o... mnmm. m nuiuuunwwnuxnwufim‘u? aux-hum": Muxtmmgmnm m COVER PHOTOGRAPH S l. Asbestos ore 8. Aluminum ore. bauxite, Georgia 1 2 3 4 2. Lead ore-Balmat Mine, NJY. 9. Native copper ore, Keweenawan 5 6 3. Chromite—chromium ore, Wash. Peninsula, Mich. 4. Zinc ore-Friedensville, Pa. 10. Porphyry molybdenum ore, Colo. 7 8 5. Banded iron formation,Palmer, 11. Zinc ore. Edwards, N. Y. Michigan 12. Manganese nodules, ocean floor 9 10 6. Ribbon asbestos ore, Quebec, Canada 13. Botryoidal fluorite ore. ll 12 13 14 7. Manganese ore, banded Poncha Springs, Colo. rhodochrosite 14. Tungsten ore, North Carolina Grade and Tonnage Relationships Among Copper Deposits By D. A. SINGER, DENNIS P. COX, and LAWRENCE J. DREW and Geochemical Exploration Techniques Applicable in the Search for Copper Deposits By MAURICE A. CHAFFEE GEOLOGY AND RESOURCES OF COPPER DEPOSITS GEOLOGICAL SURVEY PROFESSIONAL PAPER 907—A,B UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Singer, D. A. Grade and tonnage relationships among copper deposits. (Geology and resources of copper deposits) (Geological Survey professional paper ; 907—A, 907—B) Includes bibliographies. Supt. of Docs. no.: 19.16z907—A. 1. Copper ores—United States. 2. Copper—Analysis. 3. Geochemical prospecting. I. Chaffee, Maurice A. Geo- chemical exploration. techniques applicable in the search for copper deposits, 1975. II. Title. III. Series. IV. Series: United States. Geological Survey. Professional paper; 907—A, 907—B. TN443.A5854 553’.43'0973 74-23594 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, DC. 20402 Stock Number 024-001-02669-4 APPRAISAL OF MINERAL RESOURCES Continuing appraisal of the mineral resources of the United States is * conducted by the US. Geological Survey in accordance with the provisions of the Mining and Minerals Policy Act of 1970 (Public Law 91—631, Dec. 31, 1970). Total resources for purposes of these appraisal estimates in- cludes currently minable resources (reserves) as well as those resources not yet discovered or not presently profitable to mine. The mining of mineral deposits, once discovered, depends on geologic, economic, and technologic factors; however, identification of many de- posits yet to be discovered, owing to incomplete knowledge of their distri- bution in the earth’s crust, depends greatly on geologic availability and man’s ingenuity. Consequently, appraisal of mineral resources results in approximations, subject to constant change as known deposits are depleted, new deposits are found, new extractive technology and uses are developed, and new geologic knowledge and theories indicate new areas favorable for exploration. This professional paper discusses aspects of the geology of copper as a framework for appraising resources of this commodity in the light of today’s technology, economics, and geologic knowledge. Other Geological Survey publications relating to the appraisal of re- sources of specific mineral commodities include the following: Professional Paper 820—“United States Mineral Resources” Professional Paper 926—“Geology and Resources of Vanadium” Professional Paper 933—“Geology and Resources of Fluorine” Grade and Tonnage Relationships Among Copper Deposits By D. A. SINGER, DENNIS P. COX, and LAWRENCE J DREW GEOLOGY AND RESOURCES OF COPPER DEPOSITS GEOLOGICAL SURVEY PROFESSIONAL PAPER 907—A An analysis of the relationships between grades and tonnages of three types of copper deposits— porphyry, massive sulfide, and strata-bound UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975 FIGURE 1. 2. 3. TABLE FWF’!‘ CONTENTS Page Abstract ________________________________________________________________ A1 Introduction _____________________________________________________________ 1 Acknowledgments ____________________________________________________ 1 Porphyry copper deposits _________________________________________________ 1 Strata-bound and stratiform deposits in sedimentary and metamorphic rocks ____ 2 Massive sulfide deposits ___________________________________________________ 3 Geological considerations of tonnage and grade relationships ___________________ 3 Nature of data ___________________________________________________________ 4 Univariate statistical analysis ______________________________________________ 4 Correlations between grade and tonnage _____________________________________ 7 Discussion and conclusions ________________________________________________ 8 References cited __________________________________________________________ 11 ILLUSTRATIONS Page Grades and tonnages for porphyry copper deposits ____________________________________________________ A8 Grades and tonnages for African strata-bound copper deposits ________________________________________ 9 Grades and tonnages for massive sulfide deposits _____________________________________________________ 10 TABLES Page Summary statistics for the average grades of copper deposits _________________________________________ A5 Summary statistics for the tonnage of copper deposits ________________________________________________ 5 Beta and chi square goodness-of—fit statistics __________________________________________________________ 6 Correlation coeflicients between average copper grade and total tonnage ________________________________ 7 VII GEOLOGY AND RESOURCES OF COPPER DEPOSITS GRADE AND TONNAGE RELATIONSHIPS AMONG COPPER DEPOSITS By D. A. SINGER, DENNIS P. Cox, and LAWRENCE J. DREW ABSTRACT Three types of copper deposits—porphyry, strata-bound, and massive sulfide—are described, and the distributions of tonnages and grades for 267 deposits are compared with normal and lognormal frequency distributions. The relation- ships between grades and tonnages are analyzed by examining the correlation coefficients of these variables. Conclusions reached include the following: (1) Geologic factors influencing tonnage of a particular deposit type are probably distinct from those influencing grade; (2) frequency distributions of tonnages and grades approximate lognormality, making it possible to predict probability of various tonnage-grade classes and to test correlation between variables; (3) no significant correlation was found between tonnage and grade for porphyry or strata-bound deposits; (4) significant negative correlation between tonnage and grade was found for the massive sulfide subset, probably reflecting a mixture of high-grade low-tonn- age massive ores, low-grade high-tonnage stockwork, and dis- seminated ores characteristic of some massive sulfide deposits; (5) significant negative correlation was found between ton— nage and grade for the mixture of deposit types in the whole sample. Extrapolation on the basis of the negative grade-tonnage correlation shown for the mixed population seems to imply the occurrence of large-tonnage very low grade deposits, which could be important in supplying future copper needs. This ex- trapolation is misleading, however, for both statistical and geological reasons. Furthermore, large—tonnage very low grade deposits in the porphyry class of the population are found to be very rare occurrences. INTRODUCTION It has become imperative to have reliable estimates of the resource potential of various areas and com- modities so that alternatives may be considered in order to maintain an orderly supply of minerals. Estimates of the resource potential of an area or commodity must include both the unpublished ulti- mate reserves of known deposits and the undiscov- ered deposits that might be mined if found. In both cases it is useful to have estimates of the probabili- ties with which different grades and tonnages occur. This is often best accomplished by means of statisti- cal models of the grades, tonnages, and the grade- tonnage relationships of the different types of oc- currences of the commodity of interest. An analysis within each type of occurrence is necessary because the types may occur in diverse geologic environ- ments and may be characterized by distinct statisti- cal models. In this study, the grades and tonnages of three types of copper deposits are compared With theore- tical statistical models in order to build models of future discoveries. The grades of copper only are considered because of the difficulty of equating the different metals found in copper deposits. The three types—porphyry, strata-bound, and massive sulfide deposits—represent the world’s main sources of copper at present. The observed frequency distribu- tions of grades and tonnages are compared with the normal and lognormal frequency distributions. The relationships among grades and tonnages of the three deposit types are analyzed by examining the correlation coefficients of these variables. These re- lationships are useful both in building models of future discoveries and in testing the frequently im- plied inverse relationship between grade and ton- nage. This relationship is often a hidden assump- tion in large highly aggregated mineral resource models, implying that improvements in technology can enlarge the supply of minerals. ACKNOWLEDGMENTS This study was greatly helped by discussions with and data supplied by G. H. Goudarzi of the U.S. Geological Survey and various industry geologists. PORPHYRY COPPER DEPOSITS Disseminated copper deposits, genetically related to felsic or intermediate intrusive igneous rocks, are called porphyry copper deposits. Copper minerals A1 A2 may occur as fine disseminated grains, in small crosscutting veinlets, or in thin coatings on joint surfaces. In most deposits, the copper ore body is mainly within the associated intrusive rock; but in some deposits, the surrounding rocks, particularly limestone, may be replaced by copper minerals. Copper minerals may also be deposited in breccia pipes or breccia veins, or in parallel or branching quartz veins sufficiently closely spaced to be ex- tracted by mass-mining methods. All these ore types are believed to be closely related genetically and are commonly included in the porphyry copper classifica tion. Molybdenum, gold, and silver may be important byproducts, and lead, zinc, silver, and manganese de- posits may be peripheral to the porphyry deposit. Replacement and vein deposits are usually found in a distinct zonal arrangement in the rocks surround- ing a porphyry. Copper, in replacement ore bodies, may be Within a few hundred metres of the intru- sive lead-zinc silver deposits farther out, and man- ganese or gold-silver deposits may be on the outer fringes. Porphyry copper deposits generally have a com- plex history involving several stages of igneous in- trusion, mineral deposition, and hydrothermal a1- teration. In some deposits such as San Manuel, Ariz. (Lowell and Guilbert, 1970), the late magmatic and deuteric stages are well represented. In others such as Butte, Mont. (Meyer, Shea, and Goddard, 1968), hydrothermal activity that postdates cooling of the intrusive body predominates. In still others such as Morenci, Ariz. (Moolick and Durek, 1966), oxida- tion and enrichment by descending surface waters have obliterated most of the minerals that may have been deposited in earlier stages. In most deposits, evidence for all three stages is found. Factors influencing tonnage.—In deposits where mineralization is confined to an intrusive body, the tonnage of the deposit may be controlled mainly by the volume of the intrusion. Very few intrusions as— sociated with porphyry copper mineralization are of batholithic proportions; in those that are, only a small part of the intrusion is mineralized or hydro— thermally altered. In other deposits in which favor— able wallrocks are mineralized, the size of the de- posit may be dependent on the stratigraphic thick- ness of the favorable beds and on the width of the copper-rich mineral zones surrounding the porphyry deposit. This width is presumably dependent on the temperature, pressure, and hydrothermal conditions near the intrusion and on the physical character of adjacent host rocks. GEOLOGY AND RESOURCES OF COPPER DEPOSITS Finally, in some deposits, postmineral faulting followed by erosion has had an obvious effect on the tonnage of ore. Ore bodies may be cut by major faults, and a large part may have been uplifted and destroyed by erosion. Factors influencing grade—Conditions favoring oxidation and enrichment of porphyry deposits have an important influence on grade. The factors include climatic and ground-water conditions from the time of the original uplift and erosion of the deposit to that of the present erosion cycle. The degree of frac- turing of the rock is important as well as an abund- ance of pyrite that oxidizes readily to provide acid ground water. Protore grades of porphyry deposits (the copper content before enrichment by descending waters) seldom exceed 1 percent. An important factor in- fluencing protore grade may be the availability of sites favorable for deposition of copper minerals. Such sites include (1) carbonate wallrocks and (2) dispersed iron oxide and iron-magnesium silicate minerals in both wallrocks and intrusive bodies. A close spacing of fractures also influences protore grade, presumably by providing access for copper- bearing solutions to the available sites of deposition. STRATA-BOUND AND STRATIFORM DEPOSITS IN SEDIME‘NTARY AND METAMORPHIC ROCKS Strata-bound ore bodies locally cut across bedding but are confined to a single sequence of strata over large areas. Stratiform copper deposits have the form of a bed, concordant with strata of surround— ing rocks, and are laterally continuous for a great distance relative to their thickness. Copper minerals commonly are disseminated in both these ore bodies, but, rarely, in some very high-grade deposits they form massive beds. Lead and zinc ore bodies may be zonally arranged outward from the copper-rich beds or may form separate beds in the same stratigraphic sequence. Cobalt, silver, and minor amounts of nickel and bismuth are found in some deposits; uranium, vanadium, and thorium are associated . with cop-per in other types. Origin of strata-bound deposits is the subject of great controversy. Some of the largest deposits are metamorphosed and show the effects of mobilization of copper and its redeposition in crosscutting frac- tures and noses of folds. Processes including direct sedimentary deposition, early stage diagenesis, ground-water mineralization, and hydrothermal ore deposition have been suggested to explain the origin of some deposits. Sulfur-reducing bacteria may have GRADE AND TONNAGE RELATIONSHIPS an important role in the precipitation of metal sul- fides in sediments and sedimentary rocks. Metal-rich sediments underlying the hot brine pools in the Red Sea may be modern analogs of stratiform copper de- posits, but the origin of these is also a matter of controversy. Factors influencing tonnage.——A thick and lateral- ly extensive ore-bearing stratum is probably the re- sult of many processes, beginning with sedimenta- tion and ending with remobilization of copper dur- ing diagenesis or metamorphism. As in porphyry de- posits, faulting and folding can cause uplift and de- struction of large parts of a mineralized bed. Factors influencing grade—Original permeability of the sediment and abundance of organic matter, pyrite, or other possible copper-precipitating agents in the sediment are two factors that may influence grade. Abundance and activity of organisms may also be important, as well as many other factors now unknown. Downdip migration of copper during weathering may produce very high grade deposits by secondary enrichment. MASSIVE SULFIDE DEPOSITS Concordant lens-shaped deposits of copper-bear- ing massive pyrite are found in volcanic and sedi- mentary rocks. These deposits are generally more limited in areal extent than strata-bound dissemi- nated deposits. Zinc, lead, and precious metals are found in massive sulfide ore bodies and may exceed copper in total value. Deposits may have discrete boundaries, passing from massive sulfide into barren wallrocks within a few centimetres. Low-grade ore bodies in stockworks of closely spaced quartz-chal- copyrite—pyrite veinlets are associated with some massive sulfide deposits, as are sedimentary beds of low-grade ore containing disseminated mineral frag- ments. Bedded deposits of manganese oxide, barite, or gypsum, and vein deposits of gold and silver may be closely associated in time and space with massive sulfide deposits. Massive sulfide deposits are found in the eugeosyn- clinal or volcanogene parts of fold belts and in vol- canic belts in Archean shield areas. Most deposits are strongly deformed by folds and faults clearly postdating the mineralization, and their origin is thus obscured. Some younger deposits, particularly the "Kuroko deposits in the Miocene of Japan, show evidence of deposition on the sea floor by metal—rich exhalations from submarine volcanic eruptions (see, for example, Tatsumi and Watanabe, 1971). In these deposits, the low-grade ores in stockworks are believed to represent feeder channels in the under- A3 lying rock through which the exhalations passed, and the disseminated ores in sedimentary beds are believed to have been formed by the deposition of fine sulfide fragments scattered on the sea floor by the eruption activity. Factors influencing tonnage.——Massive sulfide de- posits vary greatly in physical size and tonnage. In those deposits whose origin is clear, the strength and duration of submarine fumarolic activity was the most important factor in determining tonnage. If a massive sulfide ore body is closely associated with stockwork or disseminated mineralization of economic grade, then the tonnage of the combined ore bodies may be large. Factors influencing grade.—Grade is presumably controlled primarily by the copper content of the ex- haled vapors and solutions relative to iron, zinc, and other metals. Copper content of these exhalates may be related to the type of associated volcanic rock. Anderson (1969) and others have shown that copperabearing massive sulfide deposits are more commonly associated with mafic volcanic rocks and that lead-zinc varieties are associated with felsic volcanic rocks. The inclusion of large-tonnage dis- seminated and stockwork ores in a reserve estimate will greatly lower the average grade of the combined deposits. Secondary replacement of chalcopyrite and pyrite by chalcocite during weathering and enrich- ment is important in forming high-grade deposits under certain climatic and topographic conditions. GEOLOGICAL CONSIDERATIONS OF TONNAGE AND GRADE RELATIONSHIPS From the foregoing description of copper-deposit types, it becomes clear that these deposits are geologic entities having definite geologic boundaries. These boundaries represent the outer limits of the effects of unusual geologic processes that produce mineral deposits. Beyond these boundaries, “usual” varieties of rocks are found with copper content in the 5 to 150 parts per million (ppm) range. Copper deposits are not, therefore, merely high points or anomalies in a smoothly fluctuating curve of copper abundance in rocks. The position of the deposit boundaries, and hence the tonnage of the deposit, is a function of the mag- nitude of the geologic systems involved, Whether plutonic, volcanic, or sedimentary. To use a ther- modynamic analogy, tonnage is an extensive vari- able. The ab-undance of copper minerals within these boundaries, that is, the grade, depends on a com- bination of factors relating to the intensity of the mineralization process, the concentration of the A4 mineralizing solutions, the rate of change of those concentrations in the depositional process, and the availability of sites of deposition. Most importantly, grade is affected by repeated mineralization proc- esses superimposed on each other (secondary en- richment). In thermodynamic parlance, grade is an intensive variable. Thus, it can be concluded that, in general, the grade of a deposit and the tonnage of a deposit are controlled by separate and distinct geologic factors. NATURE OF DATA The data used include the historical production plus the estimated reserves of each of 267 deposits. Copper grades used are weighted in the appropriate amount by the production and reserves. Rather than use data from the operations of a single mining unit, each deposit is considered as a geologic unit. For example, Miami, Inspiration, and Miami East constitute the geologic entity at Miami, Ariz., and San Manuel and Kalamazoo constitute the entity at San Manuel, Ariz. Grades and tonnages for the massive sulfide and strata-bound copper deposits were obtained from Manifile (Laznicka, 1973). The strata—bound de- posits were selected by using the Manifile “similar- ity type” CPBT (Copperbelt deposits, Africa- U.S.S.R.). Only African strata-bound deposits were used because other known strata-bound deposits are few and are in different economic and possibly dif- ferent geologic environments. Different economic conditions would alter the cutoff grades (lowest grade of blocks of ore considered in reserve esti- mates), which in turn would change the average grades and the tonnages. Other Manifile “similarity types”— MSCP (massive copper pyrites), MSCZ (massive sulfide copper-zinc), and KRKO (Kuroko (Japan) deposits)——were combined to provide the massive sulfide grades and tonnages. Data from the porphyry copper deposits came from many sources. Among them are prospectus of- ferings of American Smelting and Refining Co. (1969), Kennecott Copper Corp. (1971), Newmont Mining Corp. (1969), Phelps Dodge Corp. (1970), Anaconda Co. (1968), and US. Smelting Refining and Mining Co. (1968); a book on porphyries by Sutulov (1974) ; and various issues of “Metals Sourcebook” (Metals Week, 1973—74). Additional grade and tonnage estimates were made by U.S. Geological Survey geologists familiar with individ- ual deposits. An attempt was made to obtain estimates of the grades and tonnages that ultimately might be re- GEOLOGY AND RESOURCES OF COPPER DEPOSITS covered from each deposit. In general, this required using the tonnage and grade estimates associated with the lowest cutoff grade available. This effort was successful for the porphyries in the United States for which the authors have confidence in the estimates used. Most of the porphyries in Canada and in the southwest Pacific have only recently been drilled; a potential exists for higher tonnages as development proceeds. The earlier developed por- phyries of South America have been operating with relatively high cutoff grades, which means that the average operating grades are high. Because of their very large size, some of the South American por- phyries have not yet been drilled out, and estimates of grades and tonnages at low cutoff grades are not available at this time. . Although the difference is cutoff grades in difer- ent regions affects the average grades, reducing the cutoff grades to a common base would not complete- ly remove the 'differences in average grades and tonnages. For example, if the average grades and tonnages of all the porphyry copper deposits of North and South America were estimated using a 0.2 percent copper cutoff grade, the South American porphyries would probably still have the highest average grade and tonnage and the Canadian and Alaskan porphyries would still have the lowest aver- age grade and tonnage. Evidence for this statement may be found in the relatively constant difference between the cutoff grade and the average grade within a given porphyry ; the difference is relatively large for the South American porphyries and tends to become smaller for porphyries to the north. UNIVARIATE STATISTICAL ANALYSIS The purpose of univariate statistical analysis is to determine which theoretical distribution models best describe the individual distributions of the average grade and tonnage of the three types of copper deposits considered. In particular, the ability of normal and Iognormal distribution models to de- scribe dinstribution-s was examined. The adequacy with which these two theoretical frequency distribu- tions fit the observed grade and tonnage frequency distributions was determined by using both beta (Vi and b2) and chi square goodness-of-fit statistics. The beta statistics describe the shape of the fre- quency distribution: For the normal distribution, \/b—1, a measure of asymmetry, has an expected value of 0.0, and ()2, a measure of peakedness, has an ex- pected value of 3.0. The chi square statistic meas- ures the class-to-class departure of observed distri- bution from the theoretical distribution model from GRADE AND TONNAGE RELATIONSHIPS which the observed distribution is hypothesized as being a sample. The beta statistics and the chi square statistics provide complementary informa- tion about the adequacy of fit, lesser weight general— ly being given to chi square statistics because of the arbitrary decisions that must be made when an ob- served frequency distribution is constructed (Sha- piro and others, 1968). Average grade and tonnage data were obtained from 267 copper deposits. Of these, 146 are massive sulfide deposits, 103 are porphyry deposits, and 18 are strata—bound deposits. Frequency distributions of tonnage and grade were constructed, and sum- mary and goodness-of—fit statistics were computed for each type of deposit. Within the 103 porphyry deposits, 4 subdivisions were recognized which were based upon the metallogenic province within which the deposits occurred. The summary statistics are shown in tables 1 and 2, and goodness-of-fit statis- tics, in table 3. Examination of table 1 shows that the African strata-bound deposits have the highest average grade (3.78 percent Cu), followed closely by the massive sulfide deposits, which have an average grade of 2.92 percent c‘opper. The porphyry deposits have an average grade of 0.63 percent Cu, which is nearly one-fifth that of the massive sulfide deposits. Within the porphyry deposits, the average grade ranges from 0.49 percent copper for the Canadian A5 deposits to 0.99 percent copper for the South Ameri- can deposits. The average grades for the United States-Mexico deposits (0.59 percent copper) and the southwest Pacific deposits (0.52 percent copper) are close to the average grade of the Canadian deposits. The massive sulfide deposits are far more variable in average grade than the other type of deposits; they have a standard deviation of 2.35 percent cop- per. This is a consequence of the large skewness in the distribution of grade of these deposits. The standard deviation of the average grades of the stratadbound deposits is 1.18 percent copper, which is only half that of the massive sulfide distribution. The standard deviations of three of the four sub- classes of porphyry deposits range only from 0.12 percent copper to 0.17 percent copper. The fourth member of this class of deposits (South American) has a standard deviation (0.36 percent copper) more than twice that of any of the other members of this class. Porphyry copper deposits contain an average of 548 million tons of ore (table 2), about six times as much one as that contained in the strata-bound de- posits, which average 91 million tons. Massive sul- fide deposits contain an average of only 10.3 million tons of ore by comparison, or nearly an order of magnitude less than that contained in the average TABLE 1.—Summary statistics for the average grades of copper deposits Arithmetic data Logarithmic data Standard Number Mean, deviation, 0 Mean Standard Median logic logic . Type of deposit deposits grade deviation grade of grades of grades Porphyry: Canada _______________________ 21 0.0049 0.0017 0.0047 ~2.331 0.147 United States and Mexico ______ 38 .0059 .0014 .0057 —2.243 .101 South America ________________ 20 .0099 .0036 .0093 —2.033 .160 Southwest Pacific ______________ 24 .0052 .0012 .0051 —2.296 .114 World ________________________ 103 .0063 .0027 .0059 —2.233 .162 Massive sulfide ____________________ 146 .0292 .0235 .0229 —1.640 .303 Strata-bound ______________________ 18 .0378 .0118 .0362 —1.442 .132 All _______________________________ 267 .0210 .0213 .0140 —-1.855 .392 TABLE 2.—Swmmary statistics for the tonnage of copper deposits Arithmetic data Logarithmic data Mean Median Mean Median Standard (millions of (millions of (thousands of (thousand of Mean, deviation, Number metric metric metric metric logxo of logic of of tons tons tons tons metric metric Type of deposit deposits of ore) of ore) of copper) of copper) tons tons Porphyry: Canada __________________ 21 245 177 1,210 824 8.247 0.354 United States and Mexico__ 38 815 338 4,781 1,932 8.529 .629 South America ___________ 20 773 347 7,622 3,214 8.540 .610 Southwest Pacific ________ 24 203 120 1,058 608 8.080 .436 World ___________________ 103 548 234 3,452 1,368 8.369 .565 Massive sulfide _______________ 146 10.3 2.26 301 52 6.354 .828 Strata-bound _________________ 18 91 41.4 3,453 1,496 7.617 .690 All _________________________ 267 223 16.5 4,679 230 7.217 1.208 GRADE AND TONNAGE RELATIONSHIPS A6 i mam mz wfif :3 i 3“ 1 3d mz om. c: i go I 8.2 I o3 Q i £8 E .................................. 3. m2 mam m2 8.‘ a: as mz Ea m2 8. 5 m2 3 m2 mg * 3. $va 3 ma .......................... €53-38; mz 8.“ m2 owl $va 2: * $.m mz 3| 8V * «.3 3. No.2 in gm 3:; NE a; ....................... mwfam 26.32 m2 3% m2 «NW. a: 1 3m 1 5; m2 mm. 2: in S: I a; 3 SA 37; 2% m3 ............................ 333 m2 whm m2 2. £52 ma *9; "wall a I «.2 * «mm m2 3. $va 3. flu .................. 052mm umoBfisom wz 3d m2 3.! av mz m4 m2 :4“ m2 3.1 A8 m2 3 m2 8.x m2 5. av m2 3 8. .................... 85:3 58m wz aim m2 5.! 3% «NH m2 8.“ wz mo. $52 3 m2 5.” I E. 3; m2 ma mm .......... 832 is msgm v32: wz 3N mz mg 8va 3 * wwm mz 36! €va an * m3 * «3 a7; 3; a ........................... x923 Shannom 3 BB 33%" _ S E C33" 3 BB €qu 2 omEflEaMB dun—EOE omEfiCaMoA ofiwfinamfifi ammonww uo 2:8 wvuhu :85 :25. “a... Jan-H aw auaofiflumm .9. 59,2 onwusné «a «flung—Emma .« Judo—(«ENE no: .wz “Eovwmam .«o mewhwwv .mv ”on—d.» 255m in .«N "mammonwv no awn—Es: 5: ““3333? wfiicémuSocg. 9:32;. .2? 33 SQMIMN Quad. GEOLOGY AND RESOURCES OF COPPER DEPOSITS strata-bound deposit. Nearly the same relative dif- ferences in tonnages between deposit types were found when the median tonnage is used as a measure of the “center of gravity” of the distributions. Strata-bound deposits contain a median 41.4 million tons, and the massive sulfides have a median of 2.26 million tons. When considering the distribution of the tonnage of ore in each type of deposit, the me- dian is a more appropriate measure of the center of gravity because of the large skewness toward the high tonnage tail in each of these distributions. African strata-bound deposits have the largest quantity of contained copvper-fia median of 1,496,000 tons. The porphyry deposits have a median that is slightly less—1,368,000 tons—and the massive sul- fide deposits contain even less copper by compari- son—a median of 52,000 tons. Within the four sub- classes of porphyry deposits, the median metal content ranges from 608,000 tons for the southwest Pacific deposits to 3,214,000 tons for the South American deposits. The United States-Mexico de- posits have a median of 1,932,000 tons of copper metal, which is more than twice as much as the median for the Canadian deposits but only a little more than half that for the South American de- posits. The process of selecting an appropriate theoretical distribution model to describe the observed grade and tonnage frequency distributions for each of the three types of copper deposits was based upon sta- tistical tests of the beta and chi square goodness-of- fit statistics. The results of these tests are shown in table 3. With the exception of the southwest Pacific porphyry deposits, the lognormal distribu- tion model was found to be superior to the normal distributions in describing the observed distribution of mean grade and tonnage. Each of the observed distributions of tonnage of ore studied was found to be highly skewed toward high tonnages, and, as a consequence, the normal model does not provide an adequate fit to the ob- served data. The lognormal model, on the other hand, was found to fit each of the observed tonnage dis- tributions rather well. For each category shown in table 3, column 1, the skewness of the observed ton- nage distribution was within the expected range for a sample from a lognormal population, and in only one category did the peakedness depart significantly from the lognormal model. In four of the eight cate- gories, the chi square goodness-of-fit statistics were significant. However, we give less weight to the chi square test statistic because the statistic will pro- A7 duce varying results depending upon the choice of class limits. The lognormal distribution is accepted as an adequate model for the distribution of tonnage of ore in deposits from each category of deposit. As a result of the above analysis and the assump- tion that the observed data can be regarded as ran- domly drawn from their respective parent popula- tions, the lognormal models provide not only an adequate description of tonnage of ore in yet-to—be- discovered deposits of the three types of deposits studied, but also offer an adequate description of their mean grades. An additional benefit of the above results is that the correlation between the loga- rithms of the mean grade and tonnage in these de- posits can be tested statistically. CORRELATIONS BETWEEN GRADE AND TONNAGE Among the problems that must be resolved before resource models can be constructed is the relation- ship between the pairs of variables being examined. In this paper, the pairs of variables are the aver- age grades and the tonnages of the copper deposits in each of the sets of deposit types under considera- tion; the degree to which these variables vary to- gether is tested. The degree of association or inter- dependence of the pairs of variables is examined by means of the correlation coefficient for each set of samples. The values of the correlation coefficient can range between minus one and plus one, a perfect linear relationship being indicated by a minus one or plus one value, and complete linear independence indicated by zero. In this paper, the sample corre- lation coefficients are tested in table 4 against the null hypothesis that the population of grades and tonnages has zero correlation. The frequency dis- tribution of the pairs of logarithmic variables ex- amined in the last section closely approximate the bivariate normal distribution necessary for the sig- nificance tests of the null hypothesis. TABLE 4.—-Correlation coeflicients between average copper grade and total tonnage [NS, not significant; **, significant at the l-percent level] Number Correlation coefficient Type of deposit of Arithmetic Logarithmic deposits data data. Porphyry: Canada _________________ 21 —0.16NS —0.22NS United States and Mexico 38 OONS —.09NS South America __________ 20 — 07NS —.17NS Southwest Pacific _______ 24 — 05NS —.07NS World __________________ 103 09NS .05NS Massive sulfide ______________ 146 —.13NS —.42" Strata-bound _________________ 18 —.10NS —-—.19NS All __________________________ 267 —-.22" —.67” A8 GRADE AND TONNAGE RELATIONSHIPS MILLIONS OF METRIC TONS OF ORE 10.0 100.0 1000.0 10000.0 —1.7 I I I | 2.00 O —1.8 — 0 O o — 1.58 —1.9 — o — 1.26 O —2.0 — 0 ° — 1.00 O 0 O 0 ._ Z Lu 0 D O 0 0 O '5 E —2.1 — o o o — 0.79 E g 0 0 O 0. K o o0 8 o z E 0 o _> 1 —2.2 — ° ° ° ° ° ° — 0.60 E o o 0 0 o < o o 0 m 3 o 0 ° o 0 ° 0 o 0 < o on Q) o n: 5 —2.3 — o o (900 o 00 O — 0.50 E > 0 ° 0 o 00 o n. < 0 0 O 2 o 0 ° 03’ ° 0 ° 0 LU g —2.4 _ 0 o oo o o o — 0.40 g 0 o 0 o E ° 2 —2.5 _ — 0.32 -2.6 _ — 0.25 ‘2-7 — o — 0.20 O —2.8 I I I I 0.16 7.0 8.0 9.0 10.0 LOGlo TONNAGE (METRIC) FIGURE 1.-—Grades and tonnages for porphyry copper deposits. Sample correlation coefficients calculated for both the arithmetic data and the logarithmic data for each group of porphyry coppers and for all the porphyries together provide no basis for rejecting the null hypothesis of zero correlation between aver- age grade and tonnage (fig. 1). In fact, less than 5 percent of the variation of tonnage is explained by the average grade for these deposits. These re- sults also hold for the strata-bound deposits (fig. 2). The correlation coefficients for both the massive sulfide deposits (fig. 3) and the combination of all deposits are negative and highly significant (table 4). This suggests a significant negative relationship between grade and tonnage for both groups, al- though it should be noted that the combination of all deposits represents not only a mixture of geologic types of deposits, but a statistical mixture as re- flected in the significant departures of grade and tonnage from the lognormal distribution. Additional statistical analyses were performed but not included on two of the subgroups of massive sulfides; loga- rithms of the grades and tonnages from the Mani— file “similarity types” massive copper pyrites and massive sulfide copper-zinc (see p. 1—2) each had significant negative correlations. The negative relationship between grade and ton- nage for the massive sulfide deposit-s may be ac- ’ counted for by the fact that some deposits include a combination of low-tonnage high—grade massive ores and large-tonnage low-grade stockwork and dis-sem- inated ores. These ores, though genetically related, represent a mixed population with respect to the tonnage-grade relationship. DISCUSSION AND CONCLUSIONS The independence of grade and tonnage for por- phyries is convenient for modeling of grades and tonnages of yet unmined deposits. As the grades and tonnages are essentially lognormally distributed for GEOLOGY AND RESOURCES OF COPPER DEPOSITS A9 MILLIONS OF METRIC TONS OF ORE 1.0 10.0 100.0 —1.1 l I 7.9 O —1.2 — — 6.3 I'- m —1.3 L O O - 50 5 o u at: O O 5 <5 i E o E 0. u; D. o o o E Lu —1-4 — — 4o <5 <5 0 a: < u E O n. > a- < o 2 O o o O OO “1 O (9 4 s —1.5 — O - 3.2 L; o o < —1.6 b O n 25 O —1.7 l e I 2-0 6.0 7.0 8.0 8.5 LOG 10 TON NAGE (METRIC) FIGURE 2.—Grades and tonnages for African strata-bound copper deposits. the porphyries, the calculation of the probability of the occurrence of any particular range of grades or tonnages, given that a porphyry exists, is an easy task. Because of the independence of grade and ton- nage, the probability of a deposit falling in particu- lar ranges of grades and tonnages is simply the product of the probabilities of the range of grades and of the range of tonnages. For example, suppose it is desirable to find the probability of a porphyry having a very low grade and a very large tonnage. The probability of a very low grade, say less than two standard deviations below the mean (less than 0.28 percent copper), is equal to 0.023 from tables of the areas of the normal curve (Dixon and Massey, 1957) . The probability of a very large tonnage, say more than three standard deviations above the mean (greater than 11.59 billion tons), is 0.0013 from the normal curve tables. Thus, the probability of a por- phyry having a grade less than 0.28 percent and a tonnage greater than 11.59 billion tons is 0.023 X A10 GRADE AND TONNAGE RELATIONSHIPS MILLIONS OF METRIC TONS 0.001 0.01 0.1 1.0 10.0 100.0 1000.0 —0.8 I I o I I I I I I I I 15-8 o o —1.0 — o o o — 10.0 o o o —1.2 — o 0 o — 6.3 0 0 o o o 0 0 <9 0% _. O 0 O _ I- “, 1.4 — 8 oo o o oo 0 4.0 LlzJ Q 8 O O Q g 0 O O O 0 Q) n: (5 o (g; o o lIi.J a: -1.6 _ o 0 Q o o 000 o o o — 2.5 Z LIJ Q O _. 5: 0° 0 49(3) ° 0 .3 ° .- o O 8 00 009g) 0 2 o —18 ° ° 0 ° ° 16 fl: LIJ ' _ — - (5 <5 % 00c8 0 oo o g o O o o o 5 Lu 00 0 [L i 20 o o0 o 10 8 __ ‘ _ o — . 2 0 ° 93 o In 8 o 0 0° <3 -‘ o o g *2-2 — 0 _ 0.63 m > < o ‘2‘4 — o - 0.40 —2.6 — — 0.25 o “2.8 l I I i l l I I I I 0.16 3.0 4.0 5.0 6.0 7.0 8.0 9.0 ‘ LOGloTONNAGE (METRIC) FIGURE 3.——Grades and tonnages for massive sulfide deposits. 