- Structural geology and volcanism of Owens Valley region, California. 1964. (Card 2) 1. Geology-California-Owens Valley. 2. Gravity. 3. Magnetism, Terrestrial-California-Owens Valley. I. Kane, Martin Francis 1928- joint author. II. Jackson, Wayne Harold 1919- joint author. III. California. Division of Mines. IV. Title. gs (Series) UC ct gen 5\‘\BRN£( For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 % {2678 Cb v. 426-429 EARTH $CIENCES CONTENTS % LIBRARY Page ADSLTACD-. L- sik = ns as 1 2 2 Previous geophysical studies..___________. $8 Fieldwork and acknowledgments_.________________._ 3 General geographic setting......__._..___..___._._ 5 Climate and vegetation.......................2.... 5 Economic 5 nan- 6 Pre-Tertiaty ' 6 Cranitold TOCKS_. =o. cow incense =~ C 8 Sedimentary rocks of the White and Inyo - 8 MCtAMOTDhIC -- £ 8 CENOZOIC amer cell -c 9 Older Iake 9 Younger 1Ak6 DEUS 10 AllUvial a= 10 wun Volcanic rocks north of Bishop.-..___________._. 11 Volcanic rocks of Tertiary(?) age______._. 11 Volcanic rocks of Pleistocene age...... 13 Volcanic rocks of Owens Valley..____________. * .13 Structure and 14 Older structural framework....__.____________._ 14 Basin and range structural features.._________. 15 Volcano-tectonic features...___...._.__________._ 17 Evolution of the present land forms________.__. 17 Physiographic evidence.................. 18 Paleobotanic evidence_.._..._._.._...___.. 20 Geophysical survey$.......s.....oll es 21 OTtaVvIitySUIVEY 22 DeNRSILY c= 22 Fieldwork and computations..._______________ 23 Accuracy of 24 Interpretation of gravity datac_______________ 25 Gravity contour 26 Area north of Owens Valley...... 26 Northern Owens Valley.___________._. 27 Central Owens Valley._____________._ 28 Owens Lake basin................... 28 Gravity effects of dense pre-Tertiary rocks.. 28 Regional gravity and isostatic compensa- 29 Page Geophysical surveys-Continued Gravity survey-Continued Analysis of gravity profiles..._.._._______.__._ 29 Profile A--A l sl rena able lane abn ine ss 31 Profle : B-~BUe..- cer 37 Profle CHU wae -n 38 Profile 38 Profile ... 3a .n. 98 Eroflo cxc--c¥ri--al@li=-356 Relation to regional tectonic pattern...... 56 Volcanism of Owens Valley._________________. 59 Origin of Mono Basin and Long Valley...... 60 Summary of tectonics and volcanism... 62 Summary of geologic history of Owens Valley region. - 62 LiteTabuUTo.Cited .-... 63 67 ILLUSTRATIONS [Plates are in pocket] Prats 1. Combined gravity and geologic map, showing location of seismic profiles, of: Sheet 1. Area north of Owens Valley. Sheet 2. Northern Owens Valley. Sheet 3. Central Owens Valley and Owens Lake basin. 127 III IV CONTENTS Prats 2. Combined aeromagnetic, gravity, and geologic map of an area including parts of Long Valley, Volcanic tableland, and Owens Valley. 3. Analyses of seismic profiles 2, 4, 5, and 6, Owens Valley region. ~ Page Fraur® 1. Map showing geographic setting and gravity coverage. 4 9. Generalized gcologic .v. ALAMA ses i> sam vi 3-4. Cross sections showing: 3. Erosion surface of Kern County-Mount Whitney areal LLL 18 4. Erosion surfaces of Yosemite Valley assess ss 19 5. Histograms iof gravity .s. sonce uns s nel ss 24 6-7. Diagrams showing analyses of gravity profiles across: 6: Eastern Long Valley... .. . .-.. 2. _ CTCL LA coll ol oo t mnie a an m B nn a ul we Be one nline hn ip in me a I h te n Bd n te in en an n uis e ad e n lee 30 7. Nortliern extension of Owens Valley... e- ss mu cm 31 8-9. Gravity profiles across: 8. Long Valley and northern extension of Owens 32 0.- Mono Basin and Long Valley.. . . _.... ._. . _._. __ L Aves baar ats as ann s nis tnt al nt ale s fle an t 32 10-15. Diagrams showing analyses of gravity profiles: 10. In Mono enlace whe one ue alain o m on ab oo no e in far ie ie n i t ehe in e in (e e an in ie 33 11, From Mount Humphrcys to Deep Spring Valley 34 12: Near Big ananas oin be an ne anns nt ~s mle miss 35 18. North of Tungsten Hills. . nabe u tis an min ii ace mm me mie sills in a' 35 14. Bast'of Mount Whithey _. ... anni su mss aln s me pid 15. InlOwens@Lake er sss cie cem 36 16. Map comparing magnetic and gravity contours for an area northeast of Bishop.-__________________________ 40 17-18. Diagrams showing analyses of seismic profiles: 17." Southeast of Owens ics stun sal 44 18. South of Lone 46 19-20. Maps showing: 19. Fault pattern and volcanic s aes 57 20. Regional geology of southern California and southwestern 58 TABLE 4 Page TabL® 1.-Owens Valley project gravity tie sk ss sss tels bed nne. sss m slie o 23 STRUCTURAL GEOLOGY AND VOLCANISM OF OWENS VALLEY REGION CALIFORNIA-A GEOPHYSICAL STUDY By L. C. Pariser, M. F. Kaxr, and W. H. Jacrsoxr ABSTRACT Owens Valley in eastern California is one of the western- most downdropped blocks of the Basin and Range province and is important because it includes part of the boundary between the Sierra Nevada and Great Basin regions. As described in this report, the Owens Valley region includes Owens Valley, Long Valley, Mono Basin, and the slopes of the Sierra Nevada and the White and Inyo Mountains. Several small basin ranges are included in the northern part of the region. Owens Valley is terminated on the south by the Coso Range. A regional geophysical study of the Owens Valley region was made to determine as completely as possible the Cenozoic struc- tural geology and to deduce from the structural features thus described a possible explanation for the geologic processes that brought them into existence. Gravity and seismic-refraction measurements were made to determine the configuration of the interface between the Cenozoic deposits and pre-Tertiary rocks ; this determination is possible because of a marked discon- tinuity in density and seismic velocity at the interface. An aeromagnetic survey was made of part of the region to deter- mine the distribution of some of the volcanic rocks of Cenozoic age that are associated with Cenozoic structural features ; this determination is possible because the more mafic of these vol- canic rocks are more magnetic than other Cenozoic deposits. Differences in density and magnetization within the pre-Tertiary rocks also provide some information on older features. Field- work was started in 1954 and completed in 1958. The pre-Tertiary rocks of the Owens Valley region include the granitoid and the metasedimentary and metavoleanic rocks that predominate in the Sierra Nevada and the sedimentary rocks that predominate in the White and Inyo Mountains. These rocks range in age from Precambrian to Cretaceous. Clastic deposits of Cenozoic age include the lake beds and stream de- posits of Owens Valley, Long Valley, and Mono Basin, the Pleistocene moraines of the Sierra Nevada slopes, and the al- luvial fans of the Sierra Nevada, White and Inyo Mountains' fronts. Volcanic rocks of late Tertiary(?) and Pleistocene ages are widespread throughout the Owens Valley region ; they are especially abundant in the embayment of the Sierra Nevada front that includes Long Valley and Mono Basin. These Cenozoic volcanic rocks range in composition from basalt to rhyolite and in age from early Pliocene(?) to latest Pleistocene. The faults that bound the main Cenozoic structural features of the Owens Valley region may have been inherited from earlier zones of weakness that were brought into existence perhaps during the Nevadan orogeny of the late Mesozoic. Owens Val- ley is a downdropped block, or graben, between the Sierra Nevada and the White and Inyo Mountains. The physio- graphic expression of the near-linear western front of the White and Inyo Mountains is remarkably simple in form. The physi- ography of the eastern front of the Sierra Nevada is much more irregular and complex and is the expression of some warping as well as block faulting. The association of volcanic rocks with the depressions of Long Valley and Mono Basin suggests that these structural features may have been created in part by vol- cano-tectonic processes. Physiographic evidence from observations of the streams of the Sierra Nevada and fossil flora have been used by different investigators to deduce conflicting versions of the history of uplift of the Sierra Nevada and the Cenozoic deformation along its eastern front. Two periods of Cenozoic uplift have been deduced on the basis of the erosion surfaces of the Kern, Merced, and other rivers of the southern Sierra Nevada by Lawson, Knopf, and Matthes; late Tertiary fossil plants have led Axelrod to the conclusion that only one important uplift occurred-at the end of the Tertiary and in early Pleistocene time. The contrast in density between the Cenozoic deposits of Owens Valley, Long Valley, and Mono Basin and the pre-Tertiary rocks that confine these deposits is about -0.4 g per ecm*. There- fore, thick accumulations of Cenozoic deposits are expressed as gravity lows, and where the contacts between these deposits and the older rocks are steep, the gravity gradients along these contacts are large. Measurements of gravity show Owens Valley to be expressed by an elongated gravity low having a residual gravity relief of 30-40 mgals (milligals). The gravity gradients along the eastern boundary of Owens Valley are steep and nearly linear; only one important discontinuity occurs. The gravity gradients along the Sierra Nevada front of Owens Valley are discontinuous and alternate between steep and gentle. De- tailed interpretation of several gravity profiles across Owens Valley shows that a steeply dipping fault forms a common boundary between the Owens Valley depression and the White and Inyo Mountains chain. The west fault bounding the deepest wedge of Owens Valley is in general east of the Sierra Nevada front, and the deformation alternates between faulting and warping along the valley trend. The maximum thickness of Cenozoic deposits in Owens Valley is 8,000+2,000 feet. Long Valley is expressed by a large, elliptical gravity low, flanked by extremely large gradients, that has a residual gravi- ty relief of more than 60 mgals. Detailed interpretation of the anomaly shows that Long Valley subsided along near-verti- cal faults and received an accumulation of about 18,000+5,000 feet of low-density sediments and volcanic deposits of Cenozoic age. Mono Basin is a structural basin of the same type. Many smaller Cenozoic structural features are revealed by gravity anomalies. Interpretation of the aeromagnetic data from the northern part of the Owens Valley region reveals the probable existence 1 ( I 2 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. of a large amount of volcanic material buried near the center of Long Valley and also in Owens Valley northeast of Bishop. A pile of volcanic material probably also lies buried near the center of Mono Basin. Interpretation of six long seismic-refraction profiles in Owens Valley and Long Valley yields depths to pre-Tertiary rocks and velocity boundaries within the Cenozoic section that are con- sistent with those obtained from the gravity interpretation and thus greatly narrows the range of uncertainty concerning the thickness of Cenozoic deposits. A study of the volume of the deposits of Cenozoic age in Long Valley and Mono Basin that could have been transported from pre-Tertiary rock sources to these structural basins by streams compared with the combined volume of these features indicates that about two-thirds of the Cenozoic deposits are probably of volcanic origin. The Cenozoic deposits of Owens Valley are predominantly stream-transported clastic sediments. Strike-slip movement is known to have taken place along the main faults of Owens Valley, but the direction of movement is in doubt. A detailed analysis of the published record of the Owens Valley earthquake of 1872, examination of the area, and analysis of the distribution of the volcanic fields of Owens Valley leads to the conclusion that the predominant direction of hori- zontal movement has probably been left lateral. This conclu- sion is compatible with the directions of motion for the San Andreas, Garlock, and White Wolf faults, and the Walker Lane zone of faulting. The basalt flows of Owens Valley and the southern Inyo Mountains were erupted from sources in regions of relative ten- sion or stress relief near the ends of important transcurrent faults if the direction of horizontal movement was left lateral (that is, if Owens Valley is a great left-lateral shear zone). This interpretation suggests that the Sierra Nevada has been moving south with respect to Owens Valley and the Great Basin region to the east. If this is so, the embayment in the Sierra Nevada front that contains Long Valley and Mono Basin would tend to be stretched or pulled apart. Regions of low pressure, such as that inferred to exist in the Long Valley-Mono Basin} area, would be favorable to the generation of magma by the{ ( reduction of stress and possible inward migration of water and j other volatiles from surrounding regions of higher pressure; thus, the melting point of the rock materials would be reduced. Reduction of the confining pressure of the rocks over a magma chamber thus created would encourage volcanic eruption. A reory relating the tectonics and volcanism of the Owens Valley egion based on these principles is proposed. It leads to the conclusion that Mono Basin and Long Valley are volcano-tectonic depressions that subsided in response to volcanic eruptions as support was removed from a magma chamber at depth. After the emplacement of the Sierra Nevada batholith in Late Cretaceous time, the Sierra Nevada was a low mountain range, perhaps no more than 3,000 feet above sea level. Then, in a series of uplifts, it was raised to its present great heights. \ The most important of these uplifts probably took place in late Miocene or early Pliocene times and again at the end | of the Tertiary and beginning of the Pleistocene. These two major uplifts were separated by a period of quiescence. Ex- tensive faulting in the Owens Valley region took place probably during these major uplifts, but the deepest wedge of Owens Valley may have begun to subside and to receive sediments at some earlier time. Long Valley and Mono Basin probably began to subside concurrently with the early Pliocene(?) vol- canic eruptions in the area, and subsidence probably ended with the latest eruptions from Mono Craters. Glaciers in Pleistocene time sculptured the slopes of the Sierra Nevada in the Owens Valley region and deposited moraines. Finally, recent stream action modified Owens Valley to form the present landscape. INTRODUCTION PURPOSE OF STUDY Owens Valley is one of the most conspicuous physio- graphic and geologic boundary features in the United States, as well as a geologic feature of intrinsic interest. It is one of the westernmost of the downdropped blocks of the Basin and Range province. On the west, the crest of the Sierra Nevada separates the Sierra Nevada Mountains from the Great Basin region to the east. From the time of the earliest geologic studies in this area (Whitney, 1865, p. 456; Gilbert, 1875; King, 1878; Russell, 1887) Owens Valley has been described as a downdropped fault block. This conclusion, however, re- mains open to doubt because it has been based largely on physiographic evidence that, powerful though it may be, is not conclusive. Knopf (1918, p. T8), in his study of the Owens Valley area, stated that the evidence of faulting along the Sierra Nevada escarpment consists in the sharply descending slope, the topographic dis- continuity, the oversteepening toward the foot of the escarp- ment, and the dislocation of gravel-filled channels and their ovferlying lavas. In addition, triangular faceting of the moun- tain spurs, another evidence of faulting * * * has been recog- {lized along the west wall of Owens Valley in extraordinarily impressive development. Recgnt detailed geologic mapping by P. C. Bateman (written commun., 1958) and J. G. Moore (written com- mun., 1958) provided additional physiographic and geologic evidence of faulting. The evidence for faulting along most of the western front of the White and Inyo Mountains is less impres- sive than that for the eastern front of the Sierra Ne- vada, except for the remarkably near-linear trend of this front throughout its length. At best, the evidence for faulting from surface geologic mapping alone is fragmental and permits only broad speculation on the major structural trends and the magnitude of fault de- formation. Many important faults are entirely con- cealed by surficial debris and remained unknown until the geophysical study was undertaken. f The clastic rocks that fill the Owens Valley block and other similar features in the Great Basin and that have been derived to a large extent from erosion of the surrounding mountains are considerably less dense than the older rocks that confine them. Therefore, thick se- quences of such lighter, younger rocks should be revealed by pronounced gravity minimums. Fairly de- tailed gravity measurements are expected to give reliable information on the approximate depth and con- INTRODUCTION 3 .. figuration of the surface of denser, older rocks that lies buried beneath the valley fill. Faults having vertical displacement and other structural features of great vertical relief that have formed during and after deposition of the valley fill should be clearly revealed by gravity measurements. In brief, gravity surveying provides a rapid and economic geophysical method of studying the structural geology of areas such as Owens Valley. Results of aeromagnetic surveying yield valu- able clues on volcanic and other igneous features buried under valley fill because these igneous rocks are gen- erally more magnetic than the clastic rocks that sur- round or overlie them. Seismic-refraction measure- ments provide reliable information on the depth to the older rocks of higher seismic velocity, as well as depths to layers within the valley fill. This study, therefore, was undertaken to obtain as much information as possible on the structural geology of Owens Valley and its relations to surrounding geo- logic features and to the geologic history of the Sierra Nevada and the Great Basin regions. Oliver (1956) is engaged in a related but broader gravity study of the crustal structure in the Sierra Nevada region. Mabey (1958) is studying the Death Valley region to the east. PREVIOUS GEOPHYSICAL STUDIES Gutenberg, Wood, and Buwalda (1932) made a se- ries of seismic experiments in Owens Valley at Diaz Lake, east of Alabama Hills, and near the Sierra Ne- vada frontal fault near Independence in the summer of 1931. They reported that the Diaz Lake beds are 110 meters thick 46 meters east of the base of Alabama Hills and more than 100 meters thick 200 meters east of the base of the hills; bedrock was not found at a loca- tion 400 meters farther to the east. These data suggest that the fault bounding Alabama Hills dips 60° or more eastward. The thickness of the Diaz Lake beds was determined from shallow reflections 23 years before the shallow seismic-reflection method was first studied in detail (Pakiser and others, 1954). About 8 miles south- west of Independence, along the east side of the main Sierra Nevada fault, Gutenberg, Wood, and Buwalda obtained refractions from the fault surface by using a geophone spread close to the fault and three shot points at different distances from the fault. The thickness of the alluvium at this location was not determined, but it is greater than 250 meters. In the summers of 1952 and 1953, H. W. Oliver (writ- ten commun, 1957) made gravity measurements in the Bishop and Lone Pine areas in conjunction with a regional gravity survey of the Sierra Nevada. These measurements revealed that the valley block is expressed by a pronounced gravity low. FIELDWORK AND ACKNOWLEDGMENTS Gravity measurements were made during parts of each year from 1954 to 1957 to provide a fairly detailed regional gravity network of the entire modern drainage basin of Owens River from its headwaters at Deadman Creek in the Sierra Nevada to Owens Lake more than 100 miles south and from the eastern scarp of the Sierra Nevada to the western slopes of the White and Inyo Mountains. Additional information was obtained in surrounding areas, including Mono Basin to the north (Pakiser and others, 1960). In all, about 1,600 gravity stations were established in an area of about 4,000 square miles (fig. 1). Kane supervised gravity field- work done in February 1954 and the spring of 1956. Pakiser was in charge of the fieldwork done in the sum- mers of 1955-57. An aeromagnetic survey was made in the Volcanic Tableland, Long Valley, and Mono Basin in August 1956. A small, detailed survey in an area northeast of Bishop was made in 1957. The aeromagnetic work was supervised by J. R. Henderson of the Geological Survey. During the summers of 1957 and 1958, seismic-refrac- tion measurements were made in a cooperative field program of the Seismological Laboratory, California Institute of Technology, and the U.S. Geological Sur- vey. Jackson was in charge of the Geological Survey field party. We are indebted to R. E. Warrick, who made some of the seismic measurements; Donald Plouff and S. W. Stewart, who made some of the gravity measurements; W. T. Kinoshita, who did some of the planetable and alidade surveying; W. J. Dempsey and J. R. Henderson, who arranged and carried out the aeromagnetic survey ; Isidore Zietz, who provided valuable assistance and ad- vice on interpretation of the aeromagnetic data; P. C. Bateman, J. G. Moore, C. D. Rinehart, and D. C. Ross, who provided valuable geologic information and advice and assistance in surveying; and Howard Oliver, who made available gravity data from his study of the Sierra Nevada. All these men are members of the Geological Survey. We acknowledge the generous assistance of S. L. Paratt of the City of Los Angeles Department of Water and Power, who made available detailed maps and bench-mark data along the Los Angeles Aqueduct and arranged for permission to do seismic shooting. We appreciate the support of the State of California Division of Mines, with whose cooperation the work in Long Valley was done. Finally, we are indebted to Dr. Frank Press, Director, Seismological Laboratory, Cali- fornia Institute of Technology, for permission to pub- lish seismic data obtained by that organization, and to John H. Healy, who supervised the fieldwork and 38° 97° STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. 119° F R ES N 0 118° Tonopah Basalt E} JNM incase -~ A 3 g: e X ~ A> h Goldfield White MtM\ Peak 142421 , ~ Sequoia 10 20 36° \ 40 MILES \ I Panamint Springs Ficur® 1.--General geographic setting of the Owens Valley region, showing area of gravity coverage. INTRODUCTION 5 interpreted the data obtained by the Seismological Lab- oratory field party. All the manuscript except the sections on gravity fieldwork and computations, the sections on the accu- racy of gravity data, the list on principal facts about base stations, and the sections on the seismic survey was written by Pakiser. Kane wrote the sections on the gravity work, and Jackson wrote the sections on the seismic survey. Kane participated in much of the analysis of the gravity data, and Jackson participated in interpretation of the seismic data and in preparation of the final manuscript. The geologic interpretation beginning on page 48 was written by Pakiser. GENERAL GEOGRAPHIC SETTING The Owens Valley area as described in this study in- cludes the Y-shaped area between the crests of the Sierra Nevada and of the White and Inyo Mountains and extends from Glass Mountain Ridge to the south end of Owens Lake (fig. 1). It includes parts of Inyo and Mono Counties, Calif. Mono Basin is immediately to the north of the Owens Valley area, and Rose Valley is immediately to the south. Owens Valley terminates on the southeast against the Coso Range. The area in- cludes the entire modern drainage basin of Owens River, which rises in the Sierra Nevada northwest of Long Valley, is fed by various tributaries from the Sierra Nevada, and empties into Owens Lake. The Owens Valley area includes as well the intermittent drainage system that flows south along the western front of the White Mountains from a point north of Benton Station (fig. 1) into Owens River near Laws. Thus defined, the Owens Valley area includes not only Owens Valley proper, which is a narrow trough extending from the Volcanic Tableland to Owens Lake, but also Round Valley, which is a branch of Owens Valley northwest of Bishop, Long Valley, which is the depressed area that contains Lake Crowley, and the extension of the Owens Valley trough north of Laws along the White Mountains front. The Benton Range, several related mountain structures, and the Volcanic Tableland north of Bishop are also included in the Owens Valley area. The Owens Valley area is an area of great topo- graphic relief, ranging in altitude from about 3,600 feet above mean sea level at Owens Lake to more than 14,000 feet in the Sierra Nevada and the White Mountains. The area is about 120 miles long as measured from Glass Mountain Ridge in the north to the south end of Owens Lake and ranges in width (crest to crest) from 40 miles at the north end to 25 miles at Owens Lake. The mini- mum width of Owens Valley between Bishop and Big Pine is 15 miles. The total area of the Owens River drainage basin is about 3,300 square miles. 728-195-64--2 U.S. Highway 395 traverses the entire eastern front of the Sierra Nevada throughout the length of the Owens Valley area. U.S. Highway 6 follows the west- ern front of the northern White Mountains southward and joins Highway 395 at Bishop. California State Highway 190 enters Owens Valley from the east at Lone Pine. The Owens Valley area is served by the South- ern Pacific Railroad. The larger towns in Owens Valley include Bishop (having a 1950 population of 2,891), Big Pine (556), Independence (875), and Lone Pine (1,415). CLIMATE AND VEGETATION Temperatures in Owens Valley are extreme and range from hot (often more than 100°F) in the summer to very cold (less than 0°F) in the winter. Cool tempera- tures prevail throughout the summer on the higher slopes of the Sierra Nevada, however. The climate of Owens Valley and the Great Basin area to the east is arid. The climate of the Sierra Nevada is subhumid, and extensive snowfields form there during the cold winter months and remain through the summer. Descending the slopes from the crest of the Sierra Nevada eastward, one notices that the vegetation changes from a coniferous forest to a zone of pinyon and juniper pine at about 8,000 feet of altitude and gradually gives way to sagebrush at altitudes of 5,000 or 6,000 feet (Bailey, 1954). ECONOMIC DEVELOPMENT Owens Valley is an area of rich natural resources. Mumford (1954) estimated that Owens Lake Basin con- tains 160 million tons of various salts, including car- bonates, bicarbonates, sulfates, chlorides, and borates of sodium and potassium. Since 1904, five plants have been constructed for the manufacture of soda ash and one for caustic soda. All alkali operations have been discontinued except those at the Columbia Southern Chemical Corp. plant at Bartlett. Altogether, about 1,000,000 tons of alkali and 30,000 tons of borax have been produced from the brine of Owens Lake. The Bishop area contains some of the more extensive tungsten deposits of the United States (Bateman and Irwin, 1954; Bateman, 1956). The tungsten minerals that are mined are found in tactite and include scheelite and members of the wolframite group. The Pine Creek: mine in the Bishop Tungsten district accounts for about half of the current tungsten production in California, and it contains the largest tungsten reserve in the United States. Silver, lead, and zinc deposits were mined for a long time at the Cerro Gordo mine east of Lone Pine in the Inyo Mountains and at several smaller mines in the area 6 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. (Carlisle and others, 1954). Small vein deposits of gold have been mined intermittently in the Owens Valley region. (Knopf 1918). Tale, perlite, absorbent clay, and pumice have been produced from several deposits in the Owens Valley area (Wright and others, 1954). By far the most important resource of Owens Valley is the water of Owens River and its tributaries in the Sierra Nevada, which is supplied to the city of Los Angeles through the Los Angeles Aqueduct. Agriculture includes cattle grazing on the desert shrubland of Owens Valley, fruit orchards near Bishop, and some production of grain. Owens Valley is a major resort area, and all the towns serve as headquarters for a variety of summer and winter sports. Fishing, hunting, hiking, and winter sports are the major recreations to which the resort areas cater. The Mammoth Lakes area, on the Sierra Nevada slopes northwest of Bishop overlooking Long Valley, is one of the most highly developed resort areas in Cali- fornia. Many people travel through the Owens Valley area because of its spectacular scenery. As King (1878, p. 742-744) observed : The two grandest fault-lines shown in the Great Basin are those which define its east and west walls. Whoever has fol- lowed the eastern slope of the Sierra from the region of Honey Lake to Owens Valley cannot have failed to observe with wonder the 300 miles of abrupt wall which the Sierra turns to the east. GEOLOGY Owens Valley is a structural trough that has been dropped down as a graben along normal faults that separate it from the Sierra Nevada on the west and from the White and Inyo Mountains on the east (fig. 2). As the valley floor subsided to previously unknown depths below what is now the ground surface, the bounding mountain masses rose. As these mountain blocks rose and as the valley floor subsided, streams and other agents eroded the escarpments forming the valley walls, transported the resultant debris into the valley, and deposited it as alluvial fans, other stream deposits, and lake beds. During much of the time that earth movements along faults were taking place, volcanic vents and fissures poured out large amounts of lava and pyroclastic debris, much of which now lies buried with the valley-fill sediments; however, some of the volcanic material is exposed at the surface. During the ice ages glaciers flowed from the Sierra Nevada into Owens Valley and deposited extensive moraines. Owens Valley is not a simple tectonic trough, and the main part of Owens Valley is complexly faulted. Long Valley, which lies north and west of the Volcanic Tableland, is structurally separated from Owens Valley by a bedrock barrier, although Owens River flows through both valleys. North of Laws, the Owens Valley trough is uninterrupted along the White Mountains front and terminates near Benton Station. Between the northern White Mountains and the part of the Sierra Nevada escarpment that forms the south and west walls of Long Valley lies a system of low basin ranges, the most prominent of which is the Benton Range. Long Valley is structurally terminated on the north by the Glass Mountain Ridge. Mono Basin is immediately north of Glass Mountain Ridge. Owens Valley is terminated on the south by the Coso Range and by a bedrock ridge that separates Owens Valley from Rose Valley. The deformation that formed Owens Valley and the surrounding mountain masses may have begun in early Tertiary time. It has continued to very recent times; one of the great earthquakes of California and Nevada occurred on March 26, 1872, as a result of movement along the fault forming the eastern front of Alabama Hills west of Lone Pine (Richter, 1958, p. 499-503). This Tertiary and later deformation was part of the widespread block faulting that gave form to the Basin and Range province. The only comprehensive report on the geology of the Owens Valley area available in the literature is by Knopf and Kirk (Knopf, 1918), who carried out geo- logic studies in the White and Inyo Mountains and on the eastern slope of the southern Sierra Nevada in 1912 and 1913. Earlier W. T. Lee (1906) and C. H. Lee (1912) studied and reported briefly on the geology and water resources of Owens Valley. The studies of Knopf and Kirk form the basis for the present study. The gravity meter "sees" only rocks of contrasting densities; the magnetometer, only rocks of differing magnetic susceptibilities. Fortunately, these are also the rocks that can be studied to distinguish Basin and Range structural features from those of earlier oro- genies and Cenozoic volcanic features from the sedi- mentary rocks that surround them (fig. 2). The con- trast in density between the Cenozoic and pre-Tertiary rocks is marked, as is the contrast in magnetic suscep- tibility between the volcanic and sedimentary rocks of Cenozoic age. There are lesser contrasts in these physi- cal properties between the various pre-Tertiary rocks. The seismic velocity of pre-Tertiary rocks is signifi- cantly greater than that of Cenozoic rocks, and impor- tant seismic-velocity boundaries occur within the Ceno- zoic section. The discussion of the rocks of the Owens Valley area that follows is therefore highly generalized and is divided into only two main parts. PRE-TERTIARY ROCKS Comprehensive descriptions of the dense pre-Tertiary rocks of Sierra Nevada and White and Inyo Mountains © Bridgeport 119° x GEOLOGY ~ %;,Whne e fMtn 57° EXPLANATION | Clastic deposits { Bishop tuff of Gilbert (1938) Pleistocene CENOZOIC +444 44+ + + ti++44+ Rhyolite Basalt and andesite L--L-L-t-I - - k-i-t-t- Granitoid rocks CRETACEOUS PRE- CRETACEOUS § B Sedimentary and metamorphic rocks } Major fault 0 10 20 L 1 1 7 m < mt whitney y 14495<~ lel +4 T U L A RE 30 40 MILES 36° ’ FiGUrs 2.-Generalized geologic map of the Owens Valley region. 8, STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. have been written by Knopf and Kirk (Knopf, 1918), by Calkins (in Matthes, 1930), and by Anderson (1937). The rocks of the Sierra Nevada have been described in summary by Mayo (1941). The granitoid rocks of the Sierra Nevada batholith predominate in the Sierra Nevada; sedimentary rocks of Paleozoic age predomi- nate in the White and Inyo Mountains. GRANITOID ROCKS The granodiorite-granite series forms the bulk of the core of the Sierra Nevada batholith and, of this series, quartz monzonite and granodiorite predominate. Sili- ceous granite and granite porphyry also are present in many places. Hornblende gabbro and hornblende dio- rite were intruded before the main mass of the batholith was emplaced (Calkins, in Matthes, 1930; Mayo, 1941). After the emplacement of the bulk of the core, aplite, pegmatite, and some basic rocks were intruded into the batholithic mass as dikes. The granitoid rocks of the batholith in the eastern Sierra Nevada were divided into seven varieties by Bateman and Merriam (1954). For purposes of the present study it is sufficient to say that most of them are coarse and eugranitic, many are porphyritic, and they range in color from nearly white to very dark, various shades of pink and gray predomi- nating. Many authors considered the Sierra Nevada batholith to be Late Jurassic in age, but recent age studies by the zircon method (Larsen and others, 1954) indicate that these rocks are about 100 million years old, or Late Cretaceous in age. More recently, Curtis, Evernden, and Lipson (1958) showed by the potassium- argon method that the Sierra Nevada batholith near Yosemite National Park ranges in age from T7 to 95 million years and is therefore Late Cretaceous in age. Anderson (1937) distinguished two units of granitoid rocks in the northern White Mountains; presumably, these are of the same age as the Sierra Nevada batholith. SEDIMENTARY ROCKS OF THE WHITE AND INYO MOUNTAINS The sedimentary rocks of the White and Inyo Moun- tains have a total thickness of about 36,000 feet and range in age from Precambrian to Triassic. They were described by Kirk (in Knopf, 1918, p. 19-48), who made the first systematic study of these rocks in 1912 and 1913. A brief description of the rocks was compiled recently by Bateman and Merriam (1954) and is sum- marized in the following paragraphs. Rocks of Cambrian and Precambrian (?) age include 7,000 feet or more of sandstone and dolomite of Precam- brian(?) age, 5,000-6,000 feet of Olemellus-bearing limestone and shale of the Silver Peak group of Early Cambrian age, and 900 feet of limestone, quartzite, and calcareous sandstone of Middle and Late Cambrian age. The formations below the known Olenellus-bearing strata are the Campito sandstone of Early Cambrian age and the Deep Spring formation, the Reed dolomite, and pre-Reed dolomite strata of Precambrian age. Rocks of Ordovician age are limestone, quartzite, and dolomite. They include the Pogonip group, Eureka quartzite, and Ely Springs dolomite and have a com- bined thickness of about 2,200 feet. Rocks of Silurian and Devonian age are dolomite, limestone, and sandstone and include the Hidden Valley dolomite of Silurian and Early Devonian age and Lost Burro formation of Devonian age. These rocks are about 3,500 feet thick. Rocks of Carboniferous and Permian age are about 5,500 feet thick and include limestone, shale, sandstone, and conglomerate. The rocks of Carboniferous age in- clude the Tin Mountain limestone of Mississippian age, Chainman shale of Mississippian age, and arenaceous fusulinid limestone of Pennsylvanian age. The rocks of Permian age are shale, arenaceous fusulinid lime- stone, sandstone, and conglomerate. Rocks of Triassic age consist of a section of about 1,800 feet of marine shale and limestone. All the pre-Tertiary sedimentary rocks of the White and Inyo Mountains are much more dense and higher in seismic velocity than the rocks of Cenozoic age in the valley blocks, and, in the discussions that follow, they will be considered as a single sequence of undiffer- entiated sedimentary rocks of pre-Tertiary age. METAMORPHIC ROCKS The metasedimentary rocks of the Sierra Nevada are Paleozoic and Mesozoic in age and occur as roof pend- ants or septa in the main batholithic mass. The Paleozoic rocks are derived chiefly from pelitic, arena- ceous, and calcareous sediments, whereas the Mesozoic rocks contain abundant volcanic but little calcareous material. Little fossil evidence is known on which to date these metasedimentary rocks, but Mayo (1931) described crinoid stems and brachiopods of Middle Devonian(?) age that were found by a prospector in limestone on a mountainside near Laurel Creek in south- western Mono County. Rinehart, Ross, and Huber (1959) described Early Ordovician to Permian(?) fossils in the Mount Morrison roof pendant south of Long Valley and Early Jurassic pectens from a locality west of Long Valley. They believed also that the local- ity from which the fossil described by Mayo (1931) came probably contains the same marble unit as the one in which they found a suite of Pennsylvanian fossils. Metasedimentary rocks of Precambrian age, which have interbedded volcanic rocks, in the northern White GEOLOGY 9 Mountains have been identified by Anderson (1937). These rocks have been intensely metamorphosed by in- trusive rocks and include argillite, quartzite, limestone, and schist, which are unconformably overlain by rocks of Cambrian age. Metavoleanic rocks of Triassic age are found along the crest of the Sierra Nevada, in the western part of Alabama Hills, and in the Inyo Mountains east of Lone Pine. These rocks include sheared andesite and rhyo- lite flows, schistose metatuff, and intercalated red and green shales and crossbedded sandstone (Mayo, 1941; Bateman and Merriam, 1954). CENOZOIC ROCKS Rocks of Cenozoic age include the lake beds, alluvial fan deposits, and glacial moraines of the Owens Valley area and the bounding mountain slopes and include a variety of rhyolitic and basaltic flows, tuff, and breccia. Recent soil and windblown sand cover much of the Owens Valley area. Knopf (1918, p. 48-58, 72-78) described these rocks. The lacustrine and fluviatile de- posits and the pyroclastic rocks of Cenozoic age are significantly less dense and lower in seismic velocity than the pre-Tertiary rocks that confine them. These lighter rocks were deposited during and after the block faulting and warping that created the basin and range structural features of the Owens Valley area. OLDER LAKE BEDS Lake beds that may be either late Pliocene or early Pleistocene in age are exposed in several places in Owens Valley. Only one of these sequences of lake beds-the Coso formation-can be dated with any certainty, how- ever. The Coso formation of late Pliocene or early Pleisto- cene age is found in the basin between the Inyo Moun- tains and the Coso Range and extends southward across the low divide between Owens Valley and Rose Valley. It rests on an erosional surface cut in the granitoid rocks of the Coso Range. The Coso formation is the same as the unnamed sequence of lake beds south of Keeler described by Knopf (1918, p. 51), who considered it to be the oldest of the lacustrine deposits in Owens Valley. It has been studied more recently by Schultz (1937), who, on the basis of vertebrate fossils found in the coarse alluvial-fan materials of the base, considered it to be _- transitional between late Pliocene and early Pleistocene in age and by Hopper (1947), who regarded it as early Pleistocene in age and probably correlative with the McGee (Nebraskan) tills of Blackwelder (19831). The Coso formation is about 500 feet thick. The base of the formation consists of alluvial materials composed of red arkose and buff gravel, which are derived from the granitoid core of the Coso Range ; above the base is a sequence of sandstone and shale. Above these lower members is about 200 feet of thin-bedded white and light-buff lake beds and interbedded white rhyolitic tuffs. The lake beds are well-sorted silts and sands that locally contain fish bones; the tuffs also are well sorted and were probably laid down in a lake. The Coso formation is overlain with no angular dis- cordance by basaltic lava sheets; these sheets of basalt were deformed by faulting. The Coso formation on the west flank of the Coso Range dips an average of 10° (but as much as 20°) toward the Sierra Nevada. The steeper dips are largely the result of deformation during the Pleistocene (Hopper, 1947). Schultz (1937) considered the lake beds east of Big Pine to be probably almost the same age as the Coso formation. Hopper (1947), however, considered these lake beds to be younger, and probably correlative with the Sherwin (Kansan) glaciation of Blackwelder (1931). The lake beds in Waucoba Canyon were first studied by Walcott (1897), who concluded that defor- mation of these beds was evidence of post-Pleistocene elevation of the Inyo Mountains. Knopf (1918, p. 49), however, showed that some of the supposedly deformed lake beds were in reality younger alluvial-fan materials. The lake beds in Waucoba Canyon (referred to as Waucobi Canyon by Walcott, 1897) are exposed east of Big Pine in the foothills of the Inyo Mountains. They are white or light gray, are composed of shale, sandstone, conglomerate, limestone, and arkosic grits, and contain freshwater gastropods. All these beds ex- cept the limestone and grits are soft, poorly consoli- dated, and evenly stratified in thin beds that are as much as 2 feet thick. They dip west at angles of less than 6° and are about 150 feet thick. The lake beds are uncon- formably overlain by alluvial fans. Seven miles south of Big Pine, several hundred feet of evenly bedded soft sandstone and shale, containing some diatomite and a fresh-water fauna, is exposed. These beds strike N. 40° W. and dip 30° SW., mostly as a result of fault deformation. They are partly cov- ered by a basalt flow and are considered by Knopf (1918) to be probably Pleistocene in age. East of In- pedence along the flank of the Inyo Mountains, crudely layered beds containing fragments of granite, lime- stone, chert, and quartz are exposed. Most of the frag- ments are angular, but some are well rounded. These beds, possibly of lacustrine origin, dip 14°-20° W. Underlying the welded tuffs capping the Volcanic Tableland, some 70 feet of nearly horizontal beds of rhyolitic composition is exposed. These beds consist of material that ranges from particles having the fine- ness of dust to well-rounded pebbles of pumice 14-1 10 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. inch in diameter. They are evenly bedded, and the in- dividual beds are 2-3 inches thick. In places the coarser beds display crossbedding in which the foreset-type beds dip about 15° E. These lake beds are composed of rhy- olitic ash, grains of quartz, small pebbles of white pum- ice, and particles of black obsidian. Northeast of Laws are evenly stratified lake gravels that dip 14° W. The beds northeast of Laws are unconformably overlain by alluvial fans. YOUNGER LAKE BEDS Long Valley was filled with a lake during a part of Pleistocene time. According to Rinehart and Ross (1957), the extrusion of the Bishop tuff of Gilbert (1938) blocked the part of the Owens River valley south of what is now artificial Lake Crowley and impounded a lake at least 80 square miles in area. Mayo (1934) estimated the greatest depth of the lake to have been 250 feet or more. Deposition of lake beds continued until the waters spilled over from this lake and cut the Owens River Gorge into the Bishop tuff and the lake was drained (Rinehart and Ross, 1957). This inter- pretation, requiring continuity of the course of Owens River from Long Valley into Owens Valley prior to the extrusion of the Bishop tuff, is supported by gravity evidence of an old relatively mature valley buried under the Bishop tuff. The lake beds in Long Valley are at least 100 feet thick. The lower 50 feet includes strata of clay, silt, marl, and some diatomite. These lower beds are over- lain by 5-50 feet of delta beds that are made up of coarse crossbedded tuffaceous sandstone containing opal cement (Mayo, 1984). ' Some of the coarse lake beds near the west margin of Long Valley may be Pliocene in age. E Owens Lake formerly extended at least 10 miles north of Lone Pine and covered an area of about 220 square miles. Knopf (1918, p. 57-58) noted that the lower course of Owens River southeast of Independence has cut a trench 20 feet deep and 200 yards wide in hori- zontally bedded ash-gray silts and fine sands, which are about 30 feet thick. A hole drilled to a total depth of 920 feet near the center of Owens Lake penetrated beds composed predominantly of clay for the first 550 feet and of alternating beds of clay, sand, and silt be- low that (Smith and Pratt, 1957). Ostracodes and dia- toms were abundant in the cores taken from the drill hole. During the ice ages Owens Lake was one of a system of interconnected lakes that filled the Searles and Pana- mint basins and probably overflowed into Death Valley (Gale, 1915). Gale estimated, on the basis of the salin- ity of the Owens Lake waters, that the last overflow occurred about 4,000 years ago after which Owens Lake became a closed basin. Old beach deposits indicate that the Owens Lake waters had once been 220 feet higher than they were in 1912, when the lake was about 30 feet deep. In 1920 the city of Los Angeles completed an aqueduct through which the waters of Owens River are transported to Los Angeles, and Owens Lake has since receded to a small pool of brine. Late Pleistocene and Recent lake beds are also ex- posed in several places west of the front of the White Mountains north of Laws. ALLUVIAL FANS Knopf (1918, p. 52-57) mapped alluvial fans of two different ages along the front of the Inyo and White _ Mountains; he found only a single system of fans along the Sierra Nevada. j The great alluvial fans along the Sierra Nevada are 1-7 miles wide and converge to form a continuous apron at the base of the Sierra Nevada escarpment. These fans rise from 1,000 to 2,500 feet above the floor of Owens Valley and have an average slope of 6° or 7°. The alluvial-fan materials consist of coarse angular gravels that are unsorted and crudely layered; they contain blocks as large as 18 x 6 x 8 feet. According to Knopf (1918, p. 57) "the alluvial cones along the base of the Sierra Nevada attained their present height before the advent of the first glacial epoch, and the period of their upbuilding probably corresponds to that of the older alluvial cones along the Inyo Range." However, Knopf recognized only the two more recent of the glacial stages of Blackwelder (1931), so this state- ment is not applicable to the older McGee and Sherwin glaciation. The fans along the Sierra Nevada have been dissected to depths ranging from 75 to 150 feet, according to Knopf (1918), as a result of the increased erosive power of the unladen streams after glaciation and possibly also of renewed uplift of the mountain block. f The younger fans along the White and Inyo Moun- tains were derived largely from erosion and partial de- struction of the older fans, but new material eroded from the mountains has undoubtedly been added to these fans. The older fans extend back into the canyons to an altitude of 6,600 feet (2,600 feet above the valley floor). The coarse angular irregularly layered gravels of the older fans contain boulders of granite 6-12 feet in diameter. The beds of the older fans have been dis- located and tilted so that they now dip as much as 50° W. (P. C. Bateman, written commun., 1958). The younger fans at the base of the mountains are small, but pronounced, typical alluvial fans. Along the north- ern White Mountains, however, the alluvial fans are extremely well formed into a continuous apron at the GEOLOGY base of the mountains similar to that along the Sierra Nevada. Older gravels, in places covered by the Bishop tuff of Gilbert, have been mapped in the northeastern part of the Casa Diablo Mountain quadrangle by Rlne- hart and Ross (1957). Knopf (1918, p. 56-57) related the formation of the older fans to a period of increasing aridity during which the streams dropped their loads nearer and nearer to the heads of the canyons. Alluviation in the canyons was interrupted by faulting, after which, dur- ing the glacial epochs, the climate became more humid, the competence of the streams was increased, and the new fans were formed. Deposition of the new fans is now at a standstill because of the present arid condi- tions, and alluviation is going on in the canyons (Knopf, ©1918, p. 56). P. C. Bateman (written commun., 1958) concluded, on the basis of recent detailed geologic map- ping in the area surrounding Bishop, that all the older fans have been dissected because of structural move- ments and not because of climatic change ; he also noted older dissected fans along the east base of the Sierra Nevada between Bishop and Lone Pine. GLACIAL DEPOSITS Knopf (1918, p. 92-105) found evidence of two pe- riods of glaciation on the eastern slopes of the Sierra Nevada. Moraines deposited during the earlier of these two periods extend downward to altitudes of about 5,000 feet in the canyons that drain into Owens Valley, and these moraines cover the upper slopes of alluvial fans. The moraines of the later glacial period are found only on the higher slopes of the canyons, where they overlie the older moraines. Knopf (1918, p. 93) also found evidence of glaciation on the east side of the northern White Mountains. Anderson (1937) described moraines «of the Tioga glacial stage (Blackwelder, 1981) and of an earlier stage in the same area. The till of these more recent glacial stages is an unsorted and unstratified accumulation of angular boulders of granite, quartzite, schist, and basalt. Blackwelder (1931) made an intensive study of the glaciation in the Sierra Nevada and found evidence of four glacial stages, which-from older to younger-he named the McGee, Sherwin, Tahoe, and Tioga glacial stages. Blackwelder tentatively correlated the Sherwin glacial stage with the Glacier Point and El Portal glacial periods of Matthes (1930, p. 50-75) in the Yosemite Valley area and correlated the Tahoe and Tioga glacial stages with the Wisconsin of Matthes in Yosemite Valley. Blackwelder considered the McGee moraines to be Nebraskan in age, the Sherwin deposits to be equivalent to Kansan, and the Tahoe and Tioga to be Wisconsin. No evidence of Illinoian glaciation. was [11 found by Blackwelder in the Sierra Nevada, but he sug- gested that such evidence would eventually be found. Remains of the moraines of all the glacial stages of Blackwelder are preserved in the Owens Valley area, and the Tahoe and Tioga moraines are little altered. The two glacial epochs of Knopf are presumably the same as the Tahoe and Tioga of Blackwelder. The Sher- win till, which was more extensive than the tills of the Tahoe and Tioga, is exposed east of the mouth of Rock Creek canyon and in the Owens River Gorge, where it overlies basalt but is under the Bishop tuff of Gilbert (Rinehart and Ross, 1957). Tills of the Sherwin, Tahoe, and Tioga stages are especially abundant in the canyons draining into Long Valley from the Sierra Nevada, and the moraines form prominent ridges that project from the mouths of the canyons onto the slopes of the alluvial fans. Contrasted to the well-formed moraines of the Tahoe and Tioga glacial stages, the Sherwin till is almost formless. McGee till is found on McGee Mountain, which towers above Long Valley. According to Blackwelder (1931, p. 904) : the deposits on Mount McGee antedate the present eastern front of the range with its deep canyons. It is reasonable to suppose that while such great topographic changes were being brought about, the mountain peaks of that day were largely demolished and have been succeeded by others that now adorn the land- scape. The younger Tahoe and Tioga glaciers, successively less extensive, tended to follow the same canyon paths as the Sherwin glaciers. YOLCANIC ROCKS NORTH OF BISHOP Long Valley and Mono Basin have been subjected to a sequence of violent volcanic eruptions and explo- sions since the close of Miocene time. Although no volcanos are now active in this area, volcanism was probably not complete until late prehistoric time, and hot springs now discharge their vapors in many places. Gilbert (1938, 1941) made a detailed study of the vol- canism southeast of Mono Lake and tentatively corre- lated the earlier volcanic rocks with similar rocks in the Hawthorne quadrangle and the Sweetwater Range of Nevada. Chelikowsky (1940) studied the rhyolite near the center of Long Valley. More recently, Rinehart and Ross (1957) did detailed geologic mapping in the Long Valley area. Most of the discussion of volcanic rocks that follows has been summarized from the papers of Gilbert (1938, 1941). VOLCANIC ROCKS OF TERTIARY(!) AGE The oldest volcanic rocks in the area north of Bishop and southeast of Mono Lake are rhyolite. The rocks are composed of vitric-crystal tuff and consist of three 12 massive members. - An erosional surface separates them from the overlying andesite breccias, conglomerates, and flows. They are best exposed on the summit and east- ern flank of the Benton Range about 4 miles north- northwest of Benton. The lowest member, about 25 feet thick, is composed of a dull-gray matrix of poorly sorted and partly devitrified glass shards in which are scattered crystals of quartz and sanidine, larger frag- ments of light- and dark-gray pumice, and pellets of obsidian. The middle member is thicker and contains more fragments of biotite, sanidine, and quartz scat- tered in a dull-white matrix of devitrified glass; it con- tains also a few lapilli of white devitrified pumice. The upper layer is dark gray, contains more abundant crys- tals of quartz, sanidine, biotite, oligoclase, and horn- blends, and contains lapilli of porphyritic pumice as much as 2 inches in diameter; it is about 25 feet thick. Gilbert considered these rhyolitic rocks to be of sub- aerial origin and made up of material supplied by vol- canic explosions. s The andesite that unconformably overlies the rhyo- lite is composed mainly of massive beds of breccia con- taining blocks of hornblende-pyroxene andesite as much as 3 feet in diameter. The andesite is about 200 feet thick and is separated from the basalt above it by an unconformity. Along the east flank of the Benton Range, where the stratigraphic relationships are clear- est, the andesite dips about 20° E. The andesitic rocks consist of beds of andesitic conglomerate and tuff and of a single flow of hornblende-pyroxene andesite. The beds of conglomerate range in thickness from 1 to 10 feet and include large fragments that are subangular to subrounded. The finer tuff members are fairly well sorted and evenly bedded. Boulders of granodiorite, quartzite, and andesite tuff are found in a breccia about 5 miles north of Benton Station. The andesite was ejected probably from central vents from which cones were built. The location of these vents is not known, but it was probably to the north of the Benton Range, as is indicated by the general thickening of the andesite in that direction. Gilbert regarded the breccias as mudflow deposits and the conglomerates and tuffs as having been deposited by streams and floods. The olivine basalt that unconformably overlies the andesite is widely distributed throughout the area north of Bishop and southeast of Mono Lake. It consists of a series of noncontinuous flows having a maximum thickness of 600-70 on the Sierra Nevad feet east of Bald Mountain and la crest. The individual flows are 25-50 feet thick; scoria are found at the top of each flow, and vesicles are material is found in found at the bottom. Fragmental the basalt only where it underlies the Bishop tuff of Gilbert in Owens River Gorge and STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. on the Sierra Nevada crest west of Bald Mountain. The basalt has been elevated by block faulting and forms the capping of several ranges. Bedded rhyolite tuffs that have been displaced by faulting overlie the basalt north of Benton; south of Benton the tuffs lie on an eroded surface of granite. The basalt issued from scat- tered vents and flooded the broad valleys of an old ero- sion surface. - The summit and western side of Bald Mountain are covered by a dark-gray hornblende andesite. This an- desite has a flow structure of parallel crystals of horn- blende that dips into Bald Mountain at angles of 20° or 25°. Gilbert (1941) believed that the andesite was intruded into the basalt before the basin and range faulting in the area; the andesite is overlain by the Bishop tuff. The slopes of Glass Mountain and its neighboring peaks are covered by glassy and lithoidal rhyolite flows between which beds of tuff may exist. The rhyolite erupted probably from vents above the southern flank of Glass Mountain Ridge near the summit. The rhyo- lite is several thousand feet thick and has been displaced by faulting; so, the southern part now probably lies buried beneath the alluvium of Long Valley. Accord- ing to Gilbert (1941, p. 795-797) : the faulting succeeded the rhyolitic eruptions for the southern slope of the range is a fault scarp along which not only the rhyolite but the underlying granitic and basaltic rocks east and west of Glass Mountain have been displaced. [The pre-rhyolite] basement of Glass Mountain is an irregular, basinlike, structural depression, perhaps formed by downwarping and faulting during the rhyolite eruptions, perhaps after these eruptions, or possi- bly both during and after the eruptions. West of the Benton Range rhyolite tuff covers a part of Long Valley. The tuff overlies olivine basalt and is buried to the south under the Bishop tuff. In places it has been displaced by normal faults. Fragments of white pumice, chips of lithoidal rhyolite, pellets of ob- sidian, and crystals of quartz and feldspar are scattered through a matrix of glass dust. This tuff was probably erupted explosively from volcanic pipes. A bedded rhyolite-tuff sequence extends northward from Glass Mountain toward Adobe Valley (fig. 2) and east of the Benton Range. It consists of unconsoli- dated rhyolite tuff, of gravel made up of fragments of glass, quartz, and feldspar, and of pebbles and sub- angular boulders of rhyolite and obsidian. This series of rhyolitic beds was derived from Glass Mountain and was quickly spread on a surface of low relief by streams flowing to the north and east. In places it is cross- bedded. The tuff sequence overlies basalt and is over- lain by the Bishop tuff. The maximum thickness of these beds is not more than 500 feet; a section 200 feet thick is exposed near Benton. In places the tuff is dis- GEOLOGY placed by normal faults, and part of it was stripped from the ranges being uplifted and was deposited downslope on the original beds of alluvium. Correlation of the volcanic rocks southeast of Mono Lake with those in the Hawthorne quadrangle of Nevada "indicates that the lavas southeast of Mono Lake are younger than the uppermost Miocene sedi- ments exposed in the Hawthorne quadrangle, mapped by Ferguson as the Esmeralda formation'" (Gilbert, 1941, p. 801). Correlation with a similar sequence of volcanic rocks in the Stillwater Range yields a similar result. The Esmeralda formation, a sequence of lacustrine and fluviatile deposits containing volcanic rocks in the upper part, is probably early Pliocene in age, according to Gilbert (1941), and is younger than the Esmeralda of Ferguson. The older volcanic rocks southeast of Mono Lake and north of Bishop, therefore, are probably early or middle Pliocene in age. The rhyolite of Glass Mountain is younger than the Ter- tiary basalt flows and is probably middle Pliocene in C U N VOLCANIC ROCKS OF PLEISTOCENE AGE "Following the initiation of normal faulting, vol- canic eruptions continued through the Pleistocene pe- riod until recent time" in the area southeast of Mono Lake and north of Bishop (Gilbert, 1941). The oldest of the volcanic rocks of Pleistocene age is the Bishop tuff of Gilbert (1938). The age of the Bishop tuff has recently been determined by potassium- argon dating to be about 1 million years (Evernden and others, 1959). Basin and Range faulting was nearly complete when the tuff from scattered vents in the Long Valley area was deposited as ash flows on an old erosion surface. Moraines of the Sherwin (?) stage are buried by the Bishop tuff, and the tuff is overlain by moraines of the last two glacial stages. The geo- logic age of the Bishop tuff, therefore, is probably middle Pleistocene-post-Sherwin and pre-Tahoe. The total exposed area of the Bishop tuff is about 350 square miles; the total area the tuff covers is probably 400-450 square miles. It was found in the Los Angeles Aqueduct tunnel, where it underlies the Mono Craters; east of the Mono Craters it is buried under pumice. The tuff extends also beyond the exposed area under the alluvium of Owens, Long, and Adobe Valleys. The base of the tuff is exposed in Owens River Gorge, in youthful streams along the south margin of Adobe Valley, and along the south and east edges of the Volcanic Tableland. Gilbert (1938) estimated the total thickness of the Bishop tuff to be about 400-500 feet, but it is at least 800 feet thick in places (Rinehart and Ross, 1957). The total volume of the tuff is about 35 cubic miles or more. The Bishop tuff is composed of 728-195-64--3 13 thyolitic material; it is welded and poorly bedded and sorted. Its density increases with depth because of the compaction of the viscous gassy "pumice" of the hot ash flow from which it formed. Later in the Pleistocene, several basalt flows poured eastward into the western part of Long Valley, where they are probably overlain in places by domelike pro- trusions of biotite-hornblende andesite. Another oli- vine basalt sheet flowed from the base of the Sierra Nevada eastward toward the site of Mono Craters. The rhyolite eruptions near the center of Long Valley con- sist of "siliceous extrusions of the fissure type, but [they] are not tuffaceous in character. They are true lavas and only locally grade into fragmental obsidian or pumice" (Chelikowsky, 1940, p. 422). Gilbert (1941), agreeing with Chelikowsky, regarded these rhyolites as late Pleistocene in age. However, C. D. Rinehart (written commun., 1957) believed that they may correlate with the rhyolite of Glass Mountain. Mono Craters erupted late in the Pleistocene and scattered pumice over much of Mono Basin ; the volcanic peaks were built from rhyolite obsidian and pumice ejected from the vents. Putnam (1949) showed that the latest pumice covers late recessional moraines of the Tioga glaciation. The absence of old shorelines on the slopes of the northern Mono Craters (Putnam, 1949) indicates the recency of these eruptions. The early Mono Craters are about 65,000 years old, and the | youngest are about 6,000 years old, according to recent | dating by the potassium-argon method (Evernden and | others, 1959). vYOLCANIC ROCKS OF OWENS VALLEY Eruptions of rhyolitic and basaltic materials occurred intermittently during late Tertiary (?) and Pleistocene times along the margins of Owens Valley south of Bishop. The volcanic rocks of Owens Valley were first studied systematically by Knopf (1918, p. 72-78) ; more recent information on their character and distribution was added by Schultz (1937), Mayo (1941), and Hopper (1947). Knopf (1918, p. 74) found evidence that basalt flows occurred three times from late Tertiary (?) to Pleisto- cene. The earliest of these flows, considered by Knopf to be late Tertiary, is exposed in the Inyo Mountains southeast of Keeler. The basalts southeast of Keeler aggregate about 100 feet in total thickness and rest on a nearly horizontal surface eroded across dipping beds of Triassic and Carboniferous ages. Basalt sheets flowed over the lake beds between the Inyo Mountains and the Coso Range, where they now form plateaus on the east side of the Inyo Mountains (Knopf, 1918, p. T4). These basalt sheets have a total thickness of about 125 feet and are younger than the ostracode-bearing 14 beds that they cover (the Coso formation). They were displaced by the faulting that gave rise to the present Inyo Mountains and are, according to Knopf (1918, p. T4), probably late Tertiary in age. Hopper (1947) de- scribed similar basalt in the Coso Range that covers an old erosion surface and caps the Coso formation. The basalt described by Hopper is correlative with the late Tertiary (?) basalts described by Knopf; and Hopper (1947) correlated this basalt with a thin olivine basalt that in places covers the Ricardo formation 40 miles south of the Coso Range, thus dating the basalt as late Pliocene or early Pleistocene in age. Schultz (1937) found evidence of two periods of basalt extrusion in the Coso Range; faulting, followed by erosion, separated the extrusions of the basalts. Basalt of Pleistocene age, considered by Knopf (1918, p. 4) to be preglacial (pre-Tahoe), is exposed in a canyon west of Independence. Similar basalt, which is about 200 feet thick, also overlies the older alluvial fans of the east side of the Inyo Mountains and is found in isolated patches on the Sierra Nevada slopes southwest of Bishop; a few small patches are found west and northwest of Bishop. A conspicuous volcanic field of basalt, surrounding Crater Mountain and Red Mountain, may be seen on the east side of the Sierra Nevada between Big Pine and Independence. A north-trending fault connects Crater and Red Mountains (Mayo, 1941). Basalt of similar age is found along the east side of Owens Valley east of Red Mountain. These basalts are probably late Pleis- tocene in age. Massive rhyolite, probably late Tertiary in age (Knopf, 1918, p. 72-73), is exposed 8 miles south of Big Pine. Latite of late Tertiary age is found in scat- tered outcrops in the Sierra Nevada. STRUCTURE AND PHYSIOGRAPHY OLDER STRUCTURAL FRAMEWORK The metasedimentary and metavolcanic rocks of the septa or the roof pendants of the Sierra Nevada are generally more dense than the plutonic rocks of the batholith that contain them; so, the gravity data in the mountain slopes bounding Owens Valley show local evidences of the Nevadan orogeny. The very great contrast in density between the pre-Tertiary rocks and the clastic rocks of Cenozoic age that fill the valley blocks is, however, by far a greater influence on the gravity field. The gravity data therefore reveal vital information about Basin and Range structural features and directly reveal only little of the very great deforma- tion of earlier orogenies. The structural features of these earlier orogenies, however, had an important in- fluence on Basin and Range block faulting. STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. Mayo (1941) made a thorough study of these older structural features and their control over the faults along the eastern front of the southern Sierra Nevada. Structural features mapped by Mayo include isoclinal folds, cleavage, linear structures, shears, rock flowage, joints, contacts of metasedimentary and metavolcanic rocks with intrusions, inclusions in granitic rocks, schlieren, and preferred orientation of minerals. He found four important structural trends: a dominant northwesterly trend parallel to the Pacific coast, a northeasterly "cross grain," a system of generally north- northeasterly trends ranging from north to N. 30° E., and another system of generally west-northwesterly trends ranging from N. 60° W. to west. § Basin and Range faults follow the same directions, the northwesterly direction, parallel to the Pacific coast, also being the dominant one. In the Owens Valley area, for example, the dominant faults west of northern Owens Lake follow the Pacific coast directions; many small faults west and southeast of Owens Lake trend north-northeast; and the Darwin "tear fault" and the faults east-southeast of Owens Lake trend west- northwest (Mayo, 1947). Mayo (1941, p. 1064) con- cluded that "the major features of the framework of oldest structures are again reflected, this time in the arrangements of the faults along the eastern front of the Sierra Nevada." Webb (1946, p. 375), postulated that the Great West- ern Divide may "have been inherited from the Nevadan orogeny rather than to be the result of Sierran rejuve- nation." According to Webb, the Middle Kern Basin has been drained since the close of Nevadan time by a south-flowing stream ; the Great Western Divide, now transgressed by the Kern River, was the original master drainage divide, and the present Sierra Nevada, having _ the drainage divide at the crest near the eastern escarp- ment, was built by repeated movements along the Sierra Nevada fault at the eastern base of the range. Webb regarded the Kern Canyon fault as probably Nevadan in age. The direction and amount of movement along the fault is not known and all the present physiographic features in the Kern Canyon area have been affected by erosion after the faulting occurred. Lawson (1904), however, regarded the faulting as coincident with the period of canyon cutting. Drainage parallel to Nevadan structural trends is not uncommon on the western slope of the Sierra Nevada. Locke, Billingsley, and Mayo (1940) compiled a map of the tectonic trends of the Sierra Nevada region based on cleavage, bedding, flow, and shear layers in both the metamorphic and granitic rocks. They believed that the exposed edges of these layers combine into lanes, such as the San Andreas rift, and curves, such as the GEOLOGY 15 westward hook of the southern part of the Sierra Ne- vada, and concluded that the Sierra becomes a shift zone with the principal motion hori- zontal shear along the lanes, and with displacement northwest- ward on the Pacific side. Subsidiary motion starts at inequali- ties in strength and creates curves in which the inner parts are thrust upward and outward toward their convex sides. The resultant motion is clockwise in horizontal section. P. C. Bateman (written commun., 1959) summarized the significant features: (1) the prebatholith rocks were folded along north- to northwest-trending lines, with the average trend N. 30° W., (2) the long axis of the batholith is parallel to this trend, as are the axes of many individual plutons, (3) mafic dikes also trend generally about N. 30° W., but in places a set trends N. 70° W., (4) joints trending northeastward and north- westward are present along the west side of the batho- lith, but these are probably related to Cenozoic rather than earlier deformation, and (5) the average trend of Owens Valley is N. 17° W., and it cuts across the trends of folds in the prebatholith rocks. BASIN AND RANGE STRUCTURAL FEATURES The most distinguishing characteristics of Owens Valley and the related downdropped structural features to the north and northwest are the nearly linear trend of the western front of the White and Inyo Mountains and the irregularity, marked by successive offsets to the west, of the eastern front of the Sierra Nevada from south to north (fig. 2). Whether these differences are the result of differences in the ancestral structural trends, the lithologic boundaries, or the differences in the history of deformation, or all of these, is, of course, an un- answered question. The gravity data, as shall be shown on pages 22-39, confirm, in general the re- markable simplicity of the bounding fault of the east- ern side of Owens Valley, its structural extension to the north, and the notable complexity of the deformation, which involves both faulting and warping, of the west- ern boundary. Another less striking difference between the western front of the White and Inyo Mountains and the eastern front of the Sierra Nevada is the relative paucity of evidence of volcanism along the eastern boundary of Owens Valley and the abundance of such evidence along the western boundary. The western front of the White Mountains and of the transition zone between the White and Inyo Moun- tains north of Waucoba Mountain trends a few degrees west of north along a nearly straight line. Indeed, on small-scale maps a straightedge can be readily used to define this front. The western front of the Inyo Moun- tains is offset abruptly to the west and is rotated slightly in a counter-clockwise direction south of an east-trend- ing line through Red Mountain, and the linear front continues, trending a few degrees west of north, nearly to the southern terminus of the range. The White Mountains are terminated abruptly by a steep arcuate escarpment just north of the map area (fig. 2) and east of Mono Lake. On the south the Inyo Mountains are terminated by a series of basalt plateaus that are rem- nants of a continuous sheet of basalt that has been broken by normal step faults. This is the most con- spicuous volcanic field in the White and Inyo Moun- tains. Another smaller basalt flow is found along the western front of the Inyo Mountains immediately to the east of Red Mountain in the zone of westerly offset of the White-Inyo Mountains chain; the flow may be related to this offset. The near-linear trend of the western front of the White and Inyo Mountains strongly suggests that it is a continuous fault scarp, but there is little direct geo- logic evidence that this is true. Short fault segments in the alluvial fans and in the bedrock along the White and Inyo Mountains front have been mapped by Bateman, Nelson, Merriam, and Smith (Bateman and Merriam, 1954) and by Knopf (1918, p. 88-90), Hopper (1947), Anderson (1937), and Gilbert (1941), but most of these are subsidiary faults that lie east of the main bounding fault. As Knopf (1918, p. 88-90) noted, however, the White and Inyo Mountains front cuts across axes of highly folded strata. The lack of conspicuous evidence for faulting along most of the western front of the White and Inyo Mountains may indicate that the fault- ing is distributive, as Knopf (1918, p. 89) suggested, or that warping (in addition to distributive faulting) is an important mode of deformation. P. C. Bateman (writ- ten commun., 1956) demonstrated the abundance of "mountain-down" faults, as well as "valley-down" faults, along the mountain front. He regarded moun- tain-down faults as evidence of warping. The associa- tion of mountain-down faults with warping is confirmed along the Coyote warp on the eastern slopes of the Sier- ra Nevada south of Bishop but, as the gravity data indicate, a profound break in the earth's crust occurs along the front of the White and Inyo Mountains. Therefore, although mountain-down faulting may be a feature associated with warping, the warping must be incidental to major valley-down faulting. It is difficult to conceive of faulting of the magnitude of that in the Great Basin without some subordinate warping. The physiographic evidence strongly indicates a sin- gle fault of large displacement along the western front of the northern White Mountains, where the steep scarp, deeply incised by youthful canyons, and the extensive alluvial fans rival those of the steepest slopes of the Sierra Nevada. In the northern White Mountains an uplifted surface of low relief occupies a large area of 16 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. the crest, and upfaulted alluvium, most of it overlain by Pleistocene(?) basalt (Anderson, 1937), is found at altitudes as high as 10,000 feet. Anderson (1937, p. 6) described the White Mountains as a horst "with down- thrown blocks on the east and on the west * * *. The western scarp is higher and steeper than the eastern one; the mountain block has been tilted toward the east." Deep Spring Valley is representative of the down- dropped blocks on the eastern side of the White Moun- tains (Miller, 1928). The Coso Range terminates Owens Valley on the southeast ; it is a low range bounded by seven mappable faults on the west flank. An erosion surface of low relief extends over the summit of the Coso Range (Hop- per, 1947). Along the eastern front of the Sierra Nevada large westerly offsets of the front are found west of Bishop, where the Wheeler Crest escarpment is one of the most imposing along the Sierra Nevada, west of Long Valley, and west of Mono Basin ; successive westerly offsets of the eastern front of the Sierra Nevada continue north of Long Valley to the junction of the Sierra Nevada with the Cascade Range. Volcanic fields commonly occur in these offsets, as in Long Valley (Mayo, 1941), Mono Basin, and Sierra Valley far north of the mapped area. The Sierra escarpment is very steep and is in- cised by youthful valleys between Olancha and Big Pine (fig. 1). North of Big Pine to Tungsten Hills, the evidence for faulting is not conclusive and, as P. C. Bateman (written commun, 1956) showed and as the gravity data confirm, warping may be the dominant mode of deformation in this area. Knopf (1918, p. 79) recognized the physiographic contrast between the steep Sierra escarpment south of Big Pine and the relatively gentle slopes north of Big Pine; he regarded this contrast as evidence that dis- tributive faulting may have been the important mode of deformation between Big Pine and Tungsten Hills and that the escarpment to the south represents a pro- found break in the earth's crust. Examination of fig- ure 2 shows that the zone of change in the steepness of the Sierra Nevada escarpment near Big Pine, which presumably is evidence of a change in the mode of de- formation from faulting on the south to warping on the north, lies opposite the sharp shift in the western es- carpment of the Inyo Mountains and in a zone of vol- canism. Alabama Hills lie east of the Sierra escarpment and immediately west of Lone Pine. The fault along which movement caused the great earthquake of 1872 defines the eastern base of the Alabama Hills, and other recent fault scarps are found in the alluvium in this area (Hobbs, 1910). Lone Pine Creek flows across the Al- abama Hills, which presumably rose as this stream cut a narrow canyon in the bedrock and maintained its course. The conclusion of Knopf (1918, p. 90) that "Owens Valley is a great tectonic trough whose floor has sub- sided along a series of parallel faults" seems warranted on the basis of all available physiographic and surface geologic information. 