f|f0(5 5 j m l
University of Nevada Reno
The qalera Vein System, Orcopampa District, Southern Peru: Association of Tectonism, Magmatism and Hydrothermal Activity in the Formation of a Bonanza Ag-Au Deposit
A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geology
by
Peter Craig Gibson Donald C. Noble, Dissertation Advisor December 1992 The dissertation of Peter Craig Gibson is approved: Dr. Donald C. Noble, Dissertation Advisor Dr. Robert J. Watters,, Departin'rtment Chair f University of Nevada Reno December 1992 11 FRONTISPIECE View of Orcopampa pueblo, looking northwest, with the mine facilities in the foreground. The concentrator and tailings dam are in the lower left part of the photo. The low hills behind Orcopampa are composed of Quaternary lava. iii ACKNOWLEDGMENTS Financial support for field work was provided by Compama de Minas Orcopampa S.A. and grants from the Geological Society of America. Laboratoiy work was partly funded by an allotment grant from the Mackay Minerals Resources Research Institute during the early stages of the study. Certain observations during the latter stages of work were made as a consultant to Cia. de Minas Orcopampa S.A. V. R. Eyzaguirre graciously allowed time for preparation of parts of the manuscript during work for Alta Tecnologia e Inversiones Minera Metalurgica S.A. (ATIMMSA). This study would not have been possible without the logistical aid and other support provided by the personnel of Compama de Minas Buenaventura S.A. (CMBSA), and particularly by Alberto Benavides Q., Raul Benavides G., Jorge Benavides A., Mario Palla P., Oscar Mayta T., and others of the Buenaventura Group. The staff of the Departamento de Geologia of Minas Orcopampa, particu larly H. Barrionuevo T., J. Herrera S., J. Meza P., R. Orellana, J. Rodriguez R., and G. Quiroz D., deserve special thanks for their assistance at the mine and for many useful discussions. Dr. Irene Marxowa graciously provided chemical analyses. Radiometric age determinations were made possible by our collaborator E. H. McKee. R. M. Tosdal provided preliminary lead isotopic data as part of an ongoing joint study. The study was conceived by D. C. Noble, who also provided invaluable techni cal, logistical, and moral support, interfaced with CMBSA on my behalf, kept me focussed on the study, made helpful comments, and read numerous drafts of the manuscript. IV I am grateful to the members of my committee, L. T. Larson, L. C. Hsu, D. Taylor, J. Daemen, and N. Rojas. Many current and former graduate students in the Dept, of Geological Sciences, particularly K. E. Swanson, participated in numerous insightful discussions and made constructive criticisms of my work. My wife, Tatiana, and my daughter, Alexandra, were very understanding of those many hours away from them. They were the principal force that led to the completion of this study. V ABSTRACT Epithermal Ag-Au veins of early Miocene (17.7 Ma) age in the Orcopampa district, southern Peru, are hosted by N45°E to E-W striking normal faults cutting slightly older silicic and intermediate volcanic rocks. Several veins, of which Calera is the economically most important with production of more than 40 Moz of silver and 350,000 oz of gold, are spatially associated with dikes of intermediate composition that predate mineralization by about 1 to 1.5 Ma. Similarities in the paragenetic stages of the veins indicate that they are related to a single large hydrothermal system. Alteration and vein mineralization is, in general, typical of high-base metal, silver-rich adularia-sericite (low-sulfidation) type epithermal deposits. Economic mineralization is accompanied by quartz-adularia-sericite-pyrite altered wall rock. Local high-level quartz-alunite-kaolinite alteration in the Santiago vein system formed 1.5 to 1 Ma prior to mineralization and may be related to coeval intermediate volcanics. Ore shoots in the Calera vein system occur in zones of intense fracturing with abundant stockwork veins and vein splits. Five main paragenetic stages and multiple episodes of fracturing, faulting and hydrothermal brecciation are recognized. Fluids that formed the main ore shoot appear to have moved diagonally upward. Gold-rich ores of the Bonanza Stage occur preferentially near the intersections of vein splits that appear to have controlled fluid flow. Mineralization took place between 240° and 300°C from fluids with salinities of form 0.5 to 2.5 weight percent NaCl equivalent. Boiling probably occurred fre quently but was apparently not responsible for ore deposition. VI Lead isotopic ratios define a steep linear array suggestive of mixing of leads from two sources with isotopic ratios similar to the igneous host rocks. The paragenetic sequence provides a record of progressively evolving hydrothermal fluids punctuated by influx of compositionally distinct fluids that deposited much of the base- and precious-metal minerals. Close spatial and tempo ral association with dacite lavas, high /Te2//S2 ratios implied by phase assemblages, high temperatures that prevailed throughout mineral deposition, and lead isotopic patterns are all consistent with the involvement of vital magmatic hydrothermal fluids. V ll TABLE OF CONTENTS Frontispiece...... 11 Acknowledgments...... iii Abstract ...... Table of Contents...... —vii List of Figures...... ix List of Tables...... xi Part I. Introduction...... 1 Mining History...... ZZZZZZZZZZZ'Z'Z...... 3 Discovery and Development of the Caiera Vein'’i^tem Z Z Z !Z Z .’” .....5 Part II. Geologic Setting And Geochronology of the Orcopampa District Introduction...... 15 Local Geologic Setting...... 15 Structure...... 15 ....18 Overview of Economic Geology...... 19 Geochronology...... 20 Part III. The Caiera Vein System...... 25 Introduction...... Structure...... 25 ....25 Form and Distribution of Ore Shoots...... 29 Wall Rock Alteration...... * ...30 Mineralogy, Paragenesis, and Vein Stratigraphy...... ZZZZ ...37 General Features...... 37 Early Stage...... 40 Manganese Stage...... 'ZZZZZZZZZZZZ^. ...40 Quartz Stage...... ZZZZZZZ ...49 Bonanza Stage...... ZZZZZZZZZZ. ...49 Late Stage...... 61 Mineralogical Zoning, Distribution of Stages, and Grade Distribution ...66 Geochemistry of the Ores and Altered Rocks...... 69 Fluid Inclusions...... 87 Lead Isotopes...... 98 Relation to Other Veins and District Zoning...... 103 Part IV. Discussion and Conclusions...... 106 Orcopampa District...... 106 Nature and Timing of Magmatism and Tectonic Activity...... 106 Nature and Timing of Hydrothermal Activity...... 107 Caiera Vein System...... 109 Nature of the Vein and Ore Shoots...... 109 Nature of the Hydrothermal Fluids...... 113 Sources of the Chemical Constituents...... 123 Conclusions...... 130 References...... 134 Appendix 1...... 145 Fluid Inclusion Methodology and Data...... 145 viii Appendix 2...... Thermodynamic data 156 156 IX LIST OF FIGURES Fig. 1. Map showing the location of the Orcopampa district and other mining districts in southern Peru...... 2 Fig. . Production and reserves of the Orcopampa district from 1967-1992, 2 ....4 Fig. 3. Aerial photograph of the Orcopampa district...... 6 Fig. 4. Outcrop of the Calera vein...... 7 Fig. 5. Longitudinal sections of the Calera vein showing development of the principal mine workings...... 8 Fig. 6. Generalized geologic map and cross-section showing the principal veins of the Orcopampa district...... 16 Fig. 7. Radiometric age determinations for the Orcopampa district...... 22 Fig. 8. Plan of the central part of the Calera vein on the 3800 level...... 26 Fig. 9. Cross sections of the Calera vein and associated splits along A-A’in Fig. 8 ...... 27 Fig. 10. Cross section of the Calera vein and associated splits along and B-B’ in Fig. 8 ...... 28 Fig. . Example of adularized Sarpane dacite...... 11 ..32 Fig. 12. Illitized Manto Tuff and Santa Rosa formation of the upper part of the Calera vein system...... 34 Fig. 13. Wall rock alteration in the Calera vein system...... 35 Fig. 14. Paragenetic stages and substages of the Calera vein and their characteristic minerals...... 38 Fig. 15. Typical breccia texture of the Calera vein system...... 41 Fig. 16. Complex breccia ore...... 42 Fig. 17. Photomicrograph of rhodonite-quartz textures...... 43 Fig. 18. Manganese Stage veins material from the Laura vein...... 45 Fig. 19. Reflected light photomicrographs of Manganese Stage ores...... 46 Fig. 20. Specimen of Manganese Stage ore...... 48 Fig. 21. Examples of Quartz Stage material...... 50 Fig. 22. Bonanza-Stage ore...... 53 Fig. 23. Reflected light photomicrograph of Bonanza Stage ore...... 55 Fig. 24. Bonanza Stage, Ramal 1...... 56 Fig. 25. Paragenetic sequence of Bonanza Stage, Ramal 1 ...... 57 Fig. 26. Reflected light photomicrographs gold-rich Bonanza Stage ore...... 59 Fig. 27. Qualitative SEM energy dispersive spectra of zoned electrum grain from the Bonanza Stage...... 60 X Fig. 28. Plot of silver versus gold for channel samples from split Ramal 1, 3800 level...... 62 Fig. 29. Fine-grained quartz of Late Stage with leached cavities...... 63 Fig. 30. Black microbreccia or tuffisite dikes that cross cut vein material of the Manganese and Quartz stages...... 65 Fig. 31. Distribution of paragenetic stages in the main ore shoot of the Calera vein...... ^ Fig. 32. Longitudinal sections of the Calera vein showing grade isopleths...... 70 Fig. 33. Plot of Pb vs Zn for rich samples of the Calera vein...... 76 Fig. 34. Triangular plot of Cu, Pb, and Zn in weight percent for rich samples of the Calera vein...... 77 Fig. 35. Plot of 2Cu, lOAg, and Pb + Zn in weight percent for rich samples of the Calera vein...... jg Fig. 36. Plot of Ag vs Au for rich samples of the Calera vein...... 80 Fig. 37. Plot of Sb vs As for rich samples of the Calera vein...... 81 Fig. 38. Plot of Cu vs Sb for rich samples of the Calera vein...... 82 Fig. 39. Plot of Sb vs Ag for rich samples of the Calera vein...... 83 Fig. 40. Plot of Cu vs Ag for rich samples of the Calera vein...... 84 Fig. 41. Triangular plot of Cu, lOAg, and Sb in weight percent for rich samples of the Calera vein...... g6 Fig. 42. Transmitted-light photomicrographs of fluid inclusions from the Calera vein system...... gg Fig. 43. Histograms of homogenization temperatures for the Calera vein system...... g^ Fig. 44. Salinity vs. Th for fluid inclusions from the Calera vein system...... gg Fig. 45. Pb isotope data for the Calera vein system and other veins at Orcopampa...... 201 Fig. 46. Pb isotope data for veins and wall rocks of the Orcopampa district...... 202 Fig. 47. Cross section of the Calera vein system showing generalized distribution of alteration assemblages...... 112 Fig. 48. Summary of fluid inclusion homogenization temperatures for the Calera vein system...... 214 Fig. 49. Ranges of salinities and Th for selected epithermal deposits...... 117 Fig. 50. Log/CO2 -/0 2 diagram...... 219 Fig. 51. Log a 0 2 - pH diagram...... 220 Fig. 52. Log aS2 - a 0 2 diagram...... 122 XI Fig. 53. Log/Te2-/S2 diagram...... ^ 4 Fig. 54. Pb isotopic data from Orcopampa...... 227 Fig. 55. Pb isotopic provinces of the central Andes...... 129 Fig. 56. Longitudinal projection with locations of fluid inclusion samples...... 146 LIST OF TABLES Table 1. Tonnages and Grades of Selected Bonanza Ore Shoots from the Comstock and Orcopampa Districts...... 24 Table 2. Age Determinations on Specimens from the Orcopampa District...... 21 Table 3. Chemical Analyses of Ore Samples from the Calera Vein System...... 71 Table 4. Broad-spectrum minor element analyses of ores and wall rocks...... 74 Table 5. Pb isotope data for Orcopampa veins...... 200 Table 6. Fluid inclusion data for the Calera Vein System...... 147 Table 7. Thermodynamic data...... 1^7 1 PARTI. INTRODUCTION Peru, although well-known for its polymetallic ores (e.g., Petersen, 1965), pos sesses a number of important epithermal precious-metal deposits (e.g., Stephan, 1974; Kamilli and Ohmoto, 1977; Hollister and Entwistle, 1977; Kamilli et al., 1979; Noble et al., 1989; Vidal et al., 1989; Candiotti et al., 1990). Orcopampa, located about 150 km northwest of Arequipa (Fig. 1), is one such epithermal district (Gibson et al., 1990). The presence of very rich Ag-Au ore in the Calera vein, in addition to Ag-rich ore typical of the veins formerly mined in the district, prompted a study to document the evolution of this very complex vein system, to determine the details of vein for mation, the conditions under which the different types of ores were deposited, and the temporal relationships between the volcanic host rocks and the mineralization. Although little specialized analytical work was possible, the goals of the study were largely met, and further study is planned. Part I of this report is an introduction to the district that includes sections on the mining and production history and the discovery and development of the Calera vein. The regional geologic setting of the district is discussed in Part II, and the results of radiometric dating are presented. The mineralogy, stratigraphy, and zoning of the Calera vein system, as determined by detailed underground observa tions supplemented by reflected-light microscopy, are described in Part III. Sections on fluid inclusions and lead isotopes are included. In Part IV, the data from the previous parts are integrated and critically discussed, and the conclusions of the study are presented. 2 Fig. 1. Map showing the location of the Orcopampa district and nearby important precious-metal mining districts in southern Peru. The 5000 meter contour is indicated. 3 Mining History Orcopampa has been known and intermittently worked since the Spanish colonial period. The following synopsis is from Tudela (1918), who summarized the history of the district, and the twenty-fifth anniversary report of the Cia. de Minas Buenaventura S. A. (Anonymous, 1978). Orcopampa was mentioned in the "Memorias de los Virreyes de 1731 a 1781". Tudela cites reports by A. Babinski in 1879 and 1883 stating that the annual production of Orcopampa from 1786 to 1833 was about 10,000 marcos of silver or about 74,000 ounces (1 marco = 230 grams, Purser, 1971). The district was abandoned in 1842 because of a lack of experienced miners after the war for independence and subsequent flooding of the mine. In 1879, the mine was claimed by Don Mateo Moran. The claims were eventually obtained by Don Oscar Heeren, who formed the Sindicato Minero de Orcopampa, which included an ex-president of Peru, Jose Pardo y Barreda, in 1910. The Sindicato attempted to develop new ore by driving a long tunnel beneath the known workings, but the tunnel was not completed and evidently little ore was produced. Compania de Minas Buenaventura S.A. began evaluating Orcopampa in 1960, and began mining 70 short tons per day in 1967 under an agreement with the Sindicato, which receives 10% of net smelter returns. Mining continues today by Compania de Minas Orcopampa S.A., a wholly owned subsidiary of Minas Buenaventura. For many years, the district produced less than 500 short tons per day of silver ore from several vein systems. Concentrator capacity was increased to its current level of about 1,200 short tons per day when an exceptionally large tonnage of rich ore, containing gold as well as silver, was discovered in the Calera vein (Fig. 2) in the late 1970’s and early 1980’s. The increase in production, however, had the undesirable effect of rapidly depleting ore reserves and thereby decreasing the life of the mine. Cfa. de Minas Buenaventura. The production for 1992 is projected from that as of 31.Oct. as of files of from that from 1992 Data is for projected production tons. The short dry in reserves and Buenaventura. silver Minas Cfa.of de ounces troy and tons short dry in Million short tons F ig. 2. Production and reserves of the Orcopampa district from 1967-1992. Production Production 1967-1992. from district Orcopampa the of reserves and Production 2. ig. Million ounces 4 5 Orcopampa has been the largest silver producer and the largest private gold producer in Peru for the last several years. Production from 1988 through 1991 totaled almost 20 million troy ounces of silver and about 120 troy thousand ounces of gold. The production of the Calera vein system is more than the total from all the other veins mined in the district; produced ore and reserves total more than 3X106 short tons of ore containing approximately 4QX106 troy ounces of silver and 0.4X 106 troy ounces of gold. The entire production at Orcopampa by Buenaventura has been about 50M oz of silver. Discovery and Development of the Calera Vein System Exploration and development of veins in the Orcopampa district indicated that economic mineralization was hosted by generally northeast-striking normal faults with moderate to large displacements. Geologic mapping in 1972 led to the discovery of a major, largely buried normal fault, which was named the Calera fault, between the then-producing Manto and Santa Rosa-Santiago vein systems (Fig- 3)(Noble, 1972). At least 400 meters of dip-slip displacement on the Calera fault was indicated by the juxtaposition of different units of the local volcanic succes sion across the fault. An outcrop of barren vein quartz twenty meters long and several meters wide was the only surface expression of the Calera vein (Fig. 4). A tenacious exploration program was led by Ing. Mario Santillan, superinten dent of the Orcopampa unit at that time. A crosscut on the 4100 m level was driven 300 meters along a barren vein in 1975-76 (Fig. 5A). A second crosscut on the 4015 level was abandoned before reaching bedrock. In 1978, three diamond drill holes were completed to explore the vein on the 4015 m level; the third hole assayed 11 ounces per short ton (opt) Ag for a 0.89 m interval. Based on this information, the 340 m long crosscut was re-initiated on the 4015 level. Also, three more diamond 6 Fig. 3. Aerial photograph of the Orcopampa district. The mine facilities and concentrator (Con), 4015 level portal (4015), the Calera (Cal), Manto (M), Santa Rosa (SR) and Santiago (S) veins, the Blancas area (B), and the outcrop of the Calera vein (CO) are indicated. Compare with Fig. 6. 7 Fig. 4. The only natural outcrop of the Calera vein, lower left part of photograph, with adit, in Quebrada Alpajahua. A w 405?8and541TO w S inrenr^nT»niohe sh™ ing development of the principal mine workings. Numbers 3750, 3800,3880, 3950,4015, ’ . 4100> whlch represent approximate absolute elevations in meters, indicate main mine levels. The dot-dash lines outline ore shoots! haded areas indicate ore that had been mined as of the date of the respective section. The dotted lines show the traces of the intersections of vein splits, numbers 1, 2, 4 and letters L and S indicate Ramal 1, Ramal 2, Ramal 3 & 4, Laura vein, and Ramal Sur, respectively. Qa-Quaternary alluvium, colluvium, and landslide deposits. Modified after Orcopampa mine staff. A January, 1979. B. January, 1981. oo G w 3750 H w 4015 3950 3880 3800 Fig. 5. (Cont.) G. January, 1991. H. January, 1992. 1 2 drill holes were used to explore the vein on the 3800 m level in 1979; Ag values were low (the highest value was 5.5 opt Ag for 0.55 m), but continuity of the structure and the presence of vein material was indicated. The 4015 level was driven along the vein during 1979 and 1980 (Fig. 5B). Although results were encouraging, and 42,570 tons were developed with a grade of 24 opt Ag, more development was needed to confirm the continuity of the mineral ization. An attempt to intersect the vein with a crosscut on the 3900 level in 1981 failed. Therefore, in 1981 and 1982, three sublevels, one above and two below the 4015 level, were driven from ramps (Fig. 5C); reserves increased to 439,115 tons at 13.8 opt Ag. Based on the success in the upper levels, the decision had been made in 1980 to advance the main haulage (3800 m) level 1,200 m to the Calera vein from the Manto vein to the north. The 3800 m level intersected the vein in 1982 and the foot- wall splits Ramal 1 and Ramal 2 were discovered. A large tonnage of high-grade bonanza Ag-Au ore was discovered in 1983. On the 3950 level (Fig. 5D), the vein averaged almost 9 m in width and 25.6 opt Ag along 260 m of drift; one 50 m interval assayed 3.5 opt Au. On the 3800 level of Ramal 1, the vein averaged 1 m in width and more than 1 opt Au for 110 m. With this development, total reserves increased to 1,136,175 tons at a grade of 17.5 opt Ag. Subsequent development on the 3880 level in 1985 and 1986 (Fig. 5E) con firmed the continuity of the Calera vein and the splits from the upper levels to the 3800 m level and raised the reserves to 2,383,240 tons with 16.2 opt Ag in 1986. Ensuing exploration by drifting and diamond drilling has, unfortunately, yielded relatively little additional ore (Figs. 5F, G). A second much smaller ore shoot was defined to the east of the main ore shoot by 1988 (Fig. 5F); however, virtually no ore was encountered in several hundred meters of exploration on the 4015 level to the east of the second ore shoot. Several hundred meters of exploration drifting on 13 the 3950, 3880, and 3800 levels to the east of the second ore shoot has resulted in the development of little additional reserves; the only significant discovery was made in split Ramal Sur (Figs. 5G, H) The 3750 level has recently been developed to explore below the main ore shoot (Fig. 5H). Unfortunately, most of the Calera vein on this level contains very low grades and is uneconomic. A small tonnage of bonanza-grade gold-silver ore was encountered, however, below the western edge of the main ore shoot, and a diamond dnll hole that intersected the vein at an elevation of about 3720 m below the west ernmost extension of the 3800 level gave ore-grade silver values. At the time of writing (Nov., 1992), exploration being carried out to the west on the 3800 level has resulted in the discovery of several tens of meters of rich silver-gold ore. The main ore shoot developed in the Calera vein and adjacent splits con tained a very large tonnage of moderate- to high-grade ore for this type of epithermal deposit. Although the district as a whole is not large compared to world-class dis tricts of similar type such as Guanajuato (Gross, 1975) and Pachuca-Real del Monte (Geyne at al., 1990), the main ore shoot of the Calera vein compares favorably in contained metals to ore shoots in world class deposits such as the Comstock lode (Table 1). The continuity and width of the vein allowed the use of modern trackless mining techniques by the cut and fill method to increase production rates and decrease costs. Gold-rich bonanza ore that does not occur in other similar veins in the district and the region increased the profitability of the mine and prevented losses during times of low Ag prices. These and other factors contributed to the ele vation of Orcopampa to the status of the largest Ag producer and largest private Au producer in Peru from 1987 to 1991. 