0.0013 or 0.00003; that is, if 33,000 deposits were discovered, only one of them would be likely to have these characteristics—a very unlikely event indeed. It is interesting to note that if such a deposit exists, it contains about 32 million tons of copper, which is about the amount contained in the largest known porphyries, and it would require the moving of about 4 cubic miles of rock to mine it. If improvements in technology allow mining of lower and lower grade porphyry deposits, the most probable tonnage of these new deposits will be roughly the same as those presently being exploited; thus, each deposit will contain less copper. D. P. Harris (oral commun., 1973) has suggested that low-grade deposits of median tonnage are more abundant than those of higher grades. Although no geologic evidence is presently available to support this assumption, it represents an important area for future investigation. The lack of correlation of tonnage and grade for both the strata-bound and porphyry deposits has implications not only for quantitative modeling, such as mentioned above, but also qualitative modeling. Qualitative modeling is often used by those consider- ing the future supply of natural resources, and usually an assumption of a negative correlation be- tween grade and tonnage is implicit in their discus- sions. The above analysis does not support this view. Such untested beliefs could have arisen from several sources including the trend through time of lower average copper grades mined each year, which is related to improvements in technology, changing de- GEOLOGY AND RESOURCES OF COPPER DEPOSITS posit types, and the tendency for porphyries to have richer grades in the earliest mined areas. Another possible source of these beliefs could come from misinterpretation of Lasky’s (1950) work on the grade and tonnage relationship Lasky found that within porphyries the cumulative tonnage increases at a constant geometric rate as the grade decreases at an arithmetic rate. The method of using cumula- tive tonnage could be applied across porphyry de- posits, and the results would be consistent with Lasky’s results (John Whitney, written commun., 1974). Lasky’s cumulative contained copper curves become flat at a point that he called zero cutoff grade. Zero cutoff grade represents the outer limit of the effects of the special geologic conditions that gave rise to the ore deposit. Copper content does not fall to zero at this limit but tends to approach average abundance of copper in the Earth’s crust. In conclusion, an inverse relationship between grade and tonnage is often a hidden assumption in mineral-resource supply models. Use of such models for policy decisions could have a devastating effect on the orderly supply of minerals if large tonnage— very low grade deposits do not exist. From the inverse relationship between average grade and tonnage shown by the mixture of three deposit types, it is tempting to extrapolate the ex— istence of large-tonnage very low grade deposits, and it can be reasoned that such deposits can supply sufficient copper for our future needs. Such an ex- trapolation is misleading for statistical and geolog- ical reasons. It is statistically erroneous to extra- polate on the basis of a mixed population, especially because the distributions of tonnages and grades in the largest tonnage lowest grade (porphyry) subset of the population show that large-tonnage very low grade deposits are rare occurrences. In addition, dis- cussion with industry geologists familiar with por- phyry deposits indicates that a large number of such deposits is not being held in company inventories at present. If large-tonnage very low grade deposits exist, they probably will not resemble any of the three important types presently exploited. Discovery of such unconceived resources will require continuing All research on a broadly based spectrum of geologic phenomena. REFERENCES CITED American Smelting and Refining.C0., 1969, [Prospectus pro- posing a merger with Pennzoil United, Inc., January 1969]: 66 p. Anaconda Co., 1968 [Prospectus offering debentures, Nov. 1968] : 37 p Anderson, C. A., 1969, Massive sulfide‘deposits and volcanism: Econ. Geology, v. 64, no. 2, p. 129—146. Dixon, W. J., and Massey, F. J., Jr., 1957, Introduction to statistical analysis [2d ed.]: New York, McGraw-Hill, 488 p. Kennecott Copper Corp., 1971, [Prospectus offering debentures, April 1971]: 14 p. Lasky, S. G., 1950, How tonnage and grade relations help pre- dict ore reserves: Eng. and Mining Jour., v. 151, no. 4, p. 81—85. Lazinicka, Peter, compiler, 1973, Manifile; the University of Manitoba file of nonferrous metal deposits of the world: Manitoba Univ., Dept. Earth Sci., Centre Precambrian Studies Pub. 2, 3 v.: Pt. 1, 533 p.; Pt. 2, v. 1, 298 p.; Pt. 2, v. 2, 767 p. Lowell, J. D., and Guilbert, J. M., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits: Econ. Geology, v. 65, no. 4, p. 373—408. , Metals Week, 1973—74, Metals sourcebook: New York, Mc- Graw-Hill, [weekly pub.]. Meyer, Charles, Shea, E. P., and Goddard, C. 0., Jr., 1968, Ore deposits at Butte, Montana, in Ridge, J. D., ed., Ore deposits of the United States, 1933—1967 (Graton-Sales Volume), v. 2: New York, Am. Inst. Mining. Metall., and Petroleum Engineers, p. 1373—1416. Moolick, R. T., and Durek, J. J., 1966, The Morenci district, in Titley, S. R., and Hicks, C. L., eds., Geology of the porphyry copper deposits, southwestern North America: Tucson, Ariz., Arizona Univ. Press, p. 221—231. Newmont Mining Corp., 1969, [Prospectus proposing a mer- ger with Magma Copper 00., May 1969]: 59 p. Phelps Dodge Corp., 1970, [Prospectus proposing a merger with Western Nuclear, Inc., October 1970]: 8 p. Shapiro, S. S., Wilk, M. B., and Chen, H. J., 1968, A compara- tive study of various tests for normality: Am. Stat. Assoc. Jour., v. 63, no. 324, p. 1343—1372. Sutulov, Alexander, 1974, Copper porphyries: Salt Lake City, Utah, Univ. Utah Printing Services, 200 p. Tatsumi, Tatsuo, and Watanabe, Takeo, 1971, Geological en- vironment of formation of the Kuroko-type deposits: Soc. Mining Geology Japan, Spec. Issue 3, p. 216—220. U.S. Smelting Refining and Mining Co., 1968, [Prospectus of- fering debentures, Feb. 1968]: 8 p. Geochemical Exploration Techniques Applicable in the Search for Copper Deposits By MAURICE A. CHAFFEE GEOLOGY AND RESOURCES OF COPPER DEPOSITS GEOLOGICAL SURVEY PROFESSIONAL PAPER 907—B A compilation of proved and untried geochemical sampling media and techniques that may be useful in the search for new copper deposits UNITED STATES GOVERNMENT PRINTING OFFICE, W'ASHINGTON : 1975 CONTENTS Page Page Abstract ________________________________________ Bl Geochemical sampling media—Continued Introduction _____________________________________ 1 Vegetation—Continued Geochemical sampling media ______________________ 2 . BiOgeocPemica-l teChniqueS ----------------- 314 . Micro-organisms ______________________________ 16 Rocks and minerals __________________________ 2 Animals _____________________________________ 17 Soils """"""""""""""""""""" 4 Atmosphere __________________________________ 18 8011 gas ------------------------------------- 6 Remote sensing __________________________________ 19 Transported materials _________________________ 8 Statistics ________________________________________ 21 Glacial debris ___________________________ 8 Introduction _________________________________ 21 Stream sediments ————————————————————————— 8 Correlation analysis __________________________ 21 Lake sediments ——————————————————————————— 11 Regression analysis ___________________________ 22 Water and related materials ___________________ 11 Discriminant analysis _________________________ 22 Water ___________________________________ 11 Factor analysis ______________________________ 22 Ice _____________________________________ 13 Cluster analysis ______________________________ 22 Snow ___________________________________ 13 Trend-surface analysis ________________________ 23 Vegetation ___________________________________ 14 Moving-average analysis ______________________ 23 Geobotanical techniques ___________________ 14 References cited __________________________________ 23 ILLUSTRATIONS Page FIGURES 1—4. Photographs: 1. Plastic hemisphere used in soil-gas sampling _______________________________________________ B7 2 Typical desert wash used in stream—sediment sampling _____________________________________ 9 3. Mesquite roots used as a biogeochemical sampling medium _________________________________ 16 4 Plastic bags used to collect exudates for geochemical analysis _______________________________ 20 III GEOLOGY AND RESOURCES OF COPPER DEPOSITS GEOCHEMICAL EXPLORATION TECHNIQUES APPLICABLE IN THE SEARCH FOR COPPER DEPOSITS By MAURICE A. CHAFFEE ABSTRACT Geochemical exploration is an important part of copper-re- source evaluation. A large number of geochemical exploration techniques, both proved and untried, are available to the geo— chemist to use in the search for new copper deposits. Analyses of whole-rock samples have been used in both re- gional and local geochemical exploration surveys in the search for copper. Analyses of mineral separates, such as biotite, magnetite, and sulfides, have also been used. Analyses of soil samples are widely used in geochemical ex- ploration, especially for localized surveys. It is important to distinguish between residual and transported soil types. Orien- tation studies should always be conducted prior to a geochemi- cal investigation in a given area in order to determine the best soil horizon and the best size of soil material for sampling in that area. Silty frost boils, caliche, and desert varnish are specialized types of soil samples that might be useful sam- pling media. Soil gas is a new and potentially valuable geochemical sam- pling medium, especially in exploring for buried mineral de- posits in arid regions. Gaseous products in samples of soil may be related to base-metal deposits and include mercury vapor, sulfur dioxide, hydrogen sulfide, carbon oxysulfide, carbon dioxide, hydrogen, oxygen, nitrogen, the noble gases, the halo- gens, and many hydrocarbon compounds. Transported materials that have been used in geochemical sampling programs include glacial float boulders, glacial till, esker gravels, stream sediments, stream-sediment concentrates, and lake sediments. Stream-sediment sampling is probably the most widely used and most successful geochemical exploration technique. Hydrogeochemical exploration programs have utilized hot- and cold-spring waters. and their precipitates as well as waters from lakes, streams, and wells. Organic gel found in lakes and at stream mouths is an unproved sampling medium. Suspended material and dissolved gases in any type of water may also be useful media. Samples of ice and snow have been used for limited geochemical surveys. Both geobotanical and biogeochemical surveys have been successful in locating copper deposits in many parts of the world. Micro-organisms, including bacteria and algae, are other unproved media that should be studied. ' Animals can be used in geochemical-prospecting programs. Dogs have been used quite successfully to sniff out hidden and exposed sulfide minerals. Termite mounds are commonly composed of subsurface material, but have not as yet proved to be useful in locating buried mineral deposits. Animal tissue and waste products are essentially unproved but potentially valuable sampling media. Knowledge of the location of areas where trace-element-associated diseases in animals and man are endemic, as well as a better understanding of these diseases, may aid in identifying regions that are enriched in or depleted of various elements, including copper. Results of analyses of gases in the atmosphere are proving valuable in mineral-exploration surveys. Studies involving metallic compounds exhaled by plants into the atmosphere, and of particulate matter suspended in the atmosphere are re- viewed; these methods may become important in the future. Remote-sensing techniques are useful for making indirect measurements of geochemical responses. Two techniques ap- plicable to geochemical exploration are neutron-activation analysis and gamma-ray spectrometry. Aerial photography is especially useful in vegetation surveys. Radar imagery is an unproved but potentially valuable method for use in studies of vegetation in perpetually clouded regions. With the advent of modern computers, many new tech- niques, such as correlation analysis, regression analysis, dis- criminant analysis, factor analysis, cluster analysis, trend— surface analysis, and moving-average analysis can be applied to geochemical data sets. Selective use of these techniques can provide new insights into the interpretation and understanding of geochemical data that would not be possible without the use of computers. INTRODUCTION An important part of our fund of knowledge of copper resources must include techniques for the exploration for new copper deposits. Among the techniques, those using geochemistry are becoming increasingly important. For this report I reviewed much of the geochemi- cal literature. I have selected for discussion primar- ily those papers that deal With geochemical explora- tion for copper deposits in the widely differing geo- logic and climatic environments found on earth. In addition to published reports, I have also included, Wherever pertinent, information on geochemical prospecting investigations currently being conducted by scientists in the US. Geological Survey. I have B1 B2 also suggested some unproved geochemical explora- tion techniques that might be useful in the search for new copper deposits. No attempt has been made to compare and evalu- ate the various studies described in this report; too many variations exist to permit valid comparisons. Undoubtedly, some techniques have been overlooked; other methods, developed by private organizations, have not been documented in the literature, either because of their success or because of their lack of it. Those readers interested in more complete discus- sions of the large geochemical prospecting biblio- graphy should refer to the books by Ginzburg (1960), Hawkes and Webb (1962), and Levinson (1974). A review article dealing exclusively with porphyry copper deposits has also been published recently (Coope, 1973). GEOCHEMICAL SAMPLING MEDIA ROCKS AND MINERALS Geochemical sampling of rocks is the most basic type of sampling because, ultimately, it is the rock material of mineral deposits that must be studied and mined. Several types of rock samples are used in exploration. The two main types are (1) whole- rock samples and (2) mineral-separate samples. Whole-rock samples can be used to provide infor- mation about the abundance and distribution of elements or groups of elements. Variations in the concentrations of minor elements can be statistically interpreted to define mineralized areas on any scale. On a regional scale, geochemical studies alone, or in combination with geologic and (or) geophysical data, can be used to define copper-rich metallogenic prov— inces or districts. On a local scale, whole-rock an- alyses can be used for detailed studies to locate in- dividual deposits or extensions of known deposits. The use of bedrock geochemical surveys has gen- erally not found favor among geochemists for sev- eral reasons. Rock bodies have normal variations in trace elements that can be ascribed to many causes other than mineral deposits. Thus, it is difficult to collect representative samples that can be econom- ically evaluated purely on the basis of chemical abundances. Also, bedrock samples have not under- gone mechanical dispersion that might enlarge a potential target. Whole-rock analyses of bedrock samples can be useful in the search for large copper ore bodies such as porphyry-, magmatic segregation-, or bedded- type deposits. A comparison of the trace elements in samples of rock taken from intrusive rocks through- GEOLOGY AND RESOURCES OF COPPER DEPOSITS out a region can be used as a regional exploration guide. It is important, therefore, in this type of study that the samples collected from a given in- trusive rock are as representative as possible of the character of the entire rock body. Putnam and Burnham (1963) used Whole-rock analyses in a regional study of a part of the Arizona copper province. They found that samples from plutonic bodies associated with known copper min- eralization contained higher copper concentrations than did those from plutonic bodies not associated with known copper deposits. Most of the copper was found to occur in the ferromagnesian and sulfide minerals. Other applications of whole-rock trace-element chemistry as a regional exploration technique in the search for copper were carried out in British Colum- bia by Warren and Delavault (1960, 1969). These authors used hot aqua regia for extraction, and found that copper and zinc concentrations in the samples from both plutonic and volcanic rock bodies indicated the presence of genetically associated cop- per mineralization nearby. High copper—zinc ratios (>035 for volcanic rocks and >0.20 for plutonic rock-s) provided further information as to the favor- ability of certain areas. In a study undertaken by the Geological Survey of Canada, Sakrison (1971) described another re- gional geochemical survey using whole-rock samples. In that survey, copper concentrations in bedrock samples from a large area of the Canadian Shield were determined. A number of areas anomalously high in copper were found, some of which correlated with known mineralized rock bodies. Similar surveys using Whole-rock samples may be useful for appraising the potential of any region thought to contain large strata-bound copper de- posits. Many regions contain a thick stratigraphic section composed of many different rock types. ' Widely differing rock types can be expected to con- tain large normal variations in the abundances of many chemical elements. Ideally, it would be de- sirable to select rock material from only one forma— tion or bed as a representative sample of the entire stratigraphic section at a given outcrop of the sec— tion. This restricted sampling scheme would sig-' nificantly reduce the number of samples needed, and would also require sampling of only one lithology, thereby reducing or eliminating the complication of any chemical variability caused by sampling of dif- ferent lithologies with different normal abundances of various trace elements. Results of sampling of selected beds or formations should be able to show GEOCHEMICAL EXPLORATION TECHNIQUES trends in trace-element content and also to indicate those geologic formations or beds that are most likely to contain copper deposits. My experience in the vicinity of the Vekol porphyry copper-molyb- denum deposit near Casa Grande, Ariz., indicated that this technique of sampling a particular strati- graphic unit, when applied on a regional basis, could delineate localities containing epigenetic copper min- erals in favorable sedimentary host formations. In my study, rock samples showing no visible signs of hydrothermal alteration were collected from forma- tions in a Paleozoic sequence that cropped out over a large region in the vicinity of the Vekol deposit. Anomalous copper concentrations were detected in samples from several formations over 3 km ("about 2 mi) away from the deposit. Copper concentrations generally increased in samples taken progressively closer to the deposit; maximum concentration-s were found in samples directly over the deposit. Anoma- lous molybdenum concentrations were detected in only two of the formations present in the region, and then only in those samples collected directly over the Vekol deposit; the same formations in other 10- calities did not contain anomalous amounts of molyb- denum. At Jerome, Ariz., primary copper from Precam- brian ore deposits was mobilized by weathering and deposited as syngenetic copper in the superjacent Cambrian sandstone that is present at the base of a thick Paleozoic sedimentary sequence. Huff (1955) collected and analyzed samples of this basal sand- stone unit and demonstrated that this formation contained anomalous concentrations of copper as much as 3 km (about 2 mi) away from the known ore deposits. Whole-rock analyses can also be used on a local scale to identify individual deposits in a district or to find extensions of known deposits. Most large ore bodies have boundaries that are economic rather than mineralogic; that is, their boundaries are based on a grade of ore that is economic and not on a sud- den disappearance of a particular element. Looked at another way, most large ore bodies have disper- sion aureoles of various elements. The geochemical expression of a hypothetical porphyry deposit of the type common to the southwestern United States was reviewed by Jerome (1966). He noted that gold, silver, and molybdenum are commonly found en- riched in the copper-ore zone, a fact generally known. On the basis of analyses of ores, gossans, leached cappings, smelter products, tailings, and concentrates from copper deposits, Jerome concluded that antimony, arsenic, bismuth, cobalt, indium, B3 nickel, rhenium, selenium, tellurium, tin, and vana- dium may also be enriched in or around the copper- ore zone. My investigations confirm, at least in part, Jerome’s findings. Most of the elements mentioned by Jerome have been detected in the ore zone of the Kalamazoo ore body near San Manuel, Ariz. Distri- butions of the trace-element concentrations relatiVe to the known ore body indicate that many trace elements form either positive or negative primary dispersion aureoles around the copper deposit. Per- haps a statistical study of the concentrations of a group of trace elements in rock samples, together with their locations relative to this known porphyry copper ore body, can be used to predict the direction to the copper ore from any point within the host- rock body. If this type of study is successful and if a geochemical model for the trace-element aureoles can be derived for this deposit, then it may be possi- ble to use geochemical data from sample sites near other similar but seemingly barren plutonic rocks to predict (1) the presence or absence of copper deposits at depth and (2) the possible location of a blind deposit. Studies made of trace elements in rock samples near volcanogenic copper deposits in the Soviet Union revealed dispersion patterns of the ore metals, and such volatile elements as iodine, extending as far as 140 m (460 ft) from known ore bodies (Rubo, 1969). This study, when considered in conjunction with previously described studies, indicates that the use of trace-element zoning patterns in bedrock may be valid as a geochemical-exploration technique for nearly any type of copper deposit. Analysis of mineral separates is another approach to rock sampling. In their regional study of plutons in Arizona, Putman and Burnham (1963) noted that essentially all of the zinc and much of the copper in their rock samples came from the ferromagnesian minerals. Even more interesting is their observation that “in every sample in which chalcopyrite is pre- sumed to be present, the coexisting biotite has a relatively high copper content, usually above the average copper content of biotites from other sam- ples” (Putman and Burnham, 1963, p. 72). These authors concluded that the biotite and the chalcopy— rite came from the same magma. Studies of the con- centrations of copper in biotites from major copper districts in Nevada, Utah, Montana, and Arizona have since confirmed the close relationship between copper-rich biotite and copper-rich rock bodies (Parry and Nackowski, 1963; Al-Hashimi and Brownlow, 1970; Lovering and others, 1970). B4 A number of investigations of the trace-element content of magnetite have been made. Hamil and Nackowski (1971) demonstrated that the concen- tration of copper in m-agnetites was not a good cri- terion for locating porphyry-copper intrusive rocks. In contrast, they found that low concentrations of titanium and zinc in magnetite were better indica- tors of copper-related intrusive rocks. In my experi— ence, the lack of correlation of concentrations of cop- per in magnetites with intrusive rock bodies en- riched in copper-bearing minerals may be caused by the inability of the person making the study to in- terpret properly the age of the magnetite relative to the age of the copper-bearing minerals. The mag- netite may not be cogenetic with the economic cop- per-bearing minerals. When this latter situation holds true, the trace-element content of the mag- netite (or possibly of other mineral separates) prob- ably bears no useful relationship to the presence or absence of copper in the intrusive body. In another study, Smith (1970) determined the trace-element content of heavy-mineral fractions from samples collected from a group of plutons in the Plumas copper belt in northern California. He found that the highest copper concentrations were restricted to samples collected from those plutons that contained the copper deposits that produced the largest tonnages of copper ore in the region. The analysis of trace elements in metal sulfide concentrates derived from whole-rock samples has been used as a means of defining metallogenic prov- inces for several elements. Burnham (1959) an- alyzed more than 500 samples of chalcopyrite and sphalerite from 172 mining districts widely scattered throughout the southwestern United States. His data showed closely coincident zones or belts enriched in such elements as tin, silver, cobalt, nickel, indium, gallium, and germanium. The belts defined by the trace-element content of sphalerite are considered by Burnham to coincide with the trends of known copper belts. The rhenium concentration in samples of molyb- denite from porphyry copper deposits was shown to difi'er markedly from its concentration in samples of molybdenite from other types of deposits (Giles and Schilling, 1972). These differences in concen- tration suggest that the abundance levels of trace elements in ore minerals may be a useful guide in evaluating porphyry copper-molybdenum deposits and possibly other types of copper deposits. Undoubtedly many other specialized types of rock samples could be used. Obvious ones include (1) the iron- or manganese-oxide materials found in gos- sans, leached cappings, and vein fillings, and (2) GEOLOGY AND RESOURCES OF COPPER DEPOSITS fluid inclusions and geodes. Leakage halos may be present in the rocks cropping out above a mineral deposit that has not yet been exposed by erosion. In such an environment, the systematic sampling of any material suspected to be hydrothermally associated, such as iron- or manganese-stained fracture fillings or silica vei‘nlets, could provide evidence of a con- cealed mineral deposit that would not be found by the sampling of only typical bedrock material. If veins are sampled, particular attention should be paid to the relationship, if any, between the vein orientation and the trace-metal content of the veins at each orientation, because veins of certain orien- tations may be of interest economically while those at other orientations may not (Rehrig and Heidrick, 1972). Fluid inclusions are commonly thought to repre- sent samples of trapped hydrothermal ore fluids; consequently, the concentrations of various elements in the material found in fluid inclusions in rocks may also prove valuable in exploring for copper deposits. In their study of a porphyry copper prospect in Puerto Rico, D. P. Cox and J. T. Nash (written commun., 1973) found that the fluid inclusions in quartz from rocks collected over or near the copper- rich exposures contain halite; similar inclusions from rocks collected outside the mineralized area do not contain halite. From a geochemical standpoint, then, a study of the trace-element content of fluid inclusions may provide a clue 'as to the location of copper deposits. Geodes are another type of rock-sampling medium which might be useful for geochemical prospecting in the limited areas in which they occur. Because geodes crystallize from the outside to the center, the trace-element chemistry of the internal layers in a geode should reflect the chemistry of the environ- ment surrounding the geode at the time the geode began to form. In this sense, geodes can be con- sidered to be hand-specimen equivalents of fluid inclusions, and chemical analysis of geode interiors might be a useful means of studying some areas, especially those that have undergone deep weather- ing which might have leached more mobile elements such as copper from the surface rock and soil mate- rials. The practical application of geode chemistry to geochemical exploration has not been demon- strated as yet, however. SOILS The use of soils in geochemical prospecting for copper has proved both popular and successful worldwide in many geochemical exploration pro- grams. Soil sampling has been used mainly for de- tailed appraisals of limited areas rather than for GEOCHEMICAL EXPLORATION TECHNIQUES B5 regional reconnaissance programs. A review of the various routine applications of soils to prospecting is beyond the scope of this report; the reader is referred to comprehensive discussions such as those by Hawkes and Webb (1962, p. 202—226) and Brad- shaw, Clews, and Walker (1970a, 1970b; 1971; 1972). In most geochemical sampling techniques, it is important that persons collecting soil samples have an appreciation of the geology of the area being sampled and some idea of the characteristics of the soil being sampled. For example, it is important to know What soil horizon is being sampled and whether the soil is residual or transported. A soil profile is commonly developed over bedrock except in most desert areas and in areas undergoing rapid erosion. It is important that orientation studies be made in any new area to determine information such as the best soil horizon for sampling as well as the best size of soil material to retain for analysis. Where present, the B horizon has most common- ly been used for soil sampling because this horizon is normally enriched in metal-s relative to the A horizon. Under special conditions, the upper A hori- zon, which is rich in organic material, has also proved to be an effective sampling medium. Boyle and Dass (1967) reported that A-horizon soil sam- ples from the Cobalt, Ontario, district contained higher concentrations of various elements including copper than did equivalent B-horizon samples. Their A-horizon samples also showed better anomaly con- trast values. The authors attribute the higher metal values in the A horizon to biogeochemical enrich- ment processes. In another study, Curtin and others (1971) found that the ash of the forest humus layer (mull) from samples collected in the Empire district in Colorado was enriched in a number of base and precious metals as compared with concentrations of these metals in the underly- ing soils. The mull reflected known bedrock chem- istry better than did the underlying soils, because the mull was derived from trees rooted in bedrock Whereas the soils were derived mainly from trans- ported glacial till. The optimum grain size of soil material to be used for geochemical analysis should be determined for any new area being studied. The best size to use will depend on such factors as the original grain size of the minerals present, the degree to which these original grains have been eroded by weathering, and the anomaly contrast values obtained for each grain size. Hawkes and Webb (1962, p. 166) clearly dem- onstrated the effects of different grain sizes on con- centrations for nine different elements. In areas of transported cover, geochemical pros- pecting using soils has not been particularly useful. Hawkes and Webb (1962, p. 203) reported, however, that near-surface anomalies have been found over glacial till and alluvial deposits that were as much as 15 m (50 ft) thick. Apparently the metals caus- ing the anomalies migrated to the surface either in the circulating ground water or through transloca- tion from tree roots to leaves and then into the ground from the fallen leaves. Studies in humid areas (Curtin and others, 1971) and in arid areas (Chaffee and Hessin, 1971) have indicated that plants or plant materials are superior to soils as sampling media in areas containing soils derived from transported overburden. Much better results have been obtained in soil sampling programs that utilized residual soil; how- ever, the fact that the soil is residual does not assure that it will be a good sampling medium. Residual soil can form on top of transported material such as glacial till. In humid regions, deep residual soils are common, but they may have been so deeply leached by weathering that any copper minerals present would not be detected by analyzing samples for copper alone. Many of the circumpacific por- phyry copper deposits, and some others as well, are enriched in gold as well as in copper. In the environ- ment of deeply weathered residual soils in Puerto Rico, for example, the areal extent of gold-in-soil anomalies defined known porphyry copper deposits much more reliably than did the areal extent of copper-in-soil anomalies (Learned and Boissen, 1973). A thorough evaluation of relatively immobile trace elements in soils and hydrothermally associated trace elements in resistate minerals remaining in leached soils should be helpful in locating copper deposits hidden under such soils. Studies of hydrothermal quartz grains from residual soils over several por- phyry copper deposits in Puerto Rico, for example, revealed that halite-bearing fluid inclusions, which are known to be present only where copper minerals are present, are preserved in the residual soils and could be used to locate the deposits (D. P. Cox, writ- ten commun., 1973). The trace-element chemistry of these inclusions has not been determined as yet. Silty frost boils proved to be the best type of soil- sampling material to use in the search for copper deposits in the Coppermine district of northern Canada where a perennial (continuous) permafrost zone exists. The boils acted as accumulators of fine inorganic soil material and were found to have a more uniform mineralogical composition than did B6 the corresponding B-zone soils (Hornbrook and Allan, 1970). Exploration g-eochemists conducting soil-sampling programs in arid areas should consider the possi- bility that eolian-transported silt- and clay-sized ma- terial may be intimately mixed with the residual soils of an area, thereby diluting metal concentra- tions in the soil. My investigations indicate that eolian dilution of metal concentrations should not be a problem in soil-sampling programs conducted in the southwestern United States; however, in desert areas of the Near East, wind-carried dust has been found to be an important dilutive factor which has to be considered (P. K. Theobald, oral commun., 1971). Two other specialized types of soil-related mate- rial suitable for geochemical sampling are found in arid areas—caliche (calcrete) and desert varnish. Caliche is composed primarily of calcium carbonate that is deposited in warm arid climates as part of the'soil-forming processes. Caliche often contains lithic impurities. In my experience, two types of caliche can be recognized; in sampling caliche it is important to distinguish between the two types. One type is commonly found interstratified in the sedi- mentary sequences of eroded gravels filling struc- tural basins. These caliche layers represent old (now buried) and (or) modern soil horizons. Deposits of this type of caliche have never had any physical con- tact with bedrock and have no geochemical relation- ship to bedrock chemistry unless capillary action has moved mineral-rich ground water from areas of mineralized bedrock to the surface. Consequently, in most places, this first type of caliche is not suitable as a geochemical sampling medium. The second type of caliche is found at the present bedrock surface or along the bedrock-alluvium interface where bed- rock is presently buried by alluvium. This second type is analogous to a residual soil horizon because it derives most of its trace-element concentrations from the underlying bedrock; consequently, this second type of caliche can be useful in geochemical prospecting. In his study of geochemical dispersion in the Twin Buttes, Ariz., porphyry copper district, Huff (1970) found anomalous amounts of copper in this second type of caliche in the vicinity of min- eralized bedrock. Desert varnish is an iron- and manganese-rich coating that forms on exposed surfaces of rocks in arid areas. It is probably the arid-environment equivalent of the iron and manganese staining found on stream sediments and float in the humid environ- ment. In their study of desert varnish, Engel and GEOLOGY AND RESOURCES OF COPPER DEPOSITS Sharp (1958) found that it contained a variety of trace elements, including copper. Most of these ele- ments were derived from the rock under the coat- ing. In another study including areas containing mineralized rock, Lakin, Hunt, Davidson, and Oda (1963) found in many of the areas a rough corre- lation of enriched trace metals in varnish with known ore deposits containing these trace metals. No detailed study has as yet been made within a single district to establish positively Whether or not desert varnish might be a useful geochemical sam- pling medium in copper exploration. Unfortunately, the occurrence of varnish within a given area may be restricted; consequently, it may not be possible to obtain samples at all desired locations. SOIL GAS Research in recent years has demonstrated that many volatile elements and compounds are asso- ciated with mineral deposits. Weathering processes and the action of micro-organisms tend to decom- pose the metallic minerals of a mineral deposit. Under ideal conditions, gaseous products, such as mercury, sulfur compounds, carbon dioxide, and others described later, can form at depth and mi- grate upwards to the atmosphere. If these gases are present in near-surface soils, they can be collected in an apparatus analogous to a funnel, which is placed on top of the soil at a given site (fig. 1) or collected in a probe placed in a hole drilled into the soil. Any gaseous products that are present in the soil can then either (1) be passed directly, without concentration, into some type of instrument that can measure the concentration of one or more gas- eous components that have accumulated in the fun- nel or probe during a given time span; or (2) be trapped and concentrated at the funnel neck or within the probe by some chemical reaction. The trap can then be removed from the collection site and the gas analyzed for its components in a sep- arate instrument. Although it is a relatively new technique, the use of analyses of soil gas has proved feasible for the detection of various types of mineral deposits in- cluding those of copper. Where gases are present, the method offers a great potential for locating de- posits covered with thick layers of glacial debris or other transported overburden. It might also be possible to use soil gases to detect deposits overlain by postmineralization-age volcanic flows if the flows have sufl‘icient permeability. In humid areas, water will be a complicating fac- tor in soil-gas surveys because it will tend to inhibit GEOCHEMICAL EXPLORATION TECHNIQUES FIGURE 1.