'This conclusion is strongly sup- ported by the geophysical evidence at hand. _ North of Bishop, the graben between the Sierra Ne- vada and the White Mountain escarpments broadens to give a Y-shaped appearance to the modern drainage basin of Owens River. Included in the broad upper part of this Y are Round Valley, Long Valley, the structural extension of the Owens Valley block north of Laws, and several smaller Basin Ranges, the most prominent of which are the Benton Range and Blind Spring Hill (fig. 2) ; these ranges separate Long Valley from the graben to the east. Gilbert (1941) and Rine- hart and Ross (1957) studied this area and mapped the faults, for which there is abundant evidence. A bed- rock high separates Round Valley from Long Valley. Lavas and tuffs of Tertiary (?) age that cap the Benton Range have been uplifted about 1,000 feet and tilted eastward. Where volcanic rocks are absent, an old erosion surface forms the eastward-tilted slope of the range, which, although it has been battered by erosion, has only small fans at the western base. Blind Spring Hill was uplifted about 1,000 feet without tilting; its summit is an erosion surface of low relief carved in the granite bedrock (Gilbert, 1941). Glass Mountain Ridge is an east-trending mountain block bounded by a prominent fault scarp that faces the Sierra Nevada escarpment across Long Valley, which is to the south. f Although few of the faults of the Owens Valley area can be seen, those that are best revealed indicate that normal faults, whose dips range from 50° to 80°, predominate. Although some authors mention the pos- sibility of horizontal movement along the Sierra Nevada frontal fault, most seem to consider dip-slip movement to be predominant. Mayo (1941, p. 1063) observed 8 feet of left-lateral horizontal offset of a Recent mud-flow welt on Independence Creek, but he inferred predominant right-lateral offset on the basis of a shift in the course of the stream at the same place. Of the published reports on the earthquake of 1872, only that of Hobbs (1910), which was based on the map- ping of Willard D. Johnson in 1907, described right- lateral offset. The reports of Whitney (1872), Gilbert (1884), and Holden (1898) described or implied left- lateral offset. Most of the evidence of horizontal move- ment in the earthquake of 1872 has been lost, but Gianella (1959) recovered evidence of left-lateral offset GaEoLogy 17 in 3 places. Cordell Durrell (1950; written commun., 1959) found evidence of strike-slip movement along a fault in the eastern Sierra Nevada near Blairsden, Calif., having the same trend as Owens Valley. The direction of horizontal movement noted by Durrell was left-lateral, the northeast side of the fault having moved about 17,000 feet northwest relative to the southwest side; this movement is about five times the dip-slip component. Hopper (1947, p. 430) concluded that "horizontal shearing stresses were active in at least the latter part of the deformation" in the Sierra Nevada- Death Valley region. Nevertheless, dip-slip movement is important, perhaps predominant along a single fault, as the physiographic, geologic, and gravity data shows. vOLCANO-TECTONIC FEATURES Williams (1941), in his comprehensive study of cal- deras, suggested that the basin containing Mono Lake may have been formed like the volcano-tectonic depres- sions of the Pilomasin Basin of Sumatra by subsidence following extrusion of lava from a magma source below. The Long Valley basin may have been formed in the same way. Gilbert (1938) noted the possibility that the recent faulting in Long Valley is the result of the extrusion of a large volume of magma from beneath that area. Chelikowsky (1940, p. 488) stated: It is believed that the early expulsion of the great volume of basic lavas, together with the material in the welded tuffs, which are disposed on three sides of the basin, led to the col- lapse of what is now Mammoth embayment. The cooling of the early basic eruptions strengthened the initial weak zone along which the collapse occurred and shifted the site of later activity farther to the east. Subsequent volcanism accom- panied by a northwestward shift of the Sierra Navada mass caused the weak central portion of the embayment to yield and become intruded by the rhyolite * * * . It is therefore not surprising that the rhyolite fissure eruptions should have occurred in the central, most twisted part of the embayment. Mayo (1937, p. 184) described the formation of Long Valley, or what he later termed the Mammoth embay- ment, as follows: About 18 miles south of Mono Lake a great re-entrant exists in the Sierra front. The Mono volcanoes trend into this re-entrant, and many other meridional structures strike into it. It appears that here a block of the crust, weakened by many fissures, has subsided below the general level. This block is encircled by faults, and tremendous volumes of lava and pyroclastic materials have been expelled along its northern and eastern sides. Pro- trusions and stubby flows of rhyolite (Rhyolite Hills) have issued from the intensely fractured floor of the re-entrant. This interpretation in general is in agreement with present knowledge of the structure of the Long Valley block; this block has many of the features of a great caldera or volcano-tectonic depression as described by Williams (1941). The authors believe that Long Valley and Mono Basin are volcano-tectonic depressions of the type described by Williams (1941) and that subsidence was related both to extrusion of lava and the framework of Basin- and-Range faults. To a lesser extent subsidence associ- ated with volcanic extrusion may have been an influ- ence elsewhere on the structure of the Owens Valley area. EVOLUTION OF THE PRESENT LAND FORMS Because of the paucity of fossil evidence that can be used to date the various episodes of orogenic movement and the subsequent physiographic development in the Sierra Nevada and the basin ranges to the east, the pioneering geologists relied heavily on physiographic evidence to deduce the sequence of geologic events. These early geologists, with the exception of Lawson (1904), almost unanimously emphasized the antiquity of the major structural features although they recog- nized the recency of the faulting that gave rise to the present great eastern escarpments. On the other hand, recent students, relying on what little paleontologic evidence is at hand, have tended to assign virtually all the present structural and physiographic features of the Sierra Nevada-Owens Valley region to movements dating from near the close of Tertiary time. Further, they have largely discounted or ignored the possibility that the history of deformation may have been quite complex through Cenozoic time and that movements may have taken place at different times along faults occurring near each other. The most recent work, that of Axelrod (1957), is based on studies of late Tertiary floras and their behavior under changing conditions of altitude and climate and again emphasizes the recency of the uplift of the Sierra Nevada and minimizes the importance of earlier movements. Lindgren (1911, p. 41-43) postulated a complex his- tory of faulting along the eastern front of the Sierra Nevada and wrote : The main break along the east side of the range is one of great antiquity, probably dating back at least to the last part of the Cretaceous, but movements have recurred at different times, and the fault system became greatly extended by additional breaks at the close of the Tertiary. Post-Tertiary and recent move- ments have taken place in many localities * * * . To sum up, faulting has recurred irregularly along the eastern fault zone since the Cretaceous period. The subsidences along the faults are not uniform. A Cretaceous dislocation along one line may be continued by a late Tertiary fault on the extension of this line. More recent students of Basin-and-Range geology, in- cluding Nolan (1943), Jahns (1954), and Axelrod (1957), recognized that "adjacent fault blocks com- monly have had distinctly different geologic histories, 18 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. thanks mainly to the nature and timing of movements on the faults that separate them" (Jahns, 1954, p. 13). King (1878, p. 744), even before Lindgren, postulated that movement along the Sierra Nevada fault began early, perhaps within Eocene time or at the close of the Eocene. PHYSIOGRAPHIC EVIDENCE Beginning with Lawson, the pioneering geologists de- duced two important post-Eocene uplifts of the Sierra Nevada from their studies of the physiography of the Sierra Nevada uplands. All these early geologists Lawson, Lindgren, Knopf, and Matthes-recognized that the latest and probably greatest of these uplifts dated from latest Tertiary or more recent times. All but Lawson placed the initial uplift within the Tertiary ; Lawson regarded it as early Pleistocene. Lawson (1904) recognized three preglacial erosion surfaces in the Upper Kern River basin. The oldest of these he termed the Summit Upland and its related Sub- summit Plateau (fig. 3). The Summit Upland was formed, according to Lawson, by stripping away the roof of sedimentary rocks to and in places below the upper surface of the granite core of the high mountain range of the Nevadan orogeny. The Subsummit Plateau was formed in the late stages of the erosion cycle in which the Summit Upland became mature and is found, in the Upper Kern River basin, at an altitude of about 11,500 feet above mean sea level. Chrono- logically, the Subsummit Plateau correlates in part with the Summit Upland, and both surfaces antedate the uplift and faulting of the eastern front of the Sierra Nevada. MOUNT WHITNEY Altitude 14,495 feet 16,000" sub *% r i ubsummit plateau 14,000 High valley zone High valley P 12,000" Kern zone 10,000 anyon, 8000' 0 1 2 3 4 MILES Ficurs 3.-Erosion surfaces of Kern Canyon-Mount Whitney area (after Knopf, 1918). The High Valley zone of Lawson was cut after the initial uplift of the Sierra Nevada in what Lawson re- garded as the beginning of Pleistocene time. Mature valleys, about 2,500 feet deep, were formed during this stage of erosion. Representative of them are the Chagoopa Plateau, Toowa Valley, and the Little Kern Plateau. The Canyon zone of Lawson was cut after the second, greater uplift and is represented by the present deep canyons of the Sierra Nevada at altitudes too low to have been subjected to glacial erosion. The present Middle Kern Canyon was, according to Lawson, cut along a fissure or graben that formed at the same time as the canyon cutting. Webb (1946), however, postu- lated that the Kern Canyon fault predates the canyon and may have been formed in Nevadan time. Lawson estimated the duration of the Pleistocene as about 2% million years, which is much longer than present estimates of about 1 or 1%, million years (Evernden and others, 1959). He did not, however, have benefit of present knowledge of the glacial stages in the Sierra Nevada, nor did he have any fossil evi- dence. He cited no direct evidence for dating the initial uplift of the Sierra Nevada from the beginning of the Pleistocene, and indeed he divided the Pleistocene into two parts on the basis of the Sierra Nevada uplifts : the first dating from the initial uplift of the Sierra and the second from the final uplift. Thus, Lawson may have included in the Pleistocene a considerable period of time considered by Lindgren, Knopf, and Matthes to be within the Tertiary. Lindgren (1911, p. 41-43) maintained that the pres- ent fault lines of the eastern escarpment of the Sierra Nevada "were in the main established" before the rhyo- litic and andesitic eruptions and that they were in exist- ence at the time of deposition of the auriferous gravels. However, he recognized that faulting and uplift oc- curred after the close of the rhyolitic eruptions and during the andesitic eruptions of late Tertiary time. Lindgren (1911, p. 41) and others after him (for ex- ample, Knopf and Matthes) believed that the recent dis- location along the eastern escarpment of the Sierra Nevada "consists in a sinking of the eastern blocks." The rhyolite tuff to which Lindgren referred was de- posited on bench gravels of the Yuba River that he re- garded as Miocene(?) in age on the basis of fossil leaves. Following an interval of erosion, the debris that became andesitic tuffs and breccias flowed from vol- canoes along the Sierra Nevada summit down the river valleys. The rhyolitic tuffs are now termed the Valley Springs formation, and the andesites are called the Mehrten formation in the Sierra Nevada (Axelrod, 1957). Both are considered by Axelrod to be "Mio- Pliocene" in age. Lindgren regarded the upbuilding of the Sierra Nevada as one of repeated rejuvenation along eastern fault lines of great antiquity. Knopf (1918, p. 82-84) showed that the contact be- tween the granitoid rocks of the Sierra Nevada and the roof pendants is a surface of very great relief ; the sedi- mentary roof pendant at Table Mountain, between the Middle and South Forks of Bishop Creek, for example, projects at least 2,500 feet downward into the granite. Knopf (1918, p. 83) therefore rejected Lawson's hy- pothesis that the Summit Upland is, in the words of GEoLocy 19 Knopf, "a surface of differential degradation controlled by the gently undulating contact surface of the granite batholith with the rocks that formerly extended over it." The Summit Upland is, then, only the unreduced interstream portion of the ancient erosion surface repre- sented by the Subsummit Plateau; the two surfaces are continuous parts of a high mountain zone, and they were formed during the same period of erosion. Knopf (1918, p. 83-86) adopted the High Valley zone of Lawson (fig. 3, this report) and related it to the period of erosion following the initial Tertiary up- lift of the Sierra Nevada. This uplift of about 2,500 feet may, according to Knopf, be as recent as early Pliocene or as early as late Eocene. Because east-flow- ing streams that evolved a mature topography are found stranded high upon the eastern fault escarpment of the Sierra Nevada, Knopf (1918, p. 88) assumed that this initial disturbance was epeirogenic in nature and not accompanied by relative subsidence of the Owens Valley block. The present canyons were cut into the High Valley zone following uplift of the Sierra Nevada to its present heights in early Pleisto- cene, accompanied by relative subsidence of the Owens Valley block to the east. A period of stability sepa- rated the two Sierra uplifts. Knopf (1918, p. 88) wrote in summary : For the facts established by Lindgren in his study of the Ter- tiary auriferous-gravel epoch show that the drainage was re- juvenated at the close of the rhyolitic eruptions, probably late in the Miocene. This date Lindgren believes is more in harmony with the length of time indicated by the great erosional work performed since the uplift ; it assuredly does less violence to our ideas concerning the length of Quaternary time than does the assignment of a post-Pliocene age to the initial uplift and to the westward tilting of the range. The evidence from measurements of gravity of tremen- dous accumulations of clastic debris in Owens Valley and of prolonged continuous or repeated fault move- ment along the bounding faults supports Knopf's conclusion. In his study of the geologic history of the Yosemite Valley area, Matthes (1930, p. 27-31) outlined in greater detail than his predecessors the probable history of the rise of the Sierra Nevada. Recognizing that faulting along the eastern escarpment of the Sierra may have be- gun as early as the end of Cretaceous time, Matthes thought it more likely that the structures to the east were blocked out by faulting during the second half of Tertiary time and that the earlier movements were gradual upwarpings. Matthes (1930, p. 31-45) found evidence of three episodes of uplift in three stages of canyon cutting by Merced River and in a correlation of hanging valleys that discharge their waters into the Merced River from two different levels. The erosion surfaces in the can- yon of the Merced River studied by Matthes are simi- lar to those in the Upper Kern River basin described by Lawson. The highest and oldest of these episodes is the mature Broad Valley stage (fig. 4), which dates from a gradual upwarping in early Eocene time that continued inter- mittently to the end of the Eocene and was followed by quiescence through Oligocene and the first half of Miocene times. The Broad Valley, therefore, is virtual- ly a late Miocene surface where it has not been modified by glacial sculpture, although, as Axelrod (1957) pointed out, it or any other exposed surface is in reality a modern surface; the Broad Valley surface has been subjected to some modification by erosion later than the Miocene. A high system of hanging valleys is stranded above the Merced River at the Broad Valley level. 8000' 7000' 6000' 5000' 4000 0 Ya 1 MILE Fieur® 4.-Erosion surfaces oflgggimite Valley area (after Matthes, The surface of the Mountain Valley stage is incised into the Broad Valley surface; it may be correlated with the High Valley zone of Lawson. During the sec- ond half of Miocene time, the Mountain Valley surface was eroded to smoothly rounded slopes following a major uplift, accompanied by intensive faulting along the eastern margin of the Sierra Nevada. A quiescent interlude followed through Pliocene time; so, the sur- face of the Mountain Valley stage is virtually late Pliocene in age. Another system of hanging valleys at the Mountain Valley level was identified by Matthes. Finally, the Sierra Nevada was uplifted to its present height in early Pleistocene time; the uplift was accom- panied by great faulting to the east. The youthful canyons of the Canyon stage were deeply incised into the older Mountain Valley surface following this up- lift to give the present surface. Glacial sculpture has greatly augmented stream action in producing the present scenery of Yosemite Valley. The Canyon stages of Matthes and Lawson are presumably the same. The late Miocene uplift raised the crest of the Sierra Nevada 3,000 feet according to Matthes; the final, early Pleistocene uplift elevated the crest to its present height. Mount Lyell in the Yosemite Valley area, which is now 20 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. about 13,000 feet above mean sea level, was at an alti- tude of about 4,000 feet during the Broad Valley stage. Later Matthes (1933, p. 38), citing the faulting of McGee moraines as evidence of downfaulting of the Owens Valley block, believed that the "features of the eastern escarpment of the Sierra Nevada are nearly all of Quaternary age." Matthes (1947) concluded that the final Sierra Nevada uplift came at the end of Plio- cene time and carried up with it the area to the east. It was not until after the first ice age that faulting began, Owens Valley sank, and the great eastern scarp of the Sierra Nevada was formed by successive disloca- tions along the bounding faults. Thus, on the basis of glacial evidence, Matthes brought his views into accord with ideas expressed earlier by Lindgren and Knopf. However, the western bounding fault of the deep cen- tral wedge of Owens Valley is not everywhere coinci- dent with the eastern bounding fault of the Sierra; so, subsidence of this part of the Owens Valley block may have begun before the latest uplift of the Sierra Nevada. Also, in the paper of 1947, Matthes revised his deter- mination of the initial uplift of the Sierra Nevada to- ward the Recent, changing it from the last half of the Miocene to the end of the Miocene or the beginning of the Pliocene. Although he recognized the possibility that faulting may have accentuated the ruggedness of the eastern slope, he believed that the main movement was an asymmetric upward bowing of the Sierra Nevada, resulting in a short, steep eastern slope and a long gentle western slope. PALEOBOTANIC EVIDENCE Axelrod (1957) rejected the ideas of Lindgren and Matthes on the initial Tertiary uplift of the Sierra Nevada and concluded, on the basis of his study of "Mio-Pliocene" flora, that the Sierra was a broad ridge with a summit at about 3,000 feet above mean sea level just before the late Tertiary volcanism. Referring to the Broad Valley erosion surface of Matthes, Axelrod (1957, p. 21) wrote: Some geologists have expressed skepticism that the upland sur- face in the Yosemite area corresponds to that on which the Mehrten formation was deposited. They point out that there is no evidence that the Yosemite area was covered by andesite, and hence the erosion surface may have been developing since Miocene time, or earlier; in any event, the present surface is of later Cenozoic, not of Mio-Pliocene age, for it is not covered by volcanics. Others have examined the region and doubt that there is an old erosion surface, at least in Matthes' sense. Although it may be doubted that the Broad Valley sur- face was formed following an Eocene upwarping of the Sierra Nevada, as Matthes maintained, it is on the Mountain Valley and Canyon stages that Matthes based his conclusions on the post-Eocene Sierra uplifts. His observations on these surfaces are supported by the earlier studies of Lawson, the observations of Knopf, and Baker's (1912) correlation of the Ricardo erosion surface of the Mojave region with the Chagoopa Pla- teau in the basin of the Upper Kern River. Axelrod (1957, p. 21) evidently misunderstood the bases of Matthes' conclusions on the uplift of the Sierra, for he wrote: "His [Matthes'] estimate of 3,000 feet up- lift at the close of the Miocene apparently was derived from the work on the Truckee quadrangle which led Lindgren * * * to believe that there was major pre- andesite faulting in the region." Matthes actually de- duced this uplift on the basis of the surface of the Moun- tain Valley stage and the hanging valleys of this and the Broad Valley stage. Further, Axelrod (1957, p. 21) misinterpreted Matthes' ideas concerning faulting when he noted that "Matthes cited no facts in support of Mio- Pliocene faulting in the Yosemite area. Actually, no such evidence exists there, for the rocks in the uplands are pre-Tertiary crystallines and provide no data for estimating the amount of faulting at any time in the Tertiary." Matthes (1930) discussed the coincidence of uplift of the Sierra Nevada with faulting along the east- ern front of the range well to the east of Yosemite Val- ley, and later he (Matthes, 1947) minimized the im- portance of faulting during the late Miocene or early Pliocene uplift. Thus Axelrod's (1957, p. 21) conclu- sion that "the method by which his [Matthes'] conclu- sion was reached is unsound" seems unjustified. Axelrod (1957) studied both the effect of altitude on more than 20 "Mio-Pliocene" floras that were living just before or during the early eruptions of the late Tertiary andesites (Mehrten formation) and the influence of the Sierra Nevada as a climatic barrier. He concluded that the Sierra was a broad ridge having a summit about 3,000 feet above sea level just before these eruptions and that it was incised by valleys 24 miles wide and about 1,000 feet deep, some of which drained western Nevada. The lowlands of western Nevada had an average altitude of 2,000-3,000 feet above sea level. Fault deformation began in late Pliocene or early Pleistocene time, and up- lift increased southward. The total uplift in the Donner Pass area was about 5,300 feet; it was about 6,500 feet at Carson Pass just south of Lake Tahoe. About 3,000 feet of andesite was piled on top of the Sierra ridge by the close of early Pliocene time. The initial uplift of the Sierra Nevada is commonly assumed to have taken place during and probably immediately following this episode of volcanism. However, Axelrod found that floras to the lee of the Sierra Nevada did not reflect the lower amount of precipitation relative to the windward slopes expected from the creation of such a climatic barrier. Axelrod (1957, p. 42) concluded: GEOPHYSICAL SURVEYS 21 This probably was because the crust could not support the vol- canics on the Sierra Nevada and adjacent Nevada. To maintain isostatic balance, the basement would have to sink and displace a mass of sima equal to the added volcanics. Since an andesite blanket 3,000 feet thick would have to subside approximately four-fifths to come into equilibrium, the surface would be only about 500 feet higher than before vulcanism. Evidence from gravity measurements in the Sierra Nevada (Oliver, 1956) and the Basin Ranges (D. R. Mabey, written commun., 1956) and from this study clearly shows that, generally, topographic masses are only regionally compensated isostatically. Therefore, residual relief exceeding an equivalent uplift of 500 feet would probably remain after the andesite erup- tions, and at least some high peaks were possibly built up by these eruptions, even though some subsidence (with faulting along the eastern border of the Sierra Nevada) may have accompanied withdrawal of magma from beneath the area. If the andesite came from within the zone of isostatic compensation, the eruptions would have brought about no net disturbance of the iso- static equilibrium but rather a redistribution of the crustal load by piling the andesite high above the pre- eruption surface in some places. Axelrod (1957, p. 42) concluded : It was only in post-middle Pliocene time, largely in the late Pliocene to middle Pleistocene interval, that warping and fault- ing increased the altitude of the Sierran crest line from 5,000 to 6,000 feet in this north-central section. The effects of warp- ing and faulting increased southward, and 100 to 150 miles away maximum uplift was approximately 7,500-9,000 feet. Axelrod acknowledged that there is "some evidence that the range gradually decreased in altitude north- ward in Mio-Pliocene time * * *." Hinds (1956), also basing his conclusions on paleobotanic evidence, wrote : Later in the Miocene and early Pliocene, a long, dominantly explosive volcanic episode covered large sections of the northern half of the range with mudflows. Considerable elevation, pos- sibly doming of a greater area than the Sierra Nevada itself, occurred during and following this cycle, allowing erosion of canyons 1,200 to 1,500 feet deep. During the late Pliocene, the major elevation of the Sierra Nevada occurred, and greatly in- vigorated streams started erosion of the canyons of the present cycle. Noting that "dislocation of McGee moraines on the east side of the Sierra Nevada shows that graben subsidence of at least 3,000 feet followed the first glacial age and is still continuing * * *," Hinds (1956) seemed to reach conclusions in virtually complete accord with those of Matthes in his later years. Although the geophysical data suggest to us that the deepest wedge of the Owens Valley graben, which is structurally removed from the Sierra Nevada, could have been blocked out by faults well before the end of Tertiary time, we find little evidence in the litera- 728-195-64--4 ture, our own field observations, or the gravity data to refute Matthes' work. Many other geologists have contributed to our knowl- edge of the Sierra Nevada-Owens Valley region, and they will be considered with the analysis of the geo- physical data. Finally, Hudson (1955) deduced a mid- dle Eocene altitude of 5,500 feet above sea level for the Sierra Nevada summit in the Donner Pass area on the basis of the gradients of the Tertiary Yuba River. Therefore, the late Tertiary uplift of the Sierra Nevada was only about 2,000 feet in the Donner Pass area. Hudson and Axelrod seem to agree on the history of the Sierra Nevada uplift; they differ on its magnitude. GEOPHYSICAL SURVEYS The earth at an early stage in its history was divided - by some process of differentiation into its major units: the crust, the mantle, and the core (Mason, 1952, p. 53- 57). Each of these units has its characteristic density, seismic velocity, and other physical properties (Birch, 1952). Had the earth remained undeformed at the end of its original differentiation, the density and velocity layering would now be concentric. The gravity field as measured at the surface would vary uniformly with latitude, according to the International Gravity For- mula (Nettleton, 1940, p. 137), and would yield infor- mation only on the shape of the earth and its angular velocity. Seismic traveltime curves would be every- where the same. The magnetic field would also be mo- notonously smooth. But the earth has not remained undeformed. As a result of the application of internal forces, rocks of contrasting densities, seismic velocities, and magnetic susceptibilities (and other physical properties) have been moved into irregular and complex juxtaposition by faulting, folding, igneous activity, water, wind, and ice, and simply by sliding or rolling down slopes that were formed by these same internal forces. As a result, the gravity field is highly irregular, the paths of seismic waves are complex and variable, and the magnetic field is seemingly erratic. The geophysical anomalies thus created can be measured. Because they follow well- established physical laws and because they are con- trolled by the nature of the geologic deformation, these anomalies can be interpreted to reveal important in- formation on the subsurface geologic structure. In addition they may provide some clues as to the processes and forces that brought about the rock movements that in turn caused the anomalies to come into existence. In the Owens Valley region, we are concerned with the following kinematic processes: the displacement of lighter against heavier rocks by faulting; the erosion, transport, and deposition of rock particles by water, 22 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. | wind, and ice; and the movement of magma from great depths to the surface by volcanic activity. These kine- matic processes have brought about lateral inhomo- geneities in density, seismic velocity, and magnetic sus- ceptibility; we can therefore study their effects by gravity, seismic, and magnetic surveys. The movement of rock materials by these kinematic processes in the Owens Valley area has been confined for the most part to the area, for no natural outlet to the sea exists and, through much of its history, Owens River has not flowed beyond the southern limit of Owens Valley. Therefore, the Owens Valley area can be re- garded as a closed system in that little rock material has been brought in and relatively little has been carried out of the area. The total mass of rock material in- volved in the very great deformation in the Owens Valley area has therefore remained roughly constant. The geologic structures and processes of the area are particularly appropriate for study by geophysical methods. | GRAVITY SURVEY The density of the light deposits of Cenozoic age that fill the valleys and basins is considerably less than that of the pre-Tertiary rocks that lie buried beneath these lighter deposits and that rise above the valleys to form the main masses of the mountain ranges. Therefore, thick accumulations of Cenozoic deposits should be ex- pressed by pronounced gravity lows; and zones of steep gravity gradients may suggest something about the in- clination of the interface between the Cenozoic deposits and pre-Tertiary rocks. DENSITY Densities of the pre-Tertiary plutonic and meta- morphic rocks in the Owens Valley area are known with considerable accuracy. Densities of the Cenozoic clastic and volcanic rocks are highly variable and are much less reliably known. However, available seismic-refrac- tion information on depths to the pre-Tertiary bedrock floor makes it possible to place the average density of the complete Cenozoic section along common gravity and seismic profiles within a fairly narrow range of values. Although we collected rock samples only in the Long Valley area, the measured densities of the pre- Tertiary rocks there can be assumed for the entire Owens Valley area. Densities of samples of the Cenozoic de- posits collected at the surface have little significance in gravity interpretation ; the densities computed from a comparison of the seismic and gravity data are much more meaningful, and we will place our confidence on these. William Huff of the U.S. Geological Survey made the measurements of density on the rock samples from the Long Valley area. Measured densities of 10 dry samples of the plutonic rocks range from 2.42 to 2.69 g per cm*; the average density of these samples is 2.60 g per cm'. If two samples of low density are omitted, the average density of the plutonic rocks becomes 2.63 g per cm®. Measured densities of nine dry samples of metamorphic rocks range from 2.63 to 2.94 g per em?, and their average density is 2.78 g per em®. A representative density of 2.7 g per cm' is assigned to saturated pre-Tertiary rocks for purposes of interpretation. This density agrees closely with that determined by Howard Oliver (written communication, 1959) in his gravity study of the Sierra Nevada and with published data on rock den- sities. (See, for example, Birch and others, 1942, p. 14; Heiland, 1940, p. 80-81.) The contrast in average density of nearly 0.2 g per cm between the plutonic and metamorphic rocks may have some influence on the gravity field. It is exceedingly difficult to determine a reliable aver- age density for the Cenozoic deposits because of their extreme heterogeneity; the densities of samples col- lected at the surface can be misleading. Twenty-two samples of Cenozoic rocks (excluding basalt) collected in the Long Valley area range in density from a mini- mum of 1.86 for the Bishop tuff of Gilbert (1938) to a maximum of 2.37 g per em for coarse lake beds. Six basalt samples range in density from 2.59 to 2.73 g per cm* and their average density is 2.66 g per cm'. Gil- bert (1938) found that the Bishop tuff ranges uniformly in density from 1.3 g per cm at the surface to 2.32 g per cm® at a vertical distance of about 400 feet below the top of the tuff. The rocks of Cenozoic age that fill the depressions in the Owens Valley area are known to include al- luvial deposits, lake beds, glacial till, and a wide as- sortment of volcanic deposits; these rocks range in density from less than 1 for pumice to at least 2.73 g per cm> for basalt. An average density of 2.2 or 2.3 g per em is assigned to the Cenozoic deposits for purposes of interpretation. Use of this average density for computations yields de- terminations of depths to the pre-Tertiary rocks that agree closely with those obtained from the seismic- refraction results, and this average density is known to agree closely with the average density determined by a combination of seismic and gravity methods by the Shell Oil Co. in Railroad Valley, Nev. (R. J. Bean, written commun., 1958). D. R. Mabey (written com- mun., 1958) determined a similar average density after extensive sampling of the deposits of Cenozoic age in the Mojave Desert and the Death Valley regions of California. GEOPHYSICAL SURVEYS In the interpretation of the gravity data, therefore, a contrast in average density of -0.4 or -0.5 g per em® is assumed to exist between the Cenozoic deposits and the pre-Teritary rocks. (See Pakiser and others, 1960.) However, the density contrast is acknowledged to be possibly as much as -0.6 g per ecm or as little as -0.3; true depths (determined in the absence of seismic confirmation), therefore, may range from about TO to 130 percent of those computed from the gravity data. Despite this wide range of possible depths, deter- minations of the configuration of the Cenozoic struc- tures in the Owens Valley area from the gravity data remain the same. Lateral variations of density within the Cenozoic section may, however, affect the gravity field. For a more detailed discussion of the problem of the density of valley-fill sediments, see Kane and Pakiser (1961). FIELDWORK AND COMPUTATIONS A total of 1,550 gravity stations was established dur- ing five field periods: February 1954, July-August 1955, February-March 1956, July-August 1956, and July 1957. - The fieldwork in the higher altitudes north of latitude 37°30" N. was done during the summer to avoid snow; most of the work in Owens Valley was done during the winter to avoid the excessive summer heat. The field party was made up usually of four men : the party chief and computer, the observer, the surveyor, and the rodman. Gravity stations were usually set along roads and jeep trails, but some traverses were made on foot where the need for more detail was in- dicated by the data. Measurements were extended into the mountain areas so that the regional gravity gradi- ent could be determined. Bench marks of the U.S. Geological Survey, the U.S. Coast and Geodetic Survey, the California State High- way Department, and the City of Los Angeles Depart- ment of Water and Power, and spot elevations on U.S. Geological Survey topographic maps were used as lo- cations for about 55 percent of the stations. The remaining stations were surveyed by the field crews. Most of the surveying was done by using planetable and alidade, but a few stations requiring long hikes were set by using altimeters. Sixteen base stations were established at easily acces- sible and relatively permanent locations. The locations were selected as the survey expanded and as new base stations were needed. At least three readings were made at each new base, and the average of the readings was used to determine the base value. All repeated read- ings at the base stations agreed within 0.2 mgal. Base- station data and descriptions are listed in table 1. 23 TaBus 1. Owens Valley project gravity bases Lati- Alti- | Observed | Free air | Bouguer | Complete Station tude | Longitude] tude gravity ! | anomaly | anomaly | Bouguer N. w. (feet) (mgal) (mgal) (mgal) (mgal) Inyo County Bil.lsi.. 87°22.5' | 118°23.6' | 4,143 | 979,462.88 | -96.99 | -238. 14 -233. 64 37°09.5' | 118°17.3' | 4,009 | 979, 465. 28 -88. 36 | -224.95 | -217.55 36°46.0' | 118°10.7" | 3,955 | 979,465.00 | -60.65 | -195.40 | -180.63 L..... 36°40.4' | 118°05.7' | 3,738 | 979, 468. 81 -68.32 | -195.67 | -189.70 36°17.0'° | 118°00.3' 3, 646 | 979, 420. 60 -91. 58 | -215.80 | -208. 4 ©...... 6025.9" | 117°49.4' | 3,796 | 979, 477.91 -32.93 | -162.26 | -158.39 T= 36°19.9'° | 117°42.8' | 4,878 | 979, 406.06 4-5. 61 | -160.58 | -159.04 Mone County 118947.0' | 6,920 | 979, 314.48 -2.61 | -238. 37 -231. 27 118°54.9" | 7,289 | 979, 278. 80 -8.82 | -257.16 | _ -253. 64 118945.8' | 6,817 | 979,300.87 | -35.80 | -268.06 | -266.05 118°35.4" | 6,967 | 979,353.65 | 434.88 | -202.49 | -199.94 118°23.7" | 4,527 | 979, 453.91 -92. 34 | -246.57 | -239.67 119°04.4' 7, 287 | 979, 322. 51 +17. 54 | -230.73 -227. 20 119°01.8' 6, 461 | 979, 348. 95 -17.81 | -246.28 | -243.35 | 6,706 | 979,382.94 | -12.56 | -232.60 -221. 97 118°47.8' | 7,083 | 979,365.83 | 413.82 | -227.50 | -226.30 titlfeferted to U.S. Coast and Geodetic Survey "Independence" gravity pendulum station. 2 Terrain correction made. 3 U.S. Coast and Geodetic Survey "Independence" gravity pendulum station. B1. About 1.0 mile north of Bishog. Inyo County, along U.S. Highway 6 and 395, about 0.2 mile north of junction of U.S. Highways 6 and 395, on U.S. Highway 6, at service station, 5 ft west of U.S. Coast and Geodetic Survey bench mark identified by a disk stamped "V-124 Reset 1945" and set in concrete base of the pump island; on ground. At south edge of Big Pine, Inyo County, on U.S. Hlfihwa‘y 6 and 395, at southwest corner of intersection of the high- way and Bartell Avenue, 4 ft east of California Depart- ment of Highways bench mark 33-B; on ground. B3. About 2.2 miles south of Independence, Inyo County, along U.S. Highway 6 and 395, 150 ft east of California Division of Highways marker 4434-19, at U.S. Coast and Geodetic Survey pendulum gravity station U.S. 1030 Independence (Duerksen, 1949) ; on ground. B4. About 5.0 mile north of Lone Pine, Inyo County, along U.S. Highway 6 and 395, north of bridge over sglllway, near California Division of Highways station 480-485, 50 ft north of northwest corner of the bridge, 45 ft west of the center line of highway ; on ground. B5. At Olancha, Inyo County, at the northeast corner of the intersection of U.S. Highway 395 and road to Darwin (east of Owens Lake), 2 ft south of fence corner ; on ground. BG. About 20 miles south of Lone Pine, Inyo County, along California State Highway 190, at intersection of highway mnd road to Olancha, at southwest corner of the inter- section ; on ground. BT. About 8.4 miles northwest of Darwin, Inyo County, along road from Darwin to Lone Pine, on California State High- way, 190, about 40 ft south of road leading to Saline Valley, near U.S. Coast and Geodetic Survey bench mark identified by a disk stamped "4880-B 4 1905" and set in large black lava rock ; on ground by the rock. About 8.4 miles southeast of Casa Diablo Hot Springs, Mono County, on U.S. Highway 395, at bridge over McGee Creek, near U.S. Coast and Geodetic Survey bench mark identified by a disk stamped "C-124 1932" and set in the southwest corner of the bridge, on south shoulder of the highway 10 ft southeast of the disk; on ground. At Casa Diablo Hot Springs, Mono County, 274 ft southeast of the Casa Diablo store and 84 ft east of the center line of the highway, near U.S. Coast and Geodetic Survey bench mark identified by a disk stamped "Z-123 1932" and set in concrete post; on ground by e post. Along1 road between U.S. Highway 395 and California State Highway 120, Mono County, about 8.0 miles northeast of U.S. Highway 395, 6.9 miles northeast of Whitmore Hot Springs, 370 ft east of Owens River, 90 ft north and 38 ft east of a road junction, near U.S. Geol, Survey bench mark identified by a disk stamped "4-JD 1952 6818" and set in concrete post; on ground by the post. Along road between U.S. Highway 395 and California State Highway 120, Mono County, about 22.5 miles northeast of U.S. Highway 395, 21.4 miles northeast of Whitmore Hot Sgrings, T2 ft north and 45 ft east of road leading east, 41 ft north of road sign near U.S. Geol. Survey bench mark identified by a disk stamped "10-JD 1952 6968" and set in concrete post; on ground by the post. About 13.9 miles south of Benton Station, Mono County, along U.S. Highway 6, at the crossing of a power line and the intersection of a dirt road, 68 ft west of the center line of the highway, near U.S. Coast and Geodetic Survey bench mark identified by a disk stamped "A-819 1955" and set in a concrete post; on ground by the post. LBL. LB2. LB3. LB4. LB5. Fa +, 24 TABLE 1.—Owlm Valley project gravity bases-Continued LBG. About 8.3 miles southeast of Lee Vining, Mono County, along U.S. Highway 395, at junction of an old road leading south- west to June Lake, about 61 ft west of the center line of the highway, near City of Los Angeles Department of Water findthpo ex; standard cap set in concrete post; on ground y the pos MB326. About 4.7 miles east along California State Highway 120 from the intersection with U.S. Highway 395, Mono County, on a curve at the foot of an east-sloping hill, 28 ft south of the center line of the highway, near U.S. Coast and Geodetic Survey bench mark identified by a disk stamped "Q-204 1934" and set in a concrete post; on ground by the post. MB334. At Mono Lake, Mono County, on the west side of U.S. High- way 395, in the parking lot north of the post office, at the approximate location of U.S. Coast and Geodetic Survey bench mark P-123 (destroyed by flood), 25 feet north of the north dwall of the post office, 7 ft east of stone wall; on ground. f MB483. 34 miles southwest of Hawthorne, Nev. along Nevada State Highway 31, (21.6 miles northeast of U.S. Highway 395 along pole line road which connects with Nevada State Highway 31) at the California-Nevada State line, 26 ft south of the center line of the highway, near Nevada Sur- vey Mark Tablet stamped "1-7082.95" and set in concrete post ; on ground by the post. A single-loop system was used in making the gravity measurements. The initial and final readings of a series to define the meter drift. The difference in base read of measurements were made at a base station, and the' and tidal variations, but in practice the difference is treated as if it were a linear variation with a single cause. Each series of readings contained a reading at a station that had been set earlier in the same loop and a reading at a station that had been set in a previous loop. The first repeat reading was used to check (but not control) the meter drift ; the latter served as a check on the accuracy. Differences for both types of repeat readings have been compiled on histograms (fig. 5). The gravity measurements were adjusted for linear instrument drift and were referred to the U.S. Coast and Geodetic Survey gravity-pendulum station at Inde- pendence (Duerksen, 1949). Later ties to the base net- work showed this network to be within 1 mgal of Woollard's (1958) airport-base network. Corrections for latitude and altitude were made using procedures outlined in standard geophysics texts (Nettleton, 1940, p. 51-62). The corrections were made during the field- work in order to examine the progress of the survey. The drift curves were plotted daily. Terrain corrections were made later using the method outlined by Swick (1942, p. 67-68). The terrain corrections were extended through zone "O" of this system which corresponds to a radial distance of about 100 miles outward from the gravity station. A density of 2.67 g per em was used for the altitude and terrain corrections. This density is representative of the pre-Tertiary rocks of the area in which the greatest range in altitude is found. One thousand milligals were added to the complete Bouguer gravity so that the final values on the map (pl. 1) are positive. The complete Bouguer gravity with respect to the International Ellipsoid may be readily obtained by subtracting 1,000 from the contour values. .. ings is actually caused by a combination of meter drift? \ STRUCTURAL GEOLOGY AND VOLCANISM, owWENS VALLEY, CALIF. 200 El € € 180 - & 81, 5 Eh a 160 |- 140 |- | 120 |- ¥ A 100 |- ALL DIFFERENCES; MEDIAN, 0.056 MILLIGAL Z §1 so - |g 60 |- 40 |- 20 |- || 0 _1_—'—l——1——I 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 MILLIGALs res- z \ f t "V as & B &! BASE DIFFERENCES; MEDIAN, 0.056 MILLIGAL 20 - _ {1% F o = 0.00 0.05 0.10 0.15 0.20 MILLIGALS bir = asl §§ R c 9 € wi - DIFFERENCES BETWEEN METERS; (2 MEDIAN, 0.074. MILLIGAL 20 4T? 1 § -o- o 3. P 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 MILLIGALS Ficurm 5.-Histograms showing accuracy of gravity data. The gravity meters listed as follows were used in the survey : Gravity meter Period used. Mgal/dial division Winter 1954. 0. 0729 Worden 00 . 5046 120..=¢ Summer 1955_.... .._.._ . 5046 Doi. nnn Winter 1956..:....~_. . 5045 Worden Summer . 2453 Worden 220............ 1. .A. cels... . 4720 Worden 1. bummer 1957..... __. . 5533 ACCURACY OF DATA The histograms (fig. 5) are based on the differences without respect to sign of the values between original readings at a station and repeat readings. Histogram A is based on all the differences and includes 412 re- peat readings made with 5 different instruments, 11 different operators, and a time span of 5 years. The ties make up 20 percent of all gravity measurements, and this percentage is sufficient for determination of overall accuracy. The median difference, 0.056 mgal (milligal), is the probable error of any single measure- ment in the sense that 50 percent of the measurements GEOPHYSICAL SURVEYS may be repeated with a probable difference of less than 0.056 mgal and 50 percent with a probable difference greater than 0.056 mgal. Twenty-five percent of the differences are within 0.025 mgal, 50 percent within 0.056 mgal, and 95 percent within 0.190 mgal. All dif- ferences are within 0.450 mgal. Histogram 2 is based on 71 differences determined at base stations. The median difference, or probable er- ror of any single measurement, is also 0.056 mgal. Twenty-five percent of the differences are within 0.030 mgal, 50 percent within 0.056 mgal, and 95 percent with- in 0.154 mgal. All base-station differences are within 0.20 mgal, which reflects the greater accuracy of base measurements. Histogram C is made up of 70 differences that were determined by different meters; that is, the meter used in the repeat reading was different from the meter used in the original reading. The median, 0.074 mgal, is greater than those of the two previous groups. Twenty- five percent of the differences agree within 0.039 mgal, 50 percent within 0.074 mgal, and 95 percent within 0.223 mgal. The apparent decrease in accuracy is re- lated to the different meter constants, which are accu- rate within 1 part in 2,000. The decrease is small, and all differences in this group are within 0.35 mgal; the data from all the instruments, therefore, may be combined. U.S. Geological Survey topographic maps provided the control for the horizontal position of the gravity stations. Fifteen-minute quadrangle maps were used for about 75 percent of the area, and 30-minute quad- rangle maps were used for the remainder, which was chiefly in Mono Basin and northern White Mountains. Positions on these maps are accurate generally to with- in 100 feet and 500 feet, respectively. Gravity stations that were not at identifiable points such as road inter- sections, section corners, and bench marks were sur- veyed with horizontal closures of less than 500 feet. The gravity variation corresponding to 500 feet in hor- izontal position is 0.12 mgal. _ The sources of vertical control were bench marks, spot altitudes on maps, lake levels, and surveyed altitudes. Bench-mark altitudes are accurate to within 1 foot, and spot altitudes and lake levels are accurate to within one- tenth of a contour interval for steep slopes or to less than one-tenth for gentle slopes. Surveyed lines were closed within 4 feet in the valleys and within 8 feet in the mountain canyons. About 80 percent of the stations are in the valleys and therefore have altitudes that are ac- curate generally to within 4 feet. The remaining 20 per- cent of the stations are in mountain canyons and have altitudes accurate to within 8 feet. The gravity varia- - 25 tions corresponding to 4 feet and 8 feet in altitude are 0.24 and 0.48 mgal, respectively. The accuracy of a terrain correction is difficult to estimate because the correction consists of a series of approximations. The Owens Valley terrain corrections are reproducible to within 10 percent of the original and are internally consistent (that is, no erratic differ- ences are evident between stations). The total error is - probably within 10 percent of the correction, which gives accuracies of +1.0 mgal in the valleys and +2.5 mgals in the mountains. The relative error for adja- cent stations is probably less. Altimeter altitudes were used for seven stations in the Owens Lake area and for a like number of stations in Mono Basin. The altimeter altitudes have estimated ac- curacies of +20 feet, which corresponds to a gravity error of 1.2 mgals. A group of four stations was set along the crest of the northern White Mountains using altitudes that have a possible error of +100 feet. A fifth station, which was set on White Mountain Peak at a bench mark, has a terrain correction of 79 mgals. To- gether, the five stations have possible gravity errors of about +5 or +10 mgals. The three groups of stations are in areas where the determined gravity values are useful despite the relatively large possible errors. The stations of these groups are not considered in the fol- lowing table of possible errors. Error (mgal) Source Valleys Mountains ODSerVATIONL cl noelle ice. cans. +0. 10 +0. 10 Fosit10n.-.... . ere sane o 4.12 +. 12 AMitUCE.. . . . C- c..: ece ann +. 24 +. 48 c.... range ient, +1. 00 +2. 50 In the foregoing table the following may be noted : _ (1) The errors may be independently plus or minus, and a cumulative error therefore, is unlikely; (2) the maximum altitude error is two to four times the obser- vation or position error, and the accuracy of gravity values without terrain corrections, therefore, is most probably controlled by altitude accuracy, (3) the ter- rain correction error is by far the largest and controls the accuracy of the final gravity value. INTERPRETATION OF GRAVITY DATA In this study, the interpretation of the gravity data was done in the following order : 1. A qualitative study was made of the gravity contour maps (pl. 1) to reach broad, general conclusions on the subsurface configuration of the interface between the Cenozoic deposits and the pre-Terti- ary rocks. The gravity field represented by the 26 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. gravity contours was assumed to be a true repre- sentation of the gravity field on the surface of the ground, corrected for effects of altitude, latitude, and terrain. This assumption is probably cor- rect over most of the area, but in places where the ground altitude increases locally in materials of lower density than that assumed for the altitude corrections, the relative gravity field may be slightly higher than that shown on the maps. 2. Selected gravity profiles were analyzed by measur- ing the solid angle subtended from a position on the surface of the ground by a sequence of slabs of low-density Cenozoic material that could then be assembled to represent the configuration of the Cenozoic and pre-Tertiary interface. The gravity anomaly at any location was determined by multi- plying the total solid angle subtended by all slabs at that location by a constant factor that depends on the assumed density contrast and the thickness of each slab. The elongated features of Owens Valley were assumed to extend without limit along their axes, and a two-dimensional graticule was used to determine their gravity effect. The solid angles for the more nearly equidimensional structural features of Long Valley and Mono Basin were measured using a solid-angle chart. 3. The theoretical gravity anomalies determined from the measurements of the solid angles were then compared to the real anomalies taken from pro- files on the gravity contour maps, and the assumed configurations were modified, if necessary, until a satisfactory agreement between the anomalies was obtained. GRAVITY CONTOUR MAPS Inspection of the gravity contour maps (pl. 1) estab- lishes that the gravity field tends to be relatively low in the valleys and basins, where low-density rocks of Cenozoic age are exposed, and relatively high in the ranges, where denser pre-Tertiary rocks crop out. The amplitude of negative departure of the gravity field in areas of outcrop of Cenozoic rocks from the higher gravity where pre-Tertiary rocks are exposed is a guide to the thickness of the Cenozoic rocks. The magnitude of the steepest gradients along or near contacts between Cenozoic and pre-Tertiary rocks is a guide to the steep- ness of the subsurface interface between these rocks. Where the gradients are very steep, it may be inferred that the Cenozoic and pre-Tertiary rocks are in fault contact. An erosional or warped surface or distributive faulting may be inferred where the gradients are gentle. In a general way, Owens Valley is characterized by a narrow, nearly linear gravity low that extends along the entire length of the valley. However, more detailed examination shows that many interesting local features in the gravity field suggest considerable geologic com- plexity. Long Valley and Mono Basin are strikingly different in their gravity expressions from Owens Valley. The gravity anomalies associated with these two features are much more nearly equidimensional in plan, the gravity gradients that bound them are steeper, and the residual gravity relief of each is greater than that of Owens Valley. Some smaller features, such as Adobe Valley and the area southeast of Lake Crowley covered by the Bishop tuff of Gilbert, are clearly re- vealed by the gravity contours. The area north of lat 37°45 N. has been previously described in detail (Pakiser and others, 1960), and this description is summarized briefly here. The area south of lat 36°50' N. has also been previously described (Kane and Pakiser, 1961). In the discussion that fol- lows, each of the four sections of the gravity survey (pl. 1) is discussed separately (proceeding from north to south) for convenience and for an orderly presentation of the gravity data. AREA NORTH OF OWENS VALLEY The area studied north of Owens Valley extends from lat 37°30" to 38°05" and from long 118°10' to 119°15" (pl. 1, sheet 1). It includes the slopes of the Sierra Nevada, Long Valley, Mono Basin, the northern exten- sion of the Owens Valley structure, the northern White Mountains, and several smaller basin and range struc- tural features. It is an area having a history of diverse and violent volcanism, great structural complexity, and structural trends that are seemingly at odds with the simpler trend of Owens Valley. It is an area which also includes the largest offset of the eastern front of the Sierra Nevada. The geology on the map (pl. 1, sheet 1) was compiled from the following sources: Anderson (1937), in and immediately west of the northern White Mountains; Gilbert (1941), north of Long Valley and southeast of Mono Lake; and Rinehart and Ross (1957; written commun., 1958), in the Long Valley area. The geology north and west of Mono Lake is based on a brief recon- naissance by Pakiser. j The most dominant gravity features in the area north of Owens Valley are the broad gravity lows in Long Valley and Mono Basin and the narrow north-trending gravity low west of the front of the White Mountains. The broad gravity low roughly outlining the shoreline of Mono Lake has a residual gravity relief of about 50 mgals, and the steepest gravity gradients on the south- west and northwest sides of this anomaly have a maxi- mum magnitude of 15 mgals per mile. The Mono Basin anomaly has been interpreted as the expression of a large, triangular block that subsided along near-vertical GEOPHYSICAL SURVEYS 27. faults and received an accumulation of 18,000+5,000 feet of light clastic and volcanic rocks of Cenozoic age (Pakiser and others, 1960). The gravity data were used to deduce a thickness of more than 2,000 feet for Ceno- zoic deposits in Adobe Valley. The two nearly circular gravity highs (and coincident magnetic anomalies) were interpreted as probably the expressions of volcanic necks. The weak gravity high near the center of Mono Lake, together with a magnetic high at the same place, was interpreted as the expression of a pile of intrusive and extrusive volcanic rocks (Pakiser and others, 1960). The large, elliptical gravity low in the Long Valley area (south of the Glass Mountain Ridge and north and east of the Sierra Nevada) is by far the most dominant gravity feature in the entire Owens Valley region. The anomaly has an area inside the zone of steepest gradi- ents of about 150 square miles. The largest gravity re- lief (without correction for the regional gradient) be- tween the highest contour in the Benton Range to the east and the lowest contour of the anomaly is 78 mgals in a distance of only 13 miles; this is the largest local difference in gravity yet reported in the Great Basin. The steepest gravity gradient on the eastern end of this anomaly is 20 mgals per mile. By making a simple computation using the expression for the gravity at- traction of an infinite sheet and an assumed density con- trast between the Cenozoic and pre-Tertiary rocks of -0.4 g per ecm, one can conclude that the thickness of the Cenozoic deposits in this area is greater than 11,000 feet ; to obtain this result, removal of the regional gray- ity gradient was found to reduce the residual gravity relief to about 60 mgals. From the steepness of the gravity gradients surrounding the anomaly, it may be concluded that the Cenozoic and pre-Tertiary interface dips very steeply and is probably a fault contact. The gravity high near the center of the low anomaly may be the expression of a mass of dense rock within the Cenozoic section. Slightly east of south from the southern boundary of the Long Valley gravity low, a sharp change in the direction of the gravity contours coincides in part with a fault as mapped by C. D. Rinehart and D. C. Ross (written commun., 1957). The area of low gravity trending approximately along the course of Owens River southeast of Lake Crowley must represent a rela- tively thick accumulation of the Bishop tuff of Gilbert (1938) and perhaps of other light materials of Cenozoic age. The small gravity low west of the southern end of the Benton Range is clearly related to the faulting in that area. In many other places, small gravity lows in: the valleys that flank the ranges reveal moderately thick accumulations of valley-fill deposits. The broad area of high gravity embracing the Benton Range, Black Mountain, Blind Spring Hill, and some smaller features is an expression of the fact that pre- Tertiary rocks are at or near the surface in this area. The elongated gravity low trending just east of south along the eastern front of the White Mountains has a residual gravity relief of about 30 mgals and is bounded by steep gradients on both sides. It is presumably the expression of an accumulation of light Cenozoic rocks several thousand feet thick; the steepness of the gravity gradients suggests that this block of light material is probably bounded by faults on both sides. This gravity low continues with only minor interruptions to the southern limit of Owens Valley. NORTHERN OWENS VALLEY The northern Owens Valley area extends from lat 37°00" to 37°30" and from long 118°00' to 118°45'. It includes the slopes of the Sierra Nevada, the White Mountains, Round Valley, and Deep Spring Valley, in addition to Owens Valley. The geology shown on the map (pl. 1, sheet 2) was taken from the compilation by Bateman and Merriam (1954), which includes data from Knopf (1918), and recent mapping by Bateman. The geology in the Deep Spring Valley area was taken from Miller (1928). The gravity low in Owens Valley is a southward con- tinuation of the same feature occurring west of the northern White Mountains noted above. The maxi- mum residual gravity relief in this part of Owens Valley is about 30 mgals, indicating that more than 5,000 feet of Cenozoic rocks have accumulated, based on the assumption that the density contrast between the Cenozoic rocks and pre-Tertiary rocks is -0.4 g per cm'. The gravity gradients are steep along the eastern side of the valley; therefore, the Cenozoic and pre- Tertiary interface is probably a fault contact there. To the south, the gradients are also steep on the western side, but the relatively gentle gradients along most of the Sierra Nevada front suggest that the Cenozoic and pre-Tertiary contact may have been warped, as had been concluded on the basis of geologic mapping by P. C. Bateman (written commun., 1956). The Owens Valley low branches westward into Round Valley in the southern part of the Volcanic Tableland and north of Tungsten Hills and has gentle gradients on both sides of the anomaly ; this low suggests that the area may have been downwarped. A dominant gravity high interrupts the Owens Valley gravity low northeast of Bishop. This gravity high must be the expression of a mass of dense rock within or surrounded by the Cenozoic section. Deep Spring Val- ley is clearly expressed by a closed gravity low that in- dicates a fairly thick section of light valley-fill deposits. 