14 Table 1. Tonnages and Grades of Selected Bonanza Ore Shoots from the Comstock and Orcopampa Districts Ore Shoot Tons Au Ag oz/ton oz/ton Comstock 19,000,000 0.4 10.1 Con Virginia 1,391,655 1.8 29.2 Con Imperial 1,037,412 0.5 11.0 Crown Point 1,374,528 1.0 13.9 Calera, main shoot 3,000,000 0.1 15 Data on Comstock from Vikre (1989) 15 PART II. GEOLOGIC SETTING AND GEOCHRONOLOGY OF THE ORCOPAMPA DISTRICT Introduction The Orcopampa district is situated within a belt of intermediate to silicic calc- alkaline volcanic rocks of Early Miocene to Holocene age (McKee and Noble, 1989). Common lithologies include silicic ash-flow tuff and lava and volcanic breccia of intermediate composition. The geology of the Orcopampa district and surrounding areas, originally mapped by M. J. Arenas F. and D. C. Noble (Noble, 1972; Arenas, 1975), has been amplified by subsequent local and regional reconnaissance observa tions by Noble, by district mapping by geologists of the Buenaventura Group, by regional geological mapping by K. E. Swanson (unpub. data, 1989-1991), and by published and unpublished geochronological work (Noble et al., 1974; Farrar and Noble, 1976, McKee and Noble, 1989; E. H. McKee, D. C. Noble, P.C. Gibson, K. E. Swanson, and V. R. Eyzaguirre, unpub. data, 1980-1991). An interim summary of the regional geological setting, based in large part on recent unpublished mapping by K. E. Swanson and radiometric dating by E. H. McKee, has been prepared by Noble (1992). Local Geologic Setting The veins of the Orcopampa district are hosted by a sequence of intermediate to silicic volcanic rocks of early Miocene age that unconformably overlie a succession of conformable, folded Mesozoic quartzite, shale, and limestone that will not be dis cussed further (Fig. 6). The oldest unit, the Santa Rosa volcanics, formed between about 25 and 20 Ma (K. E. Swanson, E. H. McKee and D. C. Noble, unpublished data, 1990-92) and consists largely of intermediate breccias intercalated with a num ber of flows of aphanitic pyroxene andesite and units of silicic ash-flow tuff charac- 16 Orcopampa } Alhuirre Misahuanca C Blancas- Qal alluvium Tmt | Manto tuff landslide deposits | Tsr Santa Rosa volcanics Andahua volcanics Pisaca tuff iv~] Sarpane volcanics undivided sedimentary rocks |iisn't;::[ La lengua tuff Fig. 6. Generalized geologic map and cross-section showing the principal veins of the Orcopampa district. Mz = undivided Mesozoic sedimentary rocks, Tp = Pisaca tuff; Tsr = Santa Rosa volcanics; Tmt = Manto Tuff, Tit = La Lengua Member; Tsv = dacitic and andesitic lavas of Sarpane volcanics; Qav = basaltic andesite lavas of the Andahua volcanics, Q1 = landslide deposits, Gal = alluvium and colluvium. After Noble (1972), Arenas (1975), and unpublished mapping by the Orcopampa mine staff and K. E. Swanson. 17 terized by the presence of large books of biotite and quartz phenocrysts. A thick unit of tuff locally present at the base of the unit (Fig. 6), referred to as the Pisaca tuff, is probably best interpreted as an informal member of the Santa Rosa volcanics. The Santa Rosa volcanics locally contain beds of coarse conglomerate composed mainly of Mesozoic quartzite cobbles and locally contains boulders of Mesozoic limestone. The sequence of volcanic and sedimentary rocks is intruded by dikes, sills, and irreg ular bodies of lithologically similar pyroxene andesite. The Santa Rosa volcanics are overlain, generally with little angular dis cordance, by the Manto Tuff, a voluminous and areally extensive compositionally zoned unit of phenocryst-rich rhyolitic ash-flow tuff (Fig. 6). The Manto Tuff is related to the collapse of the Chinchon caldera centered about 18 km southwest of the Orcopampa district in the Chila Cordillera, at about 20 Ma (K. E. Swanson, E. H. McKee, D. C. Noble, and P. C. Gibson, unpub. mapping and radiometric dating, 1989-92). The outflow sheet of the Manto Tuff is locally 250 m or more thick in the vicinity of the Orcopampa district, but is appreciably thinner in many areas, reflecting the considerable topographic relief at the time of deposition. The La Lengua tuff, a thin unit of well-bedded volcaniclastic sedimentary rock and volcanic surge deposits with local fresh-water limestone units, locally overlies the ash-flow sequence in the area of the Orcopampa district (Fig. 6). The Manto Tuff and La Lengua tuff are overlain by domes and flows of andesite and dacite of the Sarpane volcanics (Fig. 6). This unit typically contains abundant, relatively large phenocrysts of blocky plagioclase as much as 1 cm in length, hornblende, biotite and variable amounts of quartz. These lavas contrast strongly to the lavas and breccias of the Santa Rosa volcanics, which typically contain small phenocrysts of clinopyroxene and orthopyroxene accompanied by relatively small laths of plagioclase, and lack biotite and hornblende. Preliminary chemical 18 analyses suggest that both the Manto Tuff and the Sarpane volcanics are strongly calcic members of the calc-alkalic suite, being characterized by high Ca/Fe, Mg/Fe, Ca/(Na+K), and high absolute Ca for a given silica content (K. A. Connors and D. C. Noble, unpublished data, 1991). Although lavas of the Sarpane volcanics were erupted within and adjacent to the Chinchon caldera, other vent areas for lavas of this type are situated many kilo meters beyond the topographic margin of the caldera. As discussed in more detail below, dikes of the Sarpane volcanics are closely related spatially to several veins in the Orcopampa district. The rocks described above are overlain unconformably by a number of units of silicic ash-flow tuff and lava of intermediate and silicic composition ranging in age from middle Miocene to Quaternary (Farrar and Noble, 1976; K. E. Swanson, D. C. Noble, and E. H. McKee, unpub. mapping and radiometric dating, 1989-92; Noble, 1992). The only unit that occurs in the Orcopampa district comprises flows and cin der cones of basalt and basaltic andesite of the Andahua volcanics of Quaternary age (Hoempler, 1962; Venturelli et al., 1978; Weibel et al., 1978; Kaneoka and Guevara, 1984)(Figs. 3, 6). Landslides are common in the general vicinity of the Orcopampa district (Fig. 6). Structure The rocks in the Orcopampa district are cut by a series of faults that strike N45°E to E-W, dip NW or SE, and have a large normal component of displacement (Fig. 6). These faults have strike lengths of about one to more than three kilometers and vertical displacements of as much as 500 meters. Evidence for at least two compressional events is present in the Orcopampa region. The Mesozoic sedimentary rocks that underlie the Miocene volcanic rocks 19 exhibit large-scale broad wavelength folds (K. E. Swanson, oral comm., 1990), and large overturned folds and thrust faults are observed in these rocks between Aplao and Orcopampa (Fig. 1). A second folding event occurred after deposition of the Sarpane volcanics between about 11 Ma and 6 Ma (Noble and Farrar, 1976; K. E. Swanson, E. H. McKee, and D. C. Noble, unpub. data, 1988-1992; Noble, 1992). The volcanic rocks in the Orcopampa district dip about 10-20° to the west as a result of this folding. This folding may also be responsible for the difference in elevation of the Manto Tuff on opposite sides of the Orcopampa-Andahua valley (K. E. Swanson, oral comm., 1990), a feature that has also been attributed to later normal faulting (Noble, 1972). Overview of Economic Geology Ore in the Orcopampa district is hosted by the northeast to east-west striking normal faults discussed above (Fig. 6). The productive portions of the veins are hosted by the Tertiary volcanic rocks; veins are narrow and of low grade where they pass into the underlying Mesozoic sedimentary rocks. Past production has been from the Manto, Santa Rosa, Santiago, and Tudela fissure veins or vein systems (Fig. 6); virtually all production since 1988 has been from the Calera vein system. Ore has been mined from the different veins over a range of elevation from 3690 to 4250 meters above sea level. Several studies on the ores and their vertical and lateral variations have been made by Ulrich Petersen and his co-workers (Kamilli, 1974; Petersen, 1982; Hackbarth and Petersen, 1984; Petersen et al., 1990). The Calera vein system has been described by Gibson (1987, 1988) and Gibson et al. (1990), and is the subject of part III of this report. The volcanic rocks adjacent to the productive portions of the veins have been potassium metasomatized and contain abundant adularia and sericite (Noble, 1979a; 2 0 Silberman et aL, 1985; Gibson, 1988; Gibson et al, 1990). Hypogene alunite is present in the upper part of the Santiago Norte vein (Fig. 6) (Caballero, 1988; Gibson et al., 1990), and may have occupied a high-level position in other veins in the district as well. Kaolinite is abundant in the upper part of the Santiago and the east ern part of the Calera vein systems, but the relationships with mineralization remain unclear. Adularia-sericite, alunite, and kaolinite alteration was superimposed on dis trict-wide propylitic alteration in the Santa Rosa volcanics and the Pisaca tuff, char acterized by the presence of chlorite, calcite, pynte and sericite, that appears to have formed largely or entirely prior to both the deposition of the Manto Tuff and the hydrothermal event that produced the precious-metal ores of the district. Geochronology A geochronological study of the Orcopampa district was undertaken to determine the temporal relationships between the epithermal vein mineralization, alunitic alteration, and the volcanic host rocks. Pure separates of phenocrysts from three samples of the unaltered Sarpane volcanics (biotite and hornblende) and adu- laria or hypogene alunite from eight samples of vein filling material and adjacent altered wall rocks were analyzed by E. H. McKee using the K-Ar method in the labs of the U.S. Geological Survey in Menlo Park. Two additional samples of adularia were analyzed using the 40Ar/ 39Ar method in order to confirm the K-Ar ages on adularia. The results are presented in Table 2 and Figure 7; all uncertainties are reported at the one sigma confidence level. Two ages reported on biotite phenocryst separates of the Manto Tuff by Noble et al. (1974) give an average age of 19.5 Ma using presently accepted constants. Based on additional unpublished radiometric ages obtained by K. E. Swanson, E. H. McKee, and D. C. Noble (pers. comm., 1990), 21 Table 2. Age Determinations on Specimens from the Orcopampa District Conventional K-ArAge Determinations Sample Number Material K2O ^A r* '“At* Age (wt. %) (moles/gm) (%) (Ma ± lcr) Ag-Au mineralization 1 ORCO-K Adularia-wr 6.35 1.56 40 2 3800 CH310 17.0 ± 0.5a Adularia-wr 11.78 2.901 70 17 0 + 0 5 3 CCG-49b Adularia-v 13.08 3.330 74 4 17.6 + 0.5 14.83 3.791 76 17 7 + 0 6 5 SAN 15 Adularia-v 15.30 3.963 86 17 9 + 06 Quartz-alumte alteration, Santiago Norte 6 STGO N BX Alunite 6.60 1.769 75 184 + 05 7 SAN MAYOR Alunite 7.22 1.919 38 184 + 06 8 STGONALUN Alunite 6.78 1.910 42 195 + 06 Sarpane volcanics 9 SAR-1 Hornblende 0.625 1.657 32 183+06 10 SAR-2 Hornblende 0.619 1.667 33 186 + 06 11 SARD AC Biotite 7.98 2.244 68 19.4 ± 0.6 Sample Material 40/39 37/39 36/39 40Ar* J Age Number (%) fMa + 1 ? Santa Rosa Volcanics Manto Tuff — i---- □---- 1 Sarpane Volcanics i-----□-----1 i---- □---- 1 i------o------1 Hypogene Alunite i------o------1 Vein Adularia Wall Rock Adularia 25 24 23 22 21 20 19 18 17 16 15 Age (Ma) Fig. 7. Radiometric age determinations for the Orcopampa district. Symbols indicate the following: square - Sarpane volcanics, diamond - alunite from Santiago Norte, circle - adularia from veins, and triangle - adularia from wall rocks. Error bars indicate nominal la analytical uncertainties. All analyses by K-Ar method, except filled symbols, by 40Ar/ 39Ar methods. The range of age for the Santa Rosa volcanics and the age of the Manto Tuff are from unpublished age determinations of K. E. Swanson, E. H. McKee, and D. C. Noble. 23 however, the Manto Tuff is presently believed to have been emplaced at about 20 Ma (Fig. 7). The age of 20 Ma is appreciably older than the age of 16.4 obtained by Rangon et al. (1990). This probably reflects the analysis of whole-rock materials, in a manner similar to that which resulted in anomalously young ages for the host rocks at Mina Areata (cf. Candiotti et al., 1990). Three determinations on domes of the Sarpane volcanics (Table 2, specimens 9-11) are 19.4 ± 0.6 Ma, 18.6 ± 0.6 Ma, and 18.3 ± 0.6 Ma, giving an average age for the unit of about 18.8 Ma. Eruption of the Sarpane volcanics, however, could have occurred over an extended interval of time. Alunite from Santiago Mayor, the eastern extension of the Santiago Norte vein system, yielded ages of 19.5 ± 0.6 Ma, 18.4 ± 0.6 Ma, and 18.4 ± 0.5 Ma (Table 2, specimens 6 -8 ) . Taken as a group, the ages are virtually identical to the ages obtained on the Sarpane volcanics, implying that alunitic alteration was associated with intrusion of the Sarpane volcanics. Seven ages have been obtained on adularia from both K-metasomatized wall rock and vein materials from the Calera, Manto, Tudela "G", and Santiago Norte veins (Table 2, Fig. 7). The average of these ages is about 17.5 Ma. Two of the sam ples were analyzed using the 40Ar/39Ar method in order to confirm the conventional K-Ar ages, because young ages may result from retention of argon and sequestering of argon on the walls of the fusion bottle during fusion and extraction, a problem that has been noted for determinations on sanidine (Hausbach et al., 1990). The two 40Ar/39Ar determinations, which are not affected by argon retention, are among the older ages (Fig. 7). The two youngest K-Ar ages can reasonably be discarded for the reason described, yielding a best estimate of the age of mineralization of 17.7 Ma. Note also that the four ages obtained on vein adularia cluster closely around 17.8 Ma. Mineralization therefore occurred about 1.5 to 1.0 million years after extrusion of at 24 least some of the Sarpane volcanics and the alunitic alteration in the upper part of the Santiago vein system. 25 PART m . THE CALERA VEIN SYSTEM Introduction A detailed study of the Calera vein system was made during a number of visits to Orcopampa between July, 1987 and February, 1992. During the initial part of the study between July and December, 1987, detailed observations were made to deter mine the paragenetic sequence of the complex vein system at excellent exposures in the active stopes as mining progressed. A representative sample suite was collected and a photographic record was made. Five shorter visits were made over the next four years to obtain data and collect samples from new exposures as mining and exploration proceeded. These field data and samples were used as a base for labora tory work, including petrographic, fluid inclusion and isotopic studies. Additional geochemical, mineralogical, and isotopic work is planned. Structure The Calera vein strikes N70°E to E-W and dips 50° to 70°N (Figs. 6, 8-10), shallowing with depth at a pronounced bend in the structure that rakes at about 15° to 20° west. The vein averages about 4 meters in width but locally is as much as 30 meters wide. The vein margins, particularly the footwall, are typically not well defined and consist of an almost gradational zone of stockwork veins in the wall rock. The hanging wall, however, is locally a well-defined fault surface reflecting latest movement along the composite structure. The Calera fault, the structure that hosts the Calera vein, has a known strike length of approximately 4 km, but the mineralized zone covers only about 1.5 km in the western part of the structure. The juxtaposition of Manto Tuff in the hanging wall and Santa Rosa volcanics in the footwall indicates a large vertical component N Fig. 8. Plan of the central part of the Calera vein on the 3800 level. Veins are shown in black and a dike of the Sarpane volcanics in hatched pattern. Tm-Manto tuff, Tsr-Santa Rosa volcanics. A-A’ and B-B’ indicate the locations of the cross sections given in Figs. 9 and 10. Zones 1,2 and 3 are the locations of the ore shoots in the Calera vein, split Ramal 2, and split Ramal 1, respectively. Modified after Orcopampa mine staff. 27 A Fig. 9. Cross sections of the Calera vein and associated splits along A-A’ in Fig. 8. Horizontally ruled or shaded areas are stopes mined as of July, 1989. Hatch pattern indicates dacite dike of the Sarpane volcanics. Tmt - Manto Tuff, Tsr - Santa Rosa Formation, Qa - alluvium, colluvium and landslide deposits. Modified after Orcopampa mine staff. 