—Plastic hemisphere used in soil-gas sampling. The gases are collected either by using natural convection and a chemical trap attached to the top of the hemisphere, or by drawing the gases through a tube attached to the hemis- phere and into an instrument designed to analyze gases. Scale is E30 cm long. the upward migration of gases. In arid areas, how- ever, where ground-water tables are either very deep or nonexistent, the use of soil gases to detect buried minerals should be an especially valuable technique. Mercury in air and in soil gas has received much attention as a possible pathfinder for base- and precious-metal deposits. In a review of volatile com- pounds as guides to ore, McCarthy (1972) noted that several workers have attempted unsuccessfully to find mercury anomalies in the air over porphyry copper deposits in the southwestern United States. Limited attempts to detect anomalous concentra- tions of mercury in soil gases over porphyry copper deposits have also proved unrewarding. The mercury concentrations may be too low to be detected with presently available instrumentation, or perhaps any anomalous concentrations of mercury have been dispersed well beyond the vicinity of a given de- posit, and therefore were missed because sampling did not extend far enough beyond the deposit. In the final analysis, there simply may be no anomalous concentrations of mercury associated with porphyry- type copper deposits. Gaseous products other than mercury vapor offer more promise in exploration for copper. Extensive B7 soil-gas surveys have been conducted over copper- molybdenum deposits in the Soviet Union (Shipulin and others, 1973). These authors reported concen- trations of 02, N2, H2, 002, H2S, $02, COS, and OH, in the soils overlying these deposits. Anomalous con- centrations of the sulfur-containing gases were de- tected over areas of buried copper-molybdenum min- eralization known to occur no closer than 30-60 m (100—200 ft) to the surface. These gases were not found in barren areas. In contrast, a buried barren pyrite body did not produce significant sulfur-gas anomalies. Several of the nonsulfur-containing gases were considered to be present because of atmos- pheric contamination, but their relation to the metal deposits, if any, was not described. Rouse and Stevens (1971) detected anomalous concentrations of sulfur dioxide in soil gases col- lected from the Highland Valley porphyry copper district in British Columbia. Anomalous concentra- tions were found at the top of a glacial till layer that is as much as 600 m (200 ft) thick. In their review of the use of volatile compounds in geochemical prospecting, Bristow and Jonasson (1972) indicated that the ratio of carbon dioxide to oxygen should be useful in soil-gas surveys. They noted that the gas-ratio method has been used suc- cessfully in the search for copper and other metals in various areas of the Soviet Union. Kravtsov and Fridman (1965) stated that oxidizing sulfides pro- duce soil gases that contain less oxygen and more carbon dioxide and hydrogen sulfide than do the soil gases found in normal soils. These authors found increased amounts of hydrogen sulfide "and sulfur dioxide in soils near chalcopyrite deposits. Chitayeva, Miller, Grosse, and Christyakova (1971) described the presence of iodine in the super- gene zone of the Gay chalcopyrite deposit in the Soviet Union. They found the iodine anomalies di- rectly above the copper ore body and suggested that iodine should be a good pathfinder element to use in overburden surveys. Undoubtedly other soil gases will be found associated with copper deposits. Vari- ous authors have described the detection of different volatile compounds from mineral deposits other than copper deposits. For example, Ovchinnikov, Sokolov, Fridman, and Yanitskii (1973) detected helium, neon, argon, hydrogen, carbon dioxide, nitrogen, sulfur dioxide, hydrogen sulfide, methane, and the halogen gases in rock, soil-gas, and ground-water samples. In addition to many of the gases mentioned above, Kravtsov and Fridman (1965) found com- plex hydrocarbon gases ranging from methane to hexane associated with ore deposits. B8 TRANSPORTED MATERIALS The use of transported material as a geochemical sampling medium presents especially challenging problems. Because transported materials have often migrated great distances from their original source rocks, the geochemist using these media must have a knowledge of both the local and regional geological and geochemical environments in order to interpret his geochemical data properly. In spite of the ob- vious handicaps, transported material has been used successfully in many different exploration programs. Three types of transported material are discussed: these are glacial debris, stream sediments, and lake sediments. GLACIAL DEBRIS The use of glacial debris as a geochemical sam— pling medium has found wide application in recon— naissance surveys, especially in Canada, Scandinavia, and the Soviet Union. It is important that persons conducting and interpreting geochemical sampling programs involving glacial debris have a thorough understanding of the history, nature, and mode of formation of the glacial deposits present in the areas they examine. A sampler must, for example, be able to distinguish the origin of fine sediment that might be from till fines, former lake sediments, or glaciofluvial material. Various studies have indicated that basal till material is almost analogous to re- sidual soil; lake-sediment deposits and glaciofluvial deposits, on the other hand, may be formed from material that has been transported long distances. The distance that till fragments have been trans- ported from their source is generally determined by recognizing rock fragments in the till that are unique to some known source rock upglacier. Al- though till fragments have been found as much as 1,100 km (700 mi) from their source, probably more than 90 percent of all such material has travelled only about 11/2 km (1 mi) (Chamberlain, 1883). In Sweden, about 70 percent of all till is thought to be . ‘ reconnaissance geochemical surveys. The method is locally derived (Lundqvist, 1967). The successful use of till samples in reconnais- sance surveys for copper deposits has been widely documented in the literature (Hawkes and Webb, 1962, p. 186—190; Bradshaw, Clews, and Walker, 1971; Garrett, 1971) and will not be described in detail here. In general, till is a very heterogenous type of ma- terial that ranges in size from clay-sized particles to large boulders. Different sizes of material have been used in different types of surveys. Trains composed of large rock fragments or boulders (float) in till GEOLOGY AND RESOURCES OF COPPER DEPOSITS have been used successfully in geochemical prospect- ing for copper and other metals as well as in geo- logical mapping (Dreimanis, 1958; Lee, 1971). From a purely geochemical standpoint, however, most sampling schemes have utilized fine material col- lected at the base of the till. Many metallic elements, including copper, are preferentially enriched in the fine fraction of till sediments. Shilts (1971) stressed the importance of relating the amount of clay-size material in a sample to the relative concentrations of the elements under study. To properly evaluate till anomalies, Shilts suggested removing the clay-sized material from the silt-sized material before analysis of the sample. He also suggested using a heavy-mineral concen- trate from the till sample as an alternate type of sample medium. Esker gravels, a glaciofluvial type of deposit, have been used successfully as a sample medium in Can- ada (Lee, 1965; 1971). In his study of the distance of transport of material in esker gravels, Lee (1965) found that rock fragments had been carried an average distance of 5—13 km (3—8 mi) from known bedrock sources. Sampling of esker gravels is ob- viously limited by the areal extent of esker de- posits; however, eskers are known that are several kilometres Wide and as much as several hundred kilometres long, so that relatively large areas can be prospected using esker material. The technique should be especially useful where an esker crosses the structural and(or) stratigraphic grain of a region. STREAM SEDIMENTS The most universally accepted and most success- ful of all geochemical-prospecting techniques is stream-sediment sampling. Stream sediments prob- ably reflect more accurately the upstream geology and chemistry of a large area than do any of the other types of sample media. As a consequence, the technique is extremely useful for rapid regional- also very valuable for most detailed work in drain- age basins found during reconnaissance work to con- tain anomalous metal concentrations. The technique has had wide application in the search for copper in many different geologic and climatic environments. Many examples are described in the geochemical literature. In stream-sediment surveys in which copper is the element sought, the sampling environment must be carefully considered. For example, in humid cli- mates where acid soil conditions and thorough chem- GEOCHEMICAL EXPLORATION TECHNIQUES ical decomposition of rocks generally prevail, stream sediments should be analyzed for cold-extractable copper rather than for total copper. Stream sedi- ments have been analyzed for cold-extractable cop- per as part of surveys performed both in tropical humid areas such as the Philippine Islands (Govett and Hale, 1967) , and in temperate humid areas such as Maine (Post and Hite, 1964). Less chemical breakdown of rocks takes place in arid areas than in warm humid areas; consequently, copper may be dispersed in arid areas more by me- chanical means than by chemical means (fig. 2). Also, in an arid environment, metal-rich ground water does not normally come in contact with stream sediments. Because of these facts, a total (hot-acid- extractable) analysis for copper has been used in many geochemical studies conducted in arid areas (Awald, 1971; Coolbaugh, 1971). It should be em- phasized, however, that there is not as yet enough information relating the type of copper analysis to the type of environment. Orientation studies should be run in any new area to determine which type of B9 copper analysis gives the best anomaly contrast for that area. A major concern in stream-sediment surveys has been the interpretation of the effects, if any, of secondary hydrous iron-manganese scavenging on the concentrations of other metals in the samples. J-enne (1968) made a comprehensive laboratory study and literature review of the role of hydrous iron and manganese oxides in scavenging (coprecipi- tating) heavy metals, including cop-per. He concluded that these two oxides (especially that of manganese) are the most important controls on the fixation of copper, cobalt, nickel, and zinc in soils and stream sediments. Chafi'ee, Botbol, and Hamilton (1972) applied a factor-analysis program to a data set containing analyses of more than 6,000 stream-sedi- ment samples collected throughout central Maine. In contrast to J enne’s conclusions, the factor analy- sis clearly demonstrated that, at all factoring levels, the copper concentrations in the samples (both cold- extractable and spectrographic copper) were not re- lated to either the iron or manganese concentra- FIGURE 2.—Typica1 desert wash used in stream-sediment sampling in semiarid southern Arizona. Note the poor sorting of the active sediment. B10 tions. Data from this study seemingly indicate that copper is not scavenged by secondary hydrous iron and manganese oxides. Insofar as copper is con- cerned, the relation of copper enrichment to the con- tent of iron-manganese oxides in stream sediments is not yet completely defined. The study by Chaffee, Botbol, and Hamilton (1972) indicated that iron and manganese oxides do coprecipitate and concen- trate such element-s as cobalt, zinc, lead, and molyb— denum. If any of these four elements is used as a pathfinder in the search for copper deposits, then the effect of secondary iron and manganese oxides should be considered in data interpretation. The US. Geological Survey is currently making a study of the source of the secondary metals asso- ciated with stream sediments. Present information suggests that where metal—rich interstitial ground water flows into a stream from bank soil, there are abrupt changes in the physical and chemical environ- ment. These changes cause the metals to precipitate in the stream, either as grain coatings or as small nodules, before they are transported very far down— stream. Most of the cold-extractable metals found in stream sediments are thought to have been derived from these interstitial ground waters, even though stream waters usually constitute a much larger pro- portion of the total water volume at a given stream locality (G. A. Nowlan, oral commun., 1973). As was mentioned in the section on soil surveys, the optimum size of material, in this case stream- sediment material, used for analysis should be de— termined in any new area. Most, but not all, ele- ments are concentrated in the sediment fines; how— ever, the best grain size to retain for analysis should be the one that provides the best anomaly contrast. Hawkes and Webb (1962, p. 256—258) have given an excellent discussion of this problem. Most stream-sediment surveys have used the fine« grained (—- 80 mesh) fraction for analysis; however, Fisher (1970) reported that in some areas of Aus- tralia, stream-sediment material as coarse as a —20 to +40 mesh fraction gave better results than did standard —80 mesh fractions. Different climatic, geologic, and terrain conditions were found to in- fluence the metal content of different size fractions. The importance of selecting the proper size of sample material to use in a stream-sediment survey was also emphasized by Erickson, Marranzino, Oda, and Janes (1966). Their study was conducted in an arid area of eastern Nevada not known to con- tain any significant metal occurrences. The authors concluded that the most successful method for find- ing new deposits in that terrain was analysis for GEOLOGY AND RESOURCES OF COPPER DEPOSITS base metals and mercury of float cobbles and pebbles collected in the major stream channels. Chemical data from the —50 mesh fraction of stream—sedi- ment samples failed to reveal any anomalous metal concentrations except in those samples collected from streambeds draining a previously known but abandoned small mining district. Several types of concentrates, including magnetic and nonmagnetic heavy-mineral concentrates and grain-coating concentrates, can be made from stand- ard stream-sediment samples. Analyses of these specialized samples are sometimes more useful in defining geochemical anomalies in stream channels than are the analyses of the original stream-sedi- ment samples from which these specialized samples are derived. Grifiitts and Alminas (1968) collected stream-sedimnt samples in an arid area of south- ern New Mexico and made heavy-mineral concen- trates from these samples. They found that their analyses of the nonmagnetic heavy-mineral concen- trates provided more information for locating min- eralized areas than did the analyses of the corre- sponding samples of standard —80 mesh stream sediment. Hufi' (1971) experimented with different subsamples as possible guides to a known porphyry- copper district in southern Arizona. He sampled sediments from streams draining the copper de- posits present and found that the copper concentra- tions in (1) ultrasonic concentrates of sediment grain coatings, (2) the nonmagnetic heavy-mineral fraction, and (3) hydrothermally-altered stream pebbles all located the known deposits as well as or better than did the standard ~80 mesh fraction of the stream-sediment sample. Heavy-mineral concentrates from old placer dis- tricts may be useful in the search for copper de- posits. In a study of placer minerals in Taiwan, Tan and Yu (1968) found that placer gold was present in the sediment from streams draining known copper-gold deposits. The worldwide association of gold with many copper deposits suggests that gold in stream-sediment concentrates should be a valu- able pathfinder element in the search for copper deposits. The concentrations of camouflaged trace elements in heavy-mineral fractions of stream sedimnts can also be used in geochemical exploration for copper deposits. Tan and Yu (1968) detected anomalous concentrations of copper in pyrite concentrates col- lected from streams draining areas of known cop- per deposits in Taiwan. In another study, Bell and Hornig (1970) determined that iron-oxide pseudo- morphs after pyrite may be a useful geochemical- GEOCHEMICAL EXPLORATION TECHNIQUES sampling medium in the search for sulfide deposits in deeply weathered regions. These authors found iron-oxide pseudomorphs to be a common consti- tuent in stream-sediment samples as well as in soil and saprolite samples that they collected in South Carolina. These pseudomorphs contained appreciable amounts of many trace elements. The authors sug- gested that the regional variation in trace-element content of the pseudomorphs may be a guide to areas of sulfide mineralization. In a study of alluvial magnetites from streams in central Ecuador, de Grys (1970) found that the copper and zinc concentrations in the magnetites were useful in defining various base-metal deposits. She noted that it was important to determine whether the magnetite was fresh or weathered. In fresh magnetite, all the trace elements were con- sidered to be primary; altered magnetite might con- tain secondary hydrous iron-oxide coatings that could coprecipitate other metals. As noted in the section on rocks, my experience suggests that caution should be exercised in inter- preting trace-element data from magnetites and possibly from other heavy minerals. In complex geologic terranes, different geologic formations may contain magnetites or other heavy minerals having different ages and geneses. These formations will, therefore, probably have different suites of trace elements. A knowledge of bedrock geology is thus essential for the proper interpretation of any allu- vial heavy-mineral data. Because of the large amount of information that can be generated from regional stream-sediment sampling programs, computer analysis of the analy- tical and other data is becoming more and more important for evaluating the data. Many computer programs are now available to treat large data sets. Some of these methods are discussed later in this report. LAKE SEDIMENTS In recent years, the collecting and analysis of lake sediments has been another useful geochemical- prospecting technique, especially for regional re- connaissance surveys. This method, which is espe- cially applicable to remote areas where sampling can be done from pontoon-equipped helicopters or small fixed-Wing aircraft, has been used successfully in regional—reconnaissance copper exploration pro- grams in Canada. Field studies (Allan, 1971; Allan and Crook, 1972; and Allan and others, 1972, 1973) have demonstrated that standardized samples can be taken from lake sediments and that the chemical B11 analyses from these samples can define areas en- riched in copper as well as can those from stream- sediment samples. Best results were obtained from samples of sediment collected 5 to 15 cm (2 to 6 in.) below the lake-bottom surface at depths of about 90 to 240 cm (3 to 8 ft) below the lake-water level. Although clay-sized material made the best sample, the use of clay— and silt-sized material together (about 50 micron and smaller-sized material) proved to be a useful sample that could be quickly obtained and prepared for analysis. Factors such as Eh, pH, organic content, and secondary iron- and manganese-oxide content that can affect stream-sediment sample data can also affect lake-sediment sample data. Furthermore, it is important to recognize that the lake-sediment chemistry reflects that of the sediment source area and not that of the bedrock below the lake. WATER AND RELATED MATERIALS Marine and fresh water, as well as ice and snow, represent potential geochemical sampling media, but they have not been widely used because a great many parameters can influence the chemical data obtained and because only extreme low metal con- centrations are normally found. With the develop- ment of more sensitive and accurate analytical tech— niques and new computer programs, these media are becoming more useful in geochemical studies. “VATER Hydrogeochemical prospecting techniques have been reviewed in detail by Hawkes and Webb (1962, p. 227—246) and by Boyle and others (1971). Most hydrogeochemical surveys have been conducted using lake water as the sampling medium; however, waters from streams, springs, seeps, wells, and the sea have also been used. Water has not been a popular sampling medium because a great many physical and chemical fac- tors must be considered in evaluating water an- alyses. Some of these factors are Eh, pH, the overall chemistry of the environment sampled, the distance from the source of metals to the sample site, the time of year, the duration, type, and annual rate of precipitation, the ambient temperature, the size and rate of flow or circulation of the water body, and the effect of organic matter. The form in which metals occur in water has also caused difi‘iculties in data interpretation. Metallic elements such as copper can occur in natural water in many different forms, in- cluding free ions or undissociated molecules, ions adsorbed on suspended matter, and water-soluble metallo-organic complexes. Metals may also be found B12 in organic gels, in waterborne micro-organisms, and in suspended matter such as colloids or finely di- vided inorganic sediment. Kleinkopf (1960) sampled lake waters in Maine and demonstrated that clusters of lakes there con- tained anomalous amounts of copper, molybdenum, and other metals. In some areas he found an associa- tion between the anomalous concentrations of metals in the water samples and the known geology and mineralized rock bodies. In a more recent study of lake waters conducted specifically for copper exploration, Allan (1971) and Allan, Lynch, and Lund (1972) analyzed lake-water and lake-sediment samples collected in a large region around the Coppermine district in Canada. Although the distribution of copper in lakehsediment samples gave the best indication of areas known to be en- riched in copper, the distribution of copper in lake- water samples also successfully delineated the areas of highest copper concentration in the region. Sea- sonal and intra-lake chemical variations were not found to be serious problems for interpretation. Boyle and others (1966) conducted a regional geo— chemical survey in New Brunswick, Canada, in which they collected samples from stream waters and stream sediments. Stream—water samples, even near known base-metal deposits, had very low concentra- tions of copper. Analyses of zinc and cold-extractable heavy metals in the water gave much better results; however, data for all elements from the stream-sedi- ment samples provided better information than did the comparable water data. In another study, Boyle and others (1971) showed that dispersion trains of metals in stream waters are usually more restricted than those in stream sedi- ments. Consequently, these authors recommended using stream-sediment samples for regional recon- naissance surveys, and stream-water samples for de- tailed surveys. They recommended that all water samples be collected as near as possible to the head- waters of stream tributaries. In some regions of the world the ground is frozen all year long (continuous permafrost) or at least part of the year (discontinuous permafrost). Until recently, the presence of continuous permafrost has suggested to most geochemists that metals would have little chance to migrate upwards to the sur- face; however, thawed ground is now known to exist in areas of continuous permafrost under and around streams and large bodies of water such as lakes (Allan and Hornbrook, 1971). Shvartsev (1972a, 1972b) successfully used samples of spring waters that have circulated through permafrost zones to GEOLOGY AND RESOURCES OF COPPER DEPOSITS locate buried base-metal deposits in the Soviet Union. Water from seeps and cold springs, and the pre- cipitates that form around the-m, have been found useful in geochemical surveys (Boyle and others, 1971; Shvartsev, 1972a, 1972b; Mehrtens, Tooms, and Troup, 1973). Water from hot springs, on the other hand, has not been successfully used as yet to locate copper deposits. Many—perhaps most—hot springs have no obvious relationship to potential copper deposits; however, White (1967) described the chemistry of some hot springs and hot-spring deposits that are associated with base-metal deposits in Japan and the United States. Weissberg (1969) made a similar study of some thermal areas in New Zealand. Both authors noted that base metals are not normally present in significant amounts in the waters issuing at the ground surface, but were found at depth in samples of drill core. The waters issuing at the surface commonly contained such ions as Ca+2, Na+, Mg“, Li+, F—, Cl—, Br—, 804—2, and ECG; and the discharge precipitates contained such metals as Au, Ag, As, Sb, Hg, Tl, Fe, and Mn. Appar- ently, most of the heavy-metal ions, including cop- per, are precipitated below the surface; thus, geo- chemical exploration programs using samples of hot-spring waters will probably have to rely on some combination of pathfinder elements as guides to buried copper deposits. The study of ground-water geochemistry can be valuable in geochemical prospecting. For most ex- ploration purposes, wells, drill holes, and possibly old shafts or quarries are the only feasible places for sampling subsurface waters. Wells are especially useful sources in arid areas where there is very little surface water. In arid areas, the ground water is normally alka- line except perhaps near oxidizing sulfides. In the alkaline environment, very few elements are mobile; however, in the absence of high concentrations of such ions as manganese, iron, carbonate, and sulfate, the metals chromium, molybdenum, and tungsten are fairly mobile. Copper is generally not very mobile at high pH levels. Huff (1970) was able to detect the buried copper-molybdenum deposits of the Pima dis- trict in Arizona by measuring the molybdenum con- centrations in well-water samples. He found a molyb- denum anomaly that extended as much as 13 km (8 mi) downslope from the known deposits; copper was not present in anomalous concentrations in these samples. Well waters have been sampled for molybdenum throughout much of the southwestern United States GEOCHEMICAL EXPLORATION TECHNIQUES by various private companies; however, little infor— mation has been made public. Anomalous molyb- denum concentrations in well water are known to exist downslope from known copper-molybdenum deposits, but to date no new deposits have been found using well water as the sampling medium. No information about the content of tungsten or chrom- ium in well waters is available. The mineral potential of the world’s oceans and ocean bottoms is gradually being recognized. The presence of copper in deep-sea manganese nodules, in manganese-rich encrustations on rock- and sedi- ment-grain surfaces, and in bottom deposits such as those of the Red Sea, is already well known. At pres- ent we have little detailed knowledge of the move- ment of trace elements in ocean waters and how trace-element dispersions in marine waters might be used in geochemical prospecting for nearshore or deep-sea mineral deposits. Additional fundamental chemical and physical data concerning the oceans are needed before geochemists can begin to apply geo- chemical prospecting methods successfully to sam- ples of ocean waters and bottom materials. In some deep-sea areas, the regional trends in the chemistry of near-bottom waters or of slowly upwelling waters may be useful in locating any deep-sea copper de- posits that are not buried under thick layers of bar- ren sediment. In many localities, submarine springs are known to be present. Analyses of samples of this type of spring water might be useful in determining the presence of anomalous concentrations of metals in the source aquifer. In the future, deep-sea metal-rich bottom-sediment deposits, such as those found in the Red Sea and elsewhere, may well be an economic mineral resource. Holmes and Tooms (1973) made a reconnaissance geochemical survey of part of the Red Sea area. They collected samples of surface waters as well as samples of bottom sediment and near-bottom water from depths of about 2,000 m (6,500 ft). The dis- solved metal species in the near-bottom water sam— ples and the particulate matter in these samples were analyzed separately. Significant anomalies were found in all the sample media except those from the surface waters. This study suggested that recon- naissance geochemical surveys should be successful in locating deep-sea metal-rich bottom-sediment de- posits as well as deep-sea brines that contain high concentrations of various metals. Material suspended in fresh water has recently been considered as a possible geochemical sampling medium. Perhac and Whelan (1972) compared the metal content of stream water, of bottom sediment, B13 and of the suspended colloidal and coarser particu- lates in the water. The colloidal material was found to be greatly enriched in metals, including copper, as compared with the other types of samples. Be- cause the separation of colloidal samples for analysis requires expensive equipment and an inordinate amount of time, the method is not very practical at present. Timperley, Jonasson, and Allan (1973) found that most lakes in certain parts of Canada contained or- ganic gels that may occur as much as 13 m (30 ft) above the inorganic sediment of lake bottoms. The gels are thought to include ( 1) organic precipitates of colloidal origin, (2) residual organic matter de- rived from decaying vegetation, (3) pollen, and (4) finely divided inorganic mineral matter. Preliminary data indicated that the gels contained concentrations of various elements, including copper, and that the trace-element concentrations of the gels can be re- lated to the local geology. The use of organic gels from lakes as a regional geochemical sampling me- dium seems promising. Gels also are found along shorelines Where fresh- water rivers discharge into bodies of saltwater (W. C. Overstreet, written commun., 1973). The extent of such gels along shorelines is not known, but if these gels are commonplace, then systematic sam— pling of them could be another effective reconnais— sance geochemical exploration technique. Gases are commonly dissolved in all types of nat- ural waters. Analysis of radon gas in lake and stream waters has been used in Canada as a reconnaissance hydrogeochemical exploration method in the search for uranium deposits (Dyck and others, 1971) ; how- ever, the analysis of gases in waters has not as yet been applied to copper exploration. ICE The use of samples of ice in a geochemical sam- pling survey is obviously quite limited but may be feasible. Shvartsev (1972a) has attempted to use naleds as a geochemical sampling medium. Naleds are ice sheets that form during the cold season in areas of discontinuous permafrost. They form where subsurface water is forced to the surface and flows out over the surface and is frozen. Shvartsev noted that there has not been sufficient work done on naled geochemistry to determine whether the sampling of naleds will be a useful technique for locating mineral deposits in cold regions. SNOW Traces of Cu, Hg, Zn, Pb, Ag, Mn, Cd, As, and Ni 1 have been detected in snow samples collected from B14 sites overlying several types of buried mineral de- posits in Canada (Jonasson and Allan, 1973). Their study revealed that these metals were derived at the time of the first snowfalls from the weathered min- eralized rocks below the soil horizons and not from particulate matter present in the air above these rocks. These authors concluded that snow may be a useful sampling medium but that its use is limited to detailed prospecting in areas of known mineral potential. VEGETATION Vegetation can be used both in geobotanical and biogeochemical prospecting. Geobotanical pro-spect- ing involves noting and mapping (1) the presence and abundance of, or (2) the absence of, a plant species or plant community with relation to soil and rock enriched in a given element such as copper. Geobotanical prospecting may also involve noting and mapping the presence of morphological and(or) mutational anomalies in plants. In contrast, bio- geochemical prospecting involves analyzing plant materials for their trace-metal content in order to locate areas of high metal concentrations. GEOBO'I ‘AN ICAL 'l‘l‘ZCHN IQU ES Geobotanical prospecting has been discussed in detail by Brooks (1972). Where this technique is successful, it is a rapid and inexpensive prospecting method. Brooks (1972) noted that plant communities indicative of soils enriched in certain elements or indicative of certain rock types have been recognized for more than a century; however, no plant com- munities are known that uniquely define copper- enriched soils. Several individual plant species, on the other hand, have been found to be associated with copper-rich soils in different parts of the world. Cannon (1971) listed 30 species of plants that have been used in various localities as indicators of cop- per. She noted that geobotanical prospecting has been especially successful in the search for copper in Australia, Katanga, China, and the Soviet Union. Cannon (1960) also stated that most copper-indica- tor plants belong to one of three plant groups; (1) the Caryophyllaceae, or pink family; (2) the Lab- iatae, or mint family; (3) the mosses. My geobotanical studies in the porphyry copper region of the southwestern United States as well as those by Lovering, Huff, and Almond (1950), War- ren, Delavault, and Irish (1951) , and Cannon (1960) indicate that abundant populations of the California poppy (Eschscholtzia mexicana) grow in close asso- ciation with copper-rich soils near many porphyry GEOLOGY AND RESOURCES OF COPPER DEPOSITS deposits. Unfortunately, the poppies are not found growing over all cupriferous soils of the region nor are all wild poppy populations associated with cu- priferous soils. Evidently the conditions promoting vigorous poppy populations involve more than the mere presence of high copper concentrations in the soil. During the period of flowering, localities con- taining the poppy are easily recognized both from the ground and from the air at low altitudes. Areas containing the poppy certainly warrant close ex- amination. Despite studies by many workers, no other impor- tant copper-indicator plant species has been identi- fied for the porphyry copper region of the United States. However, Cannon (1971) has described some plant species that are commonly associated with sulfur- or sulfate-rich soils; I have seen some of these species growing in soils formed from rocks containing oxidizing sulfides. Unfortunately, these species do not differentiate between biogenic-sedi- mentary sulfide and sulfate concentrations, and hydrothermally-introduced sulfide and sulfate con- centrations. A high content of copper in soils can cause mor— phologic or mutational changes in certain plant spe- cies or can cause the complete absence of vegetation. Cannon (1960, 1971) and Brooks (1972) have de- scribed areas in widely separated parts of the world in which copper-rich soils caused such effects. BIOGEOCHEMICAL TECHNIQUES Chemical analyses of plants or plant parts can be used as a geochemical-prospecting technique when it can be shown that the chemical content of the vegetation sampled has some predictable relation- ship to the chemical content of the nutrient soil or bedrock, or at least to the ground water moving through the soil or bedrock. Only some brief com- ments related to copper will be given here; for more details the reader is referred to Brooks (1972). Biogeochemical prospecting requires specialized knowledge and equipment beyond that needed, for example, for rock or soil sampling, and these re- quirements generally determine where the method can be used most effectively. The primary advantage of biogeochemical sampling over the methods of surficial sampling is predicated on the fact that many plants are deep rooted. Chemical elements absorbed by a deep well-developed root system are commonly translocated to the leaves and stems of the plant. The leaves and(or) stems can then be collected for analysis. Roots growing to depths of as much as 66 m (200 ft) have been documented (Brooks, 1972, p. 107). With these facts in mind, GEOCHEMICAL EXPLORATION TECHNIQUES it can be seen that biogeochemical sampling should be a better technique than soil or rock sampling in regions of extensive but relatively thin postmineral- ization-age overburden. This overburden may in- clude glacial deposits, talus, or landslide deposits, desert pediment gravels, or even fairly recent or- ganic accumulations such as peat or deep humus soil layers found in areas of dense vegetation. The mechanisms by which plants translocate and concentrate trace elements are not well understood. Many physical and chemical parameters such as soil Eh and pH, annual precipitation rate, mean annual temperature, aspect, soil type and texture, antago- nistic effects of elements present, and physiology of each plant species, control the ultimate locations and concentrations of each element. As a result, sam- pling and analyses of plant material should be done on a consistent basis to minimize the effects of these parameters. Probably the most common use of the technique of biogeochemical prospecting has been in areas cov- ered with glacial deposits in Canada, Scandinavia, and the Soviet Union. Wolfe (1971) concluded that this method can be an effective technique under optimum conditions, but that if impermeable layers of clay and silt are present in the glacial substrate, such layers will prevent the upward migration of metals from mineralized bedrock below the layers, and no anomalous concentrations of metals will be present in plants, even in those growing directly over ore. This author also stated that, in contrast to other regions, most trees of the Canadian Shield probably do not have root systems that extend be— low 6—16 m (20—50 ft). This maximum depth of rooting reflects primarily the presence of shallow water tables and (or) impermeable clay or silt lenses. The maximum rooting depth of plants may also be controlled by frozen layers in the zone of discon- tinuous permafrost. Biogeochemical prospecting in glaciated regions will therefore be most successful in areas covered only by thin glacial debris com- posed mainly of unfrozen, coarse, porous sediment. In a review of studies of vegetation conducted in the Soviet Union, Nesvetaylova (1961) described one case in which the iron content of the ash of birch leaves effectively outlined an area of copper- rich ore deposits. In Canada, Warren and his col- leagues (Warren and Howatson, 1947 ; Warren, Delavault, and Irish, 1949) collected samples of a variety of tree species and several different tree parts. The chemical data from these plant samples provided information that helped to delineate some copper-rich localities in western Canada. In these Bl5 studies Warren and his co—workers emphasized the use of copper-zinc ratios from the plant ash analyses as an aid in evaluating their data. Peat, another type of biogeochemical sampling medium, is commonly found in humid regions. Sam- ples of peat have been used successfully in both re- gional and local surveys in the search for copper deposits in many parts of the world. In Finland, Salmi (1967) discovered that anomalous concentra- tions of trace elements in peat ash could be used to locate copper and other types of mineral deposits buried under shallow layers of peat and glacial till. In Sweden, Brundin and Nairis (1972) found that the analytical data for copper from peat samples were superior to corresponding data from water and stream-sediment samples for delineating known min- eral deposits. In the Soviet Union, anomalous con- centrations of copper were found in samples of peat collected 300 to 600 m (985 to 1,970 ft) down drain- age from a known copper deposit. (Al’bov and Kostarev, 1968). Gleeson and Coope (1967) successfully located buried base—metal deposits in Canada by using peat as a sampling medium. They noted that the metals found in peat were probably derived from ground water circulating through the peat and that the metal content of the peat samples is related to the degree of humification of the peat and the pH of the associated waters. Biogeochemical prospecting has also been applied to the search for copper in an arid environment, particularly in the porphyry copper province of the southwestern United States. In desert regions there is an important dichotomy of plant types caused principally by the depth of the permanent ground— water table in a given area. Riparian plant species— those that grow almost exclusively in and along intermittent or permanent stream channels—are mostly phreatophytes; that is, species that have ex- tensive root systems that reach to the permanent ground-water table (fig. 3). Nonriparian plant spe- cies, on the other hand, grow almost anywhere in- cluding in intermittent stream channels and are mostly xerophytes, that is, species that have shallow root systems and that obtain their water from inter- mittent rainfall and not from the permanent ground- water table. The sampling of riparian species is somewhat analogous to collecting stream-sediment samples, and the sampling of nonriparian species is analogous to collecting soil or rock samples. Because of the generally poor development of soils in arid regions, most residual soil horizons are fairly thin; consequently, as long as any residual B16 FIGURE 3.—Mesquite (Prosopis sp.) roots are exposed in a wash near Tucson, Ariz. The deep, well-developed root sys- tem is typical of this phreatophyte and makes this species especially useful as a biogeochemical sampling medium in this semiarid region. From Huff (1970, fig. 6). soil is available, there is rarely any reason to sample nonriparian plant species. In areas containing fairly thin layers of transported overburden, however, nonriparian species may give a better indication of buried mineral deposits than can the soils derived from the transported overburden. Plots of concen- trations of copper, zinc, and molybdenum in plant ash from samples of plants growing in thin layers of transported pediment gravel deposits overlying the Vekol porphyry copper deposit in central Ari- zona showed limited but more Widespread anomalies than did plots of the corresponding soil data (Chaf- fee and Hessin, 1971; and Chaffee, unpub. data). GEOLOGY AND RESOURCES OF COPPER DEPOSITS Work by Huff (1970) and Brown (1970) and my studies clearly indicate that where phreatophytes are available they are an effective sampling medium for copper exploration. Mesquite (Prosopis juli- flora) has been the most widely used phreatophyte in biogeochemical surveys in the arid regions of the western United States. Use of samples of this spe- cies has been successful partly because mesquite grows in warm areas over a wide region extending from southeastern California to southern Kansas and partly because its root system can penetrate to great depths. A live root tentatively identified as being from a mesquite plant was collected from an open-pit mine in southern Arizona at a depth of about 53 m (175 ft) (Phillips, 1963). This depth undoubtedly represents an extreme, but roots to depths of about 15 m (50 ft) are probably common. In the Pima district in Arizona, Huff (1970) de- tected anomalous concentrations of molybdenum in the ash of mesquite stems collected as much as 13 km (8 mi) away from any outcrop of mineralized bed- rock. Brown (1970) also found anomalous concen- trations of copper in the stem ash of samples of mesquite collected over the Kalamazoo porphyry copper deposit near San Manuel, Ariz. This anomaly extended well beyond the exposed parts of the de- posit into transported colluvial gravels. I have also been successful in locating partially buried porphyry copper deposits in Arizona by using analyses of copper, zinc, and molybdenum in the ash of leaves and stems of mesquite. Analyses of the ash of other species, including those of catclaw (Acacia con- stricta) , blue paloverde (Cercidum floridum) , and a nonphreatophytic species, ironwood (Olneya tesota) , have proved nearly as effective. Plants of these spe- cies have been found to contain anomalous concen— trations of copper, zinc, and(or) molybdenum at sites at least 3 km (about 2 mi) downstream from any mineralized outcrop. Clearly, biogeochemical prospecting is an effective regional reconnaissance technique in the search for copper in an arid en- Vironment. MICRO-ORG AN ISMS Micro—organisms (mostly bacteria and algae) may be useful in geochemical prospecting. The role of micro-organisms in geological processes has been re- viewed in detail by Kuznetsov, Ivanov, and Lyalikova (1963). Essentially all the data in the literature deal with the relationship of micro-organisms to fossil- fuel, iron, or sulfur-sulfate deposits, or to the use of bacteria in the leaching of low—grade mine wastes for the recovery of base and ferrous metals. GEOCHEMICAL EXPLORATION TECHNIQUES Apparently, very little research has been done on the subject of the use of micro-organisms in the ex- ploration for ore deposits. The only practical applica- tion of micro-organisms in the search for ore deposits was done in the Soviet Union (Kuznetsov, Ivanov, and Lyalikova, 1963). This study showed that thio— bacteria were present in material of an undescribed nature overlying a molybdenum ore deposit. Circu- lating waters above the deposit were distinguished from the regional ground water by the concentra- tions of certain types of bacteria. In an analogy to vegetation surveys, micro-orga- nisms could be used in geochemical prospecting in two ways. First, the presence or absence of, or the abundance of, certain types of micro-organisms may indicate the presence or absence of certain chemical species, such as sulfur or sulfate. Second, the orga- nisms themselves can be collected and analyzed for their chemical content of any given element. It should be feasible to study soil micro-organisms by making cultures of them in various culture media. If cultures can be found that will produce organism growth or death only in the presence of anomalous concentrations of copper, for example, then it should be possible to collect small amounts of soil over areas of interest and to run cultures to determine the con- tent of copper or of some other element in those soils. This procedure has not been tested as yet; if suc- cessful, it might be more rapid than routine chemical analysis of the soils (H. W. Lakin, oral commun., 1973). Plankton are known to be concentrators of metals (Warren, Delavault, Fletcher, and Peterson, 1971). Sampling of plankton growing at the mouths of rivers might be a good reconnaissance geochemical prospecting method to use along marine or large- lake shorelines. Samples of algae collected from the mouths of rivers in Puerto Rico have been found to contain anomalous amounts of copper. Whether the anomalous samples are from the mouths of rivers draining areas containing porphyry-copper-type min- erals has not been established (R. E. Learned, oral commun., 1973). ANIMALS Studies in recent years have shown that animals can be used in two different ways in geochemical ex- ploration. First, dogs can be trained to scent sulfide minerals. Second, animals in general tend to accumu- late trace elements in their tissues and waste prod- ucts; thus, these materials can be used as a type of biogeochemical sample. 