28 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. The western front of the White Mountains is offset sharply to the west south of a line just north of the 37th parallel. The gravity contours are also deflected sharply at the same place in a direction normal to their general trend; south of the offset the gravity low continues its southerly trend. The broad but gentle gravity low south and slightly west of Tungsten Hills (in the But- termilk Country) probably represents an accumulation of alluvium about 1,000 feet thick; this gravity low is poorly defined by gravity observations. CENTRAL OWENS VALLEY The central Owens Valley area extends from lat 36°30" to 37°00" and from long 117°45" to 118°30'. The area includes the slopes of the Sierra Nevada, Alabama Hills, the Inyo Mountains, and Owens Valley. Part of the western side of Saline Valley is also included on the map (pl. 1, sheet 3). The geology shown on the map was taken from the compilation by Bateman and Mer- riam (1954), which in this area is based on the mapping of Knopf (1918). The Owens Valley gravity low is very narrow in the northern half of this area; the residual gravity relief and gravity gradients are small compared with those to the north and south. Inspection of the gravity contours in this area suggests that the Owens Valley structural basin is very narrow, and the thickness of the Ceno- zoic deposits is probably relatively small. The gravity low broadens to the south, however, where it is flanked by steep gravity gradients on both sides. The steep gravity gradient on the west lies along the eastern front of Alabama Hills and is several miles from the Sierra Nevada front. On the east, the steep gravity gradient marks the boundary between the Inyo Mountains and Owens Valley. These steep gravity gradients shown in the southern half of the central Owens Valley area probably indicate that the Cenozoic and pre-Tertiary rocks are in fault contact. The lack of pronounced local gravity relief between Alabama Hills and the Sierra Nevada suggests that the pre-Tertiary floor is shallow in this area; indeed, several outcrops of pre-Tertiary rocks project above the alluvium there. The gravity field declines sharply in intensity from the Inyo Mountains into Saline Valley. D. R. Mabey (written commun., 1958) is making a study of Saline Valley. The gravity high of Alabama Hills continues in an area of alluvial cover to the north for a distance roughly equal to the exposed part of this feature, and the elongated gravity high is sharply terminated on the north. The Alabama Hills gravity high is also sharply terminated on the south. The gravity data in the south- ern part of central Owens Valley were previously presented by Kane and Pakiser (1961). OWENS LAKE BASIN The area of the Owens Lake basin is the extreme southern part of the region studied (pl. 1, sheet 3). The area includes the slopes of the Sierra Nevada, the Owens Lake basin (the southern part of Owens Valley), the southeastern end of the Inyo Mountains, the Coso Range, and Centennial Flat. The geology shown on the map was compiled from Bateman and Merriam (1954), who obtained geologic data from Knopf (1918), from Hopper (1947), and from maps published by the California Division of Mines (Jennings, 1958; Jennings and Strand, 1958). The Owens Valley gravity low in this area flares southward into a broad gravity minimum, which is lowest near the center of Owens Lake. The residual gravity relief in this area is about 40 mgals. Computa- tion of the thickness of an infinite sheet based on this gravity anomaly and on an assumed density contrast of -0.5 g per ecm® between the basin sedimentary rocks and pre-Tertiary rocks yields a minimum depth of valley fill of about 6,000 feet. The larger density con- trast was assumed because of the probability that the basin sedimentary rocks are finer grained, better sorted, and less dense in this area than elsewhere in Owens Val- ley (Kane and Pakiser, 1961). The intricate changes _ in direction of the gravity contours for this area suggest a complex fault pattern. The steep gravity gradients along the western front of the Inyo Mountains gradu- ally die out to the south and are terminated southeast of lower Centennial Flat. The fault zone suggested by this zone of steep gravity gradients (pl. 1) has a total length from north to south of more than 100 miles with only a single interruption. On the extreme south, the Owens Valley low becomes narrow at the junction of Owens Valley and Rose Valley. GRAVITY EFFECTS OF DENSE PRE-TERTIARY ROCKS The measurements of the densities of samples of pre- Tertiary rocks and a study of the variation of the grav- ity field in pre-Tertiary terrains indicate that the meta- morphosed sedimentary and volcanic rocks of the roof pendants and septa of the Sierra Nevada are somewhat more dense than the rocks of the batholith. Therefore, large masses of these older, more dense rocks may be expected to influence the gravity field. The gravity ef- fects of these dense rocks may be determined by a direct inspection of the gravity contours for areas where these two pre-Tertiary rock types are in contact. Two such areas are Alabama Hills and the slopes of the Sierra Nevada southwest of Long Valley. In Alabama Hills metavolcanic rocks of Triassic age are exposed on the east; Cretaceous (?) granitoid rocks crop out to the west. The gravity high over the meta- GEOPHYSICAL SURVEYS 29. volcanic rocks is caused in part by the density contrast between these rocks and the granitoid rocks farther west. The residual gravity relief caused by this density contrast (assumed to be about 0.2 g per cm®) is not more than 2 or 3 mgals, however, and this residual relief is not enough to have a serious effect on analysis of Cenozoic structures based on a single density contrast of -0.4 or -0.5 g per cm®. In the slopes of the Sierra Nevada southwest of Long Valley, alternating bands of metamorphic and granitoid rocks trend into the steep southern gradient of Long Valley. Although the grav- ity contours in this area reveal small maximums over the metamorphic rocks, the effects are small and do not influence the much larger minimum anomaly of the Long Valley block caused by the density contrast between the Cenozoic and pre-Tertiary rocks. The effect is merely to superimpose small anomalies of a few milligals on an anomaly of several tens of milligals. It is thus probably safe to ignore density changes with- in the pre-Tertiary rocks. H. W. Oliver (written com- mun., 1959) is making a detailed study of the signifi- cance of these changes. REGIONAQGRAVITY AND ISOSTATIC COMPENSATION By subtracting 1,000 mgals from the contour values, the regional gravity over areas of pre-Tertiary rock out- crops with respect to the International Ellipsoid tends to be about -200 mgals. The regional gradient, de- creasing to the west, is generally about 2 or 3 mgals per mile, although it is nearly zero in the Mono Basin area (Pakiser and others, 1960). The negative complete Bouguer gravity suggests that the crust is thicker than normal in this area and increases in thickness toward the west.. Oliver (1956) is making a comprehensive study of the crustal structure and isostatic compensa- tion of the Sierra Nevada; so, only a few generalized remarks are necessary in this paper. The thickening of the crust in areas of large topo- graplnc loads in the general area of the Sierra Nevada is also suggested by analysis of the dispersion of the phase velocity of Rayleigh waves from distant earth- quakes (Ewing and Press, 1959; Press, 1956). In the Long Valley area, the complete Bouguer grav- ity is about -230 mgals if the effect of the Cenozoic rocks is removed. This corresponds to a crustal thick- ening of more than 13 km if a density contrast of -0.43 g per em is assumed between the rocks of the crust and the mantle rocks (Worzel and Shurbet, 1955). If the normal thickness of the continental crust at sea level is 35 km, the crust is probably more than 48 km thick in the eastern Sierra of the Long Valley area, and it be- comes thicker farther west. In the Owens Lake area, the complete Bouguer gravity with the geologic effect 728-195-64--5 of the Owens Lake basin removed is about -200 mgals, which suggests a minimum crustal thickness of about 46 km. The thickness of the crust also increases to the west in the Owens Lake area. Thus, the thickness of the crust on the Owens Valley region may range gen- erally from about 45 to 50 km, and this thickness cor- responds to a general altitude of the region of about 5,000-8,000 feet above sea level if regional isostatic equi- hbmum is maintained. Of-course, some of the variations in reglonal gravity may be the result of density variations within the rocks of the crust (Thompson and Sandberg, 1958), but the foregoing general conclusions should be valid ; they are supported by the results from the analysis of Rayleigh waves. ANALYSIS OF GRAVITY PROFILES Eight gravity profiles (4-4""" to H-H', pl. 1; figs. 6-15) were analyzed in detail. In these analyses, with two exceptions, the density contrast between the Ceno- zoic and pre-Tertiary rocks was assumed to be -04 g per cm*. In the analysis of profiles D-D' and H-, a density contrast of -0.5 g per cm* was used, on the as- sumption that the fine-grained well-sorted sedimentary rocks of northern Owens Valley and the Owens Lake basin are less dense than the Cenozoic rocks elsewhere in the Owens Valley region (Kane and Pakiser, 1961). This refinement may be meaningless when the many un- certainties in the gravity interpretation are considered, but it seems to bring the depths determined from gravity and seismic data into close agreement, where they can be compared. The seismic data considerably strengthen the assumptions used in the gravity interpretation and the conclusions reached. A single interface of density contrast was assumed in the analyses of gravity profiles. Variations in density within the Cenozoic section may occur and may lead to small errors in the dip and position of faults based on the gravity interpretation, as Kane and Pakiser (1961), following a suggestion of D. R. Mabey (written commun., 1956), showed. Specifically, failure to take into account the probably high density of the coarse and poorly sorted alluvial-fan materials near mountain fronts may result in a determination of the locations of faults valleyward from their true positions, a reduc- tion of the estimates of the dips of faults from their probably steeper true dips, and a determination of the upper edges of the faulted pre-Tertiary rocks down- ward from their probably shallower true depths. These uncertainties must be recognized, but they do not sig- nificantly alter the conclusions that follow. The true density contrast may range from -0.7 to -0.3 g per cm', and this uncertainty could result in a 30 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. A 7 A ar 0 0 - Computed Bouguer gravity-y a Le I < 9 S4 a a [> p -50 |- - -50 Residual Bouguer gravity T— L 1 L 1 I 1 o' 50,000 FEET A' A" o' } 0 km 500 ft =- - A p =-0.4 gm per cm® s 18,000 ft L] -| 5 km 20,000" ~ w SIMPLIFIED CONFIGURATION ASSUMED £: A' LONG VALLEY Rhyolite (Cenozoic BENTON RANGE A". VVyI/VVVVL’I/l’l’l/l/VVV = E > ohm t- C- « ' Cenozoic deposits! 5000' |- AJL AJP u - L- L- SEA LEVEL z SEA LEVEL L~ t« w- t- [~ Cenozoic deposits (p=2.3 gms per cm*) t : Pre-Tertiary rocks (p=2.7 gms per cm") ig 10,000' |- % «l io t tin ti i i t iP i b lib tt boi iL [L+ -| 4km GEOLOGIC SECTION Assumed basin outline showing location of profile A'-A" 2T" 3 L— 19 miles ————>| FicuRB 6.-Analysis of gravity profile A'-A'"' across eastern Long Valley, showing assumed subsurface structure. CEOPHYSICAL SURVEYS j 31 A" A4" a 800 |- f - 800 3 Bouguer gravity Regional gravity 3 |. o-= eas 3 g 16 % gomputed residual gravity € E ~ § w -20 - is + i g - es = -309 - S sl . G ~40 -- 3 & 750 -| 750 he 2 | - 3 o fea E. W. A” Alli 15,000 p White Mountain Peak 7] ° " 10,000 BENTON RANGE Northern structural <2 extension of Owens § Valley Pre-Tertiary rocks 7 5000' |- & (p=2.7 gms per cm?) Cenozoic deposits > 7 "-] (»=2.3 gms percm?) C SEA LEVEL E - SEA LEVEL ¥ VVVVVVVVV’VVV & 5000' F- # 10,000" |- ~ 15,000" - -] 5 km 20,000" - GEOLOGIC SECTION 0 1 2 3 4 5 MILES L..... 3. _L ___ L .._. FicurB 7.-Analysis of gravity profile A''-A'"' across northern extension of Owens Valley, showing assumed subsurface structure. range of estimated depths to pre-Tertiary rocks of from about % to 11/4 times the depths found from the gravity data. The seismic data, however, considerably narrow this range of uncertainty, perhaps to +25 percent. The major faults were arbitrarily assumed to be vertical for convenience, but this assumption is not con- tradicted by the gravity evidence. PROFILE A--A" Two sections of profile A-A4""' (pl. 1, sheet 1) were analyzed in detail (figs. 6 and 7), and the entire profile was assembled, together with the areal geology and some aeromagnetic data, to yield a regional geologic cross section along a line extending about 50 miles from the Sierra Nevada on the west to the White Mountains on the east (fig. 8). The subsurface geologic configuration along section A'-A""' was determined by measuring the sum of the solid angles from a series of points on the surface of a sequence of slabs 4,000 feet thick having the outline shown in figure 6. A solid-angle chart designed by Lachenbruch (1957) for geothermal studies was used to make the measurements of solid angle. The sum of the solid angles multiplied by a factor dependent on the thickness of the slabs and the assumed density contrast (-0.4 g per cm*) yielded the theoretical gravity anom- aly of the assumed structure. This computed gravity anomaly was compared with the residual anomaly of the actual structure, and modifications were made in the assumed subsurface configuration until satisfactory agreement was obtained. The Long Valley block in- cluded within the outline was assumed to be bounded by vertical faults; all slabs have the same size. The total thickness of Cenozoic deposits was assumed to be 18,000 feet, or 5.5 km. The method of interpretation has been previously described in detail (Pakiser and others, 1960). The configuration assumed for section A'-4""' yielded a gravity anomaly that agreed almost perfectly with the STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. 32 SVWWVS NI doud@swm pus pownsst Sujmoys 'Aot[¥A Su07 pus upseq ouopt sso10®8 ,g-g orgoid «unong I T I T T oi S37iW Ot 8 9 t 2 0. 4 Nolo3s 919071039 I Arerua|-014 . <000'0Z wy g | | . edna alah hain anted" e 3 4.000'0t [ E] ! 12 lt ter ;- 13 7: a 2 b 2|ftor 9) susodap > 7 f 13193 2 fue 2 2! perenuaiayipug ~< 3 13437 was | grive §L@_ "> die equsieuipun |7 2 F g mAnRAaAA ] vas - g Me Stinge WA unas l aa |a<====*" - - (ojozoueq) | 2/8 AJTIVA DNO - T m) m nme mrs oo. a tanks a L L129 vearry jo doysig AZ pus sifsspuy "C19 C wy tg vavazn 413 : C mae orang, e a a *g* Iz o|- 12 'N /% m 4 z' z [ z C a. ] _.|_.._ # 0OST _ \m\\\ T | _ n +0 2 § F Ajig> P ctl % | { te B a" "ugh, .~ 1 ' | i 5C w ai . key; \\\\\ a i fl M 4 0§L 3 m R o /a 3m. I 4 | & sg Jel "9p. | | Nap i n i .! 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FB qu s ® > ® tbs 1 2 < a |- -| ove " 7 a s] 108. iSte8TT 100.611 P LF: fre 4¥F, ¥. v GEOPHYSICAL SURVEYS 33 800 800 MILLIGALS 750 a o' 50,000 FEET C U AX Okm {2000 tt \ 18,000 ft Ap =-0.4 gm per cm' -| 5 km 20,000' SIMPLIFIED CONFIGURATION: ASSUMED SW. , NE. 21. 0'm 4 C Eld 10,000' (- _ SIERRA NEvADA gle $ sin Crp Pacha $l MONO Te! p> MONO LAKE Island b LAKE , I "> i s000' |- >> 1 L- I B All t I t- SEA LEVEL s } SEA LEVEL u- I AlL I a | ~- u |- # Cenozoic deposits ( a = 2.3 gms per cm*) g Pre-Tertiary rocks ( » =2.7 gms per cm*) ll: | x L- 10,000'|- s 5 2 - uwvvvvwvwvvwvwvwvrwvvvwv i: f - 1 GEOLOGIC SECTION 2 Assumed basin outline s showing location of 08" profile C-C' G FieuRr® 10.-Analysis of gravity profile C-C' in Mono Basin, showing assumed subsurface structure. STRUCTURAL GEOLOGY AND VOLCANISM,, OWENS VALLEY, CALIF. 34 'ommonI}s oovjinsqns powinsse Sujmoys doog 0; s4orqdwung qunopt wou; ,-G otgoid go #undif I J I I I S37IW OT 8 9 a a NO1L93S 919071039 ~ 0 13437 11. 1/0 syoo1 VIS [~(oiozouag) stisodap -|- |- _perenuasgyipun ___ |<7) 122220222002 E R x uing? KojjeA Sunds dagg SNIVLNNOW 3LIHM 1V311 .- _ <4,000'0t 1. 10006 . (alozouag) syisodap a porequarayyipup _| fe- \ AIIIVA SN3MmO o w m I yeseg vavA3N (mmmrw 3unop vas : -| 10006 CI 'M funes -| OSL ~ 008 00.811 / Q ST.STT SIVOI7IW 0001 + 1000'G t. 134371. 000'0t . #3n9nog GEOPHYSICAL SURVEYS 35 in o.. £ 118515 E' - < G |- - I f avity / s 800 |- Regional ___ £" - 800 o rag s & * | Cou, | -& A + Cue, é |_ Ft, Residual gravity 5 X [0 0 % bs s |- m -~20= /K ess - 2 As Computed gravity 3 750 750 5 f W.: E. E 15,000" /- A E, SIERRA NEVADA ~ &km 10,000" |- WHITE MOUNTAINS ~ OWENS VALLEY z i cH] 5000' |- Mt (Cenozoic) ies AlL "_ P. C Rete bo SSILT ~ JVVVVVVVVVVV is SEA § 10 deposits gag? |. SEA LEVEL |___ (Cenozoic) LEVEL I/I/WM _ 5000' |- } Pre-Tertiary rocks sl 10,000" |- ~I S -| 4 km u L Pace GEOLOGIC SECTION 0 1 2 3 4 5 MILES ~~Frour® 12.-Analysis of gravity profile near Big Pine, showing assumed subsurface structure. F 37°30 E" 9 - soo |- -] 800 < fe] » - a 3 0 Q B + > E < to 5 a u = el % & _ 750 |- -| 750 s S. p" 10,000' r- ; =- 3 km BENTON RANGE VOLCANIC TABLELAND ROUND VALLEY TUNGSTEN HILLS Bishop tuff of Gilbert (1938) ;, ~- 5000" |- ron mun M t p> Undifferentiated deposits (Cenozoic) ete ix il; <> M p SEA LEVEL TTV ream -e -_ RA SEA LEVEL Pre-Tertiary rocks -[ 1 fm 5000' GEOLOGIC SECTION 0 1 2 3 4 5 MILES 1 1 1 1 FIGURE 13.-Analysis of gravity profile. F-F' north of Tungsten Hills, showing assumed subsurface structure, 36 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. U sig row 340G 118°15 “Boo/f G 840 E 9 A gravity, sA" s g 820} //m‘°“‘/ { 820 < G « C = a $ § //\ & : § 5 0 W a § ssp 9 -10- _ \cComputed gravity = y 809 2 a I + é -20 - - 780 | 30 -| 780 W. £. g mount 15,000' |- WHITNEY Z g G’4k zo ol -a m 1om00' L- " alg INYO MOUNTAINS f f @ |p ca ALABAMA HILLS Owens VALLEY re" - 5000' |- *~ C wens gee * **B | Undifferentiated x~ ' SEA LEveL jg | _ ___ {ern ic SEA LEveL EI (Cenozoic) z soo0' |- c . in re-Tertiary rocks dur at lt ~ 10,000" |- -| 3 km GEOLOGIC SECTION o '< % 'a -$ Mitts 120 F3 1 __ " 1s Le} Ficur® 14.-Analysis of gravity profile G-G' east of Mount Whitney, showing assumed subsurface structure. 0 u ll 0 u 860 11800 7°50 oso h fere -I at < fol 3 840} 840 = o o £ 7 s20}- 820 > E > & , {35 © 800p 0 - 800 5 Computed gravity-_~ & as p gravity i> Fol ih em)" rea « g 780 - 3 Ts - -| 780 3 \ Residual gravity s ~|760 W. i E. H H' 15,000 ;- - sieR <5 (B RA NEVADA 20 INFO Js in f & 216 MmoUNTAINS _| T gm “VI/"— 5000' |.. P owEns vaALLEY rone Aetite *~ oc- a L wmmmtr e * | SEA Undifferentiated _ deposits F an SEA LEVEL o (Cenozoic) zer LeveL wyva—V l 5000'}- Co remmmine fone vette ket" JJP JIN t J[P 11+ Pre-Tertiary rock § 10,000:|- re-Tertiary rocks 3 mn GEOLOGIC SECTION 0 1 2 3 4 hr: scheint l receno 5 MILES FicurRB 15.-Analysis of gravity profile H-H' in Owens Lake Basin, showing assumed subsurface structure.. GEOPHYSICAL SURVEYS 37 residual gravity anomaly taken from the gravity con- tour map (fig. 6). A gentle regional gradient had been removed before the anomaly was analyzed. In particu- lar, the agreement in the zone of steepest gradients is excellent, and this agreement gives force to the conclu- sion that the system of faults that bounds the struc- ture is actually vertical, or very nearly vertical. This section is particularly significant because determina- tion of the form of the gravity anomaly is well con- trolled by gravity observations in this area. The sur- face relief is small ; so, uncertainties concerning the al- titude correction and terrain effects are small. The subsurface geologic configuration along section A"-A'""" (fig. T) was determined by measuring the angle subtended at a series of points on the surface by each of a sequence of slabs 1,000 feet thick and then by sum- ming these angles. The slabs were assumed to extend without limit along axes normal to the plane of the cross section. The gravity anomaly at each point on the sur- face was found by multiplying the sum of the angles by a factor dependent on the thickness of the slabs and the density contrast (-0.4 g per ecm*). This anomaly was then compared with the anomaly taken from the gravity contour map, and the determination of the subsurface configuration was modified as necessary, as previously described. A graticule designed by D. C. Skeels was used to make the measurements for this profile and for most of the others (see Dobrin, 1952, p. 98) ; a similar graticule, modified from Skeels by S. W. Stewart of the U.S. Geological Survey, was used for the remainder. See Hubbert (1948) for the theory of the method. The method of interpretation was previously described in detail by Kane and Pakiser (1961). Vertical bounding faults were assumed, and the thickness of Cenozoic de- posits was assumed to be 8,000 feet. The agreement between the computed anomaly and the anomaly taken from the gravity contour map is fairly good. No detailed analysis to infer the subsurface con- figuration along section A-A' (fig. 8) was made, but this configuration is clearly suggested by the smaller gravity relief and gentler gravity gradients along this line. Meticulous refinement of the interpretation along see- tion A-4' is not considered to be necessary, and it might be misleading, in view of the paucity of gravity observa- tions in the area near this line and of the relatively great effects of terrain in the slopes of the Sierra. The small gravity high near the center of Long Valley is not well defined by gravity observations; it is probably the ex- pression of dense volcanic rocks within the Cenozoic section. The sections analyzed in detail were then combined with the areal geology and the total-intensity aeromag- netic profile to obtain the regional geologic cross sec- tion A-4""" (fig. 8). This cross section shows that the depth to the pre-Tertiary floor of Long Valley is more than twice as great as that of the northern Owens Val- ley basin. The aeromagnetic data were used to infer basalt flows within the Cenozoic section of Long Valley. Seismic-refraction measurements were used to infer a buried rhyolite flow east of the outcrop of rhyolite near the center of Long Valley. The volcanic rocks shown within the Cenozoic section on profile A-4""', therefore, are based on good geophysical evidence, but the distribu- tion shown is rather highly generalized. PROFILE B-B'" A regional geologic cross section made on a line more than 50 miles long and extending southeastward from north of Mono Basin into the Sierra Nevada slopes south of Long Valley was compiled from areal geology and from gravity, seismic, and magnetic evidence (pl. 1, sheet 1; fig. 9). The structure, the velocity layering within the Cenozoic section, and the distribution of volcanic rocks in Mono Basin were deduced from gray- ity, seismic-refraction, and magnetic measurements (Pakiser and others, 1960). The bounding faults and the thickness of Cenozoic rocks of the Long Valley block were deduced from the gravity profile. A detailed analysis by graticule and solid angle measurements was made of the Long Valley gravity minimum, but it is not reproduced here. A good fit of computed gravity with the gravity taken from the map (pl. 1, sheet 1) was obtained, except at the local gravity high near the center of Long Valley. This high is assumed to be the expression of a basalt neck and a sequence of basalt flows within the Cenozoic section. The presence of this vol- canic complex is also revealed by the aeromagnetic pro- file. The offset between the gravity and magnetic maxi- mums may not be entirely real ; the gravity profile was taken from the gravity contours, which are not con- trolled by gravity observations on or near this part of the profile. The small magnetic high directly over the southern bounding fault of the Long Valley block was used as evidence for the narrow basalt feeder dike in the fault zone shown on the cross section. The existence of the volcanic rocks within the Cenozoic section of Long Valley is based on good geophysical evidence, but the distribution as shown on the cross section is highly generalized. The depth from the surface to the pre-Tertiary floor of Mono Basin is almost identical to that of Long Valley ; both of these blocks have subsided to nearly the same depth below sea level (about 10,000+=5,000 feet, or 3+1.5 km), contain volcanic rocks within the Cenozoic section, and are bounded seemingly by vertical or near- vertical faults. 38 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. PROFILE C-C" Profile C-C" (fig. 10) extends along a line from the Sierra Nevada northeastward to and beyond the center of Mono Basin (pl. 1, sheet 1). The subsurface con- figuration of the Cenozoic and pre-Tertiary interface along this cross section is nearly identical with that of the northwestern part of profile B-B'. This illustra- tion has been reproduced from an earlier paper (Pakiser and others, 1960). PROFILE D-D' A regional geologic cross section along a line more than 35 miles long, extending from the crest of the Sierra Nevada, across Owens Valley and the White Mountains to the eastern side of Deep Spring Valley was compiled from the areal geology and from gravity data (pl. 1, sheet 2; fig. 11). Only the Owens Valley gravity mini- mum was analyzed in detail. A density contrast of -0.5 g per cm' between the Cenozoic rocks and pre- Tertiary rocks was assumed, and the analysis was made using the graticule designed by Skeels, as described on page 37. The main fault that forms the boundary be- tween Owens Valley and the White Mountains was as- sumed to be vertical. 'This assumption is not contra- dicted by comparison of the computed gravity profile with the residual gravity profile as obtained from the gravity contour map. The agreement between these profiles is good. The maximum depth to pre-Tertiary rocks in Owens Valley just west of the White Moun- tains was assumed to be 8,000 feet. The pre-Tertiary floor slopes gently eastward from the Sierra Nevada to meet this fault. The thicknesses and approximate con- figurations of the pre-Tertiary floor in the valley just east of Mount Humphreys and in Deep Spring Valley were estimated from the amplitudes and forms of the gravity minimums in these two places; detailed anal- yses using a graticule were not made, mainly because the paucity of gravity observations near these segments of the profile would not justify such a procedure. PROFILE E-E* Profile Z-" (pl. 1, sheet 2) extends for more than 20 miles along a line from the crest of the Sierra Ne- vada, eastward across Owens Valley, and into the White Mountains. A regional geologic cross section along this line has been constructed from the areal geology and from gravity data (fig. 12). The structure of the Owens Valley block was determined by comparing the residual gravity minimum as taken from the gravity contour map with the computed anomaly corresponding to the configuration assumed. The block was assumed to extend without limit along its strike. The faults bounding both the east and west sides of the Owens Valley block were assumed to be vertical, and the thick- ness of the Cenozoic section was assumed to be 8,000 feet. A density contrast of -0.4 g per cm* was as- sumed between the Cenozoic and pre-Tertiary rocks. A good agreement between the computed anomaly and the residual anomaly taken from the gravity contour map was obtained. PROFILE F-F" Profile FZ-F" (pl. 1, sheet 2) was made for a line that extends for more than 15 miles southward from the southern tip of the Benton Range, across the Volcanic Tableland and Round Valley, and to the southern limit of Tungsten Hills A geologic cross section (fig. 13) was constructed from the areal geology and from the gravity anomaly. No detailed analysis was made. The depth to and configuration of the pre-Tertiary floor was inferred from the amplitude of the residual gravity anomaly (21 mgals) and from the relatively gentle gravity gradients along the profile. The configuration of the pre-Tertiary floor is that of a gentle synclinal downwarp having subordinate faults. PROFILE G-G Profile G-G' extends eastward for more than 25 miles along a line from the summit of Mount Whitney, across Alabama Hills and Owens Valley, and beyond the sum- mit of the Inyo Mountains (pl. 1, sheet 3). The re- gional geologic cross section (fig. 14) was constructed from the areal geology and from the subsurface con- figuration of the Cenozoic and pre-Tertiary interface as inferred from the residual gravity minimum. The gravity computed for the assumed configuration and the residual gravity as taken from the gravity map were compared, as described on page 37, until a sat- isfactory agreement was obtained. The depth to the pre-Tertiary floor of Owens Valley was assumed to be 8,000 feet, and the density contrast between the Cenozoic and pre-Tertiary rocks was estimated to be -0.4 g per cm*. The faults that bound the Owens Valley block were assumed to be vertical. The relatively poor agreement between the theoretical and actual residual gravity profiles west of Alabama Hills is caused probably by the dense mass of metavol- canic rock that crops out in the eastern part of Alabama Hills and in part by a relatively thin veneer of alluvial- fan materials between Alabama Hills and the Sierra Nevada front. The gravity stations that control this gravity profile between Alabama Hills and the Sierra Nevada are in the valley of Lone Pine Creek (pl. 1, sheet 3). This cross section has been modified slightly from one along the same line given in a previous paper by Kane and Pakiser (1961). GEOPHYSICAL SURVEYS 39 PROFILE H-H' Profile Z-H' extends for nearly 25 miles along a line from the Sierra Nevada eastward to the center of the Owens Lake basin and from there northeastward to the crest of the southern Inyo Mountains (pl. 1, sheet 3). The regional geologic cross section (fig. 15) was con- structed from the areal geology and from the subsurface configuration of the Cenozoic and pre-Tertiary inter- face as determined by detailed analysis of the gravity anomaly. The density contrast between the Cenozoic rocks and pre-Tertiary rocks was estimated as -0.5 g per cm*, and the maximum thickness of the Cenozoic rocks was assumed to be 8,000 feet. Vertical bounding faults were assumed. The agreement between residual and theoretical gravity profiles is excellent except on the extreme west, where the gravity field is influenced by local deposits of alluvium on which the gravity ob- servations were made. This cross section has been modified slightly from one along the same line by Kane and Pakiser (1961). DISCUSSION OF GRAVITY PROFILES The faults bounding each of the downfaulted blocks of Cenozoic rocks were assumed to be vertical. As has been pointed out, the effect of a wedge of denser alluvial- fan materials within the Cenozoic section near the moun- tain fronts would minimize the estimate of the dip. On the other hand, because of the inherent ambiguity of gravity interpretations (Skeels, 1947), modification of data on the density contrast and the subsurface config- uration could give satisfactory agreement between theo- retical and actual gravity anomalies on most profiles having bounding-fault dips as small as 60° and on a few profiles having even smaller bounding-fault dips. In any event, the dips of the bounding faults in zones of steep gravity gradient are steep, and the assumption | of vertical dip has never been contradicted on the basis of the gravity data. Indeed, for the faults of Long Valley and Mono Basin, almost the only reasonable con- clusion is that the faults are nearly vertical. Although the density contrasts between the Cenozoic deposits and pre-Tertiary rocks were assumed on the basis of few valid measurements (measurements that will represent the entire Cenozoic section are virtually impossible to make), seismic data considerably narrow the uncertainty concerning the density contrasts. The density contrast of large volumes of Cenozoic deposits may be confidentially stated to range from -0.3 to -0.6 g per cm?. Locally, dense alluvial-fan materials near the mountain fronts may contrast in density with pre- Tertiary rocks by as little as -0.2 g per cm*. The smaller limit (-0.3) would apply where a density con- trast of -0.4 g per cm® was assumed ; the larger (-0.6), where -0.5 g per cm' was assumed. This range in density contrast corresponds approximately to an un- certainty of thickness of Cenozoic deposits of about + 25 percent. The greatest depth to pre-Tertiary rocks found in Long Valley and Mono Basin was 18,000 feet ; this depth may be in error by +5,000 feet. The greatest depth to pre-Tertiary rocks found in Owens Valley was 8,000 feet. This depth may be in error by +2,000 feet. AEROMAGNETIC SURVEY An aeromagnetic survey of Long Valley, the Vol- canic Tableland, and adjoining areas was made in 1956 (pl. 2). Flight lines spaced 14-1 mile apart were flown east and west at a constant barometric altitude of 9,000 feet (about 2,000 feet above the ground). The magnetic contours were compiled with respect to an arbitrary magnetic datum; no correction was made for the regional variation of the magnetic field with lati- tude and longitude. At the same time, several long pro- files were flown north and south across Mono Basin and Long Valley at a constant barometric altitude of 14,000 feet; an interpretation of these profiles in the Mono Basin area was previously reported by Pakiser, Press, and Kane (1960). Later, in 1958, a detailed aero- magnetic survey was made over the gravity high north- east of Bishop (fig. 16) ; lines were flown east and west, as shown on figure 16, at a flight altitude of 500 feet above the ground. MAGNETIC PROPERTIES The magnetic properties of several samples of pre- Tertiary plutonic rocks and Cenozoic volcanic rocks from the Long Valley area have been measured by Wil- liam Huff of the U.S. Geological Survey. Of these samples, only plutonic rocks of dioritic composition and basalts are significantly high in magnetic susceptibilty. Plutonic rocks of dioritic composition have a measured magnetic susceptibility of about 0.003 egs unit. The more silicic plutonic rocks average about 0.0004 egs unit in susceptibility. The basalts range in susceptibility from 0.001 to 0.0037 egs unit, and they have an average magnetic susceptibility of about 0.002 egs unit. The volcanic rocks ranging from intermedi- ate composition to rhyolite (including the Bishop tuff of Gilbert (1988)) have an average magnetic suscepti- bility of about 0.0002 egs unit, and none differs signifi- cantly from this average. Some oriented samples of basalt collected from the surface have magnetic moment vectors that differ significantly from the present direc- tion of the earth's magnetic field. Thus, the magnetic contours may be expected to reveal the general distribu- tion of volcanic rocks of basaltic composition and of plutonic rocks having a dioritic or more mafic composition. 40 118°30' 37°30 118 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. 118°20' 37°25 _- 2 3 MILES 1 EXPLANATION cmm / 800 S> Total intensity magnetic contours with respect to arbitrary datum Contour interval 20 gammas. Location of flight lines shown by short lines. ————— -~ 2o_ y ~ ~ - Gravity-contour with respect to arbitrary datum Contour interval, 5 milligals Flight altitude 500 feet above ground Ficur® 16.-Comparison of magnetic and gravity contours for an area northeast of Bishop. MAGNETIC CONTOUR MAP The most striking feature on the magnetic contour map of the Long V alley-Volcanic Tableland area is the large area near the center of Long Valley where the magnetic field is anomalously high. This magnetic high consists of a broad feature on which are superim- posed two anomalies, which are small in area but have a magnetic relief significantly greater than that in sur- rounding areas. The most dominant of these, which is in the northwestern part of the broader feature, is the most intense local magnetic anomaly in the area. This broad magnetic complex presumably is the expression of a magnetically heterogeneous mass of volcanic rock within the Cenozoic section of the Long Valley block. The steepness of the gradients indicates that the top of the mass must lie relatively near the surface, and the pre-Tertiary floor is known from gravity measurements to be several thousand feet deep in this area. Southwest of the broad feature and just north and east of the Sierra Nevada front (in the southwestern part of the map, pl. 2) is an area of extremely erratic variations in the magnetic field. Basalt and some andesite crops out in this area, and the magnetic field is presumed to be the expression mainly of the basalt or the mafic andesite. To the north, at long 118°40', is a sharp magnetic high having a small area ; this high is assumed to be the expression of a volcanic neck of high magnetic susceptibility, presumably basalt. This magnetic high lies directly on the edge of a basalt flow and may represent the source of the flow. The broad magnetic high trending north along and south of the east edge of the southern part of the Ben- ton Range is presumably the expression of the Casa Diablo Mountain pluton, and the rock of this pluton must be more mafic than the granites exposed in the area. -The main mass of the pluton is probably diorite. GEOPHYSICAL SURVEYS _ 41 Samples of diorite from this general area were high in magnetic susceptibility. The magnetic low north of this broad high is in an area of outcropping granite and metasedimentary rocks that must be relatively low in magnetic susceptibility. The magnetic field is low over the northern extension of Owens Valley along the White Mountains front. The west gradient of this low prob- ably marks the eastern boundary of the Casa Diablo Mountain pluton; the low may represent, in part, the Owens Valley structural basin. - Gravity measurements in the area of the magnetic low show that the Cenozoic section is several thousand feet thick. In the extreme southeastern part of the map (pl. 2), a broad magnetic high coincides with a pronounced gravity high (pl. 1, sheet 2). This magnetic high is revealed to be a composite of two sharp high anomalies on the magnetic contour map compiled from low-level flight lines (fig. 16). Interpretation of the gravity high suggests that it is caused by a mass of dense rock that projects upward from the pre-Tertiary floor to within about 1,000 feet of the surface; the probable horizontal outline of the mass is indicated approxi- mately by the outline of the 20-mgal contour of relative gravity (fig. 16). The magnetic contours suggest a mass having approximately the same outline; the rock is more magnetic on the east and west boundaries and is very near the surface. The gravity data show that the Cenozoic section is several thousand feet thick all around this anomaly. ANALYSIS OF SELECTED MAGNETIC ANOMALIES Two magnetic anomalies were selected for detailed analysis: the complex magnetic high near the center of Long Valley and the composite magnetic high that co- incides in horizontal position with the gravity high northeast of Bishop (pl. 1, sheet 2; pl. 2; fig 16). Estimates of depth to the upper surface of the mass expressed by the complex magnetic high near the center of Long Valley have been made by Isidore Zietz (writ- ten commun., 1958). The outline of the broader part of this mass is roughly indicated by the 1,750-gamma con- tour, and the depth to the upper edge of the mass along this broader part ranges from about 4,000 feet below the flight altitude on the northwest to about 6,500 feet below the flight altitude on the southeast. The depth estimates were made by measuring the horizontal extent of the steepest gradients along this outline. Steenland (in Vacquier and others, 1951, p. 11-15) showed that for rectangular prismatic models having cross-sectional dimensions that are large compared to the depth of burial the horizontal extents of steepest gradients are approximately equal to the depth. The difference in depth from the northwest to southeast may indicate that the upper surface of the mass slopes downward to the southeast; all depth estimates are consistent with this interpretation. Or, if an uncertainty of +=25 percent is considered probable in the estimates of depth, the depth to the upper and possibly horizontal surface may be about 5,000 feet below the flight altitude. The aver- ago altitude of the ground in this area is about 7,000 feet, and the depth to the upper surface of the mass below the ground is therefore about 3,000 feet (the air- craft at 9,000 feet); this surface may slope gently downward to the southeast. Zietz found the depth to the source of the pronounced magnetic high in the northwestern part of the broader feature to be about 4,500 feet below the aircraft and the depth to the source of the magnetic high in the south- east to be about 4,000 feet below the aircraft. These masses of more highly magnetic rock are, therefore, at about the same depth as the larger mass. The mass to the northwest may be about 1 mile wide and 2 miles long, long axis extending northwestward; the mass to the southeast may be about 1 mile square horizontally. One possible interpretation of this complex feature is that the larger mass, shown in outline by the 1,750- gamma contour, consists of rock of intermediate mag- netic susceptibility into which have been intruded, or from which have been segregated, the two smaller bodies composed of more highly magnetic rock. A gravity high in the same area, which unfortunately was not fully defined, indicates that the corresponding mass of dense rock is not as large in area as the larger mass inferred from magnetic data. An alternative interpretation, which is preferred by the writers and is more consistent with the gravity data, would regard the smaller more highly magnetic masses as volcanic necks that were sources of a sequence of flows that are expressed by the broader feature. The sequence of flows would have been deposited concur- rently with the Cenozoic clastic deposits but represent only a fraction of the total thickness (18,000=5,000 feet). Thus, they would have a magnetic relief that is small compared with the necks and the nearby thick flows, which continue downward to great depths, and their influence on the gravity field would be small. The highly generalized distribution of such a sequence of flows is shown on the regional geologic cross sections (figs. 8 and 9). The larger of the masses of highly magnetic rock (the mass in the northwestern part of the broader feature) may consist actually of two or more necks whose mag- netic anomalies merge and which were intruded along a northwestward-trending zone of weakness that extends into the mass to the southeast. The upper surface of the flows may slope to the southeast, and the lava from which they were solidified may have flowed in this direc- 42 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. tion. This slope is consistent with the generally down- ward slope to the east of the pre-Tertiary floor as in- ferred from the gravity data (fig. 8). A volcanic pile similar to that in Long Valley was in- ferred on the basis of magnetic and gravity data and of the outcrop of basalt on Paoha Island, near the cen- ter of Mono Basin (Pakiser and others, 1960). A magnetic high flanked by gentle gradients coincides in horizontal position with the gravity high northeast of Bishop (pl. 1, sheet 2; pl. 2). The surface altitude in this area is just over 4,000 feet, and the flight altitude for the lines from which the map (pl. 2) was compiled was about 9,000 feet. This relatively great height above the ground accounts for the small amplitude of the anomaly and for the gentle gradients. In order to ob- tain a clearer definition of the anomaly, a small area was reflown at an altitude of 500 feet above the ground. The magnetic contour map compiled from these low- altitude lines shows that the anomaly consists actually of two separate magnetic highs with generally northerly trends and that the axes of these highs correspond close- ly with the east and west gradients of the gravity high (fig. 16). The gravity and magnetic highs are there- fore both caused by the same dense and magnetic mass or group of masses. Estimates of the depth to the top of the bodies that cause each of the two magnetic highs were made by the method described on page 41. The depth to the top of the westernmost of the two bodies was estimated to be about 1,200 feet below the flight altitude, or about 700 feet below the surface of the ground. The depth to the top of the body to the east was found to be about 2,000 feet below the flight altitude, or 1,500 feet below the surface. Although approximate, these estimates indi- cate that the depth to the top of these bodies is about 1,000 feet below the surface of the ground. This depth corresponds closely with the estimated depth to the top of the mass expressed by the gravity anomaly. If the density contrast of this mass with respect to the Cenozoic valley fill is 0.5 g per cm', the mass would be about 1,000 or 2,000 feet below the surface. The narrow magnetic highs may be interpreted as the expressions of dikelike intrusions or perhaps of rows of volcanic necks that may have been sources for buried flows in this area. The presence of a complex pile of such flows, having a large aggregate thickness, and the related dikes or necks could readily explain both the gravity and magnetic anomalies. Although a magnetic low is between the two magnetic highs, the general level of the magnetic field in this area is higher than that in all nearby areas except one-to the west. This anomaly, in which the magnetic field rises to more than 2,000 gammas, is assumed to be the expression of the southern limit of the Casa Diablo Mountain pluton described on page 40. An alternative interpretation would explain the gravy- ity high and the associated magnetic highs as the ex- pression of a large igenous intrusion in the Cenozoic rocks, which are generally about 5,000 or more feet thick in this area. The separate magnetic highs could be explained by the presence of more highly mafic and therefore more highly magnetic differentiates of the intrusive mass in which the more mafic minerals crys- tallized out along the borders early during the cooling of the mass and more silicic and less highly magnetic rock crystallized later in the core. No such intrusions of Cenozoic age are known in this area, but they may be buried under Cenozoic deposits. A second alternative would explain both the gravity and magnetic anomalies as the expression of an up- thrown block or blocks of pre-Tertiary rocks. If the anomalies are caused by a single block, the walls would seemingly be composed of material more highly mag- netic than the interior (possibly basalt intruded along the bounding faults). If the anomalies are caused by two blocks, the blocks would be in the form of narrow upthrown slivers. The block or blocks of pre-Tertiary rock may necessarily have remained more or less sta- tionary while the pre-Tertiary floor of Owens Valley subsided in the surrounding areas, and thus the sta- tionary blocks might have the appearance of being up- thrown blocks. The mechanism of such a deformation is difficult to conceive, but such seemingly anomalous movements of small masses contrary to the movement of larger blocks could possibly happen in a system of strike-slip faults. The real nature of the mass of dense and magnetic rock in this area remains unknown. Only the general outline of the mass, which is indicated approximately by the 20-mgal contour of relative gravity, the shallow depth of burial (about 1,000 feet below the surface), and its necessarily steeply sloping sides are known with confidence. § é SEISMIC SURVEY In the following discussion of the seismic measure- ments, no attempt is made to identify the individual velocity units by rock type in other than very broad terms, such as pre-Tertiary rocks, younger Cenozoic deposits, and older Cenozoic deposits. It is very dif- ficult to obtain a unique interpretation of the seismic data in terms of rock types without additional informa- tion because of the wide range of velocities possible in the sedimentary and igenous materials and the consider- able overlap in the speeds with which seismic waves travel in the materials. The interface between the Cenozoic and pre-Tertiary GEOPHYSICAL SURVEYS rocks is expressed by a marked velocity discontinuity that can be traced with reasonable accuracy through- out the Owens Valley region. The velocity in the pre- Tertiary granitoid and metamorphic rocks ranges from 15,000 to 17,750 fps (feet per second) (Pakiser and others, 1960) ; therefore, layers having velocities with- in this range can be identified with reasonable certainty as pre-Tertiary rocks. However, layers of evaporites, basalts, and rhyolite flows may also have velocities with- in this range; identification of pre-Tertiary rocks hav- ing a given velocity to some extent depends therefore on geologic relations, such as stratigraphic position. Identification of units within the Cenozoic section is much more difficult. In both Long Valley and Owens Valley, three velocity zones were observed. The rela- tively thin upper layer is composed predominantly of aerated sands and gravels having a velocity of less than 2,000 fps. The next layer, having a velocity range of 5,000-6,200 fps, may represent water-saturated uncon- solidated to semiconsolidated clastic or pyroclastic de- posits. Velocities of 6,900-11,200 fps are within the range found in older Cenozoic rocks that are more high- ly indurated than the younger deposits having lower velocities. The higher velocities in this range are often associated with flow rocks and evaporites Similar velocities were found in Mono Basin (Pakiser and oth- ers, 1960 ; see also Kane and Pakiser, 1961). SEISMIC FIELD METHODS AND INTERPRETATION oF DATA Measurements were taken along five seismic-refrac- tion profiles in Owens Valley and one in Long Valley (for location, see pl. 1). Conventional refraction meth- ods were used in which the seismometer spread was held fixed and the shots were recorded at a number of dis- tances from the spread. The method is equivalent to recording a reversed geophone spread. With the exception of one 1,100-foot spread, the pro- files were shot using 5,400-foot spreads having 12 geo- phones each. With the exception of profile 6, the shot points were located on nearly straight lines extending from each end of the recording spreads at distances that were determined by the depth to pre-Tertiary rocks. In general, the distances of the shot points from the end of each spread were increased from a few tens of feet to distances as great as 26,000 feet. Dynamite charges of 10-15 pounds were adequate throughout most of Owens and Long Valleys; however, for many of the more dis- tant shots and in a few locations on alluvial fans, charges of 20-50 pounds were required. Shot holes were drilled to below the water table where possible, using a truck-mounted auger or a rotary drill. On al- luvial fans, holes were usually dug with shovels or hand augers, and the charges were placed 5-10 feet beneath 48 the surface. Three records were taken of elevated air shots. Conventional seismic-refraction equipment was used in taking the measurements. Radio communication was established between shooter and observer ; the exact mo- ment of the explosion was transmitted by radio and re- corded on a separate trace of the record. The first ar- rivals could be read to the nearest 0.001 second for the shots nearer to the spreads and to 0.005 second for the more distant shots. In determining depths from the observed traveltime data, conventional intercept-time or delay-time comput- ing methods were used (Heiland, 1940, p. 506-533; Dobrin, 1952, p. 237-240; Nettleton, 1940, p. 250-251). The velocity layering, however, was determined only beneath the geophone spread, and only if the velocity increases with depth. The velocity layering was not known to be continuous under an entire profile, but this assumption was made unless information was available to indicate that the velocities changed laterally along the profile. If the intercept times at all shot points did not plot in a straight line, indicating that the re- fracting interfaces were not plane, the intercept times were separated into delay times corresponding to the two ends of the least-time path for each shot (Bar- thelmes, 1946). Depths to velocity discontinuities were computed using the delay times at each shot point. The delay-time method used is approximate in that it does not adequately take into account the changes in dip and velocity below the refracting interfaces along the profile. Because of the uncertainties inherent in the refraction method, however, this approximation does not introduce serious errors, and refinements yield- ing presumably more accurate results can be mislead- ing. ¢ ANALYSIS OF SEISMIC PROFILES PROFILE 1 Seismic profile 1 (fig. 17) was shot in an area where the gravity data suggest that the depth to pre-Tertiary bedrock should be relatively shallow. Two 5,400-foot spreads of 12 geophones each were placed end to end along a straight line. Seismograms were recorded on the easternmost spread from shot points 1 and 2 and on the westernmost spread from shot points 3-6, and from a shot point 11,000 feet west of this spread at ap- proximately the same location as shot point 5, profile 2. Traveltime curves for shot points 1, 2, 4, 5, and 6 are plotted on figure 19. The arrival times for shot point 3 are omitted because no shot instant was re- corded on the seismogram. Depths beneath shot points 1, 2, and 4 were computed using intercept times based on the assumption that velocity interfaces for a dipping layer were plane. 44 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. W. 1.0 - EXPLANATION a - Arrival time, shot point west of spread a = Arrival time, shot point east of spread C -- Intercept time s SP 3 * & Shot-point number 8 0.5 m < 0g l— # 0.0, SP 6 SP 5 SP 2 | 14,300 feet 0 Layer at surface, with Viy = 1000 fps, 10-20 feet thick [= . V, = 5700 fps V2 = 11,200 fps __ V, = 5700 fps meses =] [ 5 (Cenozoiz—Je—osits) flmfi? e 5 |- ¥; = 5700 fps JH Vi = 5700 fps P orn nl i erence nner tone # E1000 E =m oftememer ine !~ (Pre-Tertiary rocks) . Fi " " a.. AE & i |. = 15,600 fps [~- *H Vs, = 15,600 fps V, = 15,600 fps ~ FicurB 17.-Analysis of seismic profile 1 southeast of Owens Lake. A further assumption that a thin, discontinuous layer having a high velocity exists within the low-velocity Cenozoic rocks was made. This layer, having a velocity of 10,200 fps between shot points 1 and 2 and 11,200 fps between shot points 3 and 4, was assumed to be thin because the arrivals that had traveled horizontally in this layer were sharply attenuated with distance from the shot points. The arrivals along this layer disap- peared entirely at the sixth geophone from shot point 1 and at the third geophone from shot point 3 (not shown on fig. 17). The layer was arbitrarily assumed to be 50 feet thick in the interpretation of profile 1, and the velocity directly beneath this layer was assumed to be 5,700 fps. The discontinuities in this layer were based on the results of intercept-time calculations at shot points 1, 2, and 4 and may not be significant ; the layer may be continuous. Erratic magnetic anomalies, which are characteristic of volcanic rocks, were recorded on a trailer-borne magnetometer traverse in this area (G. D. Bath, oral commun., 1958). The depths to the 15,600-fps layer between shot points 1 and 4 were computed using formulas for a dipping layer. An increased dip in the pre-Tertiary rocks be- tween shot points 3 and 4 is indicated by the higher up- dip apparent velocity recorded from shot points 4, 5, and 6. The depths to the same layer at shot points 5 and 6 were determined from the one-way delay times at these shot points. Evidence of a fault between shot points 4 and 5 is found in the large difference in inter- cept times for the pre-Tertiary velocity segments. A possible source of error in the determination of the velocity layering between shot points 1 and 4 is in the assumed thickness of the high-velocity bed within the Cenozoic rocks. An alternate interpretation of the seismic data could be based on the assumption that the 5,700-fps layer is relatively thin and that between it and the unaltered pre-Tertiary basement is an intermediate layer (the 10,200- to 11,200-fps layer) that is composed possibly of weathered and fractured pre-Tertiary rocks. This layer might also be a thick layer of older Cenozoic materials having high absorption properties. The existence of such a layer could possibly also explain the attenuation of the first arrivals. Any increase in the assumed thickness of the 10,200- to 11,200-fps layer would also result in an increase in the calculated depth to the 15,600-fps layer; therefore, the indicated depths to the unaltered pre-Tertiary rocks should be con- sidered as minimums. If the 10,200- to 11,300-fps layer continues downward to the 15,600-fps layer basement, the depth to that layer at shot point 4 would be about 1,600 feet instead of about 900 feet. The first interpretation of the data presented is pre- ferred over several other possible alternatives. Because of uncertainties that exist, as well as those inherent in seismic-refraction measurements, the depths indicated in the cross section are not exact but represent what is probably the most reasonable of several possible depths by which the traveltime data can be explained. PROFILE 2 Seismic profile 2 (pl. 3) was shot along a 42,000-foot line where the gravity measurements (pl. 1, sheet 3) GEOPHYSICAL SURVEYS suggested that the pre-Tertiary bedrock is several thou- sand feet deep at one end (shot point 1) and much shal- lower at the other end (shot point 5). Simultaneous recordings were made by the U.S. Geological Survey and the Seismological Laboratory of the California Institute of Technology on two spreads 5,400 feet and 1,200 feet long, respectively, separated 1,200 feet to give a combined geophone spread of nearly 8,000 feet. Intercept formulas for two dipping layers were used to compute the depths to and the dip of the 8,600-fps and the 15,800-fps layers beneath shot points 3, 4, and 7. Depths to the 15,800-fps layer at shot points 1, 2, 5, and 6 were computed using delay times at these points. The velocity layering at shot points 5 and 6 was assumed to be the same as that beneath the geophone spread; however, measurements made by use of a shorter spread between shot points 1 and 2 revealed a 7,000-fps layer directly below the low-velocity weathered layer and a 10,000-fps layer at a depth of about 2,000 feet. The uppermost layer of the overburden in most of the areas described is composed of dry, aerated gravels and sands having a low velocity of about 1,000 fps. The existence of this low-velocity layer is confirmed on most of the profiles by the fact that the first linear segment of the traveltime curves do not pass through the origin, even with very close spacings. For this and subsequent profiles, calculations were made assuming a weathered layer having a velocity of 1,000 fps and a depth deter- mined primarily by the intercept time of the first linear segment of the traveltime curve. The error resulting from this assumption cannot be great, because of the shallow depths and small total traveltimes involved. The weathered layer along profile 2 is relatively thick, approximately 50 feet, as is indicated by the intercept of the 6,200-fps segments of the traveltime curve at shot points 4 and 7. The depths to the 15,800-fps layer may be in error by as much as 15 percent, but the subsurface configuration cannot differ much from that shown. As a check on the general reliability of the interpretation, a ray path (ABCDEFG) was constructed, and the time for a wave traveling from shot point 1 to the first geophone of the spread was calculated. The time required to travel this path would be 2.05 seconds; the recorded time was 2.13 seconds, a difference of less than 4 percent. In view of the many small uncertainties, especially in the thickness of the near-surface low-velocity layer, it would be mean- ingless to pursue further the cause of this discrepancy. PROFILE 3 Seismic profile 3 (fig. 18) was shot along an east- trending road just south of Lone Pine (pl. 1, sheet 3). V. P. Gianella of the University of Nevada (written commun., 1957) earlier noted that the road is now 45 abruptly offset about 16 feet apparently in a left-lateral direction in the zone of the earthquake fault of 1872. The position of the road offset falls directly on the trace of a fault as mapped by Willard D. Johnson in 1907 (Hobbs, 1910). This fault is the easternmost of two important faults along which movement took place in the 1872 earthquake, and evidence for it can still be seen on aerial photographs. The road is a modern oil- surfaced road so it could not have been offset in 1872 by fault movement, but its 1872 predecessor (perhaps a wagon trail or fence line) may have been offset by the earthquake. The profile was shot to determine if seismic evidence for a fault could be found in the posi- tion of the road offset. The seismic traveltime curves and the delay times at each geophone (Pakiser and Black, 1957) indicate that the depth to the 5,700-fps layer (assumed to coincide with the water table) in- creases from about 8 to 12 feet within 200 feet east of the third geophone from the west, which had been placed in the road offset. Faults are known to act as ground-water barriers or dams that in many places cause depression of the ground-water table on the down-gradient side of a fault (Robert C. Scott, oral commun., 1958). The finely ground material in a fault zone may be in part altered to clay having low permeability, and dissolved solids may be deposited in the remaining pore space in such a way that the flow of ground water through the fault zone is retarded and the water is ponded on the up- gradient side. The surface water from the Sierra Nev- ada in this area flows eastward. Depression of the ground-water table is east of the offset and is of an amount that would be expected if a fault actually exists at the road offset. PROFILE 4 Profile 4 (pl. 3) was located to intersect a fault scarp of late Quaternary origin described by Knopf (1918) that is approximately 3 miles east of Independence (pl. 1, sheet 3). The road along which the spread was placed makes an abrupt turn, approximately 25° N., at the number 4 geophone position. The traveltime curves have been corrected to allow for this change in direction. Depths to velocity interfaces at shot points 2, 3, and 4 were computed using intercept times on the assumption that interfaces are plane. A weathered layer 2-9 feet thick having a velocity of 1,000 fps was assumed in or- der to explain the failure of the 5,800-fps segments (shot points 2 and 3) and the 6,000-fps segments (shot points 1, 7, and 8) to pass through the origin. The 5,700-fps velocity in the upper layer is consistent with that found elsewhere in Owens Valley and was assumed to be correct at this location. A velocity of 6,500 fps in 46 FIRST ARRIVAL TIMES STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. 300 EXPLANATION 240 224 200 L Arrival time, shot-point 1 A Arrival time, shot-point 5 SP 1 89 173 151 MILLISECONDS ofips/ y 134 117 100 156 Shot-point number best 138 123 105 098 /'fil / 061 Vi = 5780 fps (Average) Gaz secs SP 5 0 + | |-- 200 feet -»} 1100 feet fe 440 feet ----->| DELAY TIMES (+- -*- o 5780 & | EXPLANATION a o . § 10ll A Delay time, shot-point 1 © $ g & 6 T7 ® A 4 20 Delay time, shot-point 5 AVERAGE DELAY TIMES Ps: EXPLANATION g 02004 sec. g 10 te # Avera ed.ela time pe - 4 -i verage y > 1 3 20 W. l»——Road offset E. 0 Aerated zone N t. 10 7 +7 V= 1000 fps ra Water table Fault? j7/////////////////////////////// t V; = 5780 fps Saturated zone 20 I ( \ \ g I \ \ I | U 16 feet ? = X . SP 2C. sp 1 1 sit." mg Center line SEV, sec. 28, T. 15 S., R. 36 E. g «R 4 $ eel ¢ tg I \ \\ Geophone spread \ FieurE 18.-Analysis of seismic profile 3 at a road offset south of Lone Pine. the layer identified as younger Cenozoic rocks was calcu- lated from the apparent 6,300-fps downdip velocity rec- orded at shot points 2, 3, and 4 and from the apparent 6,600-fps updip velocity recorded at shot points 1, 7, and 8. The 7,500-fps true velocity in the older Cenozoic rocks was obtained from the apparent velocities: 7,400 fps at shot points 2, 3, 4, and 5 and 7,600 fps at shot points 8 and 9. The velocity in the pre-Tertiary rocks has been estab- lished at other locations in Owens Valley as approxi- mately 15,700 fps. Because of the location of the geo- phone spread over a relatively complex structure, this velocity could not be calculated from direct- and re- verse-shot data. Two apparent velocities were deter- mined for the basement : 14,500 fps at shot points 10 and 11 and 12,200 fps at shot points 5 and 6. Assuming the true value of 15,700 fps for the velocity in the pre-Ter- tiary rocks, the depth to basement at shot point 5 is 1,650 feet, and the dip of the pre-Tertiary surface is ap- proximately 9°. The fault between shot points 5 and 6 CEOPHYSICAL SURVEYS 47 is inferred from the abrupt difference in intercept times of the 12,200-fps segment recorded at these shot points. The velocity layering between shot points 6 and 12 is based upon measurements taken with a short spread in this location. A major fault is well determined at the No. 3 geo- phone by the discontinuities in the 14,500-fps travel- time segments, but its location on the profile could be in error by as much as 500 feet horizontally. The depth to the pre-Tertiary basement on the downthrown side was determined by extending the Cenozoic and pre- Tertiary interface from shot point 5 to the fault at the calculated 9°. The corresponding interface was like- wise extended eastward from shot point 9 at a calculated dip of 3° to determine the depth to the pre-Tertiary basement on the upthrown side of the fault. The 32,800- fps segments recorded from shot points 5 and 6 also indicate a fault. The high apparent velocity is inter- preted as the result of arrivals that travel westward in the upper part of the pre-Tertiary rocks to the plane of the fault and then are diffracted upward to the western- most three geophones. A similar but less extreme high apparent velocity breaks away from the 7,400-fps seg- ment at shot point 4. This discontinuity may represent diffraction from a shallower layer terminating against the fault. Records from shot points 9, 10, and 11 re- veal a velocity of about 9,600 fps that may indicate a relatively high-velocity layer within the Cenozoic see- tion, whose upper surface is near the fault edge where the fault cuts the pre-Tertiary rock. Another interpretation of the data is based upon the correlation of the 32,800-fps segment as an apparent updip velocity with the 9,600-fps segment as an ap- parent downdip velocity to obtain a true velocity ap- proximately that assumed for the pre-Tertiary rocks. This would require a dip of approximately 16° E., which could indicate a system of distributive faults about 1,500 feet wide. A second alternate interpretation is based upon the assumption that the 9,600-fps and the 12,200-fps velocities are, respectively, the downdip and updip apparent velocities of a layer within the Cenozoic section. This interpretation has been rejected primarily on geologic grounds and also because consistency be- tween the overlapping east and west traveltime curves is difficult to obtain under this assumption. In computing depths at shot points 7 through 11, the intercept times were separated into delay times corre- sponding to the least time travel path for each shot. The 5,700- and 6,500-fps layers are required to thin to the west, and the 7,500-fps layer is required to be very near the surface to explain these delay times. The 3° dip of the Cenozoic and pre-Tertiary interface east of shot point 9 was computed by assuming that the 14,500- fps segment is the apparent downdip velocity corre- sponding to a true velocity of 15,700 fps. To check part of the interpretation a ray path (ABCDEF, profile 4, pl. 3) was constructed. The calculated total traveltime agrees exactly with that measured in the field. PROFILE 5 Profile 5 (pl. 3) was run along an east-trending road 214 miles south of Bishop (pl. 1, sheet 2). The maxi- mum shot-point offset of the 12-geophone, 5,400-foot spread was 13,000 feet. Depth determinations were made by use of intercept times and formulas for plane dipping surfaces. A weathered layer having a velocity of 1,000 fps and a maximum thickness of 10 feet was assumed in making the calculations. The 5,820-fps segments recorded at shot points 1 and 3 and the 6,050-fps segments at shot points 2 and 8 yield a true velocity of 5,900 fps, which is assumed to be that in the younger Cenozoic rocks. The interface between younger and older Cenozoic rocks dips to the east beneath the spread. The 6,720-fps seg- ment is taken as the apparent downdip velocity, and the 7,120-fps segment is taken to be the apparent updip velocity to give a true velocity of 6,900 fps. Velocity layering for the part of the profile between shot points 1 and 8 was determined using formulas for dipping beds; at shot points 3-7, one-way delay times were used. A precise calculation of the velocity in pre-Tertiary rocks was not possible for the profile because of small irregularities in arrival times; however, a reasonable interpretation of the data is made by taking 21,600 fps as the apparent updip velocity of the shots to the east and 12,300 fps as the apparent downdip velocity of the shots to the west. This correlation yields a true velo- city of approximately 15,700 fps, which previously had been identified as that in the pre-Tertiary rocks. No quantitative interpretation was made of the traveltime curve recorded from a shot point midway between shot points 3 and 4 because of an error in the station location ; however the data are included to supply additional evi- «dence of the 12,300 fps apparent downdip velocity in pre-Tertiary rocks. The greatest computed depth to the pre-Tertiary rocks along this profile is approxi- mately 4,800 feet (shot point 5). Shot point 5 is evi- dently in the fault zone along the front of the White Mountains. This depth together with those calculated at shot points 6, 7, and 8 are assumed to be minimum depths, and the true depths may be 500-600 feet greater than those indicated on the profile. This conclusion is based on the probability that the velocity in the older Cenozoic rocks east of the spread increases with depth. Gravity data (pl. 1, sheet 2) indicate that the thick- ness of the sedimentary rocks increases to the south. 