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The geology and evolution of the Manantial Espejo epithermal silver(-gold) deposit, Deseado Massif, Argentina Wallier, Stefan 2009

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   The geology and evolution of the Manantial Espejo epithermal silver(-gold) deposit, Deseado Massif, Argentina   by   Stefan Wallier     Diploma, The Swiss Federal Institute of Technology Zurich, 2004     A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY   in   The Faculty of Graduate Studies  (Geological Sciences)    THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)    December 2009     © Stefan Wallier, 2009  ii ABSTRACT  Quartz-adularia epithermal veins record the interplay between magmatism, tectonism, and derivative geothermal systems. At Manantial Espejo, southern Argentina, such veins form a Ag(-Au) deposit with a proven and probable reserve of 38.5 Moz Ag and 0.55 Moz Au. The district located in the Deseado Massif is part of the Middle to Late Jurassic Chon Aike Province, a large bimodal igneous province dominated by silicic volcanic rocks that was formed in association with the Gondwana break-up. The district stratigraphy consists of a more then 400 m thick andesitic to rhyolitic volcanic succession formed between 162 and 157 Ma. The precious metal veins post-date the volcanic host rocks and formed contemporaneously with normal faults at 154 ± 1 Ma. Mineralized veins are Ag-rich examples of a low- to intermediate-sulfidation epithermal deposit. In the Maria Vein, the major orebody of the district, six hydrothermal stages are identified, but only stages 1 and 4 contributed significantly metal. The distribution of the hydrothermal stages along the vein is primarily controlled by fault formation and records the growth and linkage of vein segments to form an integrated curviplanar vein. Alteration mineral zoning about the Maria Vein with proximal adularia and distal illite and illite-smectite assemblages shows higher temperature mineral assemblages extending over a broader halo and to shallower levels along a high-grade vein segment where compared to a low-grade segment. The dominant changes in whole-rock alteration geochemistry toward the Maria Vein are the gain of K and the loss of Na, Ca, and Mg. Precious metals, base metals, and pathfinder elements have typically increasing concentrations toward the vein. Boiling was the dominant depositional process and metal ratios in veins reflect the abundance of dissolved metals in solution. Barren quartz with abundant boiling textures was formed by low-salinity fluids of temperatures between 230° and 270°C. Mineralizing fluids had lower temperatures, but salinities between 4 and 10 wt % NaCl equiv. Lead isotopic constraints indicate a mostly common source for Pb in volcanic rocks of the Chon Aike Formation and Pb in hydrothermal sulfides, or that the Pb in the hydrothermal veins was derived from the Chon Aike Formation. iii TABLE OF CONTENTS  Abstract ................................................................................................................................ii Table of Contents ................................................................................................................iii List of Tables ........................................................................................................................x List of Figures .....................................................................................................................xi Acknowledgements ..........................................................................................................xvii Co-authorship Statement ...............................................................................................xviii  Chapter 1 – Introduction ....................................................................................................1 1.1.    Objectives of the dissertation ...................................................................................1 1.2.    Overview of the Manantial Espejo district and its Ag(-Au) deposit ........................2 1.3.    Epithermal ore deposits ............................................................................................3 1.4.    Characteristics of quartz-adularia epithermal Au-Ag deposits .................................7 1.5.    Overview of the dissertation .....................................................................................8 1.6.    References ..............................................................................................................11 Chapter 2 - Short-living volcanic activity in the Middle to Late Jurassic bimodal large igneous Chon Aike Province: An example from Manantial Espejo, Deseado Massif, South Argentina ...................................................................................................14 2.1.    Introduction ............................................................................................................14 2.2.    Geological setting of the Chon Aike Province .......................................................15 2.3.    Geological setting of the Deseado Massif ..............................................................17 2.4.    Geology of the Manantial Espejo epithermal Ag(-Au) district ..............................20 2.4.1.    District stratigraphic sequence .........................................................................20 2.4.1.1.    Basement rocks ...........................................................................................20 2.4.1.2.    Bajo Pobre Formation .................................................................................20 2.4.1.3.    Chon Aike Formation .................................................................................24 2.4.1.4.    La Matilde Formation .................................................................................26 2.4.1.5.    Intermediate formation ...............................................................................26 2.4.1.6.    Granite xenoliths ........................................................................................28 2.4.1.7.    Tertiary and recent sedimentary deposits ...................................................29 iv 2.4.2.    Structural architecture .......................................................................................29 2.5.    Whole-rock major element geochemistry ...............................................................31 2.6.    Geochronology .......................................................................................................34 2.6.1.    Methodology ..................................................................................................34 2.6.1.1.    Uranium-lead SHRIMP-RG geochronology ..............................................34 2.6.1.2.    40Argon/39Argon geochronology ................................................................36 2.6.2.    Geochronology results ......................................................................................39 2.6.2.1.    Inherited zircons and xenocrysts ................................................................41 2.6.2.2.    Andesite xenolith – Bajo Pobre Formation? ...............................................43 2.6.2.3.    Chon Aike Formation .................................................................................43 2.6.2.4.    Intermediate formation ...............................................................................43 2.6.2.5.    Granite xenolith ..........................................................................................44 2.7.    Discussion ...............................................................................................................46 2.7.1.    Stratigraphy and geochronology of the Manantial Espejo district ...................46 2.7.1.1.    Bajo Pobre Formation .................................................................................46 2.7.1.2.    Chon Aike Formation .................................................................................46 2.7.1.3.    La Matilde Formation .................................................................................49 2.7.1.4.    Intermediate formation ...............................................................................49 2.7.1.5.    Pre-Chon Aike and -Bajo Pobre Formation ...............................................50 2.7.2.    Manantial Espejo’s position in the age pattern of volcanic activity .................51 2.7.3.    Granite xenoliths ..............................................................................................52 2.7.4.    Extensional tectonics and faulting ....................................................................53 2.7.5.    Eruption type for large volume ignimbrites .....................................................53 2.8.    Conclusions ............................................................................................................54 2.9.    References ..............................................................................................................55 Chapter 3 - The Manantial Espejo epithermal Ag(-Au) deposit, Deseado Massif, Argentina: Vein parageneses, alteration, and age of mineralization ............................60 3.1.    Introduction ............................................................................................................60 3.2.    Geological setting ...................................................................................................62 3.2.1.    Geological setting of the Chon Aike Province .................................................62 3.2.2.    Geological setting of the Deseado Massif ........................................................62 v 3.2.3.    Geological setting of the Manantial Espejo district .........................................64 3.3.    Geology of the Manantial Espejo Ag(-Au) deposit ................................................69 3.3.1.    Sampling and analytical techniques: vein and alteration mineralogy ..............72 3.3.2.    The Maria Vein .................................................................................................77 3.3.2.1.    Structure .....................................................................................................77 3.3.2.2.    Paragenetic sequence ..................................................................................77 3.3.2.3.    Hydrothermal alteration ..............................................................................87 3.3.3.    The Concepción Vein .......................................................................................92 3.3.3.1.    Structure .....................................................................................................92 3.3.3.2.    Paragenetic sequence ..................................................................................93 3.3.3.3.    Alteration ....................................................................................................93 3.3.4.    The Melissa Vein and the Karina-Unión stockwork ........................................94 3.3.5.    Northwest Maria stockwork .............................................................................94 3.3.6.    The Mesa Veins ................................................................................................95 3.3.6.1.    Structure .....................................................................................................95 3.3.6.2.    Paragenetic sequence ..................................................................................95 3.3.6.3.    Alteration ....................................................................................................98 3.4.    Fluid inclusion study ..............................................................................................98 3.4.1.    Methodology .....................................................................................................98 3.4.2.    Fluid inclusion results .....................................................................................100 3.4.2.1.    Fluid inclusions in samples form the Maria Vein .................................102 3.4.2.2.    Fluid inclusions in samples from the Mesa Vein ..................................107 3.5.    40Argon/39Argon geochronology on hydrothermal adularia .................................109 3.5.1.    Methodology ...................................................................................................109 3.5.2.    Results ............................................................................................................111 3.5.2.1.    Adularia ages from the Maria Vein ..........................................................111 3.5.2.2.    Adularia age from the Concepción Vein ..................................................113 3.5.2.3.    Adularia ages from the Mesa Vein ...........................................................113 3.6.    Discussion .............................................................................................................113 3.6.1.    Classification of the deposit and active hydrothermal equivalents ................113 3.6.2.    Implications of hydrothermal alteration .........................................................116 vi 3.6.3.    Constraints from fluid inclusion .....................................................................120 3.6.4.    Depth of mineralization, amount of erosion, and fluid pressure ....................125 3.6.5.    Epithermal mineralization in the temporal framework of the Chon Aike Province .......................................................................................................................128 3.7.    Summary and conclusions ....................................................................................130 3.8.    References ............................................................................................................131 Chapter 4 - Distribution of hydrothermal stages in the Maria Vein of the Manantial Espejo epithermal Ag(-Au) deposit, Deseado Massif, Argentina: Result of fault formation .................................................................................................137 4.1.    Introduction ..........................................................................................................137 4.2.    Structural background: fault formation ................................................................138 4.3.    Tectonic and geologic framework ........................................................................145 4.4.    Evolution of the Maria Vein .................................................................................146 4.4.1.    Hydrothermal stages .......................................................................................148 4.4.2.    Distribution of hydrothermal stages ...............................................................150 4.4.3.    Ore-grade distribution .....................................................................................159 4.4.4.    Orientations of vein banding and vein contacts ..............................................160 4.4.5.    Vertical displacement along the Maria Vein ..................................................160 4.5.    Discussion and conclusions ..................................................................................163 4.5.1.    The formation of the Maria Vein ....................................................................163 4.5.2.    Lateral continuation of the Maria Vein ..........................................................166 4.5.3.    Structural model .............................................................................................167 4.5.4.    Significance of the structural interpretation of an epithermal district ............168 4.6.    References ............................................................................................................170 Chapter 5 - Alteration geochemistry, vein geochemistry, and Pb-isotopes of the Manantial Espejo epithermal Ag(-Au) deposit, Deseado Massif, Argentina .............173 5.1.    Introduction ..........................................................................................................173 5.2.    Geological setting of the Manantial Espejo Ag(-Au) deposit ..............................175 5.2.1.    Tectonic and geologic framework ..................................................................175 5.2.2.    Mineralization of the Manantial Espejo Ag(-Au) deposit ..............................179 5.2.3.    Alteration surrounding the Maria Vein ..........................................................182 vii 5.2.3.1.    Hypogene alteration ..................................................................................183 5.2.3.2.    Supergene alteration .................................................................................186 5.3.    Alteration geochemistry .......................................................................................186 5.3.1.    Sampling and analytical techniques ...............................................................186 5.3.2.    Whole-rock geochemistry results ...................................................................187 5.3.2.1.    Major element concentrations ..................................................................199 5.3.2.2.    Trace element concentrations ...................................................................204 5.4.    Vein geochemistry ................................................................................................209 5.4.1.    Sampling and analytical techniques ...............................................................209 5.4.2.    Vein geochemistry results ..............................................................................210 5.4.2.1    Silver- and gold-grades in the Maria and Concepción Veins ....................210 5.4.2.2.    Geochemical characteristics of the Maria Vein ........................................213 5.4.2.3.    Geochemical characteristics of the Concepción Vein ..............................217 5.4.2.4.    Geochemical characteristics of the Mesa Vein .........................................218 5.4.2.5.    Geochemical characteristics of a high-grade vein in the NW Maria stockwork .................................................................................................................218 5.4.2.6.    Vertical distribution of element concentrations and element ratios .........218 5.4.2.7.    Element ratios along the Maria Vein ........................................................219 5.5.    Pb isotope geochemistry .......................................................................................222 5.5.1.    Sampling and analytical techniques ...............................................................222 5.5.2.    Lead isotope results ........................................................................................225 5.5.2.1.    Pb isotopic composition of magmatic feldspars .......................................225 5.5.2.2.    Pb isotopic composition of hydrothermal sulfides ...................................227 5.6.    Discussion .............................................................................................................227 5.6.1.    Alteration geochemistry .................................................................................227 5.6.1.1.    General alteration observations ................................................................227 5.6.1.2.    Major element concentrations in altered rocks .........................................228 5.6.1.3.    Trace element concentrations in altered rocks .........................................232 5.6.1.4.    Effects of supergene alteration .................................................................233 5.6.1.5.    Comparison of a low- and high-grade cross section through the Maria Vein ..........................................................................................................................234 viii 5.6.2.    Vein geochemistry ..........................................................................................237 5.6.2.1.    Ore precipitation and distribution of Ag- and Au-grades .........................237 5.6.2.2.    Vertical zoning of element concentrations ...............................................240 5.6.2.3.    Element ratios and direction of fluid flow ................................................242 5.6.2.4.    Variability of metal concentrations during the evolution of the Maria Vein ..........................................................................................................................242 5.6.2.5.    Comparison of a low- and a high-grade cross section through the Maria Vein ................................................................................................................244 5.6.3.    Pb isotopic compositions ................................................................................246 5.7.    Conclusions ..........................................................................................................247 5.8.    References ............................................................................................................249 Chapter 6 - Summary and conclusions...........................................................................256 6.1.    Investigated aspects of the Manantial Espejo geology .........................................256 6.2.    The evolution of the Manantial Espejo district ....................................................256 6.3.    Scientific importance of the dissertation ..............................................................260 6.4.    Critical learnings ...................................................................................................262 6.4.1.    Relationship between intermediate and silicic volcanic rocks .......................262 6.4.2.    Nature of geothermal systems ........................................................................262 6.4.3.    Structural control on vein formation ..............................................................263 6.5.    Practical implications of the dissertation ..............................................................264 6.5.1.    Potential applications for exploration in the Manantial Espejo district ..........264 6.5.2.    Structural interpretation and timing of hydrothermal events ..........................265 6.5.3.    Implications of alteration mineralogy and geochemistry ...............................265 6.6.    Future research directions .....................................................................................267 6.6.1.    Volcanic vents, volcanic stratigraphy, and geochronology ............................267 6.6.2.    Vein geochemistry, hydrothermal mineralogy, and fluid pathways ...............267 6.6.3.    Fluid and metal sources ..................................................................................268 6.7.    