317 In the last 10 years, dogs have been used increas- ingly as an aid to geochemical prospecting in Canada and in Fennoscandia (Brock, 1972). Dogs have olfac- tory senses far superior to those of man; conse- quently, they can be trained to scent sulfide minerals. The widest application of dogs to geochemical pros- pecting has been in locating sulfides in glacial mo- raines (Nilsson, 1971). In field trials, dogs have been able to find sulfide minerals where no human could have found them by visual inspection alone. The dogs have found sulfide minerals buried under 30 cm (1 ft) of overburden (Stirling, 1972). They have also found pieces of sulfide float in the fines of stream sediments (Brock, 1972). In Finland, a dog found boulders containing pyrite and chalcopyrite that were later found to be associated with a “copper ore body of economic significance” (Stirling, 1972). Another animal that has been studied as a poten- tial aid to geochemical prospecting is the termite. Termites are found throughout the tropical and sub- tropical regions of the world and in temperate re- gions to about lat 45° N. and S. All told, these re- gions constitute about two-thirds of the Earth’s land surface (Lee and Wood, 1971). Many termite species build mounds above ground, most species construct extensive subterranean galleries. Although galleries have been reported to extend to depths of 70 m (230 ft) (Yakushev, V. M., in Lee and Wood, 1971, p. 53), most termite galleries are at much shallower depths. Watson (1972) observed termite galleries at least 23 m (about 75 ft) deep in mine workings. He be- lieved that termites constructed these deep galleries in their search for the ground-water table. These observations suggest that the material present in termite mounds might represent deeply buried ma- terial and that these mounds could be used as a sampling medium for geochemical prospecting in overburden in termite-infested regions. However, in his studies of the Kalahari sand of Africa, Watson (1970, 1972) found that most of the material from termite mounds was derived from depths of less than 3 m (10 ft), despite the fact that the termite gal- leries extended to much greater depths and into mineralized bedrock. If Watson’s observations hold true for termite mounds in other regions, then the sampling of these mounds may be a useful geochemi- cal prospecting technique for locating buried min- eral deposits, but only in areas where the soil or over- burden is less than 3 m (10 ft) thick. This technique should be investigated further. Animals can be considered to be one of many steps in the natural food chain and therefore in the geo- chemical cycle of many elements present in the B18 chain. Species of animals that move about on land or in streams only within a limited area may well collect or concentrate chemical elements available to them in their food. If the source of food for these animals is already anomalous in trace metals, then these ele- ments might be concentrated to an unusual extent in the tissue or in the waste products of the animals. Techniques that utilize animal tissue or waste prod- ucts as geochemical sampling media are described below. However, the idea of killing wildlife solely for use in geochemical prospecting cannot be recom- mended and would never be condoned by conserva- tion authorities. The base-metal content of trout livers from lakes and streams in British Columbia was studied by Warren, Delavault, Fletcher, and Peterson (1971). They noted that plankton can concentrate heavy metals and that trout feed on these plankton. The authors concluded that it may sometimes be possible to use the basemetal content of samples of trout livers to detect anomalous concentrations of metals in an area. Worthington (1968) described a study by the Colorado State Game, Fish, and Parks Department in which samples of deer antlers were collected at hunting check stations and were analyzed for gold in an attempt to discover new areas of potential gold resources. The results of the survey were not given; the approach, however, could easily be applied to the search for copper or other metal resources. Un- fortunately, from a geochemical exploration stand- point, most deer range over wide areas; conse- quently, even if metal anomalies were detected in antlers, it might be difficult to identify the source areas. In the arid lands of the Near East, much of the finer grained clastic overburden material has been transported great distances by the wind and is therefore not suitable as a geochemical sampling medium. In such areas, vegetation, although sparse, should be a useful sampling medium; however, re- gional custom dictates that living plant matter be saved for forage. Animals of the region, including camels, sheep, and goats, graze on the vegetation; consequently, animal dung may be an acceptable and useful sampling medium for regional reconnaissance in that part of the world (W. C. Overstreet, written commun., 1969). Many fish-eating birds live in large rookeries along the seacoasts of the world. If the fish have ingested metal-rich food, then analyses for copper or other metals in samples of guano from the bird rookeries GEOLOGY AND RESOURCES OF COPPER DEPOSITS might indicate the locations of nearby metal-rich waters or seafloor deposits along coastlines. Many endemic diseases of animals and man are now recognized as being related to excesses or de- ficiencies of trace elements in food or water. Webb (1971) described the use of stream-sediment chemi- cal data to identify areas in England where livestock might be ingesting forage that contained excessive or insufficient concentrations of various trace ele- ments known to affect animals. The reverse of Webb’s approach might be a useful regional recon- naissance geochemical exploration technique; that is, it should be possible to locate areas containing anomalous concentrations of trace elements by 10- cating areas high in incidence of trace-element asso- ciated endemic diseases of animals and man. Unfor— tunately, from an exploration standpoint, both man and animals are normally tolerant of anomalous amounts of copper (Scheinberg, 1970); however, excess amounts of other copper-related pathfinder elements do cause diseases. For example, the well- documented cattle disease molybdenosis (teart) is known to be related to the ingestion of excessive amounts of molybdenum. High concentrations of fluorine in drinking water cause mottling of human teeth. Other copper—related pathfinder elements that are thought to be detrimental to animals and man when present in anomalous amounts in the food chain might include lead, arsenic, mercury, cadmium, selenium, and probably others. It should be stressed that anomalous concentrations of trace elements can be caused by many natural and man-made factors. Therefore, the presence of any trace-element—related diseases within a region would not necessarily mean that the area in question has any potential for cop- per deposits. However, the application of this tech- nique, especially in the less developed countries of the world, might prove fruitful. ATMOSPHERE The results of analyses of the gases in the atmos- phere, and of the suspended particulate matter in the atmosphere, are proving valuable in geochemical exploration surveys for new mineral deposits. The detection of gases in the atmosphere can be accom— plished by using gas—analysis instruments mounted in aircraft or in motor vehicles. McCarthy (1972) and Bristow and Jonasson (1972) have recently re- viewed the use of airborne gas-analysis instruments for detecting mineral deposits, including those of copper. These authors noted that there are gases emanating from mineral deposits, and that these GEOCHEMICAL EXPLORATION TECHNIQUES emanations can be detected in samples of the atmos- phere collected above the deposits. The application of chemical analyses of the ele- ment mercury in the search for mineral deposits has received the most attention in the literature to date, probably because mercury has a known close asso- ciation with many types of sulfide mineral deposits, and because it is a very volatile element. Airborne gas analyzers have been used to detect mercury vapor in samples of the atmosphere collected above many different types of mineral deposits. McCarthy (1972) noted, however, that the concentrations of mercury in the atmosphere above porphyry copper deposits are apparently very low. He indicated that the measurement of mercury in the air above such deposits does not seem to be a useful means of de- tecting that type of copper deposit. The atmosphere above other types of copper deposits may contain higher levels of mercury. However, to date published data are not sufficient to indicate whether atmos- pheric mercury can be used to detect any type of copper deposit. Anomalous amounts of sulfur dioxide, resulting from the oxidation of sulfide minerals, have been de- tected in samples of the atmosphere collected several metres above the Tyrone, N. Mex., porphyry copper deposit, as well as above other sulfide deposits (Rouse and Stevens, 1971). The chemical variations of sulfur dioxide concentrations in the atmosphere are there- fore considered to be useful in the search for many types of sulfide deposits. Hydrogen sulfide should also be present wherever sulfide minerals are oxidiz- ing and should be detectable in the atmosphere. Because the decomposition of organic matter can also produce gaseous sulfur species, anomalous con- centrations of these compounds in the atmosphere must be carefully evaluated in order to determine the source of the gases. Other gases have been found associated with various types of ore deposits. Fridman and Makhlova (1972) discussed the use of carbon dioxide as a geo- chemical indicator of mercury ores. In addition to 002, they detected nitrogen, hydrogen, helium, and argon in the vicinity of mercury ore. The work of Fridman and Makhlova was done underground; how- ever, it does not seem unreasonable to expect that under the right conditions these same highly volatile species would reach the surface where they could escape and be detected in the atmosphere above the deposit. Halogen gases are' known to be associated with hydrothermal systems. The possibility of detecting anomalous amounts of these gases in the atmosphere B19 above copper deposits has not been adequately tested as yet. McCarthy (1972), without giving details, noted that bromine and iodine have been detected in the air over a porphyry copper deposit in Arizona. Bristow and J onasson (1972) also noted that iodine is associated both with porphyry copper deposits and with other types of base-metal deposits. The decay of 40K to 40Ar might produce anomalous concentrations of argon in the atmosphere above zones of potassium metasomatism. This argon could be analyzed with appropriate airborne instrumenta- tion. Other gases might be detected in geochemical surveys using samples of the atmosphere above cop- per deposits. It is possible that the oxides of nitro- gen could form in the zone of oxidation. And, as noted earlier, hydrocarbons, including methane and other compounds, have been found associated with ore deposits (Bristow and Jonasson, 1972; Shipulin and others, 1973). However, because many hydro- carbon compounds are formed from the decomposi- tion of organic matter, nonsignificant anomalies may also be present. Lastly, there may be cases in which the relatively high vapor pressure of such normally nonvolatile elements as arsenic, antimony, selenium, tellurium, and perhaps others, would make it pos- sible to detect these elements in the atmosphere above mineral deposits. Plants are known to give off water vapor during transpiration. Continuing studies by the US. Geo- logical Survey indicate that plants also exhale metal- lic compounds during transpiration (G. C. Curtin, oral commun., 1973). To date, the method has not been fully evaluated as a geochemical-prospecting technique (fig. 4); however, measurement of con- centrations of trace metals in the air around plants may prove to be a useful technique for detecting anomalous amounts of metals in the soil in which the plants are rooted. The results of analyses of particulate aerosols 'col- Iected from 30 to 60 m (100 to 200 ft) above the ground were used effectively as a rapid regional geo- chemical exploration technique for detecting copper and other types of mineral deposits (Weiss, 1971). The distribution of the size of the particulate matter was found to be related to the distance from the source of the material. The technique was found to be effective except over areas covered by water, snow, or thick tropical forest. REMOTE SENSING In this report the term “remote sensing” is de- fined as (1) the quantitative or semiquantitative FIGURE 4.-—P1astic bags are attached to conifer trees to collect exudates for geochemical analysis. Alpine environ- ment west of Denver, Colo. Photograph by G. C. Curtin, U.S. Geological Survey. measurement of the chemical concentrations of one or more elements or compounds in material in situ, or (2) the measurement of chemical effects on vege- tation as seen using photographs or other imagery. The remote-sensing instrument may be placed di- rectly against (or very close to) the sample to be analyzed, or the instrument may be located at a dis- tance of as much as several hundred kilometres from the sample site, as in the case of a satellite, but more likely at several thousands of metres, as from an aircraft. Remote-sensing geochemistry can be performed on land, in the air, and on or under bodies of water. Neutron-activation analysis is a remote-sensing technique that has not been widely used in geo- chemical prospecting, probably because the necessary equipment is expensive and somewhat bulky. The method involves bombarding a sample with neutrons and then measuring the resulting gamma-ray spec- trum. The technique is applicable to the detection of GEOLOGY AND RESOURCES OF COPPER DEPOSITS a number of elements, including copper. Because it measures the metal content in situ, this method offers two applications not readily available in other geochemical exploration techniques (Senftle, 1970). First, the apparatus can be adapted for down-the- hole measurements of various elements. The exact location and concentration of a given element can be readily determined. Second, the method offers means of locating copper or other metals beneath bodies of water. This second application could be particularly valuable as a relatively inexpensive method of evalu- ating the mineral potential of ocean or large-lake bottoms. Areas containing manganese nodules or other metal-rich sea-bottom deposits could be de- tected without having to collect a bottom or core sample for analysis at the surface. The remote detection of natural gamma radiation using a sensitive airborne gamma-ray spectrometer has been described by Bennett (1971) and by Mox- ham, Foote, and Bunker (1965). The airborne spec- trometer can detect natural gamma radiation emitted by potassium, uranium, and thorium. Several differ- ent types of copper deposits in Australia have been detected by this method (Bennett, 1971). Delinea- tion of the widespread zone of potassium metasoma- tism commonly associated with porphyry-type copper deposits was accomplished at several Arizona copper deposits by Moxham, Foote, and Bunker (1965). They also noted that potassium-thorium ratios should be valuable in delineating altered rock masses. Aerial photography appears to be a useful remote- sensing method for detecting and evaluating geo- botanical and biogeochemical anomalies associated with base-metal mineral deposits, especially in areas with heavy vegetative growth. Ordinary color pho- tography could be used to detect the presence or absence of, and the areal extent of, specific plants or plant communities. In addition, low-level color pho- tography could be used to detect indicator plants during their flowering period. From altitudes of about 150 to 300 m (500 to 1,000 ft) above the ground, I have observed the colorful blooms of the California poppy (Eschscholtzia mexicana) growing in masses over known copper deposits in Arizona. Areas containing these flowers should be detectable on color aerial photographs taken at similar alti- tudes. Infrared color film has been used to detect vege- tation that is unhealthy because of disease or insect damage. Some types of vegetation, however, become stressed because of an abnormal enrichment or de- pletion of trace elements in the material in which they are rooted (F. C. Canney, oral commun., 1972). GEOCHEMICAL EXPLORATION TECHNIQUES Plants growing in metal-enriched soils have spectral properties that may be detectable from remote loca- tions either by using infrared film or by using some sort of spectral scanning instrument (Marshall, 1970a ; Press and Norman, 1972). In regions contain— ing deep-rooted plants, such instrumentation could be used to detect metal anomalies in bedrock covered by relatively thin layers of transported overburden. Studies of the spectral properties of plants would therefore be especially valuable in the search for copper deposits buried by glacial debris or by gravel deposits such as those commonly present on the desert pediments in the porphyry—copper province of the southwestern United States. The technique could also be used over areas of dense vegetation, such as tropical jungles, where bedrock cannot be readily sampled and little or no outcrop can be seen on aerial photographs. The stunting or low density of vegetation in some areas may be caused by high metal concentrations in the associated soils. Areas showing these effects should be readily visible on either color or black-and- white aerial photographs. Many humid tropical areas, however, are commonly covered with clouds for much of the year and are therefore not suitable for normal aerial photography. The effects of metal stress on vegetation in such areas might be evaluated by using airborne radar imagery. This imagery can be obtained through cloud cover and could be used to map the general height of the forest canopy and the density of growth in an area (F. C. Canney, oral commun., 1973). STATISTICS INTRODUCTION The most easily discovered metal deposits have already been found in most parts of the world. In the future, scientists will be looking for ways to distin- guish subtle geochemical anomalies of possible eco- nomic significance from increasingly complex geo- chemical background data. Today we are trying not only to discriminate between natural metal anom- alies and natural background populations, but also to distinguish between these natural populations and man-made populations caused by contamination and pollution. Clearly, new approaches to geochemical exploration are needed. The use of statistical tech- niques is one example. Various fields within the social, biological, and electrical engineering sciences have provided the basic statistical techniques that are now being adapted to problems in the interpre- tation of geochemical data. 1321 It should be stressed that the ultimate evaluation of any computer output must be related back to the real world of natural systems; in other words, the data output is only as good as the data input. In short, the computer supplements but does not re- place the geochemist. Statistical treatment of geochemical data was necessarily limited before the advent of computers for data processing. Most precomputer statistical evaluations were limited to the determination of such parameters as mean, median, and standard deviation in order to establish the limits of background and anomalous populations. Graphical distributions of data sets were also studied and indicated that, in general, the analyses of a given population follow more closely a lognormal distribution than a normal one. As with any natural system, exceptions to this generalization are known. Ratio-s of elements to each other, to regional background, or to Clarke values have also been helpful for showing trends of ele- ments both laterally and with depth. In the field or in localities where computers are not available, geo- chemists have determined threshold values and the ranges of background and anomalous values for even large data sets using cumulative frequency plots (Lepeltier, 1969). Precomputer statistical analyses of geochemical data sets were generally limited to the use of only one variable at a time (univariate analysis). The advent of computer data processing means that sev- eral or many geochemical variables can be studied simultaneously (multivariate analysis). In addition to studying the chemical data of a sample, geochem- ists can now compare conditions of the physical and chemical environment of the sample site or region with the sample analytical data. In short, computer processing of data has opened up a wide range of possibilities for evaluating geochemical data. In the following discussion, some examples are given of how statistical interpretation techniques might be applied to geochemical data. Several de- tailed reviews of this field have been published re- cently (Marshall, 1970b; Agterberg and Kelly, 1971; Rose, 1972 ; Nichol, 1973). The reader is referred to these papers for more specific applications of the techniques to geochemical exploration programs. CORRELATION ANALYSIS A simple correlation analysis measures the degree of association between two or more variables taken two at a time. A multiple correlation analysis meas- ures the combined effect of several variables on a selected variable. Correlation analysis could be val- B22 uable, for example, in determining which elements or other variables associate most closely with cop- per in any given sampling medium. Elements that are found to have a high degree of positive or nega- tive correlation with copper might serve as path- finder elements in an area under study. REGRESSION ANALYSIS Regression analysis attempts to equate mathe- matically one variable (the dependent variable) with a group of other variables (the independent vari- ables). When only two variables are used, the an- alysis is called a simple regression analysis; when more than two variables are involved, the analysis is called a multiple regression analysis. The tech- nique has been used in surveys to explain the varia- tion of a given element in terms of other elements and of other physical and chemical factors. For ex- ample, the variation of the concentration of copper in a stream-sediment sample could be compared with such other variables as (1) the concentrations of other elements in the sample; (2) the grain size of the sample material; (3) the mineralogical composi- tion of the sample; (4) the pH of the stream water; or (5) the width, depth, and velocity of the stream, in an attempt to determine which factors signifi- cantly influence the variability of copper in the sample and the relative importance of each factor. Once an equation for a given data set has been deter- mined, regression analysis can be used to calculate, for example, the theoretical copper concentration at a given site based on the independent variables for that site. The differences between the calculated cop- per concentrations and the measured copper con— centrations, called residual values, can then be com- puted. One can then study both the residual data and the theoretical data in an effort to find sites containing ore-related anomalies that could not be found using only the raw data. DISCRIMINANT ANALYSIS Discriminant analysis is a classification scheme for studying geochemical data. A commonly used type of discriminant analysis creates and uses a mathematical equation based on some combination of variables to maximize the differences between two populations. This technique requires data on two different populations to begin with. New data are mathematically compared with these two popula- tions and then assigned to one or the other on the basis of all similarities and differences. An ideal example of application of this method would be one in which there were, for a given area, a data set GEOLOGY AND RESOURCES OF COPPER DEPOSITS from a mineralized sample population and a data set from a background sample population. These data sets could be acquired in an orientation study. Equations derived from these two populations could then be compared with new data from samples col- lected for the entire survey to determine whether individual sites in the extended survey more closely represented anomalous or background conditions. FACTOR ANALYSIS Factor analysis is a method for identifying com- mon factors in the variables of a data set. A factor analysis program starts with a matrix of values similar to a correlation matrix of geochemical data and assigns the various elements of the matrix to groups (factors) based on a common variability of these elements. A Q-mode factor analysis investigates the inter- relationships between samples. The analysis com- pares the set of variables at each sample site with those at every other site and groups the samples into a smaller number of “typical” samples. From this technique, the relationship between samples may be deduced. Q-mode factor analysis can be used, for example, to group data in terms of geologic para- meters such as bedrock types. An R-mode factor analysis investigates the inter- relationships between variables. This type of analy- sis groups chemical data that vary sympathetically in all samples into a set of factors, each containing one or more of the original variables. R-mode factor analysis can be used to identify groups of elements that have common associations; for example, this technique can be used to find those elements that would be scavenged by the manganese and iron oxides found in stream sediments. CLUSTER ANALYSIS Multi—element data sets can also be classified into groups by the use of cluster analysis. A commonly used version of this technique, which can also be done in a Q- or R-mode, compares pairs of variables or samples, or pairs of pairs, and so on, and then ar- ranges these pairs (clusters) into a hierarchical dendritic diagram (dendogram) in which the vari- ables or samples are grouped according to levels of pairing. These levels are based on values for cor- relation coefl‘icients between cluster pairs. As an example, the method could be applied to a data set in an attempt, on the basis of selected chemical associa- tions, to separate clusters representing samples from mineralized areas and those representing samples from background areas. GEOCHEMICAL EXPLORATION TECHNIQUES TREND-SURFACE ANALYSIS The preceding methods have dealt mainly with analysis of numerical geochemical data in order to find common associations as an aid to interpreting the data. Several techniques are also available for showing objectively, in the form of a map, the spatial distribution of geochemical data. These methods are especially applicable to large data sets from samples collected from large regions. Trend- surface analysis is similar to multiple-regression analysis but involves the fitting of mathematical surfaces to data. In trend-surface analysis the de- pendent variable, a chemical or geologic variable, is equated mathematically to geographic coordinates. The variation of this dependent variable over the region sampled defines a mathematical equation rep— resenting a plane or curved (trend) surface. This trend surface can be contoured for study in a man- ner similar to a topographic contour map. The mathematical equation for any trend surface can be expanded within practical limits by adding several more terms to the equation. This addition has the effect mathematically of more precisely defining the variations in the dependent variable. Visually, the effect of adding more terms is a more detailed con- touring of the map. Trend-surface analysis should be valuable in studying the changes in a given variable such as copper content, over an area of any size; however, adequate data-point distribution is imperative if the method is to succeed. As is the case with multiple- regression analysis, trend-surface analysis can be used to compute theoretical values at each sample site, based on regional trends, to compare with the actual measured values at the site. The difference between the theoretical and measured values at a given site is the residual value for that site. Resi- dual values can be plotted and contoured for data evaluation. In a manner similar to that used in re- gression analysis, residuals can be used to remove regional geochemical trends from local variations. Theoretically, the residuals should then delineate anomalous areas that might be related to mineral deposits. Unfortunately, in natural systems, chemi- cal trends are seldom regular enough to make use of this method without taking into account other geological and chemical information. MOVING-AVERAGE ANALYSIS The moving-average analysis (also called a roll- ing mean analysis) is another popular technique for treating data spatially. In this technique, a search area large enough to include several data points on B23 a map is moved across the map. Values within the search area at any given time are averaged, and the mean value is plotted in the center of the search area. The degree of smoothing of the original data can thus be varied by changing the size or degree of overlap of the search area and by changing the manner of weighting and averaging the data within the search area. The technique can be used to smooth regional geochemical data in order to enhance re- gional geochemical trends because the technique tends to remove anticipated sampling and analytical noise from geochemical data. 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E., 1968, Rock in the box: Mining Eng., v. 20, no. 1, p. 49. fiU.S.GOVERNMENT PRINTING OFFICE: 1975 0—211-317/17 LOGICAL s NAL PAPER- umx ”mmw COVER PHOTOGRAPHS l 2 3 4 5 6 7 8 9 10 11 12 13 14 mwa—I \IO) . Asbestos ore . Lead ore, Balmat mine, N. Y. . Chromite-chromium ore, Washington . Zinc ore, Friedensville, Pa. . Banded iron-formation, Palmer, Mich. . Ribbon asbestos ore, Quebec, Canada . Manganese ore, banded rhodochrosite 10. 11. 12. 13. . Aluminum ore, bauxite, Georgia . Native copper ore, Keweenawan Peninsula, Mich. Porphyry molybdenum ore, Colorado Zinc ore, Edwards, N. Y. Manganese nodules, ocean floor Botryoidal fluorite ore, Poncha Springs, Colo. . Tungsten ore, North Carolina Copper Deposits in Sedimentary and Volcanoganic Rocks By ELIZABETH B. TOURTELOT and JAMES D. VINE GEOLOGY AND RESOURCES OF COPPER DEPOSITS GEOLOGICAL SURVEY PROFESSIONAL PAPER 907—C A geologic appraisal of low-temperature copper deposits formed by syngenetic, diagenetz'e, and epigenetic processes UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976 UNITED STATES DEPARTMENT OF THE INTERIOR THOMAS S. KLEPI’E, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Tourtelot, Elizabeth B. Copper deposits in sedimentary and volcanogenic rocks. (Geology and resources of copper) (Geological Survey Professional Paper 907—C) Bibliography: p. Supt. of Docs. no.: I 19.16:907—C 1. Copper ores. 2. Rocks, Sedimentary. 3. Rocks, Igneous. I. Vine, James David, 1921- joint author. II. Title. 111. Series. IV. Series: United States Geological Survey Professional Paper 907-C. TN440.T68 553'.43 76—608039 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024—001-02791—7 APPRAISAL OF MINERAL RESOURCES Continuing appraisal of the mineral resources of the United States is con- ducted by the US. Geological Survey in accordance with the provisions of the Mining and Minerals Policy Act of 1970 (Public Law 91-631, Dec. 31, 1970). Total resources for purposes of these appraisal estimates in- clude currently minable resources (reserves) as well as those resources not yet discovered or not currently profitable to mine. The mining of mineral deposits, once discovered, depends on geologic, economic, and technologic factors; however, identification of many deposits yet to be dis- covered, owing to incomplete knowledge of their distribution in the Earth’s crust, depends greatly on geologic availability and man’s ingenuity. Consequently, appraisal of mineral resources results in approximations, subject to constant change as known deposits are depleted, new deposits are found, new extractive technology and uses are developed, and new geologic knowledge and theories indi- cate new areas favorable for exploration. This Professional Paper discusses aspects of the geology of copper as a framework for appraising resources of this commodity in the light of today’s tech- nology, economics, and geologic knowledge. Other Geological Survey publications relating to the appraisal of re- sources of specific mineral commodities include the following: Professional Paper 820—“United States Mineral Resources” Professional Paper 926—“Geology and Resources of Vanadium Deposits” Professional Paper 933—“Geology and Resources of Fluorine in the United States” Professional Paper 959—“Geology and Resources of Titanium in the United States” CONTENTS Page Page Abstract ........................... Cl Mesozoic disseminated copper deposits—Continued Introduction .............. 1 Triassic ...................................................................................... C12 Acknowledgments ..................................................................... 1 Connecticut Valley and southeastern Pennsylvania 12 Concepts of genesis ................................................................... 1 Nacimiento, N. Mex ..................... 13 Definition of terms ............................................ 2 Guadalupe County, N. Mex.. l4 Geochemistry ..................................................... 3 Paleozoic disseminated copper deposits .......................................... 14 Problems of genesis of copper deposits 3 Permian- .............................. 16 Copper deposits forming now ......................................................... 4 Kupferschiefer, from England through Poland ~~~~~~~~~~~~~~~~ 16 Modern bog and lake deposits ................................................. 4 W851 Ural foreland, U-S~S~R- -------------------------------------------- 17 Copper from runoff ........................................................... 4 Creta, Okla. ........................... 17 Copper from ground water ............................................... 4 Garvin County, Okla. ........... 13 Deposits from copper—rich brines... 4 Guadalupe County, N. Mex ............................ 18 Salton Sea ................................................ 4 Southern Colorado and northern New Mexico ................ 19 Red Sea ........................................... 6 Carboniferous ........................................................................... 19 Cheleken Peninsula ........................................................... 6 Dzhezkazgan, Kazakh S.S.R ............................................... 19 New Britain and the Solomon Islands ............................. 6 Precambrian disseminated copper deposits ..................................... 19 Copper deposits related to the present cycle of weathering 6 African Copperbelt, Zambia and Zaire... 19 Colorado Plateau ................................................. 6 Udokan, Siberia ............................... 21 Comer-sulfide deposits related to crustal-Plate boundariES- 8 Northwestern Montana ............................................................. 21 Plate [CCtoniC thOTY TCViCWCd ------------------------------------------------- 8 South Australia ......................................................................... 23 Island-arc environment ............................................................. 9 White Pine, Mich. ..................................... 23 Divergent-plate-boundary deposits ........................................... 9 Diagenegis and metamorphism affect Copper ................................. 24 Cenozoic disseminated copper deposits ........................................... 10 Summary; metallogenic provinces and the Copper Cycle ............. 26 Tertiary ---------------------------------------- 10 Copper deposits and brines ...................................................... 25 30160, Mexico... 10 Brines as diagenetic agents ................................................ 26 COIOCOIO, Bolivia ----------------- 11 Brines as syngenetic agents ................ 27 Mesozoic disseminated copper deposits ........................................... 11 Ground water, copper, and red beds.. 27 Jurassic(?) and Triassic(?) ......................................................... 11 Targets for prospecting ,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 28 Wyoming fOId belt ------------------------------------------------------------ 11 References cited ................................................................................ 28 ILLUSTRATIONS Page FIGURE 1. Map of the world showing major lithospheric plates and Cenozoic disseminated copper deposits ......................................... C5 2—4. Photomicrographs 2. Dolomite filling of voids in Nugget Sandstone ............................................................................................................. 7 3. Navajo Sandstone cemented by malachite and with remaining pores filled with cuprite and chrysocolla. 7 4. Spicular limestone from the Kaibab Limestone replaced by chalcedony, malachite, and chrysocolla ........................ 8 5. Diagram of plate tectonic hypothesis and its relation to copper deposits ................................................................................. 9 6. Map of the western United States showing areas of stratabound copper deposits in rocks of Mesozoic and Paleozoic ages .......................................................................................................................................................................... 12 7. Photomicrograph of Nugget Sandstone showing chalcopyrite associated with bituminous mater1al.. 13 8. Photomicrograph of sulfide replacement of fossil wood from the Chinle Formation .............................................................. 13 9. Photograph of fossil wood replaced by massive chalcocite ........................................................................................................ 14 10. Map of the world showing areas of disseminated stratabound copper deposits in rocks of Paleozoic and Precambrian ages ................................................................................................................................. . 15 11. Photograph of a sample of the Kupferschiefer showing fossil fish remains ............................................................................. 16 V VI CONTENTS Page FIGURE 12—14. Photomicrographs: 12. Chalcocite bleb in carbonate-cemented Garber Sandstone of Permian age .................................................................. C18 13. Chalcocite disseminated in arkosic siltstone from the Artesia Group of marine Permian rocks ................................. 19 14. Chalcocite and bornite interlocked with quartz and feldspar grains, and chalcopyrite and bornite in a feldspathic matrix. Quartzite from the Revett Formation ........................................................................................................ 22 15. Photograph of a sample of quartzite from the Revett Formation showing weathered rind and rich sulfide ore.. 28 16. Photograph and photomicrograph of a cupriferous drill core of Upper Callanna Beds of Precambrian age ......................... 23 17—20. Photomicrographs: 17. Native copper associated with carbonaceous films in the Nonesuch Shale of Precambrian age...., ............................ 24 18. Native copper associated with opaque bituminous matter in the Copper Harbor Conglomerate of Precambrian age .............................................. 24 19. Quartzite from the Revett Formation, showmg impermeable nature of feldspathic quartzite as a result of low- grade metamorphism ............................................................................................................................................... 25 20. Argillite from the Revett Formation, showing quartz veinlet in a sericite-biotite rock ............................................... 