48 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. Measurements taken along a profile 3 miles south of and parallel to profile 5 failed to indicate any pre-Tertiary rock even though a maximum shot-point offset of 17,000 feet was used. By assuming a velocity of 15,700 fps in pre-Tertiary rocks and making further limiting as- sumptions, the greatest depth to the Cenozoic and pre- Tertiary interface is determined to be at least 6,400 feet and possibly more. The east dip of the basement inter- face shown on profile 5 corresponds closely to that in- ferred from the gravity data (fig. 11). PROFILE 6 Profile 6 (pl. 3) was shot in Long Valley using a 12- geophone spread 5,400 feet long (pl. 1, sheet 1). Neither the spread nor the shot points lie on a straight line; however, the true shot-to-geophone distances were de- termined, and the necessary corrections in the travel- time curves were made. A thin weathered layer having a velocity of 1,000 fps was assumed in calculating depths. The velocity in the younger Cenozoic rocks was determined from the 4,900-fps segment recorded from shot point 1 by assuming an equal intercept time at shot point 5 to give a velocity segment of 5,200 fps from that shot point. The true velocity of 5,000 fps thus determined is not exact, but the error cannot be great. The 5,700- fps segment, assumed to be the apparent downdip ve- locity in the older Cenozoic rocks, is consistent over three traveltime segments, two of which are of second arrivals. This segment is correlated with the 6,300- fps segment recorded from shot point 5 to yield a true velocity of 6,000 fps. The 5,000-fps layer becomes thin- ner to the south, and beyond shot point 2 the older Ceno- zoic rocks were assumed to be very near the surface. The 10,000-fps high-velocity layer is assumed to be volcanic rock, probably rhyolite that correlates with the rhyolite near the center of Long Valley and with the rhyolite of Glass Mountain ; the rock is undoubted- ly complexly faulted, and its upper surface is highly irregular, as indicated by the seismic data. The loca- tions of the individual faults as well as their displace- ments should be considered as a schematic presentation of the probable structural features rather than an exact description. Although the irregular surface of this high-velocity layer scatters the traveltime data consid- erably, a fairly reliable determination of velocity is made by correlating the 9,200-fps and 11,800-fps seg- ments as apparent velocities to yield a true velocity of approximately 10,000 fps. The faults on either side of shot point 5 and the fault between shot points 1 and 2 were inferred from delay times at the shot points; the surface profile of the 10,000-fps layer under the spread was calculated using the delay times at each geophone (Pakiser and Black, 1957). The 31,200-fps velocity recorded for shot point 4 is believed to represent an apparent updip velocity along a tilted block having a 10,000-fps velocity. The travel path of seismic waves from shot point 4 is greatly dif- ferent from that from shot points 1, 2, and 3. This dif- ference in travel paths may explain the failure of the 31,200-fps segment to appear on other traveltime curves. Another explanation is possible: the arrivals may have been refracted off a vertical interface, such as a major fault, that bounds the area worked; however further data would be required before reaching a definite con- clusion. As a test of the interpretation, ray paths (ABCDEFG and A'B'C'D'E'F'G', profile 6, pl. 3) were constructed. Over both paths the computed and the observed travel times agree very closely. Gravity data (pl. 1, sheet 1) rule out the possibility that the 10,000-fps layer could be pre-Tertiary rock. The results of six seismic-refraction profiles shot in Mono Basin by the Seismological Laboratory of the California Institute of Technology were presented in an earlier report (Pakiser and others, 1960). Part of the data included here also was previously described and interpreted (Kane and Pakiser, 1961). GEOLOGIC INTERPRETATION OF GEOPHYSICAL SURVEYS STRUCTURAL GEOLOGY OF OWENS VALLEY REGION A nearly complete, if necessarily somewhat general- ized, description of the Cenozoic structural geology of the Owens Valley region can now be presented from the preceding review of the geology of the Owens Valley region and from the detailed presentation of the results of the gravity, magmetic, and seismic investigations. In this discussion, large-scale structural features having regional significance are emphasized. As a result of the geophysical studies, the structure of three major down- dropped blocks is now rather fully known. These are Mono Basin, Long Valley, and Owens Valley. The discussion of the gravity and seismic profiles and the gravity contour maps clearly indicates that zones of steep gravity gradient mark zones of faulting along which the Cenozoic rocks have been displaced relatively downward against pre-Tertiary rocks. The amplitude of the gravity-minimum anomalies over areas covered by Cenozoic rocks is a guide to the thickness of these rocks. The magnitude of the steepest gradient is a guide to the dip of the bounding faults. The gravity profiles (A-4""" to H-H"', figs. 6-15) and the seismic profiles (figs. 17, 18; pl. 3) are lines along which in- formation on the depth and configuration of the Ceno- GEOLOGIC INTERPRETATION OF GEOPHYSICAL SURVEYS zoic and pre-Tertiary interface is controlled. It is a relatively simple task to extrapolate the subsurface con- figuration from these lines along zones of steep gravity gradient. Thus the subsurface structure of Mono Basin, Long Valley, and Owens Valley has been determined with an accuracy sufficient for reconnaissance purposes. MONO BASIN AND LONG VALLEY The structure of Mono Basin has been previously de- | scribed by Pakiser, Press, and Kane (1960) and is only | briefly summarized here. Mono Basin is a large, roughly rectangular block that has subsided in two parts and | has received a maximum accumulation of about 18,000 « 5,000 feet (5.5+-1.5 km) of low-density sedlments and | \ volcanic deposits of Cenozoic age (pl. 1, sheet 1). The! block is bounded by vertical or near-vertical faults, and although these faults in general conform to the physio- graphic outline of the basin, they are found basinward 1 from the physiographic escarpments. The basin is di- vided into two parts: (1) a roughly triangular block forming the southwestern half, which has subsided to the maximum depth of about 18,000+=5,000 feet and (2) the block forming the northeastern half, which has sub- sided only about one-third that much. The northeast- ern half, only partly shown on the map (pl. 1, sheet 1), was described more fully in the earlier paper. These two blocks are almost completely separated by a tri- angular salient that projects northwestward from the southeastern boundary of Mono Basin. The structure of Long Valley is of the same type as Mono Basin (pl. 1, sheet 1). The valley is a closed basin, bounded on its entire perimeter by vertical or near-verti- cal faults. The locations of these faults are defined by the steepest gravity gradients along the nearly elliptical outline of the Long Valley block. Segments of this out- line are obviously straight lines (for example, the bounding fault on the south and parts of the bounding fault system on the north and northwest). Parts of the system of bounding faults may be curved, however (for example, the eastern are of the structure), or these parts may consist of a series of short line segments that ap- proximate a curve. If the perimeter of the Long Valley block as defined by its bounding faults consists of short line segments, it would be a polygon approximating an ellipse. The part of the Long Valley block outlined by the eastern arc, approximately the eastern one-third, sub- sided to a maximum depth of 18,000+=5,000 feet (5.5 1.5 km) below the surface-approximately 11,000 feet below sea level-and received a thick accumulation of light sediments and volcanic deposits of Cenozoic age (figs. 8 and 9). Mono Basin subsided about the same distance to a level below sea almost identical with that of 49 Long Valley (fig. 9). (See also Pakiser and others, 1960.) The pre-Tertiary floor of the Long Valley block slopes gently eastward from the Sierra Nevada front to reach the maximum depth about 5 miles west of the extreme eastern limit of the bounding fault system (fig. 8). A system of distributive faults has been inferred in this zone of gentle slope, but warping could explain the gentle slope as well. The maximum width of the Long Valley block is about 9 miles; the length is 19 miles. The area of the block is about 150 square miles. Thus, if the average depth of the Cenozoic deposits is estimated to be 214 +1 miles, the volume of the rocks contained in the basin is about 375150 cubic miles. The Cenozoic rocks and the pre-Tertiary rocks are in fault contact throughout the entire fault system bounding the Long Valley block, and the faults of this system are either vertical or very nearly vertical. Therefore, almost the entire section of Cenozoic rocks has been displaced against the older rocks by faulting. The displacement could have happened in only two possible ways: Either the entire Cenozoic section ac- cumulated before faulting displaced it against the pre- Tertiary rocks, or fault movements took place nearly concurrently with deposition and the lighter Cenozoic deposits were displaced against faults soon after they were laid down. The first possibility can be ruled out with confidence, and so, fault movements are concluded to have been almost continuous or repeated frequently throughout the time of deposition of the Cenozoic sec- tion. This interpretation is supported by the seismic evidence of layers within the Cenozoic section in Owens Valley that have been displaced much less by faults than has the pre-Tertiary floor (see, for example, pl. 3) ; presumably, subsidence in Long Valley was similarly prolonged. The gravity high near the center of the Long Valley f block and the corresponding magnetlc anomaly have \ been interpreted as an expression of a complex pile of ( intrusive and extrusive volcanic rocks having high , density and high magnetic susceptibility composed probably of basalt or mafic andesite. A similar gravity / high was found in Mono Basin (Pakiser and others, | 1960). Some local structural features of interest are revealed by the gravity contours in the Long Valley area. The Hilton Creek fault, mapped by C. D. Rinehart and D. C. Ross (written commun., 1956), is shown by the gravity contours to extend a few degrees west of north to meet the southern bounding fault of the Long Valley block (pl. 1, sheet 1). The gravity low trending south- eastward from the Long Valley minimum approxi- mately along the course of Owens River probably rep- 50 resents a thick accumulation of the Bishop tuff of Gilbert and, perhaps, other low-density rocks of Ceno- zoic age. The low-density deposits may have been laid down in a mature predecessor of the present youthful valley of Owens River in the area of the Volcanic Table- land. Rinehart and Ross (1957) concluded that a Pleistocene lake may have been ponded by the Bishop tuff and that the lake later broke out and the southeast- ward-flowing Owens River cut the deep Owens River Gorge, perhaps at a time of uplift of the Volcanic Tableland. South of the Owens River Gorge, deflec- tions in direction of the gravity contours are seen to cor- relate with mapped and inferred faults (pl. 1, sheet 1). The negative departure of the gravity field near the faults is probably caused by moderately thick accumu- lations of alluvium. Anomalies of this type are clearly evident along the northern end of Wheeler Crest. Many of the smaller alluvium-filled valleys that flank small basin ranges east of Long Valley are expressed as gray- ity minimums. The most marked of these is the gravity low west of the southern end of the Benton Range. Similar small gravity anomalies in the Mono Basin area were previously described by Pakiser, Press, and Kane (1960). owENS vYALLEY The Owens Valley structure as defined by the geo- physical survey extends for more than 100 miles a few degrees east of south along and west of nearly the en- tire length of the White and Inyo Mountains (pl. 1). Except on the extreme south, the east fault zone bound- ing the deepest depression of the Owens Valley struc- ture forms a common boundary with the White and Inyo Mountains. The west boundary of the valley block is generally east of the Sierra Nevada front, and the complexity of the structure along the Sierra Ne- vada front, as revealed by the geophysical survey, is in marked contrast to the relative simplicity of the struc- ture along the White and Inyo Mountains front. In the latitude of Long Valley, the westernmost fault bounding the Owens Valley structural basin is about 35 miles east of the Sierra Nevada. In the latitude of Owens Lake, the Sierra Nevada and the Owens Val- ley structural basin share a common boundary. The east fault bounding the Owens Valley structural basin is defined by the zone of steep gravity gradients along the White and Inyo Mountains (pl. 1). It ex- tends a few degrees east of south without interruption for about 50 miles from its northern limit east of the southern end of Blind Spring Hill to a point east of Red Mountain (pl. 1, sheets 1 and 2). Along this seg- ment of the fault zone, the Cenozoic rocks of Owens Valley are everywhere in fault contact with the pre- Tertiary rocks of the White Mountains. Analysis of STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. selected gravity profiles indicates that the fault dips very steeply and may be vertical (figs. 8, 11, and 12). The greatest depth to the pre-Tertiary floor of Owens Valley west of this fault is found from interpretation of these gravity profiles to be 8,000+2,000 feet. In- terpretation of the seismic-refraction profile south of Bishop (pl. 3) reveals that the greatest depth to pre- Tertiary rocks at the eastern end of this profile is more than 4,800 feet. If an undetected (and probable) see- tion of higher velocity Cenozoic rocks exists under the eastern part of this profile, the depth could be consider- ably more than 5,000 feet. The seismic line is in an area where the residual gravity relief is less than that along gravity profile D-D' (pl. 1, sheet 2) ; therefore, the depth to pre-Tertiary rocks should be correspond- ingly less. The thickness of the Cenozoic section in the general area of Bishop is probably about 5,000 feet and increases to the north and south to about 8,000 feet. The correspondence between the gravity and seismic re- sults is good within the range of uncertainty of +25 percent. East of Red Mountain the east fault zone bounding the Owens Valley structure is abruptly offset to the west by a short fault; south of the fault, the zone con- tinues 60 miles without interruption (pl. 1, sheets 2 and 3). The short fault east of Red Mountain trends a few degrees north of east at right angles to the main fault. It is defined by a steep gravity gradient that indicates a steep, perhaps vertical, dip. Immediately south of this offset the gravity minimum anomaly of Owens Valley is extremely narrow and al- most disappears (pl. 1, sheet 3). But in the latitude of Independence it is once more clearly defined ; it becomes broader and more pronounced farther south (pl. 1, sheet 3). The pre-Tertiary floor in the narrow wedge north of Independence must be relatively shallow, perhaps 1,000 or 2,000 feet. Analysis of the gravity data (figs. 14 and 15) indicates that the thickness of Cenozoic rocks in fault contact with the pre-Tertiary rocks of the Inyo Mountains south of Independence is about 8,000+2,000 feet; the fault dips steeply and may be vertical. The seismic-refraction profile east of Independence on inter- pretation revealed a depth to pre-Tertiary rocks of about 3,000 feet (pl. 3). In the Owens Lake basin, the seismic-refraction profiles indicate a thickness of Ceno- zoic rocks of about 6,000 feet (pl. 1, sheet 3; pl. 3), but none of the profiles was so located as to reveal the maxi- mum thickness; therefore, the seismic and gravity re- sults on the depth to pre-Tertiary rocks are again in good agreement within the uncertainty range of +25 percent. A few miles southeast of Independence, a branch of the main east fault bounding the Owens Valley struc- GEOLOGIC INTERPRETATION OF GEOPHYSICAL SURVEYS 51 tural basin trends almost due north from its intersec- tion with the main fault (pl. 1, sheet 3). The existence of the branch fault, which may continue farther north than shown, is based on both gravity (pl. 1, sheet 3) and seismic-refraction evidence (pl. 3). Near Keeler, an- other branch fault trends south from its intersection with the main fault, and the main displacement of Cenozoic against pre-Tertiary rocks took place on this fault (fig. 17; pl. 3). A complex system of faults diverges from this fault to form the southeastern bound- ary of the Owens Lake basin and of the narrow exten- sion of the basin to the south (pl. 1, sheet 3). This narrow extension shown on the extreme south of the map, presumably continues farther south to join Rose Valley. The main bounding fault of the Inyo Moun- tains curves to the east and is terminated in Lower Cen- tennial Flat (pl. 1, sheet 3). The existence of the southwestward-trending fault just southeast of seismic profile 2 (pl. 1, sheet 3) is based on gravity evidence ; the refracted waves in this segment of the seismic profile (pl. 3) may have traveled at shal- low depths along this fault or from the relatively shal- low pre-Tertiary floor of the Owens Lake basin along this line. The horizontal distance from the seismic pro- file to this fault is just about the same as the computed depth to pre-Tertiary rocks; therefore, the waves may have been refracted nearly horizontally from the shot points to the fault and thence to the geophone spread. The main west fault bounding the Owens Lake basin as defined by the steep gravity gradient (pl. 1, sheet 3) coincides with the Sierra Nevada front, and interpreta- tion of gravity profile H-H' (fig. 15) suggests that the displacement is distributed between two parallel faults, the easternmost of which is about 2 miles east of the Sier- ra Nevada. The areal geology and gravity contours re- veal that the westernmost fault bounding the Owens Lake basin is displaced sharply to the east by three or more short faults trending a few degrees south of east (pl. 1, sheet 3; see also Kane and Pakiser, 1961). South of these short faults, the Owens Valley structural basin forms a deep, narrow channel into Rose Valley. The western bounding fault system of the deepest part of the Owens Valley structural basin north of the Owens Lake basin is everywhere found east of the Sierra Ne- vada front (pl. 1). The west fault system bounding Owens Valley as re- vealed by the gravity data (pl. 1, sheet 3; fig. 14) forms a common boundary with the eastern front of Alabama Hills, and the dip of the main fault is very steep, per- haps vertical. This fault continues a few degrees west of north for nearly 10 miles beyond the outcrop of pre- Tertigry rocks in Alabama Hills (pl. 1, sheet 3). The structural feature including Alabama Hills thus ex- tended is seemingly terminated on the north by a short fault or system of short parallel faults trending at right angles to the main fault. The exposed Alabama Hills may also be terminated by a short fault on the north. The deflection of the gravity contours on the southern end of Alabama Hills clearly reveals that the structural feature is terminated there by a short fault (pl. 1, sheet 8). North of the buried extension of Alabama Hills, the main fault continues to join the fault trending a few degrees west of north from the latitude of Independence. This fault was mapped by Knopf (1918) and is shown on the map (pl. 1, sheet 3). Northwest of Independence, the thickness of the Cenozoic rocks is relatively small, perhaps 1,000 or 2,000 feet. The westernmost fault bounding the Owens Valley structural basin continues beyond Big Pine to the north and passes into a warp or system of distributive faults about 3 miles north of Crater Mountain (pl. 1, sheet 2; fig. 12). North of the end of the main bounding fault, the gravity data (pl. 1, sheet 2; fig. 11) show that the Sierra Nevada front is downwarped. This conclusion is verified by seismic- refraction profile 5 (pl. 3) and by geologic mapping (P. C. Bateman, written communication, 1956). The feature thus defined has been designated the Coyote Warp by Bateman (written communication, 1956). The block of pre-Tertiary rocks known as Poverty Hills (pl. 1, sheet 2) is directly on the main western bounding fault in a position that is seemingly anomal- ous if the main fault is continuous in this area. This block is postulated to be a gravity slide that broke loose from the higher slopes of the Sierra Nevada and moved downslope to a position at rest in its present location, where it was partly buried by low-density sediments. This postulation explams its contradlctory location with respect to the main fault trends. North of Tungsten Hills and northwest of the main part of Owens Valley, the Owens Valley structure branches into the west-trending synclinal downwarp and subordinate faulting, as revealed by the gravity data (pl. 1, sheet 2; fig. 18). The axis of this down- warp is buried under the Bishop tuff of the Volcanic Tableland and trends into Round Valley. The gravity data are too few to define reliably the nature of the Cenozoic and pre-Tertiary interface along Wheeler Crest, but the interface is probably a fault contact; fault segments and springs have been mapped there by P. C. Bateman (written communication, 1956). The Cenozoic section is probably relatively thin in this area. The small gravity high whose axis trends northeast- ward from the Sierra Nevada front along Pine Creek may represent a buried extension of a relatively dense glacial moraine (pl. 1, sheet 2). Many of the small 52 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. irregularities of the gravity contours in the area sur- rounding Tungsten Hills may have local geologic significance, but they are not analyzed here. The Owens Valley structural basin thus defined is seen to be bounded by a single fundamental fault or narrow fault zone having only one dislocation along most of the western front of the White and Inyo Moun- tains. Along the eastern front of the Sierra Nevada, the boundary of the Owens Valley structural basin con- sists of a complex system of alternating faults and warps, and the faults are in general found east of the Sierra Nevada front. The Owens Lake basin is bound- ed by a complex system of steeply dipping or vertical faults, and north of Tungsten Hills the Owens Valley structural basin branches into a synclinal downwarp. The main, linear structure of Owens Valley continues north to the southern end of Blind Spring Hill. A sub- dued extension of the Owens Valley structural basin probably continues farther north around the northern end of the White Mountains. } NATURE OF CENOZOIC ROCKS The regional Cenozoic structural features of Owens Valley, Long Valley, and Mono Basin have been de- scribed. Before a comprehensive analysis of the origin of these structural features and of some of the rocks as- sociated with them, the nature of the rocks of Cenozoic age that are confined within these deeply depressed structures must be considered. Much information can be obtained by examination of the upper, exposed sur- face of these rocks, and it is important to emphasize that some information in three dimensions is obtained by geologic mapping. This surface is not a plane sur- face; it generally ranges in altitude from 4,000 feet in the Owens Lake basin to 8,000 feet in the mountain slopes adjacent to Long Valley and Mono Basin. The exposed surface of the rocks is described, and this de- scription is based on the work of others as cited. In addition, some information on the nature of these rocks can be inferred from gravity, magnetic, and seismic measurements and from a consideration of the relative volumes of the depressed structural features and of the self-evident available sources of the rocks contained in them (Pakiser and others, 1960). The inferred nature of the Cenozoic rocks of Mono Basin was described in a previous report by Pakiser, Press, and Kane (1960), who concluded that the rocks consist of stream-transported sediments and materials that are of volcanic origin and have a low average den- sity. The rocks of Cenozoic age were shown to be di- vided into two units on the basis of seismic velocity : (1) a shallow layer having a relatively low velocity that may represent mostly unconsolidated clastic de- posits of Pleistocene(?) and Recent ages and (2) a deeper layer having a higher velocity that may rep- resent rather well-indurated sedimentary and volcanic rocks of late Tertiary (?) and perhaps early Pleisto- cene(?) ages. By means of an erosion-deposition budg- et, about two-thirds of the approximately 8300 cubic miles of Cenozoic rocks in Mono Basin were also shown to be probably of volcanic origin (Pakiser and others, 1960). CENOZOIC ROCKS OF LONG VALLEY Geologic mapping (Gilbert, 1938, 1941; C. D. Rine- hart and D. C. Ross, 1957, and written commun., 1956), had shown that the Cenozoic rocks of Long Valley include stream deposits, lake beds, glacial debris, and volcanic pyroclastic and flow rocks that range in com- position from basalt to rhyolite (pl. 1, sheet 1). The gravity data show only that the Cenozoic rocks of Long Valley are significantly lower in density than the pre-Tertiary rocks; the density is probably about 2.3 g per cm®, which corresponds closely to the average density of the rocks revealed by geologic mapping. The single seismic-profile shot in Long Valley (pl. 3) reveals a velocity layering similar to that in Mono Basin. Directly beneath the near-surface low-veloc- ity-weathered-layer, a layer of younger Cenozoic de- posits having a velocity of 5,000 fps was found. The deposits are probably predominantly unconsolidated lake beds and stream deposits, but some products of ex- plosive volcanic eruptions may also be present. At a depth ranging from a few hundred to 1,000 feet, a layer having a velocity of 6,000 fps was discovered. The layer is probably a more highly consolidated variety of the deposits lying above it. The shallow layer hav- ing a velocity of 5,000 fps is probably made up of vir- tually unconsolidated clastic deposits of Pleistocene ( ?) and Recent ages. The layer having a velocity of 6,000 fps is probably composed of materials of the same type but of late Tertiary(?) and perhaps early Pleisto- cene (?) ages. A layer having a velocity of 10,000 fps was found be- neath the 6,000-fps layer at depths ranging from 1,000 to 3,000 feet (pl. 3). The 10,000-fps layer is complexly faulted and probably represents a buried flow that is composed probably of rhyolite and may correlate with the rhyolite exposed near the center of Long Valley and with the rhyolite of Glass Mountain. C. D. Rinehart (written commun., 1958) inferred that these rocks were buried in the Long Valley structure; seismic data seem to support that inference. No compelling evidence for a buried extension of the Bishop tuff was found during the geophysical study, but Rinehart believed that the 6,000-fps layer may represent the tuff. GEOLOGIC INTERPRETATION OF GEOPHYSICAL SURVEYS 53 In a previous report Pakiser, Press, and Kane (1960) showed that the volume of Cenozoic rocks in Mono Basin may range from 200 to 400 cubic miles and that the volume of stream-transported sediments deposited there from pre-Tertiary rock sources may range from 0 to 200 cubic miles. These lower and upper limits of the volume of stream-transported sediments cover a wider range and are less restrictive than the true great- est lower and least upper limits, but if they are as- sumed to be the limits, the following can be concluded : If the maximum amount of eroded material had been supplied to a Mono Basin having a minimum volume, a balanced erosion-deposition budget could have been maintained without requiring volcanic material to make up the deficit. On the other hand, if the minimum amount of eroded material had been transported to a Mono Basin having a maximum volume, all the ma- terial deposited in the basin would have been of volcanic origin. But if the probable amount of eroded ma- terial (100 cubic miles) had been deposited in a Mono Basin having a probable volume of 300 cubic miles, a deficit of 200 cubic miles remained which would have been made up by material of volcanic origin. The same type of analysis can be applied to Long Valley with similar results. The volume of Cenozoic rocks confined in the Long, Valley structural basin ranges from 225 to 525 cubic miles, and the probable volume is 375 cubic miles. The maximum amount of material that could have been re- moved by erosion from the highlands surrounding Long Valley multiplied by the density ratio of pre-Tertiary and denser volcanic rocks and the assumed density of the same materials as sediments (2.7/2.3) is 800 cubic miles; this value yields a surplus of 75 cubic miles that would have been transported out of the Long Valley area by streams if a minimum volume of the structural basin is assumed (225 cubic miles). However, at least one-third of the stream-transported sediments in Long Valley would have been eroded from Cenozoic volcanic source areas (pl. 1, sheet 1) ; so, if 200 cubic miles is taken as the maximum volume of sediments to have been eroded from pre-Tertiary sources (mainly from the Sierra Nevada), an almost perfect balance between the volume of the structure and the volume of sediments of pre-Tertiary origin is obtained. The maximum amount of eroded material was found by assuming that the crests of the mountains surround- ing Long Valley once extended horizontally to meet vertical bounding faults and the difference between this surface and the present surface shown on topographic maps represents the maximum amount of material re- moved by erosion. The probable amount was taken as half the maximum on the assumption that warping and distributive faulting accounted for part of the deforma- tion and that the bounding faults may be not vertical but steeply dipping normal faults. The minimum, as- suming no erosion whatever, was zero. If the minimum amount of stream-transported ma- terial (0 cubic miles) was supplied to a Long Valley structural basin having a maximum volume (525 cubic miles), all the material buried in the structural basin would be volcanic in origin. But if the probable amount of stream-transported material (150 cubic miles) was supplied to a Long Valley structural basin having the probable volume of 375 cubic miles, then a deficit-bal- ancing volume of 225 cubic miles of material of volcanic origin must be confined in the Long Valley structural basin. Perhaps a third of the 150 cubic miles of stream- transported material may be of volcanic origin. Then, one-half or more of the Cenozoic rocks contained in the Long Valley structural basin may have been erupted or intruded directly into the subsiding basin from nearby volcanic sources. Indeed, the aeromagnetic and gravity data revealing a large pile of volcanic material of rath- er mafic composition buried near the center of Long Valley and the direct observation of a large amount of volcanic material at the surface indicate with certainty that a large fraction of the Cenozoic rocks of Long Val- ley are volcanic in origin. CENOZOIC ROCKS OF OWENS VALLEY The description of the Cenozoic rocks as they appear on the surface of Owens Valley is taken from the work of others, mainly Knopf (1918). Stream deposits and lake beds predominate, but the deposits contain some accumulations of pyroclastic debris. Also, Pleistocene glacial moraines project downward into Owens Valley from Sierra Nevada valleys. Kane and Pakiser (1961) made an analysis of the effects of changes in the degree of sorting on the density of the Cenozoic rocks of Owens Valley and on the reliability of gravity interpretations based on the assumption of a uniform contrast in density between the Cenozoic and pre-Tertiary rocks. The aeromagnetic (pl. 2; fig. 16) and the gravity data (pl. 1, sheet 2), indicate that a large pile of volcanic material (probably basalt) or an intrusive mass is possibly buried in northern Owens Valley northeast of Bishop. Basalt flows are exposed in many places along the Sierra Neva- da front and in the southern Inyo Mountains. The seismic-refraction traveltime curves (figs. 17 and 18; pl. 3) reveal velocity layering similar to that occur- ring in Long Valley and Mono Basin. A near-surface layer having a velocity ranging from 5,700 to 7,000 fps was found under the low-velocity weathered layer at almost all seismic-refraction profiles. This layer probably represents unconsolidated or semiconsolidated 54 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. clastic sediments of late Pleistocene(?) and Recent ages. Beneath this layer, generally at a depth of about 1,000-2,000 feet, is a layer having a velocity ranging from 6,500 to 10,000 fps. On most of the seismic travel- time curves, only these two layers were found to occur above the pre-Tertiary basement, but a thin layer hav- ing a higher velocity (10,200-11,200 fps), interpreted as a basalt flow, was found at seismic profile 1 (fig. 17) southeast of Owens Lake. And at seismic profile 4, east of Independence, a much more complex velocity layer- ing was found (pl. 3). At least three and probably four layers, in which the velocities increase with depth, were found above the pre-Tertiary floor. Along seismic profile 4 (pl. 3) the velocity of the near-surface Ceno- zoic layers was found to grade markedly from lower velocity near the deepest part of the depressed Owens Valley block to higher velocity to the east and west near the fronts of the Inyo Mountains and the Sierra Nevada. The complex velocity layering in the deepest part of the Owens Valley structure along this line and the lateral gradations in velocity of the layers near the surface must reflect a more complex history of structural de- formation than elsewhere in Owens Valley, for the climatic conditions, which may also affect the rate of deposition and lithification-and thus the velocity- must have been relatively constant in Owens Valley over a given interval of geologic time. The velocity boundary across which the velocity changes from the range of 5,700-7,000 fps to the higher range of 6,500-10,000 fps is characteristic of the entire Owens Valley region including Mono Basin. This boundary is found typically at depths ranging from about 1,000 to 2,000 feet. In general, the dip of this boundary is in the same direction and less than the dip of the deeper Cenozoic and pre-Tertiary interface, and where the boundary has been displaced by faulting, the displacement is invariably less than the displacement of the deeper bedrock. Thus, seismic evidence indicates that faulting and warping were continuous or repeated at short intervals concurrently with deposition of the Cenozoic rocks, a conclusion that was independently reached from interpretation of the gravity data. The older Cenozoic rocks having a high velocity must be more highly indurated than the younger rocks hav- ing a low velocity, which are probably virtually uncon- solidated. The boundary between the rocks in Owens Valley, as in Long Valley and Mono Basin, may repre- sent either an abrupt change in the rate of deposition, lithification, or both. Therefore the velocity boundary may represent a boundary in time in which either an abrupt climatic change or a renewed uplift of the moun- tain masses, or both, occurred. This boundary in time may be at or near the time of greatest uplift of the Sierra Nevada in late Pliocene and early Pleistocene times. TECTONICS AND VOLCANISM OF OWENS VALLEY As may be readily perceived, analysis has progressed from using observed facts (the mapped areal geology, measured gravity and magnetic fields, measured seismic- refraction traveltimes, and the present physiography as shown on topographic maps) to making interpretations of progressively increasing uncertainty. The horizon- tal positions of the major faults as determined from gravity data are known with a high degree of certainty; the dips of the faults are less well known. The thick- nesses of the Cenozoic rocks are known within +25 per- cent; two independent lines of evidence-the gravity field and the seismic-refraction data-were used to make these deductions. The deductions on pages 52 and 53 on the nature of Cenozoic deposits involve a wide range of uncertainty, but the limits of this range of uncer- tainty are known and have been discussed. The gen- eral conclusion that a large volume of material of vol- canic origin is buried in Long Valley and in Mono Basin is based on several lines of evidence, each of which- except the direct observation of volcanic material on the surface-involves its own uncertainty. Neverthe- less, the conclusion seems to be required by the evidence at hand. Up to this point, the presented conclusions are based on facts or on direct interpretations of facts. In the following pages, in order to arrive at some conclusions on the tectonics of the Owens Valley region and the vol- canic activity there (especially in the Long Valley and Mono Basin volcanic centers), some speculations that are not directly supportable by these facts are made without contradicting these facts, and the conclusions are necessarily uncertain. DIP-SLIP FAULTING IN OWENS VALLEY Great vertical movement along the faults in the Owens Valley region has been conclusively demon- strated. The total vertical displacement of the pre- Tertiary erosion surface in the latitude of Mount Whit- ney must have been about 19,000 feet. This is approxi- mately the difference in altitude between the summit of Mount Whitney and the buried pre-Tertiary floor of Owens Valley east of Lone Pine. The total vertical dis- placement in the area of Long Valley and Mono Basin may have been more-as much as 24,000 feet from the crest of the Sierra Nevada to the deeply buried pre- Tertiary floors of Long Valley and Mono Basin. Much of these great vertical displacements took place along the main faults bounding Owens Valley, Long Valley, and Mono Basin, but large fractions of the displace- GEOLOGIC INTERPRETATION OF GEOPHYSICAL SURVEYS 55 ments may have been distributed among systems of closely spaced faults. Warping certainly accounts for some of the displacements. The Sierra Nevada in the Owens Valley region is in general a westward-tilted block. The basin ranges east of Owens Valley, at least as far east as Death Valley, are eastward-tilted blocks. Therefore, Owens Valley seems to lie near the crest of a great arch that has been broken by block faulting (P. C. Bateman, written commun., 1958). As the crest of the arch broke, per- haps during general uplift of the Sierra Nevada and of the extreme western Great Basin, Owens Valley seem- ingly subsided as a graben. Other valleys east of Owens Valley may be merely alluviated areas on the lower ends of eastward-tilted blocks such as the Coso, Argus, Panamint, and Funeral Ranges (P. C. Bateman, writ- ten commun., 1958). STRIKE-SLIP FAULTING IN OWENS VALLEY Significant, but not extremely large, strike-slip move- ments undoubtedly took place also in Owens Valley, but the published record on the Owens Valley earth- quake of 1872 is so ambiguous and contradictory that it cannot be assumed to be known whether the strike- slip movement was right lateral or left lateral, as Rich- ter (1958, p. 499-503) showed. Gianella (1959), after a review of the literature and investigation in the field, concluded that the available evidence "indicates that the dominant horizontal movement was left lateral." Whitney and an accompanying party visited Owens Valley soon after the earthquake of March 26, 1872. Following the visit, Whitney (1872, p. 138) wrote : There are several places in the valley where fissures in the ground have crossed roads, ditches, and lines of fences, and where evidence has been left of an actual moving of the ground horizontally, as well as vertically. One of these instances of horizontal motion is seen on the road from Bend City [Kear- sarge), to Independence, about three miles east of the latter place. Here, according to a careful diagram of the locality, drawn by Captain Scoones, it appears that the road running east and west has been cut off by a fissure twelve feet wide, and the westerly portion of it carried eighteen feet to the south. The same thing was noticed by us at Lone Pine and Big Pine, with respect to fences and ditches, the horizontal distance through which the ground had been moved varying from three to twelve feet. V. P. Gianella (written communication, 1957) stated : Because of the above statement, I have visited the area east of Independence and, from visual estimates, but without careful measurements, the relative accuracy of the above statement was confirmed. There the horizontal movement was certainly left- lateral of about the magnitude stated by Whitney. Unfortunately, the diagram of Captain Scoones was not published by Whitney, nor did Whitney present any diagrams of actual measurements or photographs sup- porting his conclusion. Some years later, in 1883, G. K. Gllbert visited Owens Valley and he wrote (Gllbert 1884, p. 51), in describing the fault scarps along the eastern base of the Sierra Nevada, that one of them has been formed since the settlement of the coun- try. It occurred in 1872, and produced one of the most notable earthquakes ever recorded in the United States. The height of the scarp varies from five to twenty feet, and its length is forty miles. Various tracts of land were sunk a number of feet below their previous positions, and one tract, several thousand acres in extent, was not only lowered, but carried bodily about fifteen feet northward. Gilbert did not identify this tract, but if it is one of the fault-bounded depressions mapped by Willard D. John- son (Hobbs, 1910) between Lone Pine and Diaz Lake and if Gilbert was describing horizontal movement rela- tive to the Sierra Nevada, as seems to be implied, then the strike-slip movement was left lateral. The state- ment is ambiguous, however, and could be interpreted as implying northward movement relative to the area east of the sunken tract; that movement would require right-lateral strike slip. If the tract moved northward relative to the areas both east and west of it, the west bounding fault would be left lateral and the east bound- ing fault would be right lateral in strike-slip movement, but this movement is unlikely. Holden (1898, p. 88-92) made a review of the Owens Valley earthquake of 1872 and reported a verbal ac- count from Captain Keeler, after whom the town of Keeler was named, as follows : A fissure was opened up in the earth from about 2 miles south of Lone Pine, extending 10 miles farther north. This fissure was 4 feet wide, and the ground on the east sank from 4 to 12 feet lower than that on the west side (or the west side was raised). At the same time the ground on the east was moved bodlly 10 feet or so to the north (or the other side to the south). This was clearly shown by the position of fences running east and west. Captain Keeler gave Holden a photograph showing the shifting of a fence at a point 114 miles south of Lone Pine, but unfortunately this photograph was never pub- lished. Keeler clearly described left-lateral faulting, as related by Holden, and this faulting is probably the same faulting as that described by Gilbert. V. P. Gianella (written commun., 1958) recovered evidence for left-lateral offset of a fence line 114 miles south of Lone Pine, and this offset is probably the same as that described and photographed by Keeler. Finally, Hobbs (1910), basing his analysis on the work of Johnson without having visited Owens Valley, presented evidence that the strike-slip movement in the Lone Pine area was right lateral. A photograph taken 56 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. by Johnson and published by Hobbs showing apparent right-lateral offset of a row of trees seems to confirm this analysis. As a result, the conclusion of Hobbs has been given rather more weight by recent workers than the opposing conclusion of Whitney, Holden, and, seem- ingly, Gilbert. Evidence for left-lateral movement and an inference of right-lateral movement as reported by Mayo (1941) have already been described, as has the visual evidence (supported by seismic-refraction data) of left-lateral movement on a road offset just south of Lone Pine. Interpretations favoring both right-lateral and left- lateral strike-slip movement along the faults of Owens Valley are certainly permissible. On the basis of certain theoretical considerations to be discussed in the follow- ing sections, the interpretation that movement was left lateral is favored in this report, but the interpretation that movement was right lateral cannot be ruled out on the basis of the published record and recoverable field evidence. Perhaps both left- and right-lateral strike- slip movement actually happened. In any event, ex- tremely large horizontal displacements are unlikely to have occurred. The strike-slip and dip-slip components were probably about the same. RELATION TO REGIONAL TECTONIC PATTERN The strike of the San Andreas fault as measured in a range in latitude defined by normals to the north and south limits of the Owens Valley region is N. 40° W. The strike-slip movement along the San Andreas is known to be right lateral. However, the mean trend of Owens Valley is N. 17° W. (fig. 19) ; so, the acute angle between the San Andreas and Owens Valley is 23°. If the direction of the greatest principal (horizontal) stress in a region including both the San Andreas and Owens Valley is west of N. 17° W. but east of N. 40° W., right-lateral movement on the San Andreas and left- lateral movement on the faults of Owens Valley are compatible. The so-called Walker Lane in western Ne- vada (fig. 20) is parallel to the San Andreas, and right- lateral offset along this zone has been inferred (Gianella and Callaghan, 1934; Longwell, 1950 ; Locke and others, 1940). Again, right-lateral movement on the faults of the Walker Lane and left-lateral movement on the faults of Owens Valley are compatible. The Garlock fault south of Owens Valley (fig. 20) varies greatly in direction of strike. The Garlock is considered to be a left-lateral strike-slip fault. This movement is compatible with left-lateral strike-slip movement along the faults of Owens Valley and with a southward-moving Sierra block. The Seismological Laboratory of the California In- stitute of Technology made a comprehensive study of the major Arvin-T'ehachapi earthquake of 1952. (See Gutenberg, 1955; Richter, 1955; and Benioff, 1955.) The earthquake occurred along the White Wolf fault south of the Sierra Nevada and north of and roughly parallel to the Garlock fault (fig. 20). The strike of the White Wolf fault is about N. 50° E., and its dip is 60°-66° southeast at the focal depth of the earthquake (about 15 km), as Gutenberg (1955) showed. The southern block was thrust up with respect to the north- ern block, and the direction of strike slip was left lat- eral. The dip-slip component was determined to be about 1.4 times as great as the strike-slip component, based on analysis of the seismograms (Gutenberg, 1955). This determination agrees closely with the re- sults of leveling and triangulation made before and after the earthquake by the U.S. Coast and Geodetic Survey (Whitten, 1955). The relative dip-slip and strike-slip components were both measured to be about 4 feet. Scheidegger (1959), by a statistical analysis of the mechanisms of the aftershocks of the Arvin-Tehachapi earthquake, found the direction of tectonic motion to be N. 82° W., or approximately normal to the strike of the White Wolf fault. This direction is in agreement with the direction obtained by Gutenberg (1955) in a fault- plane solution on the main shock. The results of Schei- degger's analysis are compatible with a southward-mov- ing Sierra Nevada block and left-lateral strike-slip movement along the faults of Owens Valley. They are also compatible with right-lateral movement along the San Andreas fault and the Walker Lane and with left- lateral movement along the Garlock. Cordell Durrell (1950) mapped a fault having more than 3 miles of left-lateral strike-slip displacement near the Sierra Nevada front west of Blairsden. When this fault as described by Durrell (written communication, 1958) was plotted on a geologic map (fig. 20), it was found to strike within a few degrees of the mean trend of Owens Valley, suggesting that the main faults hav- ing similar strike along the eastern front of the Sierra Nevada, including those of Owens Valley, may be mem- bers of a system of en echelon left-lateral strike-slip faults. Right-lateral strike-slip displacements on faults in the Death Valley area were proposed by Curry (1938) and by Noble and Wright (1954). Hopper (1947) also presented evidence for right-lateral strike-slip displace- ments on faults of Panamint Valley. All these faults are oriented more northwesterly than is the trend of Owens Valley ; therefore, they are compatible with left- lateral displacements on the faults of Owens Valley. In the foregoing discussion, an attempt has been made to demonstrate compatibility of left-lateral strike- GEOLOGIC INTERPRETATION OF Bridgeport? / , 119% _ ¢g§fii J o_ LAKE 38° LeeVining\ x \y, \TUOLUMN E ”fig; \ 37° Bentong Benton Station° \ \ GEOPHYSICAL SURVEYS 57 EXPLANATION "= Axis of syncline _ a Monoclinal warp showing Volcanic rocks of Cenozoic age e-- <--- % trend and direction of dip Mapped fault s nova e ee Dashed where inferred Mean trend of Owens Valley Amma ng casa as Fault based on geophysical evidence Assumed direction of motion May coincide in part with 'of major structural blocks ~ \known or inferred faults /_—__.—\) ( T Pa * 3p, Region of relative tension or Q’OV<0 stress relief based on left- 451,27 lateral strike slip exfinuse |_ CTTCD Ap SK Direction of component of o £ \ relative tensile stress based 41, on assumption of left-lateral ~ \ strike slip T UL A RE 4 Visalia C ~ J aTulare « 7 - r m « 36° 20 1 30 1 \ 40 MILES le. FicurB 19.-Map showing fault pattern and volcanic rocks of Cenozoic age of the Owens Valley region, inferred greatest principal (compressive) stresses, and inferred local relative tensile stresses. 58 STRUCTURAL GEOLOGY AND YVOLCANISM, OWENS VALLEY, CALIF. 117° EXPLANATION F Volcanic rocks of Cenozoic age 122° 121° 120° 119° 41° slip displacement (Durrell w -\ (~$ Pyramid Lake ; 1950, and written commu tz ~ Pre-Tertiary rocks of the nication Sierra Nevada 40° e _ "M Major fault, showing direction of strike slip, if known t, r/ DCI s __ _ _ 74, Fallon o Walker Lane, showing assumed £29 direction of strike slip Q 39° 38° 6 woh 36° bas y ga A A Direction of tectonic? O motion (Scheidegger, + GO 2g 1s" Revershield - {x Ka xf. (E 50 “130 MILES 1 i 1 i | i 1 1 1 Fieur® 20.-Regional geology of southern California and southwestern Nevada showing faults 31nd volcanic rocks. (Modified from the "Geologic Map of the United States," Stose, 1932, and the "Tectonic Map of the United States," Longwell, 1944.) s GEOLOGIC INTERPRETATION OF GEOPHYSICAL SURVEYS 59 slip displacements on the faults of Owens Valley with both right-lateral and left-lateral displacements on other major faults of southern California and western Nevada. An implicit assumption was made that the greatest principal stress has been horizontal and that it has been oriented somewhere between N. 17° W. and N. 40° W. throughout the region discussed, and this assumption seems to be compatible with all available geologic and seismological evidence. However, no at- tempt has been made to apply the principles of shear in a homogeneous elastic medium (Anderson, 1951). As Benioff (1955, p. 203) pointed out: The application of stress to a rock mass having a structural weakness such as a contact or other defect produces a fracture which doesn't necessarily follow the geometry of fractures in a homogeneous medium. - Likewise, once a fracture has occurred, movements will continue on it even though the stress pattern is greatly altered from the original form which produced the fracture. This remark is particularly appropriate to the Owens Valley region, which has contrasting rock types and structural weaknesses inherited from the Nevadan orogeny. Owens Valley is probably a great left-lateral shear zone, and the Sierra Nevada has probably been moving south with respect to Owens Valley and the area farther east. Additional arguments supporting this conclusion have been presented by the senior author (Pakiser, 1960). However, the conclusion that no movement along the faults of Owens Valley has been right-lateral is not permissible. OF OWENS VALLEY In Owens Valley, volcanic activity of Cenozoic age was largely confined to three areas near the ends of im- portant faults (fig. 19). Mayo (1941, p. 1064) observed that fault movement between the granitic rocks of the Sierra Nevada and the sedimentary rocks farther east should open many channels for the extrusion of lava. It is therefore no surprise to find that volcanoes do occur along the Sierra Nevada front, but these eruptions are not evenly distrib- uted along the base of the mountains * * *. It is obvious that the volcanoes are clustered in certain favored areas. In a previous paper, the senior author (Pakiser, 1960) showed that regions of local relative tension or stress relief near the ends of or discontinuities in faults could come about as a result of strike-slip displacements. Such regions of stress relief should favor the eruption of magma. The displacement at both ends of a strike-slip fault must be zero, and at some unspecified distance from each of these ends, the displacement of one side relative to the other must be the full amount of horizontal move- ment. On one side near each end (the side on which, on a map, the displacement arrow points away from the end) the rocks will be extended, and a region of rela- tive tension or stress relief will come into existence. On the opposite side near each end (the side on which, on a map, the displacement arrow points toward the end) the rocks will be effectively shortened, and a region of compression will result. These regions will be alter- nately disposed, and a region of stress relief and a region of compression will therefore exist on each side of the fault. If two parallel faults with the same direction of strike-slip displacement are arranged en echelon and if they extend outward in opposite directions, either a re- gion of stress relief or a region of compression will be formed between them, depending on whether the region between them tends to be extended or shortened. Vol- canism would be favored if the region between the faults is a region of stress relief. In the volcanic field near the south end of the Inyo Mountains (fig. 19) the volcanic eruptions took place on the east side and near the end of the bounding fault. A region of stress relief presumably existed in the area of these volcanic eruptions, and it may be inferred, there- fore, that the Inyo Mountains moved north relative to Owens Valley and that the bounding fault is a left- lateral strike-slip fault. 'The three vents (Mayo, 1941) for the small volcanic field that flowed out of the Inyo Mountains between Big Pine and Independence near the 37th parallel (fig. 19) are located on the east side of the easternmost bounding fault, and the bounding fault system consists of two off- set segments in this area. The volcanic eruptions took place near and east of the end of the fault that continues northward to become the bounding fault of the White Mountains. Again, if the volcanic eruptions took place in a region of stress relief, the White Mountains and the Inyo Mountains may be inferred to have moved north relative to Owens Valley, and the bounding fault is a left-lateral strike-slip fault. Examination of the outcrops of volcanic rock south of Blg Pine and north of Independence shows that the vol- canic area just south of Big Pine is near the end of a major fault that passes northward into the warp mapped by Bateman and confirmed by the geophysical evidence. A region of stress relief presumably existed on the west side of this fault near its northern end, and again, left-lateral movement on this fault, which is an extension of the fault east of Independence mapped by Knopf (1918) may be reasonably inferred. The fault continues south to join the earthquake fault of 1872 that bounds Alabama Hills near Lone Pine. All vents for the Big Pine volcanic field (Mayo, 1941) are located on 60 STRUCTURAL GEOLOGY AND YOLCANISM, OWENS VALLEY, CALIF. the west side of the fault that terminates just north of Big Pine. Thus the volcanic activity and the fault pattern in the area between Big Pine and Independence seem to sug- gest left-lateral movement. Owens Valley seems to be a great left-lateral shear zone. This shear zone sug- gests that the Sierra Nevada has been moving south with respect to Owens Valley and the area farther east. If the Sierra Nevada has been moving south with re- spect to the area to the east, the area in the great offset of the Sierra Nevada front that contains Long Valley and Mono Basin (fig. 19) would tend to be stretched or pulled apart. The local component of relative tensile stress or stress relief that would be responsible for such a stretching or pulling apart would be parallel to the mean trend of Owens Valley (fig. 19). The Long Val- ley-Mono Basin area was the locus of the most intense and complex volcanism in the entire Owens Valley re- gion. This area is still volcanically active, as is shown by the hot springs (Pakiser and others, 1960). The vol- canic field in the embayment of the Sierra Nevada front east of Blairsden (fig. 20) seems to be similarly related to fault trends, and here left-lateral strike-slip displace- ments have been measured (Durrell, 1950). The mechanism described does not seem to account for the volcanic rocks in the Coso Range or for those west of the Sierra Nevada crest (see, for example, Webb, 1950), but our geophysical investigations shed little light on the structural features in these places. If Owens Valley is a great left-lateral shear zone, Alabama Hills may have been elevated along a series of east-trending reverse faults or as a result of com- pressive folding in response to local compressive stresses acting parallel to the valley. The main west fault bounding Owens Valley is sharply offset and changed in strike at the south end of Alabama Hills, and this discontinuity would cause compressive stresses to build up locally in Alabama Hills if the horizontal movement is left-lateral. Some of the short faults that strike roughly normal to Owens Valley (for example, at the 37th parallel, south of Owens Lake) may also be reverse faults. Thus the deepest wedge of Owens Valley between Independ- ence and the southern limit of Owens Lake could have been in part depressed in response to local compressive stresses. ; Two independent lines of reasoning have led to the conclusion that Owens Valley is a left-lateral shear zone and that the Sierra Nevada has been moving south with respect to the Great Basin region to the east. This conclusion is compatible with the majority of the pub- lished accounts of the earthquake of 1872 and with the regional tectonic pattern of southern California and western Nevada. The strike-slip displacements needed to bring about volcanic eruptions by relief of stress are likely to be small; certainly, horizontal move- ments comparable to those along the San Andreas can- not have happened in the Owens Valley area. The strike-slip and dip-slip displacements in the Owens Val- ley region may have been about the same; at most, accumulative strike-slip displacements can hardly have exceeded a few miles. Cordell Durrell (written com- mun., 1958) inferred about 12 miles of left-lateral strike-slip displacement through a zone 4-6 miles wide in the area near Blairsden. ORIGIN OF MONO BASIN AND LONG VALLEY We now possess a large inventory of facts and in- ferences on which a theory for the origin of Mono Basin and Long Valley can be constructed (Pakiser and others, 1960). These may be summarized as fol- lows: 1. Mono Basin and Long Valley are large structural depressions bounded by vertical or near-vertical faults that have subsided and have received accumulations of about 18,000+5,000 feet of stream-transported sediments and volcanic deposits of Cenozoic age. 2. These structural depressions are roughly equidimen- sional horizontally, and their depths are a large frac- tion of their horizontal dimensions. 3. Of the approximately 675-150 cubic miles of Ceno- zoic rocks in Mono Basin and Long Valley, a large fraction (perhaps two-thirds) is probably of direct or secondary volcanic origin, but some (perhaps one- third) is from pre-Tertiary rock sources and was transported into these two depressions by streams. 4. The Cenozoic rocks in Mono Basin and Long Valley are divided into two or more major units on the basis of increasing seismic velocity with depth. These major units are layered, as is revealed by seismic re- flections in Mono Basin and the velocity boundaries found from seismic refractions, and are presumed to have been deposited at least in part by orderly proc- esses of sedimentation and volcanism. 5. The subsidence of these structural basins has been continuous or repeatedly rejuvenated through an in- terval of late Tertiary (?) and Pleistocene time, and deposits near the surface have therefore been dis- placed by faulting relatively much less than those near the pre-Tertiary floor. The Recent deposits at the surface have not been displaced at all in most places, and as a consequence virtually no physio- graphic expression of the bounding faults of the structural basins occurs. fig Volcanic rocks are intimately associated with these ({ structural basins. GEOLOGIC INTERPRETATION 7. The Sierra Nevada may be moving south with respect to the area to the east, and this movement would create local relative tensile stresses or stress relief in the embayment of the Sierra Nevada front that contains Mono Basin and Long Valley (fig. 19). The unusually great depth of these structural basins, their roughly equidimensional outlines horizontally, and the volcanic activity associated with them make the mechanism of basin-and-range type block faulting usually assumed inadequate as an explanation for their origin. Mono Basin and Long Valley are therefore postulated to have subsided along their bounding faults as support was removed by extrusion of volcanic ma- terial from a magma chamber at depth. The physical-chemical relations of the volcanic activ- ity in Mono Basin and Long Valley are too complex to be explained fully on the basis of our present knowl- edge of the petrology and chemical composition of the volcanic rocks. However, we can attempt to ex- plain the origin of Mono Basin and Long Valley in terms of kinematic concepts (that is, movement of magma and blocks of solid rock) without making any assumptions on the nature of the energy involved. _ To make this explanation, the volcanic rocks of in- termediate and more silicic composition are assumed to have been withdrawn from a magma chamber within the earth's crust. Consideration of the basalts (except for minor products of magmatic differentiation) is ex- cluded from the explanation on the assumption that the basalts were withdrawn from a deeper source, probably in the upper mantle. The intermediate rocks and rhyo- lites considered are assumed to be less dense as volcanic rocks than were their original materials in the magma chamber before eruption. The mass of these materials before and after eruption must have remained constant ; therefore, they expanded after eruption, and they are assumed to have spread over an area larger than that of the magma chamber from which they came. If this change is so, a net deficiency of mass resulted in and above the magma chamber, and this deficiency is ex- pressed by a gravity minimum anomaly on the pre- eruption surface, such as those observed in Mono Basin and Long Valley. The problem then becomes one of finding the volcanic rocks corresponding to the mass deficiency of the source area, of finding a possible tectonic mechanism for re- lieving the pressure confining the magma in its cham- ber, and of describing the possible relations between volcanic activity and subsidence of Mono Basin and Long Valley. If the basin deposits of volcanic origin occupy a com- bined volume in Mono Basin and Long Valley of 400- OF GEOPHYSICAL SURVEYS 61 500 cubic miles, if the combined volume of the struc- tures is 600-700 cubic miles, and if all of the subsidence is assumed to have been caused by withdrawal of magma from a magma chamber or two magma chambers at depth, a simple computation shows that about 200-300 cubic miles of volcanic deposits must be found outside the basin structures. These numbers are not to be taken literally, but they do indicate the general magnitude of the volumes required. About 200-300 cubic miles of volcanic material hav- ing intermediate and silicic composition can be found in the Mono Basin and Long Valley areas; the volcanic rocks include the older rhyolite and andesite on the east flank of the Benton Range, the andesite of Bald Mountain, the large volume of rhyolite of Glass Moun- tain and its surroundings, the Bishop tuff of Gilbert (1938) , and the pumice and obsidion erupted from Mono Craters (pl. 1, sheet 1). The large volume of rhyolite piled above the general level near the center of Long Valley (pl. 1, sheet 1) can also be included. Most of these rocks are more intimately associated with Long Valley than with Mono Basin. This association sug- gests the possibility that a magma chamber extended under both structural basins and that both Mono Basin and Long Valley may have subsided in response to erup- tion of volcanic material from either of the structural basins. Subsidence may have followed eruption from separated magma chambers under each structural basin, however. The paths traveled by the magma may have extended some distance beyond the basins, perhaps by migration along a surrounding system of fissures, to the places from which the volcanic rocks were erupted. If the Sierra Nevada has been moving south with respect to the area to the east, as has been inferred, the area containing Mono Basin and Long Valley would tend to be stretched or pulled apart (Pakiser, 1960). Presumably the confining pressure at the depth to the top of the magma chamber (perhaps 10 km) would have been approximately the same as the lithostatic pressure-2,500-3,000 bars-or about the same as the partial pressure of water that could be dissolved in the magma at saturation. Reduction of the confining pres- sure by local relative tensile stresses acting in a direc- tion parallel to the mean trend of Owens Valley may have reduced this pressure to such an extent that the internal pressure of expansion in the magma chamber (assumed to be much less than the normal lithostatic pressure) could have exceeded the least principal stress plus the (essentially negligible) tensile strength of the surrounding rocks and thus have brought on eruption. If the local relative tensile stresses were large enough, open fissures may have extended downward to great depths, possibly even to the magma chamber. 62 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. The same mechanism of stress relief could, of course, also have facilitated the eruption of basalt from a deep- er source and thus possibly offers an explanation for the widespread basalt in the area (pl. 1, sheet 1). Another possible effect of the reduction of stress would be actually to bring about generation of the magma by reduction of the melting point of the rock. Reduction of the melting point could have been brought about by two effects of stress relief: the effect of the stress relief itself (Yoder, 1952; Uffen, 1959) and the in- ward migration of water from the surrounding rocks under high pressure into the low-pressure region in the Mono Basin and Long Valley area. This explana- tion of the generation of magma depends on the as- sumption that the rocks at the depth of the magma chamber normally contain water far below the satura- tion amount (Tuttle and Bowen, 1958). Raising the water content would thus have a powerful influence on lowering the melting point. Very possibly the tem- perature was raised at the same time by the conversion of mechanical energy to heat in this tectonically active region. The actual physical-chemical mechanism of eruption may have been similar to that proposed by Kennedy (1955). We conclude, as did Williams (1941) about Mono Basin, that Mono Basin and Long Valley may be re- garded as volcano-tectonic depressions that subsided along faults that may have been blocked out by the same tectonic forces that produced Basin-and-Range faulting in the area or that may have been inherited from the Nevadan orogeny. The vertical movement, however, was caused mainly by the force of gravity pulling the blocks down as support was removed from below (Pakiser and others, 1960). Without more precise information than that avail- able and that which forms the main body of this re- port, the foregoing discussion and many of those that precede it remain only plausible, however great their probability. SUMMARY OF TECTONICS AND YOLCANISM The main bounding faults of Owens Valley may be very steeply dipping to vertical transcurrent faults along which significant (but probably not large) lateral movements have taken place. The vertical movement has been large, but it may at least in part be a secondary effect of strike slip. A simple experiment using a card with cuts represent- ing faults can show that, for small horizontal displace- ments, the secondary vertical movement can be several times as great as the primary horizontal movement. If the horizontal movement along the main faults of Owens Valley was left lateral, the volcanic eruptions in Owens Valley and in the Mono Basin-Long Valley area can be accounted for by the local relative tensile stresses that would come about as a result of southward movement of the Sierra Nevada with respect to Owens Valley and the area farther east. Magma may have been generated by the melting of solid rocks in such re- gions of stress relief. Thus, an internally consistent theory relating the tec- tonics of the Owens Valley region with the volcanism has been proposed. This theory, though consistent with all the known facts, lacks clear proof and must be thus regarded as only probable. The uplift of the Sierra Nevada took place only in minor part along the faults that bound the deepest wedge of Owens Valley. This uplift was probably caused in large part by vertical forces resulting from compensation in excess of isostatic equilibrium, as has been proposed by Oliver (1956). (See also Pakiser, 1960.) SUMMARY OF GEOLOGIC HISTORY OF OWENS VALLEY REGION In late Paleozoic or early Mesozoic times the region of the southern Sierra Nevada that includes Owens Valley became the site of intense compressive forces acting in a northeast and southwest direction. These compressive forces led to the formation of a geosyneline in which thick deposits of sediments and volcanic debris accumu- lated (Vening-Meinesz, 1957). As compressive forces continued, the sedimentary and volcanic rocks of the geosyncline were folded during the Nevadan orogeny of late Mesozoic (probably late Jurassic) time. Some early mafic forerunners of the Sierra Nevada batholith were intruded into the western foothills during and shortly after folding (Curtis and others, 1958). The sedimentary and volcanic rocks of the geosyneline were dynamothermally metamorphosed during the period of intense folding as a result of increased pressure and temperature (Durrell, 1941; Krauskopf, 1953; Mac- donald, 1941). On continued deformation, the southern Sierra Nevada was intruded by a series of igneous masses that together make up the main bulk of the great Sierra Nevada batholith. Intense contact metamor- phism resulted from the intrusion of the batholith. Most of the mass of the batholith was intruded in Late Cre- taceous time (Curtis and others, 1958). Following the intrusion of the batholith, the southern Sierra may have been left in a state of isostatic un- balance (Vening-Meinesz, 1957). The mass deficiency of the deep root of the Sierra Nevada may have been greater than that needed to compensate the mass excess of the Nevadan fold mountains. So post-orogenic forces of vertical uplift may have become effective, and these forces may have elevated the Sierra Nevada in a series of movements from a surface of low relief per- LITERATURE CITED haps no more than 3,000 feet above sea level to its pres- ent great heights. The forces may still be active (Oliver, 1956). f The southern Sierra Nevada was probably relatively quiescent during early Tertiary time. Then, in late Miocene or early Pliocene time, the range underwent a major uplift and tilting that led to the formation of the mature valleys of the Kern, Merced, and other rivers of the region (Lawson, 1904; Matthes, 1980). Intense volcanic activity occurred along the crest of the Sierra Nevada and in the great Basin east of the Sierra Nevada during this late Miocene or early Pliocene period of uplift. A period of quiescence followed dur- ing the early Pliocene, and the mature valleys were eroded. Some volcanic activity continued. In late Pliocene and early Pleistocene times, the southern Sierra Nevada was uplifted several thousand feet to its present great heights (Matthes, 1930). The youthful canyons of the Kern, Merced, and other rivers of the region were cut by newly invigorated streams after this uplift. Widespread volcanism along the Sierra Nevada crest and in the Great Basin accompanied and followed this uplift. The faults of Owens Valley may have been inherited from zones of weakness brought into existence during the Nevadan orogeny (Mayo, 1941). Movement along these faults may have started in the early Tertiary, and {significant faulting probably occurred in the Owens Valley region during the late Miocene and early Plio- ene uplift of the Sierra Nevada (Matthes, 1930). The main faulting that created the great eastern escarpment £ the Sierra Nevada, however, came in late Pliocene and early Pleistocene times after a period of quiescence during the early Pliocene (Matthes, 1930). Fault move- ments, involving both dip slip and strike slip, were still continuing as late as 1872 (Whitney, 1872), and the forces that caused them are probably still active. Rich- ter (1959) is studying a series of earthquake shocks in southern Owens Valley that began in January 1959. The Owens Valley shear zone existed probably before the late Pliocene and early Pleistocene uplift of the Sierra Nevada, and Owens Valley may have begun to subside and receive sediments at some earlier time. The? subsidence of Mono Basin and Long Valley is postulated / to have begun probably with the earliest Pliocene(?) | volcanic activity in that area (Gilbert, 1941) and to have ended with the latest explosions from Mono, Craters (Evernden and others, 1959) after the final} (Tioga) Pleistocene glaciation. Because the Basin Ranges of the Mono Basin-Long Valley area are known ' not to have been blocked out by faults before late Plio- cene or early Pleistocene time (Gilbert, 1941), the sub- 63 sidence of Mono Basin and Long Valley seemingly be- gan before the elevation of these ranges. Perhaps the subsidence of these structures was initiated synchro- nously with an early Pliocene(?) uplift of the Sierra Nevada. Finally, four stages of glaciation (Blackwelder, 1931) sculptured the eastern slopes of the southern Sierra Nevada and built up impressive moraines; subsequently, in Recent time, Owens Valley has been modified by streams. LITERATURE CITED Anderson, E. M., 1951, The dynamics of faulting, 2d ed.: Edin- burgh, Oliver and Boyd, 206 p. Anderson, G. H., 1937, Granitization, albitization, and related phenomena in the northern Inyo Range of California- Nevada: Geol. Soc. America Bull., v. 48, no 1, p. 1-74. Axelrod, D. I., 1957, Late Tertiary floras and the Sierra Nevadan uplift: - Geol. Soc. America Bull., v. 68, no. 1, p. 19-46. Bailey, H. P., 1954, Climate, vegetation, and land use in south- ern California, in chap. 1 of Jahns, R. H., ed.: p. 31-44. Baker, C. L., 1912, Physiography and structure of the western El Paso Range and the southern Sierra Nevada: Cali- fornia Univ. Pub. Geol. Sci. Bull., v. 7, no. 6, p. 117-142. Barthelemes, A. J., 1946, Application of continuous profiling to refraction shooting: Geophysics, v. 11, no. 1, p. 2442. Bateman, P. C., 1956, Economic geology of the Bishop tungsten district, California: California Div. Mines Spec. Rept. 47, 87 p. Bateman, P. C., and Irwin, W. P., 1954, Tungsten in southeast- ern California, in chap. 8 of Jahns, R. H., ed.: p. 31-40. Bateman, P. C., and Merriam, C. W., 1954, Geologic map of the Owens Valley region, California, Map Sheet 11, in Jahns, R. H., ed. + Benioff, Hugo, 1955, Relation of the White Wolf fault to the regional tectonic pattern, in Oakeshott, G. B., ed.: p. 203-204. Birch, A. F., 1952, Elasticity and constitution of the earth's interior: Jour. Geophys. Research, v. 57, no. 2, p. 227-286. Birch, A. F., Schairer, J. F., and Spicer, H. C., eds., 1942, Hand- book of physical constants: Geol. Soc. America Spec. Paper 36, 325 p. Blackwelder, Eliot, 1931, Pleistocene glaciation in the Sierra Nevada and Basin Ranges: Geol. Soc. America Bull., v. 42, no. 4. p. 865-922. Carlisle, Donald, Davis, D. L., Kildale, M. B., and Stewart, R. M., 1954, Base metal and iron deposits of southern Cali- fornia, in chap. 8 of Jahns, R. H., ed.: p. 41-50. Chelikowsky, J. R., 1940, Tectonics of the rhyolite in the Mam- moth embayment, California: Jour. Geology, v. 48 no. 4, p. 421-485. Curry, H. D., 1938, Strike-slip faulting in Death Valley, Cali- fornia [abs.]: Geol. Soc. America Bull., v, 49, no. 12, pt. 2, p. 1874-1875. Curtis, G. H., Evernden, J. F., and Lipson, J. I., 1958, Age de- termination of some granitic rocks in California by the potassium-argon method : California Div. Mines Spec. Rept. 54, 16 p. Dobrin, M. B., 1952, Introduction to geophysical prospecting, ist ed.: New York, McGraw-Hill Book Co., 435 p. Duerksen, J. A., 1949, Pendulum gravity data in the United States: U.S. Coast and Geod. Survey Spec. Pub. 244, 218 p. 64 STRUCTURAL GEOLOGY AND VOLCANISM, OWENS VALLEY, CALIF. Durrell, Cordell, 1941, Metamorphism in the southern Sierra Nevada northeast of Visalia, California: California Univ. Pub. Geol. Sci. Bull., v. 25, no. 1, p. 1-117. 1950, Strike-slip faulting in the eastern Sierra Nevada near Blairsden, California [abs.] : Geol. Soc. America Bull., v. 61, no. 12, pt. 2, p. 1522. Erwin, H. D., 1934, Geology and mineral resources of north- eastern Madera County, California: California Jour. Mines and Geology, v. 30, no. 1, p. 7-78. Evernden, J. F., Kistler, R., and Curtis, G. H., 1959, Cenozoic time scale of the West Coast [abs.]: Geol. Soc. America Bull., v. 70, no. 12, pt. 2, p. 1718. Ewing, Maurice, and Press, Frank, 1959, Determination of crustal structure from phase velocity of Rayleigh waves- The United States: Geol. Soc. America Bull., v. 70, no. 3, p. 229-244. Gale, H. S., 1915, Salines in the Owens, Searles, and Panamint basins, southeastern California: U.S. Geol. Survey Bull. 580-L, p. 251-323. Gianella, V. P., 1959, Left-lateral faulting in Owens Valley, California [abs.]: Geol. Soc. America Bull., v. 70, no. 12, pt. 2, p. 1721. Gianella, V. P., and Callaghan, Eugene, 1934, The earthquake of December 20, 1932, at Cedar Mountain, Nevada, and its bearing on the genesis of Basin Range structure: Jour. Geology, v. 42, no. 1, p. 1-22. Gilbert, C. M., 1938, Welded tuff in eastern California: Geol. Soc. America Bull., v. 49, no. 12, pt. 1, p. 1829-1862. 1941, Late Tertiary geology southeast of Mono Lake, California : Geol. Soc. America Bull., v. 52, no. 6, p. 781-815. Gilbert, G. K., 1875, Report on the geology of portions of Ne- vada, Utah, California, and Arizona, surveyed in the years 1871 and 1872: Geog. Geol. Survey West of the 100th Merid- ian, v. 3, p. 17-187. 1884, A theory of the earthquakes of the Great Basin with a practical application: Am. Jour. Sci., 34 ser., v. 27, p. 49-53. Gutenberg, Beno, 1955, The first motion in longitudinal and transverse waves of the main shock and the direction of slip, in pt. 2 of Oakeshott, G. B., ed.; p. 165-170. Gutenberg, Beno, Wood, H. O., and Buwalda, J. P., 1932, Ex- periments testing seismographic methods for determining crustal structure: Seismol. Soc. America Bull., v. 22, no. 3, p. 185-246. Heiland, C. A., 1940, Geophysical exploration : New York, Pren- tice-Hall, 1013 p. Hinds, N. E. A., 1956, Late Cenozoic history of the Sierra Ne- vada, California-Nevada [abs.]: Geol. Soc. America Bull., v. 67, no. 12, pt. 2, p. 1796. Hobbs, W. H., 1910, The earthquake of 1872 in the Owens Valley, California: Beitr. Geophys., v. 10, p. 352-385. Holden, E. S., 1898, A catalogue of earthquakes on the Pacific coast, 1769 to 1897: Smithsonian Inst. Misc. Colln. 1087, 258 p. Hopper, R. H., 1947, Geologic section from the Sierra Nevada to Death Valley, California: Geol. Soc. America Bull., v. 58, no. 5, p. 393-432. Hubbert, M. K., 1948, A line integral method of computing the gravimetric effects of two-dimensional masses: Geophysics, v. 13, p. 215-225. Hudson, F. S. 1955, Measurement of the deformation of the Sierra Nevada, California, since middle Eocene: Geol. Soc. America Bull., v. 66, no. 7, p. 835-870. Jahns, R. H., ed., 1954, Geology of southern California: Cali- fornia Div. Mines Bull. 170. Jahns, R. H., 1954, Investigations and problems of southern California geology, pt. 1 in chap. 1 of Jahns, R. H. ed.: p. 5-29. Jennings, C. W., 1958, Geologic map of California, Death Valley Sheet : California Div. Mines. Jennings, C. W., and Strand, R.G., compilers, 1958, Geologic map of California, Santa Cruz sheet: California Div. Mines. Kane, M. F., and Pakiser, L. C., 1961, Geophysical study of sub- surface structure in southern Owens Valley, California: Geophysics, v. 26, no. 1, p. 12-26. Kennedy, G. C., 1955, Some aspects of the role of water in rock melts, in Poldervaart, Arie, ed., Crust of the earth-a sym- posium: Geol. Soc. America Spec. Paper 62, p. 489-503. King, Clarence, 1878, Systematic geology: U.S. Geol. Explor. 