References ............................................................................................................270 Appendices .......................................................................................................................273 Appendix A. Lithological units from the Manantial Espejo district ..............................274 Appendix B. Selected SHRIMP-RG (zircon) analytical data .........................................278 ix Appendix C. 40Ar-39Ar (hornblende) analytical data ......................................................282 Appendix D. Drill hole data from cores described in appendix E ..................................284 Appendix E. Summary of logged drill core intervals that intercept the Maria Vein ......285 Appendix F. Location of alteration samples (PIMA, thin section, XRD) ......................319 Appendix G. PIMA and pyrite data and descriptions of drill core samples from the Manantial Espejo deposit ......................................................................................321 Appendix H. X-ray diffraction (XRD) results ................................................................331 Appendix I. Photomicrographs of thick sections analyzed for fluid inclusions .............332 Appendix J. Fluid inclusion data from the Manantial Espejo deposit ............................349 Appendix K. 40Ar/39Ar (adularia) analytical data ...........................................................358 Appendix L. Location of alteration whole-rock geochemistry samples .........................365 Appendix M. Location of vein whole-rock geochemistry samples ................................367 Appendix N. Whole-rock geochemical analysis of vein material ..................................369  x LIST OF TABLES   Table 2.1 Whole-rock geochemical analyses of major elements by X-ray fluorescence (XRF). ........................................................................................32 Table 2.2 Summary of isotopic ages from the Deseado Massif. ....................................35 Table 2.3 Summary of SHRIMP-RG U-Pb (zircon) ages for volcanic and sub- volcanic rocks from the Manantial Espejo district. ........................................40 Table 3.1 Resources of the Manantial Espejo Ag(-Au) deposit. ....................................79 Table 3.2 Paragenetic sequence of the Mesa Veins. .......................................................99 Table 3.3 Fluid inclusion data for quartz and sphalerite from the Maria and Mesa Veins of the Manantial Espejo Deposit. .......................................................103 Table 5.1 Whole rock geochemical analyses of altered rocks surrounding the Maria Vein. ...................................................................................................188 Table 5.2 Whole-rock geochemical analyses of major elements by X-ray fluorescence (XRF). ......................................................................................198 Table 5.3 Pb isotopic geochemistry from magmatic feldspars from the Manantial Espejo district. ..............................................................................................223 Table 5.4 Pb isotopic geochemistry from hydrothermal sulfides from the Manantial Espejo district. .............................................................................224              xi LIST OF FIGURES  Figure 1.1 Simplified conceptual models for high-temperature hydrothermal systems, showing the relationship between epithermal environments, magmatic intrusions, fluid circulation paths, and volcanic and basement host rocks. .........................................................................................................5 Figure 2.1 Map of southern South America showing exposures and major formations of the Middle to Late Jurassic Chon Aike Province and older magmatic rocks. ..............................................................................................16 Figure 2.2 Simplified regional geological map of the Deseado Massif. ..........................18 Figure 2.3 Satellite image from the Manantial Espejo district showing major faults. .....21 Figure 2.4 Geologic map of the central part of the Manantial Espejo district. ................22 Figure 2.5 Typical rock textures and outcrops of the Chon Aike Formation in the Manantial Espejo district. ...............................................................................23 Figure 2.6 Typical rock textures and outcrops from the Manantial Espejo district. ........27 Figure 2.7 Simplified cross sections through the Manantial Espejo district. ...................30 Figure 2.8 Total alkali vs. SiO2 diagram with data points and sample numbers. ............33 Figure 2.9 Cathodoluminescense (CL) images of representative zircons with location and number of spot analyses. ............................................................37 Figure 2.10 207Pb corrected 206Pb/238U weighted mean age histograms with 2σ error bars. ................................................................................................................38 Figure 2.11 Incremental laser heating 40Ar-39Ar age spectra for hornblende. ...................42 Figure 2.12 Age vs. U-concentration plot for analyzed zircons from a granite xenolith in an andesite from the intermediate formation.................................45 Figure 2.13 Composition of the stratigraphic sequence at Manantial Espejo, derived from field mapping and drill cores studies. ....................................................47 Figure 3.1 Map of southern South America showing exposures of the Middle to Late Jurassic Chon Aike Province. .................................................................63 Figure 3.2 Simplified geological map of the Deseado Massif showing locations of epithermal ore deposits. ..................................................................................65 xii Figure 3.3 Simplified geological map of the Manantial Espejo district showing areas with hydrothermal alteration and veining. ............................................66 Figure 3.4 Compilation of the stratigraphic sequence in the Manantial Espejo district with isotopic ages. ...............................................................................67 Figure 3.5 Geology map of the central vein zone with names of localities named in the text. ...........................................................................................................68 Figure 3.6 Distribution of tabular silica bodies and silicified rhyolite dikes in the Manantial Espejo district surrounding the central vein zone. ........................73 Figure 3.7 Outcrops of tabular silica bodies in the Manantial Espejo district. ................74 Figure 3.8 Structure of the Maria Vein. ...........................................................................75 Figure 3.9 Hydrothermal stages of the Maria Vein with characteristic vein textures, gangue, and hypogene ore minerals. ..............................................................78 Figure 3.10 Photographs of outcrops, vein textures and minerals from the Maria Vein (Stages 1 and 2). ....................................................................................81 Figure 3.11 Photographs of outcrops, vein textures and minerals from the Maria Vein (Stage 3). ................................................................................................82 Figure 3.12 Photographs of outcrops, vein textures and minerals from the Maria Vein (Stages 4, 5, and 6). ...............................................................................84 Figure 3.13 Photographs of supergene enriched vein textures from the Maria Vein. .......86 Figure 3.14 Zoning of alteration minerals and alteration assemblages about the Maria Vein displayed in a cross section at 225W. ....................................................88 Figure 3.15 Zoning of alteration minerals and alteration assemblages about the Maria Vein displayed in a cross section at 425W. ....................................................89 Figure 3.16 Geology map of the Mesa area. ......................................................................96 Figure 3.17 Photographs of vein textures and minerals from the Mesa Vein. ...................97 Figure 3.18 Long section through the Maria Vein showing the location of the fluid inclusion samples. .........................................................................................101 Figure 3.19 Characteristic fluid inclusions in quartz and sphalerite from the Maria and Mesa Veins. ...........................................................................................105 xiii Figure 3.20 Fluid inclusion plot of homogenization temperature Th vs. temperature of final ice melting Tm (and apparent salinity) for fluid inclusion assemblages in quartz and sphalerite from the Maria Vein. .........................106 Figure 3.21 Fluid inclusion plot of homogenization temperature vs. temperature of final ice melting Tm (and apparent salinity) for fluid inclusion assemblages in quartz and sphalerite from the Mesa Vein. ..........................108 Figure 3.22 Photographs of vein samples for adularia 40Ar-39Ar geochronology from the Maria Vein and the Mesa Vein. ..............................................................110 Figure 3.23 Laser 40Ar-39Ar ages on hydrothermal adularia from three different veins in the Manantial Espejo district. ...................................................................112 Figure 3.24 Summary of homogenization temperatures and apparent salinities of fluid inclusion assemblages of the hydrothermal stages 1, 3, 4, and 5 from the Maria Vein and from the Mesa Vein. ............................................122 Figure 3.25 Fluid inclusion homogenization temperatures for primary and secondary assemblages from the Maria Vein plotted versus sample elevation. ............126 Figure 4.1 Map of southern South America with exposures of the Middle and Late Jurassic Chon Aike Province and the Carboniferous to Triassic Choiyoi Province. .......................................................................................................139 Figure 4.2 Simplified geological map of the Manantial Espejo district with rock formations and major faults and veins. .........................................................140 Figure 4.3 Schematic illustrations of the two models of formation of segmented fault arrays. ...................................................................................................141 Figure 4.4 2-dimensional sketch maps of nucleating, propagating, and linking normal faults of the isolated fault model in an extensional tectonic setting. ..........................................................................................................142 Figure 4.5 Schematic representation of an idealized elliptical fault. .............................143 Figure 4.6 Map and cross sections of the Maria Vein. ..................................................147 Figure 4.7 Long section through the Maria Vein showing contours of vein thickness. ......................................................................................................149 Figure 4.8 Maps of outcrops, hydrothermal stages, and ore-grades along the Maria Vein. .............................................................................................................151 xiv Figure 4.9 Drill hole locations in map view and in a long section through the Maria Vein. .............................................................................................................152 Figure 4.10 Strike-parallel long sections through the Maria Vein showing the distribution of the hydrothermal stages 1 to 4 from 34 drill core intervals that penetrate the Maria Vein. ......................................................................153 Figure 4.11 Cross sections perpendicular through the Maria Vein displaying drill core intervals that intersect the vein. ............................................................155 Figure 4.12 Gold- and Ag-grades in long sections of the Maria Vein. ............................161 Figure 4.13 Plots of lower hemisphere equal-area stereonet of structural data from crustiform-colloform banding and vein contacts along the Maria Vein outcrop. .........................................................................................................162 Figure 4.14 Contour map of the elevation of the lithological contact between a thick rhyodacite ignimbrite and younger volcaniclastic units along the Maria Vein, derived from more than 250 drill holes. .............................................164 Figure 5.1 Map of southern South America showing exposures of the Middle to Late Jurassic Chon Aike Province. ...............................................................176 Figure 5.2 Simplified geological map of the Manantial Espejo district showing the distribution of formations and major faults and veins. .................................177 Figure 5.3 Stratigraphy, formations, and lithological units of the Manantial Espejo district. ..........................................................................................................178 Figure 5.4 Cross sections through the Maria Vein with locations of alteration whole rock geochemistry samples. ..........................................................................181 Figure 5.5 Zoning of alteration minerals and alteration assemblages about the Maria Vein displayed in a cross section at 425W through a low-grade vein segment. ........................................................................................................184 Figure 5.6 Zoning of alteration minerals and alteration assemblages about the Maria Vein displayed in a cross section at 225W through a high-grade vein segment. ........................................................................................................185 Figure 5.7 Concentrations of V vs. TiO2 and Al2O3 vs. TiO2 in altered rocks surrounding the Maria Vein and in least altered rocks from the Manantial Espejo district. .............................................................................200 xv Figure 5.8 Major element concentrations in altered rocks surrounding the Maria Vein in two cross sections. ............................................................................201 Figure 5.9 Molar (2Ca + Na + K)/Al vs. K/Al plot for altered rocks surrounding the Maria Vein. ...................................................................................................203 Figure 5.10 Trace element concentrations in altered rocks surrounding the Maria Vein in two cross sections. ............................................................................205 Figure 5.11 Trace element concentrations in altered rocks surrounding the Maria Vein in two cross sections. ............................................................................207 Figure 5.12 Molar K/(2Ca + Na + K) ratios vs. element concentrations in altered rocks surrounding the Maria Vein. ...............................................................208 Figure 5.13 Gold- and Ag-concentrations and 100*Au/(Au+Ag) ratios vs. elevation for the Maria main vein and the Concepción main and high-grade vein. ....211 Figure 5.14 Gold- and Ag-concentrations and 100*Au/(Au+Ag) ratios in long sections of the Maria main vein and the Concepción main and high- grade vein. ....................................................................................................212 Figure 5.15 Summarized selected element concentrations in vein material from the Maria Vein and Concepción Vein. ...............................................................214 Figure 5.16 Summarized selected element ratios in vein material from the Maria Vein and Concepción Vein. ..........................................................................215 Figure 5.17 Long sections through the Maria Vein showing 100*Ag/(Ag+Cu) and 100*Sb/(Sb+As) ratios for the main hydrothermal stages. ..........................220 Figure 5.18 Long sections through the Maria Vein showing 100*Ag/(Ag+Pb) and 100*Cu/(Cu+Zn) ratios for the main hydrothermal stages. ..........................221 Figure 5.19 Pb isotopic geochemistry of feldspar separates from magmatic rocks, and hydrothermal sulfide separates from veins of the Manantial Espejo Ag(-Au) district. ...........................................................................................226 Figure 5.20 Molar (2Ca + Na + K)/Al vs. K/Al plots for altered rocks surrounding the Maria Vein in a low-grade cross section at 425W and a high-grade cross section at 225W. ..................................................................................236 xvi Figure 5.21 Gold- and Ag-concentrations, 100*Au/(Au+Ag) ratios, and vein thickness vs. elevation for the Maria Vein in a low- grade cross section at 425W and a high-grade cross section at 225W. .......................................245 Figure 6.1 Simplified geological map of the Manantial Espejo district showing the distribution of formations and major faults and veins. .................................257 Figure 6.2 Stratigraphy, formations, and lithological units of the Manantial Espejo district. ..........................................................................................................258    xvii ACKNOWLEDGEMENTS  Thanks to Pan American Silver Corp. and Silver Standard Resources Inc. for financial and logistical support, in particular to Michael Steinmann. I also thank Minera Triton S.A. for logistic support and hospitality during my field visits. Special thank goes to Eddy Escalante, Marcelo Parlapiano, Ken Konkin, David Landrum, and all employees of Minera Triton S.A. at Manantial Espejo. I acknowledge the diverse services from the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia, namely Tom Ullrich for performing the 40Ar-39Ar geochronology, Janet Gabites for performing the Pb isotope analyses, and Hai Lin for producing all heavy mineral separates for zircon dating. Joe Wooden and Frank Mazdab from the USGS-Stanford University laboratory are thanked for the supervision of the SHRIMP-RG U-Pb geochronology. I thank Teck Cominco Ltd. and Mit Tilkov for providing access to their PIMA. Jim Mortensen, Greg Dipple, and Kelly Russell are thanked for their input and constructive criticism during the comprehensive exam. I also acknowledge the diverse contributions from MDRU, and especially Karie Smith and Arne Toma are thanked for their help with technical and managerial matters. The Society of Economic Geologists is thanked for provided funding in form of two Newmont student research grants for field work, 40Ar-39Ar geochronology, and Pb isotope analysis. Special thanks to Dick Tosdal for supervising the project and support on all aspects of the Ph.D. degree, and for giving me so much freedom during the project. For making my live in Vancouver an unforgettable experience, I would like to thank all my new friends and colleagues from Vancouver and also my friends and relatives from Switzerland who came for a visit. Finally, I owe my thanks to my parents for support and encouragement during the study, and especially to Corinne Marti, who has been by my side through all these years even so we lived on different continents. xviii CO-AUTHORSHIP STATEMENT  The four chapters in this dissertation are written as manuscripts for publication in refereed journals. Each represents primarily my work and initiative, except where noted below or mentioned in the chapters. My thesis advisor Richard M. Tosdal is co-author of each manuscript and contributed in many important ways. He participated in all stages of the research including development of the project, critical questions and help in the field and during the entire project, guidance for analytical work, and editing of the manuscripts.  Chapter 2: Short-living volcanic activity in the Middle to Late Jurassic bimodal large igneous Chon Aike Province: An example from Manantial Espejo, Deseado Massif, South Argentina. Authors: Stefan Wallier, Eddy O. Escalante, and Richard M. Tosdal. Eddy Escalante was employed by Pan American Silver Corp. in order to establish a detailed geological map for exploration purposes of the Manantial Espejo district, and his field visits largely overlapped with my first field season. The map and stratigraphy presented in the chapter are the result of collaboration with Eddy Escalante. Richard Tosdal helped with interpretations of volcanic units and structures in the field. He gave advice for collecting geochronology samples, was an important part during the SHRIMP-RG age dating, and edited the manuscript. Both co-authors will provide revisions of the manuscript before publication.  Chapter 3: The Manantial Espejo epithermal Ag(-Au) deposit, Deseado Massif, Argentina: Vein parageneses, alteration, and age of mineralization. Authors: Stefan Wallier and Richard M. Tosdal. Richard Tosdal contributed with critical discussions in the field and helped with sampling strategy, access to analytical tools, and manuscript editing.    xix Chapter 4: Distribution of hydrothermal stages in the Maria Vein of the Manantial Espejo epithermal Ag(-Au) deposit, Deseado Massif, Argentina: Result of fault formation. Authors: Stefan Wallier and Richard M. Tosdal. Richard Tosdal helped with the structural background and interpretation of the field observations. His reviews improved the manuscript significantly.  Chapter 5: Alteration geochemistry, vein geochemistry, and Pb-isotopes of the Manantial Espejo epithermal Ag(-Au) deposit, Deseado Massif, Argentina. Authors: Stefan Wallier and Richard M. Tosdal. Richard Tosdal contributed to the chapter outline, helped with sampling strategies, assisted with data interpretation, and edited the chapter.                          Chapter 1 Introduction   1.1. Objectives of the dissertation  Geothermal systems linked with volcanism and faulting are responsible for quartz- adularia epithermal ore deposits, from which a range of metals are extracted. This dissertation constrains the geologic and temporal framework of the Manantial Espejo district located in the Deseado Massif, Southern Argentina, and investigates the hydrothermal alteration, mineralization, and evolution of the vein-hosted quartz-adularia epithermal precious metal deposit. The initially formulated major objectives of the project are:  • Understand the geological framework of the Manantial Espejo district in terms of stratigraphy and structures, and place them into an absolute temporal framework. • The description and discussion of the Manantial Espejo Ag(-Au) deposit concerning structures, alteration, and mineralization. • Define the evolution of the hydrothermal system with respect to vein formation, fluid parameters, zoning in geochemistry, fluid flows and pathways, and possible fluid and metal sources.  Mineralization at Manantial Espejo is hosted by and associated to volcanic rocks of the Middle to Late Jurassic Chon Aike Province, one of the world’s largest bimodal igneous provinces dominated by silicic volcanic rocks. The Ag(-Au) deposit is the second largest out of more than 30 precious metal deposits that have been discovered in the Deseado Massif during the last 30 years. All of them are related to the volcanic rocks of the Chon Aike Province, and the largest epithermal deposit is Cerro Vanguardia located 100 km to the east northeast of Manantial Espejo (Schalamuk, et al., 1997). Despite being host of several economic ore deposits, there are relatively few studies that illuminate the details of the geology and history of hydrothermal activity related to ore genesis in the Deseado 1 Massif. Yet there are a number of distinctive features of the region which make it stand out in the spectrum of epithermal deposits known world wide, and this includes the extent and length of epithermal veining and the widespread occurrence of silica sinters and surface- near silica bodies indicating preservation of a Mesozoic paleo-surface. This dissertation is one of the first studies that provides detailed and comprehensive information about the geologic framework and mineralization of such a deposit. The establishment of the volcanic stratigraphic sequence and temporal framework of key units and mineralized veins represents an important contribution to the understanding of the Chon Aike Province and the related precious metal deposits. Furthermore it adds to the general understanding of epithermal deposits associated to adularia-sericite alteration. Topics discussed are the formation and evolution of veins in active faults, the paragenetic sequence, fluid temperature and salinity constraints from fluid inclusions, zoning of alteration mineralogy and alteration geochemistry in volcanic rocks surrounding the veins, characteristics of the geochemical composition and zoning of the diverse vein infill, and constraints on metal sources based on radiogenic Pb isotopes.   1.2. Overview of the Manantial Espejo district and its Ag(-Au) deposit  The Manantial Espejo deposit is a medium-sized Ag deposit with by-product Au. It is located in the Santa Cruz Province of southern Argentina (lat 48° 48’ S, long 69° 30’ W) at the southern rim of the Deseado Massif. The closest towns are Gobernador Gregores about 60 km to the west and Puerto de San Julián about 160 km to the east southeast located at the Atlantic coast. Silver mineralization was originally discovered in the 1970’s by the Argentine government, but exploration drilling did not start until 1991. In 2006, the Pan American Silver Corp. became the 100 % owner of the project, and the deposit entered production in the second half of 2008. Exploration prior to the onset of mining revealed a proven and probable reserve of 7.19 Mt at 166 g/t Ag and 2.36 g/t Au totaling 38.5 Moz or 1200 t Ag and 0.55 Moz or 17 t Au (December 31, 2007). The landscape of the district is characterized by moderate topography of rolling hills with elevations mostly between 300 and 400 m above sea level. Strong, constant, and dry 2 westerly winds cause an arid climate with scarce vegetation, leading to good exposure of rocks and veins. The veins are commonly located in morphological highs where vein textures can be studied in outcrop. The more than 1000 drill holes provide close to 100 km of drill core. The four most mineralized vein systems that are subject of mining are densely drilled with an average spacing of about 25 m. Most drill core is stored and accessible in the exploration camp a few kilometers to the southwest of the deposit. Good exposure, availability of drill cores, and the advanced stage of exploration made Manantial Espejo an excellent location to investigate the volcanic succession and the epithermal environment that created an economic Ag-Au deposit. Manantial Espejo belongs to the Chon Aike Province, which consists of numerous formations in Argentina, Chile, and the Antarctic Peninsula. The volcanic activity spanned more than 30 Ma between Early and Late Jurassic (Pankhurst et al., 1998; Pankhurst et al., 2000). With an estimated volume of 235,000 km3, the province compares with the large silicic igneous province of the Tertiary Sierra Madre Occidental in Mexico or even to continental flood basalt provinces (Pankhurst et al., 1998; Riley and Leat, 1999; Pankhurst et al., 2000; Riley et al., 2001). Since the first major work presented by Feruglio (1949), researchers have been working in the Chon Aike Province for over 50 years collecting valuable petrographic, stratigraphic, geochronologic, and geochemical data, but only few of them have worked at the district-scale or their research is not public. In 1965, Cazeneuve (1965) performed the first radiometric age dating (K-Ar method) on a volcanic tuff from the Chon Aike Formation and confirmed the Middle Jurassic age that had been based on paleontology. Since then, several ages have been calculated by K-Ar, Ar-Ar, Rb-Sr, and U- Pb methods, and province-wide geochronology studies are discussed by Féraud et al. (1999) and Pankhurst et al. (2000).   