25 GEOLOGY AND RESOURCES OF COPPER DEPOSITS COPPER DEPOSITS IN SEDIMENTARY AND VOLCANOGENIC ROCKS By ELIZABETH B. TOURTELOT and JAMES D. VINE ABSTRACT Copper deposits occur in sedimentary and volcanogenic rocks within a wide variety of geologic environments where there may be little or no evidence of hydrothermal alteration. Some deposits may be hypo» gene and have a deep-seated source for the ore fluids, but because of rapid cooling and dilution during syngenetic deposition on the ocean floor, the resulting deposits are not associated with hydrothermal alteration. Many of these deposits are formed at or near major tectonic features on the Earth’s crust, including plate boundaries, rift valleys, and island arcs. The resulting ore bodies may be stratabound and either massive or disseminated. Other deposits form in rocks deposited in shallow-marine, deltaic, and nonmarine environments by the movement and reaction of inter- stratal brines whose metal content is derived from buried sedimentary and volcanic rocks. Some of the world’s largest copper deposits were probably formed in this manner. This process we regard as diagenetic, but some would regard it as syngenetic, if the ore metals are derived from disseminated metal in the host-rock sequence, and others would regard the process as epigenetic, if there is demonstrable evidence of ore cutting across bedding. Because the oxidation associated with diagenetic red beds releases copper to ground-water solutions, red rocks and copper deposits are commonly associated. However, the ultimate size, shape, and mineral zoning of a deposit result from local condi- tions at the site of deposition—a logjam in fluvial channel sandstone may result in an irregular tabular body of limited size; a petroleum- water interface in an oil pool may result in a copper deposit limited by the size and shape of the petroleum reservoir; a persistent thin bed of black shale may result in a copper deposit the size and shape of that single bed. The process of supergene enrichment has been largely overlooked in descriptions of copper deposits in sedimentary rocks. However, supergene processes may be involved during erosion of any primary ore body and its ultimate displacement and redeposition as a secondary deposit. Bleached sandstone at the surface may indicate significant ore deposits near the water table. INTRODUCTION Copper is one of the earliest discovered and most widely used metals. However, our knowledge of the distribution and origin of economically workable deposits is incom- plete and fragmentary despite long familiarity with the metal. This report will examine the great variety of copper deposits in sedimentary rocks and discuss the similarities and differences between deposits. To facilitate and clarify discussion, some words commonly used in economic geology are redefined here and other terms are inten- tionally avoided because their meaning has become blurred through the years. The main thrust of this paper is to explain the occurrence and genesis of sedimentary copper deposits, therefore resource and reserve data are not tabulated in this report. The reader interested in precise reserve data is referred to McMahon (1965), Kinkel and Peterson (1962), Cox and others (1973), and Pelissonnier and Michel (1972). Concepts of genesis definitely influence exploration for and development of copper deposits; the size, shape, and tenor of an ore deposit are controlled by its mode of forma- tion. In developing a mine, the geologist needs to know whether a deposit is likely to be large or small, spotty or persistent, whether it is associated with a ground-water table or a petroleum-brine interface, and whether the primary control of the deposit is lithologic, stratigraphic, or structural. The size and primary controls of a deposit are influenced by its genesis—Is it syngenetic, diagenetic, or epigenetic? Is it supergene or hypogene? What are the permeabilities of the associated strata? Are the associated strata marine or nonmarine? What is the structural setting of the associated rocks—Are they related to a plate margin or a stable craton? The answers to all of these questions are important in suggesting new prospecting areas as well as in developing known mineralized areas. Therefore this paper will emphasize theories of genesis and consider these many factors that influence ore deposits. ACKNOWLEDGMENTS We have benefited greatly from discussions with our col- leagues, who generously shared their knowledge of the various copper deposits of the world. While the opinions expressed here are our own, we deeply appreciate the helpful suggestions, references, and criticisms received during the writing of this report. We also thank Richard B. Taylor and Louise Hedricks for their help in preparing the photographs and photomicrographs used in this report. CONCEPTS OF GENESIS Development of concepts of genesis during the nine- teenth and twentieth centuries (see Stanton, 1972, p. 7-35, for a concise history of genetic theories) culminated in the Cl C2 domination of the epigenetic hydrothermal hypothesis for the origin of most major ore deposits. Although the veins at Butte, Mont., and Cornwall, England, are classic localities of hydrothermal ore deposition, the modern concept of a hydrothermal deposit is best illustrated by porphyry copper deposits. In a porphyry copper deposit, mineralization is irrefutably related to magmatic intru- sion; concentric zones of hydrothermal alteration center in the upper part of the intrusion and extend outward into country rock (Lowell and Guilbert, 1970, fig. 3; Sillitoe, 1973). The igneous body is generally regarded as the source of heat, metals, and volatile constituents responsible for alteration and mineralization. Ore minerals are pre- cipitated in veins or disseminated grains in a porous or chemically reactive rock by a decrease in temperature and pressure and a loss of volatiles. This hydrothermal-mag- matic hypothesis has served well in the exploration and development of many major mining districts throughout western North and South America (Sawkins, 1972; Sil- litoe, 1972b, c). However, recent oxygen isotope studies, such as those described by Taylor (1973, 1974), show that a large proportion of meteoric water is involved in the pro- cesses of alteration and mineralization of some hydro- thermal deposits. This raises the question of whether the other constituents of porphyry copper deposits and of other epigenetic hydrothermal ore deposits are derived from the magma or whether they are leached from intruded country rock, or more simply, which con- stituents originate where and in what proportions? The answer to this question may suggest new exploration targets. Many ore deposits, among them some of the world’s largest, show no relation to igneous intrusion and no hydrothermal alteration of country rock. They cannot be explained by conventional hydrothermal theories. The epigenetic-hydrothermal hypotheses have tended to dominate discussions of the genesis of ore deposits until recently. This predominance occurred even though a leading advocate of the hydrothermal hypothesis, Waldemar Lindgren (1933), included several chapters in his textbook “Mineral Deposits” on nonhydrothermal types of ore deposits, including one entitled, “Deposits Formed by Concentration of Substances Contained in the Surrounding Rocks by Means of Circulating Waters.” This is essentially what older workers called “lateral se- cretion,” but the term and concept have been so dis- credited that they are now rarely used. Instead, we have people writing in the following terms, “The * ‘1‘ * orebody is a massive hydrothermal cupriferous pyrite deposit, averaging 4 percent copper. It is of some interest in that hydrothermal wallrock alteration is almost entirely absent ’*‘ * *” (John, 1963, p. 107). A period of uranium exploration, mostly by small inde— pendent companies or petroleum companies, beginning in the early 1950’s and still active, employed petroleum geologists for uranium exploration—individuals who had GEOLOGY AND RESOURCES OF COPPER DEPOSITS no personal commitment to the hydrothermal hypothesis but who were familiar with concepts of migration of fluids and diagenesis in sedimentary rocks. The success of the application of concepts developed by petroleum geolo- gists to the search for uranium in sedimentary rocks suggests that the same concepts might be equally useful in the search for other metals that are mobile under con- ditions of low temperature and pressure, including copper. DEFINITION OF TERMS Many copper deposits in sedimentary rocks show no evidence of hydrothermal alteration. If the deposits have lateral persistence and are concordant with the strati— fication of other sedimentary rocks, they are generally called stratiform. If they are confined to specific strati- graphic horizons on a large scale but are discordant on a small scale, or if they gradually cut across local bedding planes, they are generally called stratabound. (These definitions, modified slightly, are from Stanton, 1972, p. 541.) In some districts stratiform deposits occur at several different horizons and in other districts stratabound deposits may occur in rocks of several different lithologies. One of our problems is to find unifying themes among the great variety of deposits that exist in sedimentary rocks. (See Dunham, 1969, for further discussion.) In this paper the terms “stratiform” and “stratabound” will be used for copper deposits in sedimentary rocks, instead of “red-bed” copper deposit. Red-bed copper deposit implies that the copper deposits in sedimentary rocks are in intimate association with red-colored rocks. Because this is not always so, the term ”red-bed type” has been stretched in many instances to the point that it is meaningless. For clarity, the other terms that we will be using frequently are defined: Syngenetic.—Minerals deposited or formed simultaneously with the enclosing sediment. Diagenetz'c.—Postdepositional formation of new minerals by equi- librium reactions between the original sedimentary rock constituents, both detrital and chemical, and interstitial fluids and gases from within the sequence. By implication, the elements making up the new min- erals were present in the sedimentary sequence at the time of deposition. Epigenetic.—Postdepositional formation of new minerals, especially ore minerals, by chemical reactions between the original sedimentary rock constituents and solutions from an external source. Historically the term has implied hydrothermal solutions of magmatic origin, but in recent years it has been extended to ground waters of meteoric origin, which might have been introduced into aquifers after tectonic uplift and truncation. Hypogene.—Describes ascending solutions, generally in the form of volcanic exhalations or hydrothermal waters. May produce syn- genetic deposits on the sea floor or epigenetic deposits in preexisting rock. Supergene.—Describes descending solutions, generally meteoric waters entering ground water systems. Applied most frequently to the oxidative destruction of a preexisting ore body and the formation of an enriched mineralized zone at greater depth, but it could be applied to the mobilization of disseminated ore minerals and redeposition of an ore body. Diplogenetic.—A term proposed by Lovering (1963) for SEDIMENTARY AND VOLCANOGENIC ROCKS mineral deposits whose elements are in part syngenetic and in part epigenetic. A possible example is the White Pine copper deposit in Ontonagon County, Mich., where several authors, including White and Wright (1966) and Ensign and others (1968), have suggested that the iron in preexisting pyrite was replaced by copper from copper—bearing solutions. Hydrothermal.—Means hot water. It can be either deeply circulating ground water in an area of high temperature gradient or water with components of juvenile water from igneous activity. Tons—A11 tonnages in this report are in metric tons. Most of the figures quoted are rounded and (or) estimates and the per- cent difference between short tons, metric tons, and long tons is less than the margin of error. GEOCHEMISTRY Copper is not included within the crystal structure of common rock-forming minerals; it most commonly occurs as minute grains of chalcopyrite in crystalline rocks (Goldschmidt, 1954, p. 182). Turekian and Wedepohl (1961, table 2) estimated that granitic rocks contain an average of 10-30 ppm copper and that basaltic rocks contain an average of 87 ppm. Some mafic rocks are reported to contain as much as 1,000 ppm copper (Dunham, 1972). Samples of biotite separated from felsic intrusive rock associated with porphyry copper deposits contain as much as 1 percent copper (Lovering, 1972, p. Dll), probably as minute sulfide inclusions. In the zone of oxidation and weathering, copper sulfide minerals are unstable, releasing copper ions which are carried in solution with any of the common anions, such as carbonate, sulfate, or chloride (chloride probably being the most effective under many conditions). In sedi- mentary environments, copper tends to stay in solution where oxygen amounts are sufficient to maintain a positive Eh and a low pH exists. At a pH greater than 6.3, the copper combines with carbonate or sulfate to form compounds of low solubility, which are then transported as suspended material (Strakhov, 1961). Very early in the evolution of the Earth’s atmosphere, before free oxygen began to accumulate as a result of photosynthesis (Cloud, 1971), copper sulfides, like other heavy minerals, were probably stable enough to be transported and deposited in placer deposits. In later Precambrian time enough free oxygen existed to oxidize the copper sulfides, but a high concentration of C02 in the atmosphere caused a low pH of river water (Strakhov, 1962) and copper remained in solution much longer and traveled farther than it does today. Evolution of the Precambrian atmosphere may be an important factor in the occurrence of major copper deposits in association with sedimentary rocks of middle to late Precambrian age. Some Precambrian iron deposits, such as on the Keweenaw Peninsula of Michigan (James and others, 1968; Lougheed and Mancuso, 1973), contain evidence of Precambrian organisms and are older than the copper deposits in the same region (Ensign and others, 1968; White, 1968). Bakun, Volodin, and Krendelev (1964) have actually suggested the older “Archean” iron deposits C3 in the Aldan shield as the source of copper found in the Precambrian Udokan copper deposit in Siberia. Ordinarily, during the geologic cycle of weathering, transport, and sedimentation, copper is dispersed rather than concentrated. Copper carried to the sea in streams is usually dispersed throughout a very large quantity of marine sediment, although it may be adsorbed on clay minerals, organic matter, or manganese nodules, and somewhat concentrated. Manganese nodules studied by Mero (1962) contained an average of 0.8 but as much as 2.9 percent copper. For average sedimentary rocks, Turekian and Wedepohl (1961, table 2) listed the copper contents as shale, 45 ppm; sandstone, X ppm [1—9 ppm]; and lime- stone, 4 ppm. In black shale the mean copper content was found to be 70 ppm (Vine and Tourtelot, 1970, p. 265-267); the higher copper content suggests some association with organic matter. Armands (1972, p. 20) found that uranium-rich Cambrian alum shales (which are generally organic-rich, too) from the Billingen area, Sweden, contain 180—190 ppm copper. Samples of pyrite separated from these shales contained five times as much copper as the enclosing shale (Armands, 1972, p. 58—62). In contrast, the Meade Peak Phosphatic Shale Member of the Phos- phoria Formation of Permian age in western Wyoming and eastern Idaho, which contains local concentrations of vanadium (McKelvey and Strobell, 1955) and uranium (Sheldon, 1959) and averages about 5 percent organic carbon, contains a mean of only 100 ppm copper, although it is enriched (compared to other black shales) in chromium, lanthanum, molybdenum, yttrium, ytter- bium, and zinc (Vine, 1969, p. 014; Vine and Tourtelot, 1970). The suite of elements enriched in the Meade Peak appears to represent the elements most likely to be con- centrated in a marine environment by normal marine processes, and suggests that special circumstances would be required to form a purely syngenetic marine copper deposit. PROBLEMS OF GENESIS OF COPPER DEPOSITS It is important to consider what special circumstances might produce an economic deposit of copper in sedi- mentary rocks. In many cases, there .are intriguing hints about factors important in the formation of deposits that we do not completely understand. We do know that no one theory of genesis will explain all ore deposits. Because we suspect that many copper deposits result from many different processes acting on sedimentary and volcanic rocks throughout their geologic history, and because many of these processes are obscured by later events and are difficult to decipher, we wish to avoid any classification scheme that carries connotations of genesis with it. In this paper, we will discuss first copper deposits forming today or recently formed, then massive sulfide deposits, and then generally older deposits starting with the youngest and concluding with the most ancient. Some discussion of C4 genesis will be included in the deposit descriptions, and finally we will try to sum up unifying factors. The main emphasis is on stratiform and stratabound deposits of syn- genetic and diagenetic origin. For this reason, discussion of vein deposits in sedimentary rocks and replacement deposits in limestones and dolomites has been omitted. The deposits discussed were selected on the basis of economic importance, and (or) the availability of information about them, and (or) their capabilities for illustrating important factors in ore formation. We hope to show the potential economic importance of strata- bound copper deposits and to stimulate research and exploration for them. COPPER DEPOSITS FORMING NOW MODERN BOG AND LAKE DEPOSITS COPPER FROM RUNOFF T. S. Lovering (1927) described the occurrence and genesis of spongy masses of native copper in a peat bog in the Beartooth Mountains near Cooke City, Mont. (fig. 1). The copper occurs in thin beds of black muck, but not in the in terbedded layers of sand and gravel. Several bodies of cupriferous pyrite occur in the Precambrian crystalline terrane of the surrounding mountains, and Lovering suggested that the copper is transported in solution in surface waters from the older deposits as cupric sulfate. In the bog, the biochemical action of bacteria at least partly causes the precipitation of the native copper. Near Jef- ferson City, Mont., (fig. 1), a similar deposit of native copper occurs in a peat bog downstream from copper- sulfide-bearing veins in the Boulder batholith (Forrester, 1942). However, there the copper is associated with limonitic bog iron which may be the precipitating agent for copper. In both of these instances, the deposits of native copper are small, but are clearly the result of weathering, transport, and deposition under surface conditions. COPPER FROM GROUND WATER Another copper-rich peat deposit, in southeastern New Brunswick, Canada (fig. 1), is slightly different from the two just discussed. Fraser (19613, b) said that although the forest peat contains as much as 10 percent copper (dry weight), no copper minerals were visible in the peat. He thought that the copper is organically bound, possibly as a chelate, and furthermore, rather than being brought in by surface water, the copper probably entered the swamp in ground water that picked up copper from the underlying nonmarine Pennsylvanian sandstones known to contain disseminated cupriferous pyrite. In the southern Ural Mountains, U.S.S.R., there is a pond that is colored blue by the large amount of copper it contains. As described by Igoshin (1966), spring waters feeding the pond, which are also blue, contain as much as 38 ppm copper as well as other metals. Geophysical and GEOLOGY AND RESOURCES OF COPPER DEPOSITS geochemical data are purported to indicate a local source of the spring water and its metals. The rocks near the spring are enriched in metals, and Igoshin interpreted that to mean that the metals are brought to the surface from depth. The geology of the area is diverse and the exact source of the metals is uncertain. These examples of copper-rich bogs and lakes reflect the mobility of copper in the zone of weathering and in any porous rock with oxidizing ground water flow. Con— versely, they also illustrate ways in which copper can accumulate, and they may be analogous to some ancient copper deposits. DEPOSITS FROM COPPER-RICH BRINES Besides surface waters or meteroic ground waters enriched in copper from the leaching of older copper deposits or copper-rich sedimentary rocks, other copper- rich brines are known that involve varying proportions of meteoric, connate, metamorphic, and magmatic waters. Brines enriched in metals have been discovered and studied along the East Pacific Rise (Corliss and others, 1972; Dymond and others, 1972; Bostrom and Peterson, 1966). Corliss (1971) suggested that the brines along mid- oceanic ridges become enriched in several elements in the following way—as the basalts that have been extruded along the ridges cool, they develop contraction cracks, and the chloride complexes in seawater mobilize the elements as seawater moves in and through these cracks. Moore and Calk (1971) identified microscopic spherules of iron, copper, and nickel sulfides in the glassy margins of pillow basalts. They suggested that the constituents in the sulfides are derived by diffusion from the cooling basalt. SALTON SEA The East Pacific Rise continues northward from the Pacific Ocean into the Gulf of California and then inland under the Imperial Valley of California. A well drilled for geothermal power near the Salton Sea (fig. 1) tapped a saline brine with an extremely high heavy-metal content (White and others, 1963). The brine from the well deposited siliceous scale at the rate of 2—3 tons per month containing an average of 20 percent copper and as much as 6 percent silver (Skinner and others, 1967). The brine is a Na-Ca-Cl brine and contains 500 ppm Zn, 90 ppm Pb, and 6 ppm Cu, but only 15—30 ppm total sulfide (D. E. White, 1968, p. 313). White (1968) decided that the major com- ponent of the brine was from meteoric water and that the high salinity was from the dissolution of evaporites at depth. Helgeson (1968) suggested that the high salinity was, instead, the result of evaporative concentration of connate water. Heat from magmatic activity at depth caused convective circulation of the ground water which leached the metals from the enclosing sediments (Helgeson, 1968). If undisturbed, the brines might or might not eventually form an ore deposit. C5 SEDIMENTARY AND VOLCANOGENIC ROCKS .mgzom 659980 .2 .8182 .020m .2 .mBSm USED .3833 0920—00 6 .mvnflmm COED—om 2,: v5“. 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Modified from Warren Hamilton (1969), Dewey and Bird (1970), Guild (1972a, b), and Sillitoe (1972a). either case—continent-oceanic plate collision, or con- tinent-island arc collision—where massive sulfide de- posits are present, they may be incorporated into the continental crust as part of a eugeosynclinal facies. The metamorphism that accompanies these processes may make it difficult to distinguish between those deposits that originated at a spreading center and those that formed in an island arc. Regardless of origin, eugeosynclinal facies commonly contain massive sulfide deposits, and plate boundaries are commonly the site of mineral deposits (figs. 1, 5). Guild (1973) published a map (shown in fig. 1) showing many of the massive sulfide deposits that may be associated with plate boundaries. Therefore, instead of completely cluttering his map, on figure 1 we are showing only the disseminated deposits that we discuss in some detail. ISLAND-ARC ENVIRONMENT Stratiform and stratabound copper deposits that occur in an island—arc environment may be massive and inter- bedded with calc-alkalic lavas, or they may be dis- seminated through rhyolitic tuff or through lagoonal facies shale and marlstone. Some of the ore deposits asso- ciated with volcanic arcs are syngenetic, formed by fumarolic activity that causes metal-rich solutions to pour out onto the ocean or lagoon floor. We have already discussed a possible modern example of this along the coasts of New Britain and the Solomon Islands (p. C6). Other deposits are epigenetic, formed by the alteration of volcanic rock and sediment as the solutions ascend to the surface. Many deposits, such as the Miocene Kuroko in Japan (Watanabe, 1957; Matsukuma and Horikoshi, 1970; Sato, 1971; L. A. Clark, 1971), may show complete gradation from epigenetic to syngenetic. The epigenetic part of the deposit may consist of pipelike alterations around ascending solutions. The syngenetic part of the deposit is represented by the sulfides precipitated with sea- floor mud and volcanic ash as the metal-rich solutions reacted with seawater. The range of deposits possible in an island-arc environ- ment is represented by the deposits that occur in the Tasman orogenic zone of Australia, such as Rosebery (Brathwaite, 1974) and Mount Lyell in Tasmania; Mount Morgan, Queensland; and Bathhurst and Captains Flat, New South Wales (Solomon and others, 1972). Kuroko and similar deposits in Japan (L. A. Clark, 1971 ; Ishikawa and others, 1962) are classic examples of island-arc deposits. Other probable island—arc deposits include many of the massive sulfide deposits of Ordovician age in the Appalachian region (Heyl and Bozion, 1971), including some of the deposits in Newfoundland (Baird, 1960), Stirling, Nova Scotia (Keating, 1960), Caribou (Davis, 1972), St. Stephen, Bathhurst, Newcastle (McAllister, 1960), and Brunswick Mining and Smelting ore bodies (Stanton, 1959), all in New Brunswick, and Elizabeth, Vt. (White, 1943). Sillitoe (1972a) suggested that Rio Tinto and similar deposits in Spain and Portugal are of island- arc origin. Most likely, the deposits of the Folldal district of Norway (Waltham, 1968), some of the deposits in the Caucasus (Tvalchrelidze and Buadze, 1964), the 19th Party Congress deposit in the southern Ural Mountains (Boriskov, 1966), and Altai in Kazakhstan (Shcherba, 1971) are all examples of island-arc deposits. This is by no means a complete list of possible island-arc massive sulfide deposits. DIVERGENT-PLATE-BOUNDARY DEPOSITS There appear to be two types of Stratiform deposits formed at the sites of spreading centers or rifting. The first type, generally a massive sulfide deposit, is that found in C10 ophiolite sequences and described in detail by Sillitoe (1972a). The second type is disseminated. The massive sulfide deposits are generally associated with extrusive tholeiitic basalt and probably “formed initially as accumulations of brine-soaked sulphide mud in de- pressions on the sea floor” (Sillitoe, 1972a, p. B142). The Troodos Massif in Cyprus is the best known example of this type of deposit because it has not been greatly meta- morphosed (Corliss and others, 1972; Elderfield and others, 1972; I... A. Clark, 1971; Hutchinson and Searle, 1971; Constantinou and Govett, 1973). Similar deposits are found in southeastern Turkey (Borchert, 1957), especially at Kiire (Suffel and Hutchinson, 1973) and Ergani-Maden (Griffitts and others, 1972; Sire], 1950). Other examples are Betts Cove and Tilt Cove, Newfound- land (Upadhyay and Strong, 1973); Balabac Island, Phil- ippines (John, 1963); Mugodzhar Hills in the southern Ural Mountains (Ivanov, 1965); the Amasia-Akerin de- posits in the Trans—Caucasian region (Tvalchrelidze and Buadze, 1964); West and East Shasta, Calif. (Kinkel and others, 1956), and some of the Appalachian deposits (Kinkel, 1967; Heyl and Bozion, 1971). (The preceding authors are cited for their descriptions of the deposits; they would not necessarily agree with our interpretation of genesis.) In many areas, such as the Appalachian Moun- tains, Scandinavia, the Ural Mountains, and the Philip- pines, both spreadingocenter and island-arc deposits may occur. Many of the Precambrian and also many of the younger deposits are too metamorphosed for their origins to be certain. Commonly they are associated with eugeosyn- clinal rocks and may have been tectonically emplaced after they were formed. Also, we cannot be sure that the plate tectonic regime was the same in Precambrian time as it appears today (Burke and Dewey, 1972; Sillitoe, 1972a; Hutchinson, 1973). Therefore, this pattern cannot be used to classify many massive sulfide deposits of Precambrian age, such as Otokumpu, Finland (Peltola, 1960); Mogador (Geoffroy and Koulomzine, 1960), Mattagami (Mackay and Paterson, 1960), and Noranda (Sinclair, 1971) in Quebec; Encampment, Wyo. (Spencer, 1904); Ducktown, Tenn. (Mauger, 1972); and Jerome, Ariz. (Anderson and Nash, 1972). For many other deposits, the published de- scriptions are such that classification is ambiguous. Also, various advocates of the theory of the relation of plate tectonics and ore deposits may classify the same ore deposit differently. However, thinking of these deposits as essen- tially syngenetic rather than as replacement deposits may suggest new areas and a more systematic approach for exploration. Thus, at the site of spreading centers or at island arcs, ore may form syngenetically—that is, the metals pre- cipitate at the surface because of some change in Eh or pH and are deposited with the enclosing sediments—~or the ore may form diagenetically or epigenetically if the metals are GEOLOGY AND RESOURCES OF COPPER DEPOSITS precipitated as the brine moves upward through already deposited sediments and tuffs. Both syngenetic and dia- genetic or epigenetic components can be present in the same deposit. D. E. White (1968) pointed out that many brines are sulfur deficient in comparison to their metal contents. Sulfur may be syngenetically deposited with iron by the action of anaerobic bacteria and metals, such as copper, added later by circulating brines; together, they result in the formation of deposits termed “diplogenetic” by Lovering (1963). White (1968) and also Helgeson (1968) suggested that the metals are not necessarily a juvenile contribution of the mantle or subsurface magmas but that they may have been leached from the rocks through which the brine is flowing. Although volcanism is associated with spreading centers, periods of quiescence are observed, such as in the Red Sea and Gulf of California areas today. Therefore, the deposits formed at the sites of spreading centers are not necessarily closely associated with volcanic rocks. Many of the same factors producing an ore deposit at a spreading center can operate at the site of tensional rifting within plates. Sites of rifting are sites of high heat flow. The rifting also produces grabenlike basins that re- strict circulation of water and may enhance the formation of evaporites, and thus saline brines. (For example, see Baker and others, 1972, for a detailed account of the rift— ing of eastern Africa.) The combination of high heat flow and saline brines can result in a mineralizing solution that can precipitate an ore deposit in a suitable environ- ment. In fact, the high heat flow and saline brines are the most pertinent factors in producing an ore deposit. CENOZOIC DISSEMINATED COPPER DEPOSITS TERTIARY BOLEO, MEXICO The Boleo copper district adjacent to the Gulf of California in Baja California Sur, Mexico (fig. 1) appears to be an excellent example of a disseminated copper deposit related to diverging plates. Sillitoe (1972c) sug- gested that this deposit is related to the East Pacific Rise, and Nishihara (1957) suggested a syngenetic origin. As de- scribed by Wilson (1955), the copper ore occurs at five dif- ferent horizons in soft dark beds of altered tuff in the Boleo Formation of Pliocene age. The Boleo Formation was deposited unconformably as a delta over the Comondu volcanic rocks of Miocene age. Although the district is almost mined out now, the ore mined during 1886-1947 averaged 4.8 percent copper, and about 500,000 tons of copper were produced to June 1947 (Wilson, 1955). The principal ore mineral is chalcocite; gangue minerals include montmorillonite, gypsum, calcite, chalcedony, and jasper. The ore occurs in bands, lenses, and nodules (boleos) in the tuff beds, each above a conglomerate bed. The copper deposits partly overlap the Lucifer manga- SEDIMENTARY AND VOLCANOGENIC ROCKS nese deposits (Wilson and Veytia, 1949) to the northwest. In the Boleo deposit, again there is the combination of high heat flow (the East Pacific Rise), evaporite beds (the gypsum), and reactive host rocks (the tuff beds). In- terestingly, manganese deposits have also been found in the Afar depression, Ethiopia, in Miocene sediments (Bonatti and others, 1972) associated with the rifting of the Red Sea. COROCORO, BOLIVIA The Corocoro basin forms the western part of the Altiplano between the East and West Andes in Bolivia (fig. 1). Throughout this basin there has been extensive copper mineralization; Pe’llisonnier and Michel (1972, table 25) estimated that the basin contained more than 425,000 tons of copper. According to Ljunggren and Meyer (1964), the copper occurs in a thick sequence of first-cycle Tertiary sandstones and conglomerates, in a basin 31—50 mi (50—80 km) across that received sediments from mountains on both sides of the basin. The Tertiary rocks have been deformed by diapiric folds in the underlying Cretaceous (evaporite sequence; evaporites also occur in the Tertiary sequence. Ljunggren and Meyer went on to say that the copper minerals always occur in sandy or conglomeratic layers, usually as elongate lenses, are always stratabound, and are generally associated with fossil plants. They recognized two types of ore bodies—one with chalcocite associated with plant material in structural lows, and the other with chalcocite or native copper in structural highs where the copper minerals replace cement in sandstones and conglomerates. The first type is regarded as syngenetic—it is lower grade (as much as a few percent chalcocite replacing fossil plant material) and is found throughout a stratigraphic interval of l6,000—26,000 ft (5,000—8,000 m). The copper probably was derived from the, erosion of porphyry copper deposits in the western Andes and (or) the erosion of copper-rich basalt flows of the Altiplano. The second type of deposit is higher grade (as much as 20 percent copper) and occurs on the flanks of anticlines. The second type appears to result from dissolving the disseminated copper in ground water. The copper in the ground water is then precipitated by fossil plant remains (perhaps with the aid of reducing bacteria) or iron minerals on the flanks of anticlines. Many copper deposits in other parts of the world may have received their copper from the erosion of porphyry copper deposits. Sillitoe (1972c) has emphasized that porphyry copper deposits are usually emplaced at very shallow depths and hence many pre-Mesozoic porphyry copper deposits have probably been eroded away (Sillitoe, 1972c, p. 190—191). Thus, eroded porphyries could be the source of anomalous copper contents in many sedi- mentary sequences, particularly first-cycle sandstone and conglomerate sequences. At least one such deposit, in C11 Arizona, in basin fill associated with porphyry copper has been described by Throop and Buseck (1971). There may be others. MESOZOIC DISSEMINATED COPPER DEPOSITS JURASSICO) AND TRIASSICG) WYOMING FOLD BELT Copper deposits occur in the upper part of the Nugget Sandstone of Jurassic(?) and Triassic(?) age along much of the length of the southwestern Wyoming fold belt (fig. 6). Although mining was never systematically developed, at least several tens of thousands of tons of ore were shipped to smelters in Utah from the Griggs mine in the Lake Alice district, Lincoln County, Wyo., about 30 mi (48 km) north-northeast of Cokeville (Love and Antweiler, 1973). In their samples from the Griggs mine, Love and Antweiler (1973, p. 143) reported maximum values of 6.7 percent copper, 0.12 percent silver, and 3.2 percent zinc. Areas of copper minerals in Pennsylvanian and Triassic rocks of the fold belt were described in early reports (Veatch, 1907; Gale, 1910; Schultz, 1914), but the minerali- zation that occurred in the Nugget seems to be the most im- portant. The Nugget Sandstone in this area consists of as much as 2,000 ft (600 m) of pink to white quartzose sand- stone with very little matrix or cement except quartz over- growths and a little kaolinite or calcite or dolomite. Most of the formation is crossbedded eolian sandstone that contains no fossils by which its age might be more precisely determined. Some of the upper part of the Nugget is horizontally bedded, suggesting aqueous re- working of the eolian sands, perhaps in a marine environ- ment. The Nugget is overlain by the Gypsum Spring Member of the Twin Creek Limestone of Middle Jurassic age. The Gypsum Spring Member contains gypsum and anhydrite as well as dolomite and dolomitic siltstone (Love and Antweiler, 1973). Copper minerals occur in the upper 50 ft (15 m) of the Nugget at approximately the same stratigraphic position throughout the length of the fold belt (Love and Antweiler, 1973). About 23-24 mi (37—386 km) east of the Griggs mine, petroleum is produced from a fractured anticline in the Nugget at the Tip Top field, Sublette County, Wyo. (Wyoming Geological Association, 1957, p. 456—457). Samples of ore from the Griggs mine have dark-gray streaks that may represent petroleum residue (fig. 7). Thus, the copper mineralization may have followed some persistent feature such as a petroleum-water interface and the anomalously high metal contents were probably concentrated from the section of eolian sands and red beds of Jurassic and Triassic age in the area. Love and Antweiler (1973) also have pointed out the possible relation between the petroleum, the sulfate minerals of the overlying Gypsum Spring Member, and the copper mineralization. C12 GEOLOGY AND RESOURCES OF COPPER DEPOSITS l \ r ~~~~~~~~~~ 9 1 [NORTH DAKOTA! l l x 1 SOUTH DAKOTA] 1 I M; \, NEBRASKA \3 1 T E " EXPLANATION \\ X A b X Specific locality 1 \ r“- x \ r/ x" General area of copper mineralization \ \ ‘\ 1 x o 500 KILOMETRES k I__I_1__1_|_J E‘ FIGURE 6.—Areas of stratabound copper deposits in the Western United States in rocks of Mesozoic and Paleozoic ages. TRIASSIC CONNECTICUT VALLEY AND SOUTHEASTERN PENNSYLVANIA The Connecticut Valley and southeastern Penn- sylvania deposits of copper in arkosic sandstone and shale of the Newark Group and equivalent rocks of Triassic age have been locally mined since colonial times (Lewis, 1907; Wherry, 1908; Black, 1922; Bateman, 1923; Miller, 1924, p. 31—36; Stose, 1925; Cornwall, 1945). Some of these deposits appear to be diagenetic concentrations of disseminated copper similar to many of the deposits in nonmarine rocks of the southwestern United States. Other deposits in this area appear to be structurally controlled and genetically SEDIMENTARY AND VOLCANOGENIC ROCKS FIGURE 7.—Nugget Sandstone of Jurassic(?) and Triassic(?) age from Lincoln County, Wyo., showing chalcopyrite (cp) associated with bituminous material (B). Ore minerals may follow a fossil oil-water contact. Exposures by reflected light and by light transmitted through crossed polars; sample CU—28D. associated with intrusive and extrusive basalt or diabase “trap rock.” A map and list of 55 copper localities in southeastern Pennsylvania (Wherry, 1908) and a map of copper deposits in New Jersey (Lewis, 1907) show many deposits in a zone of low-grade metamorphism adjacent to the igneous rocks in which the sedimentary rocks are de- scribed as “baked.” Other deposits, such as the Bristol mine in Connecticut as described by Bateman (1923), lie along the fault contact between Triassic sedimentary rocks and older crystalline rocks. Lewis (1907) saw both chal- copyrite and native copper in traprock and suggested that the igneous rocks were the source of the copper. In light of the modern hypotheses on the relationship between plate tectonics and metallogenic provinces (for instance, Guild, 1971, 1972a, 1973; Sillitoe, 1972a, b, c), rifting and separation of the North American plate from the European plate was accompanied by igneous activity that produced the traprock (Bird and Dewey, 1970); possibly associated volcanic exhalations formed hypogene copper deposits. Thus, the copper deposits in the Triassic basins of the eastern United States may be of different types—one type formed by ground-water concentration of dis- seminated copper, and one type formed by hypogene C13 solutions comparable to the deposits now forming in the Red Sea area and in the Salton Sea-Gulf of California area. NACIMIENTO, NEW MEXICO In the southwestern United States, many copper deposits are found in nonmarine arkosic sandstone composed of debris shed from mountains uplifted in late Paleozoic and in early Tertiary time. (Soulé, 1956). A typical example, currently being mined, is the N acimiento copper deposit near Cuba, N. Mex. (fig. 6) (Kaufman and others, 1972; Woodward and others, 1973, 1974). Scartaccini (1973, p. 52) reported that 11 million tons of ore assaying 0.65 percent copper had been developed for open-pit mining as of January 1973. Chalcocite, bornite, other copper minerals, and small amounts of native silver replace large fossil carbonaceous logs, as shown in figure 8, and are also disseminated through sandstone of the Agua Zarca Member of the Chinle Formation of Triassic age. The Agua Zarca is at the same stratigraphic horizon, at the base of the Chinle, as the uranium-rich con- glomerate, the Shinarump Member in Utah. The Nacimiento ore body, as exposed by mining, lies at and above the water table on the west flank of the N acimiento uplift. An erosional remnant of the Agua Zarca, con- sisting of sandstone that is bleached and altered to nearly pure quartz by weathering, forms Eureka Mesa. This remnant extends for several miles up the flank of the FIGURE 8.