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Clark, Frank Emerson A comparison of the rock formations of the Aurora and Tonopah mining districts, Nevada 1908 BIBLIOGRAPHIC RECORD TARGET University of California at Berkeley Library Master negative storage number: 03-67.18 (national version of the master negative storage number: CU SN03067.18) GLADIS NUMBER: 184783921H FORMAT : BK AD:991004/FZB LEVEL:b BLT:am DCF:a CSC:d MOD: EL: 7 UD:030604 /MAP CP:cau L:eng INT: GPC: BIO: FIC: CON: ARCV: PC:s PD:1908/ REP: CPI: FSI: ILC: IT:1 040 CUScCU 090 SbDISS.CLARK.MINING 1908 100 1 Clark, Frank Emerson, $d1860- 245 12 A comparison of the rock formations of the Aurora and Tonopah mining districts, Nevada. 260 Scl1908. 300 ii, 46 leaves ;$c28 cm. 500 Includes index. 502 Thesis (B.S. in Mining) --University of California, Berkeley, May, 1908. 504 Includes bibliographical references. 610 20 University of California, Berkeley.S$bDept. of Mining EngineeringSxDissertations. 690 0 Dissertations, Academic$xUCB$xMining Engineering$y1908. Microfilmed by University of California Library Photographic Service, Berkeley, CA FILMED AND PROCESSED BY LIBRARY PHOTOGRAPHIC SERVICE, UNIVERSITY OF CALIFORNIA, BERKELEY, 94720 DATE: 7/03 REDUCTION: 10 X PM-1 3%"x4” PHOTOGRAPHIC MICROCOPY TARGET NBS 1010a ANSI/ISO #2 EQUIVALENT TLE = = = l& 122 36 Es 1a ha ba l || C= 2 _— ° e Bk = l= Ii fis ps FLEE EE Ep | gp TS 2 sui git cit tt oft 6, 18 2, 10 l6, 6 18 12 hkiri OUTLINE AND INDEX, INTRODUCTION, PART ONE - THE EARI.TER ANDESITE, (a) The Original Character of the Rock with Underlying Formationssssvssovecesennnss (b) PP ACLRT III + « «on srinivas sannainnesvorncanien (¢) Alteration and Vein Formation Due to Same WaterB.cseervsvesensssenravosionnnios (d) Typical Alteration to Quartz and Sericite with Gradations into the Calcite- Bhlorite TyYpCascess vests sstrsnnnsanes Appearance of the Altered RoCKSsceecresnes Vein Material B, aaa i ves cotsetssnsannedesnens Original Composition of Altering Waters.... Effects of Alteration on the Composition of Altering WaterS. cece sens ssnnesonssnnnsss The "Propylitic"” Alterationsssessscecoscene The Comparison with Similar Rocks at Aurora BroSiorie: sect resssnsssaveannnnssnsansaneinie PART TWO « THE T.ATER ANDESITE, Ee SE (a) ®riginal Character and Comparison to the Earlier Andesit@uissesssrsssinsnsnnsnns {b) Appearance as Compared to Farlier Andesite. (c) Alteration of Later Andesite Due to Waters Directly Following Intrusions of Acid RoCkS, vests ncsernnsanrnssssonvsssscansa (d) Composition of Altering Waters of the Later Andesite as Compared with the Waters which Produced the Earlier Andesite Alteration OUTLINE AND INDEX, PART TWO - THE LATER ANOESITE (Cont.) (f) Comparison of the Alteration of the Later Andesite with that of the Earlier Andesite 26 30 (gz) Comparison of Aurora Andesites to Tonopah.... (h) The Aurora Andesite Brecciic.ciecceacccconas 35 PART THREE - RHVOILITKS AND DACITES. 38 PART FOUR =~ EVIDENCE THAT THE VEIN FORMATION WAS MIE TO ASCENDING WATERS. 41 PART FIVE - THE SEQUENCE OF EVENTS AT THE TONOFAH DISTRICT. 43 PART SIX « GENERAT CONCLUSIONS, 45 i i 1 : | i i | i : i H H 1 i ey i A COMPARISON OF THE ROCK FORMATIONS OF THE AURORA AND TONOPAH MINING DISTRICTS, NEVADA, Introduction. In this paper the comparisons have been made from a study of thin sections and hand specimens of the rocks of the two districts, with Spurr's "Monograph on the Tonopah Mining District" as a basis of compari- son, and without having obtained any knowledge from the field study of the two districts. Spurr shows in his monograph that the important mineral veins of the dis- trict, occuring in the earlier andesite, have been pro- duced at the same time and by the same waters that have caused the intense alteration of the earlier andesite. The less important veins of the later andesite for the most part are due to the ascending waters which altered the later andesite, By showing that similar rocks at Aurora have been similarly altered, the evidence wculd tend to show that the geological events at Aurora have not been very ei ei much different from those at Tonopah and that the mineralizaticns may have taken place in much the same way. Aurora is in Esmereslda County, Nevada and only a few miles from the California Nevada boundary line and about seventy-five miles northwest of Tonopah. (1) THE EARLIER ANDESITE, (a) The Original Character of the Rock with Underlying Formations. The first Tertiary volcanic rock at Tonopah, the earlier andesite, is underlain by a limestone for- mation with granitic masses intruded. The Tertiary volcanics at Aurora are underlain by a formation con- sisting of a succession of tedded cherts, shales, and limestone, The earlier andesite is now almost entirely altered, but was originally a hornblende-biotite ande- site of rather siliceous composition. It had a por- rhyritic structure with few phenocrysts of felaspar, hornblende, and biotite in a glassy