1.3. Epithermal ore deposits  Epithermal deposits are important sources of Au and Ag. About 6 % of all Au and about 16 % of all Ag mined have come from epithermal deposits, and their wide range of tonnage-grade characteristics make them an attractive target for both large and small 3 exploration and mining companies (Singer, 1995; Simmons et al., 2005). The term epithermal derives from the genetic classification of hydrothermal ore deposits proposed by Lindgren (1933) and includes the shallow and low-temperature environment of high- temperature hydrothermal systems. Epithermal deposits form from the surface to as deep as about 1.5 km, at temperatures of less than about 300°C, and most deposits form in a temperature range of about 160° to 270°C (e.g. Sillitoe, 1993; Hedenquist and Lowenstern, 1994; Hedenquist et al., 2000). Beside Au and Ag, epithermal deposits may also be important for Zn, Pb, Cu, Hg, As, Sb, S, kaolinite, alunite, and silica (Hedenquist et al., 2000). Fault hosted veins are the most common orebodies. Fewer deposits are hosted by hydrothermal breccias and diatremes, but many of these are among the largest epithermal deposits. Several classifications have been proposed in order to describe epithermal ore deposits. The most common classification is the sub-division of low- and high-sulfidation deposits, which largely corresponds to geothermal and volcanic- or magmatic-hydrothermal systems, respectively (Fig. 1.1). The terms low- and high-sulfidation describe the two end member sulfidation states derived from the sulfide mineral assemblages. Hedenquist (1987) used these terms initially to describe the oxidation state of the sulfur in the fluid, but since this is impractical for field observations, the terms are now used to refer to the sulfidation state of the sulfide assemblages (Hedenquist et al., 2000). Another classification type is based on gangue mineralogy and alteration mineral assemblages (Berger and Henley, 1989). The adularia-sericite or quartz-adularia type largely corresponds to low-sulfidation deposits and has alteration mineral assemblages with adularia and sericite, and characteristic gangue minerals are quartz, adularia, and carbonates. The alunite-kaolinite type matches the characteristics of high-sulfidation epithermal deposits and has alteration mineral assemblages with quartz-alunite-kaolinite. Low-sulfidation epithermal systems are associated to geothermal systems and form from near-neutral pH and reduced fluids that are dominated by meteoric water, and which are greatly in equilibrium with the altered host rock, resulting in a rock-dominated system (Fig. 1.1A; Giggenbach, 1992a; Sillitoe, 1993). Geothermal systems typically form in some distance from a volcanic edifice, but they are also present in areas without contemporaneous volcanic activity. Low-sulfidation deposits are normally related to 4     Figure 1.1: Simplified conceptual models for high-temperature hydrothermal systems, showing the relationship between epithermal environments, magmatic intrusions, fluid circulation paths, and volcanic and basement host rocks (from Simmons et al., 2005). A. The epithermal environment forms in a geothermal system dominated by near-neutral pH chloride waters, where there is a strong flux of deeply circulated water (mostly of meteoric origin), containing CO2, NaCl, and H2S. This type of system is analogous to those exploited for generation of electricity (e.g. Simmons and Browne, 2000). B. The epithermal environment forms in a magmatic-hydrothermal system dominated by acid fluids, where there is a strong flux of magmatic liquid and vapor, containing H2O, CO2, HCl, H2S, and SO2, with variable input from local meteoric water. This type is analogous to those existing in modern volcanoes (e.g. Hedenquist et al., 1993; Christenson and Wood, 1993). The inferred location of the underlying magma chambers in both (A) and (B) are portrayed to show the different path length that deep fluids traverse before encountering the ore-forming environment. The relatively short path to the epithermal environment in (B) means there is minimal water-rock interaction during ascent, whereas the relatively long path to the epithermal environment in (A) means there is considerable water-rock interaction during ascent. The maximum pressure-temperature gradient under hydrostatic conditions is represented by boiling point for depth (BPD) temperatures, which are also shown for reference.    5 subaerial bimodal (basalt to rhyolite) volcanic suites in a wide range of extensional tectonic settings such as intra-, near-, and back-arc, and post-collisional rifts (Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003). In most cases, intrusions likely located at more than 4 km below the surface drive the geothermal system (Hedenquist et al., 2000). The ore deposits are characterized by sulfide assemblages of pyrite-pyrrhotite-arsenopyrite and Fe- rich sphalerite that are indicative for a relatively low sulfidation state of the fluid (Hedenquist et al., 2000; Einaudi et al., 2003). They are normally associated with quartz, adularia, calcite, and sericite as the major gangue minerals. The alteration halo typically shows a mineral zoning of proximal adularia and illite to distal interlayered illite-smectite, smectite, and chlorite. High-sulfidation epithermal deposits are related to volcanic-hydrothermal systems and are formed close to volcanic vents (Fig. 1.1B). Their surface expressions are high- temperature fumaroles and related condensates of extremely acid water. The fluids are acid and oxidized and originate from SO2-rich magmatic volatiles mixed with varying amounts of meteoric water. The fluids are far from equilibrium with the host rock (Giggenbach, 1992b). Associated intrusions might be very shallow or even erupting to the surface. High- sulfidation deposits contain a mineral assemblage of pyrite-enargite-luzonite-covellite with a high sulfur/metal ratio. They are accompanied by advanced argillic alteration of a typical quartz-alunite-kaolinite assemblage, in addition to pyrophyllite, diaspore, and zunyte. Most high-sulfidation deposits are formed in calc-alkaline andesitic-dacitic arcs in a near-neutral stress state or in mild extension. They can show a close relationship with porphyry Cu deposits (Sillitoe and Hedenquist, 2003). The term intermediate-sulfidation deposits has been introduced by Hedenquist et al. (2000), Einaudi et al. (2003), and Sillitoe and Hedenquist (2003) in order to distinguish between deposits derived from geothermal systems and deposits that, despite sharing the same alteration assemblages, show a closer relationship with volcanic-hydrothermal systems. Such deposits have sulfide mineral assemblages that include tennantite, tetrahedrite, chalcopyrite, and Fe-poor sphalerite, which reflect a sulfidation state of the mineralizing fluid that is intermediate between low- and high-sulfidation (Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003). Many intermediate-sulfidation epithermal deposits are generated under similar tectonic and magmatic conditions as high-sulfidation deposits, 6 but they commonly do not show a close connection with porphyry Cu deposits (Sillitoe and Hedenquist, 2003). On the other hand, most “classic” low-sulfidation deposits also show at least locally characteristics of intermediate-sulfidation deposits. Especially the Ag-rich examples of quartz-adularia epithermal deposits such as most deposits in the Deseado Massif including Manantial Espejo might therefore rather be called low- to intermediate- sufidation deposits.   1.4. Characteristics of quartz-adularia epithermal Au-Ag deposits  Hydrothermal processes that form quartz-adularia deposits are likely similar to active geothermal systems such as the Taupo Volcanic Zone in New Zealand (e.g. Henley and Ellis, 1983; Hedenquist and Henley, 1985a and b; Brown, 1986; Hedenquist, 1990; Simmons and Brown, 2000; and Simmons et al., 2005). Quartz-adularia deposits and their active equivalents show a wide variety of characteristic ore and gangue mineral textures such as colloform and crustiform banding, druse-lined cavities, bladed calcite, and multiple-episode vein breccias (White and Hedenquist, 1995; Dong et al., 1995). Quartz is commonly intergrown with adularia and calcite. However, calcite is mostly replaced by quartz as the system cools. In many “classic” low-sulfidation deposits, electrum is associated with pyrite, marcasite, arsenopyrite, acantite/argentite, Ag-sulfosalts, and Ag- selenides. Minor chalcopyrite, Fe-rich sphalerite, and pyrrhotite are locally present. The ore zone has commonly a vertical extent of less than 400 m and the total sulfide content is typically less than 2 volume % (Sillitoe, 1993; Sillitoe and Hedenquist, 2003). Characteristic alteration assemblages are illite ± adularia proximal to the vein, smectite/mixed-layer clay ± chlorite (argillic or intermediate argillic), and chlorite-calcite ± epidote (propylitic) with increasing distance to the vein (Sillitoe, 1993). Advanced argillic alteration is formed close to the surface and includes kaolinite-alunite-(illite/smectite-native sulfur) ± opaline blankets of steam heated origin, commonly associated to chalcedony blankets (Hedenquist et al., 2000). Distinctive silica sinter deposits are locally preserved. They were formed by near-neutral pH hot spring waters and are characterized by 7 rhythmically banded textures with vertical growth structures and commonly contain abundant plant fragments (White et al., 1989; White and Hedenquist, 1995).   1.5. Overview of the dissertation  This dissertation is presented as four chapters (Chapters 2 to 5), each of which represents a manuscript to be submitted to a refereed journal for publication. Field work and sample collection represents an important part of the project and was undertaken during two field seasons. The first season was during five month between November, 2004, and April, 2005. The second season was during two month between February and April, 2006. Mapping volcanic rocks and veins in scales from 1:500 to 1:5000 is the major part of the field work. Drill core examination with focus on the vein intervals constitutes another major element. A total of about 1000 samples have been collected and sent to Vancouver for diverse analyses. The focus of each chapter lies on different aspects of the geology and mineralization of the Manantial Espejo district. However, some overlap and repetition between the chapters could not be avoided because each contribution has been prepared as a stand-alone publication.  Chapter 2 describes the geologic setting of the Manantial Espejo district. The volcanic stratigraphy was established by extensive mapping accompanied by drill core examination. The presented geology map and stratigraphy is the result of collaboration with Eddy Escalante and other exploration geologists from Minera Triton S.A., Pan American Silver Corp., and Silver Standard Resources Inc. In addition, U-Pb (SHRIMP-RG on zircon) and 40Ar-39Ar (hornblende) ages from igneous rocks place an absolute temporal framework on the stratigraphic sequence. All heavy mineral separates for zircon dating were produced by Hai Lin from the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia, Vancouver, Canada. SHRIMP-RG work on seven samples was conducted under the supervision of Joseph Wooden and Frank Mazdab at the USGS- Stanford facility, California, during two separate 5 to 6 day periods. One hornblende age was determined by Tom Ullrich at the Pacific Centre for Isotopic and Geochemical 8 Research (PCIGR) at the University of British Columbia, Vancouver. Richard Tosdal helped with constructive geological discussions and with editing of the chapter.  Chapter 3 deals with the Manantial Espejo Ag(-Au) deposit. Structure, alteration mineral assemblages, the paragenetic sequence with vein textures, ore, and gangue minerals, fluid properties based on fluid inclusions, and age relations are discussed for several veins with focus on the Maria Vein, the largest orebody of the district. The descriptions are based on outcrop studies and detailed drill core examinations. Alteration mineral assemblages were determined using the Teck Cominco Ltd. PIMA facility in Vancouver and by X-ray diffraction (XRD) at the micro-beam facility at the University of British Columbia, Canada. Vein samples from outcrops and mostly from drill cores have been collected for petrography, fluid inclusions, and 40Ar-39Ar geochronology. The preparation of polished thick sections and microthermometry on fluid inclusions was performed at the University of British Columbia, Vancouver. Seven adularia ages from three veins were determined by Tom Ullrich at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia, Vancouver. Richard Tosdal contributed with constructive geological discussions as well as to the editing of the chapter.  Chapter 4 focuses on the formation and evolution of the Maria Vein. It is tested if there is a link between the distribution of hydrothermal stages and fault nucleation and fault propagation. It is further discussed that combining the structural model of fault formation and the timing of when metals were added to the hydrothermal system can explain why and predict if a vein or vein segment is high-grade, low-grade, or barren. This chapter is based on mapping of hydrothermal stages along the entire Maria Vein outcrop and drill core interpretation. Richard Tosdal helped with insightful discussions and with the editing of the chapter.  Chapter 5 consists of three parts. The first part investigates the whole-rock geochemical characteristics and element zoning in volcanic rocks surrounding the Maria Vein. The second part deals with the geochemical characteristics of vein material from four 9 different veins. The focus lies on the Maria Vein where the samples are subdivided into the hydrothermal stages and distributed along the entire vein. The third part constrains the Pb isotopic compositions of magmatic feldspars from host rocks and hydrothermal sulfides from mineralized veins in order to obtain information about potential sources for the precious metals in the veins. The first two parts are based on drill core examinations and whole-rock multi element geochemical analyses performed by ALS Chemex, a commercial laboratory in North Vancouver, Canada. The Pb isotopic compositions were determined by Janet Gabites at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia, Vancouver. Richard Tosdal contributed by way of constructive discussions as well as to the editing of the chapter.  10 1.6. References  Berger, B. R., and Henley, R. W., 1989, Advances in the understanding of epithermal gold- silver deposits, with special reference to the western United States: Economic Geology Monograph 6, p. 405-423. Brown, K. L., 1986, Gold deposition from geothermal discharges in New-Zealand: Economic Geology, v. 81, p. 979-983. Cazeneuve, H., 1965, Datación de una toba de la Formación Chon Aike (provincia de Santa Cruz) por el método Potasio – Argón: Revista Ameghiniana, Asociación Paleontológica Argentina, v. 4, p. 156-158. Christenson, B. W., and Wood, C. P., 1993, Evolution of a vent-hosted hydrothermal system beneath Ruapehu Crater Lake, New Zealand: Bulletin of Volcanology, v. 55, p. 547-565. Dong, G., Morrison, G., and Jaireth, S., 1995, Quartz textures in epithermal veins, Queensland - Classification, origin, and implication: Economic Geology, v. 90, p. 1841-1856. Einaudi, M., Hedenquist, J. W., and Inan, E., 2003, Sulfidation state of fluids in active and extinct hydrothermal systems: Transitions from porphyry to epithermal environments: SEG Special Publication, v. 10, p. 285-313. Féraud, G., Alric, V., Fornari, M., Bertrand, H., and Haller, M., 1999, Ar-40/Ar-39 dating of the Jurassic volcanic province of Patagonia: migrating magmatism related to Gondwana break-up and subduction: Earth and Planetary Science Letters, v. 172, p. 83-96. Feruglio, E., 1949, Descripción geológica de la Patagonia: Buenos Aires, 800 p. Giggenbach, W. F., 1992a, Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin: Earth and Planetary Science Letters, v. 113, p. 495-510. Giggenbach, W. F., 1992b, Magma degassing and mineral deposition in hydrothermal systems along convergent plate boundaries: Economic Geology, v. 87, p. 1927- 1944. Hedenquist, J. W., 1987, Volcanic-related hydrothermal systems in the Circum-Pacific basin and their potential for mineralisation: Mining Geology (Kozan Chishitsu), v. 37, p. 347-364. Hedenquist, J. W., 1990, The thermal and geochemical structure of the Broadlands-Ohaaki geothermal system: Geothermics, v. 19, p. 151-185. Hedenquist, J. W., Simmons, S. F., Giggenbach, W. F., and Eldridge, C. S., 1993, White Island, New Zealand, volcanic hydrothermal system represents the geochemical environment of high sulfidation Cu and Au ore deposition: Geology, v. 21, p. 731- 734. 11 Hedenquist, J. W., Arribas, A., and Gonzalez-Urien, E., 2000, Exploration for epithermal gold deposits: SEG Reviews, v. 13, p. 245-277. Hedenquist, J. W., and Henley, R. W., 1985a, Hydrothermal eruptions in the Waiotapu geothermal system, New Zealand: Economic Geology, v. 80, p. 1640-1668. Hedenquist, J. W., and Henley, R. W., 1985b, The importance of CO2 on freezing-point measurements of fluid inclusions - Evidence from active geothermal systems and implications for epithermal ore deposition: Economic Geology, v. 80, p. 1379-1406. Hedenquist, J. W., and Lowenstern, J. B., 1994, The role of magmas in the formation of hydrothermal ore-deposits: Nature, v. 370, p. 519-527. Henley, R. W., and Ellis, A. J., 1983, Geothermal systems ancient and modern - a geochemical review: Earth-Science Reviews, v. 19, p. 1-50. Lindgren, W., 1933, Mineral deposits: New York, McGraw-Hill, 4th ed., 930 p. Pankhurst, R. J., Leat, P. T., Sruoga, P., Rapela, C. W., Marquez, M., Storey, B. C., and Riley, T. R., 1998, The Chon Aike province of Patagonia and related rocks in West Antarctica: A silicic large igneous province: Journal of Volcanology and Geothermal Research, v. 81, p. 113-136. Pankhurst, R. J., Riley, T. R., Fanning, C. M., and Kelley, S. P., 2000, Episodic silicic volcanism in Patagonia and the Antarctic Peninsula: Chronology of magmatism associated with the break-up of Gondwana: Journal of Petrology, v. 41, p. 605-625. Riley, T. R., and Leat, P. T., 1999, Large volume silicic volcanism along the proto-Pacific margin of Gondwana: lithological and stratigraphical investigations from the Antarctic Peninsula: Geological Magazine, v. 136, p. 1-16. Riley, T. R., Leat, P. T., Pankhurst, R. J., and Harris, C., 2001, Origins of large volume rhyolitic volcanism in the Antarctic Peninsula and Patagonia by crustal melting: Journal of Petrology, v. 42, p. 1043-1065. Schalamuk, I. B., Zubia, M., Genini, A., and Fernandez, R. R., 1997, Jurassic epithermal Au-Ag deposits of Patagonia, Argentina: Ore Geology Reviews, v. 12, p. 173-186. Sillitoe, R., and Hedenquist, J. W., 2003, Linkages between volcanotectonic settings, ore- fluid composition, and epithermal precious metal deposits: Economic Geology, Special Publication 10, p. 315-343. Sillitoe, R. H., 1993, Epithermal models: Genetic types, geometrical controls and shallow features: Geological Association of Canada Special Paper 40, p. 403-417. Simmons, S. F., and Browne, P. R. L., 2000, Hydrothermal minerals and precious metals in the Broadlands-Ohaaki geothermal system: Implications for understanding low- sulfidation epithermal environments: Economic Geology, v. 95, p. 971-999. Simmons, S. F., White, N. C., and John, D. A., 2005, Geological characteristics of epithermal precious and base metal deposits, Economic Geology 100th Anniversary Volume, p. 485-522. Singer, D. A., 1995, World class base and precious metal deposits - a quantitative analysis: Economic Geology, v. 90, p. 88-104. 12 White, N. C., and Hedenquist, J. W., 1995, Epithermal gold deposits: styles, characteristics and exploration: SEG Newsletter, p. 1-13. White, N. C., Wood, D. G., and Lee, M. C., 1989, Epithermal sinters of Paleozoic age in North Queensland, Australia: Geology, v. 17, p. 718-722.   13 Chapter 2 Short-living volcanic activity in the Middle to Late Jurassic bimodal large igneous Chon Aike Province: An example from Manantial Espejo, Deseado Massif, South Argentina1   2.1. Introduction  The Manantial Espejo district is situated in the Middle to Late Jurassic Chon Aike Province, a large bimodal igneous province dominated by silicic volcanic rocks. Consisting of numerous formations in Argentina, Chile, and extending to the Antarctic Peninsula, and with an estimated volume of 235,000 km3, the Chon Aike Province compares to the large silicic igneous province of the Tertiary Sierra Madre Occidental in Mexico or even to continental flood basalt provinces (Pankhurst et al., 1998; Riley and Leat, 1999; Pankhurst et al., 2000; Riley et al., 2001). In the Deseado Massif, the Chon Aike Province hosts more than 30 epithermal Ag-Au deposits of which several are of economic size. However, despite its extent and economic importance, the volcanostratigraphic and geologic framework of the province is not very well constrained. Researchers have been working in the Chon Aike Province for over 50 years collecting valuable petrographic, stratigraphic, geochronologic, and geochemical data, but only few of them have worked at the district- scale or their research is not public. The goal of this paper is to describe and reconstruct the geologic setting of the Manantial Espejo district and place a detailed temporal framework on it. We present field observations in addition to U-Pb (SHRIMP-RG on zircon) and 40Ar- 39Ar (hornblende) ages from igneous rocks that allow defining the district stratigraphic sequence.     1 A version of this chapter will be submitted for publication. Wallier, S., Escalante, E.O. and Tosdal, R.M. Short-living volcanic activity in the Middle to Late Jurassic bimodal large igneous Chon Aike Province: An example from Manantial Espejo, Deseado Massif, South Argentina.  14 2.2. Geological setting of the Chon Aike Province  The large bimodal igneous Chon Aike Province is exposed in the North Patagonian and the Deseado Massifs, in a relatively narrow band parallel to the trend of the Andes, and in the Antarctic Peninsula (Fig. 2.1; Kay et al., 1989; Pankhurst et al., 1998). Most common rock types of the province are rhyolites, but it has consistent bimodality between rhyolite and andesite/basaltic andesite (Pankhurst et al., 1998; Riley and Leat, 1999). The province is underlain by metamorphic basement rocks of Precambrian or Cambrian age, Permo- Triassic sedimentary and plutonic rocks, and volcanic and plutonic rocks of the Triassic to Jurassic Choiyoi Formation (Pankhurst et al., 1998). The most voluminous formations of the Chon Aike Province are the Marifil Formation in the North Patagonian Massif, the Chon Aike Formation in the Deseado Massif, and the El Quemado, Ibañez and Tobífera Formations along the Andean Cordillera (Fig. 2.1). Based on U-Pb, Rb-Sr, and 40Ar-39Ar ages, Pankhurst et al. (2000) propose that the emplacement of the Chon Aike Province spanned more than 30 Ma between Early and Late Jurassic, with three episodes of peak activity defined as 188-178 Ma (V1), 172-162 Ma (V2) and 157-153 Ma (V3). The oldest units of the Chon Aike Province included in V1 occur in the Marifil Formation in the North Patagonian Massif and in formations in the southern Antarctic Peninsula. Event V1 coincides with the Karoo-Ferrar mafic magmatism of South Africa, Antarctica and Tasmania. The second volcanic episode, V2, is localized along the eastern part of the Deseado Massif and the eastern coast of the northern Antarctic Peninsula. Episode V3 comprises the eastern Andean outcrops of ignimbrites and associated granite intrusions of the El Quemado, Ibañez, and Tobífera Formations. Based on major and trace element compositions and Sr, Nd, and O isotopic studies, Riley et al. (2001) proposed that vast parts of the Chon Aike volcanic rocks have been generated as the result of anatexis of hydrous lower crust, which mixed with fractionated components of arc-related mafic underplating. Long-term storage developed an isotopically uniform magma, of which fractionated magmas migrated to upper-crustal level magma chambers where they assimilated upper-crustal material and eventually erupted to the surface. However, a single petrogenetic model for the entire province is unlikely due to the large variations in age, chemistry and geological setting (Pankhurst et al., 1998). Conditions 15        Figure 2.1: Map of southern South America showing exposures and major formations of the Middle to Late Jurassic Chon Aike Province and older magmatic rocks (modified after Pankhurst et al., 1995, and Pankhurst et al., 1998). CAF = Chon Aike Formation. DM = Deseado Massif. EQF = El Quemado Formation. IF = Ibañez Formation. MF = Marifil Formation. NPM = North Patagonian Massif. TF = Tobífera Formation. Dotted areas outline the San Jorge and Magellan Basins on land and their projections offshore (Diraison et al., 1997).  16 which are favorable for the generation and emplacement of large amounts of silicic volcanic rocks may have been created by lithospheric extension marked by normal faults, horst and graben formation, and block rotation related to the Gondwana break-up, intrusion of basalts into the crust, and re-working of previously differentiated and probably crustal sources. Mantle plume impact and back-arc extension along the paleo-Pacific subduction zone are proposed to drive lithospheric extension (Kay et al., 1989; Pankhurst and Rapela, 1995; Pankhurst et al., 1998; Riley and Leat, 1999; Pankhurst et al., 2000; Riley et al., 2001).   