THE VELADERO HIGH-SULFIDATION EPITHERMAL AU-AG DEPOSIT, ARGENTINA: VOLCANIC STRATIGRAPHY, ALTERATION, MINERALIZATION, AND QUARTZ PARAGENESIS by Elizabeth A. Holley A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Geology). Golden, Colorado Date _________________ Signed:___________________________ Elizabeth A. Holley Signed:___________________________ Dr. Thomas Monecke Thesis Advisor Golden, Colorado Date _________________ Signed:___________________________ Dr. J. D. Humphrey Associate Professor and Head Department of Geology and Geological Engineering ii ABSTRACT The Veladero Au-Ag high-sulfidation epithermal deposit is located in the El Indio-Pasuca belt in Argentina. Veladero is an oxidized deposit that contained reserves of 12.2 Moz of Au and 226.2 Moz of Ag at the end of 2008. Ore is primarily hosted in silicified breccias. The volcanic package at Veladero is a coalescing complex of domes, diatremes, and hydrothermal breccia bodies, mantled by a thick apron of volcaniclastic deposits. These units are inferred to be of Cerro de las Tórtolas age (16.0 ± 0.2 to 14.9 ± 0.7 Ma) at Amable in the southern part of the Veladero area, and Vacas Heladas age (12.7 ± 0.9 to 11.0 ± 0.2 Ma) at Filo Federico in the northern part of Veladero. Emplacement of hydrothermal breccia units was accompanied or shortly followed by multiple pulses of magmatic-hydrothermal and alternating jarosite alteration from about 15.4 to 8.9 Ma. Alunite and jarosite 40Ar-39Ar ages and U-Pb zircon ages of crosscutting dikes restrict mineralization at Amable to the period from 15.4 to 12.14 0.11 Ma, probably commencing closer to 12.7 Ma. At Filo Federico, Au mineralization commenced after the beginning of alunite alteration at 11.05 0.12 Ma. The new data suggest that the maximum age of mineralization in the belt is slightly higher than previously recognized. The bulk of Veladero Ag is hosted in Ag-bearing jarosite in the jarosite- argentojarosite solid solution. Some silver occurs as < 0.1 m iodargyrite and aggregates of <0.1 m particles with variable Ag, I, Br, Cl, Hg, Se, S, and Fe concentrations. Rare Ag-Fe sulfides were identified. Veladero Au exists as >800 fineness grains of native Au. A phase of euhedral, fracture-controlled quartz with growth zone parallel iron oxide inclusions has been identified at Veladero, post-dating leaching of the host rock to vuggy quartz and pre-dating Au mineralization. The quartz is evidence that the fluids that transport the precious metals are distinct from the magmatic vapors that interact with meteoric water to produce alteration in high-sulfidation disseminated deposits, and that at least the first stage of metals are transported by a low-temperature fluid also capable of precipitating quartz. iii TABLE OF CONTENTS ABSTRACT ...................................................................................................................... iii LIST OF FIGURES ........................................................................................................ viii LIST OF TABLES ..............................................................................................................x LIST OF ABBREVIATIONS ........................................................................................... xi ACKNOWLEDGMENTS ............................................................................................... xii DEDICATION ................................................................................................................ xiii CHAPTER 1 INTRODUCTION ........................................................................................1 1.1 The Magmatic-Hydrothermal System ...............................................................2 1.2 Characteristics of High-Sulfidation Epithermal Deposits .................................3 1.3 The Veladero Deposit ......................................................................................4 1.4 Study Overview ...............................................................................................7 1.4.1 Volcanic Stratigraphy at Veladero ..................................................7 1.4.2 Veladero Alteration .........................................................................8 1.4.3 Veladero Mineralization .................................................................8 1.4.4 Quartz Paragenesis at Veladero ......................................................9 CHAPTER 2 VOLCANIC STRATIGRAPHY ................................................................10 2.1 Introduction .....................................................................................................10 2.2 Regional Stratigraphy .....................................................................................10 2.3 Local Structural Setting ..................................................................................15 2.4 The Veladero Deposit .....................................................................................15 2.5 Methods ...........................................................................................................18 2.6 Breccia Nomenclature .....................................................................................21 2.7 Major Facies ....................................................................................................23 2.7.1 Facies 1 ............................................................................................24 2.7.2 Facies 2 ............................................................................................27 2.7.3 Facies 3 ............................................................................................28 2.7.4 Facies 4 ............................................................................................28 2.8 Minor Facies ...................................................................................................32 iv 2.8.1 Facies 5 ............................................................................................32 2.8.2 Facies 6 ............................................................................................35 2.8.3 Facies 7 ............................................................................................35 2.8.4 Facies 8 ............................................................................................38 2.8.5 Facies 9 ............................................................................................38 2.8.6 Facies 10 ..........................................................................................39 2.9 Alteration ........................................................................................................41 2.10 Mineralization ...............................................................................................43 2.11 Genetic Model ................................................................................................47 2.12 Discussion .....................................................................................................50 2.13 Conclusions ...................................................................................................53 CHAPTER 3 ALUNITE AND JAROSITE ALTERATION ............................................55 3.1 Introduction .....................................................................................................55 3.2 Geology of the El Indio-Pascua Belt ..............................................................57 3.3 The Veladero Deposit .....................................................................................59 3.3.1 Mineralization ..................................................................................60 3.3.2 Deposit-Scale Alteration ..................................................................60 3.4 Methods ...........................................................................................................62 3.5 Quartz-Alunite Alteration Zones ....................................................................63 3.6 Jarosite Alteration Zones ................................................................................64 3.7 Relative Timing of Alteration Minerals ..........................................................67 3.8 Alteration Mineral Chemistry .........................................................................67 3.9 Sulfate Mineral Stable Isotopes ......................................................................71 3.9.1 Stable Isotope Results ......................................................................72 3.9.2 Discussion of Sulfate Mineral Origins .............................................73 3.9.3 Estimates of Temperature and Elevation .........................................78 3.10 Geochronology ..............................................................................................79 3.10.1 Alteration Ages ..............................................................................79 3.10.2 Discussion of Alteration Ages .......................................................81 3.11 Constraints on Timing of Mineralization ......................................................85 3.12 Conclusions ...................................................................................................87 v CHAPTER 4 SILVER MINERALOGY ..........................................................................91 4.1 Introduction .....................................................................................................91