40th Parallel Rept., v. 1, 803 p. Knopf, Adolph, 1918, A geologic reconnaissance of the Inyo Range and the eastern slope of the southern Sierra Nevada, California: U.S. Geol. Survey Prof. Paper 110, 130 p. Krauskopf, K. B., 1953, Tungsten deposits of Madera, Fresno, and Tulare Counties, California : California Div. Mines Spec. Rept. 35, 83 p. Lachenbruch, A. H., 1957, Three-dimensional heat conduction in permafrost beneath heated buildings: U.S. Geol. Survey Bull. 1052-B, p. 51-69. Larsen, E. S. Jr., Gottfried, David, Jaffe, Howard, and War- ing, C. L., 1954, Age of the southern California, Sierra Nevada, and Idaho batholiths [abs.]: Geol. Soc. America Bull., v. 65, no. 12, pt. 2, p. 1277. Lawson, A. C., 1904, The geomorphogeny of the upper Kern Basin: California Univ. Pub. Geol. Sci. Bull., v. 3, no. 15, p. 291-376. Lee, C. H., 1912, An intensive study of the water resources of a part of Owens Valley, California: U.S. Geol. Survey Water-Supply Paper 294, 185 p. Lee, W. T., 1906, Geology and water resources of Owens Valley, California : U.S. Geol. Survey Water-Supply Paper 181, 28 p. Lindgren, Waldemar, 1911, The Tertiary gravels of the Sierra Nevada of California: U.S. Geol. Survey Prof. Paper 78, 226 p. } Locke, Augustus, Billingsley, P. R., and Mayo, E. B., 1940, Sierra Nevada tectonic patterns: Geol. Soc. America Bull., v. 51, no. 4, p. 513-539. Longwell, C. R., chm., 1944, Tectonic map of the United States: Am. Assoc. Petroleum Geologists Bull.; v. 28, no. 12, p. 1767-1774. 1950, Tectonic theory viewed from the Basin Ranges : Geol. Soc. America Bull., v. 61, no. 5, p. 413-433. Mabey, D. R., 1958, Gravity study of the Death Valley region, California [abs.]: Geol. Soc. America Bull., v. 69, no. 12, pt. 2, p. 1695. Macdonald, G. A., 1941, Geology of the western Sierra Nevada between the Kings and San Joaquin Rivers, California: California Univ. Pub. Geol. Sci. Bull., v. 26, no. 2, p. 215-286. Mason, B. H., 1952, Principles of geochemistry : New York, John Wiley & Sons, 176 p. Matthes, F. E., 1930, Geologic history of the Yosemite Valley: U.S. Geol. Survey Prof. Paper 160, 137 p. 1933, Geography and geology of the Sierra Nevada, in Internat. Geol. Cong. Guidebook 16th, Excursion C-1, Middle California and Western Nevada : p. 26-40. 1947, A geologist's view, in Peattie, Roderick, ed., The Sierra Nevada, the range of light: New York, Vanguard Press, p. 166-214. Mayo, E. B., 1931, Fossils from the eastern flank of the Sierra Nevada, California: Science, new ser., v. 74, no. 1925, p. 514-515. 1934, The Pleistocene Long Valley Lake in Eastern Cali- fornia: Science, new ser., v. 80, no. 2065, p. 95-96. 1937, Sierra Nevada pluton and crustal movement: Jour. Geology, v. 45, no. 2, p. 169-192. 1941, Deformation in the interval Mt. Lyell-Mt. Whitney, California: Geol. Soc. America Bull., v. 52, no. 7, p. 1001- 1084. 1947, Structure plan of the southern Sierra Nevada, Cali- fornia: Geol. Soc. America Bull., v. 58,, no. 6, p. 495-504. iller, W. J., 1928, Geology of Deep Spring Valley, California : Jour. Geology, v. 36, no. 6, p. 510-528. umford, R. W., 1954, Deposits of saline minerals in southern California, in chap. 8 of Jahns, R. H., ed.: p. 15-22. ettleton, L. L., 1940, Geophysical prospecting for oil, 1st ed.: New York, McGraw-Hill Book Co., 444 p. Noble, L. F., and Wright, L. A., 1954, Geology of the central and southern Death Valley region, California, in chap. 2 of Jahns, R. H., ed.: p. 143-160. Nolan, T. B., 1943, The Basin and Range province in Utah, Nevada, and California: U.S. Geol. Survey Prof. Paper 197-D, p. 141-196. Dakeshott, G. B., ed., 1955, Earthquakes in Kern County, Cali- fornia, during 1952: California Div. Mines Bull. 171. Oliver, H. W., 1956, Isostatic compensation for the Sierra Nevada _ California [abs.]: Geol. Soc. America Bull., v. 67, no. 12, pt. 2, p. 1724. Pakiser, L. C., 1960, Transcurrent faulting and volcanism in Owens Valley, California : Geol. Soc. America Bull., v. 71, no. 2, p. 153-159. : akiser, L. C., Jr., and Black, R. A., 1957, Exploring for ancient channels with the refraction seismograph [Arizona-Utah] : Geophysics, v. 22, no. 1, p. 32-47. akiser, L. C., Jr., Mabey, D. R., and Warrick, R. E., 1954, Map- ping shallow horizons with reflection seismograph [Okla- homa-Kansas]: Am. Assoc. Petroleum Geologists Bull., v. 38, no. 11, p. 2382-2394. akiser, L. C., Press, Frank, and Kane, M. F., 1960, Geophysical investigation of Mono Basin, California: Geol. Soc. America Bull., v. 71, no. 4, p. 415-447. ress, Frank, 1956, Determination of crustal structure from fornia: Geol. Soc. America Bull., v. 67, no. 12, pt. 1, p. 1647- 1658. Putnam, W. C., 1949, Quaternary geology of the June Lake dis- trict, California: Geol. Soc. America Bull., v. 60, no. 8, p. 1281-1302. ilRichter, C. F., 1955, Seismic history in the San J oaquin Valley [art. 31, and Foreshocks and aftershocks [art. 9] in pt. 2 of Oakeshott, G. B., ed.: p. 177-197. 1958, Elementary seismology : W. H. Freeman & Co., 768 p. 1959, Current studies of minor earthquakes [abs.] : Geol. Soc. America Bull., v. 70, no. 12, pt. 2, p. 1743. Rinehart, C. D., and Ross, D. C., 1957, Geology of the Casa Diablo Mountain quadrangle, California: U.S. Geol. Survey Geol. Quad. Map GQ-99. Rinehart, C. D., Ross, D. C., and Huber, N. K., 1959, Paleozoic and Mesozoic fossils in a thick stratigraphic section in the eastern Sierra Nevada, California : Geol. Soc. America Bull., v. 70, no. 7, p. 941-945. San Francisco, Calif., phase velocity of Rayleigh waves, Part 1-Southern Cali- . LITERATURE CITED 65 Russell, I. C., 1887, Notes on the faults of the Great Basin and of the eastern base of the Sierra Nevada: Philos. Soc. Washington Bull., v. 9, p. 5-8. Scheidegger, A. E., 1959, Note on the tectonics of Kern County, California, as evidenced by the 1952 earthquakes: Jour. Geophys. Research, v. 64, no. 10, p. 1499-1501. Schultz, J. R., 1937, A late Cenozoic vertebrate fauna from the Coso Mountains, Inyo Canyon, California: Carnegie Inst. Washington Pub. 487, p. 75-109. Skeels, D. C., 1947, Ambiguity in gravity interpretation: Geo- physics, v. 12, no. 1, p. 43-56. Smith, G. I., and Pratt, W. P., 1957, Core Logs from Owens, China, Searles, and Panamint basins, California : U.S. Geol. Survey Bull. 1045-A, p. 1-62. Stose, G. W., 1932, Geologic map of the United States: U.S. Geol. Survey. Swick, C. H., 1942, Pendulum gravity measurements and iso- static reductions: U.S. Coast and Geod. Survey Spec. Pub. 282, 82 p. Thompson, G. A., and Sandberg, C. H., 1958, Structural signifi- cance of gravity surveys in the Virginia City-Mount Rose area, Nevada and California: Geol. Soc. America Bull., v. 69, no. 10, p. 1269-1281. Tuttle, O. F., and Bowen, N. L., 1958, Origin of granite in the light of experimental studies in the system NaAISi:Os- KAISi;Os-SiO:-H:O : Geol. Soc. America Mem. 74, 153 p. Uffen, R. J., 1959, On the origin of rock magma : Jour. Geophys. Research, v. 64, no. 1, p. 117-122. Vacquier, Victor, Steenland, N. C., Henderson, R. G., and Zietz, Isidore, 1951, Interpretation of aeromagnetic maps: Geol. Soc. America Mem. 47, 151 p. Vening-Meinesz, F. A., 1957, The geophysical history of a geo- syncline: Royal Netherlands Acad. Sci. Proc., ser. B., v. 60, no. 2, p. 126-140. Walcott, C. D., 1897, The post-Pleistocene elevation of the Inyo Range, and the lake beds of Waucobi embayment, Inyo County, California : Jour. Geology, v. 5, no. 4, p. 340-348. Webb, R. W., 1946, Geomorphology of the middle Kern River Basin, southern Sierra Nevada, California: Geol. Soc. America Bull., v. 57, no. 4, p. 355-382. 1950, Volcanic geology of Toowa Valley, southern Sierra Nevada, California: Geol. Soc. America Bull., v. 61, no. 4, p. 349-357. Whitney, J. D., 1865, Geology of the Sierra Nevada : California Geol. Survey, v. 1, pt. 2, p. 199-497. 1872, The Owens Valley earthquake: Overland Monthly, v. 9, p. 130-140, 266-278. Whitten. C. A., 1955, Measurements of earth movements in California, in Oakeshott, G. B., ed. : p. 75-80. Williams, Howel, 1941, Calderas and their origin: California Univ. Pub. Geol. Sci. Bull., v. 25, no. 6, p. 239-346. Woollard, G. P., 1958, Results for a gravity control network at airports in the United States: Geophysics, v. 23, p. 520-535. Worzel, J. L., and Shurbet, G. L., 1955, Gravity interpretations from standard oceanic and continental crustal sections, in Poldervaart, Arie, ed., Crust of the earth-a symposium : Geol. Soc. America Spec. Paper 62, p. 87-100. Wright, L. A., Chesterman, C. W., and Norman, L. A., Jr., 1954, Occurrence and use of nonmetallic commodities in southern California, in chap. 8 of Jahns, R. H., ed.: p. 59-74. Yoder, H. S., Jr., 1952, Change of melting point of diopside with pressure: Jour. Geology, v. 60, no. 4 p. 364-374. A Page -\ =o sibs us awed 8 Aeromagnetic survey, general discussion...... 3,89 Agriculture 6 Alabama Hills.... ees cs Alluvial fans...... _. 6, 10,39 Andesite, north of Bishop. . wees 12, 18 ANOIGRS IIE... ..... .. seuss 56 Arvin-Tehachapi earthquake........____..._. 56 Arelrod, D: I., 20, 21 B Basin and Range structural features, general 15 See also particular feature. Basalt, «=-- i.. | BH in Owens Valley.........~ 29-14 magnetic susceptibility of... »« 4.290 north of Bishop....._..~~~. 18-48 Beach deposits. .... £. ases 's. AD Bench gravels............. sews :.. AB Benioff, Hugo, quoted.... seee -' 60 Bibliography.... Secs: "68 Big Pine volcanic field.........___.._._..__... 59 PISRODIUN.-..._..--\-----secbuecknncsss 18, 22, 27, 50 Blackwelder, Eliot, quoted.......__.......... 11 cocco ence nees 8 Brines 5 Brosd Valley 19, 20 C (OSIIETAS .. 17 Campito sandstons........................... 8 C&RNYON 18, 19, 20 Canyon-cutting stages. . See particular stage. Casa Diablo Mountain pluton................ 40, 42 Cenozoic rocks, density of... -t- 18 general discussion.. ..... «=- 9, 68 in Long Valley:.._._._.____..__.. -> *a68 in Owens Valley........_.._._.... « < §§ seismic velocity in.... o volume of........._..~ u- ~ab8 CETTO- .. L 5 Obagoopa 20 Chainman shale............. 8 Chelilowsky, J. K., quoted.... 17 CISY. . . « =ccerces anew 6 OUMAIG: -\ ss 5 Coso formation. 8,14 15 Crinoids 8 Orust, thickness of........_._..._.__.___..... 20 D Desdmsn Cregk............_.____.___.__..«.« 3 Deep Spring formation........... 8 Density of rocks, general discussion....._..... 22 Diatoms 9, 10 DIBS LAKG 3 DiKO$. .~ 15, 42 Dioritic plutonic rocks, magnetic suscepti- DHlity Of- - ... 89 14 INDEX [Italic page numbers indicate major references] Earthquakes........... Economic development..........._._.___._... 6 Ely Springs dolomite.... 8 BH echelOH TUNIS. coca no rraccbrbcene==s=cecsen 56 Erosion surfaces, general discussion. .......... 18 See also particular surface or zone. Esmeralda 13 Burgka quartzite. .... ...... ccc. 8 Evolution of land forms, general discussion.. 17 paleobotanic evidence for................. 20 physiographic evidence for................ 18 F BGO Of.. .c 2.00 18 Basin and Range. -_ 14, 15,, 16 dip-slip...._.... 54 evidence 2, 30 ground-water barriers formed by.. 45 strike-slip............ _ 45,55 Fault scarps... 15, 16 Field party, personnel... 28 .<. -.-. .._. ««- 8,23, 43 Fish cell 9 FlOrn, fOSKIL ... .oo o nln enne ane 20 Fossils 8, 9, 20 a FAUIE. ne- cob-seneueces cane 56 (GHSHTODOGS.L.. . 9 Geography, general discussion................ 6 Geologic history, summary of...... 62 Geophysical surveys, general discussion . #1 PREVIOUS. 8 See also particular type of survey. CHlanelis, V.P., qUob@U..._.._______...-__..-. 55 Gilbert, C. M., quoted.... 12 Gilbert, G. K., quoted. ..... 55 Glacial deposits. _... 11 Glacial moraines. 6, 11, 13, 20, 51, 53 IBCIGTS. . ol 6 :... l.. 6 Grabens 6, 16, 55 Granitoid rocks, general discussion. 8 Gravity contour maps, general discussion.... 26 Gravity meters used, list Of_.___._....._....... 84 Gravity profile A-A'"'..... 81 BAB e 87 (-U .. on re orc wens 88 D-D'.. 88 E-E'.. 38 F-F' ._ 88 (+G .\ {lca c cunesss Les au aa s 38 general discussion . __ 29,89 HHH L.. ccr cusine 89 Gravity survey, accuracy of data... #4 fieldwork and computations.. #8 general discussion. _...... 3, 22 interpretation of data.._........_._....... 25 Ground-water barriers, faults acting as. ._.... _ 45 H Hanging 10 Hidden Valley dolomite...................... 8 Page High Valley 2000-. 18, 19 Hilton Cr60K TAE. ... .cc 49 Hinds, N. E. A., quoted. a 9 Holden, E+ S., QUOEOU -. coc co- 65 Horsts 16 .; .. 60 I International Ellipsold...............cclccclers 29 Investigations, present. .. 8, 23, 30, 43 «s a 8,55 purpose of.......... C 8 Isostatic compensation................___..__ 29 J Jahns, R. H., cc ACO 17 K Kern Cony 00 fAMLE. -. ccc 14 Kinematic processes, listed..._.._....._.....- 21 King, Clarence, quoted.... 6 Knopf, Adolph, quoted.................. 2, 10, 16, 19 L Lake beds, general discussion......_....~ 6, 22, 52, 53 OGEF.. coomnernrecs one 9 younger.... 10 LOK6§, PIGISEOCOIG.... . 50 Land forms. See particuler land form and Evolution of land forms. 14 Lend cc.. L. sens 5 Lindgren, Waldemar, quoted................. 17 Locke, Augustus, quoted.... 15 Long Valley, Cenozoic rocks in. 52 magnetic high in . 40, 41 origin 60 structural geology of... 49 Los Angeles Aqueduct........................ 6 Lost Burro ..... 8 M Magnetic anomalies, analysis of. .._.......... 41 Magnetic contour map, general discussion.... 40 Magnetic properties of rock, general survey... _ $9 Mayo, B. B.; 17,59 McGee moraines... McGee tills........ Mebrten formation............. Metamorphic rocks, density of. general 8 Mineral production. See Economic Devel- ment. Mono Basin, gravity anomaly................ 26 OHL Ofer. cool llc cns 60 structural geology Of.....................~ 49 Mono Craters............ 13 19, 20 16 67 68 0 Older structural framework, general discus- S10N«+ recceris re chics on Olivine basalt. See Basalt. OsHBCOUOS- Owens Lake Basin, gravity survey of. Owens Valley, area defined......... area north of, gravity survey of. Cenozoic rocks in...... central, gravity survey of fsultingin............._.. northern, gravity survey of. structural geology of...... tectonics in...... h volcanism in...... Panamint Valley.... ese vince Physiography, general discussion.... Pine Creek mine............... Plutonic rocks, density of Pogonip group.... Poverty Hills...... Pre-Tertiary rocks, density of depth to........ general discussion . . gravity affected by. seismic velocity in.... Pumice Rayleigh Waves. c.. 12. demise Ree@OIOMILG...... .. .cn Page INDEX Page Rhyolite, north of Bishop......___..____.. 11,12, 13 in Owens ake 14 Ricardo erosion surface........._.__. uc 390 Ricardo 14 8 ._... ... 5 Sedimentary rocks, White and Inyo Moun- tains, general discussion...... 8 Seismic profile, analysis of................._.. 48 J. 48 2. 44 8. 45 €.. 45 $.: 41 in eves as sews tos cedes 48 Seismic-refraction measurements. 3, 37 Seismic survey, field methods . a 43 general discussion . _... 42 interpretation of data, a 48 Shear zones...._........ 60 Sherwin glacial stage. 9, 11, 13 Sierra Nevada batholith...................... 8 Silicic plutonic rocks, magnetic susceptibility 39 Silver deposits... s 5 Silver Peak group.. 8 Stream depogits.................... 52, 45 Structural geology, general discussion. 48 regional tectonic pattern related.. 56 Subsumniit Platen. ._... css 18 Sumatra, Pilomasin Basin of................_ 17 Summit Upland........_.__..._{c...l. 18, 19 Structural fronds. 2000000002. 14 SYHCIINIOSL.C2. . 2.2 cel Clute cube cuse e- 38 p Page Tahoe glacial stage. 11 s 6 Til @ < af Tin Mountain limestone.. __....__._.__.._.__.... 8 Tioga 11,13 Topographic 5 Transportation.... ..... 00001. 5 Tungsten deposits 5 Valley Springs formation Vegetation=:.........._..... Volcanic necks Volcanic pile.... Volcanic pipes Volcanic rocks, magnetic susceptibility of..... 89 north of Bishop, general discussion. ..:... 11 Owens Valley, general discussion..._____. 18 Pleistocene, general discussion._.._______. 18 Tertiary(?) age, general discussion.. 11 NOIGBNRIG 21-22 Inno c ., 12 Volcanism, in Owens Valley.......__________. 11, 59 Volcano-tectonic features, general discussion.... 17 w Walker L416. .. cer ee eod oben rele ine suv 56 Waucoba Canyon, lake beds in.... Fr 9 Weathered layer, seismic velocities in = 45 Webb, R. W., 14 White Wolf 56 Whitney, J. D.; 55 Z TNC CEDORINNL 12s edo ren rn nece nene amen aan 5 UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 438 a % GEOLOGICAL SURVEY (tans PLATE | (SHEET | OF 3) f G28 Boll.. 3 B s §. _- ie .. 89°00 .-.. _ RATE.: 2s. ¥ EXPLANATION . lane i rent Bl in cn i tlre init nii trie in erfc orie ign Aes a iain hei a a uis + ¥ s 4 * § & Qvt ; £ | \/ § 09. a ' heart ._ R 4 j F a : rrr tas » ; eom. -T- ana... f & ; E? : s _- Clastic deposits ~ _ ~- e , I | » 24 % Mainly Recent alluvium, late Cenozoic lake beds, and moraines -I | $6 .tt". _ 'c c. ©! s* * g? E3 . k 5!" ow # ; 3 & e At * het s ~0 C r4 + at Rhyolite _- sl. Includes tuffs, flow rocks, and obsidia 16 €* 500, Pliocene (?) ages % ; | & * Basalt and andlte’, nas -~, --- = rn aA los t t adio aa v As ; T. 2nNn s and andesite tuffs and breccias of Pleistocene -__ . & and Pliocene (?) ages * , + e val 8 TL & I L T. 2 N t Granitoid rocks - Lp " tz mongzonite, g'r—janod'iorite; ; lag | 7 2+ f I f ie Loke [js } | 3, j * g jflv-a! al. ngfiw I / | I | J! | ..,: o 38°00 steel Lake 3g ~- ‘ // [ | | I | j " f ¢ s 5 R.34 E 118° 10" G: f " "ll-I H I | e a , : ( j " - - 38°00" - EU ¢ - ( f I e, \ ( \ f * t f F 28 I 12327 I B & : b 32 33 : s= ys. < {> sg §#2. | | \ \ | : | ae % # j sutta", | & u T. 2 N. t e s dimentary rocks _ _- Bs. 8 | s \ X \\ p Adobe Hills ® c @ A4 s _ rocks in the White and Inyo Mountains that [I ‘\ \ g e 4 f 7 y % tially unmetamorphosed f 8 | | \ \\ ; .$ & % h yh \ Lee Vining [ \ | * | d "m’entary; and metavolcanic rocks A f *- Truman. 7 & + R & g aul Bs :* Contact + :h Dashed, here approximately located f + *% : - ~ Pn tye: igs," K s, . ess s \ \\\ Dig *t -__ T. +X | = 3 reyes Fault based on surface evidence < | ' cal Dashed where inferred tS 11919 ~.. f f f co = CGR =- G/ >* | ; : Game Fault based on geophysical evidence $ 5g " ' * f Queried, where inferred s R. 4 C \ Hen Fiat 55 % ie - 4 8. ¢ ‘ 28 l eet ose os % e y y } ; ; [ 's /— 770\_/// rx y a & % ® =- ‘ e. & p Gravity contours / s £002 Dashed where inferred. Contour interval is 2 milligals; datum is complete Bouguer anomaly plus 1,000 milligals \\ £ d ¥ f \ 3 e % a { F i : % : f . f Pinto Hill Gravity low t | ( f g : i U Gravity station 0 LB3 Gravity base station PROFILE 2 Geophone spread 03 f j ; ___f af Shot point f u\3 1 v s 9 \ Location of gravity profiles shown on figures 6-15 § @3300 Seismic depth, in feet below surface, from California Institute of 50" Technology 45" 119° 118°30' 38° Area of Sheet 1 Area of Sheet 2 1. 35. Area of Sheet 3 40 118° 37°30 / ______ EXPLANATION Base compiled from maps 35" with scale 1:62,500 Base compiled from maps with seale 1:125,000 36°30 GEOLOGIC SOURCES . Bateman and Merriam (1954) . Rinehart and Ross (1957 and written communication) . Gilbert (1941) . Anderson (1937) . Pakiser, Press, and Kane (1960) . Miller (1928) . Knopf (1918) . Jennings and Strand (1958) . Hopper (1947) . Erwin (1934) . Mayo (written communication) I Aaa p ® w po «1 go OT m to lanl -< 36° TRUE NORTH a $ #" \ , yA nese} |.... c.... 5 s ; INDEX MAP SHOWING SOURCES OF GEOLOGIC DATA AND Base from U.S. Geological Survey >all , 3 _ if, 7 : ; : ; . : P ys ORIGINAL BASE MAP SCALES topography quadrangles 37°30" $ ‘ , R34 C Te __ INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D. C.-1964-G62204 COMBINED GRAVITY AND GEOLOGIC MAP OF AREA NORTH OF OWENS VALLEY, CALIFORNIA, SHOWING LOCATION OF SEISMIC PROFILE 6 SCALE 1:96 000 1 ¥2 0 fa. 2 3 4 5 6 MiLEs CECB-H-EH E eee m i 5 70 1 2 3 4 5 6 KILOMETERS 7 mss DATUM IS MEAN SEA LEVEL PROFESSIONAL PAPER 438 HE INTERIOR : UNITED STATES DEPARTMENT OF THE E f PLATE 1 (SHEET 3 OF 3) GEOLOGICAL SURVEY (81G PINE) R.35 E (WAUCOBA MTN) 5" nas farming! Mtn | s | ste l Shingte mill Bench L pie yanyfj‘j’} m,‘ é’aboose Pass/f) 1 { { mias. fp sang 20 \Mt Perkins: Ness: A\ / Jos e \\"’iq fv/ S, wfflfimrs Leaded Lona "lle Ram Mine FOREST . | ) g_ We f y* mn A1 § w * | 35’ wer a g I8 f ( P < i \ 6111 \ zear } tine ta 2 é! ally s i a a &) / / | , - \ i | } ¥ | Z w : < a X at $1 e pae meet s. wl hea g; \ sas # sear PA I) 6 I i mulet -> mig ~* .\ - M514, i | & wo el on | B < m. w g I k f S | \ 3 ws i 24 | h (Toe Uss | 4° 4 I ~ * M. g‘ipn‘ng / Alea De i \ \ £ vae s M} al \ I e { al i ¥ am & w y* ' y «oer | R | cose f i A f is \ g ta -+ £4. N gas? Xq | ~a ae " 3 | £ a hex fic ¥ } | f . § , ye 4 i AH € p y N ¢ s 6 | j31 32 33 | | 1 vase ( 1 ss C s " Pat Keyes % Spring 7 a" e 8 A “K sess & Mt Williamson | |" £ I/ -| J New York « P } Butte | | | Trojan Peak UeEmEBE PEAK) 122 Tunna boy/F411“ nleinyo 14k?” ine 2 z s #. itcheock Lakes Trail Crest | ‘ Mt Russell Ct eC Mt Hale a . y,’ pe t 35" ses. 3 35! Mt Young / "-- 1 am A \ "J ey £9514, { / if | C 4 | ins . f | \‘ ws “M1 if A t § AV! | *%, | Mt Muir # a fogs ul, 1 fF w Lone Pine Peak da ap ) 222 - = wx. | | | | )Consultation Lake \\ s A Mt LeConte Mt Newee 6 l \ 0 b. \ A b. \ Iridescent C L Lake __ 3 Corcoran Nouptain //_,._ A >< Mt Langley Bm 14042 0 W E N Owens Point a g ‘ 7°40' _ , “1.40 5 36°30" { < bos 2 artment" \ amet Pad - eee emits /a» _ Cirgue Lake % w Axyannod \‘ Timosea Peak \ $ Ker n ,, N . 0 W °K N S K/B 21 Meadow w, Mulkey Pass Mauh Mtn H ( p f+ I X4 chun V \ i Ki PN [ < \ / $% \ At \V . { y. 30 #] s A- L «-l f.. { , A a £ » j t! [/ pa Z 7 [} 3 ___| Soriped) "4, Z \ I \ | I 36, I C % 1 an> A Z, "A rn)» il) { FA 7 1 cs 14 ~ A 3. I | \ & \ ) »so Ima Lefts, eae. zase *% iL A 16 © & ds X ) A \ Ker: zfi’mkjky »} ap >>>.. i 3 F - N $ > ermB... 117°40' F 2 iy ~ merge ~~ %. 7 mez g- l U r supgerlk Centgrfll f_} 012“ a s Loa shin /ns, w } o Ary g N D i a w I w \ > -of" (_ [# §lL / /- \ € om / lled ilkaAL {/ atin... ¥ . -: y w ; NapLO NIT - | H ) i p I te 7! m u S.." %% eprmmene See Sheet 1 for explanation and index map showing sources of geologic data and original base map scales f-. so0Mbagy- --- mart i TRUE NORTH Base from U.S. Geological Survey APPROXIMATE ° MEAN DECLINATION, 1964 topography quadrangles f A) : 50' INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D.C.-1964-G62204 COMBINED GRAVITY AND GEOLOGIC MAP OF CENTRAL OWENS VALLEY AND OWENS LAKE BASIN CALIFORNIA, SHOWING LOCATION OF SEISMIC PROFILES 1, 2, 3, AND 4 SCALE 1:96 000 -{searies LaKE 1250000) pp 3g 1 Ya 0 1 2 3 4 z 6 MILES C S 1 5. 0 1 2 3 4 5 6 KILOMETERS == i= 1 F i DATUM IS MEAN SEA LEVEL CO N) P o a) a. < a d < Z C & & md |a 0 & a. UNITED STATES DEPARTMENT OF THE INTERIOR CN H - < 1 am Ker GEOLOGICAL SURVEY pKmv Ker 30' 119°00' EXPLANATION 770 |75O a O Rn w «-a m -< € 5p tm t s 4C 85 € S2 2a 8, m ”if“ 0 m 7 § & "-o o o 8 g o a & o 35 4 $ 2 -% aS 3% / 3 C $' \ A 3 5 9 -< w.l.w E € X \ vm e.g 1/5 £ £. 86 \ C p .5§ % E m ! p $ 22% ~ 1 a & is ik | | & 1f < 2 Ls 44 *S m im 2 & § -Z o 8 3 j*9 ! Hop o 8 p3 / 4:84 § C o. 49 / S 1 9 & # 9 4 S a 9 8 s '3 > a 35 x i= a § Klug g :g L 2 $s p o $ o > 53s 36 5 & 3 - K o o 2 & Gx & o Ry | "C & gri alle 4 By 5 2 o | J < 8 a & § $ bs] 0 3 fi s ® 9 5 goo fs g q § O p - 3 \ 23 8 c 3 Pla, o 46 2 A RD ~ H R s mess T = C S '€ € o s 8 7 © S -s [33 Fa Fa p & 8 f 8 Mm $ B 8 =] < 5 = ** 3 54 = 8° 0 est f 3 § § -= GS & snodDo EJ -YL314D0 m ® w 8 < § 2 3 C 13 #s o 7] 2 2 C 3 B 3 w s £ 8 § 2 3 8 > A 6 £ a i 2 a 5 , x 8 l12 t se | & oB | su8 | .y 13 | f | & ° 19° 8 2 18 33. | ts 'S >. "a - | o # | 3 Bs 3 £ 0 |4 o |S 3 . | 2 3 Q y | £ ¥ "| = x a bn a og cel E a a G a A a .S! 4 ml» & 8 1m 4G = Ai ' DP m dol o 5s 8 g. «A s x § S $ = 3 2 & A 3 A Y E . 3 © m puWD [ 21290189121] HLHON 3NHL Pre-Tertiary granitoid, metasedimentary, and metavolcanic rocks APPROXIMATE MEAN DECLINATION, 1964 | Q INTERIOR-GEOLOGICAL SURVEY, WASHINGTON. D. C.-1964-G62204 SCALE 1:96 000 6 MILES CECECH-H H UNITED STAT ES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY > 2.5 EXPLANATION Arrival time, shot point west of spread A Arrival time, shot point east of spread U Intercept time SP 6 Shot-point number ~ SECONDS 1.0 SECONDS bof. f TT 27: tem a 0.5 \ y FEET SECONDS FEET SECONDS 2.3 N\\\\iticl arrivals 2.0 Second arrivals SECONDS PROFESSIONAL PAPER 438 PLATE 3 EXPLANATION j Arrival time, shot point west of spread A Arrival time, shot point east of spread w Intercept time SP 8 Shot-point number 1 1.6 : /J \ 1.2 1.2; « 0 4% pa y 0.8 0.8 2 aP 0.4 A 0.4 6 « 5 I 0 « 9 PV £ 6579 0.0 . 0.0 i f SP 4 SP 3 SP 1 s a e.. eaco feet ~- _" "la ~. SP 2 SP 8 SP 7 SP 6 SP 5 3 Vw 21000 fps 7777777777777777777777777777777777777777777777777 rrr V;, =5900 fps (Younger Cenozoic deposits) 1900.7 7 ITTIIIIITT I7777 17777 4000 5000 V, =6900 fps e z V;, =15,700 fps [rarwo Cer fy M C adres > (Older Cenozoic deposits) 2 wo pr nd w- VVVM {m 2ps - i- (Pre-Tertiary rocks) GEOLOGIC SECTION PROFILE 5 SOUTH Second arrivals Second arrivals SECONDS OF BISHOP EXPLANATION U Arrival time, shot point northwest of spread A Arrival time, shot point southeast of spread U Intercept time SP 2 Shot-point number SECONDS FEET zc gi -t- gui e c- zz zc zea IIZZ777 7 % V, =7000 fps (Younger Cenozoic deposits) 777777777774. / /H. 7ILIT7TT77T7T7TTTTTTIITTIHITTTTITI1]1. 77/77). II/I/I7777. p f f / 3 / § "A | (Older Cenozoic deposits) ys V, =8600 fps (Older Cenozoic deposits) m o / / Vl/VVL/t/L/l/VI/l/l/Vl/l/L/VL/L/Vl/VVV /Ij\/x/|/\/ eties cirri" cool- --- 7///E ione etal tet § ,/// VVVVVVVVVVVVVVVVVVVVVV‘A'VVVVVVVVWW < \\\¢‘9J, //// a \\°e, V, =10,000 fps (Older Cenozoic deposits) p 2". (Pre-Tertiary rocks) #4 ara V, =15,800 fps (Pre-Tertiary rocks) E t y= 3‘F// \\\ gai// x Pad N // A P e N & A ho % V, = 15,800 fps \\\ MI f M (Pre-Tertiary rocks) !~ GEOLOGIC SECTION PROFILE 2 SOUTH OF OWENS LAKE 1.5 EXPLANATION LJ Arrival time, shot point west of spread A Arrival time, shot point east of spread _ Intercept time SP 7 Shot-point number 32,800 fps w 2 G: -1.0 O u w I/O‘t / 7 \4r L / _C SP 10 SP 8 SP. 7 SP 1 SP 2 SP 6 22,800 feet Vw £ 1000 fps F Fault mapped by Knopf (1918) 30 ft J Alt m V, =6500 fps /E j Y, = 5700 fps 390 ft L fps TFT: a/ % C _ &/ (Younger Cenozoic deposits) (Cenozoic deposits) C To / V;,=6500 fps 5+ Io/ & V;=7500 fps 777 l/ lPIN 777777 TTTTIITTTITITTITITTI777 TrTTTTTTTTTTTTITITIT777 7 7777777 ”7777777777777777-777777777777777777777-m aie VVVVVVV 7 a x' (Older Cenozoic deposits) ¥ / V, = 15,700 fps * V;= 7500 fps : f #$ 0.850 seconds for this path (Older Cenozoic deposits) (Pre-Tertiary rocks) (Pre-Tertiary rocks) uit itt V, =15,700 fps (Pre-Tertiary rocks) FEET I V;, =10,000 fps (Cenozoic rhyolite?) El ye W++++ + + t + + H+ t t+ t 4+ +ohob+ GCE I f. SP 1 SP 2 SP 3 sp 4 33,400 feet > >| Vw 21000 fps G G' A V;, =5000 fps I/ [ ~> | B/]: (Younger Cenozoic deposits) ri ,// (Cenozoic deposits) | p 7777777777I777777777_ I [+++ +++ + +++ + 4+ + + + b to p4 b+ 4 [t + 4 EZ (o V, = 6000 fps I I I \\ I a ++++++++++++++++++++ W { 7 I I & ax“/// AIT +i mir tl r Ia g rT TT £4 Ef *f + + rk L t % (Older Cenozoic deposits) I I PLC | r e> | | [3 % E | y ae (Cenozoic rhyolite?) GEOLOGIC SECTION PROFILE 4 EAST OF INDEPENDENCE ANALYSES OF SEISMIC PROFILES 2, 4, 5, AND 6, OWENS VALLEY REGION, CALIFORNIA GEOLOGIC SECTION PROFILE 6 IN LONG VALLEY 728-195 O - 64 (In pocket) UNIEED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY A i 118°40" R.30 E. 5 R.31 E. §. ; (white mountain 1125000) mTh. 1125 County Line wane Hill "z- -- *"" aet" 0 waile “new; Hiii rudy as-. { & R | 4. .....__j,..,_.w_.*¢i Tse fA ont / ‘t‘iMeacovl | 23 i an i f 10270 35° See Sheet 1 for explanation and index map showing sources of geologic data and original base map scales TRUE NORTH i nox Base from‘U.S. Geological Survey _ 118°25 DECLINATION, 1964 topographic quadrangles 37°00" £.33 C. 20 R.34 COMBINED GRAVITY AND GEOLOGIC MAP OF NORTHERN OWENS VALLEY, CALIFORNIA, SHOWING LOCATION OF SEISMIC PROFILE 5 v= { [Southbend Mine __ yen nere a in 5! PROFESSIONAL PAPER 438 PLATE 1 (SHEET 2 OF 3) R.36 E. 118°00' 37°30 1.5 8. 25 (LIDA 1:250.000) pKms 37°00 118°00' INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D. C-1964-G62204 Altered Wallrocks in the Central Part of the Front Range Mineral Belt Gilpin and Clear Creek Counties, Colorado GEOLOGICAL SURVEY PROFESSIONAL PAPER 439 Prepared on behalf of the United States Atomic Energy Commission Altered Wallrocks in the Central Part §f the Front Range Mineral Belt Gilpin and Cl/ear Creek Counties, derado By E. W. TOOKER GEOLOGICAL sURVEY - PROFESSIONAL PAPER i139 Prepared on behalf of the United States Atomic Energy Commission UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library has cataloged this publication as follows: Tooker, Edwin Wilson, 1923- Altered wallrocks in the central part of the Front Range mineral belt, Gilpin and Clear Creek Counties, Colorado. Washington, U.S. Govt. Print. Off., 1963. v, 102 p. illus., maps (1 fold. in pocket) diagrs., tables. 29 cm. (U.S. Geological Survey. Professional paper 439) Prepared on behalf of the U.S. Atomic Energy Commission. Bibliography : p. 97-99. (Continued on next card) Tooker, Edwin Wilson, 1923- Altered wallrocks in the central part of the Front Range mineral belt, Gilpin and Clear Creek Counties, Colorado. 1963. (Card 2) 1. Geology-Colorado-Front Range. 2. Petrology-Colorado- Front Range. 3. Ore deposits-Colorado. 4. Mineralogy-Colorado- Front Range. 5. Metamorphism» I. Title (Series) For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 CONTENTS Page 0. 2 el e eck. ae nana ae eaas need a ale cks 1 | Geochemistry of altered ey IntroIiiuctlon --------------------------------------- 2 Major constituent elements u ln epee Ian ds 2 ¥ ; ; f N N data.. ~- Previous investigations of altered wallrocks______. 2 Mgthods ot & i Lete Background geologic 3 F Aluminum. o.. GOL cc Ltt c _ _i 4 3 * § Saor Sodium and potassium. .: Methods of investigation. i000 _; 4 M ; i # agnesium. clt }. 0G JA Field lt. _o 4 4 a } h Calcium and barium}... -~. ul Laboratory .nl} l. 1.200 5 1 Geologic setting : 6 rre n (aoa ao eee ron eft} orr in a Afton nananana none s Manganese:-=2 C:.. c n_ ill. lc Precambrian :..? 6 Tertiary rocks 7. """"""""""""""""""" Phosphorus and .. .c. I Associated anions Mineral c {.at ect Tn o e an aran ne rrr e n= A=aree ar Cn Altered _ tut Luck. 8 Trace element Hydrothermally altered wallrocks_________________ 8 Method for presenting __ Supergene alteration and rock weathering._________ 9 Alkali and alkaline earth (regular) elements___. Width of the altered wallrock zone________________ 10 Regular clements=._>}..sc _ Factors that produce variations in the character and Transitional-metals. -. distribution of the altered wallrocks_____________ 11 Transitional and secondary transitional metals. Altered rock minerals. O -__} 002 12 Powder pH of altered wallrocks__________________._ Primary and secondary mineral stability ___ ___ 12 General _ Experimental studies of mineral stabilities . . _ .. 28 | Origin of the altered wallrocks in the central part of the Mineral structures and their inheritance. 30 Front Rance mineral belt Alteration of representative wallrocks _ __________._ 81 s g """""""""""""" Biotite-muscovite granite and microcline-quartz- Localization of alter ed‘r 06K -= noo e leer eee - oo plagioclase-biotite gneiss___________________ 31 The hydrothermal environment_.__._____._.________ Granodiorite - and - biotite-quartz-plagioclase Host rock...... @neiss.:!..... iu nl r rt Om t 34 Hydrothermal Quarts . ill l ___} 36 | Altered wallrocks as a guide to ore___________________. Amphibolite: ._. _ CCL mmc 2. 38 # 4 Garnet-quarta :". 38 Literature e choy 59 | Index L cel te wie einen a ae b uou ILLUSTRATIONS Prats 1. Map showing zoning of mineral deposits and localities sampled during this investigation..___________--- In pocket Ficur® 1. Index map of the central part of the Front Range mineral belt area, Colorado, showing location of area investigated on plate 12... 222.09. 02 oen rain na e on pe nea nn e ane ae oen le nee aaa at ane ae ana alin a ae bade eal ag 2. Diagrammatic section showing comparative widths of alteratlon zones (stippled) in different types of rocks border- ing a Vein rll. gel o 2 oo Lee ee ae ae eae eae a ae a aer aa wie aaa nn agin ae anne a an naan n e dain nian e mann o wee ok conle ae ae es t 3. Generalized ranges and amounts of the most common minerals in fresh and altered wallrocks______________-_--- 4-10. Photomicrographs: 4. Fresh and altered microcline-rith rocks cilmi rool ne oin caa iia e 6. Alteration of phenoctysts in Tertiary intrusive Dull. deben 6. Unaltered microcline along a vein in sheared biotite-muscovite GTANIteL _ ______________________-_-_----- i. Altered :.. . . . z:. .o 2 cull ce eu il eos o ae aeg i nai on De re n apneic near eave a ea 6: Altered plagioelase... ;. Len uo rion too a e oe t tien r enne eae eae en's 0: Fresh and altered hornblende: :. >: .... __ el? Lo o n e ine aer ae ae a niels aie 10. Frosh and-altcred garnet. . ._. ail nl Pei e eee nae d ae pon ei sarab a an 11. Structure of some three-layer clay ropa pn oti ipo oon renee 12. The two-layer structure of kaolinite. tuo tt dot ett O3 Page 39 39 39 42 44 44 46 47 47 49 49 49 50 50 50 82 82 83 84 85 86 88 88 91 91 93 96 97 101 Page 11 13 14 15 17 18 20 22 23 24 24 IV CONTENTS Page Ficur®E13. Smoothed X-ray diffraction traces of montmorillonite-rich clay mineral assemblages-_________________________ 25 14. Smoothed X-ray diffraction traces of illite-rich clay mineral 26 15. Smoothed X-ray diffraction traces of mixed-layer clay mineral 27 16. Smoothed X-ray diffraction traces of kaolinite-rich clay mineral assemblage 28 17. Smoothed X-ray diffraction traces of sericito. _ as an 28 1s. Photomicrographs of sericite tenes tes toes enses 29 19. Smoothed X-ray diffraction traces of clay minerals from fresh ltl lot T LIL. 30 20. Variations in mineralogic composition of rocks described for this report g2 21. Stability and relative abundance of minerals in biotite-muscovite granite 33 93. Sketch of rock alteration L see oneness ak aw ss 33 23. Diagrammatic sketch section of altered siena ns ne be, 35 24. Stability and relative abundance of minerals in altered granodioritec L 35 25. Altered gneiss sample locations, 150-foot level, Essex ae on ana aay 37 26. Stability and relative abundance of minerals in progréssively more altered quartz E2 Luc. 38 27. Comparison of methods of plotting chemical analyses data 42 98. Variations in the ratios of altered and unaltered 43 29. Variations in oxygen and oxygen plus hydroxyl ion 44 30. Summary of ion distributions in altered 45 31, Variations in aluminum and silicon in terms of 46 32. Variation in magnesium in terms of variations in 47 33. Stability ranges and oxidation states of iron-bearing 48 34. Distribution plots of total iron in altered 48 35. Conventions and notations in semiquantitative spectrographic analysis 51 36-62. Semiquantitative spectrographic analyses of rocks: 36. Fresh and altered biotite-muscovite granite 52 37. Fresh and altered 53 38. Altered microcline-quartz-plagioclase-biotite OO # 54 39. Two adjacent phases of granodiorite from 55 40. Altered quarts 56 41. Fresh and altered biotite-quartz-plagioclase gNM€ig8 - 57 42. Altered biotite-quartz-plagio¢clase 58 48. Fresh and altered biotite-quartz-plagioclase gneiss 59 44. Fresh and altered biotite-amphibole 60 45. Fresh and altered 61 46. Altered garnet-qUuartz gNCi88L 62 47. Fresh and altered gTANOGIOTIt@L 63 48. Fresh and altered microcline-quartz-plagioclase-bi0tite 64 49. Altered microcline-quartz-plagioclase-biotite 65 50. Altered microcline-quartz-plagioclase-Diotite ___ 66 51. Fresh and altered microcline-quartz-plagioclase-Di0tite 67 52. Fresh and altered qUAItZ 68 58. Altered quarts O L 69 54. Fresh and altered quartz 70 55. Altered biotite (metasedimentary) §NMCISSL pase. 71 56. Altcred biotite-quartsz 72 57. Fresh and altered biotite (metasedimentary) 73 58. Altered biotite-quarts 74 59. Altered biotite-quartz 75 60. Altered metasedimentary 76 61. Fresh and altered quartz monzonite 7G. 62. Altered cest sues ss os 78 63. Average values for elements determined by semiquantitative spectrographic analyses for granitic rocks. ll.... 79 64. Average values for elements determined by semiquantitative spectrographic analyses for metasedimentary gneisses. 80 65. Average values for elements determined by semiquantitative spectrographic analyses for quartz diorite.:_>-.... 81 66. Variation in Apparent pErC@Nt 88 67. Average values of some elements in the host rock and the vein, 90 68. Average values of some elements in the host rock and the vein, E. Calhoun mine__--_--_-__-_------------------ 91 690; Sketch of vein-altered wallrock L ean nees 91 70. ACF diagram of the amphibolite facies with plots of FOCKS SUUdi@d _ _ ___ 92 71. AKF diagram for hydrothermal assemblage _ 92 72. AKF diagrams with plots of fresh and altered 93 73. AKF diagrams with plots of fresh and altered wallrocks from Caribou and Nederland, Colo., and Butte, Mont.. 94 74. Experimental stability fields of clay 96 TABLE 1. w go st o pr m go bo 13. EM 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. CONTENTS TABLES Samples examined by chemical analysis and semiquantitative spectrographic analysis.________________________ . Principal Precambrian rock units, central part of Front Range mineral belt, in order of probable relative age___. . Widths of some small veins and their associated altered wallrock . Estimated widths, in feet, of some minor branch veins and their altered wallrock zones-______________________ X-ray data from biotite separated from zones of progressively more altered granodiorite, Hayes and Wheeler mine. Mean indices of refraction for oriented mixed-clay mineral and sericite assemblages in altered rock zones_______. X-ray diffraction powder data for sericite (-24) in zone 4, Essex Modal analyses of fresh and altered biotite-muscovite granite, Nabob and Jo Reynolds mines._______________. . Clay minerals and sericite representative of altered biotite-muscovite granite, Diamond Mountain and Nabob mings. - .c tent sss se . Modal analyses of fresh and altered microcline-quartz-plagioclase-biotite gneiss, Essex mine __________________ . Modal analyses of progressively more altered granodiorite, M. and M.-Dixie tunnel- L . Modal analyses of progressively more altered mafic border facies of granodiorite intrusive, Hayes and Wheeler funnel. e 000 aaa naan e rae aie bale a a s anon cnn d bare ts on bal atea s tik ben's on ah Representative clay minerals and sericite in altered granodiorite, Hayes and Wheeler and M. and M.-Dixie tun- Nels L2 lore ln olde a e cleus an ans ain al bale sod ane calecess an bit aer rebs scat coke ssi Modal analyses of fresh and altered biotite-quartz-plagioclase gneiss, 150-foot level, Essex mine.______________. Modal analyses of progressively more altered quartz diorite, Jo Reynolds and Nabob mines-__________________ Modal analyses of altered amphibolite, R. H. D.-McKay Modal analyses of altered garnet-quartz gneiss, Golconda mine:. Chemical analyses of fresh and altered wallrocks, Gilpin and Clear Creek Counties, Colo_____________________- A comparison of alternative methods of expressing chemical analysis data, granodiorite_______________________ Ton redistribution resulting from the alteration of igneous and metamorphosed rocks Constituents of. some silicate minerals.. .. .. _ __. 22 2. collant l o on ie J Ue Ue uce in ares acre a a ad a Distribution of constituent fons in common fron-bearing minerals __ Oxygen in fresh and altered wallrocks by weight .L sess Standard sensitivities, in percent, of elements determined by semiquantitative spectrographic methods at the U.S. Geological Survey laboratories... bek aes Amount; in percent, of some regular elements in Amount, in percent, of some transitional metals in rocks:. _L: cD 0.1L LL.. asi wav adn e Amount, in percent, of zirconium and niobium in L Amount, in percent, of some transitional and secondary transitional metals in rocks. Summary of qualitative powder pH determinations on several variously altered wallrocks-___________________- Generalized distribution of trace elements in rocks in the Front Range areal Analyses of water samples, in approximate parts per million, from springs near Idaho Springs, Colo.__________. Hardness values of mine water in the central part of the mineral Analyses of water samples, in parts per million, from Clear Creek and the Argo tunnel, Idaho Springs, Colo.... Qualitative spectrographic examination of water residues of samples from Clear Creek and the Argo tunnel, Idaho Springs, Colo... 0.2 r Llc cee cad beeen aie brain ol Page 4 6 10 10 16 24 27 31 31 34 34 34 51 82 83 84 86 8T 94 95 95 95 ALTERED WALLROCKS IN THE CENTRAL PART OF THE FRONT RANGE MINERAL BELT, GILPIN AND CLEAR CREEK COUNTIES, COLORADO By E. W. TooxEr ABSTRACT The wallrocks of fissure veins in the mining districts of the central part of the Front Range mineral belt are altered to suc- cessive zones of sericitized and argillized rock. In general, the altered wallrocks are similar throughout the region in that sericite is adjacent to the veins and clay minerals are farther out, but in detail the degree of alteration, the width of the altered zones, and to some extent the type of alteration varies because of dif- ferences in lithology and structure of the host rocks and differ- ences in the intensity of the hydrothermal environment. The wallrocks are predominantly of Precambrian age, but some are of Tertiary age. The Precambrian wallrocks include a variety of metamorphic and igneous rocks that can be grouped into two general types: (1) those containing moderately abun- dant K-feldspar, and (2) those deficient in K-feldspar but containing abundant iron-bearing minerals. The first group in- cludes biotite-muscovite granite, granodiorite, microcline-quartz- plagioclase-biotite gneiss, and biotite-quartz-plagioclase and related gneisses ; the second group in cludes quartz diorite, amphibo- lite, garnet-quartz gneiss, and biotite schist. The Tertiary igneous rocks include abundant, small, irregular intrusive dikes and plutons of alkalic and cale-alkalic rocks such as porphyritic bostonite and quartz monzonite. The ore deposits consist of a simple suite of base-metal sulfide minerals in a dominantly quartz gangue and are valuable chiefly for their gold and silver content. The ore minerals were de- posited in three closely related stages, which were, from oldest to youngest, a uranium stage, a pyrite stage, and a base-metal stage. Alteration largely preceded ore deposition. Precipita- tion of the ore minerals of the pyrite and base-metal stages yielded a concentric zonal pattern that is particularly well de- fined in the Central City district. The width of the altered rock zones that bound the veins ranges from a few inches up to a few tens of feet. In general, veins in the central zone, as at Central City, are bounded by wider altered rock zones than veins in the intermediate or peripheral zones; however, little or no relation between width of the ore zone and width of altered rock is apparent in other areas, such as at the Freeland-Lamartine district. Supergene alteration effects are superposed on the hydrothermal altered rock and can be distin- guished by the presence in the veins of secondary minerals such as limonite and chalcocite. Clay minerals formed by supergene processes have not been distinguished in this study from the hypogene clay minerals. Most of the fresh and altered rocks are characterized by similar, repetitive mineralogical-textural zones: (1) fresh rock, (2) weakly argillized rock, (3) strongly argillized rock, and (4) sericitized rock. - Fresh rock of zone 1 grades in to rock of zone 2 in which plagioclase and hornblende are altered incipiently to clay minerals. A gradual softening of the rock occurs as clay minerals completely replace plagioclase, hornblende, and some of the biotite in intensely argillized rock (zone 3). Further alteration results in the remainder of the biotite and the clay minerals being converted completely into sericite (zone 4), and in the recrystallization of quartz into aggregates of small strain- free crystals; K-feldspar is not visibly altered in zone 4. The boundary between the soft argillized rock of zone 3 and the hard, bleached, sericitized, and at places pyritized rock of zone 4 commonly is sharp. Along some veins, the alteration assem- blages differ from those in the normal altered rock zones in that the sericite zone (4) extends into the argillic zone (2) and the clay zone (3) is virtually missing. Another exception to the normal zone pattern, found where the altered rock next to the vein was originally composed almost wholly of minerals un- stable in the hydrothermal environment, is a complete absence of sericitized rock; instead the whole width of the zone is argil- lized. The clay mineral components in zones 2 and 3 consist of mixed assemblages of montmorillonite, illite, random mixed- layer montmorillonite-illite, chlorite, kaolinite, and halloysite. The iron needed for the pyrite in zone 4 was derived in part from altered magnetite (and hematite) and mafic minerals such as biotite. Carbonate minerals and quartz occur at many places in zone 4 as well as in the veins. The distribution of elements in altered wallrocks is considered to be largely the result of movements of ions in and out of the interstices of a silicon-oxygen or aluminum-oxygen framework in which the oxygen-ion positions are the most abundant and largest. Calculations from chemical analyses of samples of representative rocks show that, in the alteration of equal vol- umes of rock, small amounts of K+, Fet?, C+, H+, S-, and at places Alt} were added, and Si+4, Nat, Ca+?, Fet, and Mg* were removed. In the chemical sense, this is a hydrolysis process in which there is a replacement of chemical equivalents of metal cations by H+ ions which enter the structure and pro- duce polarized hydroxyl groups. - In the argillized zone hydration water is added also but without attendant replacements. The distribution of trace elements concealed in rock-forming minerals shows that most tend to be removed or at least displaced vein- ward. Relatively large interstitial ions with low ionization potential generally are removed, while smaller ions with larger ionization potential and higher valences are concentrated in rocks near the veins. The powder pH of fresh and altered rocks mostly gives a measure of the relative hydrolysis of the mineral components. In general, clay minerals and sericite do not hydrolyze signifi- cantly, giving lower values than feldspar or mica. In the periph- eral zone, fresh rock of granitic composition has an average powder pH of 8, whereas altered rock of the same composition has an average pH of 6. In the cental zone, in areas not greatly affected by supergene oxidation, the dominant rock-micro- cline-quartz-plagioclase-biotite gneiss-has an average powder pH of about 6 where fresh and about 4.5 where altered adjacent 1 2 ALTERED WALLROCKS, ICENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO to veins. These differences between the peripheral and central ore-zone rocks are considered to indicate greater supergene action in the central zone. The hydrothermal solutions that altered the rocks, as inferred from observable surface and underground waters and the miner- alogical and chemical changes in the host wallrocks, were relatively dilute, slightly acid, and initially carried K and subse- quently CO; and S ions or complexes. As the rocks were altered, these solutions were modified by chemicals replaced in the wall- rocks and by vein-forming ore metals. Possibly the hot springs in the area, such as those at Idaho Springs, represent spent hydrothermal solutions. Altered wallrocks are not infallible guides to ore, for alter- ation was largely accomplished before the deposition of the valuable vein minerals, and further, there is no close correlation between width of the altered rock envelope and width of the vein filling. Aside from uranium, most of the ore constituents were deposited in vein openings following the wallrock altera- tion. Primary uranium minerals are believed to have been deposited earlier in veins that were open during late stages of the rock alteration. INTRODUCTION The altered wallrocks bordering the hydrothermal vein deposits in the mining districts of the central part of the Front Range mineral belt, Colorado, were studied as a part of a detailed examination of this region by the U.S. Geological Survey on behalf of the U.S. Atomic Energy Commission. A primary aim of the study was to determine the usefulness of altered wall- rocks as a prospecting guide in the search for uranium ore. In addition, the study complements concurrent geologic studies of the ore deposits in that determina- tion of the character and extent of the altered meta- morphic and igneous wallrocks has yielded data bearing on the origin of the ore deposits themselves. LOCATION The area investigated during this study is in Gilpin and Clear Creek Counties, Colo., about 35 miles west of Den- vet (fig. 1); it lies between long 105°29"' and 105°38" W. and lat 39°50" and 39°43" N. and includes the mining districts of Central City, Idaho Springs, Freeland- Lamartine and Chicago Creek, and Lawson-Dumont- Fall River (pl. 1). The region is in the center of the Front Range segment of the Colorado mineral belt, which extends northeastward across the north-trending Front Range. It is east of the Continental Divide and is dissected by the drainage system of Clear Creek; altitudes range from 7,500 feet at Idaho Springs and 8,500 feet at Central City to 10,700 feet in the Freeland-Lamartine district. PREVIOUS INVESTIGATIONS OF ALTERED WALLROCKS Previously published reports, on parts of the Idaho Springs, Freeland-Lamartine, Lawson, Dumont, and Fall River ' districts by Spurr, Garrey, and Ball (1908) and on the Central City and parts of the Idaho Springs districts by Bastin and Hill (1917), present some generalized petrographic descriptions of the altered wallrocks. Lovering and Goddard (1950) summarize observations on altered: wallrocks associated with ore deposits in this and other parts of the Front Range mineral belt, noting (p. 77) that clay minerals become more and more prominent members of the alteration suite northeastward along the mineral belt. Several reports published during the past few years describe related altered wallrocks in other Front Range mining districts. Lovering (1941, 1950) and Lovering and Tweto (1953) described the hydrothermally altered wallrocks associated with tungsten veins at Nederland; Gonzalez-Bonorino (1956, 1959) investigated these same altered rocks. Wright (1954) described the altered rocks associated with silver-bearing base metal- uranium veins in the Caribou district. The term "argillic alteration" was first used by Lovering (1941, p. 236) for the prominently developed clay-mineral assemblage found in altered Boulder Creek granite bordering tungsten veins near Nederland. According to Lovering and Tweto (1953, p. 58), a broad zone in which sericite, allophane, beidellite, hydrous mica, dickite, and halloysite occur successively veinward is followed by a narrow zone of sericitized rock adjacent to the vein. Gonzalez-Bonorino (1959) recognized six distinctive alteration patterns, based on chemical, X-ray, and petrographic data, in the altered rocks in different parts of the mineral belt. The most complex of these found in the Nederland tungsten-Gold Hill pyritic gold districts consists of four main zones, characterized by hydromica (innermost zone), orthoclase, kaolinite, and montmorillonite. Two zones were found in the Cari- bou lead-silver-zinc mine characterized respectively by kaolinite (inner) and montmorillonite. In an earlier study of the hydrothermal alteration of monzonite at the Caribou mine, Wright (1954) described complex relationships in more detail than did Gonzalez- Bonorino. Petrographic, X-ray diffraction, chemical, and differential thermal analysis data reveal (p. 148) an outer propylitic, a succeeding argillic, and an inner sericitic zone: In Stage 1 (comprising "comparatively unaltered" rock) the ferromagnesian minerals, biotite and hornblende, were in part altered to chlorite, magnetite, and pyrite, while the felsic min- erals remained essentially fresh. - In Stage 2, chlorite replacement of biotite was important, but the chlorite disappeared in the more intensive Stages 3 and 4 (as it did also in Sales and Meyer's * * * argillized and sericitized zones). Minor sericite replace- 1 The Lawson, Dumont, and Fall River districts of Spurr, Garrey, and Ball (1908) have been combined into the Lawson-Dumont-Fall River district in this report. INTRODUCTION 105° 1 in; of plate 1 s 1Bezljtl‘al Nol « ow“ ity , |: f | I I ao E $ % Golden A daho / ** Georgeto n I H Sprmgs Y“ s q. «. * » : p 3 . f ss o Montezuma *+ e g . - o * s 6 § < EXPLANATION Paleozoic and younger rocks * Precambrian gneiss, schist, granite a+ 110 MILES FIGURE 1.-Index map of the central part of the Front Range mineral belt area, Colorado, showing location of area investigated on plate 1 ment of biotite, and kaolinite development in feldspars, occurred in the more intensively altered portion of Stage 2. Stage 3 is characterized by the development of clays, especially hydromica (replacing biotite), and kaolinite and montmorillonite (chiefly in feldspars), together with some sericite. The ratio kaolinite: montmorillonite increases with the intensity of alteration approaching Stage 4. Some kaolinite remains in Stage 4 (the most intense), but is subordinate to sericite and fine-grained quartz, the most abundant alteration products. Beyond the confines of the Front Range, classic investigations by Lovering and others (1949) and by Sales and Meyer (1948, 1949) have greatly influenced subsequent studies of altered wallrocks. A summary of field and laboratory data on hydrothermal alteration by Kerr (1955) and the discussion of altered rocks as guides to ore by Schwartz (1955) provide extensive review bibliographies on these subjects. BACKGROUND GEOLOGIC STUDIES Several reports that describe the geology and ore deposits of the mining districts in the central part of the Front Range mineral belt have been published as a result of the current investigations by the Geological Survey, and others, now being prepared, will be, published later. One of these (Sims and others, 1963) describes the uranium deposits and summarizes the data on the ore deposits from all of the districts included in the Survey's 4 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO current investigations in this region. Detailed reports that describe the geology of the Freeland-Lamartine district (Harrison and Wells, 1956), the Chicago Creek area (Harrison and Wells, 1959), and the Central City district (Sims, Osterwald, and Tooker, 1955; Sims, Drake, and Tooker, 1963) have been published; the reader should refer to these for a full description of the rock units and ore deposits of these districts. For a descrip- tion of the Tertiary intrusive rocks of the region, the reader is referred to a paper by Wells (1960). Several papers (Harrison and Leonard, 1952; Moore and Butler, 1952; Sims and Tooker, 1955; Wells, 1955; Hawley and Moore, 1955; Sims, 1956; Sims and Tooker, 1956; Tooker, 1956; and Drake, 1957) describe some details of features of the uranium deposits. ACKNOWLEDGMENTS The present study was undertaken by the U.S. Geo- logical Survey on behalf of the Division of Raw Mate- rials of the U.S. Atomic Energy Commission. Field- work and sampling were carried out from 1953 to 1955; laboratory investigations were done intermittently from 1954 to 1957. I am indebted to P. K. Sims, who directed the geologic investigation in the region, and to A. A. Drake, Jr., J. E. Harrison, C. C. Hawley, R. H. Moench, F. B. Moore, and J. D. Wells for assistance in sampling and for advice on problems of the geology of the region and of individual mines. These and other associates in the Geological Survey have been helpful in discussions during all phases of the study. The writer also wishes to acknowledge the assistance of Peter Buseck, A. R. Dearth, and J. R. McDonald of the Geological Survey in the field and laboratory. Many local mining people provided information of great value to this study, for which I am deeply grateful. METHODS OF INVESTIGATION The methods of field collection and laboratory ex- amination of samples are briefly summarized here to provide a basis for interpretation or future reinterpreta- tion of these data. Over periods of time, methods of study change and are improved, thus it is desirable to establish for the record the means by which these data were derived. FIELD STUDIES 'The wallrocks of veins throughout the region were systematically examined and sampled in conjunction with mapping of the mines. The zones of altered rock bordering the veins are generally narrow, and it was not practical to map them. Instead, samples of different kinds of altered rocks that could be distin- guished megascopically were collected and sketches were made to show the locations of the samples with respect to the veins and other structural features. 1.-Samples examined by chemical analysis and semi- quantitative spectrographic analysis [Chemical analyses are in table 18; semiquantitative analyses are in figures 36 to 62. V, wallrock caught up in vein; G, rock in the gouge zone] Altered- Sample rock Rock type Location zone I ype 1 | Biotite-muscovite granite........| Nabob and Jo Regnolds mine. o. Do. M. and M.-Dixie Do. Surface, Central City istrict. Do. Nabob and Jo Reynolds mine. R. H. D. McKay shaft. Do. Do. Almadin mine. Do. Do. ¢ Do. Kitty Clyde mine (31) and R. H. D.-Mc- Golconda mine. Do. Marys mine. o. Almadin mine. o. Hayes and Wheeler Tunnel. do Microcline-quartz-plagioclase- biotite gneiss. 0. Nabob mine. Do. Do. P Do. a Jo Reynolds mine. Do. See footnotes at end of table. METHODS 1.-Samples examined by chemical analysis and semi- quantitative spectographic analysis-Continued Altered- rock zone Rock type Location Widow Woman mine. o. Do. Bacc ex-con + 1-2 Biotéte NCIS 22 pe oci se Do. -| Cherokee mine. o. vin .[" 1-2 Biotéte-quartz gneiss. ..... ous ue Pay o. -| Banta Hill mine. o. Do. Be? o. ..] Phoenix mine. 1 Composite sample. 2 Dump sample. More than 500 samples were collected and studied during the investigation. These samples represent a diverse group of altered rocks related to a variety of geologic structures within an area of about 40 square miles. Analytical data are presented here for the representative samples listed in table 1. Because preservation of natural moisture is desirable in samples containing a high proportion of clay minerals, the samples collected were placed in watertight card- board containers and sealed by tape until ready for laboratory examination. These containers were not sufficiently airtight to prevent dehydration of the moist underground samples for more than 3 months; poly- ethylene plastic bags were found to be effective liners for the cardboard containers and retained natural moisture for longer periods of time. A LABORATORY STUDIES The samples were studied in the laboratory with the petrographic microscope and X-ray diffractometer, and were analyzed spectrographically and chemically. Sample preparation and study for each different type of altered or fresh rock varied slightly. Both thin section and oil immersion methods were used in optical studies. Friable rocks which were to be thin sectioned, including those containing clay minerals, were impregnated with Canada balsam before being cut by a diamond saw in a kerosene oil bath. A OF INVESTIGATION 5 useful, undisturbed, and unfractured specimen of friable clay-mineral rock for thin sectioning can be obtained readily when natural moisture is preserved; when soft and moist, many of the altered rocks can be sliced easily with a knife. Modal analysis of the rocks was by means of a point-count microscope stage and tabula- tor. The area studied commonly was small because the feature observed was small or the slide was difficult to make; therefore the number of counts are recorded on the tables that show modal analyses. The sodium cobaltinitrite stain for the identification of potassium- bearing minerals in thin sections (Chayes, 1952) and in rock-slab specimens, also a very useful technique, was used in the study of these rocks. X-ray diffraction was used most commonly to study the clay minerals and micas. Powder samples of clay minerals were prepared for use on a Phillips Norelco diffractometer with recording strip chart. The sample was scanned at the rate of 2° per minute between the angles 2° and 40° 20 (a range of d=44 to 2.25A) at 35 KVP, 20 m.a., 4° and 0.006-inch slits, but with variable full-scale deflections of 120 to 800 counts per second. A Phillips powder camera was used in a study of altered mica and K-feldspar. For the most part bulk samples of the clay minerals in rock units formed from altered rock minerals were studied in preference to selected microsamples of clay minerals resulting from the alteration of particular minerals in the rock. A limited number of micro- samples were examined also. While microsamples reveal important information about details of mineral alteration and are worthy of future study, this investi- gation was concerned primarily with the chemistry and mineralogy of altered wallrock units. The procedure for the separation of clay-mineral assemblages from altered rock samples and their prep- aration for X-ray examination follows. The softened rock was crushed by hand in a mullite mortar; when a jaw crusher was used on less altered samples the fine fraction was checked for tramp iron. The crushed sample was dry ground to a powder in a rotating pebble mill using flint pebbles. The resulting powder was dispersed in distilled water. No wetting or dis- persing agent normally was used; occasionally a small amount of dilute NH,OH was needed. Clay-mineral separation was effected by gravity according to Stokes Law, and the -24 clay mineral size fraction was pipetted off. The ground rock was redispersed several times (5-6 average) and the sedimentation technique repeated to obtain a representative sample. The accumulating clay mineral was kept from drying, the resulting total sample was dispersed as a thickened slurry, and a small sample was pipetted onto a glass microscope slide. When dry a clay-mineral aggregate 6 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO consisting of subparallel flakes was available directly for X-ray study. The clay slides were reversed (Schultz, 1958, p. 367), using double-coated pressure- sensitive tape and X-rayed again where differential settling was noted. These glass-slide samples were amenable for direct ethylene glycol treatment using the vapor pressure method (Brunton, 1955), and for mod- erate heat treatment of the sample. .The following notes concern the selection and size of representative fresh and altered rock samples used for chemical and spectrographic analyses. The selection of a sample representative of each fresh and altered rock type was attempted; the individual clay-mineral subzones-could not be sampled systematically. Al- though the successive phases characteristic of fresh to most altered rock can be recognized megascopically, at most places they are relatively small features. Where the layered rocks have segregations of minerals, it is difficult to obtain a representative sample. In one case the sample was a composite of the rock type within a mine from along the strike of a single zone over a distance of tens of feet. Samples selected generally weighed 500 grams or more. A limited number of standard rock and rapid rock analyses were made on representative rock suites, and semiquantitative spec- trographic analyses were made on splits from these samples. Semiquantitative spectrographic analyses also were made for additional similarly prepared suites of rock samples. Other data were gathered through mineral separa- tions, rock gravity determinations, powder pH measure- ments of dry ground rock, and mine and ground water pH measurements. - Mineral separations were by heavy liquid and water sedimentation, ultrasonic and mag- netic methods, and by hand picking using a binocular microscopé. - Bulk densities of rock were determined on wax-coated samples following the method employed by a Geological Survey rock analysis laboratory (L. C. Peck, written communication, 1957). Measurements of pH were obtained by a glass electrode pH meter. The powder pH of rocks was made on a rock powder dispersed for 24 hours in distilled water. All other pH measurements were made in the laboratory as soon as possible after sample collection. GEOLOGIC SETTING The central part of the Front Range mineral belt consists chiefly of folded metasedimentary rocks and several types of igneous rocks of Precambrian age, some of which are metamorphosed. These rocks are intruded by small plutons and dikes of early Tertiary age. Rocks of both ages are cut by many intersecting faults that contain gold- and silver-bearing base-metal sulfide vein deposits. Uranium is an important con- stituent of some veins. PRECAMBRIAN ROCKS The principal Precambrian rocks of the region, listed in the order of their relative ages, are given in table 2, which also gives the equivalent names used in other reports. The metamorphic gneisses are largely meta- sedimentary rocks that belong to the amphibolite facies; the granitic rocks are intrusive igneous rocks that range in composition from quartz diorite to granite. Most of the rocks consist principally of quartz, K-feld- spar, sodic or intermediate plagioclase, and biotite or hornblende, but in different proportions; thus they reacted similarly during alteration by hydrothermal solutions. The less abundant, iron-rich (and K-feldspar- deficient) rocks, such as quartz divrite and associated amphibolite, lime-silicate gneiss, and skarn, are less stable in the hydrothermal environment than are the more abundant, relatively iron poor, K-feldspar-rich rocks. Tasos 2.-Principal Precambrian rock units, central part of Front Range mineral belt, in order of probable relative age [Rocks are listed from youngest to oldest, except as noted] Rock units Probable Remarks origin Pogmalite:ss corso cell ilies Igneous. .... Granitic rocks: Biotite-muscovite granite....}..... do..... Equivalent to Silver Plume granite at Silver Plume, Colorado. (K-A age 1280 m.y., Davis and others, 1958) Quartz diorite and associ- |._.__ ated amphibolite. do...s.2. Probably equivalent to Boulder Creek granite. Granite gneiss and pegmatite....| Uncertain...! Also constitutes felsic layers in migmatite. Gneissic rocks: Microcline-quartz-plagio- | Metasedi- Age relations among these rock clase-biotite gneiss. mentary. units are unknown. Most of Cordierite-amphibole gneiss.|.. .._ do.v..... the rocks previously were grouped Biotite-q uartz-plagioclase |___. do...... in the Idaho Springs formation. gneiss. T he microcline-quartz-plagioclase- Sillimanitic biotite-quartz _ |..___ biotite gneiss is the granite gneiss gneiss. of Bastin and Hill (1917); it was Lime-silicate gneiss....__..._|__.__ do...... referred to as quartz monzonite Skarn and related rocks gneiss in previous reports (Sims, Amphibolite......... % 1956; Tooker, 1956). Quartz. gneiss.: do.}... The metasedimentary rocks that underlie a large part of the central Front Range are well foliated, medium grained, and range in color from light to dark gray. The most abundant of these rocks, the micro- cline-quartz-plagioclase-biotite gneiss, is a light- to dark-gray, medium-grained gneiss that occurs as a thick, folded layer in the Central City (Sims, Drake, and Tooker, 1963) and Idaho Springs districts, and as thinner layers in other areas. The other less abundant metasedimentary rocks are mainly biotite gneisses that constitute an interlayered lenticular sequence of irreg- ular thickness. The biotite gneisses may contain GEOLOGIC SETTING T. moderate or abundant granite gneiss and pegmatite, commonly as thin layers; these rocks, which are typical migmatites, are highly variable in composition and in thickness. Granodiorite occurs as a large irregular mass in the Chicago Creek area (Harrison and Wells, 1959), and elsewhere as smaller, lens-shaped bodies. It is a competent, medium- to dark-gray, medium-grained rock composed of quartz, plagioclase, K-feldspar, biotite, and a minor amount of hornblende. Quartz diorite and amphibolite are found chiefly in the Lawson-Dumont-Fall River district, but do not crop out well because they weather rapidly. The quartz diorite is dark gray, medium to coarse grained, and locally foliated; it is composed of plagioclase, hornblende, biotite, and quartz. The amphibolite is a black, medium- to coarse-grained rock composed principally of hornblende; it is generally not foliated. Biotite-muscovite granite occurs in stock, dike, and sheetlike bodies throughout the area studied, but it is most abundant in the Freeland-Lamartine area. The granite is a medium-gray to salmon-pink generally medium grained rock with an equigranular to seriate- porphyritic texture. The rock is composed of sodic plagioclase, K-feldspar, quartz, biotite, and muscovite. Large K-feldspar crystals locally produce a weak planar structure; in most of the fine-grained facies the biotite flakes form an indistinct foliation. Mortar structure was also observed in some areas. Sills, dikes, and podlike bodies of granite pegmatite cut all the older rocks. The pegmatite is medium to coarse grained, inequigranular, and composed chiefly of K-feldspar, quartz, plagioclase, and mica; biotite and magnetite are abundant locally. TERTIARY ROCKS Abundant, small, irregular intrusive dikes and plutons of radioactive porphyritic rocks of early Tertiary age cut the Precambrian rocks and are preore in age. The intrusives are particularly common in the metasedi- mentary gneisses, especially in the thicker and more widespread units of biotite gneiss. Wells (1960) has elassed them according to petrographic character, relative ages, and geographic distribution into four groups: (1) hornblende granodiorite (oldest), (2) leuco- cratic granodiorite, (3) quartz monzonite, and (4) bostonite (youngest). The hornblende granodiorite group occurs only in the western part of the area; the quartz monzonite and leucocratic granodiorite groups are more abundant in the eastern part of the area, and form small irregular plutons and radial dikes. The bostonite occurs throughout the area, mostly as dikes that at places have narrow chilled margins; these dikes are somewhat finer grained than the older rocks. Biotite-quartz latite dikes occur locally in the south- eastern part of the area where they crosscut veins formed during the main period of ore formation and thus are the youngest Tertiary intrusive rocks. STRUCTURE The gneissic Precambrian rocks are folded and cut by faults and joints. These structures, together with primary lithologic features, have produced inhomo- geneities in the rocks that have caused some variations in the character and extent of rock alteration. Thefolds, which trend northeastward, range from broad, symmetri- cal anticlines that have subsidiary open folds along their flanks, such as those in the Central City district (Sims, Osterwald, and Tooker, 1955), to complex tight or over- turned folds, such as those in the Freeland-Lamartine area (Harrison and Wells, 1956). Cataclastic struc- tures and younger northeast-trending folds are super- posed locally on the earlier folded rocks (Moench, Harrison, and Sims, 1954; Sims, Moench, and Harrison, 1959). The faulting took place at two separate times. An early period of fracturing that preceded the emplace- ment of the Tertiary intrusive rocks, produced north- west- and north-northeast-trending faults, and a later period of fracturing following the emplacement of the igneous rocks yielded east-, east-northeast-, and north- east-trending faults. - The older faults may have formed in Precambrian time, but many were rejuvenated in Laramide time; the younger faults formed during the Laramide orogeny in early Tertiary time. Metalliza- tion followed the formation of the younger faults. Except for some of the early northwest-trending faults, movements along the fractures generally were small. Recurrent movements took place along many faults during alteration and metallization (Sims, 1956; Harrison and Wells, 1959). These movements provided local open spaces in which the ore and gangue minerals were deposited. In some parts of the region the major east- and northeast-trending faults, which were the principal channelways for hydrothermal solutions, are essentially parallel to the folds and to the metamorphic fabric of the country rock. Consequently, solution movement upward along the faults and outward into the wallrocks was partly controlled by these previously established structural features. - The solutions were able to migrate a significant distance into the walls only where the fault zones provided a wide zone of broken rock, or intersected prominent joints or rock layers composed mostly of iron- and calcium-rich silicates. MINERAL DEPOSITS The ore deposits of the mining districts in the central part of the Front Range mineral belt are pyritic quartz 8 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO veins, chiefly valuable for their gold and silver content, but also containing important but variable amounts of copper, lead, zinc, and uranium. The ratio of Pb": U*" in pitchblende for the Central City area indicates an absolute age of 59+5 million years or an early Tertiary age (Eckelman and Kulp, 1957, p. 1128). The veins are hydrothermal fillings of fault fissures, and are similar in mineralogy, texture, and structure to depoZ-flts classified by Lindgren (1933, p. 530) as meso- thermal. They range from simple filled-fissures with well defined, smooth, sharp walls to complexly branch- ing lodes consisting of subparallel or intersecting fractures. The veins average 1 or 2 feet in width, but some are several feet wide. The ore shoots were de- posited in structurally controlled open spaces along the fractures (Sims, 1956; Sims, Drake, and Tooker, 1963; Harrison and Wells, 1956). The veins throughout the region contain the same suite of minerals, but they are present in different pro- portions from district to district. The common primary metallic minerals are pyrite, sphalerite, galena, chalco- pyrite, and tennantite. Other minerals that are abundant locally are pitchblende, enargite, marcasite, polybasite, pearceite, tellurides of gold and silver, and free gold or electrum. The dominant gangue is silica in several forms, but various carbonates of the calcite group are common, and fluorite and barite occur locally. The common secondary minerals are chalcocite, covel- lite, and hydrous iron oxides. Although vein filling took place during a single period of mineralization, the ore and gangue minerals were deposited in 2 main stages in most veins and in 3 stages in others (Sims, 1956, p. 745-746). The dominant mineralization, which yielded the sulfide ore minerals, began with the deposition of pyrite and quartz (pyrite stage) and concluded with the deposition of base-metal sulfide and gangue minerals (base-metal stage). Pitch- blende and sparse pyrite and quartz were deposited in some veins during an early pyrite stage that preceded the base-metal sulfide mineralization; gold tellurides were precipitated locally following the base-metal stage. Vein structures were reopened periodically throughout the area separating stages of mineralization; more localized fracturing took place during the stages of filling. The veins have been classified into two distinct types (Sims, 1956, p. 744), based on variations in the quan- tities of the principal vein-forming minerals. One type (pyrite veins) contains abundant pyrite and variable amounts of base-metal sulfides; the other principal type (galena-sphalerite veins) contains abundant galena and sphalerite and sparse pyrite. Transitional veins contain pyrite, substantial quantities of galena and sphalerite, and in addition, considerable amounts of copper minerals. . A regional concentric zonation of metallic minerals is indicated by the distribution of the different types of veins (pl. 1). The pyrite veins that are largely devoid of base-metal sulfides occur in a large, irregular core area which is designated the central ore zone on plate 1. This zone is in turn surrounded by a dis- continuous area, the peripheral ore zone, which contains predominantly galena-sphalerite veins. The veins that contain substantial quantities of both pyrite and base- metal sulfides occur in an intermediate area or zone between the two principal zones. In the Freeland- Lamartine district (Harrison, 1955) and in other places, longitudinal zonation along a single vein or a vein system is also recognized. Although a depth zonation also may be present, it is not as evident as longitudinal and concentric zonation. Most of the mines in the area expose veins for relatively short distances down the dip. The Freeland and Lamartine veins, however, are exposed for at least 2,000 feet, and Harrison and Wells (1956, p. 87) noted that on the north-northeast Freeland vein, galena occurred near the surface, but not with pyrite and gold ore at depth. ALTERED WALLROCKS The alteration of rocks into zones about vein struc- tures may vary owing to the type and proportion of rock minerals present and to the vein and rock struc- tures present as well as to the physical and chemical character of the altering solutions-whether they be hydrothermal or supergene. HYDROTHERMALLY ALTERED WALLROCKS Fresh and hydrothermally altered wallrocks along veins in the area characteristically have a sequence of mineralogical and textural zones that can be recognized megascopically. The fresh rock, zone 1 of this report, grades veinward through weakly argillized rock, zone 2, to strongly argillized rock, zone 3, to sericitized rock, zone 4. Exceptions to this zonal sequence are rare in the central part of the Front Range mineral belt, but where they do exist they can be related directly to differences in the primary mineralogy of the host rock. Most of the rocks in the region contain quartz, K- feldspar, and plagioclase as essential minerals. These rocks nearly always alter to a soft outer zone in which clay minerals are dominant, and a hard, bleached, sheared sericite-quartz-K-feldspar zone adjacent to the vein. Some of the wallrocks that are deficient in quartz and K-feldspar and which contain an abundance of iron-bearing minerals and plagioclase alter to a soft ALTERED WALLROCKS 0 rock instead of to a hard sericitized rock adjacent to the vein. The soft rock contains greenish clay and at most places a secondary biotite. Another exception to the usual alteration pattern is evident where plagi- oclase and mica are absent in the host rock; these rocks alter to a montmorillonite-rich phase. Zone 1, fresh rock.