2.3. Geological setting of the Deseado Massif  Rimed by the Cretaceous basins of San Jorge to the north and Magellan to the southwest (Fig. 2.1; Diraison et al., 1997), the Deseado Massif is located in the northern part of the Santa Cruz Province, Argentina, in central Patagonia (Fig. 2.2). Outcrops of basement rocks underneath the Chon Aike Province are restricted to relatively small areas mainly along the Deseado River in the northern part of the massif. Basement rocks consist of a wide variety of crystalline and sedimentary sequences including metapelites with Neoproterozoic-Early Cambrian detrital zircons, Ordovician to Carboniferous granitoid rocks, Permo-Triassic conglomerates, and earliest Jurassic plutonic rocks (Pankhurst et al. 1998; Pankhurst et al., 2003). Early to Middle Jurassic epiclastic and pyroclastic rocks and basalts of the Roca Blanca Formation (Fig. 2.2) outcrop in the central part of the Deseado Massif (Stipanicic and Bonetti, 1970; Pankhurst et al, 1998). They are overlain by the Bajo Pobre Formation which is composed of mafic and intermediate rocks (basaltic andesite and andesite lavas, tuffs and intrusive rocks). Although the mafic and intermediate rocks have a much smaller areal distribution compared to the silicic rocks, they define the bimodal magmatic character of the province. Field relations generally indicate that the Bajo Pobre Formation is overlain by the Chon Aike Formation, but geochronology by Alric et al. (1996) and Féraud et al. (1999) suggest that on a province-scale, the emplacement of mafic/intermediate and silicic rocks may overlap in age. 17        Figure 2.2: Simplified regional geological map of the Deseado Massif (modified from Panza and Nullo, 1994; Schalamuk et al., 1997; Echeveste, 2005).  18 The Chon Aike Formation is the most prominent formation of the Deseado Massif, where it covers an area of some 100,000 km2 and reaches a thickness of more than 500 m (Pankhurst et al., 2000; Riley et al., 2001; Sharpe et al, 2002). The dominant units are silicic ignimbrites (85 % of the outcrops), with subordinate re-sedimented volcaniclastic deposits, air fall tuffs, intercalated lavas, dikes and rhyolitic domes (Pankhurst et al., 1998). The sequence is associated with reworked volcaniclasitc material referred to as the La Matilde Formation, which can be combined with the Bajo Pobre and Chon Aike Formations to form the Bahía Laura Group (Feruglio, 1949; Stipanicic and Reig, 1957; Lesta and Ferello, 1972; Pankhurst et al., 1998). Locally, the Chon Aike Formation is overlain and intruded by units of intermediate composition (andesitic to dacitic rocks). These rocks lack a proper formation name or were generally included into the Bajo Pobre Formation. Because of the stratigraphic position above the silicic rocks, such rocks at Manantial Espejo are separated from the Bajo Pobre Formation, and are simply referred to as the “intermediate formation”. The Jurassic igneous units are unconformably overlain by continental sedimentary rocks of the Bajo Grande (Late Jurassic to Early Cretaceous) and Baqueró (Early Cretaceous) Formations, and in the northern part of the Deseado Massif by the Early to Late Cretaceous Chubut Group (Feruglio, 1949; Teruggi and Rossetto, 1963; Archangelsky, 1967; De Giusto et al., 1980; Hechen and Homovc, 1985; Panza and Nullo, 1994). Repeated transgressions during the Tertiary accumulated a shallow water sequence which is well exposed in eastern and western Patagonian regions. In the Deseado Massif, these fossiliferous horizons of the informally named “Patagonian or Patagoniense” beds are divided into the Oligocene San Julián and the Miocene Monte Leone Formations (Feruglio, 1949; Del Río, 2004; Parras and Casadío, 2006). These sedimentary rocks are in turn overlain by the Late Miocene continental Santa Cruz Formation. At about the same time, extensive Neogene plateau lavas were deposited in the eastern, central and northern part of the Deseado Massif. These 12 to 2 Ma old basalts have been related to “slab windows” associated with the collision of the Chile Ridge with the Chile Trench at 12 Ma (Gorring et al., 1997; Gorring and Kay, 2001).   19 2.4. Geology of the Manantial Espejo epithermal Ag(-Au) district  The Manantial Espejo epithermal Ag(-Au) district is located in the south Deseado Massif, about 160 km west northwest of Puerto de San Julián and about 60 km east of Gobernador Gregores in the Santa Cruz Province, south Argentina (Fig. 2.1). Mapping of about 32 km2 at mainly 1:2000 scale around the Manantial Espejo deposits supported by drill core studies resulted in the definition of a district-scale stratigraphic sequence (Fig. 2.3 and 2.4). The stratigraphic nomenclature used herein is according to Pankhurst et al. (1998) and references therein where appropriate. At Manantial Espejo, the Chon Aike Formation is a sequence of rhyodacitic to rhyolitic volcaniclastic and coherent volcanic units which are unconformably overlain by a discontinuous and heterogeneous horizon of volcanogenic sedimentary deposits of the La Matilde Formation. Intermediate rocks (andesites and dacites) are younger than the La Matilde Formation, and therefore they are not assigned to the Bajo Pobre Formation, but rather combined to the informally named “intermediate formation” (Fig. 2.4). Appendix A provides more detailed characteristics of the described units.  2.4.1. District stratigraphic sequence  2.4.1.1. Basement rocks Basement rocks underlying the volcanic suite of the Chon Aike Province do not outcrop in the district of Manantial Espejo. Nevertheless, fine-grained tightly folded quartz- mica schist xenoliths in andesite flows and intrusions and clasts in pyroclastic rocks are locally present (Fig. 2.5A). In a porphyritic andesite, locally present xenoliths of coarse- grained polycrystalline and fluid inclusion-rich quartz might originate from quartz veins in the basement.  2.4.1.2. Bajo Pobre Formation In the Deseado Massif, the Bajo Pobre Formation is described as suite of mafic and intermediate rocks (basaltic andesite and andesite lavas, tuffs and intrusive rocks) that are overlain by the Chon Aike Formation (Schalamuk et al., 1997; Pankhurst et al, 1998). 20        Figure 2.3: Satellite image from the Manantial Espejo district showing major faults (dashed where inferred). The white outlined areas represent mapped areas; the light colored area corresponds to the geology map in Figure 2.4. The dotted lines show the location of the two cross sections in Figure 2.7; the white dotted area represents the central vein zone. Star symbols indicate the location of the geochronology samples presented in this paper (1 = ME-SW-05-109; 2 = ME-SW-05-111; 3 = ME-SW-06-20; 4 = ME-SW-05-105; 5 = ME-SW- 06-54; 6 = ME-SW-04-32; 7 = ME-SW-05-118; 8 = ME-SW-06-30).  21   Figure 2.4: Geologic map of the central part of the Manantial Espejo district. Contour interval = 5 m, Gauss- Kruger (DHDN, zone 2) coordinates. The legend shows the stratigraphic sequence with ages determined by U-Pb SHRIMP-RG method on zircons (bold) and by 40Ar-39Ar geochronology on hornblende (italic). For abbreviations see text. 22  Figure 2.5: Typical rock textures and outcrops of the Chon Aike Formation in the Manantial Espejo district. A. Medium-grained rhyodacite ignimbrite (MBR) with bimodal-sized quartz and eutaxitic texture defined by abundant small fiammes; weak chloritic alteration; clast of schist from the basement. B. Coarse-grained rhyodacite ignimbrite (CBR), weak sericite alteration. C. Compound unit of equant-quartz rhyolite ignimbrite (EQR) overlaying lithic crystal tuff (LXT); weathering resistant layers are densely welded. D. Closer view of densely welded layer of EQR in photo C showing coarse columnar jointing. E. Equant-quartz rhyolite ignimbrite (EQR), weak sericite alteration. F. Top facies of equant-quartz rhyolite ignimbrite (EQR) with large circular vugs of leached undeformed pumice, intense adularia alteration. G. Equant- and bimodal-quartz rhyolite ignimbrite (EBR) with commonly equant-shaped and bimodal-sized quartz. H. Quartz-sanidine rhyolite dike (SRI), 20 m wide dike with a core showing sub-horizontal columnar joints. 23 However, none of the intermediate rocks in the Manantial Espejo district belongs to the Bajo Pobre Formation, but crystal-poor plagioclase porphyritic andesite xenoliths in andesitic units that overlie the Chon Aike Formation are tentatively identified as derived from the Bajo Pobre Formation, suggesting that it might also be present underneath the Chon Aike Formation.  2.4.1.3. Chon Aike Formation The Chon Aike Formation is composed of a sequence of rhyodacitic to rhyolitic ignimbrites and ash horizons with a total thickness of probably more than 400 m, concluded by the emplacement of rhyolitic dikes and domes. The base of the Chon Aike Formation does not outcrop in the Manantial Espejo district. Bimodal quartz rhyodacite ignimbrites and associated breccias (BQR including MBR, FBR, and CBR): The lowest outcropping unit of the Chon Aike Formation is a more than 250 m thick rhyodacite that contains abundant quartz and ferromagnesian minerals. Three sub-units are distinguished sharing similar mineralogy, mineral ratios, and the characteristic shard-shaped and bimodal-sized quartz fragments. The most voluminous sub- unit is a variably welded crystal-rich and medium-grained bimodal quartz rhyodacite ignimbrite (MBR; Fig. 2.5A) with pumice fiamme of variable aspect ratio. Densely welded zones show a eutaxitic texture and partial devitrification. Lapilli-sized lithic fragments of basement rocks are locally present (Fig. 2.5A). The top of MBR grades into a finer-grained rhyodacite (FBR) with less pumice and variable amounts of crystals and crystal fragments. To the north of the central vein zone (Fig. 2.3), the gradation from MBR to FBR passes through a clast-supported monomictic breccia of up to boulder-sized MBR clasts in FBR matrix. The breccia becomes more matrix-rich to the north and eventually grades into FBR. The third sub-unit is a coarse-grained facies (CBR; Fig. 2.5B), a more local but mappable feature within the upper part of MBR. Quartz and feldspar crystals and fragments are of up to 1 cm in long dimension. Lithic crystal tuff (LXT): This unit is a variably intercalated crystal- and lithic-rich lithified ash with horizons of very fine ash that locally contains beds of accretionary lapilli. The thickness of the tuff commonly varies between centimeters and about 30 m, but may reach 80 m locally. The lithic clasts are usually less than 1 cm and contribute 1 to 5 % to 24 the rock. Accretionary lapilli are commonly smaller than 1 cm, but locally reach several centimeters in diameter. They occur in centimeters to tens of centimeters thick horizons within a very fine ash. Equant quartz rhyolite ignimbrite (EQR): In the central vein zone (Fig. 2.3), the equant quartz rhyolite is an apparently single, up to 30 m (locally up to 100 m) thick ignimbrite with a crystal-poor but lithic-rich base, a densely welded and crystal-rich center, and a pumice-rich weakly welded top. However, in the southern part of the district, EQR consist of two thinner cooling units of the same internal structure but divided by a horizon of LXT, or it consists of a sequence of two or three 1 to 5 m thick densely welded layers with columnar joints interbedded with crystal-poor but lithic- and pumice-rich, up to 5 m thick layers (Fig. 2.5C and D). The basal part contains 1 to 10 % lapilli-sized lithics and has 15 to 20 % smaller than 2 mm quartz and feldspar crystals, placed in an ash matrix. The center and main part contains about 5 % small and flattened pumice and has 20 to 25 % crystals of plagioclase, sanidine and quartz (Fig. 2.5E). Quartz has a characteristic equant shape, commonly with resorbed edges and embayments. Ferromagnesian minerals are absent. The upper part of the unit has a similar crystal content as the center part but contains considerably more and only moderately to weakly welded pumice. Locally unwelded pumice characterize the preserved very top of the unit (Fig. 2.5F). Equant and bimodal quartz rhyolite ignimbrite (EBR; Fig. 2.5G): The ignimbrite is greater than 50 m thick. Two sub-units are distinguished: the basal to center part of the ignimbrite, which is moderately to intensely welded with few fiamme, and with local breccias at the base, and the pumice-rich and weakly welded top part. Both lithofacies contain about 30 % crystals and crystal fragments, and about 1 % sub-rounded lapilli-sized lithics of EQR and silicified clasts. EQR and EBR have a similar texture, but compared to EQR, EBR is more crystal- and quartz-rich, and it contains accessory biotite. Quartz-sanidine rhyolite dikes, domes and associated breccias (SRI): Quartz- sanidine dikes, domes and associated breccias form small outcrops throughout the district where they cut through the entire sequence of the Chon Aike Formation and mark the final magmatic event of the formation. The sub-vertical dikes commonly strike northwest to west-northwest sub-parallel to the major faults. Usually, the flow-foliated dikes are 1 to 3 m thick, but in the northern part of the district, they reach up to 30 m where they show sub- 25 horizontal columnar joints (Fig. 2.5H). The crystal content is dependent on the dike thickness and the position in the dike. In thin dikes and at the margins of thick dikes, phenocrysts can be absent, slightly thicker dikes contain a few quartz and sanidine eyes in the center, and thick dikes contain up to 1 cm phenocrysts. The matrix consists of microcrystals and devitrified glass. Up to 200 m wide domes of the same composition occur north of the central vein zone. They show flow foliation and autoclastic brecciation along their margins.  2.4.1.4. La Matilde Formation Ash lithic crystal tuff and volcaniclastic breccias (ALT): Described as the La Matilde Formation (Stipanicic and Reig, 1957; Lesta and Ferello, 1972), this discontinuous and heterogeneous unit overlays the Chon Aike Formation, and includes polymictic volcanogenic sedimentary breccias and sandstones with variable clast contents (Fig. 2.6A). Polymictic volcaniclastic breccias range from matrix- to clast-supported, and clasts size varies from gravel to boulder size. The clasts origin from the whole suite of rocks from the Chon Aike Formation in the district, but the most common clast types are BQR, LXT, and SRI, depending on the proximity of the rock source. Coarse, poorly-sorted polymictic debris breccia flow deposits are commonly present in the upper part of the unit. Volcaniclastic sandstones have variable amounts of moderately- to well-rounded lapilli- sized clasts. Clast-poor horizons are commonly present at the top of the unit and show local cross bedding and ripple marks. Breccias and sandstones are locally affected by intense hydrothermal silicification and stand out as morphological highs. In a small area north of the central vein zone, a moderately lithified volcaniclastic sandstone contains petrified wood. Such wood-bearing volcaniclastic rocks are usually included in the La Matilde Formation (Gnaedinger, 2004), however based on stratigraphic relations here, these rocks could be part of the slightly younger upper volcanic deposits (UVD, see section 4.1.5.).  2.4.1.5. Intermediate formation Fine- to medium-grain andesite and associated breccias (FMA): This unit of quartz- bearing andesite has variations in abundance and size of phenocrysts, and consists of lava flows, shallow intrusions (domes and dikes), and associated autoclastic and resedimented 26  Figure 2.6: Typical rock textures and outcrops from the Manantial Espejo district. A. Crude layering in volcanogenic sedimentary lithic sandstone from the La Matilde Formation (ALT) with lithics from rock types of the Chon Aike Formation. B. Paleo-ridge of rhyodacite ignimbrite (MBR) that has been covered by andesite from the intermediate formation (FMA); dip-symbols illustrate the orientation of the contacts. C. Dacite ignimbrite from the intermediate formation (UVD dacite-west) overlaying andesite (FMA). D. Medium-grained andesite (FMA) containting xenolith of phenocryst-poor andesite that is not outcropping and might originate form the Bajo Pobre Formation. E. Dacite ignimbrite from the intermediate formation (UVD dacite-west) with aligned biotite. F. Porphyritic andesite intrusive rock (PAI) containing a granite xenolith. G. Granite xenolith in andesite (FMA). H. Marine sedimentary polymictic breccia (MSD) with abundant clasts of silicified rhyolite dikes (SRI) and vein quartz and minor but mostly silicified clasts from other rock types. 27 coarse autoclastic breccias. The dark blue-gray to purplish andesite overlies and intrudes the Chon Aike Formation, and it is in turn overlain by a dacite ignimbrite (Fig. 2.6B and C). The majority of the unit is a medium-grained plagioclase-amphibole porphyritic andesite with scarce quartz, with a total phenocryst content of about 25 %. Occasionally, up to 20 cm in diameter xenoliths of granite, quartz-mica schist, or other andesite types are found (Fig. 2.6D). Associated monomicitc breccias are intercalated with the medium- grained andesite. A finer-grained and phenocryst-poor andesite facies (only 10 to 15 % phenocrysts) forms dikes and possibly the margins of the medium-grained andesite. Upper volcaniclastic deposits (UVD): Two, up to 25 m thick dacite ignimbrites (dacite-west and dacite-east) and undifferentiated secondary volcaniclastic deposits overlie the fine- to medium-grained andesite (FMA). The dacite in the west of the map area is a fine-grained, moderately to densely welded ignimbrite characterized by abundant aligned biotite and highly fragmented quartz (Fig. 2.6E). Rare clasts of FMA are the only observed lithic clasts, and up to 2 cm long charcoaled wood fragments are present, whereas pumice is absent. The dacite in the east of the mapped area is an unaltered, crystal-rich, and densely welded dacite ignimbrite that is texturally very similar to BQR. The 10 to 15 m thick ignimbrite contains small fiamme and lithics of EBR and FMA. Volcanogenic sedimentary deposits (UVD vsd) occur in various places of the district typically in morphological lows. They commonly have a greenish or purple-gray color and consist of weakly to moderately lithified breccias and sandstones that locally contain reworked accretionary lapilli and fossil plant fragments. Porphyritic andesite shallow intrusions (PAI): Shallow, coarse-grained, quartz- bearing porphyritic andesite intrusions are located within the FMA unit. They represent the late stage of Jurassic magmatic activity in the district. Up to 8 mm plagioclase together with amphibole, pyroxene, and scarce K-feldspar and quartz form phenocrysts in a fine-grained, microcrystalline, dark gray-purplish groundmass (Fig. 2.6F). Locally, abundant xenoliths are present.  2.4.1.6. Granite xenoliths Initially, granite xenoliths within the Jurassic volcanic units were interpreted as rocks originating from the basement, but U-Pb age dating on zircons indicates that these 28 granites have a similar age as the volcanic rocks of the Chon Aike Formation (Fig. 2.6F and G). The fine- to medium-grained K-feldspar-quartz-plagioclase granite is holocrystalline and largely equigranular, although about 20 % consist of a finer-grained crystalline matrix.  2.4.1.7. Tertiary and recent sedimentary deposits Marine and tidal fossiliferous sedimentary deposits (MSD): Related to Oligocene to Miocene transgressions, shallow marine and tidal sedimentary carbonate-rich sandstones and breccias are widespread in the Deseado Massif (Del Río, 2004; Parras and Casadío, 2006). The locally fossil-rich rocks outcrop in up to 10 m thick patches at several places in the district. The fossil-rich facies contains fragments or even intact shells of (at least) mollusca (bivalvia, gastropoda), brachiopoda, bryozoa, echinodermata (echinoidae), and shark teeth along with non-biogenic clasts from the described volcanic units and hydrothermal products. Coarse breccias contain sub-angular to moderately rounded clasts from volcanic units and abundant vein quartz and silicified rock types (Fig. 2.6H). Unconsolidated sedimentary deposits (USD): Unconsolidated deposits are present in depressions or as veneer of planes and mesas. They are commonly associated to the marine sedimentary rocks (MSD), where they consist of weathering-resistant, up to boulder-sized clasts of typically silicified rock types (especially SRI) and vein quartz.  2.4.2. Structural architecture  Late Jurassic normal faults developed during extension in conjunction with the break-up of Gondwana that resulted in the opening of the South Atlantic Ocean (Gust et al., 1985; Kay et al., 1989; Pankhurst et al., 2000; Macdonald et al., 2003). At Manantial Espejo, the trend for major faults is west-northwest, minor faults strike east to east- northeast (Fig. 2.3 and 2.4). The majority of the faults dip to the south-southwest. Many of the faults are filled with hydrothermal veins; several of them contain precious metal mineralization that constitute the Manantial Espejo epithermal Ag(-Au) deposit (Wallier et al., 2007). The normal faults define horst and graben structures (Fig. 2.7). The central vein zone (Fig. 2.3) is hosted by a horst where the stratigraphically deepest units outcrop. To the south-west of the horst, the bounding fault is the Maria fault, a major fault structure that 29        Figure 2.7: Simplified cross sections through the Manantial Espejo district. Four times vertical exaggeration makes faults appear steeper than they are. Section locations are shown in figure 2.3. A. South-southwest – north-northeast cross section through the central vein zone. B. South-southeast – north-northwest cross sections through the Mesa Vein zone, in a distance of 3.8 km to the west from section A along the Maria Fault. Abbreviations: ALT = ash lithic tuff, volcanogenic sedimentary deposits (La Matilde Formation); EBR = equant and bimodal quartz rhyolite ignimbrite (Chon Aike Formation); EQR = equant quartz rhyolite ignimbrite (Chon Aike Formation); FBR = fine-grained bimodal quartz rhyodacite ignimbrite (Chon Aike Formation); FMA = fine- to medium-grained andesite (intermediate formation); LXT = lithic crystall tuff (Chon Aike Formation); MBR = medium-grained bimodal quartz rhyodacite ignimbrite (Chon Aike Formation); MSD = marine and tidal fossiliferous sedimentary deposits; PAI = porphyritic andesite shallow intrusions (intermediate formation); SRI = quartz- sanidine rhyolite dikes, domes and associated breccias (Chon Aike Formation); USD = unconsolidated sedimentary deposits; UVD = upper volcaniclastic deposits; vsd = volcanogenic sedimentary deposits.   30 strikes 120° to 130° and dips 50° to 70° to the southwest. This structure also hosts the major orebody of the district, the Maria Vein, where the fault reaches a maximum vertical displacement of about 150 m as demonstrated by exploration drilling. To the north, the horst is delimited by the Karina fault system with a similar strike as the Maria fault but dipping steeply to the northeast. The vertical displacement of the Karina fault is greatest to the east of the central vein zone and decreases to the west-northwest. Another major fault dipping to the northeast is located in the south of the district and defines the southern limit of the graben south of the Maria fault. The major faults parallel older, sub-vertical, segmented, and discontinuous quartz-sanidine dikes (SRI) that cut the Chon Aike Formation.   2.5. Whole-rock major element geochemistry  Thirteen whole-rock samples were analyzed by X-ray fluorescence at ALS Chemex, a commercial laboratory in North Vancouver, Canada. The results are listed in Table 2.1. The compositions of these rocks range from andesite to rhyolite (57-78 wt % SiO2; Fig. 2.8). The volcanic rocks of the Chon Aike Formation are silica-rich and are divided into rhyodacite (BQR) and rhyolites (EQR, EBR, and SRI). The rocks of the intermediate formation are mainly andesites with the porphyritic andesite shallow intrusions (PAI) having a higher SiO2 content compared to the fine- to medium-grain andesite (FMA). The ignimbrites of the intermediate formation (UVD dacite-east and -west) have a dacitic composition. The andesites of Manantial Espejo have a similar major element composition as the Bajo Pobre Formation, whereas the dacite ignimbrites of the intermediate formation have a lower total alkali concentration compared to published analytical data for the Chon Aike Formation (Fig. 2.8; Gust et al., 1985; de Barrio, 1989; Sruoga, 1989; Pankhurst and Rapela, 1995; Pankhurst et al., 1998; Echavarría, 1999; Feraud et al., 1999; Echevarría et al., 2005; Echeveste, 2005).    31 Table 2.1: Whole-rock geochemical analyses of major elements by X-ray fluorescence (XRF) MBR MBR EQR EBR SRI FMAM FMA M FMA F FMA F PAI UVD W UVD E Granite wt % ME- SW- 05-95 ME- SW- 05-109 ME- SW- 05-93 ME- SW- 05-107 ME- SW- 06-32 ME- SW- 05-105 ME- SW- 06-12 ME- SW- 05-49 ME- SW- 06-25 ME- SW- 05-118 ME- SW- 06-54 ME- SW- 06-39 ME- SW- 06-20 SiO2 70.81 69.10 77.70 77.55 75.59 57.45 57.92 57.26 57.64 60.90 66.83 65.09 75.79 TiO2 0.30 0.36 0.07 0.07 0.03 0.72 0.66 0.95 0.94 0.69 0.17 0.46 0.02 Al2O3 14.91 14.81 12.13 12.34 12.49 17.77 17.09 17.64 15.89 16.54 14.59 15.47 13.37 Cr2O3 0.01 <0.01 0.01 <0.01 <0.01 0.01 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 Fe2O3 2.66 3.59 0.40 0.58 1.15 6.07 6.30 7.64 7.28 6.09 2.31 4.79 1.15 MnO 0.06 0.02 <0.01 <0.01 <0.01 0.17 0.14 0.11 0.13 0.10 0.04 0.09 0.02 MgO 0.55 0.38 0.17 0.22 0.16 2.62 2.36 2.17 1.75 2.23 0.51 1.57 0.16 CaO 2.06 0.20 0.06 0.38 0.14 5.84 5.28 6.38 5.90 4.83 2.64 3.58 0.67 Na2O 3.00 0.99 0.24 1.49 0.41 2.75 2.51 2.63 1.91 2.69 2.32 2.91 5.66 K2O 3.89 7.05 7.74 5.23 7.90 1.81 2.74 1.86 3.00 1.89 2.76 2.88 1.66 P2O5 0.08 0.09 0.03 0.02 0.02 0.16 0.16 0.19 0.19 0.17 0.06 0.11 0.02 SrO 0.03 <0.01 0.01 0.01 <0.01 0.03 0.03 0.04 0.03 0.04 0.05 0.03 0.03 BaO 0.09 0.15 0.15 0.06 0.14 0.09 0.11 0.07 0.11 0.08 0.15 0.08 0.04 LOI 1.38 2.47 0.96 1.75 1.28 4.15 4.33 2.96 4.20 3.85 7.41 2.47 0.71 Total 99.82 99.21 99.66 99.69 99.31 99.64 99.63 99.91 98.97 100.10 99.83 99.53 99.29 Analytical results from X-ray fluorescence for major elements and loss of ignition, analyzed in a commercial laboratory (ALS Chemex, Vancouver, Canada). The analyzed samples represent the least altered rocks from the major lithological units from the Manantial Espejo district. A total alkali vs. SiO2 diagram is shown in Figure 6. For abbreviations of rock types see text and: E = dacite-east, F = fine-grained, M = medium-grained, W = dacite-west. Sample numbers are shown beneath the rock type abbreviations. 