28 B B’ / / Qa — 4050 m 50 meters ■ 4000 m - 3950 m - 3 9 0 0 m — 3850 m - 3 8 0 0 m - 3750 m Fig. 10. .Cross section of the Calera vein and associated splits along and B-B’ in Fig. 8. Shaded areas are stopes mined as of Jan, 1992. Hachured pattern indicates dacite dike of the Sarpane volcanics. Tmt - Manto Tuff, Tsr - Santa Rosa Formation, Qa - alluvium, colluvium and landslide deposits. Modified after Orcopampa mine staff. (400-500m+) of displacement on the Calera fault, but the horizontal component, if any, is unknown. The amount of vertical displacement decreases greatly in the east ern part of the Calera fault about 1.5 km to the east of the mineralized zone (pers. comm., D.C. Noble and K. E. Swanson, 1989). Four major mineralized splits and many minor splits occur in the footwall of the vein (Figs. 8-10); no economically significant hanging wall splits have been dis covered. The splits are generally much narrower than the Calera vein, averaging from 1 to more than 3 meters in width. Splits Ramal 1, Ramal 2 and Ramal Sur (not shown on Fig. 8-see Fig. 5) are cymoid loops that dip steeply to the north and form a curvilinear junction with the Calera vein. The Laura vein (not shown-see Fig. 5) appears to be another north-dipping cymoid. Splits Ramal 3 and Ramal 4 are part of a south-dipping set of conjugate faults between the Calera vein and Ramal 2. The stockwork of narrow veins and veinlets in the wall rocks of the vein system also reflects this conjugate fracture set. Steeply-dipping post-mineral faults striking at high angles to the major structure offset the vein a few meters in several places. Form and Distribution of Ore Shoots Most of the ore in the Calera vein occurs in one large ore shoot, here termed the main ore shoot (Fig. 5H). The western boundary of this ore body plunges to the west, coinciding approximately with the junction of Calera with the split Ramal 1, whereas the eastern boundary is vertical. Noble (1992) inferred that the ore horizon in the Calera vein may be tilted 10° to 20° to the west because of post-mineralization folding based on information from outside of the Orcopampa district. As presently delineated above the 3750 level, the main ore shoot ranges from 200 to 500 meters in length and 150 to 325 meters in height and averages about 6 meters in width. Rich ore shoots also occur in the footwall splits near their eastern 30 junctions with the Calera vein. Other ore shoots in the Calera vein and footwall splits are narrow, elongated vertically, and contain much lower tonnages of lower grade ore. Much of the area between ore shoots contains vein material that typically is composed dominantly of quartz. The locations of the ore shoots appear to be controlled by the intersections of vein splits and, at least in part, by changes in the strike and probably the dip of the vein. Ore is commonly found in highly fractured zones that are associated with vein splits (Gibson, 1988). An important control on the localization of ore in the Calera vein may have been the position of the bend in the structure to more shallow values (Figs. 9,10); a dilatant zone would likely have formed above this bend during normal fault displacement. The bend apparently rakes at about 20° to the west, providing further evidence for post mineralization westward tilting of the district. The bend is responsible for the change in strike of the vein from northeast to east (Fig. 8), cited as an ore control by Condori (1984). The ore shoots typically have several different generations of mineral deposition and a complex history of mineral deposition, frac turing and hydrothermal brecciation. Wall Rock Alteration Based on systematic thin section and X-ray diffraction study of the Calera vein system and reconnaissance study of several of the other veins, it is clear that rocks adjacent to the veins in the district have been strongly altered and contain abundant quartz, adularia, illite, and pyrite (Noble, 1979a; Silberman et al., 1985; Gibson et al., 1990). The data strongly suggest addition of K2O, but no systematic chemical analy ses were done, as, for example, by Scherkenbach and Noble (1984). Three alteration assemblages related to the mineralizing event have been recognized: adularia alter ation, defined by quartz + adularia -I- pyrite with or without minor sericite (illite or 31 muscovite) and calcite; illitic alteration, defined by quartz + illite + pyrite with or without adularia; and propylitic alteration, defined by albite + calcite + quartz + pyrite + chlorite and/or illite/smectite or montmorillonite. Adularia-rich rocks are typically dense, dark gray to gray-green, and the original texture is preserved (Figs. 11 A, B). Optically continuous adularized feldspar phenocrysts containing inclusions of sericite grade into the adularia-rich groundmass (Fig. 11B). Rocks rich in sericite and quartz are generally white or light gray (Fig. 12), with sericite pervasively replacing feldspar and mafic phenocrysts. Original textures may or may not be preserved. Altered rocks are cut by abundant quartz veinlets, with or without adularia and/or sericite, and by less common veinlets consisting mostly of illite or adularia. Propylitically altered rocks are generally light green or white (bleached), and the texture is preserved. Small amounts of kaolinite and smectite or mixed-layer illite-smectite are present locally in all alteration types, but kaolinite is locally abun dant. Several percent of pyrite typically replaces mafic phenocrysts and groundmass material. Alteration associated with the veins was superimposed on earlier, district wide weak propylitic alteration, characterized by chlorite, calcite, pyrite and sericite, that affects the Santa Rosa volcanics but appears to predate the Manto Tuff and the hydrothermal activity that formed the precious-metal ores (Gibson et al., 1990). Study of hand samples from the surface and underground and drill core indi cates that the alteration assemblages are zoned vertically and laterally within the vein system (Fig. 13). In upper levels, above ore and in the upper part of ore shoots, illitic alteration occurs adjacent to the vein and illite-rich, chlorite-poor propylitic alter ation is found outside the illitic alteration. In deeper levels of the ore shoot, adularization adjacent to the vein is zoned outward to illitic and then chlorite-rich propylitic types. A narrow zone of illitic and/or kaolinitic alteration adjacent to the 32 Fig. 11. Example of adularized Sarpane dacite. A. Hand specimen collected adjacent to Ramal 1, Calera mine. Sample CCG-112, 3800 level stope, Ramal 1 near junction with Ramal 2. 33 Fig. 11. (Cont.) B. Photomicrograph specimen in A, showing plagioclase replaced by adularia with minor sericite, cut by quartz veinlet that contains adularia where it crosses the phenocryst; transmitted light, crossed nicols. 34 Fig. 12. Illitized Manto Tuff in the hanging wall of the upper part of the Calera vein system; overlying alluvium and colluvium has been removed for stope fill. The Calera structure is approximately located on the far side of the excavation. The ice covered volcanic edifice in the distance is Nevado Coropuna, the highest in Peru with an elevation of 6377 meters. Above Ore ILLITE + QUARTZ + pyrite Zone ± kaolinite ± adularia ALBITE + QUARTZ + ILLITE + calcite + pyrite + clay ADULARIA + QUARTZ + sericite Ore Zone + pyrite ± kaolinite ± calcite ILLITE + QUARTZ + pyrite ± kaolinite ± adularite > > :------;------;— > ------> ------> > increasing distance from structure vein1sysmmSChematlC dlagram showing the sPacial relationships of alteration assemblages in the wall rocks of the Calera 36 vein in some places appears to overprint earlier-formed adularia-bearing rock. The thickness of the alteration envelope surrounding the vein is approximately pro portional to the amount of vein material present. In places, chlorite-rich propylitic alteration occurs within and adjacent to the Calera fault between ore shoots where little or no vein filling has occurred. Detailed study of alteration zoning is compli cated by structural features, such as numerous secondary veins and fractures in the wall rock, and control by wall rock lithology. For example, abundant calcite veinlets occur almost exclusively in adularized and illitized fine-grained sedimentary rock units within the Santa Rosa volcanics adjacent to the vein. A zone of locally intense hypogene quartz-alunite-kaolinite alteration is found along the eastern and central parts of the surface trace of the Santiago Norte vein (Caballero, 1988; Gibson et al., 1990) (Fig. 6). As discussed in part II, the alunite alteration is about 1.5 to 1 Ma older than the adularization associated with epither mal veins (Table 2, Fig. 7). Alunite and quartz with lesser amounts of kaolinite occur as alteration products of the Santa Rosa volcanics, in veins, and as the matrix for bodies of hydrothermal breccia. Similar, but somewhat older, alunite-quartz alter ation is associated with a pervasively silicified breccia in the Mulanan area east of the district. Observations in trenches at Santiago Norte suggest that fine-grained, locally alunitized sediments and layers of poorly sorted angular breccia may have been asso ciated with hydrothermal explosion craters. (Noble, 1991). Intense kaolinite alter ation has been observed in the upper part of the Santiago vein to the west of expo sures of alunitic alteration and along the Calera fault east of the area of known mineralization, but the relationship with the mineralizing event, if any, is unclear. 37 Mineralogy, Paragenesis, and Vein Stratigraphy General Features Mineralization in the Calera vein system is complex and multistage. From older to younger, the following five stages are recognized: the Early Stage, the Man ganese Stage, the Quartz Stage, the Bonanza Stage, and the Late Stage (Gibson, 1988; Gibson et al., 1990) (Fig. 14). Individual stages and veinlets commonly exhibit classic epithermal open-space-filling textures, including abundant vugs, crustification, symmetrical banding, and comb and cockade textures. The vein as a whole, however, does not exhibit orderly symmetrical crustification towards the center. Rather, hydrothermal brecciation and tectonic fracturing repeatedly disrupted earlier-formed vein material during deposition of later stages. These complications have made it necessary to greatly simplify the paragenetic sequence because it is generally not pos sible to correlate individual mineral bands. Most of the Calera vein consists of quartz, manganese silicate, and rhodochrosite. The manganese silicate is mostly rhodonite based on optical proper ties, but peaks characteristic of pyroxmangite, and possibly bustamite, are observed on X-ray diffractograms of certain samples. In the following discussion, the term rhodonite is used to refer collectively to the manganese silicates. Sulfide and sulfosalt minerals are less abundant and occur mostly as distinct bands, ribbons, or pods. Sphalerite, argentian tetrahedrite (ffeibergite), pyrite, chal- copyrite and galena are the most abundant ore minerals, but electrum and a large variety of silver sulfosalt minerals were deposited relatively late in the evolution of the system. In general, the order of formation of the sulfide and sulfosalt minerals is similar in the different ore-mineral bands or stages, with early pyrite and sphalerite being succeeded and partially replaced by tetrahedrite and chalcopyrite. Galena 38 S t a g e E a r l y M n Sta ?e Quartz Stage Bonanz< L a t e Stage S u b s t a g e 1 2 3 1 2 1 2 3 4 ■ H m R h o d o n i t e Rhodochrosite — A d u l a r i a ? _____ S e r i c i t e K a o l i n i t e B a r i t e G y p s u m P y r i t e - S p h a l e r i t e Tetrahedrite _ _ G a l e n a Chalcopyrite B o r n i t e C h a l c o c i t e E n a r g i t e Stannoidite* (?) A c a n t h i t e Bournonite (?) Boulangerite P o l y b a s i t e Pyrargyrite M i a r g y r i t e E l e c t r u m ? - A n d o r i t e F a m a t i n i t e - S t e p h a n i t e Coloradoite* C a l a v e r i t e H e s s i t e S t i b n i t e C o v e l l i t e H e m a t i t e M a r c a s i t e Colloform Pyrite E v e n t s Hydrothermal brecciation x X X X x X Hydrothermal l e a c h i n g ? ___ Pebble dike A g / A u L02- 1 0 3 1 0 2 10 102- 1 0 3 Percent of vein in ore shoot R a n g e 0-5 0-5 5-95 0-10 0-20 5-95 0-10 0-10 0-5 0-10 0-10 A v e r a g e 1 65 4? 20 1? ± 2 2? ± 3 <1 a 1 _ Fig. 14. Paragenetic stages and substages of the Calera vein and their characteristic minerals. Asterisks indicate mineral identifications that have been confirmed by use of an SEM EDS system. 39 appears to have formed largely with the tetrahedrite, but also appears to have formed slightly later or earlier in some cases. The sphalerite of all stages typically contains abundant minute inclusions of chalcopyrite (see Figs. 19A, 42C, D), a feature known as chalcopyrite disease (Barton, 1978). Only some of the latest sphalerite deposited is partly free of the disease and has preserved color zoning. Petrographic and experimental work (e.g. Eldridge et al., 1983; Barton and Bethke, 1987; Eldridge et al., 1988) has shown that most chalcopyrite disease is formed by partial replacement of sphalerite by chalco pyrite. This interpretation is shown to be applicable at Orcopampa by the presence of chalcopyrite disease near free crystal faces and along grain boundaries and frac tures in sphalerite of the Late Stage. The presence of pervasive chalcopyrite disease in virtually all sphalerite of every stage except the Late Stage, in which it is incipient, suggests that the hydrothermal fluids responsible for deposition of the ore mineral bands may have evolved from zinc-rich compositions toward more copper-rich com positions. In areas with little or no vein filling, the structure is a fault, typically with about 0.3 m of gouge and an alteration envelope of variable width. In many places between ore shoots, however, the vein is composed of barren, generally fine-grained milky or gray quartz. The quartz may belong to the Early Stage and/or the Late Stage and is locally accompanied by small amounts of rhodochrosite and may be cut by pebble dikes and/or dikes of black siliceous microbreccia. The complex paragenetic sequence presented in Fig. 14 was determined from detailed underground observations and microscopic study, mostly of samples col lected in the main ore shoot. Three of the five stages are composed of two or more substages (Fig. 14) that can be recognized throughout the vein system. 40 Early Stage Early Stage material consists of quartz veinlets, with or without adularia and/or senate, rare adularia veinlets, and massive, very fine-grained quartz that cut the hydrothermally altered wall rocks of the vein. These veins probably formed con temporaneously with hydrothermal alteration. Massive quartz of this stage is rarely noted because it is nearly identical to some of the quartz in later stages. Manganese Stage The Manganese Stage is composed of several substages, each of which con tains abundant rhodonite, rhodochrosite, and/or quartz with lesser amounts of adularia (Fig. 14). Several hydrothermal brecciation events have been recognized, and hydrothermal breccia with cockade structure forms much of the material of this stage (Fig. 15). Concentric banding around fragments, termed escarapela by Peru vian geologists, is characteristic (Fig. 16). The earliest substage consists mostly of volumetrically minor infillings of rhodonite in brecciated wall rock. Fragments of the first substage are included in the second substage, which consists of several generations of banded veins and breccia fillings composed of abundant rhodonite with lesser amounts of rhodochrosite, quartz, and adularia; in places, however, rhodochrosite is more abundant. Rhodonite generally forms inter locking crystals in monominerallic bands that alternate with quartz (Fig. 17), and adularia is common as overgrowths on wall rock feldspar phenocrysts. Rhodochrosite is generally very fine grained and commonly appears to be pseudo- morphous after rhodonite. Veins and veinlets of the second substage form a stock- work in the wall rocks within a few meters of the Calera vein. The density of the stockwork increases as the main vein is approached, and within the Calera vein the second substage forms a banded breccia matrix for rotated, angular and subangular wall rock and vein fragments (Figs. 15, 16). Many complex crosscutting features are 41 Fig. 15. Typical breccia texture of the Calera vein system. Fragments of rhodonite-rich vein material of Manganese Stage, some with sphalerite-rich bands, and wall rock fragments in matrix of later Manganese Stage vein material. Split Ramal 2 near junction with Calera vein between 3800 and 3880 levels. Scale in cm and inches. Width of view is about 75 cm. 43 Fig. 17. Photomicrograph of rhodonite-quartz textures in a thin section cut from the Manganese stage shown in Fig. 16. Nearly monomineralic rhodonite (rd) with euhedral crystals in vug is infilled by quartz (qtz) and carbonate (carb). Transmitted light, crossed nicols. 44 evident between individual mineral bands, banded veins, and breccia fillings within this substage, but these cannot be correlated from one locality to another. The second substage contains several sulfide- and sulfosalt-rich layers or bands that typically contain only small percentages of gangue minerals, mostly quartz (Fig. 18). They vary from discontinuous, isolated crystals, pods, or clumps of ciystals as small as 0.1 millimeter in largest dimension, to continuous layers, as much as several centimeters wide, of coarse-grained ciystals. Detailed observations and pet rographic study indicate that the ore-mineral layers were typically deposited in strati graphic sequence during deposition of the enclosing gangue and are only rarely later- formed veins. Three general types of bands are recognized: sphalerite-rich, sphalerite-chalcopyrite-rich, and tetrahedrite-rich. Sphalerite-rich bands appear to have formed early in the substage (Fig. 18), and, in places, were deposited directly on altered wall rock. As many as four contin uous bands from about 0.5 to as much as 10 cm wide have been observed in one sec tion, and many discontinuous bands occur. Disruption of the vein by brecciation and fracturing has made correlation of the bands impossible. Sphalerite was apparently deposited with lesser amounts of pyrite; tetrahedrite, galena, and chalcopyrite replace and fill fractures in the sphalerite and pyrite (Fig. 19A). Some bands are par ticularly rich in chalcopyrite, which forms as much as 30 percent of the minerals. Elongated galena crystals that, along with carbonate, crosscut or drape the other minerals are common in some bands. The galena contains inclusions of hessite (Ag2Te) (Fig. 19B) as well as acanthite and myrmekitic intergrowths of polybasite. Marcoux and Milesi (1990) also reported altaite (PbTe). Rich ores from this stage contain lOMO3 ppm Te (see Table 4). Vein fragments composed of sphalerite, with or without adhering banded gangue or wall rock, are commonly enveloped by later banded rhodonite, rhodochrosite, and quartz (Fig. 15). Bands rich in argentian 45 < A V £ l S SX oVS^S" S rct,^ S d s “ d “ a S v 'c f qu^ T sjeTtatTb™ ™ '£of chalcopyri,e-galena’and pyri,e; rh°do" Reflected light photomicrographs of Manganese Stage ores. A. 3880 lew Kamal 4; sphalerite(sp) within tetrahedrite-rich band veined and replaced by chalcopyri i n i ’lS l T ? nj- tetrahedrite (td). B. 3800 level, Calera vein; hessite (hs) as inclusk) m galena (gn) adjacent to chalcopynte (cp). V 7 47 tetrahedrite, with Iesser amounts of pyxi.e and chalcopyrite, from less than 05 ,o 5 cm m width commonly appear to have fcm ed latex in the substage. In places, brec- ciation occurred prior to deposition of tetrahedrite-rich bands. Banded rhodonite, xhodochxosite, and quartz locally contain little ox no sul fide ox sulfosalt minerals. Electron, has been observed in some banded ore, bu, appears to locally cut across some of the bands, indicating that it may have been introduced later. Films of pyrargyrite that commonly occur on fractures that crosscut the mineral banding in some areas of the mine are thought to be part of the later Bonanza Stage (see below). The Laura vein contains material structurally and texturally very similar to that described above (Fig. 18), but with somewhat different mineralogy. Although the major minerals are the same, certain of the base metal-rich bands contain small amounts of bomite, digenite, covellite, enargi.e or luzonite, and a Cu-Fe-Zn-Sn sulfide (based on SEM EDS analysis), tentatively identified as stannoidite (Cu8[Fe,Zn]3Sn2S12). Most of these phases have not been observed in the same stages of the Calera vein or the other splits, and have not been reported from the other veins in the district. The last substage of the Manganese Stage, composed mostly of quartz with small amounts of rhodochrosite, rhodonite, and adularia (Figs. 14, 20), occurs as crosscutting veins and veinlets and as vug fillings in the previously deposited vein material. Quartz in vugs forms stubby hexagonal crystals as much as 1 cm in length. Amethystine quartz has been observed in only one exposure. Open spaces locally contain acicular manganese silicate, drusy rhodochrosite, euhedral sphalerite, and/or rare gypsum (selenite). 