-Fresh rock, as used in this report is characterized in hand specimen by the clear, reflective appearance of the constituent minerals, and in thin sections by the lack of observed alteration of plagioclase, biotite, and hornblende. The plagioclase crystals in quartz diorite, however, may be altered (deuterically?) to clay minerals, and a clouding effect along a crystal boundary, crack, or an irregular patch within the crystal area commonly is observed. Sparse and irregu- lar sericitelike inclusions also occur in many of the "fresh" rocks. The transition from zone 1 into zone 2 is gradual, but the boundary is placed where clay minerals formed from plagioclase definitely become megascopically recognizable. Evidence of widespread deuteric or early pervasive propylitic types of rock alteration, characteristic of many areas, perhaps are obscured here by regional metamorphism. Zone 2, weakly argillized rock.-The rock is hard, and the original structures and textures are preserved. The plagioclase grains, white to light gray, have a dull, chalky luster that is due to incipient alteration on cleavage and fracture faces. Hornblende, when pres- ent, also is partly altered to gray-green montmorillonite, illite, and mixed-layer clay. At the variable, often indistinct boundary with zone 3, the rock has been partly softened owing to the alteration of hornblende and plagioclase. Biotite, K-feldspar, and quartz are unaltered. Zone 3, strongly argillized rock.-Strongly argillized rock is soft, composed of light-green to white clay mineral, quartz, biotite, and K-feldspar, and it retains much of the original structure and texture. Clay minerals have nearly completely replaced plagioclase and hornblende crystals, and biotite may be incipiently altered to clay minerals, but quartz and K-feldspar remain unaltered. The strongly argillized zone may be subdivided into several subzones based on the predominant clay mineral within a mixed clay assemblage as determined from X-ray diffraction data. The outermost subzone, a, is montmorillonite-rich rock that grades veinward into a less distinct subzone, b, of illite-rich rock, that in turn grades into the kaolinite-rich rock subzone, c, adjacent to zone 4. The boundary of zone 3 and zone 4 is generally sharp, and is marked by distinct changes in texture and mineral content. Zone 4, sericitized rock.-The sericitized rock, which is the most intensely altered wallrock bordering the veins, is hard and bleached light greenish gray or white. Original structures and textures are mostly obliterated, and shearing and recrystallization are common features on the veinward side of the zone. - The rock is composed chiefly of sericite, quartz, and K-feldspar, and contains scattered crystals of pyrite that appear to be localized by altered biotite. SUPERGENE ALTERATION AND ROCK WEATHERING Supergene solutions derived from the weathering of the sulfide vein minerals probably have modified the minerals of the altered wallrocks as well as the veins, but the products of supergene alteration are not readily distinguished from those of hydrothermal alteration in the zones of argillic and sericitic altera- tion. Rock weathering processes in wallrocks may have also affected the altered and fresh rocks close to the surface, giving rise to some of the same minerals that were deposited by hydrothermal solutions. The principal observable effect of supergene solution alteration is the widespread oxidation in the vein of some iron-bearing minerals to limonite, less commonly the conversion of copper-bearing sulfides to chalcocite, and of uraninite to sooty pitchblende and other second- ary uranium-bearing minerals. - Fournier (written com- munication, 1960) points out also that supergene alteration is not limited to oxidation-reduction reactions. A very important aspect of supergene alteration, borne out by Hemley's (1959) studies, is the ability of relatively cool alkaline solutions to leach alkalies from silicates and thus form montmorillonite, mixed-layer clay, and even kaolinite. 'The oxidation of pyrite to form a very acid solution enhances altera- tion and favors the formation of supergene kaolinite. Biotite may lose iron as a result of an oxidation reac- tion, but in nonoxidation reactions supergene chlorite and vermiculite are formed. The mineralogical and chemical character of the wallrock host minerals as well as of the supergene solution therefore affects the results of supergene activity in a manner very similar to that of weak hydrothermal activity. Thus only where we recognized the results of oxidation-reduction reactions were we able to distinguish supergene action with any certainty. The depth to which supergene solutions are known to have been active varies in the districts, but averages 50 to 150 feet in most mines; however, Bastin and Hill (1917, p. 151) observed chalcocite as much as 700 feet below the surface in some mines. Some water-filled mine workings ultimately connect with old drainage tunnels (such as the Argo tunnel) through caved workings or open-vein structures and thus provide for a locally pronounced downward circulation of oxidizing solutions below the water table. 10 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Where readily oxidizable iron in the veins also is available, as at the E. Calhoun mine, limonite and limonite-stained clay minerals occur in the vein and along foliation planes that extend into sericitized microcline-quartz-plagioclase-biotite gneiss walls for a considerable distance below the surface; but in the deepest parts of mine workings limonite and pyrite are observed only in veins. In contrast to these observations, the oxidation of uranium minerals in undrained and recently water-filled workings of the Carroll mine occurs close to the surface (Sims, 1956, p. 752); uraninite is found on the 228-foot level, but is altered to sooty pitchblende and metatorbernite on the 102-foot level. Pyrite is not as abundant a vein mineral here as in central and intermediate ore zone veins. Limonite-stained clay commonly forms a thick coat on vein walls and penetrates into the walls along foliation and joint planes of walls in the upper part of the vein, but is only a thin wall coating where present on the 228-foot level. In contrast, many workings close to the surface are well drained and contain much evidence of supergene action in veins and wallrocks. The Mammoth mine workings contain sooty chalcocite and limonite in the vein, but microcline-quartz-plagioclase-biotite gneiss and bostonite dike wallrocks are relatively free of obvious supergene alteration. - The R.H.D.-McKay shaft work- ings vein, and the amphibolite and biotite-quartz- plagioclase gneiss wallrocks, contain limonite as a promi- nent alteration product of iron-rich sulfides and silicate minerals, and metatorbernite presumably altered from pitchblende (Sims and Tooker, 1955). Rock weathering at the surface produces weak ar- gillic alteration characteristic of zone 2 hydrothermal and (or) supergene alteration. Studies of oxygen iso- tope ratios offer hope that in time we may distinguish one product from another. WIDTH OF THE ALTERED WALLROCK ZONE The width of alteration selvages is roughly propor- tional to the width of the veins unless modified by an unusual wallrock composition or vein-opening geometry. Veins in the central zone at Central City have wider zones of altered wallrock (table 3) than veins in the intermediate or peripheral zones. Detailed measurements of the width of the altered zones was restricted to relatively few places. The miners carefully followed vein structures and excavated the sulfides and gangue minerals, and only as much of the sericitized-silicified wallrock as was necessary to con- tinue the operation; crosscuts away from the veins are uncommon. Thus the widths given in table 3 more commonly are for small branch veins rather than for the major veins in the district. TaBu® 3.-Widths of some small veins and their associated wallrock zones [Rock types are: A, microcline-quartz-plagioclase-biotite gneiss; B, granodiorite; C, metasedimentary gneiss] Width Ratio of Ore zone Mine! Rock altered type | Vein |Altered] rock to (feet) | rock? | vein? (feet) Central: Mammouth (7)..........- A 0.3 4.0 13. 3 Hayes and Wheeler (8)....| B . 05 1.6 32.0 Central to intermediate | E. Calhoun (9)...._.....- A 1.0 . 84 .8 f Old Stage (Mendick) (24).| C .2 . 87 1.3 Intermediate_......._. Two Brothers (12)..._.... A .2 1. 2 6.0 3.5 7.6 2.1 Intermediate to Cherokee (10)............. C 12 1.5 7.5 peripheral. . 25 1.2 4.0 Foripheral.........-.~. Dumas-Kinney (1)........ A .6 8.2 5.3 A .6 4.0 6.6 BSCX (§)... celu cued A 3.0 8.0 2.6 5.0 4.0 .8 Banta Hill (10).........__ C N: .8 4.0 M. and M.-Dixie (21)... B £ .8 3.0 B .02 . 84 42.0 B .2 2.2 11.0 ! Number refers to mine location shown on pl. 1. 2 Measurement for alteration zone generally possible only on one side of vein; altered zone generally nearly symmetrical. 3 Altered zone includes both sides of the vein. The alteration selvage generally is wider in host rocks containing appreciable amounts of calcic plagio- clase and hornblende than in the more common rocks that are felsic. The contrasts in the width of altered rock zones along veins in granodiorite and biotite- muscovite granite (table 4) illustrate this point. A generalization of the relative widths of altered wallrock zones in different types of country rock bordering a vein is shown in figure 2. The width of the openings along the vein fissures at the time of alteration may not have been of equal width at the time of metallization (p.90) ; accordingly, this factor may account for some of the anomalous altered rock to vein size ratios in similar kinds of rock such as occur in the Essex, M. and M. Dixie, and E. Calhoun mines (table 3). TaBus 4.-Estimated widths, in feet, of some minor branch veins and their altered wallrock zones [ND, not determined] Altered-rock zones Host rock Vein Mine 4 3 2 Granodiorite:. 0. 05 0. 2 0.2 1.2+| Hayes and Wheeler. ! Doll.. eet . 02 18 .16 5+] M. and M.- Dixie. ? 3) Oval .2 4 .8 1.5+ -_ Do. Biotite-muscovite granite....| 1.0 .3 2 ND | Diamond Mountain. * 3.5 ND Nabob. ? .9 .5 Do. 12.0 2.0 Jo Reynolds. ? Microcline-quartz-plagioclase- i1 .6 E. Calhoun, 6th biotite-gneiss. level, 8 DOes e ende e ae . O1 1-2 .5 1-.2 Do. (DOS: Eilen £1.5 16 4 ND Do. DO: eel iver counter .2 3-. 4 ND Do. Do. .05-. 1 . 85 v4 he Do. Do. 4 . 35 1 s Do. 1-15 A2 5 ND Do. .3 . 65 2 ND Do. 1 Central ore zone 2 Peripheral ore zone. 3 Intermediate ore zone. + Vein measured at ore stope. ALTERED : ~ ~ ~ Microcline-quartz- .:; > ~- :- _ plagiociase-biotite /: '- ~- _Metasedimentary - [ - e-! -~gneiss -_ .- _: A . a. 5 (a ~ T. v " , » Granodiorite 1 ? tige: } In! fa tg ~ N > ¥ 2 & £ % e [«] = a ® ta met ale. \ Biotite-muscovite - -~ ' - % I A \’\//\l t / 's - FIGURE 2.-Diagrammatic section showing comparative widths of alteration zones (stippled) in different types of rocks bordering a vein. FACTORS THAT PRODUCE VARIATIONS IN THE CHARACTER AND DISTRIBUTION OF THE ALTERED WALLROCKS Among the most obvious factors that produce varia- tions in the character and distribution of altered wall- rocks are differences in (1) the mineralogy and structure of the wallrocks, (2) the structure of the vein fissures, and (3) the hydrothermal and supergene environments of the veins. Variation in the character of alteration is directly related to differences in the mineralogy of the wallrock and, consequently, to the areal distribution of a given rock type. Most of the rocks in the area studied con- sist essentially of quartz, K-feldspar, plagioclase, biotite, and hornblende. Thus, the altering solutions reacted with the same host minerals, but in different propor- tions in the different rocks. Biotite-muscovite granite, 678880 0O-63--2 WALLROCKS 11 found mostly in the southwest part of the area, does not contain prominent zones of argillic alteration because most of the minerals forming this rock were not readily attacked by the altering solutions. Most altered biotite-muscovite granite is hard, and the altered rock zone is narrow. In contrast, quartz diorite and amphibolite, quantitatively only minor rocks in this region, contain only small amounts of quartz and K-feldspar but comparatively large amounts of plagioclase and hornblende, which alter readily to clay minerals. The resulting altered rock is soft, and the argillic zone commonly is broad. Differences in the temperature, pressure, and com- position of the hydrothermal solutions also resulted in variations in the distribution and thickness of altered wallrocks. In general, wider zones of altered wall- rocks border veins in the central ore zone-the center of more intense hydrothermal activity-in the Cen- tral City district than in the intermediate or peripheral ore zones, but this generalization may not be strictly applicable to other parts of the central zone, as defined on plate 1. A factor that also may contribute to the notably wide alteration halos in the central zone at the Central City district is the presence of unusually wide and more persistent veins in this area which in turn permitted a greater and more direct flow of solutions than the smaller, less continuous fractures in other parts of the region. The variations in the character and distribution of the altered wallrocks that are related to the compe- tency and structure of the wallrocks are not easily separated from those related to the relative openness of fissures. Bastin and Hill (1917, p. 95) and later workers (Sims, Drake, and Tooker, 1963; Harrison and Wells, 1956, p. 83) have noted that the relative competency of wallrock exerted a marked effect upon the extent and character of the fractures. In a general way, fractures in relatively competent granitic rocks form complexly branching, persistent, open vein fissures, whereas fractures in the less competent schistose rocks, which were affected by similar stresses, are short, closely spaced, and commonly filled with gouge clay. Veins passing from granite to granitic gneisses into schistose gneisses at a low angle to the foliation generally split into a series of small branching fractures nearly parallel to schistosity and die out; veins that cut schistosity at nearly right angles, how- ever, although not common, may persist for a sub- stantial distance into the schistose gneiss. In texturally homogenous competent rocks, such as in the microcline-quartz-plagioclase-biotite gneiss of the Central City district, the vein fissures are wide, closely spaced, and persistent, and correspondingly have wide alteration selvages. These features are especially 12 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO evident in the central and intermediate mineralogic zones (pl. 1) but are less marked in the peripheral zone. In contrast, the texturally inhomogeneous, incompetent rocks, which often are complexly folded (such as the migmatites and metasedimentary rocks characteristic of the Freeland-Lamartine district), commonly contain only narrow, moderately spaced and mostly discontin- uous fissures. Also, the brittleness of the competent rocks in Central City district fostered the formation of wide breccia zones, which permitted a pervasive penetra- tion of altering solutions where the fissure veins were close spaced. The incompetent and therefore relatively impervious, foliated wallrock of the Freeland-Lamar- tine district restricted solution flow to the principal fissures. This has resulted in the development in the schistose rocks of narrow, longitudinal altered zones that stand in contrast to the wide, altered zones typical of those bordering the veins near Central City. As may be expected, the altered zones commonly are wider at intersections of veins and along portions of the veins where movement along irregularities in the surface of the fissure has caused openings. Cross fis- sures are not common, but joints and foliation surfaces at places were paths for limited movement of solution outward from the vein fissures. However, alteration effects are generally weak along such cross-breaking features. ALTERED ROCK MINERALS PRIMARY AND SECONDARY MINERAL STABILITY Quartz, K-feldspar, and muscovite are most resistant to weathering and hydrothermal alteration, and biotite, plagioclase, and amphibole are progressively less resistant and more quickly and completely altered. Generalized ranges and amounts of minerals in fresh and progressively more altered rocks in the central part of the Front Range mineral belt (fig. 3) indicate the relative susceptibility of minerals in these rocks to alteration. Generally, the relative susceptibilities of the minerals follow Goldich's (1938) sequence of weathering susceptibility. The ranges of minerals in individual rocks are given later in this report. Quartz, Si0;, commonly constitutes about 30 percent of the rocks, and occurs uniformly across the altered rock zones. It occurs in anbhedral, strained, medium- size crystals, and is altered only in the sense that some of it is commonly recrystallized in zone 4, with a loss of undulatory extinction and the formation of smaller, complexly interlocking grains. K-feldspar, a potassium-rich alkali feldspar but not a pure potassium feldspar, generally constitutes less than 10 percent of the rocks in which it occurs; it is chiefly microcline (fig. 4) in the Precambrian rocks and is orthoclase or cryptoperthite (fig. 5) in the Tertiary intrusive rocks. K-feldspars in microcline-quartz- plagioclase-biotite gneiss and in pegmatite commonly are slightly 'perthitic, and according to Sims (written communication, 1960) contain approximately 20 percent Na-feldspar in blebs and solid solution. In general, the K-feldspar occurs as white to salmon-pink, subhedral to anhedral, tabular, well twinned phenocrysts ranging in size from 0.02 to 1.5 mm. Microcline also occurs in blebs in antiperthite crystals. K-feldspar apparently is unaltered through zones 1, 2, 3, and 4 (fig. 6). At places, small amounts of clay minerals incipiently and randomly replace K-feldspar along crystal margins, cleavages, and fractures. However, such replacement may be of supergene origin. X-ray examination of hand-picked white clay replacing K-feldspar in a pegmatite dike revealed the presence of kaolinite, illite (or sericite), and a minor amount of montmoril- lonite. Sparce amounts of anhedral adularia(?) enclosed by clay minerals occur in zone 4 in some of the rocks; however, this phase has not been identified as a prominent product of wallrock alteration in these areas. Muscovite, K(OH);Al;Siz0;,, present in biotite- muscovite granite and sillimanite-bearing metasedi- mentary gneisses in amounts less than 5 percent, is unaltered in nearly all of the alteration zones. This 2M, dioctahedral mica occurs in elongate subhedral cleavage plates (fig. 44), which may be partly recrystal- lized to coarse sericite grains in zone 4. Biotite, which according to Winchell and Winchell (1951, p. 373) may have a general composition K(OH),(Mg,Fe,Al);(Si, Al),0;», makes up as much as 20 percent of some rocks, and has an intermediate range of stability in comparison with other rock-form- ing minerals when altered in the hydrothermal environ- ment. The biotite is trioctahedral (table 5), and occurs as elongated cleavage plates (fig. 7) that are generally brown to black, or less commonly, a greenish brown. Biotite alters to a variety of alteration prod- ucts. In zone 4 it is often completely and pseudo- morphically altered to clay mineral-sericite aggregates. In many rocks the boundary between zones 3 and 4 is sharp; on one side of the contact, biotite is relatively unaltered, but on the other side it is completely altered. ALTERED WALLROCKS 13 ALTERED ROCK ZONES MINERALS 1 2 3 4 Vein Quartz tie K-feldspar ¥ Muscovite one BiOtite ITT i 374 i 3 seule Plagioclase Tiida LLE E Hornblende HL regs an I Garnet Magnetite-iImenite Hematite Pyrite * Apatite Sphene Montmorillonite e rr had tT tora als. Chlorite Mixed-layer clay Illite Kaolinite es {s 3. Y - Sericite la rer WA, Leucoxene Limonite Calcite | ae eras mbes * Accessory pyrite range is dotted (sparse), secondary pyrite range is dashed EXPLANATION O0 to 5 percent Partial or complete recrystallization 5 to 10 percent o -- --- Broken line indicates gain 0 ~~. o= >10 percent or loss of minerals FIGURE 3.-Generalized ranges and amounts of the most common minerals in fresh and altered wallrocks. 14 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO A, Zone 2-3; biotite-muscovite granite, Jo Reynolds mine; quartz (Q), microcline (ks), biotite (B), muscovite (M), and magnetite-ilmenite (mt) are unaltered; plagioclase (P) is altered partly to clay minerals, and biotite (not shown) is altered partly to chlorite; crossed nicols, 46%. B, Zone 3-4; microcline-quartz-plagioclase-biotite gneiss, Essex mine; C. Zone 4; biotite-muscovite granite, Nabob mine; quartz (Q) and quartz (Q) and microcline (ks) are unaltered, plagioclase (P) and microcline (ks) are unaltered, plagioclase (P) is altered to sericite biotite (B) are altered to clay minerals (cl), sericite (se) and an (se) and minor clay minerals (cl), biotite (B) is altered to sericite unidentified iron oxide; crossed nicols, 90%. (se), pyrite (py), and calcite (c); crossed nicols, T6X. FiGURE 4.-Photomicrographs of fresh and altered microcline-rich rocks. ALTERED WALLROCKS 15 A, Zone 1; porphyritic quartz monzonite, Silver Age mine; quartz (Q), plagioclase B, Zone 1; porphyritic bostonite, Diamond Mountain mine; unaltered K-feldspar (P), hornblende (not shown) are unaltered phenocrysts; magnetite (mt), sphene (ks) phenocrysts in quartz (Q) and weakly altered plagioclase (P) groundmass; (sp), and clay minerals (cl) are included in the groundmass; crossed nicols, 60X. crossed nicols, 130%. C, Zone 3-4; porphyritic bostonite, Banta Hill mine; plagioclase (P) phenocryst D, Zone 2-3; porphyritic bostonite, Banta Hill mine; altered zoned plagioclase (P) altered in part to clay minerals (cl); crossed nicols, 130%. phenocryst in partially altered groundmass; crossed nicols, 130%. FIGURE 5.-Photomicrographs showing the manner of alteration of phenocrysts in Tertiary intrusive rocks. 16 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO TaBu® 5.-X-ray data from biotite separated from zones of progressively more altered grano- diorite, Hayes and Wheeler mine [Fe/Mn radiation: s, strong; m, medium; w, weak; vs, very strong; b, broad lines, d, diffuse lines Sample in zone shown- 3¢ 4 45a d ang- Index stroms d ang- stroms d ang- stroms Index d ang- stroms d ang- stroms vstb).. r m gn go mto on orm O go go pon im ayer go go go 82% 19 59 po po po po po go usgeucess S10 % 1.68 Mica 10 A O -_- __ lllite (with less K 16g _ and more HzO) _L Mg(OH);' (Brlacitelike) Chlorite 14 A Ca, Na, and HzO -+- C) gogggéqggé 5.82 SHsriy - t 1 1 1 + Na+! «montmorillonite 12 A (1 water layer) Ca+2 -montmorilionite 15 A (2 water layers) U 1 1 1 1 t 1 1 A a FIGURE 11.-Diagrammatic sketch showing the structure of some three-layer clay minerals (after Grim, 1953). The indices of refraction characteristic of these clay materials obtained from oriented aggregates of clay minerals are shown in table 6. The particle size of the clay mineral is below the effective resolution size for the microscope; hence the image viewed usually is that of an aggregate of clay minerals. Grim (1953, p. 294) notes, therefore, that, "* * * the identification TaBus 6.-Mean indices of refraction for oriented mized-clay mineral and sericite assemblages in altered rock zones Index of | Altered Clay mineral ! refrac- rock Host rock Sample tion 2 zone Montmorillonite. .... 1. 542 2 | Microcline-quartz-plagio- 22 clase-biotite gneiss. 2 | Granodiorite 125 2 41 3 43 3 44 2 80a 3 79a 3 d 78 3 | Microcline-quartz-plagio- 58 clase-biotite gneiss. 3 21 3 261 3 242 G 79 V | Microcline-quartz-plagio- 51 clase-biotite gneiss. 4 | Biotite-quartz-plagioclase 28 gneiss. ' Indicates most conspicuous clay mineral constituent in the aggregate. 2 Mean value for « and y indices observed normal to 00} face. of clay minerals in thin section is usually difficult and often impossible * * *," although he feels (p. 293) that, "The petrographic microscope is * * * useful in studying the texture of clays * * *." The X-ray determinations, therefore, are combined with optical studies to form the basis for the following general O Oxygen ® Hydroxy! . Aluminum @O Silicon FIGURE 12.-Diagrammatic sketch of the two-layer structure of kaolite (after Gruner, 1932). ALTERED T grams 154 t otra T TCT fr -I V; Sample 58a Zone 2-3 (3a) Cu/Ni radiation cna. Gfarm 14.72 - Sample 60 Zone 2 Cu/Ni radiation 14.72 3.23 3.18A | 3.3 \lj 3.56 # 9 6 A < to w C/A gas! "Sue rls what N2 20 1 1 1 1 1 1 COL. 3 14 LL. 30 20 30 20 10 DEGREES 20 DEGREES 26 A B [C4 4.11 LLA FIGURE 13.-Smoothed X-ray diffraction traces of oriented montmorillonite-rich clay mineral assemblages (typical of zones 2 and 3a) that occur in altered microcline- quartz-plagioclase-biotite gneiss, Essex mine: montmorillonite, kaolinite, mixed- layer clay, illite, and chlorite are present. A, untreated; B, ethylene-glycol treated; C, heated to 500° C. discussions of the stability of clay minerals. The methods of X-ray identification and estimation of rela- tive amounts of clay minerals used in this study are described by Weaver (1958). The typical montmorillonite of the alteration zones bordering the veins in the central part of the Front Range mineral belt is indicated by X-ray studies to be chiefly of the calcium type (Ca as prime interlayer ion), which when air dried has interlayer water two molecular layers thick (fig. 11). The mineral is most common in zones 1, 2, and 3a, but it is found in small amounts in zone 4 as well. Weakly to moderately altered rock is characterized by the replacement of plagioclase crystals by a cream-white to greenish clay mineral that is primarily montmorillonite. Figure 13 shows the typi- cal X-ray diffraction patterns of a montmorillonite- rich clay assemblage in zones 2 and 2-3. The X-ray patterns are partly smoothed by reduction in scale causing the deletion of very minor extraneous instru- mental responses. Curve A is a trace of a separated but otherwise untreated clay-mineral assemblage. Curve B is the X-ray diffraction trace of the same sample after ethylene-glycol treatment, which expands the unit-cell c-dimension component and helps to re- solve the identity of illitic and montmorillonitic com- ponents. Curve C is a trace of the same sample subsequently X-rayed after heat treatment at 500° C. Incipient formation of clay minerals on cleavage surfaces of plagioclase in least altered rock (zone 1) is not readily observable in hand specimen. In thin section the twinned plagioclase crystals are mottled with specks and small irregularly shaped indistinct WALLROCKS I 20 mineral masses. - The index of refraction of large aggre- gates in thin sections is less than that of the plagioclase; birefringence is very low; and color in plane and polarized light is gray. - The most abundant clay miner- al is montmorillonite and it is formless, but the ag- gregates on most crystals are alined with the cleavage or twin planes. When observed on flat twin surfaces, the clay appears randomly distributed and exhibits no discernible crystallographic orientation. In zone 2, the aggregates of montmorillonite-rich clay materials are enlarged and at some places complete- ly cover the crystal surface; however, the plagioclase twinning, though obscured, is still visible. As the clay becomes more prominent it forms darker brown translucent masses that coalesce and broaden along cracks and along cleavage planes in the crystal. The earliest formed clay minerals appear to be replaced by a clay having a slightly higher index of refraction (> plagioclase) and higher birefringence that is possibly partly illitic in composition. Individual clay flakes are larger near the boundary with zone 3, and the altered crystals are cut by ramifying veinlets of clay. In zone 3 the clay veinlets widen, and the entire crystal is replaced. Not all of the early, low-index material is completely changed over, and "island" remnants remain to give the clay pseudomorphs a mottled appearance. In zone 4 most of the clay minerals are lost, having been replaced by sericite. Chlorite, (Mg,Fe.Al), (OH); (Si, Al),0;,, is not a com- mon secondary clay mineral; however, it was observed in thin section as an alteration product of hornblende in zones 1 and 2, and in part, of biotite in zones 3 and 4. Chlorite occurs about edges and along cleavages of these minerals (fig. 7), and it is recognized by the light pleochroic green-yellow color, low index of refraction, and low birefringence. In intensely altered rocks, chlorite is in turn replaced by the micalike clay min- erals-montmorillonite, illite, and mixed-layer clays- and ultimately in zone 4 by sericite. The X-ray trace of a sample of chlorite heated to 500° C is shown in figure 13, curve C, sample 58a. The 7A peak persists at a lower intensity, and the 14A peak emerges, follow- ing the collapse of montmorillonite 001 spacing to that of the dehydrated phase close to 10A. In general, chlorite is not an abundant product of argillic alteration in these rocks, and commonly is obscured by the more prominent clay phases. Illite, general composition K; ;; (OH) (Ra**,M5+*)Sig 57 Al ;0;» where R=Al ', Fo", or Ti", and M-Mg"~* Fet, or Lit! (Yoder and Eugster, 1955, p. 252), is the mica-type clay mineral with nonexpandable lattice. The optical properties for illite are similar to those for montmorillonite, and illite is texturally indistinguish- 26 ALTERED WALLROCKS, CENTRAL PART OF fm ef "Tt " F -T ta- rC Zone 3 Cu/Ni radiation I Sample 58b 3 o «- 14.7 3.35 < 00 C m .": 5.00 A 30 20 10 DEGREES 2 0 FIGURE 14.-Smooth X-ray diffraction traces of an illite-rich clay mineral assemblage (typical of zone 3b) that occur in altered microcline-quart?-plagioclase-biotite gneiss, Essex mine: illite, montmorillonite, mixed-layer clay and kaolinite are present. A, untreated; B, ethylene-glycol treated. able in thin section from other similar clay minerals. X-ray studies, especially of material treated with ethylene glycol or heated prior to study, clearly show the unexpanded 10 A basal order sequence and its predominance in zone 3 (fig. 14); illite occurs through- out the zones of altered rock in less abundance. Yoder and Eugster (1955) describe material that previously was believed to be illite actually to be related instead to 1Md or 1M mica polymorph crystallization or to mixed-layer clay mineral type. It was not possible to determine polymorphic form of mica in these analyses, thus the name illite was retained to describe conven- iently clay material whose nonexpanding X-ray spac- ings are more diffuse than those for sericite or mica, and whose composition may be intermediate between montmorillonite or chlorite and biotite. Mixed-layer clay minerals result from an interstrati- fication of layered clay minerals on the order of a single or a few sheets. Mixed-layer structures thus formed are as stable as those composed of symmetrically layered alumino-silicate sheets because of the close similarity between the structures of individual clay THE FRONT RANGE MINERAL BELT, COLORADO minerals (fig. 11). The composition of the mixed-layer clay is of course regulated by the composition of the constituent mixtures of clay minerals. Mixing, dis- cussed in detail by Weaver (1956), is of two types: (a) regular interstratification along the c axis resulting in a unit cell that is equal to the sum of component layers, allowing regular 001 reflections; or (b) random inter- stratification or irregular mixing, which results in a nonintegral sequence of 001 reflections. Mixed-layer clays are common in altered wallrocks in zones 1, 2, and 3 but are identified only by X-ray studies. Since mixed-layer clays were not specifically identified optically, their textural importance cannot be discerned. - Both irregular and regular mixed-layer clays occur in altered wallrocks in zones 2 and 3 but the randomly interstratified clays are most common; how- ever, mixed-layer clays are absent or in very small amounts in zone 4 rock. The precise determination of the mixed-layer type is difficult in the rapid X-ray scan used, but in figure 15 examples of mixed layering are indicated. The montmorillonite and illite reference basal reflections are shown under each trace and an asterisk indicates a change from lower to regular sensitivity for subsequent peaks. Kaolinite, (OH)sAL,Si,0,,, is most abundant in zone 3¢c as a sparse alteration product of K-feldspar or biotite, and more commonly as a replacement for earlier formed clay minerals. It occurs also in zones 1 through 4 in minor amounts. Zone 3 clays are generally white and pseudomorphically replace host- rock feldspars. Kaolinite is inferred in thin section by the presence of low index and low birefringent clay replacing montmorillonite and possibly biotite in the boundary region between zones 3 and 4. Measure- ments of indices of refraction of oriented clay-mineral aggregates rich in kaolinite from zone 3¢ are shown in table 6. The index values, slightly higher than those of Grim (1953), reflect the intermixing of clay minerals (fig. 16) common to this type of replacement. The sample shown in curve ¢, figure 16, was heat treated at 300°C. Kaolinite structure persists. Treatment at 500°C destroys the kaolinite structure, but not that of chlorite, which is a phase commonly obscured by the kaolinite X-ray pattern. Halloysite, (OH)sALSi,0;,, with basal spacing close to 7.5 A occurs sparsely in zone 3¢ and more abundantly in zone 4 of some rocks (in altered garnet-quartz gneiss). It is closely related to kaolinite in crystal structure as well as in mineral texture and appearance in thin section. The X-ray data indicate that the mineral was partly hydrated when examined (Grim, 1953, p. 54). Sericite, according to Winchell and Winchell (1951, p. 369), "is a fine scaly or fibrous kind of muscovite; ALTERED WALLROCKS the name is usually confined to white mica which is secondary." Yoder and Eugster (1955, p. 255) con- sider sericite to be a fine-grained white mica, whereas Wright and Shulhof (1957) class sericite as a clay mineral. In this report sericite is defined as a second- ary white mica within the clay-mineral particle size range which yields a well-crystallized mica X-ray pattern. X-ray diffraction data for a typical sample of sericite (table 7) indicate that it is a dioctahedral, 2M mica. Sericite is most common in zone 4 rock, in vein gouge clay, and in altered brecciated wallrock included in the vein. A transitional boundary between zone 3 illite and zone 4 sericite is difficult to delineate at Sample 22 Zone 3 Montmorillonite, kaolinite, illite, and mixed- layer (montmorillonite-illite) clays 14.48 Untreated i I iG o m a on w -o oil pubt o _ o o 56 ~ 0 wil O - «ia 1 ta 14 14 rd na : 1 1 a Ethylene glycol treated 4 j ou Fash , " Sample 58¢ Zone 3-4 Cu/Ni radiation 3.57 14.47 12 A < < 92 'W : Bass s * mols U II H AA /l\ C-_—A__I' Y NIS "N+. *A yen # mir -u sj m lr - j _: .l M: _ 30 20 10 DEGREES 2 0 FicuRrE 16.-Smoothed X-ray diffraction traces of a kaolinite-rich clay-mineral assemblage (typical of zone 3¢) that occurs in altered microcline-quartz-plagioclase- biotite gneiss, Essex mine: kaolinite, montmorillonite, mixed-layer clay, and illite are present. A, untreated; B, ethylene-glycol treated; C, heated to 300° C. most places. Sericite in zone 4 is in part a replacement of plagioclase, K-feldspar (perthitic variety), and bio- tite following their alteration first to clay minerals (fig. 17). Typical sericite textures are shown in figure 18, and result from randomly placed radial clusters, oriented elongated bunches of flakes, and random patches. Handpicked feldspar grains from granodiorite in zone 1 that contained material identified microscopi- cally as sericite was crushed, the clay-sized fraction separated, and X-rayed. This material (fig. 19) con- sists of a mixture of illite, montmorillonite, chlorite, and random mixed-layer clay. X-ray sensitivity was about two times greater than normal to obtain these traces. EXPERIMENTAL STUDIES OF MINERAL STABILITIES Recent experimental synthesis data may be useful as an aid to interpret geologic data, and several reviews THE FRONT RANGE MINERAL BELT, COLORADO of the literature by Ingerson (1955@, 1955b), Yoder (1955), Lovering (1955), and Kerr (1955) present a wide selection of such data. However, many of these data are of limited value for geologic interpretation because mineral stabilities are considered in terms of bulk compositions of sclids and as simple hydration- dehydration reactions (such as Roy, 1954) rather than as they occur in nature as assemblages in equilibrium with a single or succession of solutions. J. J. Hemley (written communication, 1960) points out that in the alteration process it is the solutions that do the work; these solutions move in an open system thus approaching equilibrium in a succession of local states over a range of chemical conditions. His experimental work (1959) shows that the Kt*/H* activity ratio and temperature are the most important controls on the fields of stability in the K,0-Al;0O;-Si0:-H.0 system. At a given tem- perature and with increasing K+/H* activity ratio, the fields of kaolinite, mica, and K-feldspar are suc- Sample 62 Zone 4 Cu/Ni radiation 3.32 A 10.04 - 4.97 w rsd 2 < m m a w < w C pop f f _t Spf n t { f piel. [f l o 30 20 10 DEGREES 2 0 FIGURE 17.-Smoothed X-ray diffraction traces of sericite in most intensely altered microcline-quartz-plagioclase-biotite gneiss, Essex mine: montmorillonite and kao- 'linite are trace constituents. A, untreated; B, ethylene-glycol treated. ALTERED WALLROCKS 29 FIGURE 18.-Photomicrographs illustrating the textures of sericite. A, Zone 4; microcline-quartz-plagioclase-biotite gneiss, Alma-Lincoln mine; quarts (Q) is unaltered, sericite (se) replaces plagioclase; area within rectangle is enlarged in B; crossed nicols, 130X. B, Enlargement of rectangle area of A, showing radial clus- ters of bladed sericite crystals in detail; crossed nicols, 320X. C, Zone 4, micro- cline-quartz-plagioclase-biotite gneiss, Essex mine; microcline (ks) and quartz (Q) are unaltered, sericite (se) replaces plagioclase; area within rectangle is enlarged in D; crossed nicols, 130X. D, Enlargement of rectangle area of C, showing elongate parallel bundles of sericite blades separated by clay minerals and quartz; crossed nicols, 320X. E, Zone 3-4; biotite-muscovite granite, Jo Reynolds mine; quartz (Q) is unaltered, the left side of the plagioclase crystal (P) is partly replaced by large, random, sparse sericite (se) or mica (mi) flakes, and the right side is almost completely altered to oriented bundles of sericite blades separated by clay minerals (C1) and quartz(?); crossed nicols, 130%. 30 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Sample 125 Sample 4 Zone 1-2 Zone 1 m ACF. toft 141 Untreated < f a && C iiif } Ethélene glyébl f -f f sf Sla et ot. fe 4-2 fa te te- {at et _ I cnl d 30 20 10 30 20 10 DEGREES 20 DEGREES 2 0 A B FIGURE 19.-Smoothed X-ray diffraction traces of clay minerals separated from fresh granodiorite by the ultrasonic separator: illite, montmorillonite, chlorite, and mixed-layer clays are present. cessively traversed. With constant Kt/H* ratio and increasing temperature, the same sequence may be obtained. MINERAL STRUCTURES AND THEIR INHERITANCE Mineral structures and the possible inheritance of structures are of limited value to thermodynamic considerations of equilibrium chemistry, yet they are significant for the understanding of mineralogical- chemical relations in alteration processes. This aspect of the alteration process will be emphasized, but an attempt will be made to relate these data to the thermo- dynamic considerations of Hemley (1959). The basic factors governing the spatial and energy relations between ions in silicate minerals are well summarized by Hauth (1951) and Henderson (1951). Of funda- mental importance in the following discussion is the observation of the large size of the O * ions compared with Ca+, Alt, and Sitt ions. - Silicate crys- tal structures are best described as regular close pack- ings of the larger oxygen ions with associated smaller ca- tions in the interstices. Mason (1952, p. 219) points out that dense structures, such as those composed of single tetrahedra, are generally more economical of space than are framework structures having a more open lattice. This explains, in part, why large ions may be replaced by smaller ions during rock alteration with- out loss of volume. The concept that parent mineral structures exert an influence over subsequent secondary mineral structures is confirmed in part by the textures of certain altered rocks seen in thin section, that is, the formation of clay minerals and sericite along certain crystal direc- tions in altered feldspar (fig. 8) and mica (fig. 7) crystals. The persistence of K-feldspar in the kaolinite argillized zone demonstrates that the arrangement of ions in stable mineral structures may impede formation of the normally expected chemical equilibrium assem- blages. Bradley and Grim (1951) have demonstrated the structural lineage in the thermal decomposition reaction of kaolinite to mullite. However, it has not been possible to demonstrate conclusively with lab- oratory methods that there is or is not structural continuity in the alteration of feldspar to clay mineral (Brindley and Radoslovitch, 1956). ALTERED WALLROCKS ALTERATION OF REPRESENTATIVE WALLROCKS The various types of igneous and metamorphic rocks in the region have altered differently, mainly because of differing mineral makeup. Six of the rocks ® or groups of rock are considered representative of those in the area: (1) biotite-muscovite granite and micro- cline-quartz-plagioclase-biotite gneiss, (2) granodiorite and biotite-quartz-plagioclase gneiss, (3) quartz diorite, (4) amphibolite, (5) garnet-quartz gneiss, and (6) bostonite. In addition to the differences between rock types, there is variability within each rock type, as is shown in figure 20. The rocks diagrammed in figure 20 may be classified into two general groups: (1) rocks in which K-feldspar is a major constituent, and (2) those in which it is a minor constituent or only an accessory. Biotite and plagioclase are constituents in both groups of rock, but hornblende is abundant only in the K-feldspar deficient rocks. BIOTITE-MUSCOVITE GRANITE AND MICROCLINE- QUARTZ-PLAGIOCLASE-BIOTITE GNEISS These K-feldspar-bearing rocks are the most abun- dant rock types observed; the granite and its altered phases were sampled along lead-zinc-silver veins in the peripheral ore zone in the Freeland-Lamartine and Lawson-Dumont-Fall River districts, whereas the microcline-rich gneiss was sampled along typical veins in the three ore zones and in all of the mining districts. The fresh biotite-muscovite granite is a medium-gray to pink generally medium grained rock composed of plagioclase, K-feldspar (microcline, and perthitic inter- growths in plagioclase), quartz, biotite, and muscovite; sphene, apatite, zircon, magnetite-ilmenite, and pyrite are present in minor amounts (table 8). _ Clay minerals, hematite, leucoxene, and chlorite are sparse secondary minerals that locally replace plagioclase, magnetite- ilmenite, and biotite in even the freshest rock. Weakly argillized granite is hard and possesses a characteristic granitic texture. In thiT) section, plagio- clase is seen to be converted partly to clay minerals found by X-ray diffraction studies to be montmoril- lonite and illite. Biotite is partly replaced by chlorite (fig. 7¢), clay minerals (table 9), iron oxide, and locally by minor amounts of epidote. K-feldspar and musco- vite are not visibly altered in zone 2. The stability ranges of the primary and secondary minerals of the biotite-muscovite granite and their relative abundance in this and subsequent zones are shown in figure 21. 3 The specific rock names used in this report conform to those selected by coworkers who mapped these units on the surface and underground, and whose reports this paper is meant to amplify. Therefore the names of rockdedescribed in the text are based on average compositions of the rock determined elsewhere in the region by modal analyses of a large number of specimens taken from a multitude of rock out- crops. 31 TABLE 8.-Modal analyses, in volume percent, of fresh and altered biotite-muscovite granite, Nabob and Jo Reynolds mines [Chemical and spectrographic analyses; samples 1, 2, and 3 are composites of 138 139, 140, 141a, and 141b] j Zones Mineral Sample................ Quartz. ..- Plagioclase K-feldspar. Biotite... Muscovite. Sphene.... Apatite. Zircon....... Hematite Calcite...... Total.... sex Points counted........ 1 Mostly clay splotches in recognizable plagioclase grains. Localities where samples were collected: 1412, Jo Reynolds mine, about 114 feet into hanging wall. 141b, Jo Reynolds mine, adjacent to 141a, but closer to vein. 138, Jo Reynolds mine, crosscut away from main vein zone. 140, Jo Reynolds mine, crosscut away from main vein zone. 154, Nabob mine, rock in horse between vein splits. 139, Jo Reynolds, red alteration stain on K-feldspar. 136, Nabob mine, along main vein zone. 143, Jo Reynolds, silicified zone 1 foot thick on hanging wall. Strongly argillized granite in zone 3 is a moderately hard rock with recognizable original rock textures in which clay minerals almost completely replace plagio- clase and biotite crystals. The large amounts of unaltered quartz, muscovite, and K-feldspar, which remain in zone 3 (fig. 4B), preclude softening of the altered granite as is commonly observed in other strongly argillized rocks. Scattered pyrite and dis- seminated blebs of calcite occur sparsely throughout the rock. TaBus 9.-Clay minerals and sericite representative of altered- biotite-muscovite granite, Diamond Mountain and Nabob mines [Relative amounts of constituents: s, sparse; m, moderate; a, abundant; va, very abundant; vs, very sparse; tr, trace] Clay minerals Altered- rock Bericite | Sample zone Mont- | Mixed- Kao- moril- layer + | Chlorite | Illite linite lonite $e r 156 m...... $r(?). :.. 151 M.. en 154 Mole 155 $.2.¢.--. fr(?).... 1302 131 ! Mixed-layer clay minerals are predominantly of montmorillonite-illite type. 2 Mixed-layer clay minerals of illite-montmorillonite type. Localities where samples were collected: 156, Nabob mine, wallrock included in vein zone. 151, Nabob mine, footwall gouge clay. 154, Nabob mine, silicified rock adjacent to a subsidiary vein. 155, Nabob mine, greenish silicified zone adjacent to main vein zone. 1302, Diamond. Mountain mine, rock from hanging wall. 131, Diamond Mountain mine, altered granite 6 in. into hanging wall. 32 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Quartz Quartz ® This report % Harrison and Wells (1959) + Sims, Drake, and Tooker (1963) (includes quartz monzonite and quartz diorite phases in body) e This report 0 Harrison and Wells (1956) K- feldspar Plagioclase K- feldspar Plagioclase A. Biotite-muscovite granite B. Granodiorite Quartz Quartz e This report a Harrison and Wells (1959) & Sims, Drake, and Tooker (1963) e This report © Sims, Drake, and Tooker (1963) K- feldspar Plagioclase K- feldspar Plagioclase C. Quartz diorite D. Microcline-quartz-plagioclase-biotite gneiss Quartz Quartz ® This report + Harrison and Wells (1956) 4 Harrison and Wells (1959) © Sims, Drake, and Tooker (1963) e This report + Harrison and Wells (1956) K-feldspar Plagioclase K- feldspar Plagioclase E. Biotite-quartz-plagioclase and related gneisses F. Amphibolite FIGURE 20.-Triangular diagrams showing variations in mineralogic composition of rocks described for this report and of similar rocks examined by others. ALTERED WALLROCKS ZONES MINERALS | Quartz K- feldspar Muscovite Plagioclase --r >.. Biotite Apatite Sphene Magnetite-iIimenite Hematite Pyrite* Calcite Leucoxene Chlorite Montmorillonite ep < r> Illite Mixed-layer clay ‘L Kaolinite Sericite *Sp§rse distribution of accessory pyrite over its range is indicated by dots, - distribution of secondary pyrite is indicated by dashed line EXPLANATION --X-- Partial or complete recrystallization O to 5 percent 5 to 10 percent - - - Gain or loss of 30--_ T mineral g=--_= >10 percent FIGURE 21.-Stability and relative abundance of minerals in the progressive alteration of biotite-muscovite granite. Sericitized granite in zone 4 (fig. 4¢) is a hard rock and is composed chiefly of K-feldspar, sericite, clay minerals consisting of sparse kaolinite, montmorillonite and illite, and partly recrystallized quartz. Residual muscovite and secondary pyrite are present also. Calcite, which occurs in shears and as disseminations in this rock, may have been introduced during the ore-depositing stage. A similar sequence of mineral changes (table 10) is obtained in zone 2 of altered microcline-quartz-plagio- clase-biotite gneiss. In contrast to those in the granite, however, zones 3 and 4 in gneissic rock are wider owing mostly to the greater abundance of unstable minerals such as plagioclase. Strongly argillized gneiss is of an intermediate hardness; clay minerals almost com- pletely replace plagioclase, but biotite retains its tex- tural character (fig. 22) and fresh greenish-black color in zone 3. Quartz and K-feldspar are unaltered. Where examined in the intermediate ore zone at the Essex mine, sericitized gneiss of zone 4 typically is hard and sheared, and contains recrystallized quartz, 33 microcline, sericite, clay minerals, and pyrite (figs. 4C and 7 G, H). Sericite occurs as radial clusters or matted aggregates of elongate flakes in clay minerals pseudomorphic after plagioclase (fig. 18, A-D). Most of this clay-sized material is sericitic in composition. Coarse sericite is most abundant adjacent to the vein and diminishes in abundance and crystal size toward the zone 3 boundary; sericite generally is not present where biotite is unaltered. The clay between sericite clusters possibly is kaolinite or montmorillonite; however, the precise position of kaolinite and mont- morillonite observed in thin sections of these rocks is uncertain. Staining of the sample with sodium cobaltinitrite reveals that K-bearing minerals are somewhat concentrated at the zone 3-zone 4 interface. Thin sections of the rock indicate the presence of micro- cline, slightly altered biotite, and an illitic clay mineral altered partly from plagioclase. Very little chlorite was formed from the biotite; the alteration of biotite is to illite, kaolinite, and sericite. Most modes and thin section observations of altered gneiss from zone 4 show that quartz, although partly recrystallized, is not markedly lost in the alteration process, and the hardness of the altered rock apparently is related to recrystallization rather than introduction of quartz. However, fine-grained quartz is commonly found within shears at vein margins and coating openings along foliation planes, and comb quartz fills some of the more open spaces in the rock. Microcline locally contains a kaolinitelike clay filling along fractures, but otherwise is unchanged. Altered rock zones .\\/ /<{ ay / A /\Rock foliation ' NZ. a[ - 7 - __. defined by unaltered «f XZ biotite flakes Square area etched by HF fumes and soaked in aqueous Na,Co(NO;), solution: K-feldspar and sericite(?) indicated by stippled areas which are stained bright we lightly silicified and 0 0.5 1.0 INCHES pyritized joint t rd o FreuR® 22.-Sketch of small-scale wallrock alteration effects along slightly miner- alized joints in the host rock. 34 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO TaBu® 10.-Modal analyses, in volume percent, of fresh and altered macrocline-quartz-plagioclase-biotite gneiss, Essex mine ire rre be o Yin ca se nese» Belev ab ade el cdc 1 2 3 3-4 4 SAMDIGI SOI cen ce ter Bes obs aw 46 47 48a 63 58 49 64 48 58a 56 Mineral C cer-. ysl 31.2 28.5 32. 2 40.7 Microcline. 14.7 27.9 12.8 17.8 Plagioclase. ... 38.9 24.9 30.8 26.7 (Antiperthite) ! 2.8 17 4.1 6. 4 Blofife........ 6.2 8.3 7.5 3. Hornblende. . 9 BB He Magnetite 3.. 1.3 .8 22 2.2 Ar ANIS LII Une cern ee neb dee bees e dese causes Fy f) Relates cove writen Leucoxene. H PO Apatite... 1.1 .8 .6 4 r-- rie iv ece rans ba siesseb's ies nees H Aer eae eevee duele lay minerals. 2.6 22 6.3 2.2 Calcite 4._. .6 .8 AD e Epidobes PE-S XO.. .cc ania ones -L ink bebe deres ense cen bn YEH eo actus MObALe ses EO Cto sede rou. .. e uou ih oe ae bade t a oen e 99. 9 99. 9 99. 9 99. 9 Points COUnbed.2. . .s 10 1 L. 20 UC an ol need cea d 837 874 500 480 1 Consists of scattered, large Na-plagioclase crystals that contain small, irregular patches of microcline. 2 Unusually biotite-rich layer in gneiss. 3 Magnetite is commonly altered to hematite. * The amount of calcite increases in samples 48a, 48, 49 and 58a due to proximity to ore veins that have calcite in gangue or disseminated in adjacent wallrock. Localities where samples were collected: 46, 3 ft into footwall of vein. 47, 114 ft into footwall of vein. 48a, 114 in. into footwall. 63, 414 in. into footwall. 49, 2 in. into footwall. 58, 8-10 in. into footwall, sheared rock. 64, 314 ft into footwall. 48, About 1 in. into the footwall. 56, 2 in. into footwall of 2-ft vein. 58a, Silicified part of sample 58 (same thin section). GRANODIORITE AND BIOTITE-QUARTZ-PLAGIOCLASE GNEISS These rocks of moderate abundance in the area rep- resent those that contain moderate to sparse amounts of K-feldspar. Fresh and altered mafic phases of the granodiorite were sampled in the M. and M.-Dixie and Wallace tunnels in the Chicago Creek area, and in the Hayes and Wheeler tunnel in the Central City district (pl. 1). Samples of biotite-quartz-plagioclase gneiss from several mines in the districts, including those from the Essex, Banta Hill, and Cherokee mines reported here, were studied. Tapur 11.-Modal analyses, in volume percent, of progressively more altered granodiorite, M. and M.-Dizie tunnel x/ |. ts e dent sa 1 2 3 4 4 124 125 126 127 128 129 32.6 34.2 30.3 30.2 16.9 32.0 24.7 26.1 38. 4 14.7 L2 (erry enn |e ier 6.2 2.1 2.5 1.5 F2 ana 22. (45 21. Z 20.1 19.3 21.8 4.5 o thas § | ' TA e recreate 1.6 A .3 .8 .3 1.3 .3 .3 *) .6 .2 .8 ________________ <4 4.0 1.5 1.9 1.9 2.4 2.3 Be ars | eee al ieee bea 2.6 4.5 28.0 38.7 53.9 45.1 68. & ________________ 2.5 3.8 3.6 1.9 2.9 2 .2 .8 1.2 .9 99. 5 99. 9 99.8 | 100.0 | 100.1 99. & 99. 8 Points counted. ---] 1,359 719 722 522 330 510 312 The fresh granodiorite is a medium- to dark-gray medium-grained, competent rock composed of quartz, plagioclase biotite, K-feldspar, and accessory minerals (tables 11, 12). A compositional layering consisting of alternating biotite-rich and quartz-feldspar rich layers in part accounts for the mineralogical variations (Ob- served in samples) of the rock. Tamu 12.-Modal analyses, in volume percent, of progressively more altered mafic border facies of gramodiorite intrusive, Hayes and Wheeler tunnel UNE IL recent i neue nes 1 2 3b 3¢ 4 css. .L. E 41 42 44a 44b 450 45b Mineral . Cle 24.3 K-feldspar........ 1.2 Plagioclase (An m-). 40.0 Antiperthite !...... .9 Biotite......... 18.7 Clay minerals. . 4.6 Calcite..... tr Chlorite.. 4 MICA 51.2. Magnetite. . 2.0 Pyrite... .6 Hemabibe .s. 0.00 see. Leucoxene.}.... 0C LLE] Fluorite. 11100200000 reocrd ean lees eens Apatite... t+ 2.7 Sphene? ...l. 000. eel corect 4.8 TObAL nll ec 100. 2 Points counted........._.__.___ 1, 045 1-Consists mcstly of microcline blebs in plagioclase. Secondary muscovitelike mica altered from biotite. ALTERED WALLROCKS V4 4-3 3 2 0 5 INCHES Fiau&E 23.-Diagrammatic section of altered granodiorite, Hayes and Wheeler tunnel. - Foliation shown by a long dash representing fresh biotite, a shorter dash represents altered biotite. Dots represent isolated remnants of biotite and no foliation. Approximate zone boundaries are marked by fine dotted lines. Note irregular contacts. The boundaries between the fresh rock of zone 1 and the weakly argillized rock of zone 2 in granodiorite are remarkably gradational and are only slightly less so between zone 2 and the soft, light-green strongly argil- lized rock of zone 3. These gradational contacts con- trast markedly with the abrupt contact between zone 3 and the hard, sericitized rock of zone 4. The bound- aries between zones at the Hayes and Wheeler tunnel (fig. 23) are irregular; and it is believed that the ragged nature of the zone contacts result from the nearly right-angle intersection of the vein and the rock folia- tion, which is concordant with compositional layering. In contrast, at the M. and M.-Dixie mine, where the vein is nearly parallel to the foliation, the contacts between zones are more regular. Granodiorite in zone 2 is moderately hard and ap- pears texturally and mineralogically similar to rock in zone 1; a slight local softening is caused by progressive alteration of plagioclase (fig. 8, C-F), and hornblende into clay minerals. Sphene is altered to leucoxene. The order of abundance of clay minerals, as indicated TABLE 13.-Representative clay minerals and sericite in altered granodiorite, Hayes and Wheeler and M. and M.-Dizte tunnels [s=sparse, m=moderate, a=abundant, va=very abundant, vs=very sparse, ?=uncertain, --=not identified] Clay minerals Altered- rock Sericite | Sample zones | Montmo-| Mixed- | Chlorite Illite | Kaolinite rillonite | layers! 4 42 126 43 127 3 44 45 129 ! Mixed-layer clay mineral, generally random illite-montmorillonite mixtures. ? Separation of illite and sericite in sample on basis of crystallinity as shown by X-ray diffraction peak. Localities where samples were collected: 4, M, and M.-Dixie, fresh wallrock on dump. 42, Hayes and Wheeler, fresh rock 5.5 in. into hanging wall from edge of vein. 126, M. and M.-Dixie, greenish clay zone. 43, Hayes and Wheeler, green clay zone 3 in. into hanging wall from edge of vein. 127, M. and M.-Dixie, white clay zone. 44, Hayes and Wheeler, white clay zone 1.5 in. into hanging wall from edge of vein. 45, Hayes and Wheeler, silicified zone 0.5 in. into hanging wall from edge of vein. 129, M. and M.-Dixie, green clay silicified zone adjacent to vein. 35 by X-ray diffraction patterns is montmorillonite> mixed-layer clay >kaolinite>chlorite (table 13). Biotite generally is unaltered except near microscopic fractures and open foliation planes now filled with calcite. Quartz, microcline, magnetite, pyrite, and apatite are unaltered in zone 2 (fig. 24); however, sphene is almost completely altered to leucoxene. Calcite, which occurs in veinlets and fractures, seems to have been introduced from the vein; some of the Ca,, though, may have been furnished through the altera- tion of the plagioclase and hornblende. Intensely argillized granodiorite of zone 3 is charac- teristically a soft clay mineral-biotite-quartz rock. The boundary with hard, weakly argillized rock of zone 2 is completely gradational, but the boundary with zone 4 rock is a sharp mineralogical-textural boundary but a less abrupt hardness boundary. Zone 3 may be arbitrarily subdivided into (a) soft, crumbly, greenish- white clay mineral-biotite-quartz rock, and (b) moder- ately soft, white clay mineral-biotite-quartz rock which may appear slightly darker gray on the veinward side. ZONES MINERALS | | I K- feldspar i Plagioclase fi mw. Biotite r-» 7! Apatite f I Sphene ‘ Magnetite-iImenite Hematite Pyrite* Calcite Leucoxene Chlorite bales Montmorillonite aterm s I NG Mixed-layer clay Kaolinite Sericite * Sparse distribution of accessory pyrite over its range is indicated by dots, distribution of secondary pyrite is indicated by dashed line EXPLANATION --X-- Partial or complete recrystallization O to 5 percent 5 to 10 percent -- - - Gain or loss of 30: Z mineral >10 percent FIGURE 24.-Stability and relative abundance of minerals in progres- sively more altered granodiorite. 36 The soft greenish-white clay mineral aggregate of subzone (a) consists dominantly of montmorillonite, some kaolinite, and less illite; a mixed-layer (illite- montmorillonite) clay mineral and chlorite are also present in sparse amounts. The whole clay mineral of subzone (b) is dominantly kaolinite. Biotite is not pervasively altered in zone 3 (fig: 7); but where it is partly altered along cleavage planes and at ragged edges it is bleached and replaced by mixed- layer silicates (illite and montmorillonite) and calcite. Chlorite is also present along the edges of some biotite flakes. Quartz is not appreciably altered, but in part is recrystallized; the number of strained crystals decreases, but the content of quartz as determined from modal analysis remains nearly constant across the entire zone of altered wallrocks. Apatite is unaltered, some magnetite is stained with hematite, and pyrite is more abundant than in zone 2. Sericitized granodiorite (zone 4) is a hard, dense, fine-grained, medium greenish-gray rock. The bound- ary with zone 3 is distinct and is marked by the absence of unaltered biotite. The appearance and modal con- tent of K-feldspar and most of the quartz are not changed, but some quartz may be partly recrystallized. Minor amounts of kaolinite and montmorillonite are found in zone 4. . Apatite, hematite, and pyrite are present; however, apatite and hematite disappear toward the vein whereas pyrite increases veinward. Biotite-quartz-plagioclase gneiss, which is similar to the granodiorite in mineralogic composition (table 14) is similarly altered. However, in the eastern part of Tamu 14.-Modal analyses, in volume percent, of fresh and altered biotite-quartz-plagioclase gneiss, 150-foot level, Essex mine Zone(®)s AAU III.. 1 2 2-3 3a - 3c 3-4 4 19 20 21b 2la 22 289 21¢ Mineral ». 38 82 40 30 37 43 40 K-feldsvar. .. 9 2 2 6 3 Plagioclase. Biotite....... Clay minerals Calcite....... Hematite. . Magnetite. .. Points counted......._. 1 2 percent antiperthite included. Localities where samples were collected: 19, Least altered gneiss, 5.5 ft into footwall. 20, Weakly altered gneiss, 4.5-5 ft into footwall. 2110},l Moderately altered gneiss, 3 ft into footwall, but close to unmineralized shear, 21a, Moderately altered gneiss (white clay-biotite rock). 22, Moderataly altered gneiss adjacent to silicified shear, 3-12 in. into footwall of main vein adjacent to silicified zone. 23a, Intensely altered gneiss on hanging wall. 21¢, Intensely altered gneiss (hard, silicified rock) along shear 18 in. into footwall from main vein. ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO the area, and in the peripheral ore zone (pl. 1), a different alteration pattern is evident in these rocks. At the Banta Hill and Cherokee mines, for example, the pronounced argillization phase is absent, and zone 2 grades abruptly into sericitized rock of zone 4. In general the entire alteration zones bounding the veins is narrow, amounting to only a foot or so. In strongly foliated biotite-quartz-plagioclase gneiss at the Essex mine (fig. 25) zone 4 follows certain planes into the rock, and sulfides were deposited farther from the vein than was possible in granodiorite. QUARTZ DIORITE Quartz diorite is not quantitatively an abundant rock and occurs chiefly in the Freeland-Lamartine and Lawson-Dumont-Fall River districts, but because of the high proportion of minerals unstable in the hydro- thermal (and weathering) environment it is more completely and abundantly altered. The fresh rock is dark gray, medium to coarse grained, and has a fair to good foliation where metamorphosed. It is com- posed (table 15) chiefly of quartz, hornblende, plagio- clase (An;s-;,), and biotite (fig. 8); K-feldspar is sparse or absent. The biotite content varies in the fresh rock (Harrison and Wells, 1956, p. 46), and no tendency toward biotitization is implied in table 14 by the appar- ent increase in biotite. Secondary minerals include clay minerals, calcite, chlorite, leucoxene, and epidote. The altered quartz diorite near silver-bearing galena- sphalerite veins is a hard, weakly argillized rock (zone 2) that in turn grades into a broad, soft-green to white, strongly argillized clay mineral-biotite-quartz rock (zone 3). A very narrow, silicified rind (zone 4) adja- cent to the vein contains little or no clay mineral, but abundant sericite. The relative distribution of minerals TABLE 15.-Modal analyses, in volume percent, of progressively more altered quartz diorite, Jo Reynolds and Nabob mines One rel ene ca e dea es ec uue 1 1 2 2 3 4 . ect 3 3 & c & = 3 w d Mineral K-feldspar... Blotife......... to PM, Sx o 8 8 9 E fer =] a a i 1 to or on ~1 - on «5 ~t 00 so im wm co i E ..o ncs coy Clay minerals............_____. Calcite Points 895 500 725 966 807 1 Fine-grained facies. 2 Megascopic estimation. ALTERED WALLROCKS 37. 21a Zone - V V V 4-3¢ 3a 10 CLL 4G .l 21¢ 4-3 21d 3 3-2 1 10 20 310 INCHES E X P L A N A T I 0 N zone commonly containing lenses of sulfides sa za } 2T Fresh foliated rock and unaltered biotite .l.. f $% ( R « n 1 % ** a 6 Disseminated pyrite Sericitized r‘ocks ea _ 2: Partially altered biotite and feldspar rocks Shope of lines indicates trace of foliation FIGURE 25.-Diagrammatic section showing altered biotite-quartz-plagioclase gneiss sample location, 150-foot level, Essex mine, Central City district, Colorado. in the alteration zones (fig. 26) follows the pattern established in other rocks. In the weakly argillized quartz diorite of zone 2, the plagioclase grains are replaced by clay minerals along fractures and twin and cleavage planes (fig. 8). With further replacement these patches and streaks of clay minerals expand until a large proportion of the crystal is enveloped. Hornblende is relatively unaltered in the outer part of zone 2, although there is incipient forma- tion of clays along cracks, edges, and cleavages of the mineral. Toward the vein, however, hornblende becomes progressively more altered and is almost com- pletely replaced by clay minerals at the boundary between zone 2 and zone 3. Biotite and quartz are not visibly altered in zone 2. Montmorillonite is a prominent secondary constituent in zone 2, but mixed-layer (random montmorillonite- illite) clay minerals, chlorite, illite, and kaolinite are increasingly more important constituents in zone 3. The strongly argillized quartz diorite in zone 3, which is unusually wide in proportion to the width of the associated vein, is composed dominantly of clay min- erals and consequently is very soft. Near zone 2 the altered rock is green because of a high content of montmorillonite, but veinward it becomes progressively lighter in color until it is nearly white which results from kaolinite displacing montmorillonite as the dom- inant alteration product. The clay minerals almost completely replace the plagioclase and hornblende; bio- tite is unchanged in the montmorillonite-rich rock and is only slightly altered on the veinward size of the zone. Quartz is unaltered. Of the accessory miner- als, magnetite is partly altered to hematite but apatite is unchanged. R Microscopic examination of the strongly argillized rock shows the clay minerals to replace the primary minerals as intimately intermixed assemblages in which the individual grains are virtually indistinguishable. In some specimens examined, however, the original plagioclase seems to be replaced chiefly by kaolinite, and the hornblende, which is recognized by its tex- tural relations with quartz (fig. 94), is replaced by a mixture of montmorillonite, illite, and random mixed- layer clay minerals. In the veins examined, the intensely altered quartz diorite of zone 4 forms a narrow, hard, dark-brown, partly silicified rind bordering the vein. The biotite and clay minerals are largely altered to sericite; mag- netite is oxidized to hematite, part of which has been replaced in turn by limonite. Quartz persists essen- tially unaltered, but some grains appear to have been recrystallized. Sericite and calcite commonly are also present in fractures and the latter also is disseminated through the sericitized rock. 38 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO MINERALS Quartz K- feldspar Plagioclase errs. Biotite Hornblende rere t C Apatite Sphene Magnetite-iimenite Hematite Pyrite * Calcite Leucoxene Chlorite Montmorillonite Illite Mixed-layer clay Kaolinite Sericite ‘ *Sp_arsg distribution of accessory pyrite over its range is indicated by dots, distribution of secondary pyrite is indicated by dashed line EXPLANATION --X-- Partial or complete recrystallization 0 to 5 percent 5 to 10 percent -- - - Gain or loss of 30:1. . mineral >10 percent FIGURE 26.-Stability and relative abundance of minerals in progressively more altered quartz diorite. AMPHIBOLITE The amphibolite, also a minor rock constituent in the area, is a dark-gray to black, medium-grained rock composed mainly of hornblende, quartz, and plagio- clase; biotite, microcline, apatite, sphene, and epidote (table 16) are minor constituents. The rock occurs mostly as small pods and lenses within metasedimen- tary gneiss units and was sampled in the R.H.D.- McKay and E. Calhoun mines in the Central City district, the Kitty Clyde and Foxhall mines in the Idaho Springs district, the Golconda mine in the Fall River district, and the Dorit tunnel in the Chicago Creek area. This rock commonly occurs also in other mines in the area studied. No distinct weak and strong argillized zones are observed; hornblende and plagioclase are altered far- thest from the vein and biotite persists closer to the vein. Adjacent to veins the amphibolite is extensively altered (fig. 9) to a soft, green rock containing mont- morillonite, illite, mixed-layer clay minerals, and resid- ual quartz (usually characteristic of zone 3). The TaBur 16.-Modal analyses, in volume percent, of altered amphib- olite, R.H.D.-McKay shaft 1-2 3 Sample. F 32 33 Mineral QUATEEL 2 11202000 OLLI .. non Ae dans 12.1 14.4 Plagioclase. 6 Al Hornblende. 48. 2 28.7 Epidote. 2B IAL. Clay minerals 30.0 60. 8 A ies 2.2 1.6 Total 100. 5 100. 5 Points counted ..... .... ... . c. oo cl ll 200 det ll I 350 200 Localities where samples were collected: 32, Weakly altered gneiss approximately 1 ft. into footwall from vein. 33, Moderately altered gneiss on footwall. quartz is not visibly recrystallized. Close to the vein sericite occurs sparsely as large, randomly oriented, fragile, biotite like flakes in the soft argillized rock. GaARNET-QUARTZ GNEISS The garnet-quartz gneiss, which occurs as moderate- sized layers or lenses in the metasedimentary rock se- quence, is a dense, weakly foliated, dark red-brown to black, fine- to medium-grained rock composed of garnet and quartz with biotite, and accessory pyrite, mag- netite, carbonate, and apatite (table 17). The garnet and biotite content is considerably varied from inter- layer to interlayer within this unit. Generally in the garnet-poor layers, biotite and plagioclase are the abun- dant minerals. - Garnet-quartz gneiss was studied only in the Fall River area, and was sampled in the Gol- conda, Almadin, and Mary mines. The alteration of garnet-quartz gneiss varies accord- ing to the rock mineralogy. The normal gneiss contains mostly garnet, quartz, and biotite. A narrow zone TaBu® 17.