ground mass with many microlites., Hornblende and biotite were about equal in amount, Augite was rare. These minerals were not abundant, The feldspar was typically an acid lab- radorite. They were small, slim, simple crystals. Apa- tite was abundant, ( (I) The Earlier Andesite, ) (b) Fracturing. Soon after the cooling of the earlier ande- site, it was complexly fractured by the volcanic for- ces which were still active below. There were no large fissures, but zones 6f small parallel fractures were formed. The main fractures in which the important veins have been formed were in an east and west direction, with a minor system of transverse fractures, (¢) Alteration and Vein Formation due to Same Vaters. Immediately succeeding the fracturing and di- rectly connected with the earlier andesite eruption, hot volcanic waters ascended through the fracture zones and caused an intense alteration of the andesite, and at the same time produced the important mineral bearing veins. “Be ( (I) The Earlier Andesite, ) These waters not only greatly altered the rock near the fracture zones, but also penetrated the rock to great distances from the circulation channels, and altered it so completely that no fresh earlier andesite has been found, The waters were active but a short time. The formation of the veins was affected only by intense replacement and interchange of material, the waters abstracting nearly all the andesitic materials, and depositing silica and the valuable minerals. All the different stages in the alteration of the andesite to vein matter in solid quartz can be seen in the fracture zones. Very often the vein consists of a done of sili- ceously altered andesite, strongly resembling the andesite of the wall rock. This zone is cut by parallel fractures having the same dip and strike as the walls, and these walls are only stronger and more persistent fractures of the same kind, The vein may consist of streaks of almost pure quartz with zones of altered andesite between, or Bw ( (I) The Earlier Andesite. ) there may be a network of quartz veinlets in altered an- desite. Generally, altered andesite composes part of the vein. (d) Typical Alteration to Quartz and Sericite with Gradations into the Calcite~Chlorite Type, The typical alteration is to quartz and seri- cite and a little adularia, and this has taken place near the fracture zones, Spurr gives the chemical analyses and mineralogical compositions of a series of rocks show- ing all transitions between this type and the calcite- chlorite tone. which is characteristic of the rocks far- thest away from the circulation channels, The veins them- selves are only a more siliceous phase of the typical quartz sericite alteration of the andesite. Rock No. 8, in Spurr's series is a rock entire - ly altered to quartz, sericite and adularia. Under the microscope, it is seen to be composed of fine to moderate- -u { (I) The Earlier Andesite. ly coarse granular quartz, with much fine muscovite. Intermixed with the quartz is adularia in characteris- tic rhombic crystals. Rock No, 1, is a typical calecite-chlorite tyre and corresponds to the "propylitic" type of altera- tion described elsewhere, except the epidote is absent. In thin sections, scattered phenocrysts of small size can be seen in a microlitic ground mass, showing flow struc- ture. The microlites are mainly feldspars. Some zircon apatite, and a few quartz grains are present. The feld- spars are the most important phenocrysts and are largely altered to calcite, with a little quartz. Abundant pseu- domorphs after hornblende, with no trace of the original mineral, consist of chlorite with some specular iron. The hornblende cleavage is still visible. The biotite has been altered to sericite, with a little calcite and hematite, The compositione of these two types, as com- pared with an average type show the changes due to alters tion. ( (I) The Earlier Andesite. ) 8, Spurr's Series, Average Type of Andesite, 62.16 16,45 No. 1 Spurr's Series. 91.40 4,31 ( (I) The ERarlier Andesite. ) The six diamond drill core specimens from Tonopah in the University collection will next be des- cribed., (Specimens from Tonopah and Aurora in the University collection will be distinguished by the sub- scripts "T" and "A" respectively.) Rock No, Ip is a light greenish gray rock, fine grained, wtth considerable pyrite in it. The phenocrysts were feldspars, hornblende and biotite, A few unaltered feldspars were determined as at least as basic as andesine. Nearly all the feldspars are almost completely altered to calcite and sericite, with a little quartz and kaolin, Others are only partly altered to sericite and caleite, Hornblende is altered to chlorite and sericite, with siderite, magnetite, and hematite marking the boundaries. Some of the chlorite has been carried into the boundaries of the feldspar pheno- crysts. The blotite has been altered to sericite, chlo- rite and siderite, There was not much original