32        Figure 2.8: Total alkali vs. SiO2 diagram (TAS; Le Bas et al., 1986) with data points and sample numbers (without ME-SW-). The analytical data are shown in Table 1. The samples represent the least altered rocks found in the district from most of the Jurassic volcanic units, except for ME-SW-06-20, which is a granite xenolith found in an andesite unit. The gray shaded areas represent the data fields including more then 200 samples of the Chon Aike Formation and more then 60 samples from the Bajo Pobre Formation from published sources (Gust et al., 1985; de Barrio, 1989; Sruoga, 1989; Pankhurst and Rapela, 1995; Pankhurst et al., 1998; Echavarría, 1999; Feraud et al., 1999; Echevarría et al., 2005; Echeveste, 2005). 33 2.6. Geochronology  Despite the Chon Aike Province being one of the world’s largest bimodal igneous provinces dominated by silicic volcanic rocks, relatively few isotopic age constraints are available. These, however, demonstrate a Middle to Late Jurassic age. The Middle Jurassic age was originally based on paleontology, and subsequently confirmed for the first time by radiometric K-Ar geochronology on a volcanic tuff from the Chon Aike Formation (Cazeneuve, 1965). Since then, several ages have been calculated by K-Ar, Ar-Ar, Rb-Sr, and U-Pb methods (Tab. 2.2), and province-wide geochronology studies are discussed by Féraud et al. (1999) and Pankhurst et al. (2000). In this study, seven samples were selected for SHRIMP-RG (Sensitive High- Resolution Ion Microprobe – Reverse Geometry) U-Pb geochronology on zircons at the USGS-Stanford University facility, California. An additional sample was chosen for 40Ar- 39Ar age dating of hornblende at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia, Canada. The goal is to constrain the temporal framework on district-scale, in order to obtain a clear image about the timing of the various volcanic activities at Manantial Espejo.  2.6.1. Methodology  2.6.1.1. Uranium-lead SHRIMP-RG geochronology Seven rock samples weighing between 300 g and 10 kg were selected for U-Pb SHRIMP-RG geochronology. The samples were crushed and ground using a Rhino jaw crusher and a Bico disc grinder with ceramic grinding plates. Zircons were concentrated using a Wilfley wet shaking table equipped with a machined plexiglas top and conventional heavy liquid and magnetic separation by a Frantz magnetic separator. Fifty to seventy zircons were isolated by handpicking. The zircon separates were mounted in epoxy and polished to expose the grain centers. Reflected light and cathodoluminescense (CL) images were taken to detect impurities, cracks, holes and inherited zircon cores within the crystals, helping to select the best locations for spot analysis. Examples for representative CL images with the location of spot analyses are shown in figure 2.9. Uranium-lead analyses 34 Table 2.2: Summary of isotopic ages from the Deseado Massif Location Analyzed material Method Age (Ma) Comments Reference Roca Blanca Ignimbrite K-Ar 160 ± 7 Cazeneuve, 1965 Bahía Camarones K-Ar 166 ± 5 Creer et al., 1972 Arroyo Page, Perito Moreno Rhyolitic ignimbrite K-Ar 155 ± 15 Baker et al., 1981 Bajo de San Julián Lithified tuff K-Ar 160 ± 10 Spalletti et al., 1982 Bajo de San Julián Lithified tuff K-Ar 157 ± 10         " Bajo de San Julián Rhyolite K-Ar 138 ± 10         " Bajo de San Julián Rhyolite K-Ar 149 ± 10         " Bajo de San Julián Lithified tuff K-Ar 161 ± 10         " Bajo de San Julián Lithified tuff K-Ar 123 ± 10         " Ea. Bajo Grande Ignimbrite K-Ar 157 ± 5 Hechen and Homovc, 1988 Ea. Bajo Grande Ignimbrite K-Ar 153 ± 5         " Rio Pinturas Ignimbrite Rb-Sr 161 ± 5 Isochron De Barrio, 1993 Puerto Deseado Ignimbrite Rb-Sr 168 ± 1.9 Isochron Pankhurst et al., 1993b Rio Pinturas Sanidine in ignimbrite Ar-Ar 151.5 ± 1.0 Alric et al., 1996 Puerto Deseado Sanidine in ignimbrite Ar-Ar 177.6 ± 1.4         " Bajo Pobre Ar-Ar 156.7 ± 4.6         " Ea. La Josefina Biotite in ignimbrite K-Ar 153.2 ± 3.6 Arribas et al., 1996 Ea. La Josefina Biotite in ignimbrite K-Ar 151.5 ± 3.6         " Ea. La Josefina Biotite in ignimbrite K-Ar 149.6 ± 3.5         " Ea. La Josefina Biotite in lava K-Ar 148.8 ± 3.6 Isochron         " Ea. La Josefina Ignimbrite Rb-Sr 150 ± 4 Errorchron Fernández et al., 1999 Zentral Deseado Massif Ignimbrite Rb-Sr 148 ± 2 Tessone et al., 1999 North Deseado Massif Sanidine Ar-Ar 168.6 ± 0.4 Plateau age Féraud et al., 1999 Northwest Deseado Massif Sanidine Ar-Ar 153.4 ± 0.3 Plateau age         " Puerto Deseado Sanidine Ar-Ar 177.7 ± 0.4 Weighted mean high temp.         " Puerto Deseado Sanidine Ar-Ar 177.8 ± 0.4 Plateau age         " Northwest Deseado Massif Sanidine Ar-Ar 154.6 ± 0.5 Plateau age         " Northwest Deseado Massif Sanidine Ar-Ar 151.5 ± 0.5 Plateau age         " South Deseado Massif Sanidine Ar-Ar 158.4 ± 0.3 Plateau age         " South Deseado Massif Sanidine Ar-Ar 157.9 ± 0.5 Plateau age         " North Deseado Massif Basaltic andesite Ar-Ar 164.1 ± 0.3 Plateau age         " North Deseado Massif Basaltic andesite Ar-Ar 160.5 ± 0.5 Weighted mean high temp.         " North Deseado Massif Plagioclase (andesite) Ar-Ar 152.7 ± 1.2 Plateau age         " North Deseado Massif Plagioclase (andesite) Ar-Ar 152.8 ± 2.6 Plateau age         " San Julian Zircon (SHRIMP) U-Pb 162.7 ± 1.1 Pankhurst et al., 2000 Río Pinturas Zircon (SHRIMP) U-Pb 156.2 ± 1.8         " Puerto Deseado Feldspar Ar-Ar 169.1 ± 1.6         " Río Pinturas Feldspar Ar-Ar 156.4 ± 2.4         " Cabo Dañoso Zircon (SHRIMP) U-Pb 168.4 ± 1.6         " Cabo Dañoso Feldspar Ar-Ar 177.8 ± 0.8 Same sample as previous         " Bajo Pobre Biotite Ar-Ar 150.6 ± 2.0         " Manantial Espejo Zircon cores (SHRIMP) U-Pb 166.0 ± 0.5 Echeveste, 2005 Manantial Espejo Zircon rims (SHRIMP) U-Pb 158.9 ± 0.5 Same sample as previous         " Manantial Espejo Zircon (SHRIMP) U-Pb 162.0 ± 1.1 This study Manantial Espejo Zircon (SHRIMP) U-Pb 161.3 ± 1.1         " Manantial Espejo Zircon (SHRIMP) U-Pb 162.6 ± 1.2         " Manantial Espejo Zircon (SHRIMP) U-Pb 160.0 ± 1.2         " Manantial Espejo Zircon (SHRIMP) U-Pb 161.1 ± 1.0         " Manantial Espejo Zircon (SHRIMP) U-Pb 156.7 ± 1.5         " Manantial Espejo Hornblende Ar-Ar 157.7 ± 1.0 Average of 2 plateau ages         " 35 were performed in two sessions. During the first session in December 2005, four samples were analyzed, and the data were collected in sets of five scans throughout the masses of ZrO2, 204Pb, 206Pb, 207Pb, 208Pb, 248ThO, 238U, 254UO, and 196HfO. During the second session in December 2006, three samples were analyzed. In addition to the masses of the first session, data were also collected through the masses of 139La, 140Ce, 146Nd, 147Sm, 153Eu, 155Gd, 179DyO, 182ErO, and 188YbO. The measured 206Pb/238U ratio was corrected for 207Pb, and by using reference zircons of a known age of 419 Ma (R33). A zircon of known composition (CZ3) was used to determine the U-concentration of zircons. Ages were calculated using the software Squid 1.02 (Ludwig 2001) and Isoplot 3.00 (Ludwig 2003). Uncertainties in mean ages and in single analyses in diagrams are reported at the 95 % confidence level (2σ errors; Fig. 2.10).  2.6.1.2. 40Argon/39Argon geochronology About 200 g of unaltered rock were crushed, washed in deionized water, dried at room temperature and sieved to obtain the size fraction between 0.420 mm and 0.595 mm. Mineral separates were hand-picked, washed in acetone, dried, wrapped in aluminium foil and stacked in an irradiation capsule with similar-aged samples and neutron flux monitors (Fish Canyon Tuff sanidine (FCs), 28.02 Ma; Renne et al., 1998). The samples were irradiated on November 14 through November 16, 2006, at the McMaster Nuclear Reactor in Hamilton, Ontario, for 90 MWH, with a neutron flux of approximately 6x1013 neutrons/cm2/s. Analyses (n=44) of 20 neutron flux monitor positions produced errors of < 0.5 % in the J value. The samples were analyzed on February 28 and March 1, 2007, at the Noble Gas Laboratory, Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia, Canada. The mineral separates were step-heated at incrementally higher powers in the defocused beam of a 10 W CO2 laser (New Wave Research MIR10) until fused. The gas evolved from each step was analyzed by a VG5400 mass spectrometer equipped with an ion-counting electron multiplier. All measurements were corrected for total system blank, mass spectrometer sensitivity, mass discrimination, radioactive decay during and subsequent to irradiation, as well as interfering Ar from atmospheric contamination and the irradiation of Ca, Cl, and K (isotope production ratios:  36     Figure 2.9: Cathodoluminescense (CL) images of representative zircons with location and number of spot analyses. For selected analytical data see appendix B. A. Andesite xenolith (Bajo Pobre Formation?) in FMA. B. Medium-grained bimodal quartz rhyodacite ignimbrite (MBR, Chon Aike Formation). C. Equant and bimodal quartz rhyolithe ignimbrite (EBR, Chon Aike Formation). D. Andesite flow (FMA, intermediate formation). E. Dacite ignimbrite (UVD dacite-west, intermediate formation). F. Shallow andesite intrusion (PAI, intermediate formation).   37        Figure 2.10: 207Pb corrected 206Pb/238U weighted mean age histograms with 2σ error bars. Uncertainties in mean ages are listed at the 95 % confidence level, including error in standard. Filled boxes represent spot analyses that were included in the weighted mean age calculations, open boxes were rejected, zircon ages that are out of the chart have 1σ errors. A. Andesite xenolith (Bajo Pobre Formation?) in andesite flow (FMA). B. Medium-grained bimodal quartz rhyodacite ignimbrite (MBR, Chon Aike Formation). C. Equant and bimodal quartz rhyolithe ignimbrite (EBR, Chon Aike Formation); D. Andesite flow (FMA, intermediate formation). E. Dacite ignimbrite (UVD dacite-west, intermediate formation). F. Shallow andesite intrusion (PAI, intermediate formation). 38 (40Ar/39Ar)K = 0.0302 ± 0.00006, (37Ar/39Ar)Ca = 1416.4 ± 0.5, (36Ar/39Ar)Ca = 0.3952 ± 0.0004, Ca/K = 1.83 ± 0.01(37ArCa/39ArK).). The plateau and correlation ages were calculated using Isoplot version 3.00 (Ludwig, 2003). Errors are quoted at the 2σ (95 % confidence) level and are propagated from all sources except mass spectrometer sensitivity and age of the flux monitor. The best statistically-justified plateau and plateau age was picked based on the following criteria: 1. Three or more contiguous steps comprising more than 50 % of the 39Ar. 2. Probability of fit of the weighted mean age greater than 5 %. 3. Slope of the error-weighted line through the plateau ages equals zero at 5 % confidence. 4. Ages of the two outermost steps on a plateau are not significantly different from the weighted-mean plateau age (at 1.8σ six or more steps only).  2.6.2. Geochronology results  An age for a potential representative of the Bajo Pobre Formation was calculated from zircons in andesite xenoliths in FMA (ME-SW-06-30). From the Chon Aike Formation, ages were determined for the stratigraphically oldest outcropping ignimbrite (MBR, ME-SW-05-109) and for the youngest ignimbrite (EBR, ME-SW-05-111). Two andesite (ME-SW-05-105 and ME-SW-05-118) and two dacite samples (ME-SW-06-54 and ME-SW-04-32) were analyzed from the intermediate formation. Sample ME-SW-05- 105 from FMA represents an andesite flow deposit and ME-SW-05-118 a porphyritic shallow andesite intrusion (PAI). Sample ME-SW-06-54 was taken from a dacite ignimbrite (UVD dacite-west) covering FMA, and sample ME-SW-04-32 is a dacite ignimbrite in the south-east of the district (UVD dacite-east). Geochronologic measurements were also performed on zircons from a granite xenolith within FMA (ME- SW-06-20). All samples except ME-SW-04-32, which was dated by 40Ar/39Ar on hornblende, were analyzed by U-Pb SHRIMP-RG on zircons. The sample locations are shown in figure 2.3; table 2.3 summarizes the U-Pb results. Analytical data for each SHRIMP-RG spot analyses and analytical data from 40Ar-39Ar step-heating are presented in appendices B and  C. Zircon crystals in all samples commonly range between 50 and 200 µm, with a maximum of 0.5 mm. Small grains are typically colorless, whereas larger grains have a 39 Rock type Sample Easting Northing No. ofanalyses Age* (Ma) 2ı Rejected zircons Andesite xenolith ME-SW-06-30 459037 4595168 12 162.6 1.2 0 Rhyodacite ignimbrite - MBR ME-SW-05-109 463668 4595065 15 162.0 1.1 7 Rhyolite ignimbrite - EBR ME-SW-05-111 462229 4593148 28 161.3 1.1 10 Andesite flow - FMA ME-SW-05-105 461219 4595831 12 160.0 1.2 2 Dacite ignimbrite - UVD dacite-west ME-SW-06-54 458855 4594338 12 161.1 1.0 4 Andesite shallow intrusion - PAI ME-SW-05-118 457214 4596511 12 156.7 1.5 2 Granite xenolith ME-SW-06-20 460828 4595937 14  All location references are given in UTM WGS 84 zone 19F coordinates. *Ages are reported as 207Pb-corrected 206Pb/238U weighted mean. no coherrent age-group Table 2.3: Summary of SHRIMP-RG U-Pb (zircon) ages for volcanic and sub-volcanic rocks from the Manantial Espejo district 40 weak yellow tint. Inclusions in zircons are locally present, and some zircons have an inherited core of an older zircon crystal that is usually visible in the CL image (e.g. figure 2.9C spot #12). Therefore, the spot measurements were preferentially performed close to the rim of the crystal, though some cores were also analyzed to gain an insight into the source of any inheritance. Twelve to twenty eight zircon grains per sample were analyzed by SHRIMP-RG. The data is presented as weighted mean age histograms with 2σ error bars (Fig 2.10). Eight to18 zircon grains with similar ages and chemistry were used to calculate the overall rock age of each sample. Ages significantly younger than the bulk are probably the effect of lead loss; ages significantly older than the bulk might represent xenocrysts or inherited zircon cores. As the zircons from the Manantial Espejo district are relatively young, only the 207Pb-corrected 206Pb/238U results can be reliably used for age determination, for two reasons: one reason is the generally poor measuring statistics on 204Pb, the normal isotope used for removing common Pb during the age calculation, and the other is the low U content of the zircons. The exception is sample ME-SW-06-20, where high U- concentrations (>2000 ppm) in most zircons do not allow a coherent age-group. Two 40Ar/39Ar analyses were preformed on hornblende from sample ME-SW-04-32. The first analysis only allowed four steps on the plateau because the hornblende released the majority of the Ar during one heating stage. The second analysis allowed six steps on the plateau. Both analyses revealed valid plateau ages with overlapping analytical uncertainties. The age spectra are shown in figure 2.11.  2.6.2.1. Inherited zircons and xenocrysts Four out of the total 105 analyzed zircons revealed ages significantly older than the bulk of the data. They range from the Early Permian to the early Middle Jurassic. The oldest age of 266.7 ± 1.4 Ma (1σ-error) was obtained from an inherited zircon core visible on the CL image from sample ME-SW-05-111 (EBR; fig. 2.9C #12). An Early Jurassic zircon (192.7 ± 1.2 Ma; 1σ-error) was analyzed in the same sample. The spot analysis performed at the rim of the zircon indicates a xenocryst. One zircon from ME-SW-05-111 (EBR) and one zircon from ME-SW-05-109 (MBR) yielded early Middle Jurassic ages 41        Figure 2.11: Incremental laser heating 40Ar-39Ar age spectra for hornblende. Hornblende is separated from a dacite ignimbrite (UVD dacite-east, ME-SW-04-32) in the southeast of the Manantial Espejo district (Fig. 2.3). For analytical data see appendix C. Sample location: 465044 E, 4592495 N (UTM WGS 84, zone 19F).  42 (178.6 ± 1.2 Ma and 179.8 ± 1.4 Ma; both 1σ-errors). The spot locations chosen at the rim of the zircon indicate these grains are xenocrysts.  2.6.2.2. Andesite xenolith – Bajo Pobre Formation? The twelve zircons from sample ME-SW-06-30 (andesite xenoliths in FMA; Fig. 2.6D) yielded a weighted mean age of 162.6 ± 1.2 Ma (Figs. 2.9A and 2.10A). None of the spot measurements was rejected. Since this unit neither outcrops nor is present in drill cores, additional crosscutting relationships are not available. The sample was taken as potential representative of the mafic to intermediate Bajo Pobre Formation that might be present underneath the Chon Aike Formation. Although the age of the andesite xenolith is slightly older compared to the ages of the Chon Aike Formation, they show overlapping uncertainties. Whether this xenolith represents a sample of the Bajo Pobre Formation is thus unclear, but considered possible.  2.6.2.3. Chon Aike Formation Fifteen clear, euhedral zircons of 50 to 500 µm length from the lowest outcropping ignimbrite (MBR; ME-SW-05-109) were analyzed (Figs. 2.9B and 2.10B). Eight zircons revealed a weighted mean SHRIMP-RG age of 162.0 ± 1.1 Ma. Three of the rejected zircons with ages of 166 Ma, 167 Ma and 180 Ma are interpreted as xenocrysts or inherited cores, four ages significantly less than 160 Ma display probable lead loss. Sample ME-SW-05-111 represents the stratigraphically highest ignimbrite of the Chon Aike Formation (EBR; Figs. 2.9C and 2.10C). The zircons are transparent and euhedral with lengths between 100 and 200 µm. Eighteen out of 28 zircons yielded a weighted mean age of 161.3 ± 1.1 Ma. Ten spot measurements were not included: three ages (179 Ma, 193 Ma, and 267 Ma) indicate xenocrysts or inherited cores, seven grains significantly younger than 160 Ma are interpreted to have undergone lead loss.  2.6.2.4. Intermediate formation Twelve zircons were analyzed from sample ME-SW-05-105, a medium-grained andesite that overlays and intrudes the Chon Aike Formation (FMA; figs. 2.9D and 2.10D). Ten of the 100 to > 500 µm long euhedral zircons define a weighted mean age of 43 160.0 ± 1.2 Ma. The two zircons not included in the calculation gave significantly older ages of around 167 Ma. The dacite-west ignimbrite (UVD dacite-west; ME-SW-06-54; figs. 2.9E and 2.10E) was dated with 161.1 ± 1.0 Ma. Eight out of twelve zircon analyses have been used to calculate the age. Two of the dismissed analyses are slightly older (163 Ma and 165 Ma), the other two ages are too young and might represent lead loss. Although the reported age is older than the age of FMA, the errors are overlapping and field observations show that the dacite-west ignimbrite is stratigraphically higher. Two 40Ar-39Ar analyses were performed on unaltered hornblende from the UVD dacite-east ignimbrite (ME-SW-04-32; fig. 2.11) that contains clasts of FMA. The ages of 158.0 ± 1.0 Ma and 157.4 ± 0.9 Ma have overlapping uncertainties. The two ages averaged give an age for the unit of 157.7 ± 1.0 Ma. The youngest age of 156.7 ± 1.5 Ma of a porphyritic andesite intrusion (PAI; ME- SW-05-118; figs. 2.9F and 2.10F) was determined on ten out of twelve zircons. The age is only slightly younger than the ages of the other dated rocks, but errors only overlap with the 40Ar-39Ar age from the UVD dacite-east. The euhedral and almost isometric zircons are 50 to 200 µm long. The two zircons not included in the calculation gave significantly older ages of 164 Ma and 166 Ma.  2.6.2.5. Granite xenolith Fourteen zircons from a granite xenolith within FMA were analyzed by SHRIMP- RG (Fig. 2.6G). The zircons did not respond to CL due to the high U-concentrations, therefore back scatter images were taken in order to choose the location for spot measurements. Analyses yielded very high U and Th contents in the zircons of up to 12,700 ppm U and 4100 ppm Th, where compared to concentration of usually < 1000 ppm U and < 500 ppm Th in all the other samples (App.B). Only three of the analyzed zircons of the granite have U- and Th-concentrations in the range of the other samples. As result of excessive U, no coherent age-group could be determined. An age versus U-concentration plot (Fig. 2.12) shows a correlation between apparent older ages and higher U- concentrations. The three zircons with low U-concentrations have ages of 163.1 ± 1.8 Ma, 164.6 ± 1.3 Ma, and 161.1 ± 1.0 Ma (1σ-errors), which might be close to the real age of the 44        Figure 2.12: Age vs. U-concentration plot for analyzed zircons from a granite xenolith in an andesite from the intermediate formation (FMA). The obtained ages for single zircons do not show a coherent age-group, which is caused by high U-concentrations in the zircons. Zircons with high U-concentrations show older ages. The three zircons with low U-concentrations might be close to the real age. Selected analytical data for each measurement are shown in appendix 2.B.     45 rock. If true, then this conceivably could represent a piece of the evolving magma chamber which sources the Chon Aike ignimbrites.   2.7. Discussion  2.7.1. Stratigraphy and geochronology of the Manantial Espejo district  The stratigraphic sequence from the Manantial Espejo district is summarized in figure 2.13. The figure shows the outcropping units and their crosscutting relationships in a composite section derived from field mapping and drill core studies. Appendix A lists the major characteristics of the lithological units.  2.7.1.1. Bajo Pobre Formation The mafic to intermediate Bajo Pobre Formation that underlies the Chon Aike Formation in the central part of the Deseado Massif is not exposed at Manantial Espejo. The andesitic and dacitic rocks present in the district are younger than the rocks of the Chon Aike and La Matilde Formations. However, xenoliths of a crystal-poor andesite hosted by the younger andesites of the intermediate formation show that andesitic magmas existed prior to the eruption of the host andesite. Although geochronology gave a slightly older age for the andesite xenoliths compared to the ages of the Chon Aike Formation, they show overlapping uncertainties, and it is therefore unclear whether these xenoliths represent samples of the Bajo Pobre Formation.  2.7.1.2. Chon Aike Formation The lowermost outcropping unit at Manantial Espejo is a more than 250 m thick variably welded rhyodacite ignimbrite (MBR as part of BQR) without obvious stratification. The thickness and homogeneity of the ignimbrite suggest that the deposit has been associated to a continuous gravitational collapse of an eruption column without significant fluctuations (Cas and Wright, 1987). Very thick ignimbrites are commonly found in, but not restricted to intracaldera settings (McPhie et al., 1993). However, no 46  Figure 2.13: Composition of the stratigraphic sequence at Manantial Espejo, derived from field mapping and drill core studies. 47 evidence is present for a caldera at Manantial Espejo, and the location of eruption remains unclear. Field observations and drill core studies show that FBR is discontinuous and variably thick suggesting that the unit underwent erosion before the deposition of LXT, the next higher unit. This interpretation is supported by the occurrence of a thin paleo-soil that marks the contact between FBR and LXT in an exploration trench, and indicates an intraformational erosion surface. LXT, consisting of variably intercalated fine to coarse and lithic ash, also shows variability in thickness that can be related to remobilization and re- deposition of the unlithified ash shortly after the primary, probably air-fall deposition. LXT likely represents the initial stage of the eruption cycle that deposited the EQR rhyolite ignimbrite, which consists of a compound cooling unit of up to three ash-flows and locally, for example in the hanging wall of the Maria Vein, it is intercalated with LXT. The basal part of EQR is formed by a finer-grained and lithic-rich facies which is interpreted as the weakly or unwelded base of the flow. The core is usually densely welded with fiamme and local columnar jointing. The top of the compound cooling unit is preserved locally, where, despite pervasive alteration, up to 8 cm rounded cavities are present that are interpreted as largely undeformed and unwelded pumice clasts concentrated at the top of the flow (Fig. 2.5F). EQR is overlain by EBR, another thick rhyolite ignimbrite which is likely a simple cooling unit with a pumice-rich top. Uranium-lead geochronology (SHRIMP-RG) on zircons from MBR and EBR point out that the more than 350 m thick sequence of ignimbrites and ash units was likely deposited in less than 1 Ma, with a maximum of 3 Ma when the analytical uncertainty is included. MBR has an age of 162.0 ± 1.1 Ma and EBR an age of 161.3 ± 1.1 Ma. The entire volcanic succession of the district was emplaced in 5 to 6 Ma between 162 and 157 Ma. Six ages between 157 and 152 Ma (one U-Pb and five Ar-Ar ages; tab. 2.2) determined by Alric et al. (1996), Féraud et al. (1999), and Pankhurst et al. (2000) show a similar time range for the volcanic activity in the northwest Deseado Massif. Such short timeframes of volcanic activity have been recorded for comparable large silicic provinces. In the Sierra Madre Occidental, Mexico, for example, ages on up to 1000 m thick silicic volcanic successions suggests that they were emplaced in only 2 Ma (Ferrari et al., 2002, and references therein). 