48 =f? a rc 49 Quartz Stage S,a Stage COnSiStS °f *” M m°re SUbStageS ,hat “ Manganese ( lg' e earlleSt SUbStage iS ComP°sed of massive, rarely banded, fine- to medtutn-gratned milky quartz with adularia and ^ ^ $ breccia fragments of Manganese Stage vein material and altered wal, rock (Rg. 21A). This substage may be gradational from the third subs,age of the preced- ing Manganese Stage. The first substage is cut by massive, locally banded, fine-grained gray quartz that contams breccia fragments of identical quartz and/or of Manganese Stage material (Fig. 21B). Veins, stringers, and masses of argentian tetrahedri.e, with or without pyrite, minor chalcopyrite and galena, and possibly rare electrum, accom panied locally by coarse-grained milky to clear quartz, occur within the fine-grained gray quartz (Fig, 21C). This material is distinct from the other stages in that it con- tarns very little sphalerite and galena. In places, the Quartz Stage is difficult to dis tinguish from the first part of the following Bonanza Stage, which it closely resembles (see below). Bonanza Stage The Bonanza Stage occurs as narrow veins, veinlets and fracture coatings deposited after the Quartz Stage (Fig. 14). Although volumetrically minor, it con tains approximately 50 percent of the values, including most of the gold, of the Calera vein system. Two types of veins are recognized. The first is typically about 10 to 20 cm wide and is composed of bands of fine-grained quartz separated by 0.1 to 10 cm wide bands of tetrahedrite, sphalerite, galena, pyrite, and chalcopyrite with a trace of covellite and small but important amounts of silver sulfosalts and electrum (Rgs. 14, 22). Miargyrite (AgSbSz), the dominant silver sulfosalt, is accompanied by pyrargyrite as well as polybasite and stephanite with electrum and small amounts of ■ ■ m n 50 subSgeof Quamtaae T S 3L A Quartz'rich vein material of first (top of D h n f n S b g with fragments of Manganese Stage vein material and wall rock Ban on s c a t e l e l cr i ^ e n p h ganeSe ^ ° re Sho°‘ Calera vein" 3880 >CTeI' 51 material^'ttekSli®sho™TF%e?6.8W d t^ ^ 3 0 c S ° KCU,‘ing MansanKe S,aS cutSfa w .H B,?nariZa"Stage ° re comP°sed largely of tetrahedrite and quartz funner left) 54 calaverite (AuTe2) and coloradoite (HgTe) (Larson, 1984; Gibson e, a]., 1990) In thin section, some of the quartz exhibits feathety or mosaic extinction, possibly indi cating recrystallization from fine-grained quartz or opahne sihca (C. J. Eastoe, ora, comm., 1988). The silver sulfosalts fill open spaces in the quartz (Fig. 23) and replace tetrahedrite, chalcopyrite, pyrite and other sulfides. Andorite (PbAgSb3S6) famatinite (stibioenargi.e), and boulangerite (Pb5Sb4Sn) and/or bouruonim (PbCuSbSs) occur with the sulfosalts and tellurides (Larson, 1984; Gibson et al„ 1990)(Fig. 14). Marcoux and Milesi (1990) also identified aramayoite (AgISb.BqS.),’ sylvamte, uytenbogaardtite (AgjAuS2), and semseyite (Pb,Sb8S21) using electron microbeam methods; however, they state that the aramayoite contains only small amounts of bismuth, and thus should be termed miargyrite. Open spaces within the veins locally contain sericite. Veins of mis *pe are typically discontinuous and can not be traced for more than a meter. In the split Ramal 1, however, a discrete vein of Bonanza Stage material about 50 meters in length and as much as 30 cm in width cuts earlier vein material (Fig. 24). A characteristic sequence of banding has been recognized in the first type of Bonanza Stage veins (Fig. 25). Fine-grained finely-banded quartz with isolated grams and narrow bands of pyrite and sphalerite with silver sulfosalts and electrum was deposited first. A band composed dominantly of tetrahedrite with small amounts of sphalerite, chalcopyrite, galena, pyrite, and electrum followed. This material resembles the tetrahedrite-rich bands in the Quartz Stage and could be mis taken for it. Milky quartz with cavities containing tetrahedrite, galena, sphalerite, silver sulfosalts, and electrum separates the tetrahedrite-rich band from a second ore-mineral band composed of intergrown quartz, sphalerite, galena, and tetra hedrite with lesser amounts of chalcopyrite, pyrite, silver sulfosalts, and coarse elec trum. Finely-banded quartz with narrow bands of tetrahedrite, chalcopyrite, silver 55 dKrf? ® ssesss irrcess c h a to p ^ e (cp) wrth small pyrite (py) inclusions. Black areas arfquS S g a S a”d 56 andF{lsni^vB|?”a,nZa Sta!C’ Ramal L View of back of slope at crosscut 115W between 3800 M a n g ^ ^ s are ° D a flX f Bona”f S,aSe « v -\ * * ft.. ’*■ / Sp 2 cm Substage Mineral Quartz Sericite Pyrite Sphalerite Chalcopyrite Tetrahedrite Galena Miargyrite Pyrargyrite Stephanite Polybasite Electron Andorite ? Famatinite Colorodoite Au-teiurides of S n a S Rrama' L A slab of « the text and shown in B as follows- 1 Neari d * the five sJaSes of deposition described in 2. Tetrahedrite S m i n o r c S S t e ’ T * w th electrum and silver sulfosalts; coarse milky quartz with central catties thJt ^ t ’-g3 P^ ltC and electrum; 3- Relatively Fine-grained quartz with tetrahedrite snhal -f 3m sPhaIe^ te> Pyrargynte, and electrum; 4. Finely banded^quartz'with stiver sulfnsaIts g^lena’ ^ Sulfosalts’ and e^ r u m ; 5. chalcopyrite This sam nle m t ' • ’ minantty miargyrite, electrum, tetrahedrite, and u p w S T sp m °/ L00 gold- Dircction of deposition fa sequence of BonaS S in A ’3S°° “ d 3880 Ievels- R g en etic 58 sulfosalts, and electrum was deposited las,; one or more of the late quartz bands commonly has a green tint, the cause of which remains unknown. T ie second type of Bonanza Stage vein, typical* less than 1 cm wide, consists of euhedral quartz crystals that project from the vein walls into open space a, the center of the vem. The quartz ays,als exhibit growfl, zoning in places. Pyrargyrite with electrum, chalcopyrite, and ,e,rahedrite(?) flu open spaces in the quartz veins and coat fractures in earlier vein materia,. Electrum commonly coats terminated pyrargyrite crystal, The major silver-bearing phase in the second type of vein is pyrargyrite, as opposed to miareyrite in the first uf gyrne in the first type of vein, suggesting that there may be two substages of the Bonanza Stage. Elec,mm locally occurs without abundant silver sulfosalt minerals in previ ously deposited vein material adjacent to Bonanza Stage veins. In one place, coarse elec,mm, along with earlier chalcopyrite and galena, mantles and replaces pyrite and sphalerite in vein material adjacent to a Bonanza Stage vein of the second *pe. Also, m a vertical ore shoot in the western part of the main ore shoot, coarse elec- tmm OCCUrs in °P“ sPa« * and, along with chalcopyrite, replaces pyrite and tetra- hedrite in veins composed dominantly of quartz and granular pyrite and lesser amounts of tetrahedrite (Fig. 26). This ore typically has a Ag/Au ratio of about 1. Variation of the silver content of electrum in the Bonanza Stage, based on color, was noted in reflected light microscopy. Electrum becomes more white as silver content increases and more yellow as silver content decreases. Some grains have a white center and yellow margin, whereas others are more gold-rich in their centers. Also, some grains have white curving Veins” that divide yellow masses, pos sibly reflecting coalesced grains. The observations were confirmed qualitatively through the use of an SEM equipped with an energy dispersive analysis system (Fig 27). 59 CaleS 4 L ^ “e' bonanza S/a«e 3880 k * l slope, replacing chalcopyrite and filling open space Rlart* C°Pynte ^ with natlve Sold (Au) spaces. mg °pen space- Black areas are quartz gangue and open 60 A Au 61 ™ e Bonanza Stage is mineralogically convex and differs greatly from the n r ,aters,ages ° f . - i d s : : u/ g ra,to w ine is an order of magmtude or ntore higher than in the Manganese I T " (R " H 28)’ Md the “ “ - — - - stdfosalt Late Stage The Late Stage is composed dominant,, of quanz, sphalerite, and gaiena , ^ Ve“ letS COntai” fi”e' ‘° medium" quanz from less than O more t an 20 cm in width, with or without a central filling of sphalerite and/or ga ena ( ,g. 14). Some of these veins posses prismatic cavities of orthorhombic or monochmc symmetry that formed from hydro,hermai .caching of an nnitnown min- e ra , poss.biy bante, tha, was originally encased in the quartz (Fig. 29). Veins as utuch as 3 cm wide composed entirely of sphalerite and galena are also present Tlte milky quartz veins are cu, by breccia or pebble dikes composed of angu- rounded fragments of wall rock, vein material, and quartzite and shale frag ments m a siliceous matrix tha, commonly contains fine-grained pyrite with or with- out marcasite and sphalerite (Fig. 14). The quartzite and sha,e fragments are pro- a y from Mesozoic rocks more than 100 meters below the breccia exposures or possibly were derived from conglomerate units in voicanic breccia tha, form the footwall of the vein. One pebble dike is a, leas, 100 meters in vertical extent Verti cally extensive bodies of syn- and post-minera, breccia have also been recognized in other veins m the district (Noble, 1979b; Gibson, 1987). A third substage consists of veins of quartz with lesser amounts of barite, mar- rasite, PJTite, sphalerite, chalcopyrite, stibnite, hematite, and minor rhodochrosite >g- M). Hydrothermally leached cavities, more equan, than those in the firs, sub- 62 SpUt Ramal 3800 lCTel- Bonanza Stage ore with low Ag/Au I about W )with : i represents the mnnng of stages having relatively high L * , L ° e S ° rCS of the Manganese Stage and other III s p a i m e n a o f C a M a s S o y i H ? , S° hd d,anrands rePresent « a y SB of selected ore in 1985 and 1986. 8 ’ nana cK represent average values for direct-shipped 64 s.age, commoniy form delicate, porous masses within large lenticu]ar ^ ^ ^ “ 7 " " 2 "e,erS 1,1 ^ dfaenSi0n- ^ « * leached masses and large euhedra, quariz crystal as ntuch as 30 cnt long, but „ 3 t0 1Q J long, line vrngs. Bante, collofomt pyrite and marcasite, stibnite, sphalerite, chalco- pynte, tntnor rhodochrosi.e and a clay(?) fi,m occur in the cavities and conttnonly coat the large quartz ctystals; in places, stibnite, barite or sphalerite are partially or wtth loose, doubly terminated quartz crystals. A few of the large quartz ctystals contam vety large fluid inclusions with vapor bubbles that can readily be seen with the unaided eye. Vems of highly fractured milky quartz with adularia and a fracture filling of dltte occur locally in the upper portions of the Calera vein. The veins cut the breccia dtke, bu, relationships to other substages are uncertain; they are tentatively included withm the substage described above. Two types of microbreccia or tuffisite dikes from 5 to 500 cm, but typically less than 15 cm, in width occur vety late in the paragenetic sequence, but timing with respect to the substage described above is unclear because crosscutting relationships have no, been observed (Fig. 14). They typically have vety sharp contacts, are irreg ular in width and orientation, and commonly bifurcate (Fig. 30). The earlier micro- breccia is typically dark gray or black with a matte texture, and contains few visible breccia fragments. In thin section, however, abundant angular breccia fragments of vein material, wall rock and quartzite typically less than 0.5 cm in diameter are evi dent ,n a siliceous matrix containing abundant adularia and fine grained pyrite. The second type of microbreccia, which is closely associated spatially with the first but is less common, contains more abundant large breccia fragments in a dark siliceous matrix. The matrix of the microbreccia is locally soft and may contain clay minerals 65 cross cut vein ma,erial °f tht level, west of junction w ith S a ll ™ W ‘S appKK' L5 “ • Calera vei”. 38« 6 6 ™.h little silica. The microbreccia commonly contains fragments of millty vein quartz containing fine-grained adularia and carbonate and in a few places is cu, by norma, faults with small displacements. Mineralogical Zoning, Distribntion of Stages, and Grade Distribution Ore shoots in the Calera vein system have several juxtaposed stages of miner alization, with the proportion of each stage depending on position within the ore shoot. The zoning of the main ore shoo, in the Calera vein and adjacent splits (Figs. 5H, 8-10), which has been studied in the most detail, is discussed below. In the Calera vein, the Manganese Stage forms most of the lower part of the main ore shoot, whereas the Quartz Stage is relatively more abundant in upper levels (Fig. 31A). In slopes of the 3800 and 3880 levels (Fig. 5), the Manganese Stage com prises about 50 to 90 % of the vein and is cut by veins of the Quartz Stage. In the stope above the 3950 level, however, the Quartz Stage comprises 70 to 90 % of the vein and contains breccia fragments and relict vein slivers of the Manganese Stage. The Late Stage is volumetrically minor and occurs throughout the vein system. These stages also have different distributions in the Calera vein relative to the splits. For example, in the stope of the 3880 level, the Quartz Stage is relatively abundant in the Calera vein but is rare in split Ramal 2. A similar relation is also evident in Ramal 1 and Ramal 2 on the 3800 level, suggesting that the spite allowed fluid flow and mineral deposition during formation of the Manganese Stage, but were more closed during the deposition of Quartz Stage, whereas the Calera vein was open during deposition of both stages. Veins and veinlets of the Bonanza Stage occur in a west-plunging zone that coincides closely with the trace of the intersection of the Calera vein with the splits Ramal 1 and Ramal 2 (Fig. 3 IB). Fluid flow during deposition of the Bonanza Stage I 67 A B T nrrit* a-' ,Dls.tributl.on of paragenetic stages in the main ore shoot of the Calera vein A patternUindirfneinthr°',eCti0n-Sh0Wing distribution of Manganese and Quartz Stages Stippled vein ShTdid the approximate area where Manganese Stage forms 50 % or more oftfie vein' SB I/?naio!d!n |CateS the aPProximate area whefe Quartz Stage forms 50 % or more of Stage Stinnled ^ Projectl° n mdlcatmg approximate distribution of the Bonanza L ip h tl'v P a t t e r n indicate the approximate distribution of Bonanza Stage veins Stfge nh r hardea 1 ‘tr tes thcJ approximate position of the richest part of the Bonanza coarse gold r e S ”8 1"d!Cates.the approximate distribution of Bonanza Stage containing (Ag/Au = l)(see text)C 3 C°PyntC’ Pynte’ and tetrahedrite with little or no silver sulfosalts 6 8 appears to have been controlled to an important degree by the intersection of the Calera vein with the splits. The riches, part of the Bonanza Stage in the Calera vein was located in the slope above the 3950 level east of the intersection with Ramal 1. Tl,e main ore shoo, of the Calera vein is mineralogicaUy zoned. Based on observations of the author and those of the mine staff, the abundance of sulfides in the Manganese Stage generally decreases with depth, and rhodonite may be more abundant relative to rhodochrosite at depth. On the 3750 level, the Manganese Stage is almost devoid of sulfides. In the Quartz Stage, tetrahedrite is more abun- dan, above the 3950 level. In the Late Stage, stibnite, marcasite, colloform pyrite and kaolinite are more abundant in the upper par, of the vein above the 3950 level, whereas sphalerite, galena, chalcopyrite and barite are more abundant in the lower levels. Mineralization in the split Ramal 1 is telescoped relative to the Calera vein. The Manganese Stage, very rich Bonanza Stage, and abundant stibnite, marcasite, colloform pyrite and barite of the Late Stage all occur on the 38X5 level, well beloJ the richest part of the Bonanza Stage and abundant stibnite, marcasite and colloform pyrite in the Calera vein. Metal ratios have been used to study ore distribution and zoning in vein systems (e.g. Petersen, 1990), including several of the veins at Orcopampa (Petersen, 1982; Petersen et al, 1990) as well other districts such as Julcani (Goodell and Petersen, 1974; Petersen et al., 1977) and San Cristobal, Peru (Campbell and Petersen, 1988), and Topia, Mexico (Loucks and Petersen, 1988). Metal ratios may be based on bulk assay data or on values obtained by microbeam and/or chemical analysis. Selection of appropriate contours of the metal ratios commonly define con volute "ore bands" that include most of the ore shoots in a vein, with antiforms in regions of high fluid flow and synforms in regions with lower fluid flow (Petersen, 69 1990). Metal ratio study of tetrahedrite zoning in the Manto and Santiago veins at Orcopampa has shown .ha, grade contours based on bulk assay data closely approximate the ore band (Petersen, 1982; Petersen et a]., 1990). Plots of Ag and Au grade contours for the main ore shoot of the Calera vein are shown in Figure 32. The contours are antiforms inclined to the east, suggesting tha, the source of the hydrothermal fluids was at depth in the western part of the vein (Petersen e, ah, 1990). The shape of the ore band in the main ore shoo, has been used to guide exploration, based on the assumption tha, the band will bend into another antiform lateral to the main ore shoot. The complexity of the paragenetic sequence of the Calera vein, however, makes interpretation of such data based on bulk assay grades difficult and equivocal, because several stages have been mixed together. For example, the majority of the values within the 0.1 opt Au grade con tour (Fig. 32B) are the result of the presence of the Bonanza Stage, but several stages provide silver values of 10 opt. Geochemistry of the Ores and Altered Rocks Chemical analyses of selected samples of rich ore from some of the stages are gi en in Tables 3 and 4. Although it is difficult to draw unequivocal conclusions from the small data set considering the large number of stages and substages present, some generalizations are possible. Precious- and base-metal contents are quite vari able m all the stages, largely because analyses were obtained on different substages and on different bands of ore within substages (Table 3). The Quartz Stage, how ever, is characterized by consistently low lead and zinc contents as well as low Pb/Cu and Zn/Cu ratios compared to the other stages (Figs. 33, 34). The lead/zinc ratios, however, are comparable (Fig. 33). In contrast to the Quartz Stage, the Late Stage has relatively high lead and zinc contents (Table 3; Fig. 33), as well as a slightly 70 Silver isoDleth32R ^ 1n^ tlJdlj lal sectlo^s °fthe Calera vein showing grade isopleths. A. 10 opt Ag grades higher tVL Pth AU 1SOpleth\ ^haded areas indicate vein material with bulk assay grades Ingher than the respective isopleth. Modified from Petersen et al. (1990). 71 Table 3. Chemical Analyses of Ore Samples from the Calera Vein System Sample Au As Cu Pb (oz/st) g Zn . As Sb (weight percent) Mn Fe Selected Hand Samples of Rich Ore Manganese Stage 15 0.620 66.4 2.26 2.58 4.44 0.04 0.06 5.8 5.3 17 0.030 61.9 1.27 0.54 0.65 0.04 1.15 9.1 2.3 57 0.075 19.7 0.46 0.94 2.29 0.03 0.13 7.3 2.3 443 0.800 189 1.30 2.13 2.28 0.07 1.34 13.3 2.0 503 0.068 16.6 3.52 4.78 0.74 0.07 0.11 2.3 4.4 520 0.106 145 1.67 1.52 2.07 0.06 0.81 0.3 1.4 553 0.068 26.3 2.79 2.82 4.31 0.07 0.06 0.3 5.2 Quartz Stage 73 0.135 45.2 0.61 0.11 0.13 0.13 0.35 0.2 2.7 79 0.105 65.7 1.62 0.06 0.24 0.10 1.12 0.4 1.6 104 0.235 136.3 2.36 0.04 0.38 0.08 1.65 0.1 2.1 524 0.195 40.6 0.26 0.18 0.25 0.07 0.22 0.3 0.5 538 0.071 53.9 0.59 0.13 0.15 0.19 0.24 1.0 0.7 Avg. 0.148 68.3 1.09 0.10 0.23 0.11 0.72 0.4 1.9 Bonanza Stage 425 14.6 575 7.78 2.13 3.45 0.24 6.38 0.2 2.6 441 20.4 245 2.70 2.80 1.61 0.30 1.04 0.2 4.5 507 1.43 437 2.41 1.72 1.65 0.21 1.59 0.3 2.0 508-1 1.20 245 0.96 1.99 2.52 0.27 0.76 0.3 2.8 508-2 9.38 680 1.54 0.45 0.78 0.11 1.45 0.3 0.8 509 1.96 138 1.37 0.19 0.45 0.15 1.22 0.3 2.0 510 7.30 299 3.07 1.89 3.50 0.17 2.77 0.2 2.6 511 8.26 482 4.34 0.15 0.91 0.22 3.23 0.2 2.9 512 8.31 504 4.63 2.61 7.77 0.25 4.28 0.2 2.3 515-1 1.16 189 1.07 0.22 0.47 0.14 1.25 0.1 2.4 515-2 3.08 562 5.13 1.60 2.55 0.19 4.68 0.1 1.7 516 11.9 556 4.00 0.68 1.49 0.19 3.76 0.3 1.3 517 28.1 374 5.02 1.46 4.54 0.22 4.01 0.3 3.9 519 15.9 129 0.92 1.45 2.78 0.11 0.95 0.2 1.4 522 10.4 443 2.44 1.20 1.94 1.18 2.21 0.3 1.3 530 70.9 579 1.45 8.33 6.20 0.19 2.16 0.6 5.7 535 2.69 545 1.91 1.53 2.11 0.23 2.28 1.0 1.0 536 6.58 276 0.86 1.96 2.34 0.11 1.08 2.6 0.9 72 Table 3. ('Cont.l 4.5 0.07 0.33 0.36 0.02 0.04 0.8 2.2 26 0.180 230 4.53 5.95 8.34 0.98 2.62 0.3 5.9 31 0.025 21.8 0.51 0.06 0.16 0.05 0.33 0.6 1.8 42 0.110 48.2 0.27 3.75 2.79 0.04 0.32 0.2 1.8 445 0.280 12.7 0.18 6.35 3.16 0.32 0.21 0.3 2.0 525 0.262 14.6 0.38 8.20 2.79 0.07 0.23 0.2 2.3 526 0.070 5.2 0.06 0.30 0.14 0.04 0.05 0.2 1.2 533 0.095 2.4 0.05 0.08 0.10 0.26 0.15 0.1 5.1 534 0.491 18.2 0.38 6.30 7.22 0.07 0.31 0.2 1.5 555 0.101 11.9 0.10 2.96 5.72 0.07 0.07 0.3 0.4 Ramal 2 0.279 29.9 0.39 3.42 5.81 0.11 0.28 0.3 1.3 Avg. (6) 0.176 23.2 0.31 4.12 3.41 0.11 0.24 0.3 2.5 Monthly Composite Mill Heads Nov-86 0.125 13.9 0.15 0.27 0.33 0.04 0.13 Jun-87 0.165 15.0 0.18 0.33 0.37 0.04 0.14 Dec-87 0.070 12.6 0.13 0.31 0.37 0.06 0.14 Jul-88 0.090 13.8 0.15 0.33 0.39 0.08 0.13 Jun-89 0.070 10.3 0.15 0.30 0.35 0.04 0.13 Average 0.104 13.1 0.15 0.31 0.36 0.05 0.13 ...... ore Sf n 5 n ? w rOUp ^ der 11,6 SUP ervision of Dr. Irene Marxowa. Samples of richri material of otherothe? stages.stafes C y coIlected t0 contamination from 73 Table 3. (Cont.) 15 17 Calera vein I I S ,leVe! S,t0pe’ spUt 3' 4> eleva«™ 3890 m. 57 Calera g£ £S £ g STS S ta tio n 3815 443 m. 503 S S 3880 level S g ore pass ! “ m' 520 Ramal 1, 3800 level stope, raise 50W + 20E. 553 Calera vein, 3880 level stope, raise 545E. 73 79 Calera v e in ' loin w ! S!°Pe’ Ce"tral part of orc body> elevation 4005 m. 104 C ateS 3 e ! pe’West end of ore body. elevation 4000 m. 3950loll level stope, west end of ore body, elevation 4000 m 524 Split between Q lera vern and Ramal 1,3800 levelstope, crosscut 65W 538 Otlera vein 4055 level stope, east end of ore body. P 425 I S r ™ 3 }’ l 6? '6611 Crosscut 115W ™d raise 50W, elevation 3840 m 441 Split Ramal 1, between crosscut 115W and raise 50, elevation 3840 m 507 Calera vein, 3800 level stope, raise 90E. m‘ 508-1 Calera vein, 3800 level stope, raise 90E. 508-2 Calera vein, 3800 level stope, raise 90E. 509 Ramal 1, 3800 level stope, crosscut 115W + 3W 510 Ramal 1, 3800 level stope, crosscut 115W + 1.5W 511 Ramal 1, 3800 level stope, crosscut 115W + 0. 512 Ramal 1, 3800 level stope, crosscut 115W + 3E 515-1 Ramal 1, 3800 level stope, crosscut 115W + 4E. 515-2 Ramal 1, 3800 level stope, crosscut 115W + 4E. 516 Ramal 1,3800 level stope, crosscut 115W + 5E." 517 Ramal 1, 3800 level stope, crosscut 115W + 7E. 519 Ramal 1,3800 level stope, crosscut 115W + 7W 522 Ramal 1, 3800 level stope, raise 50W + 20E. 530 Calera vein, 3800 level stope, raise 90E + 10W. 535 Calera vein, 3800 level stope, raise 165W + 5E.' 536 Calera vein, 3800 level stope, raise 165W + 5E. 21 Calera vein, 3800 level stope. 