biotite, The groundmass is altered to sericite, quartz, calcite, | | ! -10-~ i t ( (I) The Earlier Andesite. ) | and chlorite, with kaolin and siderite also. Several crvstals of apatite are present, with considerable mag- natite and a little hematite. Sericite, calcite, chlorite, quartz, kaolin, and siderite are the chief alteration products of the rock. The rock is not typically altered, but is a phase perhaps nearer the calcite-chlorite type than it is to the quartz-sericite type. Rock No, Sm is a dense greenish gray rock, dark- er than No. ln, with a very few small crystals of felds- par. Under the microscecpe, some feldspar phenocrysts | are seen to be partially altered to sericite, calcite, and quartz. Others are completely altered to sericite, with considerable kaolin, Some half-altered feldspars were determined as andesine. Some hornblende phenocryste are altered to calcite chlorite and siderite, with a rim of magnetite. Others are altered to the above with seri cite also. One 1s entirely altered to chlorite and sider- ite, Some probable biotite is altered to chlorite and siderite, The phenocrysts were mostly feldspar and horn- «lle ( (I) The Earlier Andesite. ) blende, with a little biotite, Some lath-shaped aggregates of magnetite, hematite and siderite show crystal cutlines, Some pyrite is present. Many small feldspar microlites, some fresh and some altered to sericite, occur in a glassy groundmass, Part of the groundmass is altered to seri=- cite and quartz with some kaolin. Chlorite and calcite from the alteration of the ferromagnesian minerals are scattered throughout the section, There are some calcite and quartz veinlets. There 1s considerable magnetite. Rock No. 4m is a greenish gray rock, darker than No. Sn with a few small phenocrysts visible, Under the microscope, the feldspars are seen to be altered to sericite, calcite and quartz with small portions remain- ing unaltered, The feldspars are andesine. There were few original phenocrysts in the section. The hornblende is altered to chlorite, siderite, and sericite. The bio- tite is altered to sericite and siderite. Chlorite oc- curs throughout the section. The groundmass 1s altered ( (I) The Earlier Andesite. ) to quartz and sericite with considerable kaolin. There 1s some apatite and considerable siderite. Veinlets of chlorite and quartz occur. The section is rather badly altered, and is very similar to the calcite-chlorite tvpe. Rock No. 5p is a dense dark rock, somewhat ba- saltic looking, with small seams of hematite. Feldspar phenocrysts can be seen, The feldspars are somewhat al- tered to sericite and quartz. Some are entirely altered to sericite. Some rather fresh ones were determined as labradorite. The feldspars vary in size from rather large ones to microlites in the glassy groundmass. Many horn- blende phenocrysts are altered to calcite, sericite and quartz. Some are altered to chlorite and sericite with heavy borders of magnetite and siderite. Others are al- tered to calcite, chlorite and quartz. Calcite, quartz and hematite veinlets occur. The section is faulted along ( (I) The Earlier Andesite. ) a small quartz veinlet. The ground mass is altered to sericite, calcite, chlorite, with some unaltered micro- lites in a glassy ground mass. The section has a great many more phenocrvsts and seems to be more basic than most of the earlier andesite, It might possibly be a later andesite, but probably represents a transition stage between the two types, and nearer the calcite-chlor- ite type than the quartz-sericite. Rock No, 64 is a rock resembling No. 3,, being a light greenish gray rock, A microscopical study shows the feldspar pheno- crysts altered to calcite, sericite and some quartz, Some are partially altered to the above with a little kao- lin. Some labradorite feldspar was determined. The hornblende is altered to calcite, chlorite, iron oxides and siderite, The biotite is altered to sericite ana sid- erite., There were not many crystals of ferromagnesian min- erals. The groundmass is altered to calcite, sericite, ( (I) The Earlier Andesite, ) chlorite, some quartz, kaolin, and iron oxides. There a are some unaltered feldspar microlites. There were not many phenocrysts in the section which is principally al- tered 40 calcite and sericite, It is a transition stage between the two types of alteration. Rock No. Tp is a dark green rock with some crystals of quartz and feldspar. A study of the minerals in thin section shows the feldspars are altered mostly to sericite., The ferromagnesian minerals are altered to chlorite and calcite, with considerable magnetite and siderite, There are some small lath-shaped feldspars al- tered to serioite and quartz. The groundmass is altered to sericite, chlorite, quartz, kaolin, iron oxides and siderite, There are veinlets of calcite with a chlorite border. The section is altered mostly to chlorite, cal- cite, and sericite, Excepting Rock No. Srp y the above