-Modal analyses, in volume percent, of altered garnet- quartz gneiss, Golconda mine . . 22. LL Ere ren Ue eate Bi bens on 1-2 2-8 4 .._ aces bebes ae neden seems en 380 382a 382b 382¢ Mi I inera $ 21 s 27 26 5 25 Me 2 a Aera ve 7 13 55 4 2 <1 ........ 1 6 2 1 1 . L O2... 9000 teen 100 100 <100 Points counted.. Ls 750 284 374 405 Localities where samples were collected: { P 380, Altered gneiss, 0.7 ft into hanging wall of vein 1.8 ft thick: ; 3822, Altered gneiss, 0.7-1.2 in. into hanging wall of veinlet 0.1-in. thick, 0.7-0.8 ft into footwall of hanging wall vein zone. 382b, Altered gneiss, 0.3-0.6 in. into hanging wall of veinlet. 382¢, Altered gneiss, 0-0.3 in. into hanging wall of veinlet. GEOCHEMISTRY OF ALTERED ROCKS along the vein a few inches thick is changed to a hard bleached quartz-clay mineral rock that locally contains abundant disseminated pyrite; no soft argillized zone is present. - The marked color changes in garnet and bio- tite are from reddish to greenish gray, and black to yellow brown. Some interlayers in the gneiss contain appreciable plagioclase, and these layers are altered to clay minerals as in other rocks. X-ray diffraction studies of bulk samples of altered garnet-quartz gneiss show that near the vein in zone 4 biotite is altered to muscovite like clay minerals and kaolinite, and garnet is partly altered along fractures to a mica-type clay mineral and quartz. - Some secondary biotite also occurs in this zone. Unaltered, '"islandlike", fragments of garnet are replaced locally by calcite near the vein. Thin-section studies indicate that biotite may be in- cipiently altered prior to alteration of the garnet, but garnet crystals are altered completely before all bio- tite grains are changed. Magnetite alters to pyrite. Quartz is unaltered, but near the veins and fractures fine-grained quartz is introduced or is the product of recrystallization. BOSTONITE Bostonite, which occurs mostly in dikes, is a lilac- to reddish-brown rock that contains abundant galmon-pink K-feldspar and sparse pyroxene phenocrysts in a fine- grained matrix consisting predominantly of plagioclase. The rock has been described by Phair (1952) and Wells (1960), and was sampled by the writer in the Mammoth, Banta Hill, Phoenix, and Diamond Mountain mines. Bostonite is not as abundant in the central and inter- mediate ore zones as it is in the peripheral ore zone; where it is cut by veins in these ore zones commonly it is bleached light gray to white, and the feldspar phenocrysts are softened and changed in color to green or chalk white. The matrix of the bostonite is com- posed largely of plagioclase which is completely altered to a soft white aggregate composed mostly of sericite and pyrite. Quartz is recrystallized. The K-feldspar phenocrysts are only partly altered. Magnetite is re- placed by pyrite. Adjacent to the vein, a narrow hard silicified and pyritized rind is common. In the eastern peripheral ore zone area, and specifi- cally in the Banta Hill mine, the prominent soft clay- mineral zone was not observed in the altered bostonite. In alteration zones 2 and 3 along the Banta Hill vein the matrix and feldspar phenocrysts are argillized but the rock is not softened, and the magnetite is altered to pyrite. Zone 4 is a granular sericite-pyrite rock which faintly retains the original porphyritic texture. GEOCHEMISTRY OF ALTERED ROCKS Hydrothermal alteration bordering vein deposits has changed the relative abundance and distribution of ele- 39 ments in wallrock minerals. In the discussion of the geochemistry of altered wallrocks the data are con- sidered in the light of three fundamental observations: (1) that the atomic structures of rock minerals con- tribute to physical-chemical behavior through (a) the unique character of constituent ions or atoms, (b) the bonding between ions, and (c) coordination possibilities between ions as set forth in Pauling's rules (Hauth, 1951); (2) that in the rocks close-packed oxygen ions, whose size is large in comparison with Ca+, Mg*, Fet, Al+} and Sit* ions, constitute as much as 92 percent by volume, and the corollary that the remaining cations and anions fill the interstices; and (3) that the partial or complete replacement of ions in a crystal structure is common in oxygen-bearing rock minerals and is regu- lated by the space requirements of atoms and ions, the temperature, ionization potential, and the crystal structure (Rankama and Sahama, 1950, p. 125). In addition, in most environments Si+t* combines with O~> to form silica tetrahedrons, (Si0,)~*. - The construc- tion of silicate minerals is based on the variable manner in which the sheets of silica tetrahedra can be joined to other silicate sheets by sharing one or more O- (Hender- son, 1951). The background data for the discussion of ion position and ion replacements in crystal structures and the average distribution of ions in wallrocks and minerals has been drawn mostly from Bragg (1937), Rankama and Sahama (1950), Mason (1952), and Green (1953). Consideration of the chemistry of altered wallrocks is facilitated by an arbitrary division of the subject into two parts: (1) the geochemistry of the elements that are the major constitutents of the alteration min- erals, and (2) the "trace element" constituents occluded with the alteration minerals. The "major constituent" elements have been determined by conventional meth- ods of chemical analysis, and the "trace element'" constituents have been determined by semiquantita- tive spectrographic methods. The powder pH of a few representative rocks also has been determined. MAJOR CONSTITUENT ELEMENTS Chemical analyses of unweathered wallrocks and their altered equivalents provide a basis for calculating the abundance and distribution of elements in progressively more altered rocks. The major distribution. trends observed in these studies are the veinward addition of K+, Fet (and total iron), C+, H*, S-, and less com- monly Alt ions concurrent with losses in Sit, Nat, Ca+, Fet, and Mg** ions. METHODS FOR PRESENTING CHEMICAL ANALYSIS DATA Several methods have been used for the rapid visual comparison of chemical data. - Gravimetric weight per- centages of oxides in the fresh and altered wallrocks 40 (table 18) are not readily adaptable to a discussion of ion movement and are not easily compared with spectrographic data (figs. 36 to 62). Many writers have modified oxide weight-percentage analyses, al- though Schwartz (1953) preferred to discuss the data as reported. Lovering (1941) computed analyses into molecular percent of equivalent volumes, and Butler (1932), Butler and others (1920), Sales and Meyer (1948), and Lovering and others (1949), converted oxide weight percent into grams per cubic centimeter. Barth (1948, 1955) calculated the number of cations and anions in a "standard rock cell." A comparison of some of these methods is afforded in table 19, and shown on figure 27. These data show that regardless ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO of the scales of measure used, the slopes of lines connect- ing plotted values are remarkably similar. In the subsequent discussions it is assumed that the movement of cations and anions is within a network of close-packed oxygen ions rather than an unordered movement of oxides in space. The movement of in- dividual ions, including oxygen, probably is variable; and it is recognized that Si and Al undoubtedly have strong structural expression (bonding) as oxide mole- cules and are therefore less mobile than the alkali, alkaline earth, and metal ions present. A further fun- damental assumption that there is no significant volume change caused by the alteration process seems warranted from thin section and other observations. TaBLE 18. -Chemical analyses, in weight percent, of fresh and altered wallrocks, Gilpin and Clear Creek Counties, Colo. [The location and rock type of all samples are indicated in table 1. Analysts: J. Theobold, 1-8; 14-18; and 31. L. N. Tarrant, 19-26; 32-34. P. L. D. Elmore, K. E. White, and S. D. Botts, 9-11. P. L. D. Elmore, K. E. White, S. D. Botts, and A, Sherwood, 12-13. H. F. Phillips, P. L. D. Elmore, P. W. Scott, K. E. White, 27-30; 35-40] Sample Analysis factor 1 2 3 4 5 6 7 8 9 10 Constituent: 71. 86 67. 56 58. 66 60. 20 55. 47 57.19 52.05 49.77 76.3 75. 4 13.19 14. 38 16. 59 14. 51 16. 87 15. 55 16. 37 16. 80 12.2 12.7 1. 24 -78 . 96 3. 56 4.11 4.47 3. 28 1.62 .9 1.0 11. 80 11.86 1 3. 28 1 5.26 15.03 1 5. 34 17.58 1 8.16 2.0 1.6 . 55 1.12 1.56 2.33 2.36 2.28 2.71 1. 68 . 48 . 50 1.29 . 98 2.15 4. 84 2.77 1.40 1. 53 1. 62 14. 1.6 2.22 1.93 .22 2.93 1. 57 16 .08 .08 8.7 3. 4 5.85 7.05 6.86 2.71 2. 58 2. 66 5.71 6.92 1.1 1.2 04 .10 . 38 .02 1.95 1.75 R 3 83 1.8 . 57 79 2. 38 70 2.92 3. 88 3 f 41 . 36 . 51 1. 48 1.37 1.66 .22 . 34 2.78 4.76 .20 1. 61 2.16 .20 . 24 11 . 39 . 63 . 51 . 69 .06 .03 .04 .16 14 14 U .02 .05 .07 16 12 .07 +00.1 -/ Mi] 2C AR INE... (0.07 . 06 14 .21 . 30 . 34 SQ: 1 001 "_ ias L 3: Ast SS. set ' _ t( _ ~ .a |.... yes 99. 76 99. 97 100. 04 99. 99 99. 95 99. 90 .03 .02 13 13 .21 ABH 3 E0 a t ORS (SAT PObAL I-22 22d s 200 Een e cn h bec t ul wen rae nee aes 99. 73 99. 95 99. 91 99. 86 99. 74 99. 72 BUC AeRSIEYL 3-22 22220! JUL Puc nueve as ere cloak esa 2. 66 2. 64 2. 52 2.78 2. 57 2. 46 FPOwWder density 2222 322 20221 el oe serena Pease Pe cena ecto cae. s 2. 66 2. 67 2.71 2. 81 2.73 2. 74 2. £20200 se eled cua wane eas aan dae a de on a a ua percent... .0 11 7.0 1.1 5.8 10.2 Sample Analysis factor 11 12 13 14 15 16 17 18 19 20 Constituent: 71.2 75.5 65.3 64.09 54.29 52. 61 51.98 48. 06 71.76 64. 69 14.1 12.1 13. 8 14.03 13. 53 15. 63 16. 88 15.00 14.37 13. 74 2.5 2.3 6.7 3. 44 5.28 2.71 4. 69 8.77 1.13 3. 56 1. 4 . 68 . 82 12.71 1 5.10 1 6. 46 14.77 1 3. 68 1. 80 3. 60 . 55 . 44 .74 1.14 2. 42 5. 67 2.90 2. 08 . 58 1.11 1.4 12 . 37 3. 91 7.22 6. 16 3. 41 5.87 2.05 3.09 2.7 vI 11 2. 81 2. 64 2.13 . 47 .99 4.95 4.16 1.2 4.6 4.7 4. g 2. 40 2.85 2. gg é g 2. g 1. 61 A .22 12 3: . ¢ ¥ 3.4 1-6 tsil{ "f 72 1.61 4.01 3. 45 . 38 .20 16 . 48 1. 31 2.37 . 91 1.33 1.91 27 . 54 <. 05 <.05 12 . 44 2.08 4.25 4.72 . 54 . 04 . 06 . 20 . 56 2.02 . 33 . 62 . 46 .04 . 06 . 01 .02 . 09 13 15 .18 .18 . 06 . 24 AT . 04 . 06 .02 + 5g . 48 . 82 . 30 . 24 108 (Porn eerie ende ene dealer n clave abana . 03 . 36 .22 . 16 . 82 99. 84 99. 74 100. 00 99. 97 100. 12 . 25 .27 .19 AZ <4 99. 59 99. 47 99. 81 99. 80 99. 94 BULK £22, rX cL ee aus Lee 2 cok ce ba uk cleans ans ob e eanen w 2.70 2. 80 2.79 2.47 2.82 Fowder 00 LILLIE 2.78 2.85 2.85 2.177 2.74 Lee coo cree oot nees eres Pear ench een ne bebe percent... 1.1 1.8 2.1 10.8 15.3 See footnotes at end of table. GEOCHEMISTRY OF ALTERED ROCKS 41 TABLE 18.-Chemical analyses, in weight percent, of fresh and altered wallrocks, Gilpin and Clear Creek Counties, Colo.-Continued Sample Analysis factor 21 22 23 24 25 26 27 28 29 30 ____________________________________________________ 65. 61 68. 61 12.12 2.0 58.1 59.5 ...... 13.05 14. 10 9. 83 1.9 14.3 14.9 ______ 3. 40 2.33 . 39 1.9 4.5 4.9 ...... 4.05 1.72 1 6. 99 3.2 5. 4 4.2 ...... T1 . 64 . 52 1.5 3.0 $2 ...... 1. 88 2. 51 . 03 *% 5.2 1.5 ______ 3.17 3. 60 .08 2.6 1.2 . O7 1. $3 % é? 3. (lag 2.2 2.6 2.6 ...... 1. f H ...... 1.31 1.22 1.29 1.0 1.8 5.6 . 66 . 54 .20 1.4 1.8 ...... 2.78 1.49 . 82 . 70 1.2 ...... AZ 11 .03 . 62 . 53 ...... . 30 14 At 7 14 14 Bulk density.. Powder densi Porosity........ Sample Analysis factor 31 32 38 34 35 36 37 38 39 40 Constituent: .O. sek bene neben ar un arie ai 5a 55.27 48. 19 51. 39 54. 62 52. 4 51. 4 49.7 43.1 52.8 49.7 16.17 15. 66 21. 80 18. 97 8.2 9. 4 7.8 9.7 11.1 10.5 2.27 3. 86 5. 87 5. 41 11.7 10.9 11.0 7.6 3. 4 4.9 1 6.30 9. 08 . 45 16 13.9 15.0 13.9 17.9 16.5 16.8 3. 42 7.17 1. 62 1. 54 2.8 3.8 1.9 2. 4 2.0 2.0 7. 49 8. 68 1. 54 1. 28 5. 4 3.0 2. 4 2.4 2.4 2.3 4. 65 .92 . 60 . 22 .10 . 09 . 06 .06 . 05 . 09 1.31 1.07 . 62 . 54 . 49 1.8 1.2 . 91 .21 . 31 o £17 falt HB } 274 .87 1.3 1.3 1.2 1.4 .98 «75 .94 79 . 82 52 . 30 42 . 42 . 38 . 48 . 01 . O1 . 01 . 25 .13 6. 4 .6 .6 8.0 . 24 12 .18 .16 .70 . 59 .94 16 . 89 52 .19 . 26 Di O1 3.5 3.2 3.0 7 . 5 . 5 Bulk density.. Powder densit Porosity......_.... 1 A calculated correction was made for FeO present as pyrite based on percent S. In making correction it is assumed all sulfur is present as pyrite. 2 Not determined. To represent the relative movement of ions or ele- ments as a result of rock alteration a simple calculation is employed to determine the weight of the ions in an equivalent volume.* It is obvious that these calcula- tions from analytical data give weights of the element. These data recalculated into percent (table 19) resemble Barth's values for the number of ions in a rock stand- ard cell. In a very loose sense the term "ion" is used here to represent these elements. _ The gravimetric cal- culation is widely accepted and these data are therefore readily compared with similar data elsewhere 4 Tons in an equivalent volume are computed from the chemical analysis by mul- tiplying that portion of the oxide weight percent due to the ion by the bulk density of the rock to obtain small whole numbers in (g per em?) X 104. } Determined by author from representative samples. + Special standard chemical method. 5 Total S as S. From these data, ratios of ions lost to those gained may be calculated (fig. 28). In these diagrams a value of less than 1 represents a loss of the ion relative to the amount of that ion in the original host rock; a value greater than 1 represents a gain of the ion. It is found empirically that oxygen varies least percentagewise in normally altered wallrocks (fig. 29). Thus the ratio of gain or loss of an ion may be plotted against that of oxygen to show the large variations in cation population gains or losses to that of a nearly constant anion popu- lation. The writer realizes that the calculated values used herein are simple approximations that are mainly useful ! in describing a very complex chemical environment 42 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO OXIDES OXIDES IONS IONS IONS (weight percent) (g/cm?) 102 (weight percent) (g/em*)x102 (standard cell) 80 q 55 4 607 160 4 30 { 70 { r | Si SiO § Si 50 554 145 SiOz 25 \’_\ 50 4 130 J sa, # "22 1' ér *~ A'203 I, ‘r l, \ A -~*~ ( y2 .* Al203 sen ,/ \ / Naive" 161 A 223% sof" _ s_./ 81. / / i 14 - 35 4 7 4 127 30 4 6 Fer? . ~ 51 Jp" - / o- K 1 « 3a ZONES FIGURE 27.-A graphic comparison of methods of plotting chemical analysis data. Samples 4 through 8 are from fresh and altered granodiorite. TABLE 19.-A comparison of alternative methods of expressing chemical analysis data, granodiorite [Bulk density, 2.78; Powder density, 2.81; Percent pore space, 1.1] Oxide molecu- Tons Ions Oxide | lar pro-| Oxide Ion Cation | (g per | in rock Constituent | (weight | portion | (g per | (weight (percent)] cu em) |standard percent)!lin equiv-) cu cm) | percent) X 10° cell? alent volume 60. 20 61.1 1. 6736 28.1 52. 4 78.1 55.1 14. 51 8.7 . 4034 TAT 14.3 21.3 15.6 3. 56 2.5 . 0990 2.5 4.7 6.9 9.5 3 5.26 9.2 .1462 4.1 7.7 11.3 4.1 2. 33 8.5 . 0648 1.4 2.6 8.9 3.1 4. 84 5.3 1345 3.5 6.5 9.6 4.7 2. 08 2.9 . 0814 2.2 4.1 6.1 5.1 2. z; 17 0752 2.2 4.1 6.3 3.1 * iP . 000 Yol ai] codes |f 1s %, +2 1.48 1.1 . 0411 .9 1.7 2.5 1.0 .20 .8 . 0055 . 05 4 v4 12 . 63 .8 . 0175 .8 .6 +4 4 14 A . 0039 AA .2 .3 'C 12 . 05 . 0033 78 .2 .3 . 04 . 21 <4. . 0058 .2 4 .6 ab 15 .03 . 0042 A 14 .4 .2 148. 6 + 95.0 2.0 4.0 120.7 155.3 280.3 | §160.0 1 Table 18, sample 4, M. and M.-Dixie mine, Chicago Creek area. 2 Barth (1955) calculations. 3 Calculated correction made for FeO present as pyrite, based on percent S. In making correction it is assumed that all sulfur is present as pyrite. + Total of cations (minus F and S). 5 Total of anions F+S+(OH)+0. found in the alteration of a diverse and unhomogeneous rock assemblage. McKinstry (1957, p. 751) points out that the representativeness of the sample itself may be questioned more strongly than the accuracy of the analyses. Chemical analyses of altered wallrocks viewed as ion distributions indicate that the major textural and structural differences between fresh and strongly al- tered rocks may be represented by a very small cation- anion redistribution. The number of ions gained or lost does not always balance (table 20), but there is generally a charge and size balance. While these summations are interesting they fail to indicate the contribution and effect of each zone to the chemistry of all the altered rock. SILICON Silicon constitutes 30 to 35 percent (calculated from grams of ion per volume) of wallrocks; however, the silicon ions are small (0.42A) and occupy a very limited volume (table 21). Silicon occurs in tetrahedral co- ordination with oxygen and is part of the fundamental silicate "building block" in quartz, feldspars, micas, amphibole, and clay minerals. Silicon generally is GEOCHEMISTRY OF ALTERED ROCKS 43 1.0 F 7 6k —————— 7 x s <| w 5 Tar += i 8 Al 5 1/ + Si { uC oest £ fo a" { - 6 1.0 53 < a ~ O| 0 0.95 4 iron 1 2 3 4 Biotite Fet2> Fes Clay minerals | Fe*s>>Fe*2 alel] eatk (with iron) Chlorite Fet2> >Fe+3 Sis u o mon C+ Soler s Magnetite Fet3>Fe+2 Hematite Fe+® ne mens nor moh. re mee e Pyrite _ Fe+2 eae EXPLANATION Primary rock constituent Secondary mineral constituent FIGURE 33.-Diagrammatic sketch of stability ranges and oxidation states of iron- bearing minerals in fresh and altered biotite-quartz-plagioclase gneiss, Essex mine. ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO hexagonally close-packed oxygen layers, and in pyrite as ferrous iron combined with S in a sodium chloride- type structure. The relative proportion of iron in varied oxidation states in several minerals common to these rocks (fig. 33) illustrates one facet of the complex problem of accounting for iron distribution. The proportion of iron in equal volumes of iron minerals varies (table 22)- pyrite requires less iron per unit volume than iron oxide minerals. Graphical plots (fig. 34) reveal a nearly regular pattern of iron distribution in altered rocks, but these diagrams fail to account for actual amounts of iron in each zone. These data do not provide infor- mation on relative areas (or volumes) of altered rock zones that are needed to demonstrate whether a balance exists between the iron lost in zone 2 and that deposited in zones 3 and 4. We can generalize from laboratory Zone 1 2 3 4 Zone 1 2 3 4 V Zone 1 2 3-4 T T T T arta T T u T T T D. Microcline-quartz-plagioclase-biotite 2+ @ 8f- gneiss % 8 |- e 1+ 3 7 - = 6 |- - 0 |- = 61 7 4 |- 3 -1}- = n ig 2 F = A. Biotite-muscovite granite 1 1 1 1 4 - \ - 0 L- - Sr - «2 a Zone 1 2 3 4 T T T T 2+ -{ -4 |- = 7 B. Granodiorite 5 ir 1p a - r- «< 7 Leal a a!: ! Bibl Rs Zésotossssny'ss * 4) g-et E & 5 | é 2 L- (_: & 1 1 1 1 i ELIOT § z Pl pa F. Amphiboli w !} g w w { < Z fia ° o} Zone 1 2 3a 3¢ 4 o he 2 Elsuf it ones. re 2, @! _ 2, gt E. Biotite-quartz-plagioclase gneiss a 2, Fone 1:2 2.3 3.4 i i bo 1 [ y 2 j j 3 2 | _L a 0 }- aa 6 |- - -~3 - a Zone 1-2 3 4 T T T= 5 |- - -4 F- m C. Quartz diorite 2 |- - 4} - "61 < 1 3 3} { sef s 0 - 2F - -10 - = -1}- - 1 - \ - -12 |- - ~2 F he Of i= 33> - -14 - - G. Garnet quartz gneiss I 1 T 1 1 T T 1 1 1 1 FIGURE 34.-Distribution plots of total iron in altered wallrocks. Values greater than zero, the starting material, indicate gain of Fe ions, values less than zero indicate loss of Fe ions. GEOCHEMISTRY OF ALTERED ROCKS TaBus 22.-Distribution of constituent ions in common iron- bearing minerals [Data from Palache, Berman, and Frondel (1944)] Total Fe Mineral Fet? Fors (g per cu 0 8 cm) x10 Magnetife:.._._ss...<......... 1. 24 2. 50 3. 74 1 Ab -L eer 20 ol ort nececes ds 3. 64 3. 64 - 180 NL .LOC. RUL HLL cui eu 2.83 .A 288 |...... 2. 67 and field observations only that a relatively small amount of iron is removed from zone 2 rock in compari- son with the greater amount of iron deposited in zones 3 and 4. The area (and therefore volume) of zone 2 rock bordering a normal vein, however, is proportionally greater than that of zones 3 and 4 rock. Biotite is believed to be the major source of movable iron in most rocks in the region, but hornblende is a significant source in a few rocks. Clay minerals formed from relatively iron-free plagioclase adsorb or incorporate iron readily, and near the vein the iron combines with S to form pyrite. In K-feldspar-rich rocks such as granite, granodiorite, and possibly in microcline-quartz-plagioclase-biotite gneiss there is a very small initial loss of iron in zone 2, a moderate concentration in zone 3, and a slight loss in zone 4. Biotite is unaltered through zone 3, thus losses of iron in zone 2 are attributed to accessory minerals, and gains in zone 3 may indicate movement of iron from zone 4. In plagioclase-rich rocks, such a amphibolite, quartz diorite, and biotite-quartz-plagioclase gneiss, a loss of iron occurs in zones 1-2, a substantial gain occurs in zone 3, and a slight loss is noted in zone 3-4. Biotite and hornblende are more abundant in many plagio- clase-rich rocks than in K-feldspar-rich rock in the area, and the argillic zone is broader in these rocks. Iron is freed from hornblende (zone 2-3) and biotite (zone 3) and is held in clay-mineral structures; however, addi- tional iron must be moved from zones 2 and 4 or intro- duced from the vein(?) to account for the marked gain of iron in the argillic zone. The distribution of iron in biotite-quartz-plagioclase gneiss (fig. 34) is complicated by the variability of the biotite content in this layered rock; and the total iron in zone 1 may be low owing to inclusion in the sample of rock with less than average biotite content. The increase in ferrous and loss of ferric iron in strongly altered garnet-quartz gneiss reflects reduction of iron to form abundant disseminated pyrite; however, there is a loss of total iron close to the vein which when added to vein solutions was available for use elsewhere. Amphibolite also contributed excess iron to the vein environment. The sequential oxidation and rearrange- ment of ferrous to ferric iron in the replacement of mag- 49 netite by hematite, and the subsequent reduction and rearrangement of ferric iron with S to form pyrite, in- volves a decrease of iron per unit volume (table 22). Thus the replacement of a magnetite grain by pyrite involves a decrease in iron per unit volume that be- comes available for combination elsewhere. MANGANESE Manganese 0.80A) is present in most rocks of the area in amounts of less than 1 percent, but is more abundant in garnet-quartz gneiss (2.7 percent). The element occurs mostly as a replacement of iron, mag- nesium, and calcium in mica, amphibole, clay minerals, garnet, magnetite-ilmenite, and apatite; in the veins it occurs in sphalerite as well as in manganese oxides or carbonates. The uncommonly high amount of man- ganese in altered garnet-quartz gneiss is contained mostly in the garnet structure. Garnet is gradually altered in zone 3, and analyses (table 18) show that manganese is not removed abruptly from the rock environment in zone 3-4. CARBON Carbon is present in minerals with CO; groups in certain silicate and phosphate minerals, but mostly as independent carbonate minerals in these altered rocks. As shown in figure 30 the carbon content of these rocks increases toward the vein. Chemical as well as petro- graphic relations indicate that CO; must be introduced by the altering solutions. These CO,-bearing solutions and released calcium, iron, and manganese ions com- bine to form the secondary carbonate minerals sparsely disseminated in the wallrocks and more abundantly filling the veins. PHOSPHORUS AND TITANIUM Phosphorus and titanium occur in amounts of 1 percent or less; the concentration of phosphorus is below spectrographic sensitivity. P ions (0.35A) occur mostly in apatite in tetrahedral arrangement with oxygen, but also may substitute for Si ions in garnet and zircon. P. K. Sims (oral communication, 1957) reports that biotite-quartz-plagioclase gneiss locally contains several percent of monazite and xenotime, minerals having zircon-type structures that are possible sites for P in these rocks. Titanium is found mostly in ilmenite, sphene, rutile, and leucoxene, and is incor- porated also in magnetite, replacing iron, and in biotite and amphibole replacing Si, Al, Fe, and Mg. The distribution of phosphorus in altered rocks is not well known but generally remains consistent with the distribution of apatite, which persists unchanged even in the zones of intense alteration. Titanium remains constant across altered rock zones until lost in the zone of most intense alteration; thus the distribution 50 pattern is consistent with the stability of magnetite- ilmenite, sphene, and leucoxene. ASSOCIATED ANIONS The associated anions include oxygen, sulfur, fluorine, and hydrogen. Oxygen (1.40A) is most abundant in weight percent as well as by volume (table 21). The weight distribution of oxygen in fresh and altered rocks is reasonably constant if one omits consideration of water lost below 100° C (H,0-); however, if all water is considered (table 23) there is a slight gain of oxygen in argillized rock and a slight loss in sericitized rock. If equivalent volumes of rock are considered, the progressive alteration of wallrocks results in only small gains and losses of oxygen. The addition of H+ ions to oxygen to form water involves no significant volume increase, and in essence there is no real loss of oxygen in the addition of H+ to form structural hydroxyl groups shown in figure 30. There is, however, the possibility that variations in the O"*/O* ratio occur in progressively more altered wallrock. Oxygen is present in excess in the hydrothermal environment and substitutions are possible. These factors do not negate the space relations that are of prime concern here. 23.-Ozygen in fresh and altered wallrocks by weight percent Altered Weight Altered | Weight Sample rock percent Sample rock percent zone zone Biotite-muscovite granite Biotite-quartz-plagioclase gneiss 48. 55 1 49.05 48. 89 2 48. 51 49, 14 3 49. 55 3-4 50. 38 v 47. 69 Biotite-quartz-plagioclase gneiss 4. 1 45. 65 5. 3a 47. 20 6 3b 48. 74 7s 3-4 47.75 1-2 49.77 4 47. 56 3a 50. 50 3b 58. 290 Microcline-quartz-plagioclase- biotite gneiss Amphibolite 54.0 50. 4 2 45. 90 50. 5 3 54. 43 49. 3 3-4 46, 29 45. 6 Garnet-quartz gneiss 1-2 47. 28 1-2 43. 36 3 49, 59 2-3 45. 32 4(?) 49. 31 3-4 | 42, 31 Very few sulfide minerals are disseminated in the wallrocks, and only in intensely altered rock is the amount of S sufficient to be noted in analyses. Pyrite is the chief sulfur-bearing mineral and generally is 1 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO texturally related to altered biotite or magnetite (fig. 7) Fluorine occurs mostly in apatite and fluorite where it is retained locally even in intensely altered wallrock in the Banta Hill mine area. F- (1.36) diadochically replaces (OH)~- (about 1.40A), CO;, and CI- in mica, amphibole, and clay-mineral structures. Fluorite and apatite resist solution, but the solubility of CaF, in- creases with the content of CO, in water (Rankama and Sahama, 1950, p. 763). TRACE ELEMENT CONSTITUENTS Trace elements 5 occur as the primary constituents in accessory minerals or in very small amounts in essential minerals by replacement (camouflage, capture, or admission) of more common elements. Thus while the mode of occurrence of trace elements in minerals is not well known, the affinity of certain elements for one another (Rankama and Sahama, 1950, p. 125; Higazy, 1954), and therefore for certain minerals is marked. The distribution problem is twofold: Trace elements in altered rocks may be either redistributed from the fresh rock or may be introduced by the altering solution. The semiquantitative spectrographic data are not statistically rigorous and are used only to indicate general trends of distibution of trace elements in altered wallrocks. The data were not obtained from statisti- cally homogeneous samples, and neither the precision of the analyses nor the small numbers of samples ana- lyzed permit a statistically small confidence interval for the values cited. It is assumed that the general and consistent distribution patterns of elements indi- cated by these data are real. Comparisons of trace elements in these rocks with similar constituents in related rocks are made but are subject to the limitations of these analytical data. METHOD FOR PRESENTING DATA Spectrographic analyses were made by the semi- quantitative method developed in the Denver and Washington laboratories of the Geological Survey; ° these analyses are presented graphically to illustrate the relations between them more easily. The value, plotted as X-looked for, but not detected-is plotted 5 Trace elements here include all elements in the rock determined by spectrographic methods except those reported by standard chemical analysis. 8 The results of spectrographic analysis are reported in ranges, each power of ten being divided into three ranges of about 1, 2, 5, 10, etc. The approximate mid-points of the ranges, 1.5, 3, and 7 are the figures used in reporting results. 'These figures represent the amount of the element in the sample that may be expected to be between 1.0 and 2.1, between 2.1 and 4.6, etc. In such a series of reported results at least 60 percent are expected to lie within the correct range. It is believed that about 20 percent of the results will be one range too high and 20 percent one range too low (A. T. Meyers, written communication, 1958). GEOCHEMISTRY OF ALTERED ROCKS 1-2 3 4 V or G T T T Zone Percent I >10 |- =I 10 |- het 7 [- 4.6 |- 3.0 |- 21 - 1.5 |- 1.0 |- 0.7 |- 0.46 |- 0.3 - 0.21 |- 0.15 |- 0.10 |- 0.07 |- 0.046 |- 0.03 |- 0.021 |- 0.015 |- 0.010 |- 0.007 |- 0.0046 |- 00032? ZPrecision range; at least 60 per- nt of results are goois |- 62 expected to lie within the correct range 0.0010 |- > 0.0007 |- Not looked for V G Vein Gouge clay 701 702 EXPLANATION ¢ Value reported O Trace, near threshold amount X Looked for, but not detected Sample 703 704 FIGURE 35.-Sketch illustrating conventions and notations used in plotting the semiquantitative spectrographic data. at the nearest unit below its standard sensitivity. The elements listed in boxes in diagrams also were looked for but not found in the sample. Other notations used in the graphical presentation of the data are given in figure 35. The standard sensitivities for elements determined by the semiquantitative method are presented in table 24. The sensitivities obtained in the Washington and Denver laboratories vary slightly, and the appropriate sensitivity has been used in the graphic-data plots. - In some individual analyses greater sensitivities than those in table 24 were possible and and the values reported have been plotted. To facilitate graphing these data, the elements have been arbitrarily grouped into four sections based on the arrangement illustrated by Barbor (1944, p. 25): a, The alkali and alkaline earth (regular) elements; b, regular elements; c, transitional metals, first group; and d, transitional metals of the second and third groups and secondary transitional metals." Semi- ? Regular elements are those with the distinguishing electron in the outermost electron shell; "transitional" metals have the distinguishing electron in the next outermost shell; "secondary transitional" metals (including rare earths) have the distinguishing electron in the third from outermost electron shell. 51 TABpE 24.-Standard sensitivities, in percent, of elements deter- mined by semiquantitative spectrographic methods at U.S. Geo- logical Survey laboratories [Denver, sensitivity as of August 1954, amended: Washington, sensitivity as of March 1956. Dashes indicate element not reported. A second exposure was required for the higher sensitivities shown in parentheses] Element Denver Washington Element Denver Washington 0.05 (0.0005) . O01 0. 01 1(0. 0003) . 001 1 03 7. (0. 007) 1 A separate exposure is required for the F estimation. quantitative spectrographic data for 112 samples of fresh and altered wallrocks are reported in figures 36-62; elements which were looked for, but not de- tected, also are listed. The resulting graphs show the distribution of elements from fresh rock to the vein; on some graphs the distance between the samples can be measured by reference to the scale shown on the figure; on others, no scale is shown as the distance was not measured, and the samples have been plotted with- out reference to true relative distance between them. Average values for some Front Range rocks (granite, microcline-quartz-plagioclase-biotite gneiss, granodio- rite, biotite-quartz-plagioclase gneiss, metasedimentary and migmatite gneisses, and quartz diorite) are shown in figures 63, 64, and 65. A comparison of these data with the average abundance of certain elements in igneous rocks is afforded by table 25. Trace elements of groups b and d in rocks of the Front Range mineral belt are slightly higher than the average. In fact the average values given by Rankama and Sahama (1950) mostly are below the analytical sensitivity obtained in these analyses. Trace elements in group c, however, are equal to or lower than average values. Elements of group a are mostly major constituents. 52 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone 1 2-3 3b Zone I T I Percent (- -| >10 - f- e : { _s." s- ~>. ' _ _e ais} 7 [« e* -~ *As" % = 1 2-3 3b I sigs. I o- *" 'e -e - - 3 - - =. 1.5 m > As, Bi, Ge, In, Sb, Sn, Te, Tl, < [- ~1 ~. (- = -> 0.15 [- ® = - 0.07 we % = f- 1 0.03 - # A |- __ e-_-___-_-L_- mg: - L en ines -o -| 0.015 |- # =I \- -| 0.007 |- P+ - / |- = 0:003 |. -¥ _e -I - -| 0.0015 |- ./0—~/—~—~0 - Ga (> -| 0.0007 |- 7 ~ =1© 0.0003 |- =I (+ -| 0.00015 |- 7 ~* s x -| 0.00007 - L- 1 #, 1 3. Sample 1 74 B -3 Sample 1 H Io & & Zone Zone 1 2 3b T T T Percent I I I F -| >10 \- - £] 7 Au, Cd, Dy, Er, Gd, f Hf, Hg, Ir, Mo, Os, ve r Pd, Pt, Re, Rh, Ru, - Sm, Ta, Th, U, W - 1.5 - kl -| 0.7 (- - - o3 _ |- E -| os |- ; -| o07 - |- -| o03 |- -| o.o15 |- -| 0.007 |- -| 0.003 |- -| 0.0015 |- _ e---._-N as as Bel . - -| 0.0007 |- ei. = -| 0.0003 |- s Ag." > -| 0.00015 |- >I © -| 0.00007 | 1 I l S Sample 1 2 3 D Analyst: P. R. Barnett, Denver r -T I x | an- F1GURE 36.-Semiquantitative spectrographic analyses of fresh and progressively more altered biotite-muscovite granite, Nabob and Jo Reynolds mines, Lawson-Dumont- Fall River district, Clear Creek County, Colorado. Analyst: P.R. Barnett. Zone 1 GEOCHEMISTRY OF ALTERED ROCKS 1 I I I T Percent I >10 <.. 7 -| _ 3 ___'\ ------- H L.5 k S] 0.7 A -| 0.3 £5 -1 o15 c ' e - Ba -] 0.07 \\q\ # -| 0.03 Spr e -| 0.015 > A -| 0.007 v~ -| 0.003 -| 0.0015 -44 0.0007 |- ( - 0.0003 |- P": "m" -| 0.00015 -| 0.00007 Sample 4 Zone 1 Zone As, Bi, Ge, In, Sb, Sn, Te, TI T I I 1 Sample 4 Zone 1 I I I I Percent | -| >10 7 % 3 Fe bd ® © 6 I Je =e 2 19 (@ Ge -<: 1 --+ ~ =| 403 "-% o -I 015 - 0.07 -| 0.03 v \% -> K- - =| 0.015 -| 0.007 -| 0.003 | Sample 4 *Special sensitivity for Zn in these samples Au, Cd, Dy, Er, Gd, Hf, Hg, Ir, Mo, Os, Pd, Pt, Re, Rh, Ru, Sm, Ta, Th, U, W -| 0.0015 |- - e-. \ o0007 |- (e- iy. -| 0.0003 -| 0.00015 -| 0.00007 I N Sample 4 Analyst: P. R. Barnett, Denver 53 FIGURE 37.-Semiquantitative spectrographic analyses of fresh and progressively more altered granodiorite, M. and M.-Dixie mine, Chicago Creek area, Clear Creek County, Colorado. Analyst: P. R. Barnett. 54 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone _ 2 3 3-4 3-4 4 V Zone 2 3 3-4 34 4 V T T J -==- Percent T T T ~ Si (> " >10 :. |- [- -| 7 \- r——A'———o——————o——Q = 3 [= A -| 1.5 \- va A - t' o7 -_ | s m 1 p3 -E s <] 016. z -Q : sl oor. - =t [= ~s?" ' --" \ ¥ st pos- | : ] / -| 0.0150 |- > \gr v -| poor |- - 0.003: |- [= ¥ -< -0.00is |- \- -| 0.0007 |- * = [> Be -| 0.0003 |- =I (s -| 0.00015 |- 7 |- : -| 0.00007 |- - | | I | I I 1 1 | I 1 | Sample 9 10 a 11 12 132 (13 Sample 9 10 11 12 132 13 Zone - 2 s 31 sa . 4 v Zone - 2 s ga 3a 4 v T I 53m r 7 Percent T T F= -~" l- -| >10 |- - - 4 7 - m |- s 3 E o Fe - 1.5 i- * -I (- [- - [- =- '0.3 - - =| 0 A8 - =4 -| 0.07 is £ -| ::0.03 - 1: 0.015 |- =~ 0.007 _ |-- = 0.003) :- -~<(~0.0015 :- c -: 90.0007 : (- -I ig ~ 0.0003 |- Yb ~ > -| 0.00015 |- o—-—._.+._._A ® ¥b ®x x Ag 0.00007 |- t | 1 | I 1 I I | | I I | Sample 9 10 c 11 12: 13@ 13 Sample 9 10 11. 12. -19a 13 I I Analyst: K. E. Valentine, Washington, D. C. FrGuRE 38.-Semiquantitative spectrographic analyses of progressively more altered microcline-quartz-plagioclase-biotite gneiss, E. Calhoun mine, Central City district, Gilpin County, Colorado. Analyst: K. E. Valentine. Percent >10 7 3 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 Percent >10 7. 3 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 Zone 1 ; } T - (M &l [- =i h C ef e 2 Mg ....................... I @ --* %f — ~- B. [< a :Y - SL,,,/,_o _{ - a : 27 : E: f Be . = l I o f 15 A Zone 1 ; . I - /. i - Bl. ze _ " t- ~~ f - - ® s* - - j l I sols 15 C GEOCHEMISTRY OF ALTERED ROCKS Zone 1 1 I I - e- Si -@ . ~- - ._ mio" ie er ie _AI_ _____ _. - |- -I - -I As, B, Bi, Ge, In, i Sb, Sn, Te, TI 7 [= 7 |- - [- =I e ®i.n _. 1 a) a F3 .—‘§.\ z- -s - Ga wi g | | Sample 14 185 B Zone i 1 T I (= 4 Ag, Au, Cd, Dy, Er, Gd, Hf, - Hg, Ir, Mo, Os, Pd, Pt, Re, g Rh, Ru, Sm, Ta, Th, U, W .\_\-&~ an t- Yb iy -e f | I Sample 14 15 D Analyst: P. R. Barnett, Denver 55 FrGURE 39.-Semiquantitative spectrographic analyses of two adjacent phases of granodiorite from surface, Central City district, Gilpin County, Colorado. - Analyst: P. R. Barnett. 56 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone 1-2 S 4 Zone 1-2 3 4 T I I Percent: I € I I Si - 1 >10 [- & bat % - unt = 7 = 0‘—~———fl——o— ——————— - W C. |- _ $-- _- e- o Gor |- - [-. e_ F le. -, 1b. i," 3 n. wa \~\Na fct -| 0.7 m = \‘\ pease? th *-- <> 0.3 [- As, Bi, Ge, In, Sb, Sn, Te, TI T - -| 0.15 - - - -: 0.07 p- x (-!. # Cs =t roos" | |= S |- es -| o.015 |- S alg Jy ~*~ =| gob7 (~!. - Li [- -41 0.003 - c B i ~ |- _ ® GA __gc ice- # .- (- 10.0007 - _ ®-~ ~ * . + p- 71"0,0003 . |- as (t te- " "95 e: -| 0.00015 |- - - - 0.00007 |- = | 1 | 1 aad. 1 Sample 16 17 18 Sample 16 17 18 A B Zone 1-2 3 4 Zone 1-2 3 4 o I I T Percent I I I > -| >10 \- - Fe |- & @ e - 7 (- - Ag, Au, Cd, Ce, Dy, Er, Gd, - = 3 > Hf, Hg, Ir, Mo, Nd, Os, Pd, - Pt, Re, Ru, Sm,. Ta, Th, U, W |- r- - 1.5 o - if es t. - 0——————|——'—f/ -| 0.7 - is - -- 0.3 ~ - "'' a yee # . -[ ois) | [- = [- -| 0.07 s -I i- -+ 0.03 [- =I (- =:"0.015 ' ' |- F |-- -| 0.007 |- -| - .(©0.003 : |- tan - =| 0001s |- _®--.__ np * at =e - I 4 (- = 0:0007 -|- TTT @----------@ - E =\ |- _ = XP o e.. 38 (- ~ :0:00016 |- ift |- - 0.00007 |- 3 I | 1 | | 1 Sample 16 17 18 Sample 16 17 18 C_ D *Special sensitivity for Zn in these samples Analyst: P. R. Barnett, Denver FrGURE 40.-Semiquantitative spectrographic analyses of progressively more altered quartz diorite, Nabob and Jo Reynolds mines, Lawson-Dumont-Fall River district, Clear Creek County, Colorado. Analyst: P. R. Barnett. GEOCHEMISTRY Zone 1 2 3 4 V V J T I T T I [- Be 3 - ma - 3 4 1 1 1 1 1 1 Sample 19 20 21 22 23 23a A Zone 1 2 3 4 V V T T T a T T L ‘/._Fe_\/‘\ J | 1 | I 1 I Sample 19 20 21 22 23 23a C OF ALTERED ROCKS Zone Percent >10 7 3 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 I I T I Sample 19 Zo Percent >10 7 3 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 | | | I I Au, Cd, Ce, Dy, Er, Gd, Hf, Hg, Ir, La, Mo, Nb, Nd, Os,Pd, Pt, Re, Rh, Ru, Sm, Ta, Th, U, W Zt, ed © * res €* -#... +-x_ \o—Y—-+— - - Ag/,' yg | £ | Sample 19 20 21 D Analyst: P. R. Barnett, Denver 1 22 23 23a 57 FIGURE 41.-Semiquantitative spectrographic analyses of fresh and progressively more altered biotite-quartz-plagioclase gneiss, Essex mine, Eureka Gulch area, Central City district, Gilpin County, Colorado. Analyst: P. R. Barnett. 58 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone 1-2 3a Sb V Zone 1-2 3a Sb . V T I Percent I . | I I - -| >10 " o- & e e =I - Jz: ik A -le cleo bs We Kul. s- - 3 - w ______ -~" Ga - Cn onn -] 1.5 |- - ®: O, 8 As, Bi, Ge, In, Sb, Sn, Te, TIJ -- Mg _.'____.-"§\ -| 0.7 - v .—»—“"-- \\ [- ©:: . {Q‘a -| 0.3 [- P.. o = %:" > aso |- a \- o—Ba—7Q\—‘——\' - 0.07 - = yy°a¢ ~ 3aep \- © ~ SSI 2 -| 0.03 - /J. = - ~ -| 0.015 |- $p -* ~ E -| 0.007 |- * = % [- -| 0.003 |- yar /fi/_ ..... % - [- -| 0.0015 |- -~ Z -| Ga - ~ 0.0007 :- = (- -44 0.0003 |- = - N - 0.00015 |- = - - 0.00007 |- = 1 | | | I | Sample 24 25 26 Sample - 24 25 26 A B Zone 1-2 Ja 3b V Zone 1-2 3a Sb V I | I | Percent I I I I |- - >10 \- - f- A 7 : Au, Cd, Dy, Er, Gd, ' Fe Hf, Hg, Ir, Mo, Nb, & o o -o =| /'3 |- Os, Pd, Pt, Re, Rh, § = 3 1 48 f: Ru, Sm, Ta, Th, U, " W f a A- _ Ti -| 0.7 -- - |- *% - 03 _ |- - (- . a> -| 015 |- - I /. -| 0.07 (- - Zr (- Mn S 2517. ity. -C ce® i [- Cu =-- TZ 10018 - 7 * " s- .m =[ O67. |- x x {La - e Z Nd: & o" he z -| 0.003 |- > + = / /7 4 - Z. "/‘/'. -| 0.0015 |- % = #4 #4 |- .SC—x'__.{ / -| 0.0007 |- - N ~=~~~ co Yb - # -- --. -_ .- % -| 0.0003 |- @- _- ___ _- -@ - - - - @ - fis ~ _ 5 // -| 0.00015 |- * i 3° -| 0.00007 |- Ato f | I Mse | | I | Sample 24 25 26 Sample 24 25 26 C D Analyst: P. R. Barnett, Denver FIGURE 42. -Semiquantitative spectrographic analyses of progressively more altered biotite-quartz-plagiociase gneiss, R. H. D.-McKay shaft, Eureka Gulch area, Central City district, Gilpin County, Colorado. Analyst: P. R. Barnett. GEOCHEMISTRY OF ALTERED ROCKS Zone 1 3-4 Zone 1 3-4 T I Percent C -| >10 |- i- - 7 [- -- _J 3 - (- = Lb (= As, Bi, Ge, In, P, I - 0.7 [- Sb, Sn, Te, TI - ~ -~0.3 - o -| 0.15 \- xs 4-4 0.07 [- [-- - 0.03 E- we -1©0.015 -- A yer f= -| 0.007 - |- yer! gue [- -| 0.003 |- rest. ¥ e e': Gg m -| 0.0015 |- o x~ [~ - 0.0007 |- [~ ~ 0.0003 - E~ -| 0.00015 |- E- -| 0.00007 |- | | 1 | Sample 27 28 Sample 27 28 A B Zone 1 3-4 Zone 1 3-4 ; T I Percent T T C =[ >10 i [- -I 7 wd Ag, Au, Cd, Dy, Er, Eu, Gd, Hf, Hg, Ho, f: s- 3 (5 Ir, Lu, Os, Pd, Pr, E~ -} AG - Pt, Re, Rh, Ru, Sm, Ta, Th, Th, Tm, U - -| 0.7 T - -| 0.3 (- |- -. "O15 [- - -| 0.07 (- - - 0.03 f= E- <©0.015 ; |- s -| 0.007 |-- - --0.003. '- & -: $" - -% tis- _._ _YB [~ -| 0.0007 |- * % [< -'0:0003 - [- -| 0.00015 |- #5 -| 0.00007 |- | | 1 1 Sample 27 28 Sample 27 28 C D Analyst: R. G. Havens, Denver 59 FrGURE 43.-Semiquantitative spectrographic analyses of fresh and altered biotite-quartz-plagioclase gneiss, Fall River district, Clear Creek County, Colorado. - Analyst: 678880 O-63--5 R. G. Havens. 60 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone 1 3-4 Zone 1 3-4 I I Percent I I >10 t- I | s Ale s l yr haga t sl =I 49. [- * 1 07. ~ | =I 3 095. -s As, B, Bi, Ge, P, § -| 0.15 m Sb, Sn, Te, TI - =( 007. . |- -I -1 003 |- & -| Gos. |- a -| 0.007 -| 0.003 (> -| 0.0015 (> -| 0.0007 [- -| 0.0003 |- - [- s. "** 's -| 0.00015 |- > [- -| 0.00007 |- - I 9 i | | i i I g 6 | I | 1 T 1 Sample 29 30 Sample 29 30 Zone 1 3-4 Zone 1 3-4 I | Percent | | [~ 0X. -| >10 |- = Ag, Au, Cd, Dy, Er, Eu, 2 L3 C Gd, Hf, Hg, Ho, Pd, Pr, 3 i -| 1.5 \- Pt, Re, Rh, Ru, Sm, Ta, i Ti Tb, Th, Tm, U, W t- - <3. 0:7 - = (> -< 0.3 (- £ . e-_-ML_____-se 73:0.15 [= a- i -| 0.07 [~ = -| 0.03 |- i - sl -o.018 ' |- g-: . _ 3 o =I -| 0.007 |- y - -| 0.003 |- #7 - -| 0.0015 |- -= == nn" -e - \- -| 0.0007 |- - \- b -| 0.0003 |- -| I -| 0.00015 |- e -| 0.00007 [- = 1 I | | Sample 29 30 A Sample 29 30 C D Analyst: R. G. Havens, Denver FIGURE 44.-Semiquantitative spectrographic analyses of [res]; and altéed biotite-amphibole gneiss, Fall River district, Clear Creek County, Colorado. Analyst: R. G. Havens. | GEOCHEMISTRY OF ALTERED ROCKS 61 Zone 1 2 3 3 3. V Zone 1 2 3 3 9 V e Percent aT I I I 17 >10 & Si Al sL. 7 - @------- - -- 1 -- 3 |- -I 1 «1.5 f- us, Bi, Ge, In, Sb,Sn, Te, Tl, P = {0.7 - ~s -] 03 _ |- - Sods. "l o -| 0.07 |- a pies \_ -| 003 |- Pp/ \o e Z - 0.015 d 44 e 0.007 a -| 0.003 -| 0.0015 U -| 0.0007 |- - -| 0.0003 |- [- K -| 0.00015 - \- -| 0.00007 1 | 1 I f" Syl 1 Sample - 31 32 esa 38 "a Sample _ 31 32 & $.. w \ \\\ cr bos | | 1 6 1 J I 1 t Zone 1 2 3 3 3 V Zone 1 2 fies Percent I I |- -| >10 \- -I 5 m ge 7 (~ |Au, Cd, Ce, Dy. Ef, Gd,: 5 - + 3 a Hf, Hg, Ir, Mo, Nd, Os, j E Pd, Pt, Re, Rh, Ru, Sm, (- -< 1.5 (~ [Ta, Th, U, W i [- o_ = 0.7 (- > t- Jnr ' ~s ='~/0.3 E - - :O0.15 (- ; - - -I ~t/:0,.03 [- - = 0.015 .. - - -|: 0.007... - =| 0.003 |- -| 0.0015 - = 0.0007 - - 1. 0.0003 |-- -| 0.00015 [- -j 0.00007 |- I Sample _ 31 32 33a 33 34 Sample 31 32 33a 33 34 Analyst: P. R. Barnett, Denver FIGURE 45.-Semiquantitative spectrographic analyses of fresh and progressively more altered amphibolite, Kitty Clyde mine and R. H.D.-McKay shaft, Idaho Springs and Central City districts, Clear Creek and Gilpin Counties, Colorado. Analyst: P. R. Barnett. 62 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone 1-2 2-3 3-4 j Zone 1-2 2-3 3-4 I I T I I I Percent I I I I I I (- -| >10 [> 0---O----O---@----@---o = <1 Sys _- A. 0 000 - mik toc vow ts" e s 1.5 |- - =~ 07 @ As, B, Bi, Ge, P, ; y -| 0.3 & Sb, Sn, Te, Tl -| 0.15 - 0.07 -| 0.03 = < -| 0.015 -| 0.007 -| 0.003 - 0.0015 - 0.0007 1 0.0003 |- x S] -| 0.00015 |- g - 0.00007 |- is I l I | I I I I I I Sample - 35 36 37 38 39 40 Sample - 35 36 37 38 39 40 Zone 1-2 2-3 3-4 Zone 1-2 2-3 3-4 aJ I I I I Percent | I I I I I -e @ - ---_--8 -- >10 t- = @ C < - = ' 7 © Ag, Au, Cd, Ce, Dy, Er, = |- C 3 - Eu, Gd, Hf, Hg, Ho, Ir, (% V Lu, Nb, Nd, Os, Pd, Pr, - Ma =! 'ts [ Pt, Re, Rh, Ru, Sm, Ta, + Tb, Th, Tm, U, W (- 0.16 Ts = - -| 0.07 - - - 0.03 E- -I -| 0.015 |- aI -| 0.007 |- M =I -# 0.003 |- ——+__;‘._=La__.‘______.___"§‘ 2] -| 0.0015 |- ->> 4+ 5 % al -| 0.0007 |- s [> -| 0.0003 |- f 2] [ -| 0.00015 [- - 0.00007 1 1 T 1 1 1 | 1 | I 1 |- Sample 35 36 37 38 39 40 Sample - 35 36 37 38 39 40 C D I 1 I | Analyst: R. G. Havens, Denver FIGURE 46. -Semiquantitative spectrographic analyses of progressively more altered garnet-quartz gneiss, Fall River district, Clear Creek County, Colorado. Analyst: R. G. Havens. GEOCHEMISTRY OF ALTERED ROCKS 63 Zone 1 2 3 '4 4 Zone 1 2 3 14:4 e fs > ", | T T T ] Percent I Si T [(- =| >10 - y oy a[ agit 8 l ~~ [2 s Vig wos 2] S| ois" "I= g el - 0.07 (- As, B, Bi, Ge, Sb, Sn, Te, Tl 23 -| 0.015 7. o -| 0.007 i f A'0o0g "L ewm-._lA ig- -a - s -| 0.0015 \- -| 0.0007 |- Ry" = T 1 I - . | f | | L-- - 0.0003 |- x Lar g- inning BB -@---® - 0.00015 A - -| 0.00007 I ¥ 12. rss =-4 %> Sample 41 A 42 434445 Sample 41 B 42 434445 Zonek My Zone k l"’T’?l/|"§jl)4 Percent | - j / -| >10 \- [€ ~ & a Sk .g (- Au, Cd, Dy, Er, Gd, a € Hf, Hg, Ir, Mo, Os, |- -| - 3 I Pd, Pt, Re, Rh, Ru, a osteon. o **. sl L _ (om To Th. Law id t- ®----@ - 0.7 -- - [- S| ga. [!~ =] |- o——————M"/\—1 -/ 0150 |- > (3 -I gor != gos |- e -| 0.015 |- 0.007. | |- 9.003 | |- & C - 0.0015 e NP s... ." %.. %>: ©:; 3s © *~ # - : -| 0.0007 - f - 0.0003 a -| 0.00015 |- e + (- -| 0.00007 |- € as- 41 # 4! lo |! @ | |: |! |= Tl' feal ; ® f ®.% Al | Machi S T § | | | | | | 9 1+ | | € | I £ Sample 41 42 43 44 Sample Jp 42 43 44 C 0 2 4 6 8 10 INCHES Analyst: N. M. Conklin, Denver FIGURE 47.-Semiquantitative spectrographic analyses of fresh and progressively more altered granodiorite, Hayes and Wheeler tunnel, Central City district, Gilpin County Colorado. Analyst: N. M. Conklin, 64 ALTERED WALLROCKS. CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone 1 g 2 3+ 4) iG Zone - 1 2 3 F T Percent f I T3] - 1310 |- =g ---! - if y ia B 6 - [g" t> a {us, s (+ # 07". '- st :os - l- =] 1 0156 |- i -t 007 | |= - -| 0.03 |- - = -| 0.015 = \\ 74..0,.007, . !~ i © ® -| 0.003 (> - 0.0015 --it _ ~~ _.. ts <4 De- i =|. 0.0007 |- X 8 i -| 0.0003 f Es e -| 0.00015 aad -| 0.00007 £". I I 1 Sample 46 47 _ 48 49 50 Sample 46 47 LAs, E, Bi. Ge, In, P, $b 9h, To, fl I I I | I I @ 4 « ¢ T | I | ~a §C B L w al 0 Zone 1 2 3. 4~ "G Zone 1 4 7 T Percent I I [=> >10 E § S - -- 7 \- -I -| $ -| @ 3 - 4 1.5 |- C4 Ag, Au, Cd, Dy, Er, Gd, Hf, 0.7 [- Hg, Ir, Mo, Os, Pd, Pt, Re, - Rh, Ru, Sm, Ta, Th, U, W 0.3 T - 0.15 - -I o.o7 - |- s Zt, 0.03 [- - 0.015 . - 0.007 |- 0.003 |- 0.0015 |- alt sua -| 0.0007 |- X_o-- fas | 5 0.0003 |- e&-~ i 0.00015 - -| 0.00007 |- ad | I L._s= 4 | I ts fees Sample 46 47 € 48 49 50 Sample 46 47 48 49 50 D 0 10 20 INCHES 1 gs 12" [- ncn I | Analyst: N. M. Conklin, Denver FIGURE 48.-Semiquantitative spectrographic analyses of fresh and progressively more altered microcline-quartz-plagioclase-biotite gneiss, Essex mine, Central City district, Gilpin County, Colorado. Analyst: N. M. Conklin. GEOCHEMISTRY OF ALTERED ROCKS 65 Zone V V V G V 4 4 Zone V V V G V 4 _ 4 T I I I I I I I I Percent an 23, ale Sis" "B 'a ~Llos . ale <] ube -| 0.07 /.! -| 0.015 [2 -| 0.007 c.) ty f -| 0.003 i \ p* -e- 3 -| 0.0015 (s \ / s -| 0.0007 - sh og '®--e - -| 0.0003 |- T I - >10 t- =] [>;. . ~s" No > L- \ if -| 0.00015 |- As, B, Bi, Ge, In, P, Sb, Sn, Te, TH - -| 0.00007 |- 1 1 I | I I 1 Sample: 61.52 53 54 $5 56 57 Sample 51 52 53 54 55 56 57 Zone V V G V 4 4 Zone B v _V ; v'. "0 vy. _ 4 4 V I I # I "I I I Percent T -| >10 \- I I I I I I I - -I 7 - S 3 -- Au, Dy, Er, Gd, Hf, Hg, Ir, Mo, Os, Pd, ig Pt, Re, Rh, Ru, Sm, Ta, Th, U, W -| 1.5 \- -| 0.7 - - :O.3 (= -> 0.15 -- =- 0.07 - ~ 0.03 [- ~/'0.015 '- -1©0.007 - I- 1::0.003 |- -1.©0.0015 |- -| 0.0007 |- - 0.0003 |- -| 0.00015 |- [- - 0.00007 |- Sample 51 52 53 54 55 56:57 __ Sample 51 :52: b3 :-64 55 56 57 0 10 20 30 INCHES LOE Analyst: N. M. Conklin, Denver FIGURE 49.-Semiquantitative spectrographic analyses of progressively more altered microcline-quartz-plagioclase-biotite gneiss, Essex mine, Central City district, Gilpin County, Colorado. Analyst: N. M. Conklin. 66 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone 3 4 4 4 '. 'G Zone 3 4 4 a .C I | T T TST Percent F T 1 |- - >10 t- '>———4—'T\\§\ ys -A- [~ \ K //‘\ /. ~ 7 - -< o - y- J 1.2 T & -| ~1.5 - -L. 0.7 [- ~:0.3 (= - '0.195 |- f- 4 0.07 (- Re - 0.03 t- = ~i>0.015 : |- [- : #. Sp _ _ @- -#- -e ~1 0.007 [- -- 'T -- =-©0,003 |-- En -| 0.0015 |- 0.0007 nd =~0.0003 |- e [~ - 0.00015 W [~ 0.00007 | | | | L- 1 | | | {:- Sample 58 59 60 61 62 Sample 58 59 60 61 62 I 4 I I Zone 3 a' A 1 . G Zone 3 4 a. 4a 'C T T T T T7 Percent T T T T F3 |- -| >10 i- a Au, Cd, Ce, Dy, Er, Gd, Hf, - ~ 7 - Hg, Ir, Mo, Os, Pd, Pt, Re, = Rh, Ru, Sm, Ta, Th, U, W (- Fe MHS [= > |- -= 1.5 t- = |- -| 0.7 T - -| 0.3 I - 10.15 \- 34 - 0.07 |- - - "0.03 [(- 10 - .s\ @- 7f'fl ame Al | \ e 10 (- - s 1 {3 E L - Fe - 3 -- w » Au, Ce, Dy, Er, Gd, Hf, Hg, 1.5 - | Ir, Mo, Nd, Os, Pd, Pt, Re, ~ 0.7 |_ | Rh, Ru, Sm, Ta, Th, U, W a 03) «/- Ll gis. - o- $o7 «|- 324 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 |- N- -~ ~~ 0.00007 |- x" 1 | 1 1 1 1 1 1 A 1 Sample 63 64 65 66 67 68 Sample 63 64 65 66 67 68 C D 0 1 2iFEET EILL Analyst: N. M. Conklin, Denver FrGuRE 51.-Semiquantitative spectrographic analyses of fresh and progressively more altered microcline-quartz-plagioclase-biotite gneiss, Essex mine, Central City dis- trict, Gilpin County, Colorado. Analyst: N. M. Conklin. 68 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO L - Zone 1 2 3 c 34V eee e> 7A Be T -.. -I 73 | \a\1\, j Sample 69 A 72 Z 1 A- -m i \\\\\75 Sample 69 71 72.73 74 0 Percent >10 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 | Sample 69 70 B Zone Percent >10 7. 3 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 | | I | T .S -T T I I I I I F. Ag, Au, Cd, Dy, Er, Gd, Hf, Hg, Ir, Mo, Os, Pd, Pt, Re, = Rh, Ru, Sm, Ta, Th, U, W o- -e- -?- - @ _ e -e - fa Sample 69 70 71 72 7374 1 2 FEET D Cox sip d® LIL LLL FrGurE 52.-Semiquantitative spectrographic analyses of fresh and progressively more altered quartz diorite, Jo Reynolds mine, Lawson-Dumont-Fall River district Analyst: N. M. Conklin, Denver Clear Creek County, Colorado. Analyst: N. M. Conklin. » GEOCHEMISTRY OF ALTERED ROCKS 69 Zone 2-3 3 3-4 G Zone I | | I Percent >10 ~ <7, 1 A8 -] 1.5 =i 0.7 - 0.15 . 0.07 =~ 0.03 ~ 0.015 -| 0.007 - ©0.003 -| 0.0015 -| 0.0007 (- Be -| 0.0003 E- -| 0.00015 (- -| 0.00007 | | | | 1 2-3 I I T I & As, Bi, In, P, Sb, Sn, Te, Tl H Sample 76 77 78 79 Sample 76 Zone 23 3 3-4 G Zone l | I | Percent f- -| >10 = 7 = 3 -|- 1.5 -| 0.7 -|- '0.3 =| 0.15 ~: 0.07 ->0.03 3 0.015 -| 0.007 - 0.003 > et. -| 0.0015 [- - 0.0007 - - 0.0003 [- - 0.00015 p- -| 0.00007 I I I L B 2-3 3 3-4 G Au, Cd, Ce, Dy, Er, Gd, Hf, Hg, Ir, Mo, Nd, Os, Pd, Pt, Re, Rh, Ru, Sm, FL Ta, Th, U, W Sample 76 77 78 79 } Sample 76 77. 78 79 0 1 2HFEET fois d cues. (hs, darem nt. Lana Analyst: N. M. Conklin, Denver 53.-Semiquantitative spectrographic analyses of progressively more altered quartz diorite, Nabob mine, Lawson-Dumont-Fall River district, Clear Creek County, Colorado. Analyst: N. M. Conklin. 70 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone x 3 4 Zone 1 3 4 I I I Percent I . I I - '> . 41-710 s - ~ ‘\ Ca & % I 7 t mas x' k - .\5,\_ \\\ i 3 ~: =I |- Mg‘Na .\\\\'=—_—=-—:' -| 1.5 - - \- -| 0.7 |- - As, B, Bi, Ge, In, P,Sb, Sn, Te, Tl I =i ~0.3 - -I o - 0.15 p- -I - - 0.07 |- =I |- - ~0.03 - = (- = ~0.015: |- ig i- ={ 0.007 - - i -41 0.003 .|- Ga - - -1- 0.0015 |- Bil)“ - TT---Q-------0 t- +1: 0.0007 |- - - <.0.0003 - = [- -| 0.00015 |- - o -| 0.00007 |- s -| | 1 1 I | I Sample 80 81 75 Sample 80 81 75 A B Zone 1 3 4 Zone 1 3 4 I I I Percent xP I I i- -| >10 [- -I Ag, Au, Ce, Dy, Er, Gd, [- Fe =I 7 [= Hf, Hg, Ir, Mo, Nd, Os, ; |.. I 3 Bs Pd, Pt, Re, Rh, Ru, Sm, " Ta, Th, U, W |- -| 1.5 i - - -| 0.7 t- - - =~ 0.8 |- ~ - 44 0.15 - - [- -| 0.07 t- - - -+ 0.03 t- - - = 0.015 |- - as 10.007 _ - ~- (~ 1: 0.003: - -I [- A ases. -{ 0.0015 - -f (> Tg ~~ =| 0.0007 |- s = .Zn - -1~ 0.0003 |- e_ - ml. //. [- -| 0.00015 |- e. - \\ PLA [- -| 0.00007 |- me = __ I | I | 1 Sample 80 81 75 Sample 80 81 75 C D Analyst: N. M. Conklin, Denver FiGurE 54.-Semiquantitative spectrographic analyses of fresh and progressively more altered quartz diorite, Jo Reynolds mine, Lawson-Dumont-Fall River district, Clear Creek County, Colorado. Analyst: N. M. Conklin. Zone 1-2 GEOCHEMISTRY OF ALTERED Be Sr e Pics 1 Fe Zone 1-2 ROCKS Percent >10 - 7 f- 3 w 1.5 t- 0.7 E= 0.3 - 0.15 " 0.07 - 0.03 as 0.015 0.007 0.003 0.0015 0.0007 0.0003 |- 0.00015 0.00007 T I T I I 1 Bi, Ge, P, Sn, Te, Tl I | Sample 82 Zone 1-2 83 84 85 B Percent I 3 4 I TI Sample 82 >10 7 3 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 Sample 82 Au, Dy, Er, Gd, Hf, Hg, Ir, Os, Pd, Pt, Re, Rh, Ru, Sm, Ta, Th, U, W INCHES 84 - 85 Analyst: N. M. Conklin, Denver FIGURE 55.-Semiquantitative spectrographic analyses of progressively more altered biotite (metasedimentary) gneiss, Widow Woman-Cherokee mines, Central City district, Gilpin County, Colorado. Analyst: N. M. Conklin. 72 FiGurE 56.-Semiquantitative spectrographic analyses of progressively more altered biotite-quartz gneiss, Cherokee mine, Russell Gulch area, Central City district, Gil- pin County, Colorado. Analyst: ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone 4 Percent -| >10 7 3 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 | Percent >10 7 3 1:5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 -| 0.0003 -| 0.00015 -| 0.00007 | | Sample 86 87 88 89 C 90 91 92 Zone 4 4 V 4 4 V 3 3:2 |- | As, Bi, Ge,P, Sb, Sn, Te, TI 8 Sample 86 87 88 89 90 91 Zone 4 4 V 4 4-V 3 3-2 | | I | | | | I I I I I I I Au, Dy, Er, Gd, Hf, - | Hg, Ir, Os, Pd, Pt, S Re, Rh, Ru, Sm, Ta, Th, U, W \ \ | | | | | Sample 86 87 88 89 90 91 92 Analyst: N. M. Conklin, Denver N. M. Conklin. 7 GEOCHEMISTRY OF ALTERED ROCKS 73 Zone 1 2 S 4&NV Zone 1 2) 3 V | (urea a aiid | ca ~ // I I T I I Percent T T Fal I - >10 | --- T gI Al -| 1.5 - -| 0.7 t- = 0.3 - - 0.15 t- - 0.07 - -. 0.03 - |- &~ Sy -| 0.015 f- Ne -| 0.007 -| 0.003 -| 0.0015 f- -| 0.0007 it - '0.0003 |- c __ -| 0.00015 Z (- -| 0.00007 |- > ts - P I I I T I I I I I Percent i ] I TH | (- ¢ >10 \- -| ].}. 7 |- | Au, Dy, Er, Gd, Hf, Hg, j # Ir, Os, Pd, Pt, Re, Rh, = 3 - | Ru, Sm, Ta, Th, U, W = mare "o[ I -| 07 |- - -| 03 _ |- - -| 015 _ |- 2 -| 0.07 _ |- -| 0.03 |- e- -| 0.015 |- -| 0.007 a -| 0.003 -| 0.0015 -| . 0.0007 I - 1 1 # a P 4 P » D ~ D 1.5J 6 - -| 0.0003 |- &- -- -& Lo -| 0.00015 \- -| 0.00007 l I 1 i> SS 1g; Sample 93 C 94 95 96 Sample 93 0 1 2 FEET L 1 1 1 N 1 J I Analyst: N. M. Conklin, Denver FIGURE 57.-Semiquantitative spectrographic analyses of fresh and progressively more altered biotite (metasedimentary) gneiss, Banta Hill mine, Russell Gulch area, Central City district, Gilpin County, Colorado. Analyst: N. M. Conklin. 74 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Percent I R I (s - >10 t- g -=- Fikes. #- e p - ie age, Jo y: - +4 8 |-- s \ /\ a «::: o., \ -ef 1.8 (s © .> t- T s cdk . © =| 07 - / - t Fa e 1 os: . f - \ 0.15 L_ # - f= ®. -: 0.07" != - g Ba * Pb # E - 0.03 |- s/ -] \- -| 0.0150 |- e - 3 $ \ 0.015 ( =|" g.oo7 - |- Ae -| 0.003 |- F5 \ -| 0.0015 |- (s \_ © 0.0007 |- o 1 _ sl 9.0003 - (- .___B_e_—‘\O\} -| 0.00015 |- ~ 3 -| 0.00007 |- As, B, Bi, P, Sn, Te, TI L I 1 | 1 F | 1 1 Sample 98 99 100 101 Sample 98 99 100 101 A B Zone 1-2 3 4 V Zone 1-2 3 4 V I J 73 Percent I I 17 Zone 1-2 3 4 V Zone 1-2 3 4 : V I I t ~3 - ~>10 |- # 3 Dy, Er, Gd, Hf, Hg, ir, | ._—_——._———.7Z - 7 S Mo, Os, Pd, Pt, Re, Rh, - / / f Ru, Sm, Ta, Th, U, W - 3 * - S| Aig | = 10 t= ay %R: x- 7 [-- = 3 Ho % a Zone _ 1-2 3 4 4-V Zone 1-2 $ 3 4 4-V I & ~ 1.5 (= . - =] 0.7 (= f - -| 0.3 |- yf # 7 015 |- A ¢ t -| 0.07 _ |- e- -e # - -| 0.03 |- ~6 - |- ___ e -2---- o_ -| 0.015 -| 0.007 -| 0.003 |- -| 0.0015 |- -| 0.0007 -| 0.0003 |- -| 0.00015 |- = - 0.00007 |- As, B, Bi, P, Sn, Te, Tl s | | | | | 1 I I Sample 102 103 a 104 105 Sample 102 103 B 104 105 - Ca I w s jo | I x 1 Be Zone 1-2 3 4 4-V Zone 1-2 3 4 4-V I | I Percent I I I I \- -| >10 o - 7 W "l T [c Aerdv Dyi— Err Gd, ml i Hf, Hg, Ir, Mo, Os,|" [2 § Pd, Pt, Re, Rh, Ru, -| 1.5 I Sm, Ta, Th, U, W oss __ -| 07 __ }- - [- @- -_- ~~ -# ¢ ~o ~©:0.8 % - i £ =- ois. . != S (= sl o07.. := -t l oo3 . |= =| pols. |- =|: 0007 _ t t' | != =| o.0018 |- =.0.0007 |- C -| 0.0003 = -! 0.00015 - -| 0.00007 |- al % S | Sample 102 103 104 105 Sample 102 103 104 105 C D Analyst: N. M. Conklin, Denver FIGURE 59.-Semiquantitative spectrographic analyses of progressively more altered biotite-quartz gneiss, Cherokee mine, Russell Gulch area, Central City dish-10]; Gilpin County, Colorado. Analyst: N. M. Conklin. 678880 O-63--6 « 76 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO T T | T T | Percent I [- =| >10 [- o -- <[. . 9 f- o ~ Al ~~ ----e_ a Zone - 1-2 3 4 G V Zone 1-2 3 4 G V I I Rs 3 f- ~ R -| 0.7 t- = - "0.3 - - -| 0.15 - - - 0.07 o ing -. 0.03 - |- ‘4—.\\Sr /,0\\ -| 0.0156 |- -i 9.007 > Pw * - -! /- * =>0.0015 |- *> 1 0.0007 |- \- -| 0.0003 (s Be -| 0.00015 a x -| 0.00007 1 | | 1 &] I 1 1 1 1 Sample 106 107 108 109. ' 110 Sample 106 107 108 i095 *~ tto A B Zone 1-2 3 4 G V Zone 1-2 3 4 G V I I I I I Percent T I I I t- -| >10 |- - Fe Cd, Dy, Er, Gd, Hf, o -~ . 7. - Hg, Ir, Mo, Os, Pd, = Pt, Re, Rh, Ru, Sm, £ f A 2 80 t.. "| uw § (s s 18s - S s' og" O| 4 s os". - 1 ois | |- <] E oor - |- [ obs :- -| o.015 -| 0.007 -| 0.003 -| 0.0015 |- -| 0.0007 s -| 0.0003 (s -| 0.00015 LS -| :0.00007 |- | I | | | I 1 I I | Sample 106 107 108 109 110 Sample 106 107 108 109 110 C. D Analyst: N. M. Conklin, Denver I I 1 - I | I 1 FrGurE 60.-Semiquantitative spectrographic analyses of progressively more altered metasedimentary gneiss, Phoenix mine, Idaho Springs district, Clear Creek County, Colorado. Analyst: N. M. Conklin. GEOCHEMISTRY OF ALTERED ROCKS Zone 1 2 3 4 I I I I ~ >10 K - @- -- =-- @ -- --- -@- - -- - ~ : 7 je . -| ' 3 - o ~~ ~\~ '/' -| 1.5 (= *e" =! 0.7 Mg So Saor Sce e - - 0.3 s- ca 1 0.15 fe Ca-Sr - 0.07 he -| 0.03 - -| 0.015 E- -| 0.007 s , -| 0.003 Zone Percent | - -| 0.0015 E- -| 0.0007 -- - 0.0003 |- (- U &e & h - 0.00015 [- -| 0.00007 |- 1 f. 3 a ~" ¢ I K "PB \—————.// - Ga I ~ Sample 111 112 113 114 Sample 111 112 113 114 A Zone 1 2 3 4 Zone 1 h -| >10 (~ 2. 7 [- Sl s o <--~*-s a. . :]! 15 21.07 I 0.3 -| 0.15 SI 0.07 -| 0.03 -| 0.015 -| 0.007 -| 0.003 -| 0.001 wa @- - - -#~ ~- -| 0.000 (< -| 0.000 Sample 111 112 113 114 -: 9.0007 ) (- -< AB - 0.0003 |- | I 1 1 Percent | Au, Cd, Dy, Er, Gd, Hf, Hg, Ir, Nd, Os, Pd, Pt, Re, Rh, Ru, F4 Sm, Ta, Th, U, W 5 |- 15 07 I Sample 111 112 113 114 BD Analyst: N. M. Conklin, Denver 77. FrGURE 61.-Semiquantitative spectrographic analyses of fresh and progressively more altered quartz monzonite prophyry, Banta Hill mine, Russell Gulch area, Central City district, Gilpin County, Colorado. Analyst: N. M. Conklin. 78 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone 1-2 3-4 Zone 1-2 3-4 G:: V ] | ~. Percent I § I I I [- | &- -- BL - eo- - -| >10 |- & -R ====== ---# -- i ai -| _ 7 (> fe > \- o// 'a (= ‘zx = & ] 1.5 (> 2 & "07 % [2 al -| 0.3 - - < 0.15 |- > - 0.07 (- -| 0.03 |- [- Sr/ X -I qos. |- [- x 3 i 0.007 [- 's" , ' -( obos |- E -| 0.0015 |- [- -| 0.0007 |- - o < o e - 0.0003 |- (~ -| 0.00015 As, In, P, Te, Tl, (- -| 0.00007 - | 1 | | 1 > 1 | Sample 115 116 117 118 Sample 115 116 117 118 B -= G5 6 -| < I I Zone 1-2 3-4 G V Zone 1-2 3-4 G V I I | I Percent I I I I a t 719° ~ [ € E- so 9 [x Au, Cd, Dy, Er, Gd, Feo Hf, Hg, Ir, Os, Pd, (- - 3 \- Pt, Ra, Rh, Ru, Sm, - Ta, Th, U, W \- -| 1.5 i - - -| 0.7 t- - - -| 0.3 [-- -I -| 0.15 (- -I -| 0.07 (- -I -| 0.03 [-~ '/0———0—' -I Ce =- 0.015. |- n s ne ae m -- -. & a ~A -| 0.007 |- T2 «No ~=@ al |= o 3 =e - - =I 0.0015, ! s - #I glooo7 | ~" "x x" * -| 0.0003 |- - -| 0.00015 | 'e - '= [< -| 0.00007 1 1 I | I | | 1 Sample 115 116 117 118 Sample 115 116 117 118 C D Analyst: N. M. Conklin, Denver I x x. | FrGurE 62.-Semiquantitative spectrographic analyses of progressively more altered bostonite, Phoenix mine, Idaho Springs district, Clear Creek County, Colorado. Analyst: N. M. Conklin. GEOCHEMISTRY OF ALTERED ROCKS Zone 1 2 3 4 G-V I | I I I - .\ J Ba tee ~- - -a., "% (- ® ---# g re- --- \\ i% \- +-~-~>4.- ~:: *~ s Tse. - \\. [- t (= o- o s -* «---» I | | | | A Zone 1 2 3 4 G-V | I I I I ~ + fe 2 m a ws & & & & L F 2% * &2 / C *Averages of six samples or less Percent >10 7 3 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 Percent >10 7. 3 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 Zone 1 2 3 « G-V | | I I J I - o———-—o————o——s'——o-—-——-0 - [(- L & s«~*-s -@ - - ge - I- / a T J I I [_ As, Bi, Ge, In, Sb, Sn, Te, m | / Zone 1 2 3 4A I I Au, Cd, Dy, Er, Gd, Pd, Pt, Re, Rh, Ru, Sm, Ta, Th, U, W Analysts: P. R. Barnett and N. M. Conklin, Denver and K. E. Valentine, Washington, D. C. Hf, Hg, Ir, Mo, Os, = 79 FIGURE 63.-Average values for elements determined by semiquantitative spectrographic analysis of 44 samples of fresh and progressively more altered granite, grano- diorite, and microcline-quartz-plagioclase-biotite gneiss. Analysts: P. R. Barnett, N. M. Conklin, and K. E. Valentine. 80 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO Zone 1 2 3 4 GV Zone 1 2 3 4 GV T T T T 1 Percent T T T T T |- [- 1: -: :s -S- ' _s" _. 2 52 al Al al ¢ % - ‘——¢—0—0\' 3 = 1.5 E- loys -|- As, Bi, Ge, P, Sb, Sn * / g ;}. £ 2 {gas (f ,A = +I p07... |- 7 x -| 0.03 E: @. ~- ~- -- -- # . ig = G08 |- = 10.007) :| = -| 0.003 |- B..s® - 7 -| 0.0015 n #-_ > ( -| 0.0007 Tg\' 7 i -| 0.0003 |- 4 f Be -| 0.00015 -: -| 0.00007 |- aa. i P b I | Zone a N t p M < - Zone 1 2 3 4 GV I I T | T Percent T T T T 1 i- -| >10 i- - T <*> f Dy, Er, Gd, Hf, Hg, Ir, 3] = 3 - Os, Pd, Pt, Re, Rh, Ru, =- +l t:e : Sm, Ta, Th, U, W a a 07 7 Au, Cd, Mo* El (- -f = 0:15 - £- - 0.07 (- -I -+ 0.03 - -< '0.015 |- -t 0.007 | / -- -| 0.0015 |- e-.-.es-_NB_.q-._-.-ezi-_-e - "e - 0.0007 |- __ e- * 4\' &l (- -| 0.0003 |- - ad -| 0.00015 [- -| 0.00007 I | 1 | | | | I I | C D I '\‘ > \\m \ \ & I I | *Sparse amounts in 1 to 4 samples only Analysts: P. R. Barnett and N. M. Conklin, Denver FIGURE 64.-Average values for elements determined by semiquantitative spectrographic analysis of 31 samples of fresh and progressively more altered biotite-quartz- plagioclase gneiss, biotite gneiss, and metasedimentary and migmatitic gneisses. - Analysts: P. R. Barnett and N. M. Conklin. GEOCHEMISTRY OF ALTERED ROCKS 81 Zone 1 2 3 4 GV Zone 1 2 3 4 GV T | I T | percent | T 1 | I >10 |- [~ Q _K =I 7 (- Ca ~-. iy ong leger fonts : o [- #8=- if =---Q- -8 =8 - 3 ax #:, 2 te.... me _Ns ne - ME ge: ® -| L5 [> As, Bi, Ge, In, P, a x wer. ~ e a I l I ® ® x | I Zone 1 2 3 4 GV Zone 1 2 3 4 GV 7 Percent I l | ' | a < - - >10 (- - Fe 7 E- - Au, Cd, Dy, Er, Gd, & ~. 2 x Hf, Hg, Ir, Mo, Os, ~ |: 21 ' {.g S Pd, Pt, Re, Rh, Ru, ] Ti Sm, Ta, Th, U, W |- ___ @ --- -=- =-=-@----@----@ -| 0.7 |- s \- - 03 - |- - I # ® $ | - g -| 0.15 |- a - eh -| 0.07 _ |-. . Ce* Ir -I f - o03 |- Nd; > & -| 0.015 |- BX Q“\ = -| 0.007 |- ex 'x f -| 0.003 |- _ e @ -~@- o ---# - =| 0.0015. |- @------- @ -- ----- @ ----~< ~@#-7 --- -- $ - ( -| 0.0007 |- /. ~A |- -| 0.0003 |- te [> -| 0.00015 I \- -| 0.00007 |- PS - | | | 1 | | | | 1 C D *Average of two samples Analysts: P. R. Barnett and N. M. Conklin, Denver $ I > . ® I FIGURE 65.-Average values for elements determined by semiquantitative spectrographic analysis of 17 samples of fresh and progressively more altered quartz diorite. Analysts: P. R. Barnett and N. M. Conklin. 82 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO TaBu® 25.-Amount, in percent, of some regular elements in rocks Element ‘ 1 l 2 3 I 4 l 5 6 I 7 M: ese Ple enne: ene ease leger vend 0.00065 | 0.01792 | 0.007 Bell.... 0.0001 | 0.0001 | 0.0001 | 0.0002 | .0002 | .00036 | .00055 ost | .o2 .o7 .o7e | .0300 {fig }.03 Be |-: ors | os 15 | .o2s0o | .0670 | .oss .................... 002 | :0003 3 | lool5 0016 003 002 002 __________________ f : ©0003 .................... 3 ©00015 - 0003 ©000012 ____________________ ©0045 rane pang © 00004 Bi EAR Cl:" 1 (. 0002) 1 Requires further checking. . Biotite-muscovite granite and granodiorite, average of 5 samples (this report). Metasediments, average of 14 samples (this report). . Quartz diorite, average of 3 samples (this report). . Dike rocks, average of 2 samples (this report). . Igneous rocks, average, Green (1953). Granite, average, Rankama and Sahama (1950). . Acidic rocks, average, Vinogradov (1956). ALKALI AND ALKALINE EARTH (REGULAR) ELEMENTS Lithium, beryllium, strontium, and barium occur in trace amounts that generally decrease toward the vein. The distribution of sodium, potassium, magne- sium, and calcium ions is noted earlier. Lithium is rarely detected in these wall rocks, but where present in microcline-quartz-plagioclase-biotite gneiss (fig. 38) no marked redistribution is noted. Lithium (Lit, 0.68A) can be admitted into the lattices of silicate minerals in 6-fold coordination as replacement for Mg+* and Al" ®; biotite, muscovite, and their clay-mineral alteration products are the persistent potential lithium- bearing mineral structures in the rock. Lithium is readily leached and is not concentrated in zone 4. Beryllium is present in amounts close to its analyti- cal sensitivity in all of the wallrocks; and in all rocks except quartz diorite there is little variation across the zone of alteration. Beryllium is concentrated slightly toward the vein in quartz diorite. The beryllium ion is small and occurs in most types of silicate structures, where Bet" (0.35A) most generally replaces tetrahe- drally coordinated Sit, as well as in some oxide and phosphate mineral structures. Beryllium is readily leached and removed from zone 4 rocks by thermal waters. Strontium and barium are moderately abundant trace elements in this group, and barium is detected in some rocks by standard analysis. Spectrographic analyses indicate that both elements are depleted in rock adjacent to the vein. Barium (Bat", 1.34A) most commonly replaces potassium in mineral struc- tures, and strontium (Srt?, 1.12A) has an affinity for calcium ; their relative distributions here correlate directly with the abundance and distribution of potassium- and 8 Here and in the following discussion of sizes and position of ions in mineral lat- tices the data from Bragg (1937), Green (1953), and Rankama and Sahama (1950) are the principal sources of reference. calcium-bearing minerals, especially feldspar and mica. Bray (1942) has shown that strontium is more abun- dant in feldspar than in mica in granodiorite from Boulder County, Colorado. Strontium also replaces calcium in carbonate and fluoride structures. A loss of strontium, shown especially well in figures 37 and 47, occurs in rock in which the clay minerals are being enriched in potassium at the expense of calcium. Strontium is more soluble than barium in thermal water and is more likely to be removed from the system ; barite is a common gangue mineral in the vein and may be disseminated in adjacent walls along fracture and foliate surfaces. REGULAR ELEMENTS - Boron, gallium, and lead are important trace con- stituents in this group (aluminum, silicon, and phos- phorus are discussed earlier), but germanium, arsenic, indium, tin, antimony, and bismuth were identified in sparse amount in some rocks; thallium and tellurium were looked for but not detected. Boron is sparse, occurs mostly in rocks containing abundant biotite, and is concentrated veinward. The element is small (B+, 0.23) and is believed to be a replacement for silicon in hydroxyl-bearing minerals such as biotite and amphibole. The boron data are too sparse to permit assignment of boron entirely to veinward concentration by leaching from host rock minerals or by direct deposition from vein solutions. Gallium occurs in most Front Range rocks in amounts comparable to those elsewhere, and well above its spectrographic sensitivity. Ga+ ions (0.62A) may replace Al" in minerals having a mica structure 'as well as Fet and Znt* in magnetite and sphalerite. In moderately argillized wallrocks gallium may be concentrated slightly, but it is generally leached from sericitized rock. This suggests that gallium may be concentrated in clay minerals as a result of rock alteration. Indium is sparse in most Front Range rocks, in part because it occurs in amounts close to the spectro- graphic sensitivity limit for the element. Metasedi- ments that contain abundant biotite, however, com- monly contain indium in amounts comparable with averages in other rocks. The distribution of indium in general follows that of gallium, but Int" (0.814) may replace Fet, Mn+*, and Znt* in sulfide minerals as well as AI" in micas. Slight increase in indium occurs in vein gouge clay and in wallrocks caught up in some veins (figs. 55 and 56). Germanium is of sparse occurrence in fresh bostonite and in sericitized metasedimentary rock in and border- ing veins. The analytical sensitivity for germanium is higher than average values in comparable rocks else- GEOCHEMISTRY OF ALTERED ROCKS where. Ge+* (0.53A), which resembles gallium and is found most commonly in quartz replacing Sit, also is in simple silicates, topaz, garnet, and ilmenite. The element is readily weathered at the surface and is leached from argillized rock. Tin is present in small amount in bostonite and meta- sediments from the Phoenix mine. The rocks in this area contain less tin than averages of other igneous rocks. Sn** (0.71A) can occur in plagioclase, clay min- erals, fluorite, and sphene as a replacement of Fet, Ca+, Sct, Sit, and may also occur in some sulfide minerals. No pattern of tin distribution could be detected from these data. Lead is a common trace constituent in most rocks in this area and occurs in amounts comparable with aver- ages of analyses in table 25. The amount of lead in 1gneous and metasedimentary rocks of Precambrian age increases veinward whereas the amount of lead in dike rocks of Tertiary age decreases veinward. In addition to a site in galena, Pb+* (0.84A) can occur as a replacement of Ca+® in apatite, aragonite, epidote, and biotite, or of K+ in feldspars and micas. Most of the lead in vein rock and gouge clay is believed to occur in galena. Small fractures into sericitized rock may also contain galena. Arsenic, antimony, and bismuth are very rarely found in the altered wallrocks in the central part of the Front Range mineral belt, and all are rare in the structures of silicate minerals. These elements more commonly combine with sulfur as sulfo-salts of the heavy metals (Cu, Fe, Ni, Co). Arsenic 0.46) is most common in pyrite and arsenopyrite but may replace phosphorus in apatite. Bismuth (Bits, 0.74A) may replace calcium in apatite and with (0.62) occurs in an unknown granite mineral. Antimony, arsenic, and bismuth also can occur in galena structure. These elements occur in or close to the vein in wall- rocks such as bostonite (Sb, Bi) and metasedimentary gneiss (Bi) at the Phoenix mine, biotite-quartz-plagio- clase gneiss (As, Sb) at mines in the Russell Gulch area, and in microcline-quartz-plagioclase-biotite gneiss (Bi) in the Essex mine. TRANSITIONAL METALS The first group of transitional metals, characterized by progressive filling of electrons in the next outermost shell, comprises those from scandium through zinc (atomic numbers 21 through 30); the distribution of iron, titanium, and manganese was noted previously. Zinc, and less markedly copper, are concentrated vein- ward, but the heavy metal group, V, Cr, Co, Ni, and Se, are closely related in their distribution and concen- trations within these rocks; they tend to be removed from rocks adjacent to the vein. 83 Seandium is present in most rocks of Precambrian age in amounts greater than average. It is sparse or absent in dike rocks of Tertiary age. Sct (0.81A) can replace Mg*, Fet, or Al*® in ferromagnesian minerals. In general scandium is lost as rocks are altered, and in quartz diorite it is lost gradually, but is concentrated in and adjacent to the vein. However, argillized meta- sedimentary gneisses (zone 3) often show a concentra- tion of seandium whereas the adjacent sericitized rock in and bordering veins appears to have lost secandium. Vanadium is most abundant in igneous and metasedi- mentary rocks of Precambrian age, but is less abundant in later dike rocks. The average abundances of vana- dium in these and other rocks in table 26 are compar- able. Vat** (0.63) is most common in igneous rock minerals replacing Tit, Fet, and Alt"; V+*5 (0.59A) commonly replaces Alt} in clay minerals and P+" in apatite. Vanadium has been detected in amphiboles, micas, sphene, rutile, and magnetite-ilmenite. TaBur 26.