—Sulfide replacement of fossil wood from the Agua Zarca Member of the Chinle Formation, at the Nacimiento mine, Sandoval County, N. Mex. Reflected light; sample CU—IOJ. C14 mountain east of the mine. In this weathered sandstone, evidence of the former existence of fossil wood is found as empty molds, except for a few places along the cliff margin of the mesa where chalcocite-bearing fossil wood has been exposed in long-abandoned prospect pits. Small deposits of copper probably existed throughout the Agua Zarca before the area was eroded to its present level. Most of the copper present updip from the Nacimiento mine may have been flushed down to the mine area, which now repre- sents part of a dynamic interface between oxidized and unoxidized sandstone, similar to the Wyoming roll-front type of uranium deposits. Moreover, the massive chalco- cite replacements of fossil wood, as shown in figure 9, suggest a possible zone of supergene enrichment similar in occurrence and mode of origin to the zones of supergene enrichment associated with various hydrothermal porphyry, vein, and replacement deposits. At the Naci- miento mine, bleached and altered quartz sandstone overlies the main ore body. Similar bleached sandstone may overlie other ore bodies and might be an aid in pros- pecting. FIGURE 9.—Fossi1 wood replaced by massive chalcocite at the Nacimiento mine, Sandoval County, N. Mex. Massive replacement may be evidence of supergene enrichment near the water table. Sample provided through the courtesy of Lyle Talbott, Geologist, Earth Resources Co. GUADALUPE COUNTY, NEW MEXICO Near the town of Santa Rosa, Guadalupe County, N. Mex., a tributary of the Pecos River cuts through the copper—bearing Santa Rosa Sandstone of Late Triassic age and exposes a section of rocks of the Artesia Group of Guadalupian Permian age that includes a basal gypsum bed overlain by a fine—grained marine sandstone that has GEOLOGY AND RESOURCES OF COPPER DEPOSITS been mined for copper at the Pintada mine (fig. 6), as will be described later (p. C18). On the south side of the canyon, copper has been mined from fluvial deltaic sandstones of Late Triassic age at the Stauber mine (Harley, 1940; Holm- quist, 1947; Soulé, 1956, p. 24—28). The Triasic deposit forms a tabular ore body about 1,500 by 320 ft (520 by 110 m) that C. B. Read, R. D. Sample, and J. S. Sheldon (written commun., 1943) described as a fossil sinkhole in the Triassic rocks, once the site of a bog containing abundant organic matter. They further suggested that the copper sulfides were precipitated from epigenetic copper- bearing solutions that flowed through the surrounding permeable sandstones. The Triassic deposit may also represent a syngenetic deposit formed in a manner similar to the formation of the copper-rich forest peat bog in New Brunswick, mentioned before (p. C4) and described by Fraser (1961a, b); this peat bog formed by ground water rising from the underlying copper-rich Permian rocks. The Permian marine rocks across the canyon contain ir- regular patches or streaks of chalcocite disseminated through the matrix of fine-grained sandstone or siltstone that contains very little organic matter and is cemented chiefly by gypsum and dolomite. Possibly the same solutions flowed through both deposits, but because the chemical environment of precipitation in each was dif- ferent, the resulting deposits differ in general form and appearance. PALEOZOIC DISSEMINATED COPPER DEPOSITS Small (containing less than 10,000 tons) stratabound deposits of copper occur in sandstone and shale in non- marine strata containing red beds in the Appalachian Plateau, the Appalachian fold belt, and the basins formed by Triassic block-faulting in- the eastern United States (Kinkel and Peterson, 1962). Many old prospects in the Catskill Formation of Devonian age were examined during the search for uranium (McCauley, 1957; Klemic, 1962; Klemic and others, 1963), along with similar prospects in the Mauch Chunk Formation of Missis- sippian and Pennsylvanian age and the Pottsville Forma- tion of Pennsylvanian age. None of the prospects were found to contain economically valuable amounts of either copper or uranium, but they do demonstrate the mobility of copper in the red-bed environment. In eastern Canada, copper deposits in rocks of Carboniferous age were mined locally in the late 19th century (Papenfus, 1931). Minerali- zation in the Pictou Formation of Late Pennsylvanian age occurs over an area of 500 mi2 (1,300 km?) in northern Nova Scotia. Although individual samples may contain as much as 67.7 percent copper (as logs replaced by chalco- cite), not enough copper occurs in any one locality to make mining economical (Brummer, 1958). Figure 10 shows the general location of Paleozoic and Precambrian dis- seminated stratabound copper deposits. C15 SEDIMENTARY AND VOLCANOGENIC ROCKS £qu USE 21:5 .2 .m__mbm:< £30m £50qu smuEflww< d define—2 .mzwoavw :om .w .miufiw £13034 K .BRN min «5sz J—fiSQQOU cwuifiw 6 .M.m.w 5—3an RammvafiucNQ Am .meum «>02 Cymsfioz :9 .3335 092200 6 .530 NEEO .m .M.m.m.D REE—Bow .92 33$ .N finflom smacks“ vim—ya .Ewowcumaflmzfi 4 .vaw :ESESBL HEN Egan—mm mo 318 E mimoaww fiance “522.3536 “0 wmeg o >5§EEOUEQ we NEON v.00: Eacwca I ZO—H "01/ / \500 /¢ 4} c PPERAS HILLC be 1 1k O NO TH 43°29’— K 03 mavens — mos r GRANI E ILL mm: aumv 0 .5 1 2 KILOMETERS l I l I I V I I 0 v2 1MILE CONTOUR INTERVAL 100 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 GEOLOGY AND RESOURCES OF COPPER DEPOSITS EXPLANATION ‘ ,- Essexite I'bmblende-bioflte syenlte Pulasklte Sodafite—mpknlite syenite + + 1 Augite-syenite @ Country lock not distingubhed except limestone, which is shown by pattern FIGURE 5.—Geologic map of the Cuttingsville pluton, Shrewsbury and Wallingford T0wnships, Rutland County, Vt., showing location of the Granite Hill molybdenum prospect. Modified from Eggleston (1918). PORPHYRY COPPER-MOLYBDENUM DEPOSITS, EASTERN UNITED STATES lite(?), titanite, magnetite, pyrrhotite, apatite, and zircon. The molybdenite prospect is along the west- ern margin of the pulaskite phase and separated from the main pulaskite mass by a thin screen or lens of essexite (fig. 5). Northward-trending streaks of very coarse grained phases of pulaskite occur near the western edge of the pluton near the ridge crest of Granite Hill, including the vicinity of the molyb- denite prospect. The pulaskite generally weathers to a rusty brown color that is locally related to the decomposition of sulfides; whether or not the brown color is everywhere related to disseminated sulfide is not certain. The analysis of copper and molyb- denum in some of the coarse phase of the pulaskite is shown in table 5. TABLE 5,—Copper and molybdenum analyses of four samples from the Cuttingsville pluton, Rutland County, Vt. {Copper was determined by atomic absorption spectrometry. Molybdenum was determined photometrically. Analysts: P. Aruseavage and E. Camp- bell, U.S. Geological Survey] Lab. No. Field No. Cu Mo (ppm) (ppm) Description W~191906-___JX20210____ 5 3.9 Hornblende biotite-syenite, grab sample from waste, quarry on Granite Hill (fig. 5). Hornfels or border phase at edge of pluton, 30 m north of molyb- denum prospect. Hornfels near contact, crest of Granite Hill, 110 m north of prospect. Coarse pulaskite, about 120 m north of quarry on Granite Hill. 7-__- 5---_ 6 5.8 8__-- 6-___60 6.0 9____ 8____ 5 8.8 Augite-syenite (nordmarkite) makes up the north- ern end of the Cuttingsville pluton (Eggleston, 1918, p. 398—401). Mostly medium grained, the grain ranges locally from fine to coarse. Seventy-five per— cent or more of the rock is feldspar, mostly micro- perthite, and also microcline, orthoclase, and albite. Green augite locally rimmed by green hornblende is the major dark mineral, and aegirite-augite, olivine, and primary quartz are minor constituents. Acces- sories are magnetite, zircon, apatite, and perhaps ilmenite. Although Eggleston detected no sulfide in the rock, he suspected it was there and reported that the rock gives a sulfurous odor on breaking, and weathers to a pale buff or very light brown. Essexite occurs in a large patch between the pulaskite and the augite-syenite, and also in several small scattered lenses and pods (fig. 5). It is a dark rock containing hornblende, pyroxene, and biotite in about equal quantities, and 60 percent feldspar, most of which is plagioclase (An=30—70 percent) (Eggle- ston, 1918, p. 388). Accessory minerals are olivine (partly serpentinized), magnetite, titanite, apatite, pyrite, and probably pyrrhotite. Eggleston (1918, p. 388) considered this phase to have been the first to crystallize. It is not known to be sulfide bearing. E19 The hornblende-biotite syenite is a gray medium- grained rock consisting of light-gray feldspar speckled by hornblende and biotite (Eggleston, 1918, p. 392—395). More than half the feldspar is plagioclase, and most of the orthoclase is present as antiperthitic intergrowths in the plagioclase. Biotite and hornblende are about equal in quantity, and to- gether make up about 10 percent of the rock. Acces- sories are apatite, titanite, magnetite, zircon, and pyrite. No quartz was noted. The biotite is fresh and no claylike alteration of the potassium feldspars is present. Pyrite is locally more abundant, making up perhaps as much as 0.5 percent of a few blocks in the waste dump of the old quarry (see fig. 5). The copper and molybdenum analysis of a sample of the hornblende-biotite syenite is shown on table 5. The sodalite-nepheline syenite phase was described by Eggleston from only one relatively small lens near the east edge of the pluton. It is medium-coarse— grained gray rock consisting of 90 percent feldspar, and pyroxene, sodalite, and nepheline. More than half the feldspar is microperthite, orthoclase, and microcline, and the rest is plagioclase near albite in composition. Accessories are apatite, magnetite, ti- tanite, and pyrite (Eggleston, 1918, p. 397). Molybdenite occurs at one prospect at the west edge of the pluton (fig. 5) ; I know of no other oc- currence in the Vicinity. Jacobs (1937, p. 24) cited “a mineralized zone over 3100 feet long and some 125 feet wide,” but I think that sparse pyrite rather than molybdenite probably determined the major boundaries of his zone. Sparse molybdenite can be seen in bedrock at the prospect. High-grade material in the dump probably came from the small shaft, which is too dangerous for thorough examination and is also flooded to perhaps 2 m from the surface. Relationships of the high-grade material to the en- closing rock must be deduced from the dump mate- rials. Country rock at the prospect consists of fine to coarse pulaskite, made up mostly of perthitic ortho- clase, and a fine—grained sugary hornfelsic rock, probably mostly metamorphosed wall rock but per- haps partly a chilled marginal phase of the intru- sive rock. Some specimens seem to be fine hornfels injected along and across foliation by abundant pulaskite, and some mixing of the two rocks has taken place in the contact zone. The molybdenite mineralization seems to be very close to, if not in, the contact zone, because both rock types are present in the sides of the small prospect shaft. The horn- felsic rock at the shaft and dump and also in nearby outcrop contains from a trace to 3 percent pyrite, E20 and little molybdenite was observed in it. The copper and molybdenum content of two samples of the hornfels is shown in table 5. Molybdenite is disseminated in irregular patches in fine and medium-grained biotite pulaskite frag- ments found on the dump of the prospect pit. Some material is estimated to contain more than 10 per- cent molybdenite, a few percent pyrite, and perhaps some chalcopyrite. The amount of this high-grade material on the dump is small but it probably has been preferentially removed by mineral collectors. It is assumed that the “analyses showing as much as 30 percent molybdenum” (Jacobs, 1937, p. 24) were made on selected pieces of material. Traces of see- ondary molybdenum minerals are present in weath- ered dump fragments, but no secondary copper min- erals were observed. Hand specimens of the richest molybdenum-bearing pulaskite are weakly radio- active, but the source of the radiation has not been determined. Although there is widespread weak sulfide miner- alization and local high-grade molybdenum-bearing rocks, the Cuttingsville pluton completely lacks the pervasive hydrothermal alteration that one should expect in a porphyry-type deposit. It is proposed that the molybdenum is of the molybdenitic deposit type as defined by Khrushchov (1959). This type of de- posit generally has a much lower potential for ex- ploitation than a porphyry deposit, but a small high- grade deposit in the Cuttingsville pluton is still a possibility. Previous prospecting was probably con- trolled entirely by observations on natural outcrops, and I am not aware of any use of geochemical pros- pecting methods to explore the mineral potential of the pluton. Many unexposed areas are probably near the pluton margin that are large enough to obscure ore bodies of significant size. Drift cover is probably mostly thin, and geochemical soil sampling should be a rapid and efficient way to seek other molyb- denum-bearing parts of the pluton. OTHER MOLYBDENUM OCCURRENCES Molybdenum has been reported from several other localities in southern and eastern Maine in addition to the Cooper mine and the Catherine Mountain de- posit (King, 1970, p. 7). Although the brief de- scriptions given by King indicate that some of these are associated with pegmatites and small sulfide veins, some of them probably are deposits of the ~ molybdenitic type. I know of no evidence that any of these localities are deposits of the porphyry type. Hollister, Potter, and Barker (1974, p. 625) dis- cussed the sparse molybdenite mineralization that GEOLOGY AND RESOURCES OF COPPER DEPOSITS may be associated with a narrow breccia zone at Cadillac Mountain on Mount Desert Island, Hancock County, Maine. PORPHYRY-TYPE DEPOSITS IN THE SOUTHEASTERN UNITED STATES Several disseminated copper-molybdenum deposits associated with felsic stocks are known in eastern North and South Carolina, but information on them is somewhat limited. One prospect, the Conner- Neverson quarry area, is tentatively classified as the porphyry type, but none are proved to be of that type. These prospects are in, or close to, rocks of the Carolina slate belt, a sequence of metamorphic rocks derived from volcanic flows in tufl‘s intermixed and intercalated with rock waste. These slate-belt rocks and their presumed equivalents, extending from Alabama to Maryland, represent a very exten- sive belt of late Precambrian and early Paleozoic volcanism, perhaps related to an island arc or to the edge of a paleocontinent. One tectonic model for the southeastern Piedmont region would have had two continental masses collide about 450 my ago (Glover, 1976), and the subduction zone dip south- eastward. The distribution of small late-orogenic or postorogenic plutons in a thick sequence of volcanic sediments and volcanic tufi's and flows along the southeast edge of the Piedmont is compatible with this View. The subduction of oceanic crust prior to collision would have resulted in volcanism and per- haps the formation of porphyry copper-type de- posits in a linear belt above the subduction zone. Small plutons within the slate belt may indicate the site of active volcanism and subvolcanic intrusion above the subduction zone, and I consider the small plutons to be the areas of highest potential for the occurrence of porphyry copper-type deposits. In an alternate interpretation, Spence and Car- penter (1976) considered the western part of the slate belt as the site of a major volcanic island arc during late Precambrian and early Paleozoic time associated with an arc-trench gap and trench to the east, now hidden under Coastal Plain sedimentary strata, and a westward-dipping subduction zone. Black and Fullagar (1976) cited Rb/Sr age dates to indicate that subduction and volcanism began before 705 my and ended 613 my ago, then resumed in the early Paleozoic, and ended finally before the “Taconic” orogenic period. Presumably the inter- pretation by Black and Fullagar would permit more than one subduction zone, perhaps two of opposite dip. The extensive belt of volcanic rocks in the Pied— PORPHYRY COPPER-MOLYBDENUM DEPOSITS, EASTERN UNITED STATES mont, though not well exposed in many areas, and as yet only partly mapped on an adequate scale, includes several small plutonic intrusive bodies, but has surprisingly few subvolcanic intrusive bodies known in it. The deep saprolitic weathering in the region does not favor the identification of felsic subvolcanic rocks in felsic volcanic country rock. There are many gold deposits and prospects, at least some of which are probably of volcanogenic origin (Worthington and Kiff, 1970), and some of these occurrences contained associated copper, lead, zinc, bismuth, and arsenic; a few such as the Haile and Blackmon mines, Lancaster County, SC. (fig. 6), l _______,_ __ VIRGINIIA I_ J ‘ ' — ' ' — — Ems TENNESSEE I my swarm \ 36° ’ " K J NORTH CAROLINA M “Wk” "MIX — f smpswec amnv x I Raleigh Wilson . Conwd coma mortar, Charlotte I NEWHJ. ”WSW GUAM? 3507/ _‘——_1’ SPECT \ I Chesterfield ‘ SOUTH .1 .mu— - -.(—\ N 6“ CAROLINA mv: >§¢>§ \ 00 fi\ MU "NE Hm "N5\ .00 GA] ' Camden #/ \ I Columbia_ I \ 9“; | 0 100 200 KILOMETERS EXPLANATION - City or town 0 50 100 MILES X Mine or prospect area FIGURE 6.—Index map of mines and mineralized prospects in North and South Carolina. See text for discussion. have associated potassic rocks that may indicate substantial hydrothermal potassic alteration (Par- dee and Park, 1948, p. 112, 114). The disseminated molybdenum-copper mineraliza- tion at the Conner prospect and the Neverson quarry in North Carolina is sufficiently extensive to be of the porphyry copper type, but the intensity and extent of rock alteration are not known; hence the classification is tentative. The Brewer mine, Chesterfield County, SC. (fig. 6), is in a large zone of intense hydrothermal altera- tion that may be regarded as a deposit of porphyry copper affinity, possibly similar to the Mount Pleas- ant, New Brunswick, and the Cornwall, England, deposits. Three prospects in Halifax County, NO. (the Moss-Richardson, Boy Scout-Jones, and Ellis), are not now classified as porphyry-type deposits because rock alteration and mineralization seem too limited, E21 but more information about these prospects could easily change their category. Stratabound polymetallic mineralization of the type described by Maucher (1972) probably is present in the slate belt also (US. Geological Survey, 1973, p. 2). Perhaps this is related in some way to the prob- ably volcanogenic gold-base metal deposits. Neither deposit type can be considered directly associated with or indicative of porphyry copper-type mineral- ization, but both suggest that this long volcanic belt had metal-bearing hydrothermal emanations asso- ciated with it. Geologists working for private companies have searched for at least the last 10 years in the south- eastern Piedmont for deposits of the porphyry type. Although none of the mineralized areas they found have yet proved to be minable, some may fit the porphyry model well enough to encourage further search. Six mineralized areas that I think deserve consideration as porphyry—type deposits are de- scribed here. DEPOSITS THAT MAY BE OF THE PORPHYRY COPPER-TYPE CONNER-NEVERSON QUARRY AREA The Conner stock is in Wilson and Nash Counties, N.C.,‘about 53 km east of Raleigh. Exploration at the Conner stock was described by Cook (1972). Porphyritic biotite granite has intruded argillite and tufi‘ of the Carolina slate belt. Part of the stock is overlapped by thin sediments of the Coastal Plain. Three areas of anomalous metal content were identified and two were explored. Two samples were taken and analyzed from the third area, the Never- son quarry near Sims, but no exploration was car- ried out. The areas explored were on the northwest and west edges of the intrusive body. Pyrite-chalcopy- rite-molybdenite mineralization was intersected in altered granite and in rock described as greisen. Quartz veins bearing minor galena, sphalerite, chal- copyrite, and pyrite were found at the granite-slate belt contact. All analyses of drill samples were less than 175 ppm Mo and 650 ppm Cu, averaged over 10-ft (3.05 m) intervals. Locally pyrite makes up as much as 15 percent of the “greisen” zone. The most intriguing part of the pluton is the Neverson quarry where no drilling was done. Cook reported that argillic alteration is present here and alteration seems more intense than at the drilled locations. Some rock is abnormally red. According to Cook (1972), “Quarry blocks and greisen float con- tain up to 0.380 and 0.192 percent molybdenite re- AND RESOURCES OF COPPER DEPOSITS GEOLOGY E22 T 34° 40' \ I 0” .‘\ c I,” I \\ a o” 71 5 I. ~ ..... n I; / ‘| a” . . . I: '-/ I ‘I'I .-' I ,9 _/ _/' 47 y 45 2/ _/ ' | 45 W 45_.- / 4 5: l,’ \\ 57/./ U 545%,}? Compiled nom Fries (1942), Peyton and Lynch [1953), Minard (1971). and unpublished information by A. R Kinkel, .11.. 1967 1| KILOMETER l 1/4 1/2 MILE D—'—D FIGURE 7.—Sketch map of the Brewer mine area, Chesterfield County, 8.0. PORPHYRY COPPER-MOLYBDENUM DEPOSITS, EASTERN UNITED STATES 25 _4_ E23 EXPLANATION MlDDENDORF(?) FORMATION (UPPER CRETACEOLB)—Unconsolidated light-gray, yellow-brown, and reddisl’rbrown clay, silt, sand, and, espechlly at the base, gravel METAMORP'HOSED VOLCANO‘SEDIMEN- TARY ROCKS—Metamorphosed ielsic tuft, tufhceous sand and argllite, lelsic porphyry, and volcanic breccia; metamorphosed rocls of undetermined 0min include diabase, hom- blende gneiss, and amphibolite. Some areas covered by Middendort Formation are not delineated ARGlLLlTE—Largely greensh—gray (brownbh- gray when weathered) fine-grained laminated tufbceous argllite Contact Strike and dip of bedding Inclined Vertical 70 Strike and dip of foliation __ inclined + Vertical W Gossan (data from J. T. Pardee in Fries, 1942, pl.6l - Topaz rock (data from J. T. Pardee in Fries, 1942, pl. 6) Silicified rock (data from J. T. Rardee in Fries, 1942, pl. 6) 80 US. Bureau of Mines diamond drill hole. All holes contain significant fluorine believed to be present only as topaz (Peyton and Lynch, 1953). Hole 8 contains the most topaz (:3 Outline of open pit — — - - Vehicle trail 100 MILES 100 KILOMETERS INDEX MAP OF SOUTH CAROLINA, SHOWING BREWER MINE AREA FIGURE 7.—Continued. E24 spectively.” J. B. Mertie, U.S. Geological Survey, prepared a panned concentrate from finely crushed rock collected at the Neverson quarry crusher in 1947. Molybdenite is a major constituent of the heavy minerals in the concentrate. I examined thin sections of five specimens of po- tassic quartz monzonite and granite taken from the Neverson quarry in 1949 by J. B. Mertie. Plagioclase appeared mostly unaltered, and biotite had been partly altered to chlorite and a little epidote. These mineral changes indicate rather weak alteration compared with major porphyry-type deposits else- where. The most reasonable alternate classification of the sulfide mineralization here is probably the molybdenitic type mineralization of Khrushchov (1959). I also examined thin sections of two specimens collected by J. B. Mertie from a quarry about 4 km south-southwest of Sharpsburg in Wilson County, NC (about 23 km northeast of the Never- son quarry). The rock is very potassic, containing perhaps 50 percent potassium feldspar. Feldspars do not appear altered but the high potassium feld- spar content may itself indicate a form of high- intensity potassic alteration. I know of no evalua- tion of the Sharpsburg quarry for contained sulfides. DEPOSITS OF PORPHYRY COPPER AFFINITY THE BREWER MINE AREA The Brewer mine, a producer of gold from about 1828 until 1940, is in part of a much larger zone of highly altered copper-bismuth-tin-bearing volcanic rocks. The most outstanding alterations are quartz- andalusite and quartz-topaz greissens. The large alteration zone (a minimum of 1.4 km in one dimen- sion), the evidence for a subvolcanic origin, and the metal suite provide only a sketchy picture of the overall geology of the hydrothermal system, but they suggest and permit consideration of a large system such as Mount Pleasant, New Brunswick, and some of the deposits of Cornwall, England. Although not a typical porphyry copper system, it has some features in common with one. Gold-mining operations at the Brewer mine, near Jefferson in Chesterfield County, 8.0. (fig. 7), at first by placer and later by openpit and underground, have been described by Leiber (1858, p. 63—68), Nitze and Wilkens (1896, p. 762—767), and McCauley and Butler (1966, p. 36—40). The Brewer mine area is in felsic volcanic rocks of the Carolina slate belt, close to and partly under the overlap of Coastal Plain strata. Arthur R. Kin- kel, Jr. (written commun., 1968) described the GEOLOGY AND RESOURCES OF COPPER DEPOSITS country rocks in the mine as strongly silicified rhyo- lite, part of which is tectonic breccia, part pyroclas- tic breccia, and part tuff. According to Kinkel, the gold ore consists of silicified topazized pyritized rhyolite, in which local areas several centimeters across consist entirely of fine-grained granular py- rite. Small grains of pyrite are widely disseminated in the rock associated with the gold deposit and may constitute 2—5 percent of the rock (Pardee, Glass, and Stevens, 1937, p. 1058). Topaz was identified in the altered volcanic rocks of the Brewer mine by Pardee, Glass, and Stevens (1937). In general appearance, the topaz-rich ma- terial here is difficult to distinguish from dense siliceous rocks unless the high specific gravity is noted, and the material was generally called flint'y quartz or blue hornstone in the reports of earlier examinations. The topaz occurs as disseminated grains, as patches and streaks of aggregates, and as massive topaz. All these are made up of rounded in- dividual grains only a few microns in diameter. Pardee, Glass, and Stevens (1937, p. 1062) reported that only a few grains had a euhedral form. I ob- served only rounded shapes. The identification of topaz in the mine led to a drilling program by the U.S. Bureau of Mines in 1951—52 to determine the extent of the topaz reserve and the possibility of the use of the topaz as mullite and for the production of calcium fluoride. The results of this exploration program were described by Peyton and Lynch (1953). The cores of the 10 holes drilled on this pro- gram, retained by the U.S. Bureau of Mines, are the best source of information on the nature and extent of the rock alteration northwest of the old mine workings. The locations of the holes are shown on figure 7. Compilation of the full suite of base metals identi- fied in the mine depends mainly on old reports. Pardee, Glass, and Stevens (1937, p. 1058) observed that “small grains of enargite are sparingly scat- tered through parts of the rock.” Other authors noted covellite, cassiterite, bismite, native bismuth, chalcanthite, and chalcopyrite, but these were not seen by Pardee, Glass, and Stevens (1937). Grab samples of pyritic rock from drill hole 1 close to the old open pit (Brewer pit, fig. 7) contained as much as 0.3 percent arsenic, 1.0 percent copper, 500 ppm bismuth, and 200 ppm tin (analyses performed in laboratories. of the U.S. Geological Survey). One sample in another hole near the mine contained 0.07 percent molybdenum. Sulfides make up an estimated 1—5 volume percent of all the rock in the cores from 9 of the 10 U.S. PORPHYRY COPPER-MOLYBDENUM DEPOSITS, EASTERN UNITED STATES Bureau of Mines drill holes. In two cores taken near the old Brewer pit, average sulfide content was close to 5 volume percent, and in one hole (N0. 1) the estimated sulfide content over an interval of 33.2 m was 15 volume percent. No primary rock textures seem to have survived the rock alteration in the Brewer drill cores. Al- though volcanic textures have been described from the mine, primary textures were probably not every- where recognizable. Kinkel (written commun., 1968) described a variety of breccias in the mine, and breccias were common in three drill holes. Much more needs to be known about the detailed geology of the area around the Brewer mine. Little has been described in earlier studies except the mine exposures and the occurrence of gold, but we know that strong hydrothermal alteration has affected a much larger area, accompanied by introduction of much disseminated sulfide, trace amounts of bismuth and tin, and as much as 0.3 percent arsenic, 0.07 percent molybdenum, and 1.0 percent copper. If the Brewer mine area is part of a large hydrothermal system of the Mount Pleasant type, the style of alteration and the associated metal suites may vary considerably in the different parts of a system that may be as large as several kilometers in each di- mension. Until the perimeter of the alteration zone is defined, and variations in both alteration and mineralization style are known, the Brewer area cannot be said to be evaluated for its mineral potential. PROSPECTS OF COPPER AND MOLYBDENUM NOT INCLUDED IN THE PORPHYRY TYPE EASTERN HALIFAX COUNTY, NORTH CAROLINA, PROSPECTS At least three copper-molybdenum prospects in Halifax County, N.C., have been explored by dia- mond drilling (fig. 6). The Moss-Richardson and Boy Scout-Jones pros- pects were explored by the US. Bureau of Mines in 1942—46 (Robertson, McIntosh, and Ballard, 1947 ). The Ellis prospect was explored by Perry, Knox, Kaufman, Inc., about 1970. Cores from the latter exploration have been examined by the US Geological Survey. Mineralization at the Moss-Richardson and Boy Scout-J ones prospects is confined mostly to veins and is mostly near the contact between a small granitic body and siliceous chlorite slate, schist, and gneiss (Robertson, McIntosh, and Ballard, 1947). Sparse disseminated sulfide also occurs in wall rocks within 20—60 cm of the veins. E25 The ore minerals are molybdenite and chalcopy- rite; some ferrimolybdite, chalcocite, and covellite are present as secondary minerals. Traces of lead, silver, and vanadium were detected spectrographi- cally. The paucity of ore minerals, the seeming lack of extensive hydrothermal alteration, and the coarse- ness of the small intrusive body make it seem un- likely that these mineral occurrences have any rela- tionship to a porphyry-type deposit, though molyb- denum and copper are here associated with a felsic intrusive rock. Drilling at the Ellis prospect was mostly in meta— morphosed felsic volcanic rocks and small bodies of alaskite. Much of the volcanic rock that was drilled is silicic and probably rich in potassium feldspar, probably as a result of hydrothermal alteration. Some phyllic alteration is present also. The effects that I attribute to hydrothermal alteration are most- ly in the volcanic rocks, as is most of the visible sul- fide. Some alaskite is altered and sparsely mineral- ized, but much is fresh and contains only traces of sulfide. Ore minerals are molybdenite, chalcopyrite, and scheelite; traces of lead, zinc, and bismuth are also present. The molybdenite and chalcopyrite are mainly disseminated and are probably most abund- ant where pyrite is locally as much as 2—5 percent. Some of the molybdenite and chalcopyrite are in films on fractures and in small veinlets. Both dis- seminated and fracture and vein sulfides seem to be more common where the quartz-potassic alteration is greatest. Rough visual estimates of chalcopyrite content indicate that of 1,003 m (3,291 ft) of core examined, 180 m (590 ft) probably contained more than 0.1 percent but less than 0.5 percent copper, and 18 m (59 ft) contained at least 0.5 percent cop- per. Because of their crudeness, these estimates must be used with caution. No estimates were made for molybdenum. Scheelite was looked for only in relatively few pieces of core. The most abundant scheelite was in relatively unaltered laminated volcanoclastic rock, concentrated in a 2-mm-thick lamina and 2-cm-thick brecciated bed. The scheelite is probably volcano- genic, not hydrothermal, and resembles the wide- spread stratabound occurrences in mostly mafic metavolcanic rocks of Late Cambrian to Late Silu- rian age in Europe and Korea, as described by Maucher (1972). NEWELL PROSPECT, NORTH CAROLINA The Newell copper-molybdenum prospect is about 15 km south of Concord, in Cabarrus County, N.C. E26 (fig. 6), in the vicinity of the 01d Dixie Queen gold- copper mine. Anomalous amounts of copper in stream sediments led in 1964 to exploration for cop- per ore. More than 760 m of drilling was performed in exploring an area of weakly mineralized rock near the old Dixie Queen mine by Bear Creek Mining Co., and the results of this work have been de- scribed by Worthington and Lutz (1975). Worthing- ton and Lutz interpreted the deposit of sparsely mineralized rock to be a lean porphyry copper- molybdenum deposit, in part because of what they believe to be extensive supergene sericite in same- lite. The drill core from this exploration is now in the collection of the US. Geological Survey and I have examined and logged the drill core. The Newell prospect is in a region of gneiss and schist and felsic to mafic intrusive rocks generally known as the Charlotte belt. Country rocks at the prospect consist of diorite and several kinds of quartz monzonite (termed granite at Bogers Chapel by Bates and Bell, 1965) (fig. 8). Most of the min— eralized rock is part of a pluton of quartz monzonite and alaskite (about 1.2><2.4 km (Worthington and Lutz, 1975, p. 4—6) in which the weak mineralization (0.03~0.04 percent Cu, 0.01—0.02 percent Mo) out- lined by the drilling is perhaps 600 m in diameter. Details of the company chemical analyses of drill- ing samples were not sought for this study. Within the area of quartz monzonite and alaskite, metamorphosed potassium-rich granitic and rhyolit- ic rocks occur in several places, but I do not know their relationship to mineralization. Examination of thin sections shows that some of these rocks contain quartz, microcline, and biotite as major minerals, and plagioclase as only a minor constituent. In other specimens, part of the microcline seems to have been deposited as a late rim on the outside of plagioclase grains, suggesting potassium enrichment of the quartz monzonite toward the end of crystallization of the pluton. I observed no potassic feldspar that looks like the product of hydrothermal potassic a1- teration at typical porphyry copper deposits. The analyses for K20 in the rocks at the Newell prospect range from 1.8 to 6.8 percent (table 6). I identified quartz-sericite and argillic alteration only in some very thin zones along fractures and veins. The origin of the sulfides is not clear. There is no evidence for hydrothermal mineralization since the time of metamorphism. If some hydrothermal alteration took place before metamorphism, the metamorphic grade is high enough that the fabric may have been considerably altered and perhaps the origin disguised. GEOLOGY AND RESOURCES OF COPPER DEPOSITS The association with a felsic pluton, the widely disseminated enrichment by potassium, copper, and molybdenum, and the quartz-sericite and argillic a1- terations are features characteristic of a porphyry alteration system, but the scantness of metal en- richment and quartz-sericite alteration makes the classification of the Newell prospect as a porphyry rather problematical. A zone of sericitic alteration based on clays that developed during formation of saprolite, as described by Worthington and Lutz (1975), does not, in my opinion, increase the like- lihood of the prospect being of the porphyry type. Until more information about the geology of the Newell prospect is available, I prefer to not include the prospect with deposits of the porphyry type. The interpretation of the origin of mineralization at the Newell prospect is more important than just for understanding of that prospect alone. It lies outside the Carolina slate belt, the area ,I think to be of highest potential, and, if the prospect is classified as one of the porphyry copper type, then the poten- tial area for the occurrence of lower Paleozoic por- phyry copper deposits is larger than I have postu- lated. There seems to be no good reason to change the prospecting model on the basis of present knowl- edge of the Newell prospect. SUMMARY AND CONCLUSIONS Past discoveries of “porphyry copper deposits in eastern North America indicate that there is poten- tial for finding new deposits of exploitable size, al- though the potential is considerably lower than in younger, less eroded, and less deformed porphyry- bearing regions. I propose that porphyry-type mineralization in the New England-Maritime Provinces area is re- lated to a northwest-dipping paleosubduction zone marking the line of collision of a European-African continent with North America during the Devonian Period. The collision line is near the Maine-New Brunswick coast; the most favorable structural zone for the occurrence of related porphyry copper deposits is parallel to the collision line and 130—320 km northwest of it, especially in western New Hampshire, northwestern Maine, and southeastern Quebec. The upper Precambrian or lower Paleozoic volcanic piles included in the Carolina slate belt are proposed as favorable prospecting areas. Few subvolcanic intrusive rocks have been delineated in the metavol- canic rocks, but the geologic conditions are not fav- orable to easy recognition of such rocks. I believe that further detailed mapping and geochemical E27 PORPHYRY COPPER-MOLYBDENUM DEPOSITS, EASTERN UNITED STATES :V .J.,.% _ if? ’, ..,," ,1; . 1/. ,. y. //g L ..l ‘ . ./ _ _ . \ x. \ vat'IOOII “\SWMN\\OHOO“ 0' W9“, 1” l/o’r / . x _ / . _. f V {9/ SSS! Vol/$9.“? 77V? .. 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I. .. \\ way“. 7»...%VVVA/ J... A 4S . . Z; Baseiifrom US. Geological Survey, 2 KILOMETEBS l 1 MILE V2 CONTOUR INTERVAL 20 FEET NATIONAL GEODEHC VERTICAL DATUM OF 1929 EXPLANATION Diamond drill hole—Numbers are keyed to table 6 O The Newell mine is about 15 km south of Concord, N.C. —— Contact—Approximately located 3 1:24.000. Concord SE, North Carolina, 1969 m 7m. 13 of the Newell mine shaft (old Dixie Queen mine shaft) and the exploration drill holes; modified from FIGURE 8.——-General geologic map of the Newell prospect, Cabarrus County, N.C., showing the locations Worthington and Lutz (1975). GEOLOGY AND RESOURCES OF COPPER DEPOSITS E28 e; V 9H V 94 V QM 6A 6% V Wu 9N wA w." .H V ill 7:93 «-113 .mv .fi mg. .o: .o: .3 in .E .8” n; ma. V iAEaB 1.6: .2; .cs .8 .oum .Sn .3: .ocn .8” 58 8a emu .lAEn5--:=o 2. 3. one. «8. fl. 2.... 3. 33. «u. 25. as. {Afiaélimm am. A: S. S. 3.. fi. 1.. mm. .3. §. 3. -Snmobétim 2.. 3 2: 3 3 2: ma 2: 3 2: 2: :138. we. «a. an. a. no. V .3. V 2. mm. no. V E. 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AE ”.31“..va AE waning AS “Eli: c: mfilfid AE «Slade AE 3733 FE 3:72: 3 2o: 2 ion 2 3o: 3 Son 2 39m 2 ion F in: a so: m 32.— .. is: n so: unfit; NEE; 5&2; 3323 33:3 waSB 53:? 83:3 33:3 «33:5 «wa53 :32 "2:5: c: w 2:: .U .2 £23.23 ”2:11:23 ea? .3 DO ”3552: 2505323.:3.5x 5.33:. .5337.“ .3 .5 .5: 52 .E 2.7.?! 155:». 1:: .::.:3 <— ._. 5:57. ._=E.§..: 5.35:.— .A— :— .._ .3 A..__..::z .5: 293%.; mama—.5: air—.35“: 635365 1:23.33 H.395: E 12:15:. 6.2 £3380 Esaso £85.25“ 3352 3: EEK PS... :23 kc 332%» NH \0 wmmfisgs Nafifigblé mania. PORPHYRY COPPER-MOLYBDENUM DEPOSITS, EASTERN UNITED STATES sampling will locate more mineralized subvolcanic felsic intrusive bodies, and that some of them have a reasonable chance of being exploitable. The extensive Pleistocene glacial drift on Pre- cambrian rocks in Minnesota, Wisconsin, and Michi- gan handicaps prospccting for porphyry-type de- posits in the covered areas to the extent that it does not seem advisable at the present time. However, if exploration for other types of mineral deposits reveals evidence for deposits of the porphyry copper type, this evidence should not be overlooked. REFERENCES CITED Albee, A. L., and Boudette, E. L., 1972, Geology of the At- tean quadrangle, Somerset County, Maine: U.S. Geol. Survey Bull. 1297, 110 p. Allcock, J. B., 1974, Gaspe copper: A Devonian skarn-por- phyry copper complex [abs]: Econ. Geology, v. 69, p. 1175—1176. Anderson, F. D., 1954, Preliminary Map, Woodstock, Carle- ton County, New Brunswick: Canada Geol. Survey Paper 53—33, 3 p. Ayres, L. D., Wolfe, W. J., and Averill, S. A., 1973, The Early Precambrian Setting Net Lake prophyry molyb- denum deposit [abs]: Canadian Mining and Metall. Bull., v. 66, no. 731, p. 48. Bates, R. G., and Bell, Henry, III, 1965, Geophysical inves- tigations in the Concord quadrangle, Cabarrus and Mecklenburg Counties, North Carolina: U.S. Geol. Sur- vey Geophys. Inv. Map GP—522. Bird, J. M., and Dewey, J. 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Zinc ore. Edwards. N. Y. Michigan 12. Manganese nodules, ocean floor 9 10 6. Ribbon asbestos ore. Quebec. Canada 13. Botryoidal fluorite ore. 11 12 13 14 7. Manganese ore, banded Poncha Springs, Colo. rhodochrosite 14. Tungsten ore, North Carolina The Nature and Use of Copper Reserve and Resource Data By DENNIS P. COX, NANCY A. WRIGHT, and GEORGE j. COAKLEY GEOLOGY AND RESOURCES OF COPPER DEPOSITS GEOLOGICAL SURVEY PROFESSIONAL PAPER 907—F A discussion of the use and limitations of copper resource data for production estimates, trends, and forecasts UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1981 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretmy GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Cox, Dennis P. The nature and use of copper reserve and resource data. (Geology and resources of copper deposits) (Geological Survey professional paper ; 907—F) Bibliography: p. Supt. of Docs. no.2 I 19.16:907—F 1. Copper cores—United States. I. Wright, Nancy A., joint author. 