sections show that there were originally a limited number ( (I) The Earlier Andesite. ) of phenocrysts, and the average alteration has been to calcite, sericite, chlorite, and smaller quantities of quartz, iron oxides and siderite, The rocks were prob= ably at a considerable distance from the main circulation channels as the alteration more nearly approaches the calcite-chlorite type than it does the quartz-sericite. (e) Appearance of the Altered Rocks. Spenimens of the Andesite altered largely to caleite, are generally light-colored, dense, prophy- ritic rocks with small feldspar phenocrysts. The chlorite gives the rock a greenish tinge. When the original ferromagnesian minerals were abundant and have altered to chlorite, the rock is generally dark green, The rock that has been typically altered to quartz and sericite is usually very light-colored and closely resembles a rhyolite. The six diamond drill ( (I) The Earlier Andesite, ) spedimens in the University collection, thin sections of which were described under the last topic, are light greenish gray rocks, dense, and contain a few feldspar crystals and some pyrite. (f) Vein Materials. In the unoxidized quartz vines, quartz is the predominating gangue mineral, with some adularia, less sericite, and a very little of the carbonates of calcium, magnesium, manganese and iron. Silver sulphide is tne chief metallic mineral and sometimes contains antimony and arsenic, Silver selenide, gold in some unknown form, chalcopyrite, iron sulphide and perhaps silver chloride OCCUr. In the oxidized regions of the veins, the metallic minerals are mostly secondary and due to des- cending surface waters, They include argentite, ruby sil ver, horn silver and free gold. ( (I) The Earlier Andesite, ) (g) Original Composition of Altering Waters. The mineralizing waters were charged with sili- ca and potash, together with silver, gold, antimony, ar- senic, copper, lead, zinc, selenium, etc., in small amounts. They were deficient in iron, since that metal was removed from the vein zone and adjacent rock. Sul=- phides and carbonates were formed both in the veins and the altered rock, showing that carbon dioxide and sul- phur were in the altering waters. Some original silver chloride shows that chlorine was also present. (h) Effects of Alteration on the Composition of Altering Waters. The maximum effect of the altering waters wes the formation of veins by a process of replacement by which the original rock constituents were dissolved out in the order of their solubility in the altering waters, -]l18= ( (I) The Earlier Andesite .) and silica and potash were precipitated in their places. The order of solubility was lime, magnesia, soda, iron and alumina. As the waters left the main circulation channels, their abstracting power decreased and smaller | and smaller amounts of lime, magnesia, soda, iron and alumina were dissolved. The waters were depositing sili- ca and potash constantly, so the amounts of these con- stituents continually decreased. After the waters had penetrated the rocks to a considerable distance and had lost their excess of silica and potash, they were still charged with carbon dioxidg¢, soda in excess of potash, important amounts of lime and magnesia, some iron, and were still capable of decomposing the rock and forming more stable compounds under the new conditions. These waters altered the rock to calcite, chlorite, quartz, and the result was the so-called "propylitic" alteration. i | | | | “10. ( (I) The Earlier Andesite, ) (i) The "Propylitic" Alteration. The "propylitic" alteration is characterized by alteration of the feldspars to calcite and quartz, with the development of epidote; and the alteration of the hornblende, biotite and pyroxene to chlorite, while the groundmass is altered to a granular aggregate of feldspar, quartz, chlorite, epidote and calcite, Some sulphides are formed. This 1s a widespread type of al- teration and the occurrence of rich mineral veins in the "propylitic type" of altered rock has been explained by the fact that the waters which effected the alteratim of the rock, at the same time produced the veins, (j) The Comparison with Similar Rocks at Aurora. Rock No. 3p from Aurora, is a light colored dense rock with a few pyrite crystals. It is a rock that ( (I) The Earlier Andesite, ) has been much altered and no phenocrysts remain, There are no good pseudomorphs, so the character of the origin- al minerals cannot be tolé., The alteration is chiefly to quartz and sericite, with a very little chlorite, some pyrite, siderite and hematite, Pvrite crystals are prac- tically the only ones remaining. There is some kaolin. From the fact that the alteration is to quartz, sericite and pyrite, typical alteration product of the earlier an- desite, the rock is classed as being similar to the ear- lier andesite at Tonopah. It may be a later andesite en=- tirely altered. The evidence in favor of its being simi- lar