48 2.7.1.3. La Matilde Formation Succeeding the Chon Aike Formation, volcanogenic sedimentary deposits of the La Matilde Formation (ALT) indicate a phase of erosion, transport, and sedimentation. The deposits are generally heterogeneous and immature suggesting a proximal source of the components. Although the paleotopography was probably not very steep, gravity-driven mass flow deposits with up to boulder-sized clasts were generated. These coarse polymictic debris breccia flow deposits grade laterally into finer-grained breccias. Crude stratification of finer-grained breccias and lithic sandstones as well as cross-bedding and ripple marks in sandstones in the upper part of the deposit indicate partial shallow sub-aqueous sedimentation. Vast parts of the upper part of these deposits are well preserved and outcrop prominently because of intense silicification. These silica bodies are interpreted as paleo- water tables that channeled lateral flow of a hydrothermal system leading to silica precipitation. They are related to the Manantial Espejo epithermal Ag(-Au) deposit and mark the top of the hydrothermal system (Wallier et al., 2007; Chapter 3).  2.7.1.4. Intermediate formation Mafic and intermediate rocks from the Deseado Massif are usually included in the Bajo Pobre Formation, which underlies the silicic rocks of the Chon Aike Formation in the central part of the Deseado Massif (Stipanicic and Bonetti, 1970; Pankhurst et al, 1998). Geochronology by Alric et al. (1996) and Féraud et al. (1999) has already indicated that not all andesites pre-date the Chon Aike Formation. Since the intermediate rocks (andesites and dacites) from Manantial Espejo are younger than the rhyodacite and rhyolite units from the Chon Aike Formation, we do not include these rocks in the Bajo Pobre Formation. Instead, we name these intermediate rocks collectively and informally “intermediate formation”. In future, a proper formation name might be appropriate that clearly differentiates those rocks from the Bajo Pobre Formation. Based on the geochronologic data, longer lasting but less productive volcanism formed the intermediate formation. Activity initiated around 160 Ma with the emplacement of andesitic dikes, domes, and flows (FMA). The exact length of the time window in which the volcanogenic sedimentary deposits of the La Matilde Formation were formed is not resolvable by geochronological method due to overlapping analytical errors, but it was 49 barely longer than 2 Ma. The field of andesite flows, domes, and associated autoclastic breccias and syn-eruptive volcaniclastic deposits are present in the west and northwest of the district in a coherent area with only small windows where silicic rocks of the Chon Aike Formation outcrop. Field observations show that in the south, FMA is overlain by an up to 25 m thick fine-grained and welded dacite ignimbrite (UVD dacite-west). Despite this fact, U-Pb geochronology on zircons gave an older age for the dacite compared to FMA. However, the ages have overlapping analytical uncertainties. A second dacite ignimbrite (UVD dacite- east) occurs in the southeast of the district. In contrast to all other Jurassic rock types, this unit is unaltered and barely weathered pointing toward a much younger age, but 40Ar-39Ar geochronology on hornblende gave an age of 158 Ma. A porphyritic andesite (PAI) that is present as shallow intrusions within FMA is the youngest Jurassic unit recorded in the district. Crosscutting relationships between PAI and the dacite ignimbrites are missing and consequently the stratigraphic position is based on the U-Pb geochronology on zircons that gave an age of 157 Ma.  2.7.1.5. Pre-Chon Aike and -Bajo Pobre Formation Scarce information about rock types underlying the Chon Aike and possibly Bajo Pobre Formations at Manantial Espejo is available from xenoliths, xenocrystic zircons, and inherited zircon cores. Xenoliths consist of tightly folded quartz-mica schist which might be a similar lithology as described by Pankhurst et al. (2003) from the Dos Hermanos metapelite outcropping about 200 km northeast of Manantial Espejo, for which U-Pb (SHRIMP I) geochronology on detrital zircons gave ages ranging from 560 to 1430 Ma. However, a correlation of the rock types is not adequate due to the distance and the lack of exposed basement in the Deseado Massif. Although the origin of xenocrystic or inherited zircons is unknown, four spot analyses of the analyzed zircons in this study gave significantly older ages than the late Middle to Late Jurassic units, but on the other hand those ages are also significantly younger than the Dos Hermanos metapelites. The oldest zircon of an age of 267 Ma is Early Permian, but no similar age has been reported by Pankhurst et al. (2003). This age coincides with the emplacement of the Carboniferous to Triassic granitoids and rhyolites of the Choiyoi Province in the North Patagonian Massif 50 and farther north (Kay et al., 1989). Two zircons of 179 and 180 Ma and one slightly older crystal of 193 Ma coincide with the first episode of peak activity V1 (188-178 Ma) of the Chon Aike Province present in the Marifil Formation in the North Patagonian Massif and in formations of the southern Antarctic Peninsula (Pankhurst et al., 2000). Two 40Ar-39Ar ages on sanidine from rhyolite ignimbrites in the very east of the Deseado Massif near Puerto Deseado also have ages of 178 Ma (Tab. 2.2; Féraud et al., 1999), and are by far the oldest rocks of the Chon Aike Province in the Deseado Massif.  2.7.2. Manantial Espejo’s position in the age pattern of volcanic activity  Pankhurst et al. (2000) proposes that the emplacement of the Chon Aike Province spanned more than 30 Ma between Early and Late Jurassic, with three episodes of peak activity defined as V1 (188-178 Ma), V2 (172-162 Ma), and V3 (157-153 Ma). Whereas V1 includes rocks from the Marifil Formation in the North Patagonian Massif and formations in the southern Antarctic Peninsula, V2 occurred along the eastern coast of the northern Antarctic Peninsula and in the eastern part of the Deseado Massif. The third peak activity (V3) was in the west Deseado Massif and as volcanic-plutonic activity in the eastern part of the South Patagonian batholith, showing a westward migration of the magmatic activity toward the proto-Pacific margin. The U-Pb and 40Ar-39Ar ages of 162 to 157 Ma from the Manantial Espejo district are exactly between the episodes V2 and V3, suggesting complication with the punctuate volcanic sequences proposed by Pankhurst et al. (2000). Due to the geographic position of Manantial Espejo between the areas of the peak activities V2 and V3 in the Deseado Massif, it is proposed that the volcanic activity was a more continuous process and the distribution of the ages implies a wave of volcanic activity that started in the eastern Deseado Massif between 172 and 162 Ma and moved westwards crossing Manantial Espejo between 162 and 157 Ma and waning in the west between 157 and 153 Ma.     51 2.7.3. Granite xenoliths  Granite clasts in pyroclastic rocks and granite xenoliths in lava flows and shallow intrusions are present locally in silicic and intermediate rocks from Manantial Espejo. They were originally interpreted as plutonic rocks from the basement, but U-Pb (SHRIMP-RG) geochronology on zircons from one of these xenoliths indicates an age rather coinciding with the local volcanic units. Although the single zircon ages do not show a coherent age- group due to high U-concentrations (Fig. 2.12), it is suggested that co-genetic granites were emplaced at depth. Modern examples for co-genetic intrusive complexes that were sampled by erupting magmas are given by Bacon and Lowenstern (2005) and Bacon et al. (2007), where they describe plutonic xenoliths of a similar age as the lavas and pyroclastic deposits that transported the plutonic rocks to the surface during the 7700 yr B.P. Mount Masama eruption in Oregon (Crater Lake) and the 3700 yr B.P. eruption of Mount Veniaminof (Alaska). Middle to Late Jurassic plutonic rocks have not been reported in the Deseado Massif, but plutonic rocks related to the Chon Aike Province are exposed along the eastern margin of the South Patagonian batholith, in Tierra del Fuego, and in the Antarctic Peninsula. The oldest plutonic rocks within the South Patagonian batholith have an age of 157 ± 3 Ma and are coeval with volcanic products of the V3 stage of volcanic activity present in the El Quemado and Ibáñez Formations in the north and the Tobífera Formation in the south (Hervé et al., 2007). Two-mica granites coeval with the deposition of the Tobífera Formation are also present beyond the eastern margin of the batholith in the Darwin granite suite in Tierra del Fuego (Mukasa and Dalziel, 1996). These granites with an age of 164.1 ± 1.7 Ma were exhumed as part of the Cordillera Darwin metamorphic core complex during a period of extensions in the latest Cretaceous. Similar ages for subvolcanic plutons were also recorded on the Antarctic Peninsula (Pankhurst et al., 2000). It is therefore likely that crystallized magma chambers, where the magmas became fractionated, are present beneath the Deseado Massif, but uplift and erosion has not been sufficient to expose them.   52 2.7.4. Extensional tectonics and faulting  The area of the Manantial Espejo district was likely under extension already during the period of volcanic activity, but most normal faults developed after the emplacement of the volcanic and shallow intrusive rocks. Epithermal veins were formed syn-genetic with the formation of normal faults and cut through the entire volcanic sequence (Wallier et al., 2007). 40Ar-39Ar ages on hydrothermal adularia from three different veins confirm that the hydrothermal veins post-date the magmatic activity (Chapter 3). Faults of minor extent were already present during volcanism as indicated by the rhyolite dikes (SRI), but the discontinuity, the wide spacing, and the minor vertical displacements along the dikes suggest only minor extension. The locally vein-filled faults are interpreted as a set of linked normal faults with a strike generally trending west-northwest. The majority of the faults dip to the south- southwest, but a few faults also dip to the north-northeast. These conjugate sets of normal faults developed a horst and graben setting with vertical displacements that locally exceed 100 m. Deeper units like BQR and hydrothermal veins that reflect higher-temperature epithermal mineral assemblages outcrop in the horst exposing the central vein zone (Figs. 2.3 and 2.4), where shallower units and silicified rocks interpreted as the surface-near expression of the hydrothermal system are present in the graben structures. Most faults and veins show a curviplanar geometry and segmentation, which indicates linkage and interaction of extensional faults (e.g. Acocella et al., 2000; Peacock, 2002). Extension in the Manantial Espejo district was of low-magnitude. For example assuming that the major parallel normal faults are present at a 2 km spacing, dip 65°, and have a vertical displacement of 100 m, the finite extension is less than 3 %.  2.7.5. Eruption type for large volume ignimbrites  Although voluminous ignimbrites with welding textures are typically associated with calderas in modern volcanic environments (e.g. Cas and Wright, 1987), there is no evidence for a caldera in the region of Manantial Espejo, and only few calderas have been suggested in all the Chon Aike Province (Sruoga, 1994; Panza et al., 1996; Echavarría et 53 al., 2005). A similar situation exists for the even larger silicic volcanic province of the Tertiary Sierra Madre Occidental in Mexico and south-western USA, where less than 15 calderas have been identified but more than 350 should be expected if compared to the middle Tertiary San Juan volcanic field in Colorado. As an alternative, Aguirre-Diaz and Labarthe-Hernandez (2003) proposed that many large-volume ignimbrites in the Sierra Madre Occidental were derived from linear fissures along basin and range faults. The small number of suggested calderas in the Chon Aike Province and the presence of rhyolite dikes and domes along the trend of the major faults in the Manantial Espejo district also suggest that fissure eruptions are a potential eruption type for the large volume ignimbrites.   2.8. Conclusions  The more than 400 m thick volcanic sequence of the Chon Aike Province at Manantial Espejo consists of silicic units from the Chon Aike Formation that are overlain by volcanogenic sedimentary rocks from the La Matilde Formation, which in turn are older than andesitic to dacitic rocks from the informally named intermediate formation. Uranium- lead geochronology on zircon and 40Ar-39Ar dating of hornblende show that the volcanic activity lasted from 162 to 157 Ma, but probably less than 1 Ma was needed to deposit the vast part of the volcanic products represented by the voluminous rhyodacite to rhyolite ignimbrites and volcaniclastic sandstones from the Chon Aike Formation. Geochronology on zircons from a granite xenolith suggests co-genetic intrusions at depth. The lack or very uncommon occurrence of caldera structures in the Chon Aike Province suggests that the large volume ignimbrites were erupted along fissures rather from calderas. Most extension and faulting in the district post-date the volcanic activity. The west-northwest trending normal faults form a horst and graben structure with vertical displacements that locally exceed 100 m. The faults are locally filled with hydrothermal veins forming the Manantial Espejo Ag(-Au) deposit.   54 2.9. References  Acocella, V., Gudmundsson, A., and Funiciello, R., 2000, Interaction and linkage of extension fractures and normal faults: examples from the rift zone of Iceland: Journal of Structural Geology, v. 22, p. 1233-1246. Aguirre-Diaz, G. 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Ph.D. thesis, Universidad nacional de La Plata, Argentina. 58 Sruoga, P., 1994, El Complejo Caldera La Peligrosa, cordillera patagónica austral, (47°15' L.S.): Actas del VII Congreso Geológico Chileno, Concepción, 1994, p. 1219-1223. Stipanicic, P. N., and Bonetti, W., 1970, Posiciones estratigráficas y edades de las principales floras jurásicas argentinas. II. Floras doggerianas y málmicas: Ameghiniana, v. 8, p. 101-118. Stipanicic, P. N., and Reig, O. A., 1957, El "Complejo Porfirico de la Patagonia Extraandina" y su fauna de anuros: Acta Geológica Lilloana, v. 1, p. 185-230. Teruggi, M., and Rossetto, H., 1963, Petrología del Chubutiano del codo del río Sengerr: Boletín de Informaciones Petroleras, 354, p. 18-35. Tessone, M., Del Blanco, M., Macambira, M., and Rolando, A., 1999, New radimetric ages of the Chon Aike and Bajo Pobre formations in the central zone of the Deseado Massif, Argentina: II Simposio Sudamericano de Geología Isotópica, Córdoba, 1999, p. 132-135. Wallier, S., Tosdal, R. M., and Escalante, E. O., 2007, The geology of the Manantial Espejo district and its vein-hosted epithermal Ag(-Au) deposit, Deseado Massif, Argentina: "Digging Deeper", Ninth Biennial SGA Meeting, Dublin, 2007, p. 707-710.   59 Chapter 3 The Manantial Espejo epithermal Ag(-Au) deposit, Deseado Massif, Argentina: Vein parageneses, alteration, and age of mineralization2   3.1. Introduction  The Deseado Massif located in southern Argentina is a relatively new epithermal precious metal region with more than 30 mineralized areas that have been mostly discovered during the past 20 years. Despite being host of several economic ore deposits, there are relatively few studies that illuminate the details of the geology and history of hydrothermal activity related to ore genesis in the Deseado Massif. Yet there are a number of distinctive features of the region which make it stand out in the spectrum of epithermal deposits known world wide, and this includes the extent and length of epithermal veining and the widespread occurrence of silica sinters and near-surface silica bodies indicating preservation of a Mesozoic paleo-surface.  Manantial Espejo is situated in the Santa Cruz Province of southern Argentina (lat 48° 48’ S, long 69° 30’ W) at the southern rim of the Deseado Massif, about 160 km west northwest of Puerto de San Julián and about 60 km east of Gobernador Gregores. Silver mineralization was originally discovered in the 1970’s by the Argentine government, but exploration drilling did not start until 1991. In 2006, the Pan American Silver Corp. became the 100 % owner of the project, and the deposit entered production in the second half of 2008. Exploration prior to the onset of mining revealed a proven and probable reserve of 7.19 Mt at 166 g/t Ag and 2.36 g/t Au totaling 38.5 Moz Ag and 0.55 Moz Au (December 31, 2007). Silver-gold mineralization was formed in conjunction with the emplacement of the Middle to Late Jurassic Chon Aike Province, one of the world’s largest bimodal silicic- dominated igneous provinces. The Manantial Espejo district hosts the second largest  2 A version of this chapter will be submitted for publication. Wallier, S. and Tosdal, R.M. The Manantial Espejo epithermal Ag(-Au) deposit, Deseado Massif, Argentina: Vein parageneses, alteration, and age of mineralization.  60 epithermal deposit in the Deseado Massif, ranking behind Cerro Vanguardia. It consists of Ag-rich epithermal quartz-adularia veins with characteristic vein textures and vein minerals such as crustiform and colloform banding, lattice-bladed quartz, and abundant adularia. The veins are associated with quartz-adularia-illite-smectite alteration assemblages. Vein textures, ore and gangue minerals, and alteration assemblages mostly correspond to low- sulfidation epithermal deposits, but it also shares characteristics of intermediate-sulfidation epithermal deposits (Sillitoe, 1989 and 1993a; White and Hedenquist, 1995; Hedenquist et al., 2000; Einaudi et al., 2003; Sillitoe and Hedenquist, 2003; Gemmell, 2004) classifying Manantial Espejo as a low- to intermediate-sulfidation epithermal deposit. Excellent exposure of mineralized veins and host rocks and dense exploration drilling make the Manantial Espejo deposit an outstanding object for studying this type of epithermal precious metal mineralization. This paper describes and discusses the geology of the Ag(-Au) deposit including structures, vein minerals, paragenetic sequence, and alteration, with focus on the Maria Vein, the largest orebody of the district with a continuous outcrop of 920 m and a thickness of locally more than 20 m, and more than 250 drill holes that intersect the vein. The Maria Vein is compared to other mineralized veins of the same type of the deposit, but also to veins of a contrasting base metal sulfide-rich epithermal mineralization that is present in the district. Microthermometry on fluid inclusions from vein quartz and sphalerite constrains the temperature-salinity properties of the mineralizing fluids and illuminate the complex evolution of the geothermal system with significant fluctuations in temperature, pressure, and salinity, which are generally not observed in active geothermal equivalents such as the Taupo Volcanic Zone in New Zealand. 40Ar/39Ar geochronology on hydrothermal adularia yielded some of the first reliable ages for the hydrothermal activity in the Deseado Massif and places the hydrothermal system into the temporal framework of volcanic activity during the formation of the Chon Aike Province.      61 3.2. Geological setting  3.2.1. Geological setting of the Chon Aike Province  The Manantial Espejo epithermal Ag(-Au) district is part of the Chon Aike Province, a large bimodal igneous province that extends from the North Patagonian Massif south to the Antarctic Peninsula (Fig. 3.1, Kay et al., 1989; Pankhurst et al., 1998). Rhyolitic rocks are the most prominent in the province, but there is a consistent bimodality between rhyolite and andesite/basaltic andesite (Pankhurst et al., 1998; Riley and Leat, 1999). Underlain by a basement of Precambrian and Cambrian metamorphic rocks, Permo- Triassic sedimentary and plutonic rocks, and Triassic to Jurassic volcanic and plutonic rocks of the Choiyoi Formation, the Chon Aike Province has an estimated volume of 235,000 km3 and was emplaced during more than 30 Ma between Early and Late Jurassic (Pankhurst et al., 1998; Pankhurst et al., 2000). The origin of the Chon Aike volcanic rocks is proposed to be the result of anatexis of hydrous lower crust, which mixed with fractionated components of arc-related mafic underplating. Long-term storage developed a uniform magma, of which fractionated magmas migrated to upper-crustal level magma chambers where they assimilated upper-crustal material and eventually erupted to the surface (Riley et al., 2001).  Lithospheric extension related to the Gondwana break-up, mantle plume impact, and subduction along the paleo-Pacific margin may have been important for the generation and emplacement of the large volume of silicic volcanic rocks (Kay et al., 1989; Pankhurst and Rapela, 1995; Pankhurst et al., 1998; Riley and Leat, 1999; Pankhurst et al., 2000; Riley et al., 2001).  3.2.2. Geological setting of the Deseado Massif  The earliest volcanic rocks in the Deseado Massif of ages around 178 Ma are recorded near the Atlantic coast, but the bulk of the igneous rocks was formed between 169 and 152 Ma (Alric et al., 1996; Féraud et al., 1999; Pankhurst et al., 2000; Chapter 2). Early to Middle Jurassic pyroclastic and epiclastic rocks and basalts of the Roca Blanca Formation are overlain by intermediate rocks (basaltic andesite and andesite lavas, tuffs and 62        Figure 3.1: Map of southern South America showing exposures of the Middle to Late Jurassic Chon Aike Province (modified after Pankhurst et al., 1995, and Pankhurst et al., 1998). DM = Deseado Massif. NPM = North Patagonian Massif.   63 intrusive rocks) of the Bajo Pobre Formation, which is in turn overlain by the Chon Aike Formation, the most voluminous formation of the Deseado Massif (Fig. 3.2; Stipanicic and Bonetti, 1970; Pankhurst et al, 1998). The more than 500 m thick sequence of the Chon Aike Formation consists of silicic ignimbrites with subordinate volcanogenic sedimentary deposits, air fall tuffs, intercalated lavas, and rhyolitic dikes and domes (Pankhurst et al., 1998; Pankhurst et al., 2000; Riley et al., 2001; Sharpe et al, 2002; Chapter 2). The Chon Aike Formation is overlain by volcanogenic sedimentary rocks of the La Matilde Formation (Feruglio, 1949; Stipanicic and Reig, 1957; Lesta and Ferello, 1972; Pankhurst et al., 1998). Intermediate volcanic and sub-volcanic rocks (andesites to dacites) conclude the volcanic succession of the Chon Aike Province in the Deseado Massif (Chapter 2). Hydrothermal systems were active throughout the Deseado Massif in association to the Late Jurassic volcanism. Hydrothermal veins fill mostly normal faults and are locally mineralized, forming more than 30 epithermal precious metal deposits. The largest among them are the Cerro Vanguardia Au-Ag deposit, the Manantial Espejo Ag(-Au) deposit, and the Mina Martha Ag deposits (Fig. 3.2; Schalamuk, et al., 1997). The Jurassic igneous units and hydrothermal veins are unconformably overlain by Late Jurassic and Cretaceous continental sediments, Neogene shallow marine deposits, Late Miocene continental sediments, and Neogene plateau lavas (Feruglio, 1949; Teruggi and Rossetto, 1963; Archangelsky, 1967; De Giusto et al., 1980; Hechen and Homovc, 1988; Panza and Nullo, 1994; Gorring et al., 1997; Gorring and Kay, 2001; Del Río, 2004; Parras and Casadío, 2006).  3.2.3. Geological setting of the Manantial Espejo district  The Manantial Espejo district is located along the southern rim of the Deseado Massif (Fig. 3.2). The geology consists of a silicic-dominated bimodal volcanic sequence emplaced between 162 and 157 Ma. The district geologic framework and geochronology are described and discussed in Chapter 2. The lowermost outcropping unit of the Chon Aike Formation is a more than 250 m thick rhyodacite (BQR; Figs. 3.3, 3.4 and 3.5) that is divided into three lithofacies. These are a homogeneous, moderately welded, medium-grained rhyodacite ignimbrite MBR that 64        Figure 3.2: Simplified geological map of the Deseado Massif showing locations of epithermal ore deposits (modified from Panza and Nullo, 1994; Schalamuk et al., 1997; Echeveste, 2005; Chapter 2). 1 Manantial Espejo; 2 Mina Martha; 3 La Pilarica; 4 La Manchuria; 5 La Valenciana; 6 La Marcelina; 7 La Esperanza; 8 La Josefina; 9 Ea. Sol de Mayo; 10 Piche; 11 Lejano; 12 Rio Pinturas; 13 El Macanudo-El Mirasol; 14 Bajo Pobre; 15 Cerro Negro; 16 La Mariana-Eureka; 17 Cerro Saavedra; 18 Huevos Verdes; 19 La Emilia; 20 La Sorpresa; 21 El Pluma; 22 Tres Hermanas; 23 Cerro Chato; 24 Microondas-Martinetas; 25 Bajo de La Leona; 26 Cerro Moro; 27 Chispas; 28 La Paloma; 29 El Dorado-Monserrat; 30 Cerro Vanguardia;  31 Laguna Guadalosa.   65        Figure 3.3: Simplified geological map of the Manantial Espejo district showing areas with hydrothermal alteration and veining.   