111. Title. IV. Series. V. Series: United States. Geological Suvey. Professional paper ; 907—F. TN443.A5C69 553.4’3’0973 80—607780 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 APPRAISAL OF MINERAL RESOURCES Continuing appraisal of the mineral resources of the United States is con- ducted by the US Geological Survey in accordance with the provisions of the Mining and Minerals Policy Act of 1970 (Public Law 91—631, Dec. 31, 1970). Total resources for purposes of these appraisal estimates include currently minable resources (reserves) as well as those resources not yet discovered or not cur- rently profitable to mine. The mining of mineral deposits, once discovered, depends on geologic, eco- nomic, and technologic factors; however, identification of many deposits yet to be discovered, owing to incomplete knowledge of their distribution in the Earth’s crust, depends greatly on geologic availability and man’s ingenuity. Consequently, appraisal of mineral resources results in approximations, subject to constant change as known deposits are depleted, new deposits are found, new extractive technology and uses are developed, and new geologic knowledge and theories indicate new areas favorable for exploration. This Professional Paper discusses aspects of the geology of copper as a frame- work for appraising resources of this commodity in the light of today’s tech- nology, economics, and geologic knowledge. Other Geological Survey publications relating to the appraisal of resources of specific mineral commodities include the following: Professional Paper 820—“United States Mineral Resources” Professional Paper 926—“Geology and Resources of Vanadium Deposits” Professional Paper 933—“Geology and Resources of Fluorine in the United States” Professional Paper 959—“Geology and Resources of Titanium in the United States” FIGURE 1. TABLE #630!th O‘IhODNI-i . Copper resources of the United States as of 1978 . Potential annual production from US. copper deposits as of 1978 CONTENTS Abstract Introduction Acknowledgments Definitions of terms Sources of data and methods of analysis Reserve data Total ore and contained copper Resource data Production data Anticipated copper production Requirements for new sources Conclusions References cited ILLUSTRATIONS Graph showing us. reserve estimates and annual production 1925-1977 . Plot showing tonnage and grade of US. copper deposits . Graph showing declining yield of US. copper mines, 1936-1976 . Diagram showing classification of US. copper resources and reserves . Plot showing relation of annual production capacity to total copper in ore for 34 deposits . Diagram comparing copper demand with US. production capacity projections 1976—2030 . Graph showing cumulative reserves and resources at various grades in the United States, Chile, Peru, Zambia, and Zaire, as of 1977 Pine F1 F1 F1 F3 F10 F13 F14 F15 F19 F19 mine production TABLES . Published reserves and resources of copper deposits in the United States . The sensitivity of US. copper reserves to economic conditions, in 1975 . Total copper contained in typical deposits . Bar chart showing tonnage of copper in reserves and number of deposits brought into production that are potentially producible, graph showing cumulative copper in developed ore reserves, cumulative consumption, and cumulative Pu! F11 F12 F13 F15 F17 F18 Pace F3 F10 F14 GEOLOGY AND RESOURCES OF COPPER DEPOSITS THE NATURE AND USE OF COPPER RESERVE AND RESOURCE DATA By DENNIS P. Cox, NANCY A. WRIGHT, and GEORGE J. COAKLEYI ABSTRACT Copper reserve, resource, and production data can be combined to produce disaggregated resource estimates and trends and, when com- bined with demand forecasts, can be used to predict future explora- tion and development requirements. , Reserve estimates are subject to uncertainties due mainly to incom- plete exploration and rapidly changing economic conditions. United States’ reserve estimates in the past have been low mainly because knowledge of the magnitude of very large porphyry-copper deposits has been incomplete. Present estimates are considerably more reli- able because mining firms tend to drill out deposits fully before min- ing and to release their reserve estimates to the public. The sum of reserves and past production yields an estimate of the total ore, total metal contained in ore, and average grade of ore orig- inally in each of the deposits known in the United States. For most deposits, estimates of total copper in ore are low relative to the total copper in mineralized rock, and many estimates are strongly affected by the economic behavior of mining firms. A better estimate of the real distribution of copper contained in deposits can be obtained by combining past production data with resource estimates. Copper resource data are disaggregated into categories that in- clude resources in undeveloped deposits similar to those mined 1n the past, resources in mines closed because of unfavorable economic con- ditions, resources in deep deposits requiring high-cost mining meth- ods, and resources in deposits located' 1n areas where environmental restrictions have contributed to delays 1n development. The largest resofic'e'is’located in the five largest pOrphyry deposits. These de- posits are now being mined but the resources are not included in the present mining plan. Resources in this last category will not con- tribute to supply until some future time when ores presently being mined are depleted. A high correlation exists between total copper contained in deposits and annual production from deposits. This correlation can be used to predict roughly the potential production from undeveloped deposits. Large deposits annually produce relatively less metal per ton of cop- per contained than do medium and small deposits. Dividing reserves by annual production gives a depletion date for each copper mine. The sum of annual production capacity of all mines not yet depleted at any year of interest gives the minimum production capacity for that year. A graph of minimum production capacity by year combined with curves representing potential capacity from unde- veloped identified resources can be compared with various demand scenarios to yield a measure of copper requirements from new sources. Since 1950 reserves have been developed in the United States at a rate of about 1 million tons of copper per year. Since 1960 the number of deposits developed per 10-year period has greatly increased without , acommensurate increase in tonnage of copper. This is in part due to the : fact that recent exploration successes have been increasingly repre- sented by smaller and (or) lower grade deposits containing less metal. INTRODUCTION The objectives of this report are to describe the na- ture and limitations of the data on copper resources, to ‘ US. Bureau of Mines, 2401 E Street N.W., Columbia Plan, Room 1102, Washington, DC. 31241. show how the data can be used to produce resource estimates, and to illustrate some of the ways in which trends and forecasts can be derived from the data. Throughout this report, the reader is shown re- peatedly that the data on copper resources contain in- herent limitations on accuracy and that projections of future rates of resource development made from the data are affected by economic and technological factors that are themselves difficult to project. Although we acknowledge the limitations of such resource analysis, we feel that the analysis does illustrate methods of data treatment that may become useful parts of more com- plex models of copper supply. Many comprehensive reviews of US copper reserves, technology, and supply have been made by specialists in the US. Bureau of Mines. Prominent among these are McMahon (1965), Everett and Bennett (1967), Bennett and others (1973), and Schroeder (1977). In contrast to these reports, our study focuses on some alternatives to the standard ways of treating copper-resource data and presenting results of analysis. Pioneering work on such alternatives, carried out by the Canadian Department of Energy, Mines, and Resources (Zwartendyk, 1974; Martin and others, 1976) has provided valuable stimulus to this study. The reader of this report will find useful a general knowledge of the geology of copper deposits, such as is reviewed in Titley and Hicks (1966), Cox and others (1973), Singer and others (1975), and Tourtelot and Vine (1976). The history of development of the US. copper- mining industry, as described by Parsons (1933) and Joralmon (1973), is also useful as background information. ACKNOWLEDGMENTS The authors wish to thank Donald A. Singer, John H. DeYoung, J r., Ted G. Theodore, Robert Gordon Schmidt, William J. Moore, Edward M. MacKevett, Jr., and S. Cyrus Creasey of the US. Geological Survey, and John D. Morgan, J r., Harold J. Schroeder, and Harold J. Bennett of the US. Bureau of Mines for contributions to data on which this study was based, as well as for valu- able discussion and criticism. We also acknowledge the cooperation of industry geologists, too numerous to mention, who have contributed information used in this study. Fl F2 GEOLOGY AND RESOURCES OF COPPER DEPOSITS DEFINITIONS OF TERMS Reserve and resource terminology has been standard- ized by the US. Bureau of Mines and the US. Geological Survey (1980). The definitions as they apply to copper are as follows: Resource. A concentration of naturally occurring cop- per in or on the Earth’s crust in such form and amount that economic extraction from the concen- tration is currently or potentially feasible. Identified resources. Copper resources whose lo- cation, quality, and quantity are known or estimated from physical measurements or are inferred from geologic evidence. Undiscovered resources. Copper resources, the exis- tence of which are only postulated, comprising de- posits that are separate from identified resources. Reserve base. That part of identified resources whose location, quality, and quantity are known or esti- mated from physical measurements. Reserve. That part of the reserve base that could be economically extracted or produced at the time of determination. Usage within the copper industry differs from the foregoing definitions. An informal canvas of eight mining company chief geologists and consultants (J. J. Hemley, written commun., 1978) revealed that the term “reserve” is used for any deposit whose location, quality, and quantity are known. Those reserves that have been determined to be economic are termed “economic reserves.” Those reserves for which such a determination has not been made are called “geologic reserves.” In this study, the following terms are equivalent. US. Bureau quim and US. Geological Survey Identified resource Reserve Industry usage Geologic reserve Economic reserve Other terms used in this study deal with meas- urements of the copper content of rock. These include the following: Ore. Copper-bearing rock that meets the criteria of reserves. Grade. The copper content, in weight percent, of a body of rock or ore in the Earth. Cutoff grade. The lowest grade of ore that can be eco- nomically mined and milled in a specific deposit; that is, the lowest grade of ore that can be included in a reserve estimate. Yield. The net quantity of copper produced expressed as a percentage of ore mined, allowing for losses in mining and processing. Two classes of copper production are considered: Primary copper. That part of total production or con- sumption derived from mining. Secondary copper. That part of total production or consumption derived from recycling of various forms of scrap. Four main geologic types of copper deposits are re- ferred to in this report: Porphyry-copper deposits. Masses of rock, tens of millions to billions of tons each, containing copper and molybdenum sulfides in closely spaced veinlets or in disseminated small grains. The deposits are irregu- lar to roughly cylindrical and are in most places associated with an intrusive igneous rock that has a porphyritic texture. Massive sulfide deposits. Deposits of compact massive copper, iron, zinc, and lead sulfide, and associated dis- seminated sulfide, having tonnages on the order of tens of millions of tons. These deposits have tabular or podlike form and are interlayered with marine vol- canic and sedimentary rocks. Sedimentary deposits. Layers of sandstone or shale containing disseminated copper minerals and, com- monly, valuable amounts of silver. I Copper-nickel deposits. Disseminated or compact masses of copper and nickel sulfide in mafic igneous intrusions. SOURCES OF DATA AND NIETHODS OF ANALYSIS Data on copper-ore reserves were taken mainly from Metals Sourcebook, a former bimonthly publication of McGraw Hill Company, and Mining Annual Review, an annual publication of Mining Journal. These publica- tions summarize information from other mining- industry periodicals and company annual reports. Company prospectuses, prepared in compliance with the US. Securities and Exchange Commission, also were consulted for reserve data on several major de- posits. Data on total past production were taken mainly from geologic reports on deposits discussed in Titley and Hicks (1966), Ridge (1968), and other publications too numerous to be named here. Data on current annual production were mainly from Mining Annual Review and company annual reports. 'All data in this report were collected before January 1, 1979. Quantitative data are presented in English and met- ric units in tables and figures and in English units in the discussion sections. English units are used because of US. industry usage and because US. Bureau of Mines reserve data are in those units. For conversion pur- poses, 1 metric ton equals 1.101 short tons. Because manyof the estimates are not precise, metric and Eng- lish units expressed in tons are for practical purposes interchangeable. Analysis of data for this paper was assisted by the use of the Computerized Resource Information Bank (CRIB) of the US. Geological Survey and by the Min- erals Availability System (MAS) of the US. Bureau of THE NATURE AND USE OF COPPER RESERVE AND RESOURCE DATA Mines. CRIB is a computer file that contains location, exploration, mineralization, deposit-description, and geological and mineralogical information on many min- eralized areas. Thedata file used in this study, a subset of CRIB, is accessed by the Geologic Retrieval and Syn- opsis Program (GRASP) (Bowen and Botbol, 1975). GRASP allows the user to retrieve data on selected copper deposits on the basis of given conditions and statistically to analyze and test the reliability of the data. Computer programs that use the copper file have been writtento calculate estimated depletion dates or to determine the total production capacity of copper in the United States for any years of interest. The Minerals Availability System (MAS) was estab- lished to determine the availability of minerals to the United States from known domestic and foreign sources and to express this potential supply as a function of cost relative to time. In the process of identifying and evalu- ating significant mineral deposits, MAS is capable of analyzing the conditions that influence the use of min- erals and of evaluating current technology as applied to F3 the development of the reserve or resource. It can de- velop and cost related mining and processing systems using the MINSIM 4 financial-analysis program, and can perform financial analysis; it can be used to conduct comprehensive availability studies. An early version of the financial-analysis methodology was described by Bennett and others (1970). RESERVE DATA Ore-reserve estimates for selected U.S. deposits are shown in table 1. In interpreting the significance of such published reserve data, the reader should be aware of three facts. First, published copper-reserve figures fre- quently represent the total amount of copper contained in situ, and only a certain amount of these reserves is recoverable and will ultimately enter into the available supply. Second, the value of associated metals is im- portant in the economics of copper reserves. Third, min- ing companies tend to be conservative in publishing reserve figures, and these figures should be considered as describing a minimum available supply. TABLE 1.—Published reserves and resources of copper in the United States [From sources available as of January 1977; data in thousand short tons] Deposit name Company Type of Ore Grade Copper Associated Reference depositl (percent copper) content metals A. RESERVES IN OPERATING MINES AND ANNOUNCED DEVELOPMENTS Arizona Twm Buttes ______ ANAMAX Mining Co____ OP—P 344, 000 0.71 2,445 0.03 percent Mo Securities & Exchange D0 __________ _-___ do ___________________ 62, 000 1.11 688 Commission,(SEC), mer- gar registration Form 14 No. 2—52105, Atlan- tic Richfield Co. (ARGO) and The Anaconda 00., Sept. 14, 1976. Mission ___________ ASARCO Inc _____________ OP-P 87,100 .68 592 Ag, Mo ________ ASARCO Incorp., Prospec- Sacaton, West-___ _____ do ___________________ OP—P 32, 700 .74 542 Ag _____________ tus on debenture offering Sacaton,.East ____ _____ do ___________________ ~ UG-P 16, 700 1.23 205 ________________ of May 7, 1975. SEC file San Xav1er ____________ do ___________________ OP-P 152,100 .51 776 ________________ No. 1—164. Do _______________ do ___________________ OP—P 27, 900 1.06 84 ________________ Silver Bell _____________ do ___________________ OP-P 29,700 .66 196 Ag, Mo ________ Palo Verde (1978) Eisenhower Minin o. OP—P 125,000 57 713 ________________ ARGO-Anaconda merger, (-ASARCO ANA AX). SEC filing Sept. 14, 1976. ASARCO, 3rd Quarter 1976, Report to Stock- holders. Pinto Valley ______ Cities Service Co. ________ OP—P 350,000 44 1,540 ________________ Cities Service Co., Prelimi- ..... do -_-_--____-----____ P nary prospectus, June 24, 1975. Ortacgle Ridge/Con- Continental Materials P 11,300 2.28 257 .64 oz. Ag ______ Engineering & Mining Corp. (Union Miniere, Journal, v. 176, No. 4, 45 percent interest). April 1975, page 170. Bagdad ___________ Cyprus Mines Corp ______ OP-P 297,000 .49 1,455 Ag, Mo ________ erus Mines Corp., 1975, Do _______________ do ___________________ 228,000 .35 98 ________________ nnual Re rt, SEC Do _______________ do ___________________ ”88,000 .22 194 ________________ Form 10-K led March Bruce __________________ do ___________________ UG-M 195 3.65 12 9 percent Zn 30, 1976. Johnson _______________ do ___________________ OP-P 213,000 .50 65 ________________ Esperanza ________ D1821; Corp. (Pennzoil OP-P 31,000 40 125 Mo _____________ Pennzoil, 1975, Annual Re- . . port. Sierrita ________________ do ___________________ OP-P 523,000 .32 1,672 .036-percent Mo Mineral Park __________ do ___________________ OP—P 60,000 .29 175 ............. Ins iration (in- Ins iration Consolidated OP—P ‘189,000 ‘.50 943 ________________ Ins iration Consolidated c udes L1ve Oak, pper Co pper Co., 1975, Annual Thornton, and Report. Recoverable cop- Red Hill Mines). per reported Christmas ________ OP-P ‘25,200 ‘.50 126 UG-P ‘56,800 ‘.50 284 OP— ‘9,000 ‘.50 45 F4 GEOLOGY AND RESOURCES OF COPPER DEPOSITS TABLE 1.—-Published reserves and resources of copper in the United States— Continued Deposit name Company Type of Ore Grade Copper Associated Reference deposit‘ (percent copper) content metals A. RESERVES IN OPERATING MINES AND ANNOUNCED DEVELOPMENTS—Continued Arizona—Continued Ray _______________ Kennecott Copper Corp__ OP-P 667,000 .79 4,400 Mo _____________ Reserves at 75.9 percent re- covery. Excludes copper recoverable from waste dump leaching. Kenne- cott Co 1' Corp., Letter to stoc olders, Nov. 26, 1976. San Manuel Kala- Magma Copper Co. (New- UG—P 1,000,000 .70 7,000 +0.03—percent “Magma Facts,” Magma mazoo. mont Minmg 00.). Mo Copper 00., Oct. 1, 1976. Magma/ Superior _____ do ___________________ UG-O 10,000 4.50 450 ________________ Morenci ___________ Phelps Dodge Corp ______ OP-P 662,500 .80 5,300 ________________ Phelps Dod e Cor ., Pro- s tus, EC Fi e 1—82, Metcalf ___________________________________ OP—P 416,000 .77 3,200 ________________ ay 22, 1975. New Cornelia (Ajo) _____ do ___________________ OP-P 126,600 .63 798 ________________ Bluebird __________ Ranchers Exploration & OP—P 275,000 .50 375 ________________ Ranchers Exploration & Development Co. Development Corp., 1974 annual report. Form 10K SEC File 1-6367, Sept. 30, 1975. Michigan White Pine _______ Copper Range ____________ UG—S 5405,000 1.23 4,981 0.16 oz. Ag ___- Coplpfir Range, 1975, Annu- a eport. D0 _______________ do ___________________ UG—S 694,000 1.20 1,227 __ Do _______________ do ___________________ UG—S 6128,000 1.06 1,361 ________________ Montana Berkeley __________ The Anaconda Co ________ OP—P 152,000 .67 1,018 ________________ ARGO/Anaconda, Merger Continental area- _____ do ___________________ OP—P 17,000 .49 83 ________________ Filing. Continental _____ do ___________________ OP— 253,000 .60 1,518 ________________ (North, Center, West). Battle Mountain Duval Corp, Pennzoil ___ OP—P 7,300 .63 46 ________________ Pennzoil, 1975, Annual Re- (includes Cop- port. per Basin and Copper Can- yon). Ruth/Ely _________ Kennecott Copper Corp-- OP-P 29,330 .79 185 Mo _____________ Kennecott, Prospectus April 15, 1971. Reserves at 74.7 percent recovery, adjusted for production shown in 1971—75 annual reports. New Mexico Chino/Santa Rita Kennecott Copper Corp__ OP—P 443,000 0.73 2,400 Mo _____________ Reserves at 74.2 ercent re- covery. Exclu es copper recoverable from waste dump leaching. Ken- necott Copper Corp., Let- ter to stoc holders, Nov. 26, 1976. Tyrone ____________ Phelps Dodge Corp ______ OP—P 344,400 .79 2,720 ________________ Phelps Dodge Prospectus, May 22, 1975. Continental/ Bay- UV Industries Inc ________ UG-P 19,300 1.97 380 ________________ UV Industries Inc., 1975, ard (North). Annual Report. Continental / Bay- _____ do ___________________ OP-P 17,500 .86 150 ________________ ard (Smith) Tennessee Copperhill ________ Cities Service Co _________ UG-M ‘40,000 ‘1.00 ‘400 2.0 percent Zn Estimates based on “20- year reserves at planned rate of production” re- _ ported in Cities Service 00., Preliminary pro- spectus, June 24, 1975. Utah Carr Fork ________ The Anaconda Co ________ UG-P 61,200 1.84 1,126 ________________ ARGO/Anaconda, merger filing, Sept. 1976. THE NATURE AND USE OF COPPER RESERVE AND RESOURCE DATA TABLE 1.—Published reserves and resources of copper in the United States—Continued F5 Deposit name Company Type of Ore Grade Copper Associated Reference deposit‘ (percent copper) content metals A. RESERVES IN OPERATING MINES AND ANNOUNCED DEVELOPMENTS—Continued Utah—Continued Bingham/Utah Kennecott Copper Corp__ OP-P 1,602,000 .70 9,500 Mo,“ Au, Ag Reserves at 84.7 percent re- Copper covery. Excludes copper recoverable from waste- dump leaching. Kenne- cott Co r Corp., Letter to stoc olders, Nov. 26, 1976. B. IDENTIFIED RESOURCES IN UNDEYELOPED DEPOSITS AND CLOSED MINES na Helvetia-East/ ANAMAX Mining 00.... P 222,000 .55 121 ________________ ARGO/Anaconda, merger Rosemont filing Sept. 14, 1976. Helvetia-East/ _____ do ___________________ P 337,000 .54 1,820 ________________ Rosemont. Helvetia-West ___ _____ do ................... P 724,000 .75 180 ________________ Lakeshore ________ Hecla Mining Co., and El Paso Natural Gas Co. UG—P 470,000 .75 3,500 ________________ Hecla Mining 00., 1969, An- nual Report. Reserves include tactite, oxide, and disseminated, un- changed through 1976. Miami East ______ Cities Service Co _________ UG-P 55,000 1.95 1,073 ________________ Cities Service Co., Prelimi- nary prospectus, June 24, 1976. Pima ___________________ do ___________________ OP—P 181,000 .49 887 Ag, M0 ________ Cyprus Mines Corp., An- nual Report, 1975. SEC Form 10-K filed March 30, 1976. Casa Grande Area Coastal Mining Co. (Getty UG—P 250,000 1 0 2,500 ________________ Mining Magazine, Jan. Oil 00., Hanna Mining 1977, p. 47 (Mining Co.) _____________________ Engineering, Nov. 1976, p. 15). Florence/Poston CONOCO _________________ OP—P 800,000 .4 3,200 ________________ Skillings Mining Review, Buttes. Jan. 17, 1976, p. 6. Sanchez (Safl’ord) Ins iration Consolidated P 200,000 .4 800 ________________ Inspiration, 1975, Annual opper Co. Report. Safford ___________ Phelps Dodge Corp ...... UG—P 400,000 .72 2,800 ________________ Phelps Dodge, Prospectus, _ , May 22, 1975. Cog er Basin __________ do ___________________ P 175,000 .55 963 .02-percent Mo 01 eliable Mine Ranchers Exploration & OP/UG-P 4,000 .74 10 .02-percent Mo Reserves in Recoverable Development Co. Copper, Ranchers 1974— 75, Annual Report, Form 10—K, Sept. 30, 1975. Maine Bluehill/ Kerr Addison Co _________ UG—M 520 1.46 8 3.4-percent Zn Noranda Mines Ltd., 1975, Blackhawk. Annual Report. Michigan Western Syncline AMAX Inc ________________ SS 105,000 1.3 1,365 ________________ SEC merger registration White Pine Dis- _____ do ___________________ SS 62,000 1.35 840 ________________ Form S-14 No. 2—54286, trlct. AMAX, Inc., and Copper Range C0,, July 29, 1975. Minnesota Babbit Lake ______ AMAX Inc -__ CuNi 100,000 .9 770 0.4-percent Ni Skillings Mining Review, Ely/Spruce Rd--- Inco Ltd __________________ CuNi ‘100,000 4.9 1,000 .25-percent Ni July 19, 1975. Montana Butte District (9+ The Anaconda Co ________ UG-P 1,679,000 .72 avg. 12,068+ Mo, Ag, Zn ARGO/Anaconda, merger de carts). and vein filing, Sept. 14, 1976. Hed leston ____________ do ___________________ P 93,000 .48 446 .02-percent Mo Stillwater _____________ do ___________________ CuNi 151,000 .25 378 .25-percent Ni Nevada Victoria __________ The Anaconda Co ________ OP/UG—P 1,900 2.37 45 ________________ ARCO/Anaconda, merger Yerington District: filing. Yerlngton _____________ do ___________________ OP-P 222,400 .32 77 ________________ ARQO/Asnaconda, merger MacArthur ____________ do ___________________ 13,000 .43 56 mmg’ ept' 14’ 1976‘ F6 GEOLOGY AND RESOURCES OF COPPER DEPOSITS TABLE 1.—Published reserves and resources of copper in the United States—Continued Deposit name Company Type of Ore Grade Copper Associated Reference deposit' (percent copper) content metals B. IDENTIFIED RESOURCES IN UNDEVELOPED DEPOSITS AND CLOSED MINES—Continued Nevada—Continued Ann Mason _______ The Anaconda Co. _______ P 495,000 .41 2,029 ________________ Lyon ______________ Anaconda/US. Steel _____ UG-P 32,000 1.22 390 ________________ ARGO/Anaconda, merger filling, Sept. 14, 1976. Bear ______________ The Anaconda Co ________ P 500,000 .40 2,000 Mo, Au, Ag Hall ____________________ do ___________________ P 54,000 .46 248 52 million tons 0.19-percent Mo Wisconsin Crandon __________ Exxon _____________________ UG—M 60,000 1.00 600 6.5-percent Zn The Capital Times, Mad- isogg Wisc., Sept. 8, 1976, - p. . Lad smith/ Kennecott ________________ OP-M 11,000 4.0 450 ________T _______ Skillings Mining Review, F ambeau. April 10, 1976. 'Mining method: OP, open pit, and UG, underground. Geological type: P, porphyry, includes disseminated, stockwork, and skarn; SS, stratabound sedimentary; M, massive sulfide including volcanogenic deposits; Cu-Ni, magmatic copper-nickel deposits; 0, other types. 2Oxide-ore reserve. 'Stockpiled oxide ore. Acid soluble copper content shown. ‘Estimate. “In situ and undiluted resource. Not included in total. ‘Reservee at 57-percent extraction and 9-percent dilution. 1Mixed oxide and sulfide reserves. Losses take place at nearly all stages of mining and processing. Good industry-wide data on mining losses are not available, but as an extreme example of such a loss, in the sedimentary copper deposit at White Pine, Mich., only 52 percent of a mineable reserve block is extracted because of the layered form of the ore deposit and the mining methods used. In the large open-pit mines on porphyry deposits, however, dilution and losses during mining are less significant. Greenspoon and Morning (1976) showed that, in 1975, 82 percent of the contained copper was recovered from the milling and flotation of copper and copper-molybdenum ores treated in the United States. An additional 4 percent of the contained copper in concentrates is lost during smelting, and negligible amounts are lost at the refining stage. The net result is that less than 79 percent of the in-ground copper reserves is eventually converted to usable refined copper. Losses are also substantial in the recovery of copper by in-situ and dump leaching of oxide ores. A partial ofiset to reserves lost in mining and processing is provided by copper recovered in leaching of waste rock containing copper and in-situ leaching of mined underground workings; such recoveries amounted to 9 percent of copper-mine production in 1977. Coproducts, such as molybdenum, gold, or silver, can be important in making a particularly low—grade copper deposit economic. For example, the Sierrita porphyry- copper mine in Arizona at the end of 1975 contained 523 million tons of ore reserve, which had an average grade of 0.32 percent copper and 0.033 percent molybdenum per ton. At a price of $0.70 per pound for copper and $3.20 per pound for molybdenum, the 0.033 percent of molybdenum in the ore can generate a value equivalent to 0.15 percent copper. This gives the Sierrita deposit a “copper equivalent” reserve grade of 0.47 percent copper, which can be increased further when gold and silver by- product credits are considered. In this study, only actual copper content was considered in the calculations. Considerable uncertainty exists in any reserve esti- mate. Even for a single deposit under static economic conditions, two firms may make estimates differing by 50 percent or more. Several sources of underestimations can be recognized. Conservative (too low) estimates re- sult from incomplete exploration of a deposit. Data on the subsurface extent of a mineral deposit are acquired by drilling or tunneling. These activities are very costly, and sound economic reasoning demands that they not be undertaken until the information is needed. Ex- ploration expenditures are thus made at a rate adequate for an assured production, and the true tonnages of many deposits are not known until the deposits are in the last stages of being explored. Reserves and pro- duction data for the Bingham porphyry-copper deposit are excellent examples of this type of underestimate. A reserve estimate for Bingham in 1930 shows approxi- mately half the present reserve (Committee on Mineral Resources and the Environment, 1975). Another source of underestimation results from the existence of State and local taxes on reserves. Exploration efforts may be limited in order to show a reserve large enough to at- tract financing but not so large as to invite excessive taxation. The effect of these types of underestimation may be large relative to a national reserve estimate. Because the tonnage of total ore in copper deposits is distributed lognormally (Singer and others, 1975), the largest deposits contain copper in amounts an order of magnitude more than the mean. Thus, an error in the estimate for the largest deposit may seriously affect the national estimate. THE NATURE AND USE OF COPPER RESERVE AND RESOURCE DATA Some of these errors have been reduced because in recent years mining firms have tended to drill out and evaluate deposits fully and have initially designed the pit or underground mine to extract most of the ore body. In addition, most of the very large deposits were devel- oped in the early part of this century and now are fairly well understood and their total dimensions known. Most important, the national-reserve estimate now tends to be overestimated, at least temporarily. Since 1972, inflation has caused mine and mill operating costs to rise more than 50 percent, whereas copper prices have stayed low because of large worldwide inventories. More than a third of the US. copper mines operated in 1975 at costs per pound equal to or above the average annual copper price of 64.5 cents per pound. During such periods of low copper prices, operators of unprofitable mines who stay in production can be assumed to do so because they anticipate a resumption of more favorable price-cost ratios and are avoiding the high costs of closing and reopening their operations. Thus, even by restricting our usage of the term “reserve” to ore in operating mines, we are still uncertain how much of this ore is being extracted profitably and can be defined as reserves in the strict sense. If we use a somewhat more flexible approach that allows for rapidly changing costs and prices, reserves may be defined by anticipated eco- nomic conditions in the near term. Under this defin- ition, most of the operating properties can be considered as having reserves. Moreover, deposits being considered for development may or may not have reserves as defined by anticipated economic conditions at the time they are expected to come into production, usually 4 to 8 years from the time of the decision to develop (Burgin, 1976). To show the sensitivity of reserves to the economic conditions prevailing at the end of 1975, MAS was used to generate the data shown in table 2. The table lists estimates of the amount of measured and indicated cop- per reserves, in short tons, recoverable at various price levels and at discounted cash-flow rate of return calcu- lated in terms of 1975 dollars. The rates of return were selected to show reserve levels ranging from a break- even cost point (0 percent) to a profit level of 18 percent. TABLE 2.-—The sensitivity of as copper reserves, in millions of short tons, to economic conditions, in 1.975 [Data from H. J. Bennett. written commun., 1977] Cop r price Rate of return (1975 difllm/mn) (percent) 0.60 0.65 0.70 0.75 0.80 82 89 93 102 107 77 78 85 90 94 46 69 76 80 85 40 47 51 56 74 The national reserve estimate is thus somewhat of a moving target, which is underestimated as a result of inherent economic, geologic, and engineering factors and overestimated as a result of falling price/ cost ra- F7 tios. Any reserve estimate must therefore be accom- panied by a statement of the assumptions on which it is based. For example, on the basis of data in this report (tables 1, 4), the following two different U.S. reserve estimates can be made: First, recoverable copper in de- posits in production in 1977 is estimated at 49 million short tons. This most conservative estimate reflects a lack of confidence in new mine development under con- ? ditions of low copper prices and high capital and oper- ating costs prevailing in 1977. It also reflects the loss of copper in the mining, milling, and smelting process. Second, the sum of the reserves in the foregoing cate- gory plus reserves in other deposits whose location and geologic, mineralogic, and engineering characteristics are similar to those being mined during the 1970’s is estimated at 93 million short tons. This estimate is 3‘ made on a deposit-by-deposit analysis of the foregoing factors. The deposits included are those that could be readily brought into production by improved economic conditions or as strategic requirements necessitate. This estimate is the one given in such publications of the US. Bureau of Mines as Mineral Commodity Profiles (Schroeder, 1977). This estimate includes cop— per that may be lost in mining and mineral processing but excludes copper recoverable from leaching waste rock, from dumps and from byproduct copper recov- erable from noncopper ores. These two estimates made by empirical methods com- pare favorably with the estimates generated by using MAS (table 2). The first, conservative, estimate com- pares with MAS estimates of copper available at $060—$070 per pound at a 12—18 percent rate of return. The second, more liberal, estimate compares with esti- mates of copper available at $0.75 to more than $0.80 per pound and at a 6—12 percent rate of return. Copper-reserve estimates of the second, more liberal, type that have been made over the past 45 years are shown in figure 1. Figure 1 shows that from 1930 to 1960, reserve estimates equalled about 27—39 times annual production, indicating that, on the average, mining firms performed sufficient exploration to provide a 30-year supply of primary copper. Reserve estimates made since 1960, however, have approached 49-64 times annual production. This increase probably reflects an increased understanding of the magnitude of the early- discovered large porphyry deposits, such as Bingham ' ' and Morenci, an increased willingness of mining firms to make public their reserves, and an increase in discov- eries resulting from heavy investment in exploration during the 1960’s. It is interesting to compare historic reserve estimates with a curve representing reserves in operating properties as they are now measured and credited back to the year of first production. This curve, whose derivation will be discussed in a later section, can be thought of as approximating the actual amount of copper available through time in developed deposits. In )4. GEOLOGY AND RESOURCES OF COPPER DEPOSITS 100 I I I I l I 90 — . u- e 100 so — . ,° 70 — ’_—\\§ / ~“\ /’\ 50 - 40 ~ 30 e . ' ' O. . 20 — o ' O m a ° 5 E I— E 2 o 10 7 E I 9 — — 10 Lu m 2 ID 8 i .9 7 ~ é g 6 — Z a 5 , 33‘ > > s 4 ~ °= LLI a: m LIJ Lu 2 3— °= z 9 < < Z 2 — z 9.; Q o '6 :> 3 o o g o D: D. 0. _l 1 _ _' < .9 — — 1 < D 8 _ ~.9 3 z - 2 <2: .7 — 7-8 z .6 — 7 '2 it .5 — ' —.5 .4 — —.4 .3 — —.3 .2 — ~.2 _1 I I I l I I _1 1910 1920 1930 1940 1950 1960 1970 1980 YEARS 1925—1977 FIGURE 1.—U.S. reserve estimates and annual production 1925—1977. Dots represent historic reserve estimates plotted from Everett and Bennett (1967), Bennett and others (1973), and the US. Bureau of Mines (1977). The solid line represents annual production from the US. Bureau of Mines Minerals Yearbook. The dashed line represents the actual reserves of operating mines taken from modern data and is calculated from the curves in figure SB. THE NATURE AND USE OF COPPER RESERVE AND RESOURCE DATA the early 1960’s, estimates jumped above this curve as a result of better knowledge and the inclusion of un- developed properties whose reserves were defined by anticipated economic conditions. TOTAL ORE AND CONTAINED COPPER The sum of reserves and past production yields an estimate of the total ore, total metal contained in ore, and average grade of ore originally in each of the de- posits known in the United States. Graphical display of these values (fig. 2) demonstrates the relative mag- nitude of deposits of various types and facilitates a com— parison of the amount of copper in a typical deposit with the annual domestic consumption. Each year, the United States mines the equivalent of all the ore in a large porphyry deposit. A similar chart for worldwide deposits was published by Cox and others (1973) and updated for inclusion in the COMRATE report (Com- mittee on Mineral Resources and the Environment, 1975). These charts clearly reveal that small high-grade deposits, such as those of the massive sulfide type, have contributed little to the national resource total. The very large deposits of this type, of which only a few examples exist, contain about as much copper as a F9 medium-size porphyry deposit. Medium-tonnage mas- sive sulfide deposits contain an order of magnitude less copper. Table 3 summarizes these relationships. TABLE 3.—Total copper contained in typical deposits [Short or metric tons] Deposit Small Medium Large Porphyry ___________________ 105 108 107 Massive sulfide ____________ 10‘ 105 10° Sedimentary _______________ 10‘ 105 106 The values. for total ore and total contained metal accurately reflect the extent and intensity of natural mineralizing processes only Where all the following con- ditions hold: 1. The copper grade falls off rapidly at the boundaries of the deposit, so that a lowering of cutoff grade does not substantially change the tonnage. 2. The form and depth of the deposit are such that all of it can be included in a mining plan and thus be- come an ore reserve. 3. Parts of the deposit were not removed by faulting, erosion, or other postmineralization processes. Clearly, these three conditions are found in few de- posits, so that most estimates of copper contained in ore 100 , EXPLANATION Io ' Porphvrv type ’0 .0, - ‘90 qu o Sedlmentary-type 00 o A Massive sulfide type + Other types E Kennecott, Alaska + o. n_ 10 — __ O U u. Magma, Ariz. O AJerome, Ariz. A ’2 A A m A A A + 8 A + ‘o‘.’ A A + . g Crandon, WIS. White Pine . 1 — Mich. ' . . — g . . o. o . ..... . .MoienCI,'Anz. < o o 0.. o .Bmgham, Utah 0 g 0 Butte, Mont. 0 ° °Florence, Ariz. . 'Sierrita, Ariz. .1 ' 1 1 Million 10 Million 100 Million 1 Billion ORE, IN SHORT TONS FIGURE 2.—Tonnage and grade of reserves plus past production of US. copper deposits. Only published data are shown. Both grade and tonnage scales are logarithmic. Diagonal lines show tons of contained copper in each deposit. F10 are low relative to the total copper originally in miner- alized rock and estimates may be strongly affected by the economic behavior of mining firms. For this reason, Phillips (1975) has objected to the use of such estimates in making geologic deductions or in estimating the ton- nage and grade of undiscovered deposits. A better ap- proximation to the real distribution of copper contained in deposits can be obtained by combining past pro- duction with resource estimates (discussed in the fol- lowing section). RESOURCE DATA Identified resources include mineralized rock that varies greatly in tonnage, metal content, and depth of overburden in widely differing locations. Metal can po- tentially be won from each deposit classed as an identified resource but at great differences in costs per unit and under widely differing technologic and eco- nomic conditions. Consider, for example, as number of tons of copper in mineralized rock in Alaska, y number of tons of metal in a deposit 5,000 ft below the surface in Arizona, and z number of tons of metal in a very low grade deposit for which a metallurgical recovery proc- ess is not known. The sum of 90, y, and z is not a mean- ingful number because it does not tell the user how much metal could be won if only one or two of the following events were to take place: (1) a road and power network were built in Alaska, (2) an economic-recovery method for deep deposits were devised, or (3) a metal- lurgical process for low-grade deposits were discovered. Useful resource estimates should be disaggregated (divided into categories) to the maximum extent per- mitted by the availability of data (Singer, 1975). A fac- tor limiting disaggregation of data is the requirement that reserve data on certain deposits be kept confiden- tial; data obtained in confidence are aggregated with other data to protect company proprietary interests. In this study, identified copper resources were classi- fied into seven categories (table 4). Category 1 refers to known resources. Category 2 is composed of deposits similar to those profitably mined during the early 1970’s. These deposits would be transferred to the re- serve category as soon as a firm announces plans for development. Category 3 represents a large quantity of copper in resources that are of lower grade than that permitted by the present operation and (or) that require an un- evaluated milling technique or represents ores that are deeper or have a higher stripping ratio than do those exploited in the present operation. These resources may be transferred to the reserve category at some future time when ores now being exploited in these deposits become depleted. This category includes about 8 million tons of copper estimated to exist in the White Pine deposit in Michigan in beds that are too thin and (or) too deep for economic mining at present. GEOLOGY AND RESOURCES OF COPPER DEPOSITS Category 4 represents some of the most readily acces- sible sources of copper, that are in mines that could be TABLE 4.