to the earlier andesite is not conclusive, Rock No. 6, 1s a rather fine-grained, porphy- ritic rock with decomposed crystals of feldspar, some chlorite and pyrite. The color is a light greenish gray. As seen under the microscope, the feldspar phenocrysts are almost ehtirely altered to sericite and a very little quartz, The ferromagnesian minerals have ———— pn ———___ 2] ( (I) The Earlier Andesite. ) been altered to chlorite and siderite. Nearly all of the microlites in the groundmass have been altered to sericite and quartz. There is some kaolin and siderite in the section. Also a very little magnetite. The rock very closely resembles some of the rocks in Spurr's series of the earlier andesite, and is not very differ- ent from some of the diamond core's specimens except that it has been altered more, and no calcite is.pres- ent, The presence of but few phenocrysts and the type of alteration indicate that it may be similar to the earlier Tonopah andesite, but the evidence is not strong. (k) Erosion. After the alteration of the earlier andesite it was eroded, and the mineral veins were exposed. Then the later andesite was erupted, and in some places com- pletely capped the earlier andesite. or i St (II) THE LATER ANDESITE. (a) Original Character and Comparison to the Karlier Andesite. The later andesite is a biotite-augite ande- site of a more basic composition than the earlier ande=- site, It is often found fresh, and is often almost com=- pletely altered, The phenocrysts of feldspar, biotite, augite, and hornblende are larger and more abundant than those in the earlier andesite. There is usually a graded Srvatalil zation from the larger phenocrysts to the micro- lites of the groundmass. The feldspars are usually be- tween andesine and labradorite and are larger and show more complex twinning than those of the earlier andesite. Biotite in large crystals is sometimes found fresh, though usually bleached. The augite is sometimes found fresh, but seldom. The groundmass is of glass, with feldspar and pyroxene microlites, ( (II) The Later Andesite.) (b) Appearance as Compared to Earlier Andesite. The later andesite has generally been some- what altered and is usually a dark green color, The surface rock has a characteristic rich purvlie color, though typically the rock has a medium dark color and is mottled with crystals of feldspar and biotite, The later andesite is generally darker than the earlier andesite, It is characteristically coarser=-grained, and contains large and more abundant porphyritic crystals. The porphyritic crystals of feldspar in the earlier andesite are usually slim, of simple form, and almost rectangular; while those of the later andesite are apt to be stout and complex as a result of twinning, Fresh crystals of biotite can often be seen in the later andesite, and but seldom in the earlier andesite. ( (II) The Later Andesite. ) ( (II) The Later Andesite, ) | (¢c) Alteration of Later Andesite Due to Waters the waters themselves were very basic, charged with much of Acid Rocks. : ic elements, The waters which produced the earlier an- | I | | Directly Following Intrusions magnesda, lime, and iron and small quantities of the acid- | desite alteration followed the eruption of the earlier | There were several eruptions of dacites and ] » . ¥ : andesite, a rock of medium composition, and were highly rhvolites: and hot vclcanic waters, following along the ab y ’ ’ charged with silica and potash, elements characteristic tact th ntrusiy I : : contacts of these acid intrusives, produced the alteration Of thi most noid recits. of the later andesite and also produced some of the veins of the later andesite, (e) Vein Formation, (d) Composition of Altering Waters of the Later The velns of the later andesite are not, as a | Andesite as Compared with the Waters rule, very high grade, Gold values predominate. The | | which Produced the Earlier veins are composed mostly of a jaspery quartz and were | Andssite Alteration. formed along the contacts of acid intrusives. Some of the small veinlets of calcite and quartz are due to The waters which produced the alteration of lateral secretion. the later andesite followed the eruption of such rocks very siliceous rock; while as the Oddie Rhyolite, a ( (II) The Later Andesite. ) (f) Comparison of the Alteration of the Later Andesite with that of the Earlier Andesite, The feldspars of the later andesite are usual- ly partially altered to calcite, chlorite, quartz and kaolin. The biotite, in large crystals, is sometimes fresh; but usually bleached or altered to sericite, pyrite, calcite and siderite, and sometimes to a chloritic aggregate, The hornblende is always altered to chlorite, quartz, siderite, calcite, and sometimes sericite and kaolin, The augite alteration produdte vary, but are usually serpentine, chlorite, more abundant than in the earlier andesite, The ground mass of glass with feldspar and pyroxene microlites is altered to