66        Figure 3.4: Compilation of the stratigraphic sequence in the Manantial Espejo district with isotopic ages (Chapter 2). All ages except for UVD dacite-east (40Ar-39Ar on amphibole) were determined by U-Pb SHRIMP-RG on zircons. Abbreviations: ALT = ash lithic tuff, volcanogenic sedimentary deposits (La Matilde Formation); CBR = coarse-grained bimodal quartz rhyodacite ignimbrite (Chon Aike Formation); EBR = equant and bimodal quartz rhyolite ignimbrite (Chon Aike Formation); EQR = equant quartz rhyolite ignimbrite (Chon Aike Formation); FBR = fine-grained bimodal quartz rhyodacite ignimbrite (Chon Aike Formation); FMA = fine- to medium-grained andesite (intermediate formation); LXT = lithic crystall tuff (Chon Aike Formation); MBR = medium-grained bimodal quartz rhyodacite ignimbrite (Chon Aike Formation); PAI = porphyritic andesite shallow intrusions (intermediate formation); SRI = quartz-sanidine rhyolite dikes, domes and associated breccias (Chon Aike Formation); UVD = upper volcaniclastic deposits.    67        Figure 3.5: Geology map of the central vein zone with names of localities named in the text. Gauss-Kruger (DHDN, zone 2) coordinates. Abbreviations: ALT = ash lithic tuff, volcanogenic sedimentary deposits (La Matilde Formation); BQR = bimodal quartz rhyodacite ignimbrite (Chon Aike Formation); EBR = equant and bimodal quartz rhyolite ignimbrite (Chon Aike Formation); EQR = equant quartz rhyolite ignimbrite (Chon Aike Formation); FMA = fine- to medium-grained andesite (intermediate formation); LXT = lithic crystall tuff (Chon Aike Formation); SRI = quartz-sanidine rhyolite dikes, domes and associated breccias (Chon Aike Formation); UVD = upper volcaniclastic deposits.   68 grades into a fine-grained lithofacies FBR at the top. A coarse-grained facies CBR is present in the upper part of MBR. The main lithofacies MBR has an age of 162.0 ± 1.1 Ma (SHRIMP-RG U-Pb on zircons). A variably intercalated lithic crystal tuff (LXT) with thin beds of accretionary lapilli is overlain by an equigranular and equant-shaped quartz rhyolite ignimbrite (EQR). The top ignimbrite of the Chon Aike Formation is an equant-shaped and bimodal-sized quartz rhyolite ignimbrite (EBR) that was dated at 161.3 ± 1.1 Ma. The volcaniclastic sequence is intruded by sanidine-quartz rhyolite dikes and domes (SRI). The Chon Aike Formation is overlain by the La Matilde Formation, which is composed of mainly reworked and re-sedimented volcaniclastic rocks of the Chon Aike Formation. This ash lithic crystal tuff (ALT) includes coarse- to fine-grained volcanogenic sedimentary breccias and sandstones with variable clast contents. Around 160 Ma, a period of andesitic to dacitic magmatic activity initiated. Earliest recorded fine- to medium-grained porphyritic andesite flows, shallow intrusions and associated breccias (FMA) were dated at 160.0 ± 1.2 Ma. They are overlain by dacite ignimbrites (UVD dacite-west, 161.1 ± 1.0 Ma; UVD dacite-east, 157.7 ± 1.0 Ma), and intruded by a porphyritic andesite with an age of 156.7 ± 1.5 Ma, which is the youngest recorded Jurassic magmatic unit of the district. Dominantly west-northwest striking normal faults are the result of mild extension. The faults mostly postdate the emplacement of the volcanic rocks. The major faults have vertical displacements exceeding 100 m, and formed a horst and graben geometry. Hydrothermal activity was contemporaneous with the fault formation (Chapter 4) and produced widespread alteration and formed numerous hydrothermal veins that locally contain precious metal mineralization forming the Manantial Espejo epithermal Ag(-Au) deposit.   3.3. Geology of the Manantial Espejo Ag(-Au) deposit  Most hydrothermal veins in the Manantial Espejo district are Ag-rich examples of low- to intermediate-sulfidation quartz-adularia epithermal precious metal veins with characteristic vein textures of crustiform and colloform banding, bladed calcite replaced by quartz, and abundant adularia associated to adularia-illite alteration. A second set of 69 epithermal veins and vein breccias enveloped by the same alteration assemblages exhibits a different style of mineralization with coarse-grained base metal sulfides and commonly clear comb quartz. The deposit consists of several areas with veins and hydrothermal alteration covering more than 25 km2 (Fig. 3.3). The principal regional lineament strikes 120° to 130° and consists of conjugate dipping sets of linked normal faults with vertical displacements of tens to about 150 m forming a horst and graben geometry with mild block rotation. Two secondary sets of faults with shorter strike lengths and generally smaller displacements strike 090° to 115° and 060° to 070°. They probably developed in the transfer zones of overstepping normal faults. Principal normal faults are locally mineralized. The Maria Vein for example, the richest vein of the district, is located in one of these faults, and also the Unión-part of the Karina-Unión stockwork is mostly hosted by such a fault. However, broad parts of these faults only have minor hydrothermal infill and commonly exhibit tectonic breccias with or without hydrothermal matrix. The secondary faults are usually filled by thin unmineralized quartz veins, but the economic and up to several meter thick Concepción and Melissa Veins, and the Karina-part of the Karina-Unión stockwork are hosted by such faults. Styles of mineralization include massive quartz veins, vein breccias, sheeted and stockwork veins, replacements, and minor dissemination. Quartz is the most common gangue mineral. Adularia is common and more abundant in deeper parts of veins. Carbonates and possibly barite were also precipitated but replaced by quartz and minor adularia. Hypogene ore minerals are acanthite/argentite, silver sulfosalts, electrum, native silver, uytenbogaardtite, iodargirite, stromeyerite, pyrite, galena, sphalerite, chalcopyrite, tetrahedite, marcasite, arsenopyrite, bornite, Sn(-Pb)-sulfides, enargite, hematite, and magnetite (Espinosa and Núñez, 1996; Echeveste, 2005). Hypogene mineralization is mainly related to early gray and white crustiform and colloform banded quartz, and late veins, vein breccias, replacements, and open space infill. Supergene mineralization and enrichment include goethite, jarosite, hematite, acanthite, native silver and gold, covellite, chalcosite, bornite, and malachite (Echeveste, 2005). All four economic vein systems and hundreds of weakly or unmineralized veins and stockworks of variable size are located in the central vein zone in an area of only about 70 5 km2 and hosted by a horst bounded by the Maria Fault to the south-southwest and the Karina Fault system to the north-northeast (Fig. 3.3 and 3.5). The horst underwent more erosion than the surrounding grabens and therefore stratigraphically deeper units and veins showing higher temperature mineral assemblages crop out. The horst with the central vein zone exposes silicic rocks of the Chon Aike Formation that underwent extensive hydrothermal alteration showing alteration assemblages with adularia, illite, illite-smectite, pyrite, and chlorite. The Maria Vein is the largest structure with a continuous 920 m long outcrop and variable thickness locally exceeding 20 m. It has been drilled to 300 m below outcrop. The vein strikes 120° to 130° and it dips 50° to 70° to the southwest, showing a characteristic curviplanar geometry of an integrated normal fault. The Karina-Unión orebody is a system of sheeted veins and stockworks, with dips from near vertical to 60° north. The veins have a surface exposure of 850 m length and are recognized to a depth of 150 m. The whole system has a width of over 100 m. The Concepción Vein consists of a system of several sub-parallel veins with a general strike between 060° and 070° and a dip of 45° to the south. The largest of these veins has a strike length of about 600 m, a thickness between 1 and 12 m, and a vertical extension of at least 200 m. The Melissa Vein system strikes 090° to 115o with subvertical dips and a recognized length of about 600 m. The thickness reaches 3 m, and it extends to about 100 m below surface. Hydrothermal veins and alteration are also present at various areas outside the central vein zone (Fig. 3.3). Several of these areas lay along the Maria Fault structure. To the west of the Maria Vein at Nueve Vetas and Mesa, sphalerite-galena-chalcopyrite-rich veins of generally low Ag- and Au-grades have been discovered. Farther to the west, the structurally complex Candelaria spreads out over an area of about 800 by 400 m, with a major vein of about 350 m strike length and up to 9 m thickness that shows abundant lattice-bladed quartz of variable but generally low Au- and Ag-grades. The westernmost area and still in the trend of the Maria Fault is the so-called Candelaria Extension with thin but probably more than 2 km long west-northwest trending veins. About 10 km to the east- southeast of the Maria Vein in the same trend of the fault, the La Gruta area hosts several veins of likely economic mineralization. To the north of the central vein zone, a 2 to 3 km wide graben preserved stratigraphically higher lithological units of which large parts underwent intense silicification forming blankets of chalcedony and microcrystalline 71 quartz. These silica bodies are interpreted as paleo-water tables that channeled lateral flow of the hydrothermal system leading to silica precipitation. Such shallow epithermal features are well developed at La Flecha just north of the central vein zone and have the largest extent at the Laguna Breccia (Fig. 3.6 and 3.7). Exploration drilling underneath the Laguna Breccia to a depth of 250 m below surface discovered scarce base metal sulfide disseminations along with silicification. Similar dissemination was also found to the west of the Laguna Breccia in a circular depression about 50 m deep. Further to the north in the neighboring horst, the Gaby area shows several up to multiple meters wide and several kilometers long veins with abundant breccias and mostly massif white quartz, but to date none of these veins showed consistent elevated ore-grades. South of the Maria Fault, the locally mineralized Veronica Veins are located at the northern rim of the neighboring horst southwest of the central vein zone (Fig. 3.3). This study focuses on the central vein zone with quartz-adularia precious metal veins, and the Mesa Veins in the western extension of the central vein zone with a base metal sulfide-rich style of epithermal mineralization. In the central vein zone, the focus lays on the Maria Vein, by far the largest and most mineralized vein of the district.  3.3.1. Sampling and analytical techniques: vein and alteration mineralogy  The paragenetic sequence and the vein minerals were determined by outcrop and drill core studies (logging of 34 drill core intervals that penetrate the Maria Vein; results are summarized in appendix E), hand samples petrography, and microscopy of polished thin sections and rock specimens. Few sections were also analyzed by a scanning electron microscope (SEM) at the University of British Columbia using a Philips XL30 electron microscope with Princeton Gamma-Tech energy-dispersion X-ray spectrometer and image analysis systems. Ore grades are derived from exploration drill core assays (Au and Ag) and geochemical analyses on representative samples from the specific stages. The vein geochemistry is studied in more detail in Chapter 5. Alteration is examined on drill cores from three veins, the Maria Vein, the Concepción Vein, and the Mesa Vein, whereas the focus lies on two cross sections perpendicular through the Maria Vein (Fig. 3.8). Those two sections were chosen based on 72        Figure 3.6: Distribution of tabular silica bodies (black) and silicified rhyolite dikes (red) in the Manantial Espejo district surrounding the central vein zone. A, B, and C indicate the location of the outcrops shown in figure 3.7.  73        Figure 3.7: Outcrops of tabular silica bodies in the Manantial Espejo district. The locations are shown in figure 3.6. A. Silicified volcanogenic sedimentary breccias of the La Matilde Formation at the Laguna Breccia. B and C. Gently dipping silicified laminated ash.    74        Figure 3.8: Structure of the Maria Vein. A. Outcrop map of the Maria Vein with location of cross sections in figures B and C. Drill holes are shown of which the vein intervals are logged and reported in appendix E; six additional holes to the east are not shown on the map. The red drill holes are used for alteration studies (Gauss Kruger, DHDN, zone 2 coordinates). B. Cross section through the Maria Vein at 425 W. C. Cross section through the Maria Vein at 225 W. The red drill holes are used for alteration studies. Abbreviations: ALT = ash lithic tuff, volcanogenic sedimentary deposits (La Matilde Formation); EQR = equant quartz rhyolite ignimbrite (Chon Aike Formation); FBR = fine-grained bimodal quartz rhyodacite ignimbrite (Chon Aike Formation); fEQR = fine-grained equant quartz rhyolite ignimbrite (Chon Aike Formation); LXT = lithic crystall tuff (Chon Aike Formation); MBR = medium-grained bimodal quartz rhyodacite ignimbrite (Chon Aike Formation); USD = unconsolidated sedimentary deposits; UVD = upper volcaniclastic deposits.  75 the following criteria: The sections have a well established geology. Each section goes through a high-grade and low-grade vein segment respectively. The high-grade vein segment contains the complete record of the paragenetic stages, and has a variable thickness between less than 4 and more than 12 m. The low-grade vein segment contains only the paragenetic stages 3 and younger, and has a thickness of mostly between 4 and 8 m. More than 300 samples were analyzed by a portable infrared mineral analyzer (PIMA) at the Teck Cominco Ldt. PIMA facility in Vancouver, Canada. Spot analyzes were evaluated with the software PimaView 3.1 (1999) using the provided standard libraries and the purest samples of the own sample suite. The PIMA is a field-portable infrared spectrometer that allows rapid and cost effective identification of alteration minerals in a large number of samples with minimal sample preparation (Pontual and Merry, 1995). The spectral absorption features observed in the PIMA spectral range (1300-2500 nm) are the result of lattice vibrations that occur at longer wavelength due to bending and stretching of molecular bonds in hydroxyl, water, carbonate and ammonia, and between Al-OH, Mg-OH, and Fe-OH. These molecules are found as major components in phyllosilicates (including clay, chlorite, and serpentine minerals), hydroxylated silicates (such as epidotes and amphiboles), sulphates (alunite, jarosite, and gypsum), and carbonates. Five samples were also analyzed by a SpecCam at Spectra-Map Ldt., Oxfordshire, United Kingdom. The SpecCam uses the same infrared absorption features as the PIMA, and the two devices gave largely identical results and the SpecCam results are thus not reported. Since adularia is not detectable by infrared techniques, few samples were treated by “HF-staining”, a technique where the surface of a cut rock specimen is edged by hydrofluoric acid (HF) before K- feldspar is stained yellow with sodium cobaltinitrite. To test the PIMA results, 13 samples from the Maria Vein were also analyzed by X- ray diffraction (XRD) at the micro-beam facility at the University of British Columbia, Canada. The interest was on the qualitative analysis of the clay minerals. The samples were ground into fine powder and smeared on to a glass slide with ethanol. Step-scan X-ray powder-diffraction data for the samples were collected over a range 3-80° 2θ with CoKα radiation on a standard Siemens (Bruker) D5000 Bragg-Brentano diffractometer equipped with an Fe monochromator foil, 0.6 mm (0.3°) divergence slit, incident- and diffracted- beam Soller slits and a Vantec-1 strip detector. The long fine-focus Co X-ray tube was 76 operated at 35 kV and 40 mA, using a take-off angle of 6°. Four samples were also analyzed after solvated with ethylene glycol (glycolated). Mineral identification was done using the International Centre for Diffraction Database PDF-4 and Search-Match software by Siemens (Bruker). The sample location, the mineral phases identified by the PIMA, and the mineral phases identified by XRD are given in appendices F, G, and H.  3.3.2. The Maria Vein  3.3.2.1. Structure The largest vein in the Manantial Espejo district in terms of dimensions and resources is the Maria Vein, a Ag-rich epithermal quartz-adularia vein that fills a complex normal fault (Fig. 3.8; tab. 3.1). The curviplanar vein dips 50° to 70° to the south-southwest and has a continuous outcrop of 920 m strike length with a variable thickness of up to 17 m. The vein is documented by exploration drilling to a depth of about 300 m below outcrop, and it shows abrupt changes in thickness of locally less than 4 m to more than 20 m. The upper part of the vein marks the contact between a thick rhyodacite ignimbrite in the footwall and younger rhyolitic to dacitic volcaniclastic rocks in the hanging wall. Throw is variable along the vein but highest to the southeast where it reaches about 150 m. The Maria Vein is located in the main fault from which various syn- and antithetic branches splay into the hanging wall. Many of these splays contain a hydrothermal infill but the veins are mostly blind and only to the southeast do they reach considerable thickness and outcrop locally.  3.3.2.2. Paragenetic sequence The Maria Vein shows a large variety of hydrothermal stages and vein textures. The paragenetic sequence is divided into six main stages of hydrothermal activity (Fig. 3.9) defined by crosscutting relationships and characteristic textures and minerals that are largely consistent along the entire vein. However, some stages or sub-stages are restricted to certain segments or levels of the vein. More detailed mineralogical studies are presented in Espinosa and Núñez (1996) and Echeveste (2005). 77        Figure 3.9: Hydrothermal stages of the Maria Vein with characteristic vein textures, gangue, and hypogene ore minerals. Line weight indicates relative abundance.  78 Table 3.1: Resources of the Manantial Espejo Ag(-Au) deposit Tonnage Ag Au Ag Au Ag / Au Mt ppm ppm Moz Moz ratio Measured 2.89 151.6 2.69 14.09 0.25 56 Indicated 2.24 130.3 2.15 9.38 0.15 61 Measured and indicated 5.13 142.3 2.45 23.47 0.40 58 Measured 1.72 194.8 1.89 10.77 0.10 103 Indicated 0.75 160.9 1.72 3.88 0.04 94 Measured and indicated 2.48 184.5 1.84 14.71 0.15 100 Measured 0.10 487.9 7.04 1.57 0.02 69 Indicated 0.15 453.0 6.28 2.18 0.03 72 Measured and indicated 0.25 466.9 6.58 3.75 0.05 71 Measured 0.33 192.4 2.18 2.04 0.02 88 Indicated 0.54 195.7 1.95 3.40 0.03 100 Measured and indicated 0.87 194.5 2.03 5.44 0.06 96 Measured 5.04 175.6 2.47 28.45 0.40 71 Indicated 3.68 159.2 2.20 18.84 0.26 72 Measured and indicated 8.73 168.7 2.35 47.35 0.66 72 Concepción Vein Totals Ore body Resource calculations by December 2005, according to technical report for the Manantial Espejo project, Canadian Standard National Instrument 43-101. Resource Category Maria Vein Karina-Unión Veins Melissa Vein 79 Stage 1 is restricted to two vein segments of 175 m and 285 m strike length. It is commonly present in the upper part of the vein where it has a thickness of up to 6 m, and reported to a depth of about 200 m below surface. It consists of crustiform and colloform white and gray millimeter to several centimeter wide bands of dominantly quartz and very minor adularia (Figs. 3.10 A and B). The gray bands contain pyrite, sphalerite, galena, minor chalcopyrite, tetrahedrite, acanthite/argentite, Ag-sulfosalts, and occasionally electrum. The ore minerals are fine-grained and intergrown with equally fine-grained quartz, and only at deeper levels the minerals are slightly coarser-grained and coincide with fine adularia needles. Breccias are uncommon but are present locally and more abundant towards the end of the stage. Stage 1 contributes significant Ag and Au to the total mineralization. Ore grades vary between 100 and 3000 ppm Ag and 1 to 40 ppm Au. The oldest hydrothermal stage is overgrown or cut by younger stages and the contacts are locally brecciated (Figs. 3.10 C, D, and E). Stage 2 overgrows stage 1 or forms the cement of clast- to matrix-supported breccias with stage-1-clasts (Figs. 3.10 C, D, and E). The second hydrothermal stage includes largely barren massive fine- to medium-grained white quartz with abundant replaced bladed calcite and massive to vuggy white sugary quartz (Fig. 3.10 F). Towards the end of the stage, quartz is also present as coarser-grained pale amethyst. Former calcite blades are commonly 1 to 4 cm, but locally they reach 10 cm. Calcite blades have been replaced by mostly quartz, but also locally by fine-grained adularia. Very scarce pyrite is the only opaque mineral. Stage 2 overlaps the two vein segments with stage 1, but has longer strike length and extends farther to the northwestern part of the vein. It is commonly 0.5 to 2 m thick. Brecciation during stage 2 was rare and only few breccias occur with stage-1-clasts and stage-2-matrix (Fig. 3.10 E). Silver-grades are commonly less than 10 ppm and Au-grades less than 0.1 ppm. Stage 3 commonly overgrows or cuts stages 1 and 2 with generally sharp contacts. Locally, it has a brecciated contact with wall rock or older stages (Fig. 3.10 F). Stage 3 is present along the entire strike length of the Maria Vein and forms the thickest vein infill of up to 14 m (Fig. 3.11 A). It consists of colloform and crustiform banded quartz and adularia of variable shape and grain-size, generations of amethyst, and bands of quartz pseudomorphs after platy calcite and/or barite (Fig. 3.11). Quartz-adularia bands are 80      Figure 3.10: Photographs of outcrops, vein textures, and minerals from the Maria Vein (Stages 1 and 2). A and B. Colloform-crustiform gray and white banded quartz of stage 1; gray bands contain fine-grained ore minerals. C and D. Crustiform banded gray and white quartz of stage 1 overgrown and cut by massive bladed quartz of stage 2. E. 15-cm clast of stage 1 in a matrix of massive white quartz of stage 2 that cuts through stage 1. F. Lattice-bladed calcite replaced by quartz from stage 2 overgrown by colloform-crustiform banded quartz-adularia from stage 3. Adularia is altered to kaolinite and Fe-hydroxides (brown).  81        Figure 3.11: Photographs of outcrops, vein textures, and minerals from the Maria Vein (Stage 3). A. Outcrop of several meters thick colloform and crustiform banded quartz-adularia from stage 3 in about the center of the Maria Vein. B and C. Colloform banded quartz and adularia from stage 3 in outcrop and drill core; adularia partially altered to kaolinite. D. Gray and white colloform-crustiform banded quartz from stage 1 is cut by similar looking colloform-crustiform banded quartz-adularia of stage 3. D. Drill core photograph of coarse-grained massive adularia intergrown with quartz and sulfides of stage 3 high-grade (3hg). Abbreviations: ad = adularia; qz = quartz. 82 millimeter to centimeters wide, bands with lattice-bladed texture are commonly a few centimeters wide but locally they reach up to 30 cm with thin up to 18 cm long blades. Millimeter thick colloform bands with pyrite and scarce sphalerite, galena, chalcopyrite, and Ag-minerals are locally present. Aggregates of quartz associated with adularia or its weathered products have locally acicular or pseudo-acicular textures (Dong et al., 1995). Generation of coarse euhedral and usually zoned comb quartz are present towards the end of stage 3. The grain-size and abundance of adularia increase downwards to locally massive adularia below 200 m depth. Stage 3 also occurs as cockade vug infill and breccia matrix where it commonly cements clasts of the same stage but locally also clasts of wall rock or older paragenetic stages. This stage is low-grade (mostly < 50 ppm Ag and < 0.2 ppm Au), but it serves as a good host for younger hypo- and supergene mineralization where open spaces are filled or calcite and adularia replaced by Fe-hydroxides and silver minerals. Locally at the deepest levels of the Maria Vein and predominantly at the southeastern end of the vein, a Cu-Pb-Zn-Ag-Te-rich mineral assemblage is intergrown with massive coarse- grained adularia, and Ag-grades can exceed 2000 ppm (Fig. 3.11 E). The common opaque minerals are Fe-rich sphalerite, galena, chalcopyrite, and bornite. Although Te is abundant, telluride mineral phases have not been identified. Stage 4 is the main brecciation stage where clasts from older hydrothermal stages and wall rock are cemented by a fine-grained and commonly dark quartz-sulfide matrix of low Ag- and Au-grades (commonly < 50 ppm Ag and < 0.2 ppm Au; Figs. 3.12 A and B). Most of the stage 4 low-grade breccias (4lg) have an individual thickness between few centimeters and 1 m, but locally they reach up to 4 m and the total thickness may exceed 6 m. The majority of the breccias are polymictic and clast-supported with angular to sub- rounded clasts of commonly less than 2 cm diameter. However within 10 to 20 cm intervals, the breccias are locally matrix supported. The clasts mostly consist of vein quartz from older stages and in proximity to the hanging and footwall they are composed of mainly wall rock. The commonly light- to dark-gray matrix consists of a fine-grained cement of mainly quartz and pyrite with minor sphalerite and galena and uncommonly chalcopyrite. The abundance of galena and sphalerite results in elevated Pb and Zn concentrations of up to 3000 ppm, whereas Cu-grades are less than 100 ppm. Although most of the stage-4-breccias are low-grade, high-grade mineralization during this stage 83      Figure 3.12: Photographs of outcrops, vein textures, and minerals from the Maria Vein (Stages 4, 5, and 6). A. Stage 4 low-grade breccia that brecciates coarse-grained quartz of stage 3. B. Low-grade breccia with fine- grained quartz-sulfide cement from stage 4 and wall rock clasts. C. Stage 4 high-grade ore; stage 3 brecciated by stage 4 with a sulfide- and precious metal-rich cement, which is rebrecciated by light fine-grained quartz. D. Stage 4 high-grade breccia vein that cuts through stage 3. E. 5-mm wide quartz-galena-sphalerite-pyrite- chalcopyrite veins in adularia altered rhyodacite ignimbrite. F. Low-grade stage 4 breccia cut by thin quartz- sphalerite-galena vein of stage 5. G. Green chalcedony-microcrystalline quartz vein from stage 6 cutting older quartz vein in rhyodacite ignimbrite. Abbreviations: ad = adularia; gn = galena; qz = quartz; sl = sphalerite.  