-Amount, in percent, of some transitional metals in rocks 1 2 3 4 5 6 Verne 0.014 | 0.010 | 0.017 | 0.003 | 0.015 0.004 £ y # . 0015 . 020 . 0025 . 0015 . 0023 . 0005 . 005 . 008 . 0008 . 078 . 007 .003 .05 . 0051 . 006 1. Biotite-muscovite granite and granodiorite, average of 5 samples (this report). 2. Metasediments, average of 14 samples (this report). 3. Quartz diorite, average of 3 samples (this report). 4. Dike rocks, average of 2 samples (this report). 5. Igneous rocks, average, Green (1953). 6. Acidic rocks, average, Vinogradov (1956). A trend toward concentration of vanadium in argil- lized zones of some biotite-rich rocks is observed, but the general distribution trend is a loss of vanadium toward the vein. The sericitized zone of most rocks is leached, and vanadium also is removed during weather- ing as evidenced by the significant content of the element in water emerging from the Argo tunnel (table 34). In the central part of the Front Range mineral belt, granite, granodiorite, some biotite-quartz-plagioclase gneisses (all biotite-rich), and amphibolitee have a higher content of vanadium than microcline-quartz- plagioclase-biotite gneiss and porphyry dike rocks. Bray (1942) noted that biotite in monzonite from Jamestown, Colorado, areas contained 0.0670 percent V; plagioclase from the same rock contained 0.0007 percent V. It is inferred from these data that biotite and amphibole are the prime host minerals for vanadium in these rocks. As amphibole is altered in zone 3 and biotite is altered in zone 4, vanadium is released. Chromium is present in most rocks in amounts well above the spectroscopic sensitivity limit but in amounts 84 comparable with other rocks. It is probably concealed in oxide and silicate structures where Crt® (0.634) replaces Alt} and Fet outside of the silicate framework. Chromium in magnetite-ilmenite is not released as rapidly by weathering as chromium in hornblende, mita, and garnet; Bray (1942) reports 0.11 percent Cr in biotite and 0.05 percent Cr in muscovite. The abundance of chromium generally decreases veinward but locally it increases slightly in the clay-mineral zone and in the vein. Cobalt and nickel, present in most rocks, are most abundant in quartz diorite; nickel is more abundant than cobalt. In general, these data suggest that the elements are slightly less abundant in rocks of the. Front Range than in comparable rocks elsewhere. Hornblende and biotite are the host mineral structures unless sulfide-mineral structures are present. Nit (0.69) is liberated from host minerals readily during weathering and is stable in aqueous solution, but can be fixed in clay-mineral structures. Cot (0.72), which is soluble also, commonly does not remain in hydrated silicates. These elements generally are removed from rocks during alteration as hornblende and biotite are altered. Rocks that produce a prominent montmorillonite altera- tion phase appear to localize nickel slightly, and nickel often is concentrated in vein-gouge clay-perhaps in a sulfide mineral. Copper is an abundant trace constituent in these rocks in amounts generally higher than the average from rocks elsewhere. The porphyry dike rocks con- tain the most copper, and metasediments contain more than the igneous rocks. Cut (0.72) may replace Fet* and Mgt* in silicates but more often occurs in accessory amounts in pyrite disseminated in zone 4 rock. Copper is readily soluble but reprecipi- tated in the vein in the presence of S. In general copper is concentrated veinward in all rocks except porphyry dikes, where there appears to be a decrease veinward. Zinc is not detected in amounts less than 0.02 per- cent, thus is reported only in the dike rocks whose content of zinc is considerably above average. Znt" (0.74A) can replace Fe and Mg in magnetite, amphibole, and biotite. These data suggest that zinc is leached from the argillized rocks and concentrated in the sul- fide phase disseminated in rock caught up in the vein. TRANSITIONAL AND SECONDARY TRANSITIONAL METALS Of the large group of transitional and secondary transitional metals, yttrium, zirconium, niobium, neo- dymium, silver, lanthanum, and ytterbium are found in very small amounts; molybdenum, cadmium, gold, and cerium occur in some rocks in small amounts very ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO close to analytical sensitivity limits; and ruthenium, rhodium, palladium, osmium, iridium, platinum, hafnium, mercury, tantalum, tungsten, samarium, gadolinium, dysprosium, erbium, thorium, and uranium were looked for but were not detected. Zirconium is most abundant in granite, granodiorite, and quartz diorite, less abundant in the metasediments, and least common, in the porphyry dike rocks. Values for Front Range rocks in table 27 are higher than average values for the earth's crust but the values for granitic rocks are comparable with average values given for granite. Most of the zirconium is assumed to reside in zircon, but minor amounts may be incor- porated in mafic rock minerals. Zr+* ion (0.79A) has coordinations of 6 and 8; however, this ion is not in- corporated in mineral structures during early stages of erystallization or after alteration. Once formed, zircon is very stable under mechanical and chemical weathering and mild hydrothermal rock alteration conditions. In general there is a decrease in Zr toward the vein; the loss is small, and the variation across zones of altered rock is not great. Zirconium is not reported in available analyses of mineral water. Taus 27.-Amount, in percent, of zirconium and niobium in rocks ‘ 1 2 3 4 5 6 7 L+ de 0.043 | 0.025 | 0.033 | 0.015 | 0.028 0.046 0.02 NDL .t.! 002 001 | .0015 | .0022 | .0024 . 003 . 002 1. Biotite-muscovite granite and granodiorite, average of 5 samples (this report). 2. Metasediments, average of 14 samples (this report). 3. Quartz diorite, average of 3 samples (this report). 4. Dike rocks, average of 2 samples (this report). 5. Earth's crust Sisler, Van der Werf, and Davidson (1949, p. 706). 6. Granite, Rankama and Sahama (1950 p. 566). 7. Acidic rocks, average, Vinogradov (1956, p. 49—51) Niobium was detected in about equal amounts in most rocks, but in amounts only slightly above ana- lytical sensitivity. These numbers are comparable with averages in other rocks. The NbtS ion is of medium size (0.69A) and has a coordination of 6. The element is rare in sulfides, but is common in silicates as replacement for Ti in sphene, rutile, biotite, and clay minerals, and for Zr in zircon. Tat (0.68A), which commonly occurs with niobium, was looked for but not detected in these rocks. Niobium content decreases slightly in more altered rock along veins in the Front Range and parallels the distribution of zirconium in the same rocks. Trace quantities of silver were reported in altered wallrock and vein rock in about 70 percent of the locali- ties sampled; gold was reported in vein rock in only two mines. Agt (1.26A) and Aut (1.37A) are large ions not easily taken into mineral structures, and there- fore are most commonly found in sulfides, sulfosalts, GEOCHEMISTRY OF ALTERED ROCKS 85 tellurides, or in the native state. In fresh rock, silver was not found in amounts above the 0.0001 percent analytical sensitivity limit. Veinward, in the argillic and sericitic zones, silver content increases on the average to 0.0018, and in the vein rock and gouge clay sampled up to 0.07 percent. Two samples yield 0.003 percent gold in vein and gouge material. In igneous rocks the average amount of silver is 1 X10~" percent, and of gold is 5 X 10-" percent (Green, 1953). Silver may be dissolved and moved as a sulfate, but gold generally is inert and remains in the native state. Silver may be reprecipitated in the vein environment as a secondary mineral. Molybdenum, a rare constituent in most rocks, occurs in altered rocks in the peripheral ore zone in the Russell Gulch, Fall River, and the Idaho Springs (Phoenix mine) areas. Mo+* (0.70A) possibly is concealed in sulfide structure of accessory minerals, and only where concentrated from 0.001 to 0.007 percent in the argillic zone is it detected. Molybdenum is dissolved readily, and when released in the absence of S it complexes with O. Thus no secondary deposits are formed, and the element is depleted in zone 4 and in the vein. Cadmium is rare in most wallrocks of the mineral belt; Rankama and Sahama (1950, p. 712) report an average content of 0.00002 percent Cd in granite- well below the analytical sensitivity for spectrographic analyses. Biotite and other ferromagnesian minerals, plagioclase, and apatite are possible carriers of cad- mium, but in an unknown manner. Cd+ (0.97A) more commonly occurs concealed in sphalerite. Cad- mium is dissolved readily as rocks are altered, but has a great affinity for S and may be enriched in the vein as greenockite (CdS). However, intensely altered metasedimentary wallrock and ore from veins in the peripheral ore zone in the Russell Gulch area commonly contain abnormal amounts of Cd. The maximum amount reported was 0.3 percent, the average content is 0.10 percent. Sphalerite is judged to be the host- mineral structure. Yttrium, lanthanum, cerium, neodymium, and ytter- bium are more abundant in rocks in the Front Range than in the rocks listed in table 28. The sizes of these ions are large (Y+, 0.924; Lats, 1.14; 0.94A; Nd+*3, 1.04A; Yb*3, 0.86A), their coordination ranges from 6 to 8, and their chemical properties are very simi- lar. Yttrium and the lanthanides (La, Ce, Nd, and Yb) are common low-concentration constituents of minerals in igneous and sedimentary rocks. They have an affinity for phosphorus and fluorine in oxide- and silicate-mineral structures in apatite, xenotime, fluo- rite, zircon, monazite, garnet, and biotite. In many of these minerals yttrium and lanthanide ions replace Ca+ (0.994) ions as in calcite, sphene, apatite, amphi- bole, garnet, and plagioclase; La may replace K in K- feldspar. The distribution of individual transitional metals varies: Y and La generally are lost in rocks adjacent to the vein; Ce and Nd occur in amounts close to ana- lytical sensitivity in only 60 percent of rocks sampled and have no pronounced trend toward gain or loss across a zone of altered rock; Yb, which is more com- monly captured by replacement of Ca, is frequently concentrated veinward. There is no marked concen- tration of these elements in clay-mineral structures. TaBus 28.-Amount, in percent, of some transitional and secondary transitional metals in rocks 1 2 3 4 5 6 7 Transitional metals SOX dio 0.003 | 0.0023 | 0.002 |...... 0.0005? 0.00013 | 0.0007 I aes 0078 | .004 004 | 0.0022 | .0017 |...... .002 MA L sis .027 . 005 . 004 003 | .0019 | .0043 . 0046 Secondary transitional metals CRs lt lin 0:040) {-L cbes 0028 (| 0.:00247 |...... 0.006 NUE :.. neces AEL AL "ec 20018 . 004 iiss et cal bok cs .0008 | 0.0011 | .0003 | 0.0002 | .00027 |...... . 0002 1. Biotite-muscovite granite and granodiorite, average of 5 samples (this report). 2. Metasediments, average of 14 samples (this report). 3. Quartz diorite, average of 3 samples (this report). 4. Dike rocks, average of 2 samples (this report). 5. Igneous rocks, average, Green (1953). 6. Granites, Rankama and Sahama (1950). 7. Acidic rocks, average, Vinogradov (1956). Yttrium and the lanthanides are readily removed by solution when the minerals containing them are altered. The ions are carried off and generally accumulated in carbonate sediments (Rankama and Sahama, 1950). The spectrographic data suggest, therefore, that these ions are more commonly concealed in structures of accessory minerals that are resistant to alteration even in the argillic zone. POWDER pH OF ALTERED WALLROCKS The relative acidity or alkalinity of water, an import- ant property in chemical reactions, is expressed in terms of hydrogen-ion concentration, or pH. Rocks are com- posed of variable amounts and types of hydrous min- erals and contain water-filled pore spaces. When pulverized and suspended in a standard water solution, the rocks impart a hydrogen-ion concentration to the suspension (Stevens and Carron, 1948); this hydrogen- ion concentration then is assumed to be characteristic for the rock environment. It is not this simple, how- ever, and Hemley (written communication, 1960) sug- gests that pore water is the least contribution to the powder pH. It is mostly a measure of the extent of room temperature hydrolysis of the powdered minerals involved; fresh rocks give high values, argillic and sericitic minerals give low values. These low values 86 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO may reflect low hydrolyzing character and (or) in- cluded traces of acid sulfates from oxidized sulfides. Nevertheless, powder pH data are a measure of these values and should serve as an aid in the evaluation of abnormal rock oxidation. A qualitative measure of the hydrogen ion concentra- tion of progressively more altered powdered wallrocks indicates that rock pH decreases toward the vein (table 29). Thus the pH of rocks known or inferred to have been exposed to extensive migration of oxidizing waters is significantly lower than the pH of rocks not so exposed. The pH values from rocks sampled in old, abandoned, commonly wet or water-filled mine workings are generally lower than pH values obtained from the same types of rocks in newer and (or) drier workings. The pH values obtained from granitic rocks on the fringes of the fractured and mineralized area are high (pH of 8), and the writer infers that the high pH value reflects the fact that these rocks have never been ex- posed to extensive migration of oxidizing waters. Intensely altered wallrock in the central ore zone has a pH of only 4.5, whereas "fresh" rock in the same zone has a pH of about 6, equivalent to the pH of the same + rock where intensely altered in the peripheral ore zone. This may indicate also that rocks of the central ore zone had more intense hydrothermal permeation than those of the peripheral ore zone. TaBus 29.-Summary of qualitative powder pH determinations on several variously altered wallrocks Zone pH! Mine Sample Granite 4 6.7,7.8 | Nabob : 154 4 8.4,8.0 |__... ne seee chas s nne 149 a ].: - o Re 151 G $:9,8.B 1:40 .. 12 22 nec 144 v 7.1,6.9 | Nabob 155, 156 v do % 146 Granodiorite 2 6 | Hayes and 42 3 7.8.8.4 |_... do 43 3¢ |....- eos rin lessen eres ch ere cants 44 4 8.8 |. do a. 45 Quartz diorite 3 8. 4,8. 7 T7b 4 8.2, 8. 4 78 G 7.6, 7.4 79 2 8.8,9.0 16 2 8. 5,8. 3 71 3 8. 4 73 8 8. 4,8. 4 72 3-4 8.4,8.3 74 v 8.5,8.1 75 Granite pegmatite 3-4 3.6 | Alma Lincoln.. 2 411 4 2 T 412 v BA do 416 v 8.1,8.7 | Carroll. 203, 206 4-V 5 1 CRETORCR . 20. ec U Comunes nabbed Tapur 29.-Summary of qualitative powder pH determinations on several variously altered wallrocks-Continued Zone pH! Mine Sample Bostonite 3 MammibbR .tc .. cose 63 LPE ACOA 3 443 €]: (45,8. 2 /..... do..... een 444 4 y 439 a-V * 440 ¥ |}}. 46,40]... 10. ods ERL ERE edu ilu nn ene 441 Microcline-quartz-plagioclase-biotite gneiss 3 8.0] Carroll, BO Level: :o t icle. 2C. ees 205 4 ew e 7.5 v 7.4 3-4 5 4 5 G 4.5 v 4.5 3 4.8, 4.9 4 5.7, 4. 4 v 5.6, 4.3 2 6.0, 8.3 2-8 6.0, 8.2 3 6.0, 8.0 v 5.5, 7.6 2 5 3 4.6 4 4.2 v 3.5 Biotite-quartz-plagioclase and related gneisses 2-3 8.0 3 7.9 4 7.0 2 4.5 3 4.5 a 4.5, 5.0 4-V 4.5 2 8.2 2-3 8. 2 3 8.2 3 7.9 3-4 8.6 1-2 4 v 4 3 6 4 4 a-v 3 Amphibolite 1-2 5 | Foxhall 403 5:6 |-... do 404 Garnet-quartz gneiss 2-3 6 | Golconda 386 3 O do... 385 3-4 6 do 384 4-V 0 {. -e-. do 390 G 4.5 do 383 v 5 do 389 * The second value represents determination in the same sample after it was allowed to stand for 1 month. 2 Near adit 3 Near surface. 4 Very strong ground-water movement, near surface. 5 Ground-water movement strong. GENERAL CONCLUSIONS The alteration of a varied assemblage of metamorphic and igneous wallrocks by mild hydrothermal solutions in the central Front Range mineral belt left a more informative record than intense hydrothermal altera- tion of a single rock type might have done. The dis- tribution of major and trace elements in these rocks may be related to (1) the stability of host minerals, (2) the geochemical nature and response of the ele- GEOCHEMISTRY OF ALTERED ROCKS ; 87 ments to the altering hydrothermal solutions, and per- haps (3) the physical and chemical nature of the ore solution. Some significant conclusions concerning rock alteration may be deduced from these chemical data. 1. Oxygen ions comprise nearly 90 percent of the vol- ume of fresh rocks, and the weight of oxygen in equivalent volumes of rock remains nearly con- stant across altered zones. Any apparent loss is adequately accounted for by the addition of H* to oxygen. The introduction of sparse F and S ions locally in zone 4 must displace some O ions if equal volume relations are maintained as assumed. 2. Most other ions are small and fit into the inter- stices of the close-packed oxygen network in a manner governed by geochemecial factors such as ion size, charge, concentration, solution pH, crys- tal structure, and temperature. Because a ma- jority of the rocks are composed of minerals with sheet-type silicate structures, only minor shifts in interstitial ions need be postulated to occur when H+ ions in acid waters invade the rock and con- vert O-" ions to (OH)~. In general K, Fet" (and total iron), C, H, S, and some Al were added to the altered rock while Si, Na, Ca, Fet, and Mg are lost from the wallrock. 3. The structures of the host and newly formed min- erals may influence ion distribution. Interstitial ions with low ionization potential are removed first, hence alkali and the alkaline earth ions 4. The decrease in K+ fixation in vein walls of K- feldspar-deficient rocks is thought to relate to the K+/H*+ activity ratio in the hydrothermal system. Thus local changes in the composition of the hydro- thermal solution occasioned by a more rapid local alteration reaction in the host rock could effect large-scale local changes in the alteration environ- ment. - Solutions moving in K-feldspar-rich rocks should tend to adjust close to the K-feldspar-mica- quartz equilibrium curve and additions of K+ tend to produce sericite in these rocks. A similar solu- tion moving in rapidly altering K-feldspar-poor rocks would be farther from the equilibrium curve in the clay-mineral field and a similar addition of K+ would produce montmorillonite or illite. 5. Most of the trace elements occur in amounts equal to or slightly greater than their average con- tents in similar rocks elsewhere. Therefore the evidence for geochemical partition of trace-element ions during rock alteration is not clear. In gen- eral the smaller ions (in each period) with larger ionization potential and higher valences occur or tend to be concentrated in rock near the vein (table 30). These elements coincidentally have a pronounced affinity for sulfur, and several are TaABLu® 30.-Generalized distribution of trace elements in rocks in the Front Range area [Asterisk indicates major rock-constituent ions] (except for potassium which has a large ionic radius and almost certainly was introduced to zone 4 of certain rocks by altering solutions) are fewer toward the vein and are leached during rock alteration. K-feldspar is stable under all alteration conditions, and potassium is not re- leased; biotite is altered in zones 3 and 4, and the clay-mineral structure that results retains much of the potassium that was released. Plagi- oclase is partly altered in zone 2, and all ions in excess of those required to form (or to be acceptable in) clay minerals-such as sodium and calcium-are freed. - The anomalous loss of stron- tium in zone 3 and its reappearance in zone 4-V (figs. 37 and 47) is correlated with (a) its release from altered plagioclase, (b) its incompatibility in clay-mineral structures, and (c) its stabiliza- tion as a replacement for calcium in calcite. The alteration of biotite in zone 4 is correlated in part with the release of barium, lead, gallium, boron, all of which have an affinity for iron, and perhaps some rare earth elements. Montmoril- lonite and illite clay structures may retain trace elements such as vanadium, zinc, titanium, and chromium. Ions concentrated mo unaltered rock (or leached from altered rocks) stly in Indeterminate or variable ion concentration Ions concentrated mostly in altered rocks adjacent to veins Li, *Na, Be, *Mg, *Ca, Sr, Ba.... *81/Ga. In......l..s..... ¥, Or, Co,. Ni, Bo......~ * Ge, Pb, As, Sb, Bi. Cu Nb, Mo, 04, ¥, ._.... x: -| Zn, x Ag, Au, Yb. commonly major constituents or concealed trace elements in sulfide ore minerals. Elements with low ionization potential are relatively soluble and are more readily removed from zone 4 rock into the vein solutions. 6. Powder pH measurements generally show a decrease of H ions in rocks away from the vein, but this is mainly the effect of differential rates of hydrolysis of the minerals involved. Lower than average pH values are most useful as indicators. of more intense rock oxidation near the vein. The pH data sug- gest also that rocks in the central ore zone were subjected to a more intense and acid hydrothermal and (or) supergene alteration environment than were similar rocks in the peripheral ore zone. 7. Very sparse data indicate that ions in addition to K and S may be introduced (in an unknown form) into zone 4 rock by vein solutions; their identity 88 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO and proportions seem to be consistent with vein mineralogy; intensely altered metasedimentary and igneous rocks in the intermediate ore zone contain copper, lead, zinc, and silver in amounts above average. Quartz diorite, which crops out almost exclusively in the peripheral ore zone, contains average amounts of copper and lead and above average amounts of zine and silver. There is no compelling chemical evidence to suggest that introduced ions (except H) penetrated appreciably into the wallrocks beyond the zone 3-zone 4 interface. ORIGIN OF THE ALTERED WALLROCKS IN THE CEN- TRAL PART OF THE FRONT RANGE MINERAL BELT The sequence of the hydrothermal activity that produced the altered wallrocks in the central part of the Front Range mineral belt was compiled from detailed observations of the altered-rock environment and from some reasonable inferences. LOCALIZATION OF ALTERED ROCK The location and character of the altered rocks studied is governed, at least in part, by several geo- logical factors, including location within an area con- taining a favorable combination of rock structures, a sequence of mild hydrothermal solutions, and variable rock mineral-chemical relations. First, it is assumed that the source of the hydrothermal solutions is at depth. Second, it is inferred that the openings along the fissure veins were produced by dilatancy caused by regional shear stresses; these openings were loci of deposition of the vein-forming minerals during early Tertiary time (Eckelmann and Kulp, 1957, p. 1128). Rock alteration took place outward from openings, which range in size from large vein faults a foot or more wide to pore spaces of microscopic width, at a depth of at least 1 or 2 miles below the existing land surface.* Thus some channelways were throughgoing whereas others permitted only slow percolation of solutions and ions. These openings provided access for primary hydrothermal solutions to the rocks and an outlet for altered hydrothermal solutions that resulted from the exchange of constituents between the rock and the solution. Finally, the vein faults locally provided a locus for deposition of the ore deposits. The principal routes of access for rising hydrothermal solutions were first along deep-reaching faults, then ° From geomorphic evidence Murray (1956) suggests that the Continental Divide in early Tertiary time extended northwestward from Pikes Peak (14,274 ft) through the Tarryall Mountains, Mount Evans (14,260 ft), Gray-Torreys (14,274 ft), and northeastward to James Peak (13,260 ft), passing across the mining district area. The present position of the Continental Divide is the result of the headward erosion westward of the Platte and Arkansas Rivers since Tertiary time. - The ore deposits at Idaho Springs and at Central City, which range in altitude from 7,000 to 10,000 feet, underlie the Tertiary land surface by at least 1 to 2 miles. along less pervasive faults that connected with the main channels, and finally out into the bedrock along minor openings such as foliation planes, joints, small fractures, and pore spaces. The principal channels resulted from repeated faulting that formed open spaces and allowed access to rising solutions. The continuity of these faults and their numbers depends to a large extent on the lithology and kind of folding present in local areas within the whole area. As examples, the Central City and Freeland-Lamartine districts (Sims, Drake, and Tooker, 1963; Harrison and Wells, 1956) are sharply contrasting. At Central City, broad open folds in an alternating sequence of thick competent and thin less competent gneisses contain continuous, wide, closely spaced vein faults. At Freeland-Lamartine, a tightly folded, in part overturned, and generally distorted series of thin competent and thicker incompetent meta- sedimentary gneisses produces fewer, less continuous, and narrow vein faults. At Central City, the width of altered walls and altered rock zones in the central zone are consistently greater than along veins with a similar ore-mineral assemblage in the Freeland-Lamarine district. The penetration of solutions into the walls away from the faults is either restricted or aided by foliation planes, joints, and similar structures. - Promi- nent foliation parallel to the vein fissure restricted the ease of migration of solutions into the walls, whereas prominent foliation at a large angle to the vein fissure, cross fractures related to faulting, and joints aided the migration of solutions into the walls. I I he I —| P .0 - 20 |- al 5 f 8 4 3 # $ ; ' i o _» ® € c 9 PX a ] U 5 19 § o fui a] ( ¥, s gil sn ans cli o ito e; x # a / L ofr .. 00s $o. 9 | I §. 1 1 d 2 3a 3¢ 4 ALTERED ROCK ZONES FiGurE 66.-Plot showing variation in apparent percent porosity calculated from bulk and powder density values (table 18). Dashed line indicates an average value, dotted lines indicate the range in values. ORIGIN OF THE ALTERED WALLROCKS 89 It appears that significant pore space for the mi- gration of fluids and deposition of vein minerals was not formed by leaching, as noted in particular at East Tintic district, Utah (Lovering and others, 1949, p. 27). First, the narrow alteration envelopes in the Front Range area studied, indicate either that hydrothermal solutions were not able to alter the rocks very far from the veins or that they did not freely move very far into the walls. There is no evidence that zone 4 rock has been extensively leached, and the formation of mont- morillonite, the principal alteration product of plagio- clase and hornblende in zones 2 and 3, may well have restricted solution movement away from the vein. Second, although pore-space measurements, determined from bulk and powder density data, indicate a veinward increase in pore space of as much as 20 percent in the clay zones (fig. 66), thin sections of these rocks fail to show any evidence of open space " of this order of magnitude. The absence of disseminated base-metal sulfides in altered rock zones also indicates that the apparent increase in porosity did not result in permeable rock; such open space likely would have been filled by sulfides during the ore-depositing stages. Bastin and Hill (1917, p. 103) were puzzled by the contrast be- tween the localization of vein minerals and those formed by replacement of wallrock minerals. "One of the most striking exemplifications of these contrasts is the scarcity or entire absence of galena and sphalerite in the walls of many fissure fillings in which these minerals are very abundant; in such walls pyrite is the dominant sulfide. Carbonates may be present in a filled fissure but absent from its walls. Conversely, sericite the commonest alteration product in the walls, is never found in fissure fillings." In a very general way the spectrographic data sum- marized in figures 67 and 68 show that some essential elements in ore minerals are only moderately concen- trated, if at all, even in strongly altered wallrock in- cluded in the vein zone. The data also suggest, however, that lead and zinc are more abundant in ¢ The method for obtaining the powder density determination may in part explain this seeming discrepancy in argillized rock. The gravity of the powdered rock is usually determined in a pycnometer in water solution (L. C. Peck, written communi- cation, 1957). - Small errors are possible owing to air bubbles entrapped in a finely ground powder and the possibility that some colloidal material may be lost in the manipulation of the pycnometer. - A fundamental error, however, may be caused by the clay minerals in the samples. The water associated with clay minerals is an in- tegral part of the structure, is partially ordered, and is structural water (Grim, 1953). 'The amount of water adsorbed by a fluffed-out powder may not necessarily be directly proportional to that which can be adsorbed on the same surface in the confined-rock environment. (This problem also is of concern in the analytical determination of H;O- in argillized rock; it is not possible to judge the proportions of pore water to structural water that may be lost below 100° C.) Because the ground and air-dried clays adsorb water, probably more than that originally held, their density per volume should be reduced or their volume increased. This cannot be evaluated in the com- putations, so the powder density value is effectively increased, and the calculated pore space is thereby increased. The more clay minerals that are present, especially montmorillonite, the greater this discrepancy may become. The unusually high pore-space values on figure 66 occur in the clay-mineral zones in altered wallrocks. wallrocks of galena-sphalerite veins. There is little or no abnormal increase in base-metal vein-mineral con- stituents in zones 1 to 3 of most rocks, and zinc and lead are, if present at all, in amounts below the sensi- tivity limits of detection. Copper, lead, zinc, and silver are concentrated locally in small amounts in zone 4 rock, but this may be partly a leakage outward from the vein along small fractures. The progressive con- centration of iron is less marked because it is a major constituent in the rocks. These sparse data suggest that most elements which are in trace amounts in sul- fides are not greatly concentrated or dissipated from their normal abundance in the wallrocks. The sequence of rock alteration and metallization events is not wholly clear, but the bulk of the evidence indicates that most wallrocks were altered prior to base- metal ore deposition. Such a sequence of hydrother- mal activity provides a possibility that some localiza- tion factors for the alteration phase may have differed from those of ore deposition. The observed ore-mineral occurrences, for example, generally are restricted to a vertical range of 2,000 feet along the vein fissures," whereas the enclosing altered wallrocks essentially are unchanged where they are observed along veins above and below the ore horizon. In addition, most base- metal sulfide ore occurs in shoot structures along veins, and there is no indication of any difference between alteration of essentially barren wallrocks and those enclosing ore. Crosscutting relations between veins, such as those reported at Butte (Sales and Meyer, 1948) which may demonstrate synchronous alteration and metallization stages, are not observed here, and the low-angle vein intersections observed in the central part of the Front Range mineral belt are not informative. Some small- scale vein structure relations also suggest different ages of alteration and metallization. One such example, sketched in figure 69, from the back of a drift in the Houston mine (Kitty Clyde area), shows a change in vein direction which is not reflected in the altered walls. This may be reasonably interpreted as alteration of rock along a vein structure, subsequent reopening glong a slightly different path, and finally vein filling by an agent not capable of modifying the walls further. A regional temperature gradient may be responsible for the ore-mineral zonation" and also for the greater amounts of altered rock in the central zone as con- ! Harrison and Wells (1956, p. 85) show that many ore shoots in the Freeland- Lamartine district pinch out before reaching the surface (10,450 feet altitude) and that values in the ore decrease with depth-none have been minable below an altitude of 7,900 feet. - Similar decreases in ore values were discovered in veins which the Argo tunnel intersected under the Central City district at an altitude of 7,500 feet (Lovering and Goddard, 1950, p. 171). 12 Sims (oral communication, 1960) has some evidence for a thermal gradient which accounts in part for the areal zonal distribution of ore minerals. ALTERED WALLROCKS, CENTRAL PART OF 90 Zone 1 2 3 4 G V o* I I I I I I I bed & (- _. e o o-S o =< 0-8-9. .> ve "se Percent I >10 - 1.5 Ch Zn /'/¢/ ve o 7 ' ; - A _ |- F 4 - 0.15 |- ’ 0.07 _ |- -| 0.03 |- -| 0.015 |- =| 0.007 . |- -I 0.003 |- 0.0015 |- -| 0.0007 |- -| 0.0003 |- 0.00015 |- - 0.00007 |- - THE FRONT RANGE MINERAL BELT, COLORADO 1 2 3 4 G v o* i I T T T I f wl 7 - / == Fe

10 7 3 1.5 0.7 0.3 0.15 0.07 0.03 0.015 0.007 0.003 0.0015 0.0007 0.0003 0.00015 0.00007 91 I | | I | 1 I G=gouge clay, V=strongly altered wallrock fragments included in veins, O=ore. * values from P. K. Sims (written communications, 1957) FIGURE 68.-Average values from semiquantitative spectrographic analyses of some elements in microcline-quartz-plagioclase-biotite gneiss wallrocks and in pyrite ore from E. Calhoun mine. rich rocks such as granite or microcline-quartz-plagio- clase-biotite gneiss. THE HYDROTHERMAL ENVIRONMENT HOST ROCK The chemistry and mineralogy of a changing host- rock environment in progressively more altered rock are conveniently summarized by means of phase diagrams; however, pressure and temperature relations are not directly observed and are based on less well-established data. Eskola's metamorphic facies principle is very useful to describe and illustrate the metamorphic-rock environment. Creasey (1959) proposes to extend the idea to include the action of hydrothermal solutions on rocks and thereby to describe a hydrothermal-mineral assemblage. Turner (1948, p. 54-59) describes a meta- morphic facies as including rocks of any chemical com- position that have reached chemical equilibrium during metamorphism under a particular set of physical con- ditions." The altered-rock type that most closely !? Petrographic evidence for an approach to equilibrium is obtained if it can be observed (1) that the same mineral assemblage develops from different parent rocks under constant physical conditions that have been attained from different directions, (2) that a mineral assemblage is of wide distribution in metamorphic rocks of different ages, (3) that the mineral assemblage conforms to the requirements of the mineral- ogical phase rule, and (4) that disequilibrium, such as one mineral obviously in the process of replacement by another, is absent. 678880 O-63--T7 approaches these requirements is the sericitized rock formed in zone 4 adjacent to the vein; most of the altered rocks we describe do not fully meet Turner's facies requirements. Triangular composition diagrams used by Eskola relate rock chemistry and mineralogy. Eskola selected Al;0;, CaO, and (Mg, Fe)O as three components mainly responsible for the observed mineralogy, and mN AL., mem o Nem ic mons hme lucks Sac < lron-stained altered wallrock sa | haas iand ag nl 1 wont, nun i (mms -~ ~ - -- wo le -_- n reac m - A io ien 2 INCHES ° L_ FiGuRE 69.-Sketch of small-scale example of vein-altered wallrock relations, Hous- ton mine, Idaho Springs district, Colorado. 92 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO plotted them on an ACF diagram (Turner, 1948, p. 57). For rocks containing excess Al;0; and SiO; (for example, rocks above the anorthite-biotite join on the ACF dia- gram) Eskola used the AKF diagram (Turner, 1948, p. 82). A plot of the Front Range fresh rock types on an ACF diagram (fig. 70) indicates that quartz diorite, amphibolite, and garnet-quartz gneiss are deficient in Al;0;, and that biotite-quartz-plagioclase gneiss and granodiorite are close to the borderline. Biotite-mus- covite granite, - microcline-quartz-plagioclase-biotite gneiss, biotite-quartz-plagioclase gneiss, and granodio- rite, which are the predominant host rocks in the region, are plotted in the AKF diagram. Curiously muscovite is a megascopic mineral only in the granite, and it is assumed that the corresponding microscopic clay mineral (illite or deuteric sericite ?) proxies for muscovite in the other rocks. As proposed by Creasey, the progressive alteration of rocks by a hydrothermal solution may be compared on an AKF diagram (fig. 71). Ideally field 1 contains a clay mineral-quartz equilibrium assemblage, and field 2 a K-feldspar-sericite-quartz assemblage. - K-feldspar, muscovite, and biotite are unaltered in field 1, and K- feldspar and muscovite are recrystallized partly in field 2. Disequilibrium occurs in the Front Range altered rocks, and the triangular diagram is not rigorous; A C ca di ac anth F A=Al;s0s+Fe:03-(Na;z0+K20) muse=muscovite ac=actinolite C=Ca0o gr=grossularite - anth=anthophyllite F=MgO+FeO+ MnO an=anorthite bi=biotite A+ C+F=100 percent ca=calcite hbn=hornblende di=diopside FIGURE 70.-ACF diagram of the amphibolite facies (low temperature subfacies) for rocks with excess SiO: and (Turner, 1948, p. 78) showing plots of fresh rocks from the Front Range area studied. K-feldspar and quartz are additional phases. Biotite-muscovite granite (1), granodiorite (4), microcline-quartz-plagioclase-bio- tite gneiss (9), quartz diorite (16), biotite-quartz-plagioclase gneiss (19 and 24), amphibolite (31), and garnet-quartz gneiss (35). A ka se 2 bi K F mr A=Al0s-(CaO+Naz0+K20) ka =kaolinite K=K;0 se=sericite (or three-layer clay minerals) F=FeQ+MgO+MnO mr=microcline A+K+F=100 percent bi=biotite FIGURE 71.-AKF diagram proposed for the hydrothermal assemblage for rocks with excess SiO; and H;O (Creasey, 1959); field 1 contains a lower temperature mineral assemblage than field 2. nevertheless, it is useful to show the chemical trends as an equilibrium assemblage is approached in zone 4. The trends of chemical changes (fig. 72) parallel mineralogical changes in progressively more altered wallrocks. Samples 1, 4, 9, 19, and 24, which are fresh rock, are unstable in the hydrothermal environ- ment and are argillizsed. Weakly argillized rock con- tains more Al;,0;,; exclusive of that related to CaO, Na,0, and K,0 in feldspar. Strongly argillized rock is enriched even more in Al. Sericitized rock, however, is enriched in K at the expense of Al. Where pyrite is present in sericitized rock the Fe is also enriched. The alteration of biotite-quartz-plagioclase gneiss (fig. 72, samples 19-23, and 24-26) follows a similar but more variable trend. The samples of biotite muscovite granite deviate from this pattern in that few clay minerals form in a rock composed predominantly of quartz, microcline, biotite, and muscovite-minerals which are not altered in field 1 and (except for biotite) are stable in field 2. The loss of alkali and alkaline earth ions, largely from altered plagioclase, shown in figures 28 and 30, results in effective enrichment in Al. Loss of K and Fe is most marked in biotite- and hornblende-bearing rock. In some rocks Fe and Mg are lost gradually; but, in those rocks in which biotite alters to magnetite ORIGIN OF THE ALTERED WALLROCKS 93 ks FrGurRE 72.-AKF diagrams A and B show plots of fresh and altered wallrocks in the central part of the Front Range mineral belt: biotite-muscovite granite, 1-3; grano- diorite, 4-8; microcline-quartz-plagioclase-biotite gneiss, 9-13; and biotite-quartz-plagioclase gneiss, 19-23, and 24-26. Quartz is an additional phase in all samples and anorthite and various other minerals are additional phases in the fresh rock which is shown only for reference. Rocks of zone 2 and 3 also contain irregular amounts of these partially altered phases. Lines connect related analyses and do not necessarily indicate intermediate paths; microcline (ks), biotite (bi), sericite (se), and kaolinite (ka). and to chlorite, Fe and Mg do not escape readily from the host rock. These results conform in a general manner with those from diverse rocks cut by vein deposits of similar type elsewhere. The AKF diagram plots (fig. 73) for fresh and altered Boulder Creek granite, syenite from the Caribou stock in Colorado, and for quartz monzonite at Butte district, Montana, indicate a trend toward Al during argillization and, except for Butte where no marked increase in K is reported, a trend toward K with more intense sericitization. AKF diagrams suggest that possibly only the final sericitized rock assemblage is in equilibrium with the altering environment, and that the paths of chemical changes in the host rock environment are dependent on the composition and quantity of unstable minerals. Although the paths may vary, the end product of hy- drothermal alteration of Al,0,;-rich rocks and their equi- librium assemblage is a regular feature-a narrow zone parallel to the muscovite-Fe join. Not shown, but indicated by the mineralogy of altered Al,;O;-deficient rocks is their high-intensity assemblage of three-layer clay minerals, kaolinite, and quartz, which is com- parable to the nonequilibrium transitory field 1 assem- blage noted above. HYDROTHERMAL SOLUTIONS The complex nature of the hydrothermal solutions that altered the wallrocks and formed ore deposits can- not be deduced completely from wallrock data alone; however, these data and those obtained from surface hydrothermal and meteoric waters provide partial in- sight into the hydrothermal environment in the area studied. First we shall define hydrothermal (after White, 1957a, p. 1638, and 1957b, p. 1661) as pertain- ing to any water that is relatively warmer (plus 5° C) than its enclosing environment. In this region hydro- thermal solutions are inferred to be mixtures of waters of deep origin (for example, juvenile or magmatic water) which are diluted and contaminated nearer the surface by meteoric and metamorphic water. Thus we may infer also, as does Korjinsky (1936), that a mov- ing solution and immobile interstitial pore and bound water solution are mutually involved in the continuous interchange of matter by diffusion of ions and mole- cules along a concentration gradient. The classical idea (Kerr, 1955) that acid emanations react with wallrock at depth to produce neutral to alka- line solutions and that these may revert to acid char- acter by oxidation at or near the surface is borne out within the limits of observation in ground-water and hot-spring data at Idaho Springs, Colo. (table 31), and where drilled to a moderate depth at Wairakei, New Zealand (Grange, 1955; Steiner 1953). The radio- active warm springs at Idaho Springs may represent late stages of a long period of hydrothermal activity in the region, and are, as they issue at the surface, worked- out, oxidized, diluted hydrothermal solutions. These solutions are bicarbonate-, sulfate-, and chloride-bear- ing. In addition they carry abundant Na and Si as 904 ALTERED WALLROCKS, CENTRAL PART OF THE FRONT RANGE MINERAL BELT, COLORADO a, Fresh Boulder Creek granite, 5th level, Coldspring mine, 35 feet from vein: Quartz, 15-25 percent; ortho- clase-microline, 20-30 percent; andesine-oligoclase, 20-30 percent; biotite and hornblende, 10-20 percent; ilmenite and apatite, calcite and sericite minor and recalculated. constituents. w, Kaolinization zone. b, Argillized granite (montmorillonite-rich) 10 feet x, Sericitization zone. from vein. c, Argillized granite (kaolinite-rich) 3 feet from vein. d, Sericitized granite one-half inch from vein. 0, Fresh wallrock. p, Kaolinite zone. a, Orthoclase zone. r, Hydromica zone. 8, Vein. B B t, Fresh quartz monzonite, 2800 level, a Northwest vein. Values approximate as scaled from diagram u, "Green' zone (montmorillonite-rich). v, Boundary "green'" zone and kaolinization zone. y, Boundary sericitization zone and vein. i, Fresh syenite, middle section, 1040 level, Caribou mine, 1.7 feet into hanging wall: Orthoclase-plagio- clase, 65-75 percent; augite, 10 percent; biotite, 10 percent; quartz <5 percent; apatite, pyrite, sphene, accesories. j, Weakly altered syenite (montmorillonite-rich) 1 foot into hanging wall. k, Moderately altered syenite (kaolinite-montmoril- lonite-rich) 0.6 feet into hanging wall. 1, Intensely altered syenite (sericite-rich) 0.2 feet into hanging wall of vein 0.5 feet thick. m, Moderate-intense altered syenite 2.2 feet into footwall. n, Intensely altered syenite 0.6 feet into footwall of vein. FIGURE 73.-AKF diagrams show plots of fresh and altered wallrocks in (A) granite, Nederland district, Colorado (a to d, Lovering, 1941, p. 242; and o to s, Gonzalez-Bonorino, 1959, p. 74), (B) quartz monzonite, Butte district, Montana (Sales and Meyer, 1948, p. 30), and (C) syenite, Caribou district, Colorado (Wright, 1954, p. 133). Quartz is an additional phase; microcline (ks), biotite (bi), kaolinite (ka), and sericite (se). TaBum 31.-Analyses of water samples, in approximate parts per million, from springs near Idaho Springs, Colo. Sample Constituent or analysis 72.9 412. 35 1, 467.05 1 Measured by W. Niles, U.S. Geol. Survey, Apr. 19, 1956. 2 Measured by D. Skinner, U.S. Geol. Survey, Apr. 19, 1956. Sample data: a. Hot Soda Spring, No. 85 (Radium Hot Springs?); analysts: H. A. Curtis, R. M. Butters, P. M. Dean and H. R. Mosley (George and others, 1920). b. Cold Soda Spring, No. 86; analysts: H. A. Curtis, R. M. Butters, P. M. Dean and H. R. Mosley (George and others, 1920). c. Blue Ribbon Spring, No. 87; analysts: H. A. Curtis, R. M. Butters, P. M. Dean and H. R. Mosley (George and others, 1920). d. Radium Hot Springs, north end of Radium Hot Springs Hotel; analysts: N. F. Witt and W. B. Peitenpol. well as a minor amount of Fet and Al. One analysis reports H;S and correspondingly contains more Fe iqns. Temperature at the radioactive springs range from 58° to 115° F, and the pH of the Radium Hot Springs was 6.5. The more active nearly boiling springs at Wairakei, have 270° C temperature measured in drill holes at 2,000 feet (Grange, 1955, p. 53), and are of the sodium-chloride type (White, 1957b, p. 1665). Ellis (1955) reports a general increase of pH with depth from 2.5 to 8.0 where drilled to 100 feet and estimates from chemical equilibria that a pH of 9.2 is required at 2,000 feet. Some mine waters in the Front Range region (Clarke, 1924, p. 644, 646) are slightly acid; however, pH measurements (table 32) of mine water throughout the region shows that moderately to slightly alkaline water occurs in may mines in the intermediate and peripheral ore zones. Ground water at the Running Lode mine (Headden, 1903, p. 179) contains sulfate- carbonate-chloride components as well as moderate to minor amounts of dissolved alkali and alkaline earth ions, silica, iron-alumina, and manganese oxide. In contrast, extremely acid water (tables 33 and 34) from the Argo tunnel, which taps water from mines where pyrite is highly oxidized, contains high sulfate, low halogen ion concentration, moderate amounts of alkali, alkaline earths, silica, aluminum, and iron, and ORIGIN 32.-Hardness values of mine water in the central part of the mineral belt Area and ore zone Mine Hardness pH) Central City district: Central (pyrite) zone..___._..______.. re 0620s feck 3.4 Argo (drainage). . excel 2:6 Intermediate zone..._......_._.._... Widow Woman.. 64 Banta Hill...... 6/7 Peripheral (galena-sphalterite) zone.| Carroll.._______________....--- 6.8 % Two Sisters (near R.H.D.- | 6.9 . McKay shaft). Idaho Springs district: Intermediate zone................... Stlallfilcy (near Alma-Lincoln | 7.4 ne). Alma 7.4, 7.7 Sunnyside (near - Phoneix | 7.7,7.9 mine). Lawson-Dumont-Fall River district: Peripheral JO REYHONGS-.1 -. 8.2 Watt-Sitemple (near Almadin | 7.6 ne). TaBur 33.-Analyses of water samples, in parts per million, from Clear Creek and the Argo tunnel, Idaho Springs, Colo. [Samples collected by Lyman Huff, U.S. Geol. Survey; Analyst: F. J. Flanagan, U.S. Geol. Survey.] Sample Constituent or analysis factor 9 10 Silica (S102)... 7.0 38 Aluminum (Al) 4 30 Tron (Fe+?).___. Di 25 ____________ 175 1.0 60 4 16 __________________ 5 ...... 18 350 7.4 125 5.3 27 1.8 5.4 0 0 Bicarbonate (HCO;). 36 0 Sulfate (SO4)......... 54 2, 300 Chloride (C1). * .8 7.0 Fluoride (F)... * .6 4.4 Nitrate (NO3).... a 1.0 .9 Dissolved solids.... 2s a 117 3, 460 Specific conductance (KX105 at 25° C.) a 19.3 364 Colon... R 1 70 * 6.7 2. 65 Acldlty (as HSO): ............................................................. 1,040 Free ............................................................... 130 Localities where samples were collected: Clear Creek, Idaho Springs, Colo., just above Argo tunnel. 10 Drainage from Argo tunnel at mouth, Idaho Springs, Colo, 34. -Qualitative spectrographic examination of water residues of samgles from Clear Creek and the Argo tunnel, Idaho Springs, Colo [Samples collected by Lyman Huff, U.S. Geol. Survey. Analyst: K. J. Murata, U.S. Geol. Survey] Elements in sample- 9 10 Major elements. .. Ca, Mg, Mn, Fe, Al. Minor elements.. . Na, Cu, Zn, Si. P1;¢ Cu, Zn, Fe, Al, Y'B Yb Ni Co, Ti, Ba, r. a iz sears Be, Ti Co, Y, Yb, | FD;, 133, plus others. ! plus others.! Total solids (g per gal)....-- 0. 484 12. 257 y 1 {fickred for but not found: TI, As, Sb, Mo, W, Ga, Ag, Bi, In, Cd, Cr, Re, La, r, Cb, P. Localities where samples were collected: Clear Creek, Idaho Springs, just above Argo tunnel. 10 Drainage from Argo tunnel at mouth, Idaho Springs, Colo. OF THE ALTERED WALLROCKS 95 trace amounts of other ions. Ferric iron is more abundant than ferrous iron. The acid waters (table 34) carry more Cu, Zn, Mn, and Fe, and also carry Co, Ti, Y, and Yb which are not found in near- neutral meteoric and ground water (George and others, 1920). These are mostly supergene effects and therefore should not be confused with the hypo- gene environment. Data concerning pH must be extrapolated to depth with caution because,, as Hemley (1959) indicates, reactions which required low pH at high temperatures may be accomplished at higher pH values at lower temperatures. A mild hydrothermal solution environment postulated from mineral assemblages may have a range of actual temperatures, but the temperature in any one place was probably only moderately above that of the wall- rock. Host-rock mineral structures are not completely destroyed by such solutions; K-feldspar grains are unaltered, quartz is partly recrystallized, and most other silicate minerals are argillized. The sericite zone produced on the vein wall is thin, and leaching here of all elements except silicon and oxygen (thereby creating a porous mass) was not observed. Synthesis data demonstrate that minerals can be formed artificially in the presence of a solution within certain temperature, pressure, and composition limits. Most of the critical wallrock mineral assemblage may be formed between 100° and 500° C. Some of the extensive laboratory findings are reported by Folk (1947), Ingerson (1955@a), Yoder and Eugster (1955), Morey and Chen (1955), and Brindley and Radoslovich (1956). Possibly the most appropriate study for our consideration is that by Hemley (1959) which was concerned specifically with the hydrothermal environ- ment and relates temperature and composition of ore solutions and wallrocks. A lateral temperature gradient for formation of sphalerite in veins from 620° C, in the central ore zone, to about 380° C, in the peripheral ore zone, has emerged from studies by Sims and Barton (Sims, written communication, 1960). The chemical relations of these wallrocks imply that the altering solutions were slightly acid "* and slowly differentiating at a source so that their chemistry changed with time. Physical and chemical gradients initially favored migration of H ions a moderate dis- tance into slightly alkaline walls. Ions with low ionic potential were replaced, moved, and added to vein solutions. Solution acidity would thus be reduced up- vein initially, but as all attainable fresh wallrock was !* Lovering (1950, p. 235) states, " * * * the pH range usually considered of geologic interest, say from pH 7 to a pH of 1 (tenth-normal HCl has a pH of 1.1, one-normal HCl has a pH of 0.1), is very small-from 428 parts per million to about 422 parts per million in HCI." 96 ALTERED WALLROCKS, CENTRAL PART OF 600 500 A ~ K f & 400 < | Py-Bogh\ Mica\\ K-feldspar | Z & 300 ~ ® l u [__ Kaolinite & 200 L‘J - a : 100 o J 10 o: Ior 10 10° 10) 10" 10: EQUILIBRIUM QUOTIENT mKCI /mHCI FrGuRE 74.-Experimental stability fields of clay mineral-mica-feldspar in the K2O- Al?Os-Si0>H:0 system after Hemley (1959 p. 246). Py, pyrophyllite; Boch, boehmite. altered down vein and temperature of the solutions increased, the acid solutions rose higher up the vein system. Na, Ca, Mg, Si, Al, and Fe must all have been deficient in the solution initially, and gradually were enriched. K ions were present in the altering solution during its later evolution, but probably were sparse in early differentiates because potassium was leached from unstable minerals in certain plagioclase- rich rocks. ~Sulfur also appeared late and combined with iron during the K ion-rich phase of rock alteration. The textural position of calcite in fractures implies that CO; ions appeared at still a later phase of rock alteration. Under strong hydrothermal alteration con- ditions all of these delicate yet significant features are obliterated. The early solution which penetrated and argillized the walls either was incapable of depositing Cu, Pb, Zn, and Ag in argillized rock, or, in the early stages of differentiation of the solutions at the source, these elements (which are subsequently concentrated in the veins) were not present. (Sparse data indicate that the later telluride depositing solution also did not penetrate and deposit in the walls.) Semiquantita- tive spectrographic data show that the sericite zone does contain minor amounts of these elements above normal rock background. - Either sericite-forming solutions con- tained these ions in addition to K and S and incorporated them, or they result from subsequent leakage from the vein along small fractures. If they were present in a sericite-pyrite forming phase, disseminated copper, lead, or zinc sulfides should have been observed. The stability fields of clay mineral, mica, and feld- spar in the K,0-Al;0;-8i0,-H,0 system (Hemley, 1959, p. 246), reproduced in figure 74, relate variations in THE FRONT RANGE MINERAL BELT, COLORADO the equilibrium quotient mKCl/mHCl and tempera- ture. If one assumes near isothermal conditions (Lovering, 1950), variations in the K+/H* ratio of pore fluids may account for most mineral changes. Similar alterations are possible, holding the ratio con- stant and changing temperature. The absence of pyrophyllite and boehmite in the altered rocks has two possible explanatons. If formed under the tem- perature and equilibrium conditions suggested by altered wallrock mineral assemblages and sphalerite geothermometry, the pyrophllite inverted to kaolinite on subsequent cooling. Hydrothermal pyrophyllite, however, occurs along faults (Ehlmann and Sand, 1959) and some relict pyrophyllite, if present here originally, should have been discovered. If, however, the K*+/H*+ ratio remained within a restricted range kaolinite, mica (sericite and three-layer clay minerals), and K-feldspar could form with increased temperature, and the pyro- phyllite field be bypassed. K-feldspar is a metastable mineral in many of the host rocks studied, but almost no secondary K-feldspar was observed in altered wall- rock in this area. It was reported by GonzAlez- Bonorino (1959) in certain Boulder County ore de- posits. It is the writer's conclusion, based on these data, that a vanguard solution argillized the walls that were sub- sequently sericitized, pyritized, and partly silicified. Lastly, the solutions filled the veins with ore and gangue minerals. Possibly these distinct steps happened at the same time in different levels of the system, but not at one time on one level. Therefore these solutions and vein structures more closely resembled those which produced alteration by "stages" (Lovering, 1941, 1950; Lovering and others, 1949) than those which produced simultaneous fronts (Sales and Meyer, 1948, 1949). Movements of supergene solutions and normal surface water reached lower with time and erosion, and also acted on the system at a much later time. The fluids were near neutral in the intermediate and peripheral zones and thus did not greatly modify the alteration mineral assemblages. Strongly acid mine waters in the central ore zone have caused minor cation exchange, leaching, physical transportation of rock materials, and thin replacement by limonite and chalcocite. ALTERED WALLROCKS AS A GUIDE TO ORE In general, the altered wallrocks of the central part of the Front Range mineral belt cannot be considered to be a useful guide to base-metal sulfide ores. No particular wallrock alteration mineral assemblage ac- companies ore. The width of the envelope of altered wallrock about veins outside of the central and inter- mediate zones at Central City is generally small, meas- ured in a few inches to a few feet. Schwartz (1955, LITERATURE CITED 97 p. 316) pointed out that "very narrow zones of altera- tion are of limited help in searching for ore because finding the altered rock is about as difficult as finding the ore." The chances are best that altered rock may be a useful guide if the alteration and metallization processes are nearly contemporaneous. In these areas most rock alteration must have preceded ore formation. Postalteration, but preore strike and perhaps some dip- slip movements on curved vein faults rendered zone faults wholly or partly closed during one phase or the other, thus wide ore-filled veins may adjoin narrow al- tered rock zones, and often do. However, one may show that there is a local correlation between wide veins and widest altered rock zones in the central and inter- mediate ore zones in the Central City district. Whereas little correlation is evident between location and width of base-metal sulfide ore shoots and the loca- tion and width of altered granitic wallrocks, altered wallrocks such as amphibolite and garnet-quartz gneiss that contain large amounts of iron-bearing minerals and small amounts of quartz and microcline have a spatial coincidence with some pitchblende deposits in the veins. Regardless of the source, the uranium may have been deposited in open spaces as a consequence of reaction with excess iron in solution that possibly was derived from rock alteration. The close paragenetic relations between pitchblende and early pyrite (Sims, 1956) strongly suggest that the hydrothermal solution of the late and most intense al- teration phase, which altered biotite and provided a source of iron for subsequent early pyrite formation, locally carried uranium. The relation between iron- bearing wallrock and pitchblende metallization also has been noted under slightly differing environment condi- tions in the Front Range by Adams and Stugard (1956) and Hawley and Moore (1955). Secondary uranium minerals, such as metatorbernite, also appear to have an affinity for the iron-bearing clay mineral (illite- montmorillonite) structures in altered amphibolite and biotite-quartz-plagioclase gneiss (Sims and Tooker, 1955). Whether this relationship will be useful in finding uranium ore is questionable from the foregoing com- ments, but this study of alteration mineralogy within a geological framework provides an answer to the seem- ingly irrational distribution of uranium ore deposits in a well-zoned area of sulfide ore deposits. LITERATURE CITED Adams, J. W., and Stugard, Frederick, Jr., 1956, Wallrock control of certain pitchblende deposits in Golden Gate Canyon, Jefferson County, Colo.: U.S. Geol. Survey Bull. 1030-G, p. 188-209. Allen, V. T., 1956, Is leucoxene always finely crystalline rutile? discussion: Econ. Geology, v. 51, p. 830-833. Barbor, J. A., 1944, A periodic table based on atomic number and electron configuration: Jour. Chem. Education, v. 21, p. 25-26. Barth, T. F. W., 1948, Oxygen in rocks: Jour. Geology, v. 56, p. 50-60. 1955, Presentation of rock analyses: Jour. Geology, v. 63, p. 348-363. Bastin, E. S., and Hill, J. M., 1917, Economic geology of Gilpin County and adjacent parts of Clear Creek and Boulder Counties, Colorado: U.S. Geol. Survey Prof. Paper 94, 379 p. Bradley, W. F., and Grim, R. E., 1951, High temperature thermal effects of clay and related materials: Am. Miner- alogist, v. 36, p. 182-201. Bragg, W. L., 1987, Atomic structure of minerals: Ithaca, N.Y., Cornell Univ. Press, 292 p. Bray, J. M., 1942, Spectroscopic distribution of minor elements in igneous rocks from Jamestown, Colorado: Geol. Soc. America Bull., v. 53, p. 765-814. Brindley, G. W., and Radoslovich, E. W., 1956, X-ray studies of the alteration of soda feldspar, in Swineford, A., ed., Clays and clay minerals: Natl. Acad. Sci., Natl. Research Coun- cil Pub. 456, p. 330-336. Brunton, George, 1955, Vapor pressure glycolation of oriented clay minerals: Am. Mineralogist, v. 40, p. 124-126. Butler, B. S., 1932, Influence of the replaced rock on replacement minerals associated with ore deposits: Econ. 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L., 1957, Uranium-lead method of age determination. Part II; North American localities: Geol. Soc. America Bull., v. 68, p. 1117-1140. Ehlmann, A. J., and Sand, L. B., 1959, Occurrences of shales partially altered to pyrophyllite, in Clays and clay minerals: New York, Pergamon Press, p. 386-391. Ellis, A. J., 1955, Effect of temperature on chemical equilibria in geothermal chloride waters: New Zealand Dept. Sci. Indus. Research, Geothermal Chemical Rept. No. 9, 8 p. Folk, R. L., 1947, The alteration of feldspar and its products as studied in the laboratory: Am. Jour. Sci., v. 245, p. 388-394. George, R. D., Curtis, H. A., Lester, O. C., Creek, J. K., and Yeo, J. B., 1920, Mineral waters of Colorado: Colorado Geol. Survey Bull., v. 11, 474 p. Goldich, S. S., 1938, A study in rock weathering: Jour. Geology, v. 46, p. 17-58. 98 Gonzalez-Bonorino, Felix, 1956, Hydrothermal alteration in tungsten and gold-pyrite veins of Boulder County, Colorado [abs.]: Geol. Soc. 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N., and Winchell, Horace, 1951, Elements of optical mineralogy: New York, John Wiley and Sons, 551 p. Wright, H. D., 1954, Mineralogy of a uraninite deposit at Caribou, Colorado: Econ. Geology, v. 49, p. 129-174. Wright, H. D., and Schulhof, W. P., 1957, Mineralogy of the Lone Eagle uranium-bearing mine in the Boulder batholith, Montana: Econ. Geology, v. 52, p. 115-131. Yoder, H. S., Jr., 1955, Role of water in metamorphism, in Poldervaart, A., ed., Crust of the earth: Geol. Soc. America Spec. Paper 62, p. 505-524. Yoder, H. S., Jr., and Eugster, H. P., 1955, Synthetic and natural muscovites: Geochim. et Cosmochim. Acta, v. 8, p. 225-280. [Where several page numbers appear, major references are in italic] A Page Acidity, altered wallrock.._......__....______._. 86 hydrothermal solutions 94, 95, 96 Acknowledgments_...._....... Alkali and alkaline earth elements. Alma-Lincoln mine............. Alteration, guide to ore hydrothermal. ... localization.. supergene. ......... variation in character width of zones.... Aluminum........... Amphibolite, alteration. lithology....... Analyses, of wallrock. of water...... granodiorite. .. hot-spring water.... methods of presenting. . seee spectrographic. Antimony........ Argillic alteration..._......._._._. Argo tunnel, ground-water analyses...... nere ube o enkele cevned cine B (Bald Hagle vein. 90 Banta Hill mine, biotite-quartz-plagioclase gneiss. . fur 04 granodiorite. . 34 bostonite. .. tees (109 EC recliner res ce uble an 47,82 Barnett, P. R., analyst... 52, 53, 55, 56, 57, 58, 61, 79, 80, 81 Bastin, E. S., Hill, J. M., quoted...._.._....... 89 Beryllium 82 iBiotibe, deSeripkion. :.... 12 Biotite-muscovite granite, alteration............ 11, 31 AIFhOIOEY L. s oI IAT Ace ece decl. 7 Biotite-quartz latite dikes, Tertiary............ 7 Biotite-quartz-plagioclase gneiss, alteration of.. _ 34 LEI AT T 2s sae eae en een cae e cein emaiecs 83 Bochmite ¥ 96 (BOFOH;\: cn chante eos 82 iBostonite, 39 See also Tertiary rocks. Botts, 8. D.; analyst. 40 Gutters, R. M., analyst_.._.............l0l.... 94 C 4s . 00 2003000 AI recte e. 85 LLIN eT ne dis edes eels ene 21 Calcium.. 47 CATDONAC CEATEC ei ecenics als 49 Carroll .. ...... 112.0... 200800000. 10 (Ceritim - rest PLI LIL .LA -_ 84,85 7,8 Chemistry, host rock............. 91 .. 16, 25 ICRFORIUN AEL SAC ECT IO.. 83 Clay minerals, abundance in biotite-muscovite $TARINE. ...... 31 35 -_ 24, 25 5 stability fields........ 96 structure..._....... 21, 26, 87 X-ray diffraction traces...._.......... 25, 27, 28, 30 INDEX Page Jens noe neces ans s 84 Conclusions, geochemical. 86 Conklin, N. M., analyst.... 63, 64, 65, 66, 67, 68, 60, 70, 71, 72, 78, 74, 75, 76, 77, 78, 79, 80, 81 ...o ells c ee 84 Curtis, H. A., cz ick. 94 D Dean, P. M.. analyst. c IL 94 Diamond Mountain mine, modal analyses 31 Dikes, Tertiary... .. ERE ack cn 7 Dorit tunnel-..........l.clecc dle clone 38 E E. Calhoun mine, alteration zones._............ 10, 90 effects of supergene alteration.__....__._..._. 10 spectrographic analyses.._._.._..___._.__.____ 91 Elmore, P. L. D., analyst. 40 Eskola's metamorphic facies principles. 91 Essex mine, ion redistribution.........__._____. 42 modal analyses.....___.. - 34, 36 sericifized 33 spectrographic analyses._.____.__________._. 90 veins and wallrock zones.._.______________. 10, 36 F up | 1) rn nie oe a cname oin 7 Field studies, 4 Flanagan, F. J., analyst. 95 Fluorine ": 2.992 TCL CeeCee, 50 Folds 2 Tors 7 Foxhall mine.............. é 38 Freeland vein 22. . 3.0.90, ICT C ILL 8 G Galoha .... 00 Reet 7,8 Galena-sphalerite veins.. 8 32 Gangue minerals _______________________________ 6 Fo s o Ape coder aed De men enim 17, 21, 38, 49 Garnet-quartz gneiss, description_._....___..._. 16, 38 fOR CORbenbc2 L= cor eac once. 47 THAMIGANCNE. . L (.. Al. el cen nares. 49 Geochemistry of altered rocks, major elements... 39 Sesociated anions. -I 50 frace 60, 87 Geologie ..... cie. 6 2.22. 20020. ce- nc aces on 82 Golconda mine, garnet...._......._...__ 17 modal 38 OMC ELE AT rere re s ol eee RAN ree ece enas» » 6, 8 Granite, argillized..........._...._._ 31 6 7, 31 Powder 86 sericitized....... 33 Granodiorite, 35 0C ec . cous oul ber ab beeen ne ous we 34 iron content. 47 MEKHOTOEY .$ 22 : .c. noose. eo pone bends 7 SErICIAIz@d .. ._ 01.2. IIL seen na 36 Orim, R. E., .. CIT. econ. 24 H L2 IAT. We cence el e beter 26 Hayes and Wheeler tunnel, model analyses.... 34, 35 Havens, R. G., analyst_._..._._________._. 59, 60, 62 Hill, J. M., and Bastin, E; S., quoted........... 89 Page Hornblende, description................_....... 17 in quarts dloribe.. cels 200.0200 CA .e. 37 Hornblende granodiorite. See Tertiary rocks. Hotiston ming: . 2 Y e. 89, 91 Hydrothermal alteration. .. - 8,88 Hydrothermal environment...... 91 Hydrothermal solutions, acidity and temper- clas | n M a 94 alteration stages. 96 defined 1. 2.2 lise veces 93 structural control of movement. 7, 88 synthesis of minerals._......... 95 I Bbe oo cense HLA CEG ie rene 17, 25 Indium. 202.0. ALA ral 82 Tons, calculation of movement... 40 distribution............. 39, 87 Introduction...._.......... 2 Intrusives, classification .. 7 Tron, distribution......... 47 ovidation states.: -- 48 J JOI L EL OLA ILOIL : lun ae ale 7 Jo Reynolds mine, modal analyses_.._.......... 31, 36 K Knolinibe. .. 12, 16, 17, 26, 30, 33, 44 . : 110.010.0002. cen a=» 6/8/12 Kifty Clyde mings... IE 38 L Laboratory studies, methods......._.___..._... 5 Leads 61800 L 2200220002 cele 84 Leucocratic granodiorite. See Tertiary rocks. LeUCORANC... 2.0000 OAI neck cs 21 Timonite .. IROL Lincoln vein:. . 00A AIIRA en. Lithium vik Lithology, hydrothermal alteration sequence.. . 8 Procambrint 6,7 LocalisabiOn-.2.. c Cee 00 LILLIE ICR Adc recane 88 LOCAHION . ... 1 one seus oleae eee ne arenes 2,8 M s . 21-27. 0.0001 NCCE Er ders cen 46 Major constituent elements, chemical analysis... 40 spectrographic analyses.._......_...._...... 50 M. and M.-Dixie tunnel, clay minerals_........ 35 modal 34 Mammoth mine.............. 10 201. I2 EAL LER . cdc ll 49 MoetallizabiOn. 02. cos revlon nene ece es 8, 89 Metals, transifional.. .. .s.02022..00..00.2200002 83 Metasedimentary rocks, Precambrian.......... 6 Mefatorbernibe. «cure n. oo. ecole nebo cu cual cl 10, 97 Methods of investigation. .........._.._._...... 4 Mica. . 26 Microcline-quartz-vlagioclase-biotite gneiss, al- SEFAEIONL :. 1 niet 202. F3 usu- ceca ne ao 31 MeSCrIppon. .... . . . .oo ere Siecle baa 6 Mineral . .. nec 7 Mineral stabilities, experimental studies. 28 primary and secondary..................... 12 101 102 Page Mineral structures and inheritance. ......-~~ 30, 40, 87 Mineral 2-- 8 Mineralogy, host rock.._............~- 11, 90, 91 triangular composition diagrams 91 Modes, amphibolite. 38 biotite-muscovite granite.......... 31 biotite-quartz-plagioclase gneiss. 36 garnet-quartz gneiss..._........-.- 38 gTAROGiOFIt@. . 34 microcline-quartz-plagioclase-biotite gneiss. 34 85 12, 16, 17, 26, 47 Montmorillonite-illite.. yess 16,37 Mortar -. ccllco colle E2 l 7 Mosley, H. R., analyst..._..._....___..... 94 Murata, K. J., analyst.. 95 'MASCOVibG. .._ L2... i eco oe cab awk 12 N Nabob mine, clay 31 modal 36 84, 85 NHCKON:..1 LI. novi 84 NIODIIT ... . - 2. Lele nene cloe ake 84 0 (Ore 2. -.. siles 7, 88, 89, 96 Origin of altered wallrocks...... 88 Oxidation-reduction reactions...._............-- 9 Oxygen, weight percent in wallrocks...........- 50, 87 P Pauling @ PUIG§.20 2. 202 ee teu cba 39 Peltenpol, W. B., analyst.._......___:____.___._.. 94 pH of wallrocks.......... 85 Phillips, H. F., analyst-.....___.___.__.___.____ _. 40 Phoenix mine; 83 P ACCEL ERICA: Gere aan ee 49 Pitchblende, age relations and deposition....... 8 relation to pyrite.. 8, 97 Plagioclase......... 16, 17, 31, 90 Plutons. . Polymorphism, clay minerals............---..-- 31 PONG cL. 89 INDEX Page P obASSIUI : . :. Lo AA dvi cee 44, 87, 92, 98, 96 Potassium feldspar....~...- - 7, 9, 12, 30, 31, 33, 39 Potassium-hydrogen ion ratio......._.... 28, 46, 87 Precambrian rocks, description_......_........~ 6 T Previous investigations.........._.....- 2 E:. cll. resten 8, 10, 16, 31, 80, 92 Pyrite veins. a 8 Pyrophyllite. . 2. ..-... 96 Q UAL I2. 12, 33, 36, 30 Quartz diorite, altered.....................- e 87 36 CORbeNL. 2 :. «220. 20s ETTI. ce ne 47 MISHOIORY .. cen 4200 ev cane ol ara sua 7 Quartz monzonite. - See Tertiary rocks. R Radioactivé springs. 93 Radium: Hot Springs. 94 R. H. D.-McKay mine, effects of supergene AL dures 10 modal analyses. 38 Running Lode mine, ground-water analyses.... 94 S .. .c. 920042 lab 83 Scott, P. W., 40 Secondary minerals...................._....._.. 8 SENIGID LLL .l. ool 26, 33, 37 Sericitic alteration.... 9 Sherwood, A., 40 Silicate minerals, structure..........-...------- 39 Silicon: -se Silver... Sodium. Sphalerite. Sphene... Stokes Law. Page SuIpHULS: L eee cevedcun s receapance 50 Supergone 12... 9, 96 T. Tarrant, L. N .. 40 Temperature gradient. 89 coo ins 8 Tertiary, rocks, description.._......._.......... T 'Theobold, J., analyst.... 40 PIM ] NAL AL Conceded IN. ann nene 83 .L .. .- AAs eo 49 Trace elements, spectrographic analyses. 50 "Two 'Brothers LET 00; 90 U ATARIAIEC ... . .-»: rnt ith cn ee nae 10 rus Se nies 6, 97 v Valentine, K. E., analyst.... __.. 54,79 s... . 00 up eed even eure 83 Variations in distribution and character of altered wallrock ..-..._._..l.lscclcle. 11 8 . | _. 10 W Wenthering. 9 White, K. E., analyst.. 40 Wright, H. D., quoted... 2 X X-ray diffraction traces, clay minerals. 25, 26, 27, 28, 30 ¥ . . 84, 85 NIE! rr 84, 85 Z TANC-7. . 2 oo. ooo vel snene Zones, HAIMOTAI-L c 10000 000000» sau sue hes Zones of alteration, lithology. width.... 39°45" UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY PROFESSIONAL PAPER 439 PLATE 1 105°35" 105°30' \ I p \f\O\ INDEX OF MINES p e wTHUH (90 } _" l 1. Dumas-Kinney 17. Kitty Clyde \\ G Q4, * ® 4, | 2. Bobtail 18. Dorit LPIN cofié 4 / Ss, s g 3. R.H.D..McKay 19. Martha E. Jy" *A 6 3 e cn 4. Carroll 20. Diarmond Mountain Q4 ©, Su, || Clear f R& Opprw C Ch 1 5. Essex 21. M. and M.-Dixie REE 3 P 6. Lacrosse (Patch) 22. Wallace / # 7. Mammoth 23. Lamartine <- 7 8. Hayes and Wheeler 24. Old Stagg a a 4 I 9. E. Calhoun (Wood) 25. New Era T , 10. Cherokee, Banta Hill, _ 26. Nabob ( f if | 4 Al I | O! m E Widow Woman 27. Jo Reynolds .)(;Mt Pisgah of C al p b- S| \\L ’ 11, Silver Age 28. Golconda % 6 e - - Sn _g 12. Two Brothers 29. Almadin f RE- Z . Cartel of ___ \ 13. Foxhall 30. Mary eyeka 15 o 2 ~- ma L-- 14. Phoenix 31: Blackhawk .~ =- * - ."" . _ "® " s s t oak taf gll cda angrier oo" has "o . (.. a :o Rent. dit, o_ o nle ae net ail o BRA N C ew a he poe an ( hone o HHT . S -~ f 15. Alma-Lincoln 16. Argo tunnel Lawson ws YO .............. ...... ................. ......... .................. ....... 2.19031: | | | | 2) -G tL f/Z—‘/ I | | | x: Z Ht" 2x :CNf mba 7ZSUN AND II : | Fes: 8 .......... f- " £954 Pare o t aye +e , \/. R BAE WAW C # 08 ceca eg ...... d - rs" - «|. -k} s Cte ; * -| 39°45" xz» "EXPLANATION in a % Lat . Shes wu Areas prédominantly cfinfaining pyrite veins devoid of base met- als, and pyrite veins containing w MAP SHOWING ZONING OF MINERAL DEPOSITS AND LOCALITIES SAMPLED DURING THIS INVESTIGATION SCALE: 1:48 000 4 copper mingftgls; r Lamartine + % \ * Alps Mtn a. A. Areas predominantly containing ) pyrite veins with sphalerite, ga- // 22 - lena, and copper minerals / wip. - // Cg: 21 y **, f ”Area's’bnedominantly containing f 2 Fre". 3 MINING DiIsTRicts Relena-sphalerite veins [ " OP \a. Central City fas. ® Y + £3 tp C o s : ~._. Cn C & ///