quartz, chlorite, serpentine, siderite, pyrite, calcite, and sericits. In general, the alteration of the rock is to quartz, chlorite, calcite, pyrite, and si derite, but may ( (II) The Later Andesite. ) be altered to quartz and sericite-like aggregates of kaolin and hydrargyllite, The earlier andesite is typically altered to quartz and sericite with a little pyrite. It may be im- possible to classify a specimen of andesite by studying its type of alteration, as both earlier and later ande- sites may be similarly altered. Rock No, 11, a greenish gray later andesite from Tonopah not as dense as most of the earlier andesite 1s altered principally to sericite and chlorite, The feldspar phenocrysts are not as badly altered as in the earlier andesite and were found to be labradorite. Some are altered to sericite, kaolin and quartz. Feldspar is the chief phenocryst. The ferromagnesian minerals are al}- tered to chlorite, sericite, magnetite and siderites. The groundmass is partially altered to sericite, quartz, and ( (TI) The later Andesite,) chlorite, with some unaltered feldspar microlites. There are many feldspar phenocrvsts of various sizes. There is considerable magnetite, hematite, siderite and kaolin, with some quartz and apatite. The section is not badly altered and the type of alteration is not altogether dif- ferent from that of several of the later andesite speci- mens described by Spurr, and resembles somewhat some of the diamond drill core sections of the earlier andesite, except that there were more phenocrysts than in the ear- lier andesite specimens and there was no calcite in this section, Rock No. 124 shows the typical later andesite surface rock at Tonopah, and is a purple rock, with white feldspar phenocrysts, giving the rock a mottled appear= ance, A great many feldspar phenocrysts of various sizes and nearly all unaltered, occur in a section which ee —— ( (II)The Later Andesite.) has a dark color due mainly to the great amount of iron oxides present. Teldspar is the principal phenoecryst, and appears to be labradorite, A few feldspars are slightly altered to kaolin, others are slightly altered to sericite and quartz. Considerable biotite occurs throughout the section. It has a rim of magnetite and some seams of bleached hiotite., There are apatite inclu- sions in the biotite. Some of the biotite crystals are resorbed. One partial augite crystal occurs. The ground- mass is partly altered to quartz with sericite-like ag- gregates, There is no calcite nor chlorite in the sectim The biotite and the number of phenocrysts would serve to identify the section as the later andesite; though cal=- cite, chlorite, and pyrite, typical alteration products of the later andesite, are missing. ( (11) The Later Andesite.) (g) Comparison of Aurora Andesites to Tonopah. Later Andesites, with Conclusions. Rock No. 1, is a dark gray rock with small white feldspar crystals and considerable oxide of iron. It has a mottled appearance due to a considerable number of crystals. The feldspar phenocrysts are mainly altered to calcite and sericite, while some are altered to cal- cite and quartz, The feldspars are not completely al- tered, but nearly so. There are no unaltered ferro- magnesian minerals, nor good pseudomorphs, but their de~ composition products, serpentine, iron oxide, siderite, quartz, and sericite, with a little chloride are scat- tered throughout the section. The groundmass is altered to sericite, and considerable serpentine. A few felds=- par microlites, in a glassy groundmass are unaltered. «l] = .( (II) The later Andesite.) There is considerable quartz in the section. The section is altered mostlv to calcite, sericlite, serpentine, hema- tite, quartz, and siderite, The whole section is more or less colored by the hematite, with considerable limonite present, Serpentine does not occur as an alteration product of the earlier andesites at Tonopah. This rock has heen more altered than the ordinary later andesite at Tonopah and also has had a greater number of phenocrysts than the earlier andesite usually has. There is consid- erable sericite and quartz. The number of phenocrysts, and the occurrence of serpentine would seem to be fairly good evidence that the rock is similar to the later Tono- pah andesite, though the alteration is far from being a typical one, Rock No. 2, has a mottled appearance due to mary white feldspar crystals in a dark groundmass. 