26 3815mV6m’ 3800leVCl St°pe’ eaSt 6nd°f °reb0dynearraise 90’ elevati°n 31 Calera vein, 3880 level stope, east end of stope, elevation 3885 m. 42 Ca era vern, 3800 level stope, west end of stope, elevation 3805 m. 445 Calera vern, 3800 level stope, raise 90E + 15E 525 Split between Calera vein and Ramal 1, 3800 level stope, crosscut 65W. 526 bplit between Calera vein and Ramal 1, 3800 level stope, crosscut 65W 533 Calera vein, 3800 level stope, raise 165W + 35E 534 Calera vein, 3800 level stope, raise 165W + 35E. 555 t j . ^ ^Calera “ vein,------^»vi 3880 aiuuu level taut stope, raise 545. Ramal 2 Split Ramal 2, 3800 level, stope 50E. Sample Mn Stage 6 1 7000 37700 41800 45900 90 38 25700 61900 540 22800 2 393 93 <2 14600 14800 1650 53900 81 148 <0.8 469 <4 Quartz Stage 80 <2 16.6 249 19 67 217 7800 28100 2800 5600 1390 21200 120 670 66 3.4 Bonanza Stage 15.6 175 12 120 2666 21500 27100 64900 76200 2180 31600 71 78 >741 124 1853 10400 12900 10700 12400 <2 7.2 557 21 1320 16100 163 291 143 <2 Calera Outcrop 35.8 105 25 44501 0.011 29.6 374 26.6 8.83 36.5 50.7 0.435 33.0 2.12 1.58 Altered Wallrock 17.9 0.154 <0.493 44502 0.002 12.9 4075 HW <0.0005 0.385 0.296 4.87 1.05 0.362 <0.096 <0.478 4075 FW 0.007 2.35 <0.098 0.797 <0.977 0.678 0.141 <0.488 B 0.0005 0.180 <0.096 3.64 1.51 3.82 0.583 <0.481 HW 0.002 0.822 <0.097 0.506 <0.970 0.327 <0.097 <0.485 3800HWR1 0.001 0.788 <0.097 0.645 <0.966 0.402 0.098 <0.483 43 0.083 4.59 <0.098 <0.489 <0.978 0.288 0.236 0.526 <0.489 91 0.093 16.6 1.05 <0.971 0.416 1.00 1.08 94 0.046 15.8 3.78 3.25 2.98 <0.242 7.48 1.45 1.84 110 0.014 8.82 1.97 2.60 <0.241 8.54 1.30 Subacuoso 0.088 3.76 2.16 2.48 2.61 1.19 2.37 <0.097 1.27 1.54 <0.966 1.20 1.11 0.577 75 Table 4. (Cont.) Sample Locations: 906 t^alera Calera vdnvein, Iran3880 level i?0JeYeI stope, st0Pe’ elevation elevation 3890 3890m. m. 67 Calera vein 3880 level stope, central part ore body elevation 38Q0 m 120 Sph, Ramal 1, between window 115 aSd c h i n l y t o ^ S g o 124 Split Ramal 1, between window 115 and chimney 50, elevation 3820 44501 Calera vein outcrop 44502 Adjacent to Calera vein outcrop 4075 HW Hanging wall adjacent to Calera vein, surface 4075 FW Footwall adjacent to Calera vein, surface B Hanging wall, near Calera vein, surface HW Hanging wall, near Calera vein, surface 3800 HW R1 Hanging wall of Ramal 1,3800 level stope, junction with Ramal 2 43 H a tin g wall adjacent to Calera vein, SsVlevel s t o p T w S r f 91 Hanging wall of Calera vein, 3880 level stope 94 Footwall of Calera vein, 3880 level stope 110 Footwall of Calera vein, 3950 level stope access, Subacuoso Footwall of Calera vein, 3800 level stope, east end of orebody Fig. 33. Plot o f Pb vs Z n for rich samples of the Calera vein. Calera the of samples rich for n Z vs Pb f o Plot 33. Fig. Pb (wt. %) 10- KT 10 101 101 - ° •i'i UK* Late 0 Bonanza ♦ ^ ° Quartz A Manganese a A o ♦ ♦ Zn (wt. %)(wt. Zn O 10 ° ♦ ♦,» ♦ ♦ O ♦ o o t 0 ft o 10 ‘ 7 6 77 Cu Zn Calera vein TnanguIar pIot of Cu’ Pb> and Zn in weight percent for rich samples of the 78 higher Pb/Zn ratio, and high Pb/Cu and Zn/Cu ratios (Fig. 34). Samples from the other stages generally have intermediate Pb/Cu and Zn/Cu ratios. A ternary Cu-Ag-Pb+Zn diagram further illustrates the mineralogical differ ences m the stages and, in some cases, substages (Fig. 35). Samples of the Bonanza Stage and the Quartz Stage, as well as tetrahedrite-rich Manganese Stage plot on the left side of the diagram, whereas samples of the Late Stage and base-metal sulfide- rich samples of the Manganese stage plot on the left side of the diagram. Srlver/gold ratios of most of the stages are variable, but are mostly between 100 and 1000, except for the Bonanza Stage, in which they are lower by as much as an order of magnitude or more (Fig. 36). Several samples of the Late Stage also have silver/gold ratios of less than 100 (Fig. 36); some of the samples assigned to the Late Stage may be Bonanza Stage, but were incorrectly identified because of poor para- genetic control and low percentage of sulfides. The precious-metal contents reported for the Late Stage require further study because no abundant precious- metal-bearing minerals have been identified. Antimony is much more abundant than arsenic; arsenic content is relatively constant within an individual stage and is lowest in the Manganese and Late Stages (Fig. 37). Antimony/arsenic ratios are variable within individual stages, largely because of variations in antimony content. Antimony correlates positively with copper, because tetrahedrite is the most abundant copper-bearing mineral in most samples, except for samples of Manganese Stage that contain a chalcopyrite-rich band and have higher Cu/Sb ratios (Fig. 38). Relationships similar to those for copper and antimony exist between antimony and silver (Fig. 39) and copper and silver (Fig. 40), but the correlations are not as good, probably because silver is present in other minerals, such as sulfosalts. The samples for the Late Stage plot in the same array as the other samples, indicating that the 79 2Cu Pb + Zn Fig. 35. Plot of 2Cu, lOAg, and Pb + Zn in weight percent for rich samples of the Calera vein. 80 104 r D Manganese A Quartz ♦ Bonanza < f'/ 103 ° Late - y * o,' ♦ T3co a ♦ °y' A 3 i°2 W> /\ < A °a a , □ 3 101 ?' % s* 10 ° 1 0 ' 10'1 10° 101 102 Au (oz/dst) Fig. 36. Plot of Ag vs Au for rich samples of the Calera vein. Compare with Fig. 28. 81 101 i_ i 10° D ♦ D,'' b ' / ' o o JD A , ' CO 10'1 □ ,'□0 o Manganese O' A Quartz * v ♦ Bonanza ° Late # 10 - io- 101 10° 101 As (wt. %) Fig. 37. Plot of Sb vs As for rich samples of the Calera vein. 82 101 r O* V' V □ □ V D S A O A % O O / a O 'O 10’ 1 □ O/' Manganese \ A Quartz ' ' / ♦ Bonanza < ? / O Late 10“ ± -d lO- Hf1 10» 101 Sb (wt. %) Fig. 38. Plot of Cu vs Sb for rich samples of the Calera vein. 10' n Manganese a Quartz ♦ Bonanza r< ♦V ° Late o A ^ / i 9 10° a o / ^ x> a co -s° ' A 10'1 / □ ° □ ,0 & s ' 10" 10“ 101 jo2 103 Ag (oz/dst) Fig. 39. Plot of Sb vs Ag for rich samples of the Calera vein. 84 101 r o ♦ * ♦ □ A / ♦ ♦ A q,'" ♦♦ 10° a -♦ □ A^/ □ N o o V—'o 3 A° u ° / / 10'1 / ° o / □ Manganese ,&/ A Quartz p > ' y ♦ Bonanza o Late 10------1----j-—i—i—i i i 11 , , 10“ 101 102 103 Ag (oz/dst) Fig. 40. Plot of Cu vs Ag for rich samples of the Calera vein. 85 sdver in the Late Stage may be in tetrahedrite. On a ternary diagram of the three elements (Fig. 41), most of the data plot near the center in a relatively restricted range; some Bonanza Stage samples, however, have higher Ag/5b and Ag/Cu ratios probably because of the presence of abundant silver sulfosalts, and some Manganese Stage samples have higher Cu/Ag and Cu/Sb because of abundant chalcopyrite. Table 4 includes analyses for several additional elements in the Manganese, Quartz, and Bonanza Stages. Tellurium and selenium contents are relatively high for all samples; one sample of the Bonanza Stage contains very high selenium and high Se/Te and may contain a selenide. Mercury contents are variable but are generally high. Bismuth content is low, a feature observed in many adularia-sericite systems. The samples also contain significant amounts of molybdenum and thallium. Cad- mmm is present in appreciable amounts and correlates with zinc, suggesting that it occurs mostly in solid solution within sphalerite. Table 4 also includes broad-spectrum geochemical analyses of altered vol canic rocks sampled adjacent to the vein system from underground and surface expo sures as well as the vein outcrop. Most of the rocks contain anomalous amounts of copper, lead, zinc, molybdenum, and tellurium, moderate antimony, and low gold, arsemc, mercury, and thallium. The vein outcrop contains relatively high values of silver, copper, molybdenum, and tellurium. Chemical analyses of five monthly composite mill-head samples are shown in Table 3. These analyses reflect average values of the ore mined during the month - small-scale variations in chemistiy due to exploitation of ore from different areas of the mine are smoothed out. In 1986, ore was being extracted largely from the upper levels (4015 and 3950) of the mine, but the proportion extracted from the lower levels (3800 and 3880) increased to almost 100 percent by 1989. The general decrease in Au and Ag grade and increase in Ag/Au ratio and in manganese content 86 Cu Calera vein. TnanguIar plot of Cu’ 10A& and Sb in weight percent for rich samples of the 87 through time reflect, therefore, the position of the richest and most abundant part of the Bonanza Stage in the upper part of the vein and more abundant Manganese Stage in the lower part of the vein. The content of base metals has been relatively constant with time and is somewhat lower than in some other epithermal vein deposits containing significant amounts of base metals for which data are available, such as Topia (Loucks and Petersen, 1988) and Areata (Candiotti et al„ 1990). Fluid Inclusions A fluid inclusion study was carried out on fourteen samples of several of the paragenetic stages of the Calera vein system. Doubly polished sections, generally about 0.1 mm or less in thickness, were studied in detail with a standard petrographic microscope in both transmitted and reflected light. Subsequently, selected parts of the sections were used for microthermometric study on a U.S.G.S,type gas-flow heating-freezing stage obtained from Fluid Inc. Microthermometric measurements were typically obtained by bracketing, such that the homogenization temperature (Th) or ice melting temperature (T„) falls between two values, generally 5°C or less for Th and 0.2°C for TM (SeeAppendix 2). Replicate measurements indicate a reproducibility of ± 5°C for TH and ± 0.2”C for TM. Calibration of the heating freezing stage was performed using synthetic fluid inclusion standards provided with the stage ( 0 C and -56.6°C) and a solid chemical standard, potassium dichromate (398°C). Ice melting temperatures were generally obtained prior to homogenization temperatures. The first pyrargyrite sample studied, however, cleaved during freezing causing the inclusion to leak; therefore, other inclusions in pyrargyrite were heated before freezing. Also, as reported by other workers (cf. Simmons et al, 1988), pyrar- gynte became darker as heating progressed, making it difficult to see the vapor bubble. Usable fluid inclusions were observed in quartz, sphalerite, and pyrargyrite. Three types were recognized: type I inclusions contain an aqueous liquid and a small vapor bubble that comprises less than about 20 to 30 volume percent of the inclu sions (all volume percentages reported are visual estimates); type la inclusions are similar to type I inclusions but also contain an irregular, platy or fibrous birefringent daughter mineral, possibly dawsonite, as has been reported from other epithermal systems (e.g. KamiUi and Ohmoto, 1977; Smith et al„ 1982); type II inclusions are vapor dominated with more than about 50 volume percent vapor and an aqueous liquid rim. Some type II inclusions contain nearly all vapor. All reported micro- thennometric measurements were made on type I and type la inclusions. Quartz contains abundant inclusions. Primary type I and la inclusions, gener ally less than about 15 Mm in size, are commonly observed on growth zones (Fig. 42A), but abundant secondary and/or pseudosecondaiy inclusions in trains along healed microfractures make petrography difficult. Although type I and II mclusions occur locally in the same inclusion population, nearly ubiquitous necking down of the inclusions, indicated by the presence of liquid-filled inclusions, precludes proof of boiling. The presence of a vapor phase is locally indicated, however, by planes of type II inclusions. Euhedral quartz ciystals of the latter part of the Late Stage as much as 25 cm in length contain very large fluid inclusions as much as 1 cm in largest dimension with vapor bubbles visible to the unaided eye. Fluid inclusions in sphalerite are type I and are commonly shaped as negative crystals as much as 50 ^m in size, but are generally less than 20 Mm in largest dimen sion. The inclusions are typically opaque around the edges, but in many cases the vapor bubbles move when part of the incident light beam is obstructed. Opaque 89 Fig. 42. Transmitted-light photomicrographs of fluid inclusions from the Calera vein sys tem. A. Primary inclusions along a growth zone in quartz of the Bonanza Stage. Sample CCG-14, Calera vein, 3800 level stope. 90 Fig. 42. (Cont.) B. Inclusion in sphalerite from the Bonanza Stage with fine dusting of chalcopyrite disease and some clear areas. Sample CCG-14. 91 Fig. 42. (Cont.) C. Sphalerite of the Manganese stage with coarse chalcopyrite disease, cut by clear sphalerite veinlet with fluid inclusions and finer chalcopyrite disease selvage. Sample CCG-503, Calera vein, 3880 level stope. 92 Fig. 42. (Cont.) D. Banded sphalerite of the Late Stage enclosing solid galena inclusions, some of which have attached primary fluid inclusions (FI). Sample CCG-116, split between the Calera vein and Ramal 1, crosscut 115W. 93 inclusions could be type I or II, but no explicit evidence for type II inclusions was observed. Most of the inclusions appear to be primary, but almost all the sphalerite observed is affected by chalcopyrite disease (Fig. 42B, C), indicating replacement and recrystallization of the sphalerite (Barton and Bethke, 1987; Eldridge et al., 1988). Fluid inclusions are locally attached to chalcopyrite inclusions in sphalerite, a feature used as evidence of primary origin (Roedder, 1984); however, in this case, the inclusions may be primary to recrystallized sphalerite. These inclusions are con ceptually similar to fluid inclusions in recrystallized quartz, termed pseudoprimary by Sander and Black (1988). Only late sphalerite is free of chalcopyrite disease in places and has preserved compositional banding (Fig. 42D). Inclusions are also present in clear sphalerite veinlets with chalcopyrite diseased selvages that cut diseased sphalerite (Fig. 42C). The significance of microthermometry on fluid inclu sions in diseased sphalerite is unclear; however, the fluids trapped should represent hydrothermal fluids responsible for transport of copper and deposition and/or recrys tallization of base-metal sulfides. Usable fluid inclusions in pyrargyrite are uncommon. Primary type I inclu sions 20 fim in largest dimension are spherical or elliptical and are typically nearly opaque because of internal reflection. The few inclusions in which vapor bubbles are visible contain less than 20 volume percent vapor. Other similar inclusions that are completely dark and thus appear to be filled with vapor (type II) may actually be liquid-rich. Other minerals evaluated for possible use in microthermometry include rhodonite, adularia, tetrahedrite, and miargyrite. Few inclusions were observed in rhodonite, partly because of the relatively fine grain size, the high index of refraction, and the morphology of the crystals. Vein adularia contains fluid inclusions but many appear to be on cleavages and liquid filled inclusions are common, indicating neck- 94 mg. Tetrahednte and miargyrite were too opaque to observe fluid inclusions even in very thin sections; however, these minerals would likely yield meaningful data if stud ied with an infrared microscope (Campbell et al., 1984). The results of microthermometry on twelve doubly polished plates are shown in Figures 43 and 44. Data for individual inclusions as well as sample locations are contained in Appendix 1. The majority of homogenization temperatures (TH) and ice melting temperatures (TM) fall in the ranges from 220°C to 300°C and -1.6 to -0.2, respectively, for all the samples studied. The TM values correspond to salinities of less than 3 weight percent NaCl equivalent for all inclusions, using the equation of Hall et al. (1988). A single sample of adularia in altered wall rock yielded consistent homogenization temperatures of about 150-180°C (not shown in Figs. 43, 44). More data is needed to confirm these values, because only a few necked inclusions in vein material homogenized below 200°C. Five samples from the Manganese Stage were studied. A single primary inclusion in rhodonite from substage 1 yielded a TH between 195°C and 200°C, but this inclusion was probably necked with a nearby, vapor-rich inclusion, indicating that the temperature measured is probably low. Inclusions in four samples of sphalerite with chalcopyrite disease from substage 2 were analyzed; ninety percent of all the TH values fall in the range 240-290°C (Fig. 43) with corresponding TM between -1.4 and -0.6°C or salinities of from about 1 to 2.5 weight percent NaCl equivalent (Fig. 44). A fluid inclusion in argentian tetrahedrite from an unknown location at Orcopampa, measured using an infrared microscope by Campbell et al. (1984), yielded a homo genization temperature of 255°C. This sample is most likely from the Manganese Stage and the Th agrees well with the values obtained from sphalerite. Primary^ pseudosecondary, and secondary inclusions in one sample of quartz from the Man- 95 20 - | A 1 0 - 0 T Fig. 43. Histograms of homogenization temperatures for the Calera vein system. A. Manganese Stage; clear - quartz, black - sphalerite with chalcopyrite disease, gray- late clear sphalerite veinlet (see text). B. Quartz Stage. C. Bonanza Stage, clear - early quartz, crosshatched - later quartz, black - sphalerite with chalcopyrite disease, diagonal lines - pyrargynte. D. Late Stage, diagonal- primary inclusions at center of quartz crystal, clear - primary and pseudosecondary inclusions in overgrowth on quartz crystal, crosshatched - quartz with sphalerite, black - primary sphalerite, gray - secondary sphalerite. 96 3 a>cr 0cd £ £ *g ” c d CO Fig. 44. Salinity vs. TH for fluid inclusions from the Calera vein system. Triangles and diamonds indicate data collected from quartz, squares and circles from sphalerite, and plus signs trom pyrargynte. Open symbols indicate primary and pseudosecondary inclusions filled symbols secondary inclusions, and half filled circles inclusions in sphalerite with chalcopyrite disease. A. Manganese Stage; data collected from primary inclusions in a late sphalerite veinlet (Fig. 25C) fall at the high range of salinity. B. Quartz Stage. C. Bonanza Stage; upright and inverted triangles represent inclusions in the early and late growth zones of a single crystal, respectively. D. Late Stage; upright and inverted triangles represent inclusions in the early and late growth zones of a large crystal, respectively. 97 ganese Stage yielded TH values similar to those of the sphalerite (Fig. 43), but with slightly lower salinities of less than 1 weight percent NaCl equivalent (Fig. 44). A single sample of the Quartz Stage was studied. Primary type 1 fluid inclu sions, present m a growth zone in a quartz grain within interlocking, fine-grained quartz, gave homogenization temperatures that ranged from 160°C to 350°C, with most in the range from 270° to 350°C (Fig. 43). The high temperatures are suggestive of boiling, but the only evidence for this is the fine grain size of the quartz. Some of the inclusions with TH above 300°C were necked, however, and the true homogeniza tion temperature is probably 270 to 300°C. Tm was measured between -1.0 and -0.6 C, corresponding to salinities of about 1 to 1.5 weight percent NaCl equivalent (Fig. 44). Inclusions contained in quartz, sphalerite, and pyrargyrite from the Bonanza Stage were measured (Figs. 43 and 44). Most of the data was collected from one sample with a quartz vein containing pyrargyrite + electrum in central cavities that crosscuts material rich in sphalerite with chalcopyrite disease, pyrite, chalcopyrite, and electrum. TH in six inclusions in the sphalerite averaged about 225°C, with cor responding Tm of -1.2 to -0.4. A single euhedral quartz crystal from the quartz vein with primary inclusions on two growth zones yielded TH of 240 ± 20°C and TM -1.2 to -0.6°C for inclusions on the earlier growth zone, and TH of 300 ± 20°C and TM of -0.4 to -0.2 C on the later growth zone (Figs. 43, 44). Two primary inclusions in pyrar gyrite in a central cavity of the quartz vein gave TH of 215-220°C and 242-243°C and Tm of -1.0°C to -0.6°C, respectively. An inclusion in a second pyrargyrite crystal gave a much lower Th of 125°C to 130°C, probably as a result of necking. Sphalerite with chalcopyrite disease from a different sample of Bonanza Stage from Ramal 1 gave Th in the range 280 ± 20°C with TM -0.6°C to -0.4°C. 98 Quartz and sphalerite of the Late Stage were analyzed (Fig. 43, 44). A single large quartz crystal was studied in a section cut normal to the c-axis. Relatively large, irregular primary type I inclusions in the center of the crystal yielded Th of 265°C to 270°C and TM of -1.0°C to -0.8°C. Elongated primary type II inclusions present far ther from the center of the crystal were not measured because they were very dark and contained no visible liquid, indicating that a vapor phase was trapped. A narrow overgrowth on the crystal contained abundant small primary and pseudosecondary Type I inclusions that yielded TH of 250 ± 20°C and TM of -1.6°C to -1.0°C. Petro graphic study of similar quartz crystals from throughout the vein system show similar relationships between the type of fluid inclusion present and the growth zone, but microthemiometric data have not been obtained. Three samples of Late Stage sphalerite were measured. The sphalerite in at least one sample (CCG-116) formed after the large quartz crystals of the Late Stage, upon which they were perched. The other samples were associated with fine grained quartz. All of the samples were mostly free of chalcopyrite disease except along grain boundaries. Most of the primary, pseudosecondaiy, and secondary inclusions in all three samples gave consistent TH of about 290 ± 15°C with TM of -1.0°C to -0.2°C (Figs. 43, 44). Primary inclusions in a small euhedral quartz crystal that grew on one of the sphalerite crystals homogenized in the range between 250°C and 290°C and gave TM of -0.8°C to -0.6°C. Lead Isotopes A lead isotopic study of the Orcopampa district is in progress. The study is unfinished, but preliminary results are presented below. Eleven samples of galena, tetrahedrite, and/or pyrite from the Manganese, Quartz, Bonanza, and Late Stages of the Calera vein system, as well as one sample 99 each from the Tudela, Santiago, and Blancas vein systems (Fig. 6), have been analyzed by R. M. Tosdal in the laboratories of the U.S. Geological Survey in Menlo Park. The lead isotopic ratios of each sample are presented in Table 5. Limited data on volcanic host rocks and Mesozoic basement rocks are available (Tosdal et al., 1992). Feldspar separates of the volcanic and intrusive host rocks, additional sulfide and sulfosalt minerals from the Calera vein system, as well as pyrite and adularia from the altered wall rock are being analyzed at the time of this writing. The lead isotopic ratios for the Calera vein system are slightly variable between stages and plot in a steep trend above the Stacey-Kramer growth curve (Tosdal et al., 1992) (Fig. 45). The more radiogenic end of the trend is formed by the Manganese Stage, whereas the least radiogenic end of the trend is formed by the Bonanza and Quartz Stages. The Late Stage is distributed throughout the trend. No Early Stage or alteration minerals have been analyzed to date. The analytical un certainties associated with the isotopic measurements are 0.1 percent (R. M. Tosdal, written comm., 1991). Although the uncertainties comprise much of the variation of the Pb isotopic data and result in an array with a similar slope, the variations are con sidered to be real. This conclusion is supported by the clustering of the data from individual stages. With the present data, it appears that the lead isotopic ratios of the Calera vein represent a mixing line, with relatively more radiogenic Manganese Stage and less radiogenic Bonanza and Quartz Stages. Lead isotopic ratios of samples of simi lar stages from some of the other veins at Orcopampa are generally not in the same range of values as the corresponding stage in the Calera vein (Table 5), but these samples do plot on the same mixing line (Fig. 45). Lead from the Mesozoic basement rocks and volcanic host rocks (units not known) are more radiogenic than the ore leads (Tosdal et al., 1992)(Fig. 46). The 100 Table 5. Pb isotope data for Orcopampa veins. Sample Mineral Stage 206/204 207/204 208/204 Calera vein CCG-6 tetrahedrite Manganese 18.607 CCG-85 15.645 38.735 tetrahedrite Quartz 18.607 15.636 38.705 CCG-98 galena Late 18.612 CCG-120 15.637 38.702 tetrahedrite Bonanza 18.608 15.635 38.695 CCG-300 tetrahedrite Quartz 18.603 CCG-308 15.632 38.693 galena Late 18.599 15.635 38.694 CCG-316 galena Manganese 18.620 CCG-403 15.651 38.750 galena Manganese 18.617 15.648 38.734 CCG-442 galena Bonanza 18.603 CCG-445 15.626 38.670 galena Late 18.619 15.647 38.740 25571 galena Bonanza 18.612 25571 15.644 38.735 tetrahedrite Bonanza 18.605 15.628 38.671 Tudela vein TCG-1 tetrahedrite Manganese 18.608 15.634 38.691 Santiago vein SAN-3 tetrahedrite Quartz 18.628 15.641 38.734 Blancas 2 veinl B-2 tetrahedrite Manganese!?) 