-Coppe'r resources, in 1}illions of tons, of the United States as o 1978 Copper Categories Short tons Metric tons 1. Reserves in place in operating mines or in 64 58 deposits for which development plans have been announced. 2. Resources in drilled-out deposits awaiting 15 30 development. Mainly porphyry deposits in Arizona, New Mexico, Nevada, and Montana. 3. Resources in operating mines but not in- 75 68 cluded in mining plan. Mainly in five of the largest porphyry deposits in Utah, Arizona, and New Mexico and in sedi- mentary deposits in Michigan. 4. Resources in mining properties closed since 4 10 1974 because of unfavorable economic conditions. 5. Resources in drilled-out deposits at depths 27 25 requiring high-cost underground mining methods or as et undeveloped in-place leaching meth 5; mainly in Arizona and Utah. About half the total is in deposits having grades of more than 0.75-percent copper. 6. Resources in drilled-out deposits in Wash- 4 4 ington, Wyoming, Minnesota, and Wis- consin, where environmental restrictions have contributed to delays in devel- opment. 7. Resources in drilled-out deposits in remote 5 5 locations in Alaska. About 60 percent of the total is in deposits that have grades of more than 0.75-percent copper. List of deposits included in resource categories Category 1. Arizona: Ajo, Bagdad, Bluebird, Bruce, Esperanza-Sierrita, Inspiration, John- son, Magma, Marble Peak, Mineral Park, Mission, Morenci Metcalf, Palo Verde, Pinto Valley, Ray, Socaton, San Manuel-Kalamazoo, San Xavier, Silver Bell, Twin Buttes. Michigan: White Pine. Montana: Black Pine, Butte-Berkeley. New Mexico: Chino, Continental-Bayrad, Tyrone. Nevada: Copper Basin Tennessee: Duckt'own. Utah: Bingham, Carrfork. 2. Arizona: Cactus, Carpenter, Chiliw, Copper Basin, Florence, Helvetia-Rose- Mont. Miami East, Mineral Butte, Sanchez, Van Dyke, Vekol. Montana: Heddleston-Lincoln, Spar Lake, Stillwater, Twin Bridges. Nevada: Ann Mason, Bear, Lyon, MacArthur. New Mexico: Pinos Altos. Oklahoma: Mangum. . Arizona: Ajo, Christmas, E speranza-Sierfita, Inspiration, Morenci, Ray, San Manual, Silver Bell, Twin Buttes. Michigan: White Pine. Montana: Butte. Nevada: Ely-Ruth. New Mexico: Chino. Utah: Bingham. 4. Arizona: Lakeshore, Pima, Christmas, Yerington. Copper Canyon. Nevada: Ely-Ruth, Victoria. New Mexico: Nacimiento. Oklahoma: Creta. . Arizona: Red Mountain, Safiord-Kenneeott, Safiord-Phelps Dodge, West Casa Grande. Utah: Tintic South. . Minnesota: Babbitt Lake, Ely-Spruce Road. Washington: Flagg Mountain, Glacier Peak, Margaret, North Fork, Sultan. W' 'C.‘i-‘“PL“’. Wyoming: Kerwin. Alaska: Arctic Camp, Bornite, Brady Glacier, Horse Creek, Picnic Creek, Orange Hill. 03 O! 03 :'~I THE NATURE AND USE OF COPPER RESERVE AND RESOURCE DATA brought back quickly into production under favorable economic conditions. The longer the period of closure, however, the more difficult it is to reopen the mines because of the dispersal of trained workers to other projects and the inevitable decay of excavation slopes or underground workings. Category 5 constitutes resources that are most sensi- tive to the effects of technological development. The role of technological change in the copper industry is diffi- cult to assess. The expectation that technological ad- vances will transform low-quality resources into copper ore has some historical precedent. From 1904 to 1913, the application of large-scale mining machinery and new mineral-concentrating techniques to copper mining made possible large economies of scale (Parsons, 1933; Joralmon, 1973). These permitted a reduction of eco- nomic grade from about 6 percent in previously mined vein deposits to 2 percent in large-tonnage porphyry deposits. Since that period, ores of continually declining yield have been profitably mined (fig. 3), testifying to the continuing improvement in mining and beneficiation 1.5 F11 techniques. The 1904—1913 changes in technology were major breakthroughs; the changes after that period were brought about by fine tuning of existing techniques. For most of the deposits in category 5 to become econom- ically attractive, major technological breakthroughs are required. These breakthroughs must not depend on ex- travagant uses of energy, because of its rising cost (Rosenkranz, 1976). In-place fracturing and leaching of deposits represent a potential for important tech- nological change, but many problems are still unsolved. Because deposits in category 5 are low in quality, ex- ploration firms are reluctant to spend the effort neces- sary to quantify their resources fully. For this reason, large but unknown quantities of copper are excluded from the estimate. Principal among these are resources of native copper in deep extensions of mines closed since the 1950’s in the Upper Peninsula of Michigan. These resources could potentially equal in magnitude the quantity shown in category 5. Category 6 illustrates a recent trend toward in- creased restrictions on mineral development brought ._‘ _. _\ _| —a o L. N w h 2.0 ANNUAL YIELD OF COPPER MINED, IN PERCENT .1 l | l l | | i l l l l | i | l l | I l 6 8 1940 2 4 6 8 1950 2 4 6 8 1960 2 4 6 8 1970 2 4 6 YEARS FIGURE 3.—Declining yield (percent of metal produced from ore mined) of US. copper mines, 1936-1976. F12 about by environmentally concerned groups at the local, State and Federal levels of government. The effects of environmental concern are felt most strongly when new deposits are discovered in regions without longstanding copper-mining traditions. In Minnesota, Washington, Wisconsin, and Wyoming, creative exploration concepts and willingness to apply modern technology to hitherto unexploited deposits (copper-nickel resources near Dul- uth, Minn.) stimulated investment in new projects in the 1960’s and early 1970’s. Most of these projects have been abandoned or indefinitely postponed, partly be- cause of unfavorable economic conditions but mainly because of resistance from groups concerned with threatened changes in the natural environment. The resource total for category 6 includes only drilled-out deposits and does not include large resources of copper estimated to exist in the Duluth Complex of Minnesota (Bonnichsen, 1974). Deposits in Alaska are placed in a separate category (7) because of the special conditions placed on devel- opment by their remoteness and by the harshness of the environment. Costs of mining low-grade ores in Alaska in 1973 were 67 percent higher than those for similar deposits in Arizona (Bottge, 1974). Even such a high- grade deposit as Kennecott, which was profitably mined between 1911 and 1938, would be uneconomic under 1973 conditions if the operation had to bear the cost of road construction to the mine site (Maloney and Bottge, 1973). Figure 4 shows diagrammatically the relative eco- nomic feasibility of exploitation and the amount of geologic uncertainty of these categories of identified re- sources. The figure is based on the resource-classification diagram introduced by McKelvey (1972) and discussed GEOLOGY AND RESOURCES OF COPPER DEPOSITS by Schanz (1975) and Brobst (1979). Rectangles of vary- ing sizes have areas proportional to the tonnage esti- mates in the various categories. These rectangles are arranged vertically with likelihood of future production decreasing downward and uncertainty of the estimate increasing to the right. The arrangement is somewhat subjective, and readers having special knowledge might prefer to shift some of the rectangles up, down, or side- ways. The relative position of the rectangles may change with time, depending on breakthroughs in min- eral technology and changes in domestic economic con- ditions and international commodity prices. Figure 4 differs from McKelvey’s original diagram in two ways. First, because a scale is used, no lower or right-hand margin can be drawn, as this would imply quantitative knowledge of the total amount of copper to be discovered and mined in the United States. Second, the division between economic and subeconomic depos— its is not shown as a single line because it changes with economic conditions. This division is shown graphically by a curve representing an estimate of the trend in cost/price ratios for the last 25 years. This curve is based on published U.‘S. reserves for 1950, 1960, and 1964; on the intensity of mineral exploration in late 1960’s; on the tonnage, grade and location of targets of interest during that period; and, finally, on the pre- viously mentioned estimate that in 1975 a third of the US. copper mines were operating at a loss. The vertical axis of the plot has no scale, but a careful economic analysis of the copper industry could produce a scale based on copper prices and mining costs. On the right side of figure 4, rectangles represent undiscovered resources. Their dimensions are based on estimates by Cox and others (1973) of about 100 million Identified Undiscovered Measured/indicated Inferred Hypothetical Speculative l \\\ Economic / g \ 1 I < \ / a: \\ // 4 2 / § \ / / _ _ _ _ _ _ _ _A m \ [/6 [y/ & 53 8 I/ §/ /7 3 (D , Z Subeconomic / (D < u‘J ALE' U SC . Z =1o,ooo,ooo Short tons, l 1 l l 1 9,000,000 Metric tons 1950 1955 1960 1965 1970 1975 FIGURE 4.—Classification of US copper resources and reserves. See text for explanation. THE NATURE AND USE OF COPPER RESERVE AND RESOURCE DATA tons in each of the two categories, hypothetical and speculative. Because of the large amount of copper known to exist in identified deposits, no further analysis of undiscovered resources has been made by the US. Geological Survey. Undiscovered resources are large; what must be stressed is that a large amount of effort is required to find the undiscovered deposits. PRODUCTION DATA For each mine-mill unit in the United States, records of annual production capacity, in terms of copper metal, as well as records of copper production of past years, have been collected. Total capacity of the 38 producing deposits is 2 million tons of recoverable copper annu- ally. On the basis of these data, future annual pro- duction capacity has been estimated following methods described by Zwartendyk (1974) and by Martin and others (1976). Annual production capacities are compared with total contained copper in ore on a logarithmic plot (fig. 5). Capacities range from less than 1,000 tons to 300,000 tons of copper per year and have a geometric mean of F13 33,000 tons per year. Logarithms of production capaci- ties of metal mined per year show a high correlation with the logarithm of total contained copper per deposit. Two regression equations are given as follows. One predicts annual production capacity, 0, given total con- tained metal in ore reserves and past production, M. A second predicts M given C: log C = 0.65210gM + 0.572 log M = 1.310 log 0 + 0.134 The correlation coefficient R for these equations is 0.925. Other regression equations relating annual ca- pacity to total ore and to total ore and grade were tested and gave no significant improvement in the correlation coefficient. Lines of planned mine life can be drawn through the data shown in figure 5; all deposits falling on one such line have the same ratio of production capacity to con- tained metal. The geometric mean of the data falls on the 30-year mine-life line. Note that the slope of the regression line is less than one, indicating that large deposits tend to have a lower ratio of production to 6.0 , 9" o LOG ANNUAL PRODUCTION CAPACITY (C), IN SHORT TONS P o l I 5.0 6.0 7.0 8.0 LOG RESERVES PLUS PAST PRODUCTION (M), IN SHORT TONS FIGURE 5.—Log annual production capacity (C) in relation to log total copper in ore (reserves plus past production (M ) for 34 deposits (+). The solid line is the regression line. Dashed lines show ratios of annual capacity to contained metal or years of production per deposit. F14 contained metal than do medium to small deposits. For example, the plot shows that 2 million tons of copper in 10 deposits (each having 200,000 tons contained copper) would be expected to yield slightly more metal per year than would one deposit containing 5 million tons. The above relationships describe the past behavior of U.S. mining firms in selecting the scale of operations for new mine developments. If this behavior is the same as that of mining firms in other countries, the relationships may be an important consideration in relating supply to reserves. Where most of a nation’s reserves are con- tained in a few very large deposits, the reserve estimate could be as large as 100 times the annual production capacity. Where reserves are generally contained in many small deposits, reserves could be as small as 10 times the annual production capacity. Conversely, if past planning behavior continues into the future, we can say that the discovery of several medium-size deposits would yield a higher annual production than the equiva- lent tonnage in one large deposit. The prediction of copper-production potential has serious limitations. We assume, for example, that the behavior of mining firms in selecting production rates will remain unchanged when, in fact, this behavior de- pends on such economic variables as inflation rate and cost of capital investment, as well as on new environ- mental regulations and improvements in extraction technology. Another limitation is the high variability of the data, masked by the logarithmic scale used in figure 5. Note, for example, that for a l-million-ton deposit, capacities range from 15,000 to 60,000 tons per year. Although precise statements cannot be made about the confidence limits in the real data because logarithms are used, the predicted value of capacity is probably within a factor of four of the real value. Table 5 shows the results of applying the regression equation of figure 5 on a deposit-by-deposit basis to some of the resource categories of table 4. Estimates are not made for categories 3, 5, and 7 because it is not realistic to apply the regression equation to resources that cannot be exploited in the near future or to Alaska, where mining conditions are so different from those of the lower 48 States. The totals in table 5 are the result of solving the regression equation for each deposit. Ap- plication of the equation to the sum of the copper re- sources in any category gives a much smaller aggregate annual production potential because, by this method, the contribution of medium and small deposits is underestimated. ANTICIPATED COPPER PRODUCTION Future production of primary copper depends on the aggregate mine and mill, smelter, and refinery capaci- ties and on the reserves of the commodity. If plants are operating near capacity, supply is slow to respond to GEOLOGY AND RESOURCES OF COPPER DEPOSITS TABLE 5.—Potential annual production, in 1,000 tons per year,from US copper deposits as of 1978. Short or metric tons per year 1 Operating minesl 2,000 2 Good deposits 500 4 Closed mines 300 6 Mines having environmental problems ________________ 150 ‘ Actual capacity plus planned increases in capacity as of 1?”. Categories increases in demand because existing mines and plants require significant time to expand and because grass- roots development of drilled-out deposits requires 4—8 years from investment decision to production (Burgin, 1976). Future supply may be increased by new mineral technology that may make possible the conversion of subeconomic resources into reserves and by mineral ex- ploration that converts undiscovered resources into re- serves. Because the rate of technologic advance and mineral discovery is difficult to predict, no great confidence can be placed in estimates of total future supply. Considerable confidence can be placed, however, in a minimum estimate of future production. This estimate is made by first assuming that no new deposits will be developed, that no extreme fluctuations in demand and price will take place, and that no changes in productive capacity will be made other than the planned changes already announced by mining firms. For each copper mine in the United States, a depletion date was calcu- lated assuming production at full capacity. For any year, the sum of the individual annual production capacities for deposits not yet depleted provides an estimate of minimum U.S. production capacity. For ex- ample, a deposit having 100 million tons of reserves in 1978 at a grade of 1 percent copper will contain 1 million tons of copper. If production capacity is 40,000 tons of metal per year and if 80 percent of the copper is recov- ered from the ore, then the life of the deposit operating at capacity is 20 years, and its depletion date is 1998. The capacity of this mine is thus part of aggregate U.S. production for all years up to and including 1998. Figure 6 shows a curve of minimum production for 1976—2030, based on 1978 data. The curve rises between 1976 and 1979 because of planned increases in capacity and announced development of new deposits, then falls steadily as existing small- and medium-tonnage depos- its are depleted. Some of the largest deposits in the United States can be expected to supply copper until the year 2030. The area under this curve is equivalent to the United States reserves and resources in categories 1 and 4. Zwartendyk (1974) has emphasized that this type of predictive calculation is more useful than that which simply compares the national reserve to anticipated cu- mulative consumption. A family of curves may be drawn, each having a sim- ilar slope and enclosing increasingly larger areas. These areas represent tons of copper available under different THE NATURE AND USE OF COPPER RESERVE AND RESOURCE DATA F15 | l l l l AProbable total demand from mines and old scrap (Schroeder, 1977) KP / robable primary demand 10,000,000 Short (D Z O ’— j.— 0: O I (I) TO. 2 from mines (Schroeder, tons Copper 2 3 _ 1977) . o . j l: Copper requrred from 0 new sources 8 T \ O \ Capacity from reserves plus E resources in categories 2 _ \2 and 5 (Table 4) — O: u.| Capacity from deep\\ & or low grade reserves 0 in operating mines, \ 0 category 3 (Table 4) \ A ................. k- \ g 1 H ' . ............ 5 ........ _ 2 Historic mine production production capacrty from \ ............... z \ < \\ \ l l l mm 1950 1960 1970 1980 1990 2000 2010 2020 2030 YEARS FIGURE 6.—Comparison of copper demand with US. production capacity projections, 1976-2030. (Demand projection from Schroeder, 1977.) assumptions about the rate at which they will be con- verted into reserves. One of these curves, enclosing an area of 47 million tons, is shown in figure 6. This curve was made by assuming that the previously mentioned identified deposits, similar to those being mined (cate- gory 2), and those in deeply buried deposits (category 5) will be developed between 1985 and 1994. Other curves enclosing various categories of subeconomic deposits, as well as hypothetical and speculative resources, could be drawn, each with increasing uncertainty. Deposits of category 3, identified subeconomic resources in oper- ating properties, will probably have no significant effect on production until the reserves in those deposits are nearly depleted. Thus, the tonnage of copper in this category will tend to raise the curve of minimum pro- duction capacity at its lower end and extend the curve considerably into the future. REQUIREMENTS FOR NEW SOURCES Two demand projections for copper, also shown on figure 6, have been taken from Schroeder (1977). The total demand projection is the amount of copper needed regardless of source. Because the amount of copper that can be recovered from old scrap is limited by cost and energy availability, 3 large part of total demand must be met by mined copper, either domestic or imported. This part is called primary demand. The most likely primary-demand projection based on a 2.9 percent per year growth rate indicates that demand is expected to be greater than mine capacity from reserves in category 1 by about 1983 and to be greater than total possible capacity from deposits in categories 2 and 5 by 1994. From 1994 on, the graph shows a widening gap between the primary-demand projection and the annual production-capacity curve. This gap represents the re- quirements of copper from new sources. The magnitude of requirements for new sources can- not be precisely determined because some past and cur- rent discoveries of mining firms are kept confidential. Discussion with mining-company geologists indicates that the number and size of deposits in this discovered but unpublicized category is small in relation to the gap shown in figure 5. A greater uncertainty in the mag- nitude of the requirements from new sources is the slope of the demand projection. Although it is beyond the scope of this study to examine the assumptions be- hind such projections it is important to note that projections vary widely. Figure 6, used as an inter- pretive tool, may be combined with any appropriate demand projection. The projected 3 percent growth in primary demand shown in figure 6 is based on Schroeder F16 (1977). If a different projection is used, such as the 1.9 percent growth rate forecast by Malenbaum,2 require- ments from new sources are considerably smaller. Requirements from new sources can be met in one or a combination of four ways: by increasing imports, by increasing plant capacities at operating properties, by technological improvements converting subeconomic resources to reserves, and by discovery and develop- ment of new deposits. Imports of copper are likely to increase in the next 20 years as costs of domestic production rise above average costs of production in the rest of the world. In the devel- oping countries, copper mining and exploration have been less intensive than in the United States, and major low-cost reserves are still being discovered and devel- oped. Chile’s identified copper reserves of 107 million tons (Sutulov, 1977), for example, have doubled since the 1960’s and now are greater than US. reserves. These reserves are in ore that has an average grade of 1.0—1.2 percent copper compared with 07—08 percent for US. resources. The relationship between grade of reserves in the United States, Chile, Peru, Zambia, and Zaire is shown in figure 7. Not only the decreasing grade of do- mestic production but also the increasing depth of de- posits and the increasing stringency of environmental controls are driving up US. mining costs relative to those in the rest of the world. What effect increased imports will have on prices and assured copper supply for US. industry is difficult to predict. Although a cartel-like organization has been formed by Chile, Peru, Zaire, Zambia, and other copper-producing nations, its success in controlling prices is believed to be in- creasingly unlikely in view of the large number of coun- tries now known to have important copper resources (Council on International Economic Policy, 1974). Ra- detzki (1977) has shown, however, that six developing countries having large low-cost copper reserves (Chile, Papua-New Guinea, Peru, Philippines, Zaire, and Zam- bia) could gain a 51-percent share of the world market by the year 2000. Investment in expansion of. production of high-cost reserves in the United States and Canada, as well as in new production from countries that have no present copper industry (Argentina and Panama), would be curtailed in the face of increased production from the six countries, according to Radetzki. Increases in plant capacities at presently operating properties can temporarily raise the minimum production-capacity curve. Production increases, how- ever, increase the rate of mine depletion and make the slope of the curve steeper. The supply problem is only postponed by a few years. As discussed earlier, technological improvements, such as those in the field of in-place leaching of ores, may contribute new production to fill the widening gap. ‘ Malenbaum. Wilfred, 1977, World demand for raw materials 1985-2000: National Science Foundation unpublished report, p. 116. GEOLOGY AND RESOURCES OF‘ COPPER DEPOSITS Energy- intensive methods for winning metals from low- grade deposits, however, will make only small con- tributions to increasing production. Discovery and development of new deposits can pro- vide sufficient production to fill the widening gap. If we assume a 3 percent growth in demand, by 2000, the gap between production capacity from known deposits and the most probable primary demand will approach 1 mil- lion tons of annual production. This gap could be closed by the discovery and development of 3 giant deposits or about 30 average-size deposits (1 million tons of con- tained copper each). Because of the 4—8 year lead time between discovery and first production, this 30 million tons of new copper reserves must be identified by 1985 or 1990, calling for a rate of discovery and development of about 2 million tons of copper in ore per year. To put this requirement in proper perspective, we may wish to know at what rate copper ore has been discovered in the past. Reliable data on discovery dates are difficult to obtain, but as a close approximation, dates of first production can be used. In general, the first production year follows, by 4—8 years, the discovery or first recognition of the economic value of a deposit. Fig- ure 8A shows the total tonnage of copper in reserves brought into production in 10-year periods. The vertical bars are totals of past mine production and reserves as they are now known, credited back to the year of first production. Black bars represent the number of deposits developed in the same lO-year period. The bar graph shows a very large peak at the begin- ning of the century that represents the development of the Bingham deposit in Utah in 1904, Morenci in Ari- zona in 1907, and the Robinson District in Nevada in 1908. From 1910 to 1920, the Miami, Inspiration, Ray, and Ajo deposits in Arizona and the Chino deposit in New Mexico were brought into production. A lull during the depression years was followed by increased devel- opment in the World War II era, marked by the opening of San Manuel in Arizona in 1956. The bars for the 1980—1990 period are projected on the optimistic as- sumption that all the known deposits of categories 2 and 5 will be developed within that period. Since 1960, the number of deposits developed per 10-year period has increased greatly without a commen- surate increase in tonnage of copper. This trend may result partly from inadequate knowledge of the true tonnage of recently developed deposits. In recent years, however, firms increasingly have tended to drill out re- serves fully before mining is begun. The‘ trend more probably reflects decreasing grade (and copper content) of deposits and increasing numbers of small deposits containing small amounts of copper. For example, the average grade in the 38 producing mines in the United States is 0.74 percent copper compared with an average grade of 0.53 percent in the undeveloped deposits of categories 2 and 5. THE NATURE AND USE OF COPPER RESERVE AND RESOURCE DATA I | I | I I I I | | I I I I I I I I 110- _ o) 100— - LU 2 Z I _ _ <3 3 90 .— ‘1’ < "’ 80— LIJ '— — cc < O I I— _ E Z 70 United States — 95 s g I— 60— — O c: ”E 5 9 50 — — (I) CC 3 o 40— _ *— o u_ F— O _ w 2' 30 — I) Z o 8 3 20— _ :‘ 2 Zambia 10— Zaire _ | l I I I | I I I I I I | | I I I I 0.3 0.4 0.5 0.6 0.7 0.8 0.91.0 1.2 1.4 1.61.82.0 2.5 3.0 4.0 5.0 6.0 COPPER GRADE, IN PERCENT FIGURE 7.——Cumulative reserves and resources at various grades in the United States, Chile Peru, Zambia, and Zaire, as of 1977. US. reserves and resources include categories 1, 2, 4, 5, and 6 (table 4). For the other four countries, all published reserves estimates were included. Figure 8B shows the historic development of copper in ore and is derived by plotting cumulatively the same data used to make the bar graph in figure 8A. This cumulative curve is compared with cumulative mine production and consumption. For any year, the US. re- serve in producing mines can be found by subtracting the amount of copper shown by the cumulative mine- production curve from the cumulative copper in devel- oped ore. The reserve values thus calculated are plotted on figure 1 and compared with historic reserve esti- mates. The early estimates are low mainly because the magnitude of reserves in some of the largest US. de- posits have only become known in recent years. The curve of cumulative copper in developed ore rises sharply in the 1900—1920 period because techno- logical breakthroughs in material handling and min- eral processing affected economic evaluation of copper deposits that had been discovered many years before. The slope of the curve between 1950 and 1980 is equal to slightly more than a million tons of contained-cop- per per year and represents the development of de- posits whose discoveries required greater exploration effort. The copper deposits now being mined were mainly recognized in surface outcrop. Many are surrounded by a halo of smaller base- and precious-metal deposits, which drew the attention of prospectors in the late 1800’s. Deposits to be found in the future will be those deeply covered by postmineral sedimentary and volca- nic rock or that have no halo of other deposits. Thus, future discoveries will be considerably more difficult to make than were the discoveries of the deposits making up our present reserves. Future discoveries must also meet increasingly severe production-cost criteria in order to obtain financing for development in com- petition with low-cost foreign deposits. F18 ' GEOLOGY AND RESOURCES OF COPPER DEPOSITS 0 4 7 35 _ EXPLANATION Copper (10° tons) in are developed during 10-year periods - Number of deposits 30- Identified undeveloped resources 25— 20~ 15— 10— 160 — 7‘ 140 V 9/ _ 120— Copper in developed are 100 _ 80- 70* 60- 50w 40* 30~ 20~ 10— CUMMULATIVE COPPER, IN 10° SHORT TONS I I I I I I I I I I 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 B THE NATURE AND USE OF COPPER RESERVE AND RESOURCE DATA CONCLUSIONS Some of the more notable conclusions that can be drawn from this study are as follows: 1. US. reserve estimates in the past have been low mainly because of incomplete knowledge of the magnitude of the very large porphyry deposits. Present estimates are considerably more reliable, mainly because mining firms tend to drill out de- posits fully before mining and because many of these companies are willing to make reserve esti- mates public. The effect of economic fluctuations on reserve estimates can be minimized if near- term anticipated economic conditions rather than present conditions are used to define reserves. 2. The wide variation in tonnage, grade, and contained metal in copper deposits depends largely on the geologic type of deposit. Of the four common de- posit types, porphyry deposits are the most abun- dant, have the highest metal content, and largest aggregate production potential. 3. Resource data should be presented in the most disag- gregated form possible because of the wide variety of conditions required to transform resources into reserves. Of the various resource categories dis- cussed in this report, the largest tonnage of copper is contained in the deep parts of a small number of deposits known to be the largest in the country. These resources probably cannot make contribu- tions to our supply until well after the year 2000, when present mining operations have nearly de- pleted the reserves and a changeover to a new mine plan can be made. ‘ 4. A high correlation exists between logarithms of an- nual production capacity and tonnage of contained copper in past production and reserves. A re- gression equation can be used to calculate the probable production capacity of a group of un- developed deposits, but the large scatter of the an- tilog data makes such estimates accurate only within a factor of four. Analysis of production data shows that reserve requirements are smallest if annual production comes from medium- to small- tonnage deposits or, conversely, that large deposits yield a disproportionately small annual production relative to their large metal content. 5. A minimum future production capacity can be calcu- lated by totaling, for any year, the production cap- F19 acities of all deposits not yet depleted by that year. A curve can be drawn through these points show- ing declining production as smaller deposits are depleted. Such a curve, when compared with projected primary consumption is useful in esti- mating the requirements of copper from such new sources as imports or deposits not yet discovered. A comparison with one demand projection (Schroeder, 1977) suggests that if the new sources are to be from copper in domestic deposits, this copper must be found and developed at the rate of 2 million tons per year. Discovery and development of copper in the United States since 1950 has been at a rate slightly greater than 1 million tons per year. REFERENCES CITED Bennett, H. J ., Moore, Lyman, Welborn, L. E., and Toland, J. E., 1973, An economic appraisal of the supply of copper from primary domestic sources: US. Bureau of Mines Information Circular 28598, 156 p. Bennett, H. J ., Thompson, J. G., Quiring, H. J ., and Toland, J. E., 1970, Financial evaluation of mineral deposits using sensitivity and probabilistic analysis methods: US. Bureau of Mines Information Circular 8495, 82 p. Bonnichsen, Bill, 1974, Copper and nickel resources in the Duluth Complex, northeastern Minnesota: Minnesota Geological Survey Information Circular 10, 24 p. Bottge, R. G., 1974, Comparative porphyry copper mining and proc- essing costs—Alaska and Arizona: US. Bureau of Mines Informa- tion Circular 8656, 55 p. Bowen, R. W., and Botbol, J. M., 1975, The geologic retrieval and synopsis program (GRASP): US. Geological Survey Professional Paper 966, 87 p. Brobst, D. A., 1979, Fundamental concepts for the analysis of resource availability, in Smith, V. K., ed., Scarcity and growth recon- sidered: Baltimore, Md., published for Resources for the Future by the Johns Hopkins University Press, p. 106—142. Burgin, L. B., 1976, Time required in developing selected Arizona copper mines: US. Bureau of Mines Information Circular 8702, 144 p. Committee on Mineral Resources and the Environment, National Re- search Council, 1975, Resources of copper, in Mineral resources and the environment: Washington, DC, National Academy of Sciences, p. 127—187. Cook, Earl, 1976, Limits to exploitation of nonrenewable resources: Science, v. 191, no. 4228, p. 677—682. Council on International Economic Policy, 1974, Special report critical imported material: Washington, DC, US. Government Printing Office, 110 p. Cox, D. P., Schmidt, R. G., Vine, J. D., Kirkemo, Harold, Tourtelot, E. B., and Fleischer, Michael, 1973, Copper, in Brobst, D. A., and Pratt, W. P., eds., United States mineral resources: U.S. Geologi- cal Survey Professional Paper 820, p. 163-189. (_ FIGURE 8.—Plots showing rate of development of copper in ore in the United States, 1900 to 2000. A, Tonnage of copper in reserves and number of deposits brought into production and potentially producible, by 10-year periods, 1900—1980. Tonnage of copper is shown in crosshatched bars. Number of deposits developed is shown by black bars. Bars shown for the period 1980—1990 represent copper in identified deposits potentially producible (categories 2 and 5, table 4). B, Cumulative copper in developed ore reserves, cumulative consumption, and cumulative mine production. The copper-in-ore curve is projected to 1990 and 2000. The consumption curve is projected at a 3-percent growth rate (Schroeder, 1977), and the mine production curve is projected at the rate of past increase, 2 percent. For any year, the difference between cumulative copper in developed ore and cumulative mine production equals total US. ore reserves for that year, as calculated from modern reserve data. F20 Everett, F. D., and Bennett, A. J., 1967, Evaluation of domestic reserves and potential sources of ores containing copper, lead, zinc, and associated metals: U.S. Bureau of Mines Information Circular 8325, 78 p. Greenspoon, G. N., and Morning, J. L., 1976, Froth folation in 1975—advance summary: U.S. Bureau of Mines Mineral Industry Surveys, 23 p. Joralmon, I. B., 1973, Copper: Berkeley, Calif, Howell-North Books, 407 p. McKelvey, V. E., 1972, Mineral resource estimates and public policy: American Scientist, v. 60, no. 1, p. 32—40. McMahon, A. D., 1965, Copper, a materials survey: U.S. Bureau of Mines Information Circular 8225, 340 p. Maloney, R. P., and Bottge, R. G., 1973, Estimated costs to produce copper at Kennecott, Alaska: U.S. Bureau of Mines Information Circular 8602, 35 p. Martin, H. L., Cranstone, D. A., and Zwartendyk, Jan, 1976, Metal mining in Canada to the year 2000: Resources Policy, v. 2, no. 1, p. 11-24. Metals Week, 1973—74, Metals sourcebook: New York, McGraw Hill, (bi-weekly publication). Mining Journal, 1972-75, Mining annual review: London, Mining J our- nal Ltd. Parsons, A. B., 1933, The porphyry coppers: New York, American Institute of Mining and Metallurgical Engineeers, 581 p. Phillips, C. H., 1975, Discussion: A resource analysis based on por- phyry copper deposits and the cumulative copper metal curve using Monte Carlo simulation: Economic Geology, v. 70, no. 8, p. 1484. Radetzki, Marian, 1977, Mineral commodity stabilization: The produc- ers view: Resources policy, v. 3, no. 2, p. 118—126. Ridge, J. D., ed., 1968, Ore deposits of the United States, 1933—1967 (Graton-Sales Volume): New York, American Institute of Mining Metallurgical and Petroleum Engineers, 2 v.: 1880 p. GEOLOGY AND RESOURCES OF COPPER DEPOSITS Rosenkranz, R. D., 1976, Energy consumption in domestic primary copper production: U.S. Bureau of Mines Information Circular 8698, 22 p. Schanz, J. L., 1975, Resource terminology: an examination of concepts and terms and recommendations for improvement: Electric Power Research Institute (Palo Alto, Calif.), EPRI report 336, 116 p. Schroeder, H. J., 1977, Copper: U.S. Bureau of Mines Mineral Com- modity Profiles, no. 3, 19 p. Singer, D. A., 1975, Mineral resource models and the Alaskan Mineral Resource Assessment Program, in Vogely, W. A., ed., Mineral materials modelling: Washington, D.C., Resources for the Future, p. 370—382. Singer, D. A., Cox, D. P., and Drew, L. J., 1975, Grade and tonnage relationships among copper deposits: U.S. Geol. Survey Profes- sional Paper 907—A, p. Al—All. Sutulov, Alexander, 1977, Chilean copper resources said to be world’s largest: American Metals Market, v. 85, no. 150, p. 18. Titley, S. R., and Hicks, C. L., eds., 1966, Geology of the porphyry copper deposits southwestern North America: Tucson, Ariz., Uni- versity of Arizona Press, 287 p. Tourtelot, E. B., and Vine, J. D., 1976, Copper deposits in sedimentary and volcanogenic rocks: U.S. Geological Survey Professional Paper 907—C. 34 p. U.S. Bureau of Mines, 1977, Copper, in Commodity data summaries, 1977: Washington, D.C., p. 46—47. U.S. Bureau of Mines and US. Geological Survey, 1980, Principles of a resource/reserve classification for minerals: U.S. Geological Survey Circular 831, 5 p. Zwartendyk, J ., 1974, The life index of mineral reserves—a statistical mirage: Canadian Mining and Metallurgical Bulletin, v. 67, no. 750, p. 67—70. fiU.S. GOVERNMENT PRINTING OFFICE: |98I- 361-6I4/287 Geology and Resources of Copper Deposits GEOLOGICAL SURVEY PROFESSIONAL PAPER 907 This volume was published as separate chapters A—F UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director CONTENTS [Letters designate the chapters] (A) Geology and Resources of Copper Deposits—Grade and Tonnage Relationships Among Copper Deposits, by D. A. Singer, Dennis P. Cox, and Lawrence J. Drew (B) Geology and Resources of Copper Deposits—Geochemical Exploration Techniques Applicable in the Search for Copper Deposits, by Maurice A. Chaffee (C) Geology and Resources of Copper Deposits—Copper Deposits in Sedimentary and Volcanogenic Rocks, by Elizabeth B. Tourtelot and James D. Vine (D) Geology and Resources of Copper Deposits—Fluid-Inclusion Petrology—Data from Porphyry Copper Deposits and Applications to Exploration, by J. Thomas Nash (E) Geology and Resources of Copper Deposits—The Potential for Porphyry Copper-Molybdenum Deposits in the Eastern United States, by Robert Gordon Schmidt (F) Geology and Resources of Copper Deposits—The Nature and Use of Copper Reserve and Re- source Data, by Dennis F. Cox, Nancy A. Wright, and George J. Coakley