30 - ( (IT) The Later Andesite.) Numerous fresh feldspar phenocrysts predominate, dome were determined as labradorite. Some are slightly altered to calcite, with a very little sericite and quartz. No unaltered ferromagnesian minerals occur, but these minerals are represented by their alteration pro- ducts, serpentine, hematite, magnetite, and siderite, and some chlorite. Feldspar microlites in a glassy ground mass are but slightly altered. Iron oxides and siderite are very thickly disseminated throughout the section. This rock very closely resembles the typical later ande- site from Tonopah as described under Rock No. 124, except there is no fresh biotite in the section. Rock Yo. 8, is a purple rock with many white feldspar crystals. The many feldspar phenocrysts are practically unaltered and are of various sizes. labradorite was de- termined, Some of the feldspars are slightly altered to ( (IT) The Later Andesite.) kaolin, There are many small feldspar microlites in a brown wlnany aronndngse. There are no unaltered ferro magnesian minerals, chlorite and serpentine are absent. The whole section is very dark owing to the great amount of hematite, magnetite and siderite. The ferromagnesian minerals are probably represented by bunches of iron oxides, showing crystal outlines. This rock very closely resembles No. 12q, the tvpical later andesite surface rock at Tonopah, except there is no fresh biotite or au- gite present, It is undoubtedly a rock very similar to the Tonopah later andesite. Rock No, 9, is a somewhat fine-grained dark gray rock, with many feldspar crystals. Inder the microscope, the many feldspar pheno=- crysts are seen to be almost entirely altered to sericite and calcite. There are a few imperfect pseudomorphs of chlorite after some ferromagnesian minerals. The ground- I os is MI 0 0 { (11) The later Andesite.) mass is altered to calcite, chlorite, and sericite. There is considerable magnetite, siderite, hematite, and pyrite in the section. The section is much altered, though not typically altered later andesite. Though the evidence is not very strongly in favor of its being a later ande- site, the great number of phenocrysts of alteration and the calcite-chlorite type would seem to indicate that the rock is similar to the later andesite at Tonopah. Several of the Aurora rocks are very similar to the later andesite at Tonopah, and in all probability, these rocks were erupted at the same time as the later an- desites at Tonopah and from the same general magma. Prob- ably the geological condition governing the alteration of these rocks were much the same as those at Tonopah. ( (II) The Later Andesite.) (r.) The Aurora Andesite Rreccia. Rock Xo. 4, 1s a brecciated rock with light gray fragments of a rhyolitic looking rock cemented to- gether by andesitic material of a dark-purplish color and mottled appearance, The fragments have feldspar and biotite crystals in a white groundmass. The cement has many feldspar orys tals, with considerable magnetite and chlorite. Under the microscope, the fragments are seen to contain+feldspar phenocrysts altered to calcite and gsericite, with some quartz. There were not many original phenocrysts, RBiotite is altered to sericite, siderite, chlorite and some quarté. There was considerable origin- al biotite. Many feldspar mierolites occur in a glassy groundmass. The groundmass is partially altered to (TI) The Later Andesite.) kaolin, quartz and sericite, with some siderite, magne- tite and hematite, There is considerable sericite in the fragments with a few apatite crystals. Some of the fragments are altered to chlorite, sericite and quartz, with considerable glass present and a few unaltered mi=- crolites, Apparently there was very little original horn- hlende., The section is considerably altered and the fragments, perhaps, represent a very acid type of earlier andesite, though the evidence in support of this is not conclusive, the feldspars and other phenocrysts being badly altered. The fragments may be of rhyvolitic material, The cementing material has feldspar phenocrysts of various sizes, much altered to sericite, calcite and quartz. The ferro-magnesian minerals have been altered to calcite, chlorite and siderite, They were hornblende and biotite. A few microlites in the groundmass are un- HT Ny ( (IT) The later Andesite.) altered, though the groundmass is considerably altered to quartz, sericite, calcite, with considerable chlorite. There is much kaolin and hematite, some magnetite and siderite, The cementing material has a very dark color due to the iron oxide present and is chiefly altered to calcite, chlorite, quartz, sericite and kaolin, with much or the iron oxides and serpentine. The cementing material closely resembles the later andesite at Tonopah, and is not very different from the typically altered type, If the fragmental material does not correspond to the earlier andesite at Tonopah, and assuming that the ce- menting material is a rock that is similar to the later andesite, then at Aurora there must have been some volcan=- ic rock erupted to which we have no corresponding type at Tonopah of the same relative age as regards the later an- desite, {I11) RUVOLITES AND DACITES, Rock No. 5p is 2 1izht colored fine crained rock with porphyritic crystals of feldspars and biotite. Considerable pvrite can he seen, A few of the feldspar phenocrysts are fresh, and are oligoclase and orthoclase. Some small unaltered feldspar microlites occur