18.599 15.626 38.666 101 15.66 ■ Manganese * Quartz ♦ Bonanza 15.65 - • Late T Other veins 0HJO 15.64 JO CL, 15.63 15.62 38.76 . —- i . i --- '--- r i i----1--- ,------1----1--- ■ Manganese ■ A Quartz 38.74 ♦ Bonanza • • Late ■ ♦ ■ ▼ T Other veins JO CL, 38.72 - - P-, A 38.70 - • - • A ♦ . ▼ - • • - ♦ ♦ T ■ - 38.66 ---1----1----1--- 1----.__ i__ ■ —i__ .__ 1- ■ 15.60 X O. . + X 15.55 • P-, % X + 4• • • Veins • X x Volcanic rocks + Mesozoic rocks 15.50 ------1 39.0 i i i t | i r——i----- 1----- T----- >------'------'------1------T------r - + - - • Veins 38.9 x Volcanic rocks - + Mesozoic rocks x X - Ph X - 38.8 X - X Pi + • X _ A . + + _ 38.7 - i - - 38.6 ------1------*------1------»______1___ — i------1------i______i 18.6 18.8 19.0 19.2 19.4 19.6 206pb/ 204pb Fig. 46. Pb isotope data for veins and wall rocks of the Orcopampa district. 103 volcanic rocks may have been contaminated with lead from the Mesozoic rocks. One sample, however, plots along the trend of the ore leads at more radiogenic values and may represent one of the mixed sources (Fig. 46). The sources of the lead at Orcopampa are unclear, but limited data suggests that the volcanic host rocks may be the more radiogenic source. Further work is necessary to confirm the ranges of lead isotopic values of the stages of the Calera vein and to provide more information on the sources of the lead in the Orcopampa ores. • Relation to Other Veins and District Zoning Although a large amount of microprobe and related mineralogical and chemi cal work has been done on samples from other veins of the Orcopampa district (e.g., Petersen, 1982; Petersen et al., 1990), little detailed paragenetic and petrographic information is available for these veins. The paragenetic sequence of the Manto I' ||| vein, which is currently inaccessible, was described by Kamilli (1974) based on underground observations and hand-specimen study. A reconnaissance study of the parageneses, mineralogy, and vein characteristics of accessible portions of the other veins has been carried out by the present author. Information from the mine staff and study of polished sections were used to supplement field observations. Vein outcrops typically consist of massive iron oxide stained or milky quartz; large quartz crystals, identical to those in the Late Stage of the Calera vein, are common. All of the veins in the district that have been explored underground have paragenetic stages virtually identical to the Manganese Stage of the Calera vein, and stages indistinguishable from one or more of the substages of the Late Stage occur in most of the veins. In addition, a stage very similar to the Quartz Stage occurs in the upper part of the Santiago vein, above Manganese Stage material, as in Calera. The Bonanza Stage was not observed in any other vein, but gold-rich ore that was mined 104 in small amounts in the Tudela "G" vein may be equivalent. Textural and zoning relationships of the other veins are also very similar to those of the Calera vein. However, the Manto vein, unlike Calera, exhibits symmetric crustification of stages with relatively little hydrothermal brecciation (Kamilli, 1974). The elevations of the economic portions of the veins vary between the north ern and central parts of the district. Ore in the Manto vein was mined between 3690 and 4040 meters elevation (the upper elevation is an erosion surface). Ore shoots in the Calera vein occur between 4100 and 3750 meters elevation, the lowest level at the time of writing, but most ore is between 3775 and 4100 meters elevation. Recent exploration indicates that small amounts of ore-grade material locally extend down ward to the 3720 level. Ore in the Santiago and Santa Rosa veins to the south is much higher and occurs between 4080 and 4350 meters and 4030 and 4250 meters, respectively. The elevation differences appear to be real because no large-scale post mineral faulting has been recognized in the district. In all cases to date, ore occurs in the volcanic rocks that overlie Mesozoic sedimentary rocks. This contact is at higher elevation in the central and southern parts of the district because of premineral normal faulting (Fig. 6). An important empirical control on mineralization would, therefore, appear to be the elevation of the contact between the sedimentary and volcanic rocks. The generally poor accessibility and the advanced nature of the exploitation of most of the veins in the Orcopampa district make a district zoning study difficult; however, a few generalizations are possible. The veins in the northern half of the dis trict, Manto, Calera, Tudela, Santa Rosa, Santiago, Rayo and Magaly (Fig. 6), all contain significant percentages of manganese-bearing gangue minerals. Veins in the southern half of the district, however, contain much higher percentages of quartz, apparently in the Manganese Stage equivalent (e.g. Blancas vein, Fig. 6). Also, at least in the Calera vein, the Manganese Stage contains more quartz in the eastern ore shoots than in the main ore shoot. The Calera vein system, and possibly the nearby Tudela vein system, in the center of the productive veins, contain the only recognizable quantities of gold-rich Bonanza Stage material. It would therefore appear that the district is zoned, with gold-rich Bonanza Stage present only in the center and manganese-rich material in the center and surrounding areas, but with lesser percentages of rhodonite and rhodochrosite in the farther reaches. 106 PART TV. DISCUSSION AND CONCLUSIONS Orcopampa District Nature and Timing of Magmatism and Tectonic Activity Adularia-sericite type veins in the Orcopampa district are hosted by early Miocene volcanic and volcaniclastic rocks. The oldest unit, the Santa Rosa volcanics, was deposited and pervasively propylitized within a few million years or so before eruption of the Manto Tuff about 20 Ma ago. The eruption of voluminous interme diate lavas and associated rocks and their alteration shortly before the onset of large- volume calc-alkalic silicic volcanism is reminiscent of the style of Cenozoic volcanism m various parts of the western United States (e.g., Lipman et al., 1970; Steven and Lipman, 1976; Best et al., 1989). The local presence of boulder conglomerate con taining abundant blocks of Mesozoic quartzite and limestone within the Santa Rosa volcanics suggests that early Miocene tectonism (Quechua 1 pulse, Megard et al., 1984; McKee and Noble, 1989) had begun by about 20 Ma (Noble, 1992). Eruption of dacite and andesite of the Sarpane volcanics took place within about 0.5 to 1.5 Ma after eruption of the Manto Tuff. It is unclear to what extent these intermediate rocks are related to the magma system responsible for the Manto Tuff and the Chinchon caldera. Dikes of Sarpane dacite parallel the veins in the district and are spatially asso ciated with the Calera vein and probably the Manto vein. Intermediate lavas of the Sarpane volcanics are cut and offset by various of the Calera and Manto structures. Intermediate volcanism and normal faulting, therefore, appear to have been in part contemporaneous, although movement on the vein-hosting structures continued during vein deposition. 107 The east-northeast to northeast trend of the veins of the Orcopampa district lies at a high angle to the approximately N50°W trend of the Andean chain in this part of southern Peru. The orientation of these normal faults and the associated dikes suggests that, at least locally, extension was nearly parallel to the trend of the belt during Quechua 1 tectonism (McKee and Noble, 1982). Sebrier et al. (1985) have documented similar extensional Neogene stress regimes, albeit with extension normal to the belt, in other parts of the high Andes. Late Miocene tectonic activity has resulted in tilting of the Orcopampa district about 20° to the west. This folding event is bracketed between about 11 and 6 Ma (K. E. Swanson, E. H. McKee, and D. C. Noble, unpub. radiometric ages, 1988-1992; Noble, 1992) and may be related to the Quechua II or III tectonic pulses (Megard et al., 1984; Noble et al., 1990). Nature and Timing of Hydrothermal Activity Evidence for three discrete hydrothermal systems are preserved at Orco pampa. The first, represented by the widespread and pervasive propylitic alteration of the Santa Rosa volcanics, appears to have died out before deposition of the Manto Tuff at about 20 Ma. A similar relationship between stratovolcano formation and alteration prior to caldera formation has been described at Nevado Portugueza in Peru (Noble and McKee, 1982). Quartz-alunite alteration, present along the surface trace of the Santiago Norte vein at Santiago Mayor, formed at about 19.5 to 18 Ma. The radiometric ages suggest a close relationship between the dacites and andesites of the Sarpane vol canics; a similar conclusion was reached for early alunitic alteration in the wall rocks of the Comstock lode (Vikre et al., 1988; Vikre, 1989). The timing is reminiscent of the very short time between magmatic activity and alunite-forming hydrothermal 108 activity at a number of enargite-gold-silver or acid-sulfate type deposits, for example Summitville (Mehnert et al., 1973), Julcani (Noble and Silberman, 1984), Goldfield (Silberman and Ashley, 1970), and Ccarhuaraso (Vidal et al., 1989). The economic veins of the Orcopampa district, however, show all the mineralogical and other features of the adularia-sericite or low-sulfidation (Bonham, 1984; Hayba et al., 1985; Heald et al., 1987; Bonham, 1988; Hedenquist, 1987) type of system (Gibson et al., 1990). Ore deposition and formation of adularia±sericite in surrounding wall rocks appear to have taken place about 1 to 1.5 m.y. after intermediate magmatism and near-surface quartz-alunite alteration. Such an interval between volcanism and adularia-sericite type hydrothermal activity is common, and perhaps characteristic, of adularia-sericite type hydrothermal systems (e.g., Silberman et al., 1972; Bethke et al., 1976; Noble and Silberman, 1984; Heald et al., 1987; Candiotti et al., 1990; McKee et al., 1992). Although the quartz-alunite alteration at Orcopampa was initially interpreted as the near-surface representation of a hydrothermal system of adularia-sericite type (e.g., Vikre, 1985; Rye et al., 1989; 1992; Candiotti et al., 1990), the relatively large difference in their ages indicates that they are probably not directly related. The productive veins occur in the northern half of the Orcopampa district; veins that have been explored in the southern part of the district, e. g. Blancas, are subeconomic. The district is apparently zoned from the productive portions of the veins with ore largely associated with manganese-bearing gangue minerals outward to subeconomic material composed largely of quartz and calcite. Vikre (1989) describes a similar relationship in the Comstock lode and suggests that collapse of the margins of the hydrothermal system and influx of colder meteoric water is the cause of the central position of the richer orebodies. 109 Calera Vein System Nature of the Vein and Ore Shoots The blind ore shoot in the Calera vein was discovered by the application of a model for mineralization that was based on geologic information obtained from the other veins in the district. The most important factors in the discovery were the recognition of the Calera fault during district-scale geological mapping and the per sistence of management during the early stages of exploration by tunnelling and dia mond drilling. The presence of several stages of mineralization, the continuity of the grade, and the width of the vein allowed exploitation by modern trackless mining techniques, thereby decreasing production costs. Without the Bonanza Stage, par ticularly rich ore-mineral bands would probably have been mined selectively, as they were, for example, in the large San Cristobal vein at Cailloma (Stephan, 1974; V. R. Eyzaguirre, H. Candiotti R., D. C. Noble, and P. C. Gibson, unpub. observ.). Conse quently, the tonnage exploited would have been much lower, as would have been the total metal production. The formation of a complex and diverse sequence of paragenetic stages in the Calera fissure vein system involved repeated alteration and interplay of mineral deposition, fracturing, and hydrothermal brecciation. The variations in the nature and relative abundance of mineral phases suggest an overall history of fluid evolution punctuated by abrupt shifts in the solution composition. In the Manganese Stage, abundant early rhodonite is replaced first by rhodochrosite and eventually by quartz as the dominant gangue mineral to be deposited. This trend was interrupted repeat edly by the deposition of relatively thin bands composed almost entirely of sulfide and/or sulfosalt minerals, some of which have high precious metal and tellurium con tents. These bands reflect profound and recurring changes in solution chemistry; 110 similar conclusions have been reached by workers studying other systems, for exam ple, Tayoltita (Clarke and Titley, 1988). The stages deposited subsequently contain quartz as the dominant gangue mineral, but are otherwise distinct. The Quartz Stage contains bands and stringers of argentian tetrahednte with pyrite and chalcopyrite but very little galena or sphalerite. The Bonanza Stage, the most striking and economically important, contains large amounts of sphalerite and galena in addition to argentian tetrahedrite, several silver sulfosalts, and electrum. The Late Stage is characterized by sphalerite, galena, stib- nite, and marcasite with only a minor amount of tetrahedrite. The wall rocks are potassium metasomatized and contain abundant adularia, sericite (illite and muscovite), and pyrite. Three types of alteration are associated with the vein mineralization (Fig. 47). Quartz-adularia-pyrite alteration occurs at depth near the vein margins. A wide zone of illitic alteration occurs adjacent to the vein in upper levels and lateral to adularia-rich alteration at depth. A narrow zone of illite-kaolinite alteration that locally occurs near the vein at depth may overprint adularia alteration. Propylitic alteration generally is found outward of illitic alter ation, but occurs adjacent to the structure where little or no vein filling has occurred. The zoning of alteration assemblages in the Calera vein system resembles that of the model for epithermal mineralization of Buchanan (1981). The form of the ore shoot, qualitative zoning patterns, and the grade distri bution suggest a diagonally upward flow of hydrothermal fluids during deposition of the main ore shoot. Steeply rising hydrothermal fluids were redirected and focussed by structural features, particularly the junctions of splits with the Calera vein. Although the style of fluid movement was intermediate between the dominantly upward flow characteristic of many epithermal vein systems (e.g., Loucks and Petersen, 1988; Petersen et al., 1990) and the subhorizontal flow demonstrated at / Q a / - 4 0 5 0 m 50 meters . / Fig. 47. Cross section of the Calera vein system showing generalized distribution of alter ation assemblages. Refer to Fig. 10 for wall rock lithologies. 112 Creede (Steven and Eaton, 1975; Barton et al., 1977; Bethke, 1988; Plumlee, 1989), Julcani (Herminia subdistrict, Petersen et al, 1977), and Tayoltita (Clarke and Titley, 1988), the lateral component was imparted by structural control and not by hydrologic control. If post-mineral tilting of the district 10-20° to the west inferred by Noble (1992) is correct, the fluid flow in the main ore shoot of the Calera vein would have been subhorizontal. Economic values in the Bonanza Stage are not vertically restricted above a well-defined bottom as are similar exceptionally rich stages in some veins, such as the Finlandia vein at Colqui, Peru (Kamilli and Ohmoto, 1977). Rather, high-grade ore of the Bonanza Stage in the Calera vein extends as a narrow shoot below the eco nomic portions of the other stages (Fig. 31B). Although the richest and most abun dant part of the Bonanza Stage occurred between the 3950 and 4015 levels, the stage extends downward, albeit with decreasing abundance, to at least the 3750 level, and probably to the 3720 level, where the Manganese Stage is waste. Also, the richest part of the Bonanza Stage in the split Ramal 1 occurs about 100 meters below the richest part in the Calera vein. Boiling has been inferred to have caused mineral deposition of rich stages in some deposits (e.g, Finlandia, Kamilli and Ohmoto, 1977), but the Bonanza Stage exhibits no explicit evidence for boiling, and ore depo sition may have been, at least in part, the result of fluid mixing. Exploration to the east of the main ore shoot resulted in the development of relatively little ore (Fig. 5). Metal ratio data based on chemical zoning in tetra- hedrite (Wu and Petersen, 1977; Hackbarth and Petersen, 1984), shown to be useful in delineating convolute ore bands that encompass ore shoots in the previously ex ploited veins at Orcopampa (Petersen, 1982; Petersen et al, 1990; Petersen and McMillan, 1992), are not available for the Calera vein system. Therefore, the form of the grade contours for silver and gold (Fig. 32) and the observed distributions of 113 the Bonanza Stage (Fig. 3 IB) have been used to guide recent exploration to the west of the main ore shoot (Gibson, 1992). A diamond drill hole that explored the Calera vein at the 3720 level below the western end of the 3800 level (Fig. 5H) encountered ore-grade mineralization, and rich silver ore has been developed on the 3800 level of the Calera vein about 350 meters west of the main ore shoot (D. C. Noble, oral comm., 1992). Nature of the Hydrothermal Fluids Fluid inclusion data indicate that the temperatures of the hydrothermal fluids that deposited the vein minerals fluctuated between about 240°C and 300°C during mineral deposition (Fig. 48). The stated temperatures are uncorrected Th values, because the temperature correction for pressure is very small at the depths indicated below (Haas, 1971; Potter, 1977) and the fluids were probably at or near the boiling curve most of the time. Homogenization temperatures of inclusions in two growth bands of a single quartz crystal from the Bonanza Stage, however, span nearly the entire range of measured values. The temperatures that obtained during early wall rock alteration may have been lower (Fig. 48), at least locally, but more information is needed to confirm this. Relatively high temperatures obtained through the latest stages of mineral deposition, in contrast to some epithermal systems, such as Finlan dia (Kamilli and Ohmoto, 1977) and Sunnyside (Casadevall and Ohmoto, 1977), where a decrease in homogenization temperatures occurred with time. Inclusions in sphalerite and quartz from the latest substage measured (substage 3 of the Late Stage, Fig. 14) homogenized between 270°C and 300°C. That