University of Nevada Reno F|F0(5 J M L the Qalera Vein System
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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 <n 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 <t. /Les, 0 Dr. Kenneth W. Hunter, Dean, Graduate School 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