in a glassy groundmass, Some of the glass is altered to sericite and quartz, Some probable biotite is altered to chlorite, sericite, quartz, and siderite, There is some apatite in the sec- tion, some hematite and siderite, The rock has heen bad- lv crushed, The rock corresponds very closely to the description of the Oddie rhyolite, though not as fresh as the unaltered Oddie rhyolite, Rock No. 7, is a rather coarse grained, light colored rock with porphyritic crystals of quartz, feldspar and biotite, Under the microscope, phenocrysts of orthoclase : | i i i h i i | § { § me —————— ee ——— ( (III) Rhyolites and Dacites.) and oligoclase feldspars and quartz occur in a glassy groundmass which has a very low index of refraction. Fragments of broken glass in the groundmass were probab=- ly caused by movement when the rock was solidifying. The rock is a vesicular rhyolite, with fine flow structure, A considerable number of biotite crystals of various sizes occur throughout the section. There is considerable hema- tite magnetite, apatite, and some zircon. This rock closely resembles the Tonopah-rhyolite dacite, though the feldspars determined were not as basic as those of the Tonopah rock in which they ranged from andesine to orthoclase. No. 10, is a grayish rock with white feldspar crystals, The feldspar phenocrysts are albite and ortho- clase and are partly altered to sericite. The groundmass ( (IIT) Rhyolites and Dacites,) is altered to quartz, with considerable sericite, The viotite is altered to chlorite and sericite with some siderite, There is considerable pyrite and magnetite and much siderite, The rock corresponds very closely to the altered Oddie rhvolite. (IV) EVIDENCE THAT THE VEIN FORMATION WAS DUE TO ASCENDING WATERS, The abundance of silica and potash in the veins; the absence of sodium, magnesium, calcium, and iron-- elements characteristic of the andesite; the presence of comparatively large amounts of gold and other rare metals; the extremely small amount of the metals commonly found in veins, such as, lead, copper, and zinc; seem to show that the mineral veins were not the result of lateral secretion, but due to ascending waters, Only the slightest traces of gold and silver are found in the andesite, The intense alteration near the circulation channels, with decrease in alteration as the distance from the fracture zones increased, is evi- dence that the waters were ascending ones. In the later andesite, traces of nickel have been found in the veins, This would probably not be true (V) THR SEQUENCE OF EVENTS AT THE if tre veins were due to lateral secretion. TONOPAH DISTRICT, At the Beginning of the Tertiary, the formation was a limestone one with intruded masses. The earlier andesite eruption, Fracturing. Alteration of the earlier andesite, Erosion of the earlier andesite and exposure of veins, The later andesite eruption. Probable erosion, Heller dacite intrusion. Fraction dacite breccia. Tonopah rhyolite dacite eruption. Siebert tuffs, Elevation of tuffs, Tilting of region. Contemporaneous eruption of rhyolites and Brougher dacite acccmpanied by the main faulting, wile (v) (The Sequence of Events at the Tonopah District.) The main faulting of the district which af- fected all preceding rocks was due to the Brougher dacite eruption, After the intrusion of the dacite into the enormous volcanic necks, there was a collapse as the molten material commenced sinking at the vents and carrying ' with it blocks of the adjacent rock. This increased the faulting due to the eruption and caused some of the down | faulting in the vicinity of the volcanic necks. districts, and the similarity of the types of alteration, wlH=- (VI) GENERAI, CONCLUSIONS. The rocks of Aurora, similar to the earlier an- desite and later andesite at Tonopah, have apparently been altered in much the same way 28 the Tonopah rocks themselves, There is nothing to prove that the Aurora rhyolites and dacites have not been altered in much the same way as the similar rocks at Tonopah, The resemblance of the rock at Aurora, which is probably a later andesite, to the Tonopah later andesite, is much more striking. Several specimens of the Aurora rock correspond very closely to the different phases of the altered later andesite at Tonopah. The absence of biotite in the Aurora rocks is the chief difference. The vesicular rhyolite from Aurora is essential- ly the same as the Tonopah rhyolite-dacite in an unaltered condition, The general resemblances of the rocks of the two (vi) ( General Conclusions.) might lead one to believe that the waters which produced the alteration in the two districts had practically the same chemical constituents and were hot ascending waters of magmatic origin. The evidence at hand is not strong enough to establish the facts in the case, but it is enough to perhaps justify the above conclusions. Probably the veins of the two districts were also formed in much the same manner, END OF TITLE