such high temperature fluids deposited barren quartz after most of the ore stages would seem to indicate that the heat source that drove the hydrothermal system was long-lived or was reju venated, for example by continued magmatic activity. A similar theory was proposed 114 Stage Fig. 48. Summary of fluid inclusion homogenization temperatures for the Calera vein system. Squares represent the arithmetic average of homogenization temperatures. Error bars represent the range of measured homogenization temperatures. Dashed lines indicate the range of homogenization temperatures believed to be representative of the particular hydrothermal fluid. r to explain high homogenization temperatures in barren quartz that formed after ore stages at Huachocolpa, Peru (Bruha, 1983). The measured salinities of all inclusions in quartz, sphalerite, and pyrargyrite are below 3 weight percent NaCl equivalent (Fig. 44). Salinities of inclusions in sphalerite are equivalent to or less than those in quartz except in the Manganese Stage, where they are slightly higher (Fig. 44). Most fluid inclusion studies on relatively sulfide-nch adulana-sericite systems indicate that fluids that deposited sphalerite had relatively high salinities of about 3-5 weight percent NaCl equivalent or more (e.g. Kamilli and Ohmoto, 1977; Woods et al, 1982; Loucks et al., 1988; Simmons et al., 1988; Vikre, 1989). Exceptions include Lake City I ore that contains inclusions in some sphalerite with salinities less than 3 weight percent NaCl equiva lent (Slack, 1980), the Rayas mine at Guanajuato with less than 3 weight percent NaCl equivalent (Mango et al., 1991), Areata, Peru, with less than about 3.5 weight percent NaCl equivalent (Candiotti et al., 1990), and base-metal ore at Yatani, Japan that averages 0.6 weight percent NaCl equivalent (Hattori, 1975; Hedenquist and Henley, 1985). Also, many studies indicate similar, relatively high salinities for fluids that deposited both quartz and coexisting sphalerite (e.g. Woods et al., 1982; Vikre, 1989). The significance of microthermometric data on inclusions in sphalerite from Calera is unclear due to ubiquitous chalcopyrite disease. A study by Pisutha-Amond and Ohmoto (1983) on a Kuroko-type massive sulfide deposit showed that fluids responsible for chalcopyrite disease were higher temperature but of comparable salinity to the fluids that deposited the sphalerite; however, the authors did not explicitly indicate whether data was collected on fluid inclusions in diseased sphalerite. A subsequent study by Foley (1986) on diseased sphalerite from a Kuroko-type ore body supported the results of the earlier investigation. In the 1 1 6 Calera vein, the formation of chalcopyrite disease could have been a result of a reac tion between sphalerite and chalcopyrite during the relatively high temperatures that were sustained at various times throughout mineral deposition. Figure 49 shows ranges of homogenization temperature and salinities for selected high-base metal adularia-sericite type vein deposits, as well as Julcani, an acid-sulfate type vein system. The Calera vein system is readily seen to be part of a group of deposits with low salinity fluids. Creede and Finlandia have two distinct fluids, one with relatively high salinities and a second with low salinities. Other deposits, such as the Comstock lode and Santo Nino, span the region between high and low salinities. Geological relationships and fluid inclusion studies indicate that the depth of mineralization was in the range of 500 to 1000m. Although no evidence for C02 in the fluid inclusions was observed, small quantities that are undetectable by the means available are significant for pressure estimates (Takenouchi and Kennedy, 1964; Hedenquist and Henley, 1985; Bodnar et ah, 1985). The presence of large quantities of rhodonite in the vein systems at Orcopampa, however, indicate that the hydrothermal fluids that deposited the Manganese Stage had a very low /co2, on the order of 2.6, or rhodochrosite would have been deposited (Fig. 50)(Candia et ah, 1975; Abrecht, 1988). A fluid with a salinity of 2 weight percent NaCl equivalent and PC02 °f 2.5 bars would boil at about 500 meters under hydrostatic conditions (Haas, 1971) or about 445 meters under hydrodynamic conditions that exceed hydrostatic pressure by 10 percent because of fluid flow (Hedenquist and Henley, 1985). Repeated episodes of explosive boiling greatly disrupted the materials deposited by the hydrothermal fluids, but appear to have had little or no influence on 1 Pyroxmangite should actually be the stable phase under the p-T conditions given (Maresch and Mottana, 1976). Metastability or ionic substitutions may explain the presence of rhodonite. Fig. 49. Ranges of salinities and Th for selected epithermal deposits. The stippled area is the range of values considered to represent the fluids that deposited the Calera vein. Data from Casadevall and Ohmoto (1977); Kamilli and Ohmoto (1977), Woods et al. (1982V Loucks (1984) Clark and Titley (1988); Simmons et al. (1988); Vikre (1989); Candiotti et al.’ (1990); Deen (1990); and Mango et al. (1991). 118 ore deposition other than providing open space. Although fluid inclusions in quartz are locally indicative of boiling, no fluid-inclusion evidence for boiling has been encountered in the ore-mineral bands, and features that are commonly cited to be indicative of boiling, such as fine mineral grain size and a vertically restricted ore horizon with a flat base do not occur. Also, abundant kaolinite in the upper level of the eastern portion of the vein system, a feature that could be cited as evidence of boiling, is probably older than the economic mineralization. Fluid inclusions with TH above 300°C in the Quartz Stage may be a result of boiling, but these could also have resulted from necking down of some inclusions. The mineralogy of the diverse stages provides some constraints on the chem istry of the fluids that deposited them, assuming equilibrium conditions obtained. For gangue mineral bands in the Manganese Stage, the presence of abundant rhodonite in some ore bands suggests a lower limit o n /o 2 of IO-35 (Fig. 50), because no alabandite is present. As stated previously, the presence of rhodonite also con strains the/co 2 at less than about 10&« (Fig. 50). In general, rhodochrosite is more common in epithermal vein systems with abundant manganese-bearing gangue min erals than are manganese silicates or manganese sulfide. Other similar epithermal vein deposits described as having a significant percentage of manganese silicates are the Sunnyside mine, Colorado (Casadevall and Ohmoto, 1977; Heald et al., 1987), Tayoltita, Mexico (Smith et al., 1982; Clarke and Titley, 1988), and Areata and Cailloma, Peru (Candiotti et al., 1990; Stephan, 1974), suggesting that hydrothermal fluids with the low /co2 required for stability of manganese silicates are not common. The abundance of adularia and sericite in the wall rocks and early vein materials suggests that the pH of the hydrothermal fluids was about 5 to 6 (Fig. 51). The lack of pyrrhotite, enargite or famatinite (stibioenargite), hematite, mag netite, chlorite, alabandite and other phases such as bomite and covellite constrain 119 Fig. 50 Log f c o 2 - f 0 2 diagram showing stability fields of manganese minerals and pyrite-pyrrhotite at 250°C with anfS2 = 1CYn. The inferred conditions of formation of the Manganese Stage are shown. Thermodynamic data from Robie et al. (1978) and Barton and Skinner (1979). ' 120 r J ' 1' 51; a° 2 \,pH dia*ram showing stability fields of minerals important in the sprite”pnaiente al SS 250 £ C, 2.S - 0.02 molal,sMur and 1 molal salinity with“ ° Na/K“ ™ of= 9. ir™ The “ “stinnled “ * of d ^ t ^ ,H dlC | the aPProximate conditions of formation of most of the Calera vein the dark shading mcheates the approximate conditions of formation of the ore mineral band in S S ‘he 121 the fs 2 and f o 2 of the hydrothermal solutions that deposited the ore mineral bands between 1(H* to 10-9 and 10® to 10* at 250°C, respectively (Fig. 52). The presence of bomite + pyrite, digenite + covellite, and enargite in at least one ore mineral band from the Laura vein, however, indicates that/S2 locally increased to about lO-7 (Fig. 52). A Cu-Fe-Zn-Sn sulfide, provisionally identified as stannoidite, also present m the Laura vein assemblage, is apparently not consistent with this high sulfur fugacity at temperatures in the range from 200-350°C (Lee et al, 1975). However, the phase may actually be mawsonite, which is stable at higher sulfur fugacities (Lee et al., 1975). Zoning of silver in electrum grains in the Bonanza Stage provides possible evidence for fluctuating fs 2, but no quantitative data on electrum compositions are available. The Fe content of sphalerite is unknown. Determinations by Marcoux and Milesi (1990) indicate that the Fe content averages 0.2 to 0.3 mole % FeS. They do not state, however, whether data were obtained on sphalerite free of chalcopyrite disease or on replaced sphalerite, either between chalcopyrite inclusions or including chalcopyrite inclusions. Barton and Bethke (1987) and Eldridge et al. (1988) have shown that the Fe content of sphalerite is lowered during formation of chalcopyrite disease, but that the overall Fe content and gross compositional banding in the composite sphalerite-chalcopyrite grains are preserved. The presence of pervasive chalcopyrite disease with no preserved compositional banding at Calera seems to indicate that the original Fe content was relatively constant and may have been moderate, indicating moderate fs 2 and f 0 2 in the ranges inferred from the phase assemblages (see above). The presence of telluride minerals such as hessite, altaite, coloradoite, sylvan- ite, and calaverite in certain of the ore-mineral bands indicates relatively high/Te^s^ with /Te2 on the order of 1017 to 109, for the hydrothermal fluids that deposited the 122 Fig 52. Log aS2 - a02 diagram at 250°C, 1 molal salinity with Na/K = 9 and auartz siTem anHrOUSfhOUt’ f ° mng the stability fields of minerals important in the Calera vein system and contoum of iron content of sphalerite. The conditions of formation of the Calera itS^rat^K 1Cfi!ed^byith uSti? pled pattern’ the conditions of formation of the Laura vein are icated by the dark shading, and the conditions of formation of alunitic alteration are indicated by the diagonal lines. After Heald et al. (1987). alteration are 123 tellunde-beanng ores of the Manganese and Bonanza Stages (Fig. 53). T h e / T e ^ apparently increased as sulfide and sulfosalt minerals were deposited, resulting in deposition of tellurides late in any given ore band, a feature that has been reported in several epithermal Au-Ag deposits, e.g. Mahd Adh Dhahab, Saudi Arabia (Afifi et al., 1984), Golden Sunlight, Montana (Porter and Ripley, 1985), Tongyoung, Korea (Shelton et al., 1990), and possibly the Sunnyside mine, Colorado (Casadevall and Ohmoto, 1977). Sources of the Chemical Constituents Stable isotopic studies of many adularia-sericite type epithermal veins indicate that large volumes of shifted meteoric water, connate water, or a combination of the two were involved in their formation (e.g. Creede, Bethke and Rye, 1979; Foley et al., 1989; Finlandia, Pern, Kamilli and Ohmoto, 1977; Sunnyside mine, Casadevall and Ohmoto, 1977; and National, Nevada, Vikre, 1987). The presence of magmatic waters, however, is indicated for some deposits (e. g., the Comstock lode; Vikre, 1989; and Creede, Wetlaufer, 1977) and a small magmatic component cannot be ruled out for others (Hayba et al., 1985). Diverse sources for carbon, sulfur, and lead (and presumably other metals), however, are proposed for many deposits (e.g. Norman and Rye, 1983; Hayba et al., 1985). It is clear that the fluids that deposited the bulk of adularia sericite veins are not, in general, the same fluids that deposited the bands of economic minerals within the veins. The high sulfidation assemblage present in the Manganese Stage of the Laura vein suggests that some of the hydrothermal fluids were similar to those that form similar assemblages in acid-sulfate (high-sulfidation or enargite Au-Ag) type deposits. The close association of acid-sulfate deposits with igneous rocks and stable isotopic studies indicate that these deposits typically have a large component of 124 , ^ 3; L° g /I’e2 -fS2 diagram showing stability fields of teUurides and sulfides significant tor the Calera vein system, at 250°C. The stippled patterns show the conditions of formation °t ore mineral bands in the Calera vein system as inferred from mineralogy. Data from Afifi et al. (1988a), Ahmad et al. (1987), and Barton and Skinner (1979). 125 magmatic fluid (Hayba et al., 1985; Deen et al., 1987; Deen, 1990). By analogy, ore- bands within the Manganese Stage may have been deposited from fluids having a significant magmatic component that have been diluted by mixing with circulating meteoric water. The presence of tin in the Laura vein also is suggestive of a mag matic component, because tin is commonly associated with silicic magmas and related mineral deposits (Kelly and Tumeaure, 1970; Sillitoe et al., 1975; Lehmann, 1982; Duffield et al., 1990; Heinrich, 1990), and stannoidite and mawsonite have been reported in vem deposits associated with granitic intrusives in Japan (Imai et al., 1975). The source of the manganese in the vein minerals is uncertain. Abundant manganese-bearing gangue is characteristic of certain relatively silver and base-metal rich adularia sericite type systems (Heald et al., 1987), and seems to be particularly common in the silver-rich epithermal systems in Peru. Study of oxygen, hydrogen and carbon isotopes indicate that pre-ore rhodochrosite at Creede was formed from deep seated hydrothermal fluids (Wetlaufer, 1977; Bethke and Rye, 1979), possibly indicating a magmatic source for the manganese. Oxygen and hydrogen isotopic data from late rhodochrosite at the Sunnyside mine indicate a meteoric water source, but carbon isotopic data may be compatible with deep seated fluids (CasadevaU and Ohmoto, 1977). Elevated concentrations are also present at Purisima-Concepcion, Peru (Alvarez and Noble, 1988) and Star Pointer, Nevada (mine staff, oral comm., 1992), deposits that have a strong magmatic affiliation. Tellurium is commonly thought to be indicative of a magmatic fluid compo nent in hydrothermal systems (Afifi et al., 1988b), and some telluride-bearing epithermal deposits have been documented as having formed largely or partly from magmatic fluids (e.g. Golden Sunlight, Porter and Ripley, 1985; Emperor, Fiji, Ahmad et al., 1987). Other epithermal deposits that contain tellurides, however, 126 have been documented as having formed from dominantly meteoric waters, such as the Sunnyside mine, Colorado (Casadevall and Ohmoto, 1977) and Tongyoung, Korea (Shelton et al., 1990). Isotopic studies on these deposits, however, have been dominantly performed on fluids and minerals (e. g. quartz) deposited by fluids are potentially different from those that deposited the precious metals. Afifi et al. (1988b) hypothesize that small components of Te-rich magmatic fluid would not be detected during isotopic studies. Shelton et al. (1990) do not rule out a magmatic source for the tellurium, and, indeed, postulate a magmatic source for the sulfur. The lead isotopic data obtained on Orcopampa sulfides show little variation. When plotted on 2W b vs 208Pb/204Pb and 207Pb/204Pb diagrams? ^ data form a steep linear array (Fig. 45), indicating mixing of lead from two sources. For the Calera vein, Manganese Stage leads form the more radiogenic end of the array, whereas Bonanza Stage leads form the least radiogenic part of the array (Fig. 45). A detailed lead isotopic study of the Creede mining district showed that paragenetically early, gold-rich stages also had a slightly less radiogenic lead isotopic signature than later silver- and base-metal-rich stages (Foley and Ayuso, 1991). The sulfide lead values from Orcopampa are less radiogenic than those obtained from whole-rock samples of the pyroclastic host rocks of the district, which have probably been contaminated by the Mesozoic sedimentary rocks that underlie the region (Fig. 54)(Tosdal et al., 1992). One sample, however, plots along the same trend as the sulfides but at more radiogenic values and may represent one of the end members (Tosdal et al., 1992). The source of the other end member composition, represented by the less radiogenic end of the mixing line formed by sulfides from the Bonanza and Quartz Stages, is unknown. Several more sulfide samples from the Calera vein, including alteration pyrite and adularia, as well as feldspar separates of a tuff in the Santa Rosa volcanics, the Manto Tuff, and a dome of the Sarpane vol- 127 A B Fig. 54. Pb isotopic data from Orcopampa. Modified from Tosdal et al. (1992) 128 camcs are currently being analyzed. The data currently available, however, suggest that the metals in the veins in the Orcopampa district originated from a magmatic source similar to the coastal batholith and/or from volcanic rocks; the small varia tions in isotopic ratios preclude contamination from Mesozoic sedimentary basement rocks with high isotopic ratios (Fig. 54)(Tosdal et al„ 1992) or metamorphic rocks with low isotopic ratios (MacFarlane et al„ 1990) that may occur at depth. Lead iso topic studies of adularia-sericite type systems in other areas commonly indicate that the lead originated from the volcanic host rocks, but with a component derived from Precambrian or Phanerozoic basement rocks (e.g. San Juan volcanic field, Doe et al., 1979; Sanford, 1992). MacFarlane et al. (1990) defined several lead isotopic provinces in the Cen tral Andes with characteristic lead isotopic ratios of ore deposits and crustal rocks. The provinces are generally parallel to the Andean Cordillera (Fig. 55), reflecting the age and geology of the underlying rocks in a manner analogous to the lead isotopic provinces of the western United States (Zartman, 1974). Province I, the coastal region and western part of the Andes occupied by the Jurassic to early Tertiary coastal batholith and associated volcanic rocks, has isotopic ratios of 206Pb/204Pb = 18.21 to 18.82, ^ P b /^ P b = 15.55 to 15.69, and 208pb/204pb - 28.11 to 38.95. Province II, the high cordillera with Tertiary volcanic rocks underlain largely by Mesozoic miogeoclinal rocks, has isotopic ratios of 206Pb/204Pb = 18.76 to 18.90, 2°7Pb/204pb = 15.62 to 15.73, and 2««PbPPb = 38.63 to 39.16. Province III, the East ern Cordillera of southern Peru and northern Bolivia underlain by Paleozoic clastic rocks, has isotopic ratios of 206Pb/2°4pb = 17.97 to 25.18, ^ P b /^ P b = 15.51 to 16.00, and 208Pb/204Pb = 37.71 to 40.07. Provinces I and III are further divided into three and two subprovinces, respectively (Fig. 55). MacFarlane et al. (1990) place Orco pampa within province II (Fig. 55), but the Pb isotopic ratios are less radiogenic than Fig. 55. Pb isotopic provinces of the central Andes. After MacFarlane et al. (1990) 130 their stated range for province II values. The Orcopampa leads are in better agree ment with some province I values because the Mesozoic sedimentary rocks evidently contributed little or no lead to the mineralizing solutions. Conclusions The present study allows the following conclusions: 1. Mineralization in the Orcopampa district occurred at about 17.7 Ma, and post-dated intrusion of the youngest dated exposed premineral igneous rocks by about 1 to 1.5 Ma. Alunitic alteration in the upper part of the eastern Santiago vein system was evidently related to intrusives of the Sarpane volcanics and was not high- level alteration contemporaneous with ore formation and adularia-sericite alteration. 2. Orcopampa is a single, multistage hydrothermal system, as shown by the recognition of certain of the paragenetic stages in various of the producing, for merly producing, and explored vein systems. The centrally located Calera vein system is particularly complex in that it contains all of the paragenetic stages recog nized in the district, contains abundant hydrothermal breccias, and is not symmetrical banded. 3. The epithermal veins at Orcopampa are of the relatively base-metal rich adularia-sericite or low-sulfidation type (Hayba et al., 1985; Heald et al., 1987, Hedenquist, 1987). Characteristics that support this conclusion include abundant adularia and sericite alteration, abundant manganese-bearing gangue minerals, typi cal ore mineralogy, low bismuth content, and a time gap between volcanism and mineral deposition. Local deviations from the adularia sericite "norm" occur, how ever, in the Calera vein system; these include the presence of high-sulfidation, low pH mineral assemblages in the Manganese Stage of the Laura vein, a large percent- 131 age of manganese silicate gangue, and, perhaps, the presence of telluride minerals in some ore bands. 4. Metal-ratio studies indicate that the hydrothermal fluids at Orco- pampa generally rose steeply along normal faults in zones of high permeability (Petersen, 1982; Petersen et al., 1990). Fluid flow in the Calera vein system, how ever, was controlled by the junctions of splits; initially steeply rising fluid were diverted along a relatively shallow-plunging zone of high transmissivity as indicated by the distribution of paragenetic stages and grade isopleths. 5. Although metal ratios and grade contours may be used to guide explo ration in certain of the veins at Orcopampa and similar epithermal vein systems, the complex paragenetic sequence of the Calera vein makes the utilization of metal ratios and grade contours based on bulk assay data tenuous. 6. The paragenetic sequence of the Calera vein reflects evolution of the hydrothermal fluids from relatively manganese-rich to manganese-poor composi tions, punctuated by the repeated influx of several different types of base and precious metal-rich solutions. The mineral bands within the Manganese Stage that are rich in base metals, silver and/or tellurium were the product of volumetrically minor and temporally restricted pulses of metal-rich fluids. Short-term evolution of pulses of hydrothermal fluid during deposition of the ore-mineral bands is suggested by the similarity in paragenetic sequence and by the increase \nfTc2JfS2 through time in some bands. 7. The Bonanza Stage, the economically most important because it con tains most of the gold in the vein system, represents a fundamental change in the chemistry of the hydrothermal solutions; the gold/silver ratio is an order of magni tude or more higher than that for other stages and silver sulfosalts and electrum are abundant. 132 8. The Calera vein system formed within the temperature range of 240° to 300°C. Salinities of the hydrothermal fluids were evidently less than three weight percent NaCl equivalent throughout mineral deposition, but the significance of fluid inclusion salinity data obtained on sphalerite that has been affected by chalcopyrite disease is uncertain. 9. Boiling of the hydrothermal fluids, reported to have aided mineral deposition in some epithermal veins and districts (e.g, Finlandia, Peru, Kamilli and Ohmoto, 1977; Guanajuato, Mexico, Buchanan, 1981), was evidently not an impor tant mechanism for ore deposition. Conclusive evidence for boiling was observed in fluid inclusions in quartz, however, and episodic explosive boiling clearly provided abundant open space for mineral deposition. 10. The small variations of lead isotopic composition and the steep array defined by the data suggest that metals were derived from two sources with well defined and limited isotopic compositions. The source of the lead, and presumably other metals, in the Orcopampa district, was apparently crystallizing magmas and/or igneous host rocks. Hydrothermal fluids did not leach significant amounts of lead from Mesozoic sedimentary rocks or Precambrian metamorphic rocks. 11. The high-sulfidation mineral assemblages bomite + pyrite + chalco pyrite + enargite and covellite + digenite that occur in Manganese Stage material in the Laura vein suggest that the hydrothermal fluids locally had a high sulfur fugacity typical of enargite-gold-silver deposits, a feature that has not been reported from other adularia-sericite type systems. Such fluids, found at Julcani to be dominantly of magmatic origin (Deen, 1990), could be the source of the metals in the ore mineral bands. 12. The source of the hydrothermal fluids that produced the economic mineralization and the spatially and temporally related potassium metasomatism is 133 unclear. Most studies of adularia-sericite systems indicate that the hydrothermal fluids responsible for mineralization are largely meteoric and/or connate in origin (White, 1981). Although the same may be true for the Calera and similar vein sys tems, it is clear that the fluids that deposited the bulk of the vein material were greatly different than those that deposited the various types of bands of ore minerals. The close spatial association of slightly older dacite dikes and flows, the presence of significant amounts of Te in precious-metal rich bands within the Manganese, Quartz and Bonanza Stages in conjunction with the high f i t # s2 ratios implied by some of the observed phase assemblages (Afifi et al., 1988a), the presence of high sulfidation mineral assemblages in the Manganese Stage, and the apparent lack of contamina tion of lead by basement rocks are consistent with the involvement of hydrothermal fluids from a magmatic source. Also, homogenization temperatures of about 295°C recorded in late sphalerite indicate that temperatures remained relatively high throughout the evolution of the hydrothermal system, possibly indicating the pres ence of repeated intrusions of magma. A model similar to that of Simmons et al. (1988), in which metal-rich fluids with an important magmatic component are episodically injected into a dominantly meteoric convective system, is plausible. In this model, the fluids that deposited the ore may have been related to late intrusives of the Sarpane volcanics or to a younger magmatic system for which there is no sur face expression. Mineral deposition could have been brought about by a combina tion of factors, such as cooling, and/or mixing and dilution of hydrothermal fluids. 13. The presence of the gold-rich Bonanza Stage almost exclusively in the Calera and possibly the Tudela vein systems remains unexplained. 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Geol., v. 75, p. 963-991. Smith, D M., Albinson, T and Sawkins, F. J., 1982, Geologic and fluid inclusion Geok v°77 pS 1120-1145 S1 VCr'g°ld vein deposit> Durango, Mexico: Econ. Stephan, S., 1974 Genese der subvulkanischen Silbererzgange von Caylloma in Peru Geol JahrdbrD v 8 i ^ p 6*6 hedhlgten bergbaubchen Nutzungsmerkmale: Steven, T. A., and Eaton, G. P., 1975, Environment of ore deposition in the Creede mining district, San Juan Mountains, Colorado: I. Geologic, hydrologic and geophysical setting: Econ. Geol., v. 70, p. 1023-1037. 6 ’ 143 Steven T A., and Lipman P. W , 1976, Calderas of the San Juan volcanic field southwestern Colorado: U. S. Geol. Survey Prof. Paper 958, 35 p ’ m en o u c h i S and Kennedy, G. C., 1964, The binary system H20-C 0 2 at high temperatures and pressures: Am. Jour. Sci., v. 262, p. 1055-1074. S Tosdal, R. M Bouse R. M Gibson, P. C., Wallace, A. R„ Moscoso D., R., Cuitino Ti™ ,v^Crn, M^ ksTa?v> v -> Vilca N., G, Quispesivana Q., L., Tejada G. R. Lf eca’ J‘ L>’ Murillo> F-> Hardyman, R. F., and Koeppen, R. AnH^9M f ^ d D0trP1C C01K s?tlons as tracers in ore deposits in the central Andes of Peru, Bolivia, and Chile: Symposium on Ore Forming Processes of Precious Metal Deposits, Proceedings V&ume, in press. 8 rr0C6SSes 0t Tudela M1918 Minas de Orcopampa: Unpub. rept. in the files of Cia. de Minas Buenaventura S.A., Lima, 11 p. (transcribed from the handwritten original). Ventur^li, G^ Fragipane, M., Weibel, M and Antiga, D., 1978, Trace element dis- ^ a ^ tiie Camozoic.lavas of Nevado Coropuna and Andagua valley, central Andes of southern Peru: Bull. Volcanol., v. 41-1, p. 213-226. ^ Vidal, C E., Noble, D. C McKee, E. H Benavides, J. E., and Candiotti de los Rios, H., 1989, Hydrothermally altered and mineralized late Pliocene-Quatemarv central volcanoes m the Andes of southern Peru [extended abs.l: 28th Intemat. Geol. Cong., Abstracts, v. 3, p. 3-297. 1 Vikre, P. G 1985, Precious metal vein systems in the National district, Humboldt county, Nevada: Econ. Geol., v. 80, p. 360-393. Vikre, P. G., 1987, Paleohydrology of Buckskin Mountain, National district, Humboldt county, Nevada: Econ. Geol., v. 82, p. 934-950. Vikre, P. G., 1989, Fluid-mineral relations in the Comstock lode: Econ. Geol., v 84 p. 1574-1613. ’ Vikre, P G. McKee, E. H., and Silberman, M. L., 1988, Chronology of Miocene hydrothennal activity and igneous events in the western Virginia range Washoe, Storey, and Lyon counties, Nevada: Econ. Geol., v. 83, p. 864-874 ’ Weibe\M -, Fragipane-Geysel, M, and Hunziker, J., 1978, Ein Beitrag zur Vulkanologie SUd-Perus: Geol. Rundschau, v. 67, p. 243-252. Wetlaufer, P. H., 1977, Geochemistry and mineralogy of the carbonates of the Ereede mming district, Colorado: U. S. Geol. Survey Open-File Report 77- White, D. E., 1981, Active geothermal systems and hydrothermal ore deposits- Econ Geol., 75th Anniv. Vol., p. 392-423. Woods, T. L., Roedder, E., and Bethke, P. M., 1982, Fluid-inclusion data on samples from Creede, Colorado, in relation to mineral paragenesis: U S Geol Survey Open-File Rept. 82-313, 77 p. 144 Wu’ L’r ^ S r i terSSn’ U-’ 1977’ Geochemistry of tetrahedrite and mineral zoning at Casapalca, Peru: Econ. Geol., v. 72, p. 993-1016. g Zartman R E 1974, Lead isotopic provinces in the cordillera of the western United States and then- geologic significance: Econ. Geol., v. 69., p. 792-805. 145 APPENDIX 1 Fluid Inclusion Methodology and Data Samples for fluid inclusion study (Fig. 56) were prepared as thin plates, gen erally about 0.1 mm thick, polished on both sides and mounted on standard thin sec tion glass with "super" glue soluble in acetone. The polishing procedure was carried out in a manner that minimizes excessive heating of the sample. Petrography in transmitted and reflected light was carried out to find areas with useable fluid inclusions and to determine the textural relationships and para- genetic sequence of the minerals. Areas with useable fluid inclusions were mapped out and numbered for future reference. After detailed optical study was completed and photographs taken, the areas selected for microthermometry were cut out of the plates using a thin diamond saw blade. The sample was subsequently removed from the glass plate by dissolving the adhesive in acetone. Microthermometric data (Table 6) were obtained on a modified gas-flow heating-freezing stage manufactured by Fluid Inc., Denver, CO. Freezing runs were generally performed prior to heating. The temperature at which melting com menced, the eutectic temperature, TE, was generally not visible due to the small size of the inclusions, although the temperature at which a melt was first visible was noted in some cases. Ice was typically not visible in the inclusions. Measurements of TM, the temperature at which the last ice crystal melted, were bracketed in 0.2 °C incre ments, which is the resolution of the thermometer, by observing the vapor bubble during heating and re-cooling; the temperature at which the vapor bubble jerked and no longer moved upon re-cooling was taken as TM. Measurements of the homo genization temperature, TH, were generally bracketed over intervals of 5 °C or less by watching the behavior of the bubble upon cooling. 146 canSSc5'c Lo1nglludmai Proj.ection of the Calera vein with locations of fluid inclusion samples. Samples from the splits are projected to the Calera vein. 147 Table 6. Fluid inclusion data for the Calera Vein System No. Origin1 Th2 (°C) -Tm2(°C) Salinity3-Tf (°C) -Te4 (°C) Comments Low High Low High MANGANESE STAGE CCG-6 sphalerite ? 1 232.6 233 0.6 0.6 0.99 2 ? ? 3 265 270 0.1 0 0.08 4 9 280 285 5 9 215 220 6 9 270 275 6a 9 270 275 6b 9 280 285 7 9 235 240 8 9 285 290 9 9 255 260 10 9 285 290 11 9 300 305 CCG-101 sphalerite 1 ? 268 269 1.4 1.4 2.31 2 ? 281 282 1.4 1.4 2.31 CCG-101 quartz la ps 255 260 lb ps 275 280 2 ps 250 255 3a ps 0.4 0.2 0.50 3b ps 275 280 0.4 0.2 0.50 3c ps 275 280 0.4 0.2 0.50 4a ps 4b ps 275 280 4c ps 5a ps 272 275 5b ps 272 275 0.4 0.2 0.50 5c ps 270 272 0.4 0.2 0.50 6a ps 269 270 0.4 0.2 0.50 6b ps 266 267 0.4 0.2 0.50 7a p 7b p 265 270 in rhodonite ? 8a p 295 300 0.6 0.4 0.83 8b p 260 265 8c p 263 265 0.6 0.4 0.83 9a p 265 267 0.2 0.2 0.33 9b p 272 275 0.4 0.2 0.50 9c p 268 269 0.2 0 0.17 10 p 307 309 0.4 0.2 0.50 11 p 260 265 0.4 0.2 0.50 148 Table 6. (Cont.) Fluid Inclusion Data No. Origin1 TH2 (°C) -Tm2 (°C) Salinity3-TF (°C) -TE4 (°C) Comments Low High Low High 12 s 248 250 13 s 240 245 0.6 0.4 0.83 14 s 265 270 0.6 0.4 0.83 15 s 265 268 0.6 0.4 0.83 CCG-503 sphalerite, area 1 1 ? 249 250 1.2 1.2 1.98 2 ? 245 246 1 0.8 1.49 CCG-503 sphalerite, area 2 1 ps 250.6 250.6 1.2 1.2 1.98 2 ps 254.6 254.8 1.4 1.2 2.14 3 ps 264.6 265 1.6 1.2 2.31 4 ps 278.2 278.6 CCG-508 sphalerite, area 1 1 ? 261 262 2 ? 263 263.2 0.6 0.6 0.99 33 3 ? A 9 v. dark 4 [ v. dark 5 ? 256 256.2 0.8 0.8 1.32 6 ? 257 260 1 0.8 1.49 7 ? 270 275 CCG-508 sphalerite, area 2 1 ? 276 277 0.8 0.6 1.16 27 2 ? 268 270 0.6 0.6 0.99 31 3 ? 263 265 0.6 0.6 0.99 35 4 ? 280 282 1 0.8 1.49 5 ? 6 ? 240 245 7 ? 288 290 QUARTZ STAGE CCG-3 quartz 1 P 285 290 2a p 335 340 1 0.8 1.49 31 2b p 305 310 3a p 345 350 0.8 0.6 1.16 3b p 3c p 335 340 4 p 310 315 0.8 0.6 1.16 5 p 290 295 0.8 0.6 1.16 6 p 330 335 l P 8 p 240 245 149 Table 6. (Cont.) Fluid Inclusion Data No. Origini1 Th2 (°C) -Tm2 (°C) Salinity3 -' Low High Low High 9a 345 P 350 0.6 0.6 0.99 necked 9b P 160 165 9c necked P 325 330 0.6 0.6 0.99 necked? 10a P 10b P 315 320 11 P 270 275 12 P 275 280 necked BONANZASTAGE CCG-14 sphalerite 1 9 258.8 259 0.4 0.4 ? 0.66 2 225 230 0.4 0.4 0.66 3 9 220 225 0.4 0.4 0.66 4 9 205.8 205.8 1.4 1.2 2.14 5 9 leaked 6 9 230 232 CCG-14 quartz, area la 1 P 307 310 0.4 0.4 0.66 la P 302 305 2 P 322 323 3 P 307 310 0.4 0.4 0.66 4 P 294 295 0.4 0.2 0.50 5a P 301 302 0.4 0.4 0.66 5b P 290 292 5c P 137 140 necked 6 P 293 294 0.4 0.4 0.66 7 P 307 310 8 P 125 130 necked 9a P 280 282 0.4 0.4 0.66 9b P >336 10 P 288 290 0.4 0.4 0.66 11 P 0.4 0.2 0.50 12 P leaked? 13 P 234 235 0.4 0.2 0.50 14 P 278 280 0.4 0.2 0.50 CCG-14 quartz, area lb 1 P 278 280 1 0.8 1.49 2 P 277 280 1.2 1 1.82 necked 3 P 140 145 1 0.8 1.49 necked 4 P 5 P 6 P 245 248 1 0.8 1.49 7 P 241 242 8 P 233 235 0.8 0.6 1.16 150 Table 6. (Cont.) Fluid Inclusion Data No. Origin 1 TH2(°C) -Tm2 (°C) Salinity3-Tf (°C) -Te4 (°C) Comments Low High Low High 9 P 265 267 10a P 235 236 1 0.8 1.49 10b P 236 237 1 0.8 1.49 10c P 232 235 1 0.8 1.49 11 P 220 225 12 P 220 225 0.8 0.6 1.16 13 P 245 248 14 P 250 255 15 P 270 273 necked CCG-14 quartz, area 2 1 P 323 324 0.6 0.4 0.83 2 P 286 287 0.6 0.4 0.83 3 P >336 CCG-14 pyrargyrite 1 1 P 215 220 0.8 0.8 1.32 39 2 v. dark 3 P 242 243 1 0.8 1.49 v. dark CCG-14 pyrargyrite 2 1 v. dark 2 p 125 130 3 flinc? 4 flinc? 5 flinc? 6 flinc? 7 flinc? 8 flinc? 9 flinc? CCG-325 pyrargyrite 1 P leaked CCG-441 sphalerite 1 ? 275 276 0.4 0.4 0.66 <21 2 ? 298 299 0.6 0.4 0.83 2a ? 277 280 3 9 269 270 0.4 0.4 0.66 3a 9 277 280 LATE STAGE Loose quartz, area 1 1 P 0.4 0.2 0.50 leaked 2 P 264.6 264.6 0.8 0.8 1.32 31 Table 6. (Cont.) Fluid Inclusion Data No. Origin1 Th2(°C) -Tm2(°C) Salinity3 -Tf (°C) -Te4 (°C) Comments Low High Low High 3 P 264.8 264.8 0.8 0.8 1.32 4 P 264.6 264.8 0.8 0.8 1.32 5 P 264.8 264.8 0.8 0.8 1.32 6 P 265.2 265.4 0.8 0.8 1.32 7 P 268.8 268.8 1 0.8 1.49 8 P 240 240 necked ? 9 P 220 240 necked ? 10 P 269.8 269.8 necked 11 P 270.6 270.6 necked 12 P 265.2 265.4 necked Loose quartz, area 2a 1 P 233.2 233.2 1.4 1.2 2.14 2 P 255 256 1.2 1 1.82 3 P 174 175 1.2 1 1.82 4 P 285 290 1.2 1 1.82 5 P 250 255 necked 6 P 260 265 1.6 1.6 2.63 necked 7 P 255 257 1.4 1.2 2.14 necked 8 P 235 240 1.4 1.2 2.14 9 P 10 P 240 245 1.4 1.2 2.14 11 P 12 P 13 P 14 P 15 P 265 270 16 P 238 240 17 P 229.8 229.8 1.6 1.4 2.47 18 P 1.6 1.2 2.31 19 P 250 252 1.4 1.2 2.14 20 P 252 255 1.4 1.2 2.14 21 P 247 250 1.4 1.2 2.14 22 P 240 245 2 1.6 2.96 23 P 300 300.6 1.4 1.2 2.14 24 P 220.6 220.8 1.4 1.2 2.14 necked 25 P 245 250 Loose quartz, area 2b 1 P 235 236 1.2 1.2 1.98 2 P 265 268 1.6 1.2 2.31 necked? 3 P 261.6 261.6 1.2 1.2 1.98 4 P 267 268 1.2 1.2 1.98 5 P 290 300.6 6 P 260 263 7 P 245 250 necked? 8 P 250 255 2a 2 1 lshlrt,ae 1 area sphalerite, -l R 914 23 22 21 1 20 20 19 18 17 16 15 14 13 9 12 11 8 10 7 6 5 4 3 2 Loose quartz, area 3 area quartz, Loose 20 O O n U> 1 19 18 17 16 15 9 14 13 12 No. 11 10 Origin1 ps ps ps ps P ps P ps 256 295.6 >320 295.6 p p ps ps ps 26 298 296 p ps ps 27 300 298 297 297 p p ps ps 222 292.2 292.2 p ps ps ps ps ps ps ps ps ps ps P P P P P P P P P P P P >215 9 294 293 256.6 244 250 245 170 250 222 225 250 250 245 240 250 222 237 250 220 245 245 233 285 228 236 215 254 253 255 240 254 245 255 290 o High Low TH2(°C) 256.8 245 255 250 175 255 225 226 253 251 246 255 245 223 240 253 222 250 250 234 290 230 238 217 255 254 260 245 255 246 300.8 258 bl . Cont) udIcuinData a D Inclusion luid F t.) n o (C 6. le ab T 0.4 0.6 0.4 1.4 1.4 1.2 1.4 1.4 1.2 o High Low 1.2 1.2 1.2 1.2 1.2 -T m 2(°C) 0.2 0.4 0.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Salinity3-TF(°C) -Te4 Comments (°C) 0.50 0.83 0.50 2.14 2.14 1.98 2.14 2.14 1.98 1.98 1.98 1.98 1.98 1.98 necked? necked 1 52 153 Table 6. (Cont.) Fluid Inclusion Data No. Origin1 TH2(°C) -TM2(°C) Salinity3 -TF (°C) -TE4 (°C) Comments Low High Low High 8 P 290 295 9 P 298 298.6 9a P 296 298 10 P 296 297 11 P 303 305 12 P 13 P 14 P 301.8 302 0.4 0.2 0.50 15 P 914 R-l sphalerite, plane 1 1 s 292 293 0.6 0.4 0.83 2 s 292 293 0.6 0.4 0.83 3 s 292 293 0.6 0.4 0.83 4 s 280 285 0.6 0.4 0.83 5 s 300 301 0.6 0.4 0.83 6 s 293 294 0.6 0.4 0.83 7 s 294 295 0.6 0.4 0.83 8 s 294 295 0.6 0.4 0.83 9 s 280 285 0.6 0.4 0.83 914 R-l sphalerite, plane 2 1 s 290 295 0.6 0.2 0.66 2 s 294 295 0.6 0.2 0.66 3a s 298 299 0.6 0.2 0.66 3b s 298 299 0.6 0.2 0.66 3c s 298 299 0.6 0.2 0.66 3d s 297 298 0.6 0.2 0.66 3e s 298 299 0.6 0.2 0.66 3f s 296 298 0.6 0.2 0.66 3g s 298 299 0.6 0.2 0.66 4 s 298 299 0.6 0.2 0.66 5 s 296 297 0.6 0.2 0.66 6 s 298 299 0.6 0.2 0.66 7 s 296 297 0.6 0.2 0.66 8 s 289 290 0.6 0.2 0.66 9 s 299 300 0.6 0.2 0.66 10a s 277 280 0.6 0.2 0.66 10b s 306 309 0.6 0.2 0.66 10c s 290 295 0.6 0.2 0.66 lOd s 277 278 0.6 0.2 0.66 lOe s 303 305 0.6 0.2 0.66 11 s 276 277 0.6 0.2 0.66 12 s 295 296 0.6 0.2 0.66 13 s 312 313 0.6 0.2 0.66 14 s 297 298 0.6 0.2 0.66 154 Table 6. (Cont.) Fluid Inclusion Data No. Origin1 TH2(°C) -Tm2(°C) Salinity3-TF(°C) -Te4 (°C) Comments Low High Low High 914 R-l quartz 1 P 252 254 0.8 0.6 1.16 2 P 265 270 dark 3 P 259 260 0.8 0.6 1.16 4a P 265 270 0.8 0.6 1.16 4b P 272 275 0.8 0.6 1.16 5a P 280 285 0.8 0.6 1.16 34 5b P 135 140 necked 5c P 285 290 0.8 0.6 1.16 6 P 265 270 0.8 0.6 1.16 7 P 270 272 0.8 0.6 1.16 8a P 270 272 0.8 0.6 1.16 8b P 275 280 0.8 0.6 1.16 <24 8c P 280 285 0.8 0.6 1.16 8d P 285 290 0.8 0.6 1.16 9 P 272 275 0.8 0.6 1.16 10 P 264 265 0.8 0.6 1.16 11 ps ? 310 311 0.8 0.6 1.16 12a ps ? 257 258 0.8 0.6 1.16 12b ps ? 160 165 0.8 0.6 1.16 13a ps ? 155 160 0.8 0.6 1.16 13b ps ? 300 302 0.8 0.6 1.16 14 ps ? 320 325 0.8 0.6 1.16 15 ps ? 325 CCG-116 sphalerite, area 1 1 p 298 299 0.2 0.2 0.33 very dark 2 p 280 285 1 0 0.83 very dark 3 p 300 301 0.6 0.2 0.66 very dark 4 p 285 290 5 p didn’t see 6 p 235 240 7 p 299 300 7a p 285 290 8 s 275 280 9 s 275 280 10 s 275 280 CCG-116 sphalerite, area 2 1 p or ps 299 300 2 p or ps 300 302 3 p or ps 0.2 0.2 0.33 30 4 p or ps 5a P 5b P 285 287 0.6 0.4 0.83 6 p or ps 300 302 7 p or ps 295 297 0.2 0.2 0.33 155 Table 6. (Cont.) Fluid Inclusion Data No. Origin1 Th2 (°C) -Tm2(°C) Salinity3-Tf (°C) -Te4(°C) Comments Low High Low High 8 p or ps 300 302 9 p or ps 300 302 10 p or ps 295 300 CCG-116 sphalerite, area 3 1 s leaked 2 s 260 261 0.4 0.2 0.50 edge of cpy disease 3 s 270 275 0.8 0.6 1.16 37 <23 4 s 275 280 0.8 0.8 1.32 35 5 s 275 280 0.8 0.8 1.32 6 s 270 275 0.8 0.8 1.32 7 s 240 245 0.4 0.2 0.50 edge of cpy disease 8 s 270 275 9 s 282 283 0.8 0.8 1.32 10 p ? 240 245 0.2 0.2 0.33 11 p ? 286 287 0.4 0.2 0.50 23.2 <20 12 p ? 240 245 0.2 0.2 0.33 CCG-308 sphalerite 1 p 285 290 0.6 0.4 0.83 38 dark 2 p 290 295 0.6 0.4 0.83 dark 3 s 282 283 dark 4 s or p 294 295 5 s 285 290 6 s 283 285 0.6 0.4 0.83 7 s 281 285 0.6 0.4 0.83 8 s 290 295 9 s 290 295 10 s 285 285 11 s 290 295 1 p = primary, ps = pseudosecondary, s = secondary, ? = unsure origin because of chalcopyrite disease. 2 Low and High indicate the lower and higher temperatures, respectively, used to bracket the phase disappearance; the average was used to construct Figs. 43 and 44. 3 Salinity in weight percent NaCl equivalent, calculated from Hall et al. (1988) using the average 4 The eutectic temperature was typically difficult to measure 156 APPENDIX 2 Thermodynamic data The thermodynamic data for reactions involving tellurium-bearing minerals (Table 7) were obtained from Afifi et al. (1988a), and are largely consistent with data compiled by Ahmad et al. (1987). The values for Log K were calculated from Gibbs free energy values and used to construct Figures 30 and 33 by writing the appropriate reaction in terms of the important constituents. For example, the reaction between rhodonite and rhodochrosite may be written: MnCC>3 + SiC>2 = MnSiC>3 + C02(g). The appropriate reaction is then: Log Kj-c-r(j = Log/C 02, if the solids are assumed to have unit activity. 157 Table 7. Thermodynamic data used to construct Figures 30 and 33. Reaction Log K (250°C, 1 bar) Reference Te2(g) = 2Te(s) 7.83 1 S2(g) = 2S(i) 4.73 2 2Pb + Te2(g) = 2PbTe 20.99 1 2Pb + S2(g) = 2PbS 23.54 2 2PbS + Te2(g) = 2PbTe + -2.55 calc 2Zn + Te2(g) = 2ZnTe 30.23 1 2Zn + S2(g) = 2ZnS 43.69 2 2ZnS + Te2(g) = 2ZnTe + S2^g) -12.54 calc 2Hg + Te2(g) = 2HgTe 12.89 1 2Hg + Sjft, = 2HgS 12.75 2 2HgS + Te2(g) = 2HgTe + 0.14 calc 4Ag + Te2(g) = 2Ag2Te 17.41 1 4Ag + S2(g) = 2Ag2S 13.92 2 2Ag2S + Te2(g) = 2Ag2Te + S2(g) 3.49 calc Au + Te2(g) = AuTe2 9.42 1 2FeS2 = 2FeS + S2 -13.64 2 Cu5FeS4 + 4FeS2 = 5CuFeS2 + S2 -8.48 2 MnC03 + Si02 = MnSi03 + C02(g) 0.41 3 2M nC 03 + S2(g) = 2MnS + 2C02(g) + 0 2 -24.62 3 2MnSi03 + S2(g) = 2MnS + 2Si02 + 0 2 -25.43 3 1 Afifi et al. (1988a) 2 Barton and Skinner (1979) 3 estimated from the data of Robie et al. (1978) calc - indicates value calculated from other reactions