84 (4hg) is present mostly at the deeper levels (below 100 m below outcrop) in the form of breccia cement, vug-infill, replacements of adularia, and in thin veins (Figs. 3.12 C and D). Silver-grades are commonly more than 1000 ppm and exceed locally 10,000 ppm. Gold- grades are variable and range from less than 1 ppm to about 30 ppm. High Ag-grades are associated with Cu-, Pb, and Zn-concentrations in the percent-range. Pyrite, mainly Fe-rich sphalerite, galena, chalcopyrite, acanthite/argentite, Ag-sulfosalts, stromeyerite, native silver, and uncommonly electrum are the opaque minerals. SEM-analysis also detected Sn- sulfide and Sn-Pb-sulfide phases. Stage 4 high-grade mineralization produced the highest Ag-grades in the district, but it usually has a width of only few centimeters to about 1 m. Stage 5 consists of thin (< 1 cm) veins and open space infill of clear quartz and base metal sulfides (mainly coarse-grained Fe-poor sphalerite and galena, minor chalcopyrite and pyrite; Figs. 3.12 E and F). These uncommon veins cut through the stages 1 to 4, but their volume and precious metal contribution is insignificant. Stage 6 is also a small-scale and low-grade feature represented by post- mineralization brown and green chalcedony veins, vug infill, and breccia matrix (Fig. 3.12 G). It commonly has a thickness of less than 10 cm. Supergene alteration and mineralization in the hanging wall close to the vein and in the vein itself commonly has a wide range of elevated Ag- and Au-grades of several 10s to 100s of ppm Ag and several ppm Au. Iron-hydroxides form gray-brown crack-infill and coating of fracture and crystal surfaces, and they locally contain small inclusions of native gold. Hypogene sulfides in the wall rock and the veins are oxidized or leached, vein quartz is gray where supergene ore fills intergranular spaces and replaces together with clay minerals hypogene adularia (Fig. 3.13). Supergene ore minerals are mainly acanthite, native silver, and native gold. The highest Au-grades in the Maria Vein of more than 100 ppm coincide with supergene enrichment where visible gold forms yellow coating on fractures or quartz crystal surfaces or occurs as grains in Fe-hydroxides.      85        Figure 3.13: Photographs of supergene enriched vein textures from the Maria Vein. A. Bladed calcite replaced by quartz from stage 2 turned grey by fine-grained Fe-hydroxides and Ag-minerals filling open and intergranular spaces. B. Supergene alteration and enrichment of colloform banded quartz-adularia form stage 3. Adularia is replaced by kaolinite, Fe-hydroxides, and Ag-minerals. 86 3.3.2.3. Hydrothermal alteration Most volcanic rocks that host the Maria Vein are intensely altered with all igneous minerals except quartz and accessory zircon replaced by alteration minerals (98-100 % replaced except quartz and accessory zircon). Despite the complete replacement of igneous minerals, the primary volcanic rock textures are largely preserved. A higher percentage of the original minerals is only preserved in the distal parts of the vein. The alteration halo surrounding the Maria Vein is asymmetrically developed due to the 50° to 70° inclination of the vein to the south-southwest. Hanging wall alteration is intense to a distance of more than 150 to 200 m, whereas the footwall only shows 5 to 10 m of intense adularia-illite alteration before it passes into moderate but pervasive chlorite alteration. The hypogene alteration shows largely a typical horizontal and vertical zoning of adularia alteration proximal to the vein and at deeper levels grading into illite-smectite alteration, and into distal and commonly pervasive chlorite alteration. These alteration assemblages are locally overprinted by kaolinite alteration of mostly supergene origin. Shallow-level blankets of intense silicification are present at various places in the district. A variety of alteration minerals was formed adjacent to the Maria Vein. The occurrence, spatial distribution, and the temporal relationships of selected alteration minerals are described below and their distribution is shown in figures 3.14 and 3.15.  Quartz: Quartz is the most abundant alteration product throughout the hydrothermal system. It is present as small grains replacing igneous and hydrothermal minerals and rock matrix. Tabular massive silica bodies where >99 % of the rock was replaced by microcrystalline quartz or opaline silica are broadly distributed in the district replacing lithological units of a high stratigraphic position and forming morphological highs (Fig. 3.7). The silica bodies are extremely hard and have variable color from black, gray, brown, red, blue, green, to white. Silicified ash layers exhibit fine lamination. Silicified breccias from the La Matilde Formation form the massive silica bodies of the Laguna Breccia. Such intensely silicified horizons are not adjacent to the Maria Vein, but they are present a few hundred meters to the south and north of the vein in the graben structures.  87   Figure 3.14: Zoning of alteration minerals and alteration assemblages about the Maria Vein displayed in a cross sections at 225W (Fig. 3.8). Wall rocks consist of volcaniclastic dacites to rhyolites. Alteration minerals were determined by drill core and thin section petrography, HF-staining, and PIMA, SpecCam, and XRD analysis. PIMA Sample locations are shown with diamond symbols. Alteration data and all sample locations are shown in appendices F, G, and H. 88   Figure 3.15: Zoning of alteration minerals and alteration assemblages about the Maria Vein displayed in a cross sections at 425W (Fig. 3.8). Wall rocks consist of volcaniclastic dacites to rhyolites. Alteration minerals were determined by drill core and thin section petrography, HF-staining, and PIMA, SpecCam, and XRD analysis. PIMA Sample locations are shown with diamond symbols. Alteration data and all sample locations are shown in appendices F, G, and H.  89 Adularia: Adularia is the only observed hydrothermal feldspar in the Manantial Espejo district. Its distribution is proximal to the Maria Vein. Intense adularia alteration is up to 50 m wide in the deeper parts of the hanging wall and thins out toward the shallower parts (Figs. 3.14 A and 3.15 A). Adularia together with quartz replaces mainly plagioclase and the ash matrix of the volcanic rocks. As demonstrated by HF-staining, adularia replaced locally most of the wall rock except for quartz. Adularia is accompanied by minor amounts of pyrite and illite, the latter usually indicates the outward or upward transition to alteration dominated by clay minerals. Adularia is locally overprinted by illite and/or kaolinite. In places, this overprint is complete. Adularia altered rock has a light brown-pink color and tends to break along conchoidal fractures.  Illite and interlayered illite-smectite: Illite is the most widespread alteration mineral surrounding the Maria Vein. It is most ubiquitous at the shallow levels proximal to the vein and envelopes approximately the zone of intense adularia alteration at the deeper levels (Figs. 3.14 B and 3.15 B). Illite replaces plagioclase, mafic minerals, and also commonly overprints hydrothermal adularia. The rock matrix is commonly strongly affected by illite alteration. Toward the shallow levels, illite contains more and more interstratified smectite. This transition is shallower in the 225 W section of the Maria Vein compared to the 425 W section, where near the surface illite and smectite have approximately equal amounts (Figs. 3.14 D and 3.15 D). Illite is commonly associated with variable amounts of pyrite and traces of chlorite. The texture of the gray-white rock remained largely intact. Especially near the Maria Vein and along the fault fracture mash of the hanging wall, illite is largely replaced by kaolinite, locally completely.  Pyrite: Intact, oxidized, or leached pyrite is ubiquitous in minor amounts of commonly less than 1 vol% except for the very surface-near regions where pyrite is almost absent (Figs. 3.14 E and F and 3.15 E and F). Mimicing the stratigraphy of the hanging wall, pyrite is most abundant (up to 5 vol%) within fine ash layers of the lithic crystal tuff (LXT) and also in the bimodal quartz rhyodacite ignimbrite (BQR) in section 425 W where pyrite replaces ferromagnesian minerals. Pyrite is mostly present as less than 1-mm cubes, but may also be present as up to 3 mm spherical aggregates. Along the Maria Vein, and 90 along the fault fracture mash of the hanging wall, pyrite is commonly oxidized to goethite or jarosite or completely leached forming cubic cavities.  Kaolinite: Kaolinite is commonly present in the hanging wall of the vein, where it is most intense in proximity of veins, faults, and fracture zones (Fig. Figs. 3.14 G and 3.15 G). It replaces igneous feldspars, hydrothermal adularia and illite, and the rock matrix. Kaolinite commonly replaces the rock completely except quartz. It is commonly accompanied by iron-hydroxides, and it mostly overlaps with oxidized or leached pyrite. Such rocks are commonly soft, of brown color, and the rock texture is greatly obliterated. The overlap of kaolinite and oxidized pyrite suggests that most of the kaolinite is of supergene origin, but locally and especially in the vicinity of the main vein where kaolinite is partly present together with intact pyrite, the kaolinite might have formed by a late stage hydrothermal overprint altering adularia and illite into hypogene kaolinite. Supergene alteration has been detected to a depth of more than 200 m below surface.  Calcite: Although calcite was precipitated in abundance as a gangue mineral as indicated by lattice-bladed textures, calcite is only a very minor alteration mineral in the alteration halo of the Maria Vein. It is locally present with smectite-rich interlayered illite- smectite and minor chlorite in the shallow and distal parts of the hanging wall. Calcite is a more important alteration mineral in andesitic host rocks of the Mesa Vein, but they are not present along the Maria Vein.  Chlorite: Chlorite is widespread in the Manantial Espejo district coloring most of the volcanic rocks green. It was formed distal to the hydrothermal veins and commonly replaces ferromagnesian minerals such as amphibole and biotite. It was also formed in the rock matrix, but plagioclase remained mostly intact. Chlorite alteration is most intense in the footwall of the Maria Vein a few meters away from the vein where the rock is dark green. It also occurs as traces in the hanging wall of the vein together with illite and illite- smectite.  91 The distribution in cross sections of hypogene alteration assemblages are summarized in figures 3.14 H and 3.15 H. The assemblage adularia + quartz + pyrite ± illite formed during the main stage of alteration proximal to the fluid conduits at relatively high temperatures. The variable amounts of illite reflects overprinting relationships of the illite + quartz + pyrite ± adularia alteration assemblage that envelopes the adularia + quartz + pyrite ± illite assemblage. At the shallow levels and distal to the vein, adularia is absent and the alteration assemblage is dominated by illite-smectite in an assemblage of illite-smectite + quartz ± pyrite ± chlorite. The transitions are gradual and show overprinting relationships. Chlorite + illite ± pyrite ± calcite alteration is the distal hypogene assemblage. It is most intense in the footwall of the vein but is present as weak but pervasive alteration throughout the district. Intense silicification forming tabular silica bodies formed at the shallow and peripheral levels of the hydrothermal system (Figs. 3.6 and 3.7). They are interpreted as steam-heated paleo-water tables and places of sub-surface lateral flow of the hydrothermal system into relatively permeable rock hosting aquifers, where the host rock was partially or completely replaced by chalcedony (Schoen et al., 1974). Late stage hypogene kaolinite may locally overprint the previous alteration assemblages in relatively narrow halos around the Maria Vein. Supergene kaolinite + Fe- hydroxides overprint the hypogene alteration assemblages intensively along the veins and the fault-fracture mash of the hanging wall where locally hypogene adularia and illite is completely replaced by kaolinite.  3.3.3. The Concepción Vein  3.3.3.1. Structure The main vein of the Concepción Vein system has a length of about 600 m and a thickness between 1 and 12 m. It has been drilled to a depth of 200 m below surface. The set of sub-parallel veins fills a normal fault that strikes between 060° and 070° and dips 45° to the south, which makes it the shallowest dipping, major vein of the district. The outcrop of the vein marks the contact between a rhyodacite ignimbrite and a rhyolite ignimbrite. The maximum total vertical displacement along the vein based on the stratigraphic offset might be more than 150 m. 92 3.3.3.2. Paragenetic sequence The paragenetic sequence of the Concepción Vein is similar to the sequence of the Maria Vein, but the variety of vein textures is less developed. The majority of the vein infill is barren fine- to medium-grained massive white to milky quartz with minor adularia and replaced calcite blades. Adularia becomes more abundant toward the end of the massive quartz and commonly turns into a poorly developed colloform-crustiform banding of quartz and adularia. Texture and mineralogy largely correspond to stages 2 and 3 of the Maria Vein, but with the massive quartz as the dominant vein infill. Silver- and Au-grades of such vein infill are mostly less than 50 ppm Ag and less than 0.5 ppm Au. Mineralization occurred relatively late in the paragenetic sequence in narrow veins and vein breccias that cut through wall rock and earlier vein quartz, and filled open spaces. The breccias are mostly clast-supported with a cement of white to light-gray quartz intergrown with sulfides. The ore minerals are pyrite, marcasite, sphalerite, chalcopyrite, galena, acanthite/argentite, and Ag-sulfosalts. Ore-grades range between 200 and 6000 ppm Ag and locally more than 100 ppm Au. Based on texture, mineral assemblages, and the paragenesis, this late mineralizing event is correlated with the high-grade event of stage 4 in the Maria Vein. Locally, the vein is cut by post-mineralization microcrystalline quartz veins similar to stage 6 of the Maria Vein. Compared to the paragenetic sequence of the Maria Vein, the early gray and white colloform-crustiform quartz-sulfide stage is missing, and also the quartz base-metal sulfide veins of stage 5 are not observed at the Concepción Vein.  3.3.3.3. Alteration The alteration halo about the Concepción Vein shows a less developed zoning compared to the Maria Vein. Most of the hanging wall was affected by intense adularia or adularia-illite alteration with a minor overprint of kaolinite. Kaolinite commonly overlaps with oxidized pyrite and abundant iron-hydroxides, and therefore it is likely mostly of supergene origin. Assemblages dominated by illite are only present in narrow intervals of a few meters close to the surface, but the outcropping rhyolite in the hanging wall mostly shows intense adularia alteration. Chlorite-rich alteration was formed in the footwall of the main vein, but drill holes that penetrate this chloritic zone revealed again illite-adularia assemblages. 93 3.3.4. The Melissa Vein and the Karina-Unión stockwork  The Melissa Vein and the Karina-Unión stockwork show similar textures and paragenetic sequence as the Concepción Vein. However at Karina-Unión, gray and white crustiform and colloform banding of fine-grained quartz with precious metals comparable to stage 1 of the Maria Vein is present locally. As in the Concepción Vein, the main mineralizing event is late in the paragenetic sequence commonly in the form of quartz- sulfide veins and breccia matrix. Few millimeter thin quartz-base metal sulfide veins of the type of stage 5 in the Maria Vein cut through the high-grade ore of the Karina-Unión stockwork. The Melissa Vein is the smallest of the four economic vein systems, but it shows the highest average Ag- and Au-grades (Tab. 3.1).  3.3.5. Northwest Maria stockwork  The Northwest Maria stockwork is located about 0.5 km northwest of the Maria Vein, strikes 110° to 120° and encompasses an area of about 1000 by 300 m (Fig. 3.5). Hosted by a mainly adularia altered rhyodacite ignimbrite (MBR), the network of centimeter up to 0.5 m thick veins ends abruptly towards the northwest at the contact with andesites of the intermediate formation. The stockwork consists of mainly massive medium- to coarse-grained quartz with scarce adularia. It is mostly unmineralized, however, at its southwestern end, few veins with abundant acanthite/argentite and electrum outcrop locally. The massive dark-gray mass of medium-grained quartz and ore minerals overgrows medium- to coarse-grained quartz and adularia. This very local mineralizing event produced the highest Au-grades of the deposit of about 1000 ppm, and Ag-grades in excess of 10,000 ppm. However, the presence of goethite and acanthite in open spaces and cracks indicates that parts of the high concentrations of Au-Ag may reflect supergene processes.     94 3.3.6. The Mesa Veins  3.3.6.1. Structure The Mesa Veins are followed along a discontinuous outcrop over a distance of 1500 m and probably fill the western extension of the Maria fault (Figs. 3.3 and 3.16). The structure consists of a roughly east-west striking set of linked and overstepping normal faults filled with up to 15 m wide vein networks and breccia veins. The western end of the structure consists of two sub-vertical parallel curviplanar veins. There, the veins are hosted by fine- to medium-grained andesite of the intermediate formation at surface, and at depth they mark the contact between andesites in the hanging wall and a rhyodacite ignimbrite of the Chon Aike Formation in the footwall. The central part of the vein dips 80° to the south where it is overlain by a veneer of post-Jurassic marine and unconsolidated sediments. To the east, the vein reappears hosted by andesite, but it steps over by 100 m to the north, where it forms the faulted contact between andesite in the hanging wall and rhyodacite in the footwall. The orientation changes from west to west-northwest and it dips with 50° to 65° to the south. The trace of the vein disappears to the east, but likely continues to the Nueve Vetas, which share the characteristics of the Mesa Veins.  3.3.6.2. Paragenetic sequence The Mesa Veins and Nueve Vetas show a different style of epithermal mineralization compared to the veins of the central vein zone. The Mesa-style mineralization consists of quartz and coarse-grained base metal sulfide assemblages of sphalerite, galena, chalcopyrite, and pyrite in veins, breccias, vug infill, and replacements (Fig. 3.17). Adularia and calcite are present locally. Sphalerite is usually yellow to brown indicating a low Fe-content. Quartz occurs either in microcrystalline gray aggregates or comb-textured where it is medium- to coarse-grained and milky to clear. Precious metal- grades are variable but commonly low (<50 ppm Ag and <0.2 ppm Au, only locally up to 1000 ppm Ag and 1 ppm Au). Lead- and Zn- concentrations are up to several percents, Cu- grades are commonly less than 0.1 %. Mineralogy, textures, and metal-grades correspond to the epithermal classification of an intermediate-sulfidation deposit as defined by Hedenquist et al. (2000). The paragenetic sequence for the western part of the Mesa Vein is 95        Figure 3.16: Geology map of the Mesa area. Contour interval = 5 m. Gauss-Kruger (DHDN, zone 2) coordinates.   96      Figure 3.17: Photographs of vein textures and minerals from the Mesa Vein. Paragenetic events E1 to E10 are listed in table 3.2. A. Andesite cut by thin pyrite veins and quartz veins of events E1 and E2. B. E3 quartz- galena-sphalerite-pyrite-chalcopyrite assemblage cut by E4 breccia. C. E4 breccia cut by E5 quartz-sphalerite- galena-pyrite-chalcopyrite vein, and gray microcrystalline quartz of E6 filling open spaces. D. Quartz and coarse-grained amber sphalerite of E5 overgrown by E6 gray microcrystalline quartz that contains fine- grained sulfides, which in turn is overgrown by clear comb quartz. E. Coarse-grained quartz-sphalerite- galena-pyrite-chalcopyrite as cement of breccia with andesite clasts and open spaces. F. E10 quartz-bladed calcite in andesite. Abbreviations: cc = calcite; gn = galena; py = pyrite; qz = quartz; sl = sphalerite.  97 derived from drill cores and has ten hydrothermal events, which are summarized in table 3.2 and illustrated in figure 3.17. The term hydrothermal event is used instead of hydrothermal stage because a hydrothermal stage as defined for the Maria Vein contains various sub-stages, and the paragenetic sequence of the hydrothermal events in the Mesa Vein may be comparable to one such stage. The assemblage of coarse-grained base metal sulfides and quartz resembles the stage 5 of the Maria Vein.  3.3.6.3. Alteration Hydrothermal alteration is considerably weaker around the Mesa Vein compared to the Maria Vein. Nonetheless, a zoning in alteration is present. Since the hanging wall of the vein consists of andesites, drill cores mainly sampled this lithology, and silicic rock of the Chon Aike Formation is only drilled for a few meters into the lower footwall of the vein. PIMA analyses on andesite samples from a drill core show a much lower reflectance of the altered andesite compared to altered silicic rocks. Petrography and PIMA analysis show an illite + adularia + quartz + pyrite ± chlorite assemblage that is present proximal to the vein up to a distance of about 10 to 15 m of the main vein and superimposed on the base metal sulfide-rich zone. This assemblage grades outward into an illite + pyrite ± chlorite assemblage, and into an illite-smectite + calcite ± chlorite assemblage. Significant hydrothermal alteration was detected to about 50 m distance from the main vein.   3.4. Fluid inclusion study  3.4.1. Methodology  Fluid inclusions were studied from vein and breccia matrix quartz and translucent sphalerite crystals from four hydrothermal stages of the Maria Vein with focus on the metal-bearing stages of epithermal ore deposition. A smaller number of fluid inclusions were analyzed from the Mesa Vein. Microthermometry was performed to obtain homogenization temperatures (Th) and final melting temperatures of ice (Tm) of fluid inclusion assemblages following the petrographic definitions of Goldstein and Reynolds 98 Table 3.2: Paragenetic sequence of the Mesa Veins Event Characteristics Figure E1 Fine pyrite veins, <1 mm 12A E2 Fine-grained milky to gray quartz veins (<1cm), commonly as stockwork 12A E3 Galena-sphalerite (reddish to yellowish)-pyrite- (±chalcopyrite)-quartz breccia cement and veins, locally drusy 12B E4 Matrix- to clast-supported polymictic breccia with gray fine-grained mainly hydrothermal matrix with abundant fine-grained pyrite 12B and C E5 Drusy quartz-sphalerite (mainly yellow)-pyrite- galena-chalcopyrite 12C, D, and E E6 Gray chalcedony as breccia cement, veins, and vug infill 12C, D, and E E7 Drusy or comb-textured quartz veins and subordinated breccia cement, ± scarce base metal sulfides; at depth, bladed calcite replaced by adularia might be present locally -- E8 Thin quartz-galena veins -- E9 Carbonate crystals (ankerite?) as overgrowth of quartz (event 6) or galena (event 7) -- E10 Carbonate (calcite) veins and vug infill, locally with bladed texture 12E 99 (1994). In order to obtain reliable and statistically relevant data, 34 doubly polished thick sections of 100 to 300 µm thickness were produced and petrographically studied, of which 16 sections were chosen for microthermometry. All samples are from drill cores except for one surface sample. Samples were also selected based on the location in the vein in order to obtain a data set that covers the entire vertical extent of the Maria Vein with focus on the 225W and 425W cross sections that were studied for alteration. The sample locations are shown in a long section through the Maria Vein in figure 3.18. Photomicrographs of the analyzed thick sections with the location of the measured fluid inclusion assemblages/groups are shown in appendix I. Microthermometric measurements were carried out on a Linkam THMSG-600 heating-freezing stage, calibrated to ± 0.2°C for the melting point of CO2 (-56.6°C), and melting point of H2O (0.0°C), and to ± 3°C for the critical point of pure H2O (374.1°C). The apparent salinity in weight-percent NaCl equivalent of fluid inclusions was determined from the final melting temperature of ice (Tm) using the equation of Bodnar (1993).  3.4.2. Fluid inclusion results  At room temperature, the studied samples contain dominantly liquid-rich liquid- vapor inclusions with 80 to 90 % liquid, and minor vapor-rich inclusions with very little liquid. All analyzed fluid inclusions are of the liquid-rich type because neither freezing nor homogenization could be observed in the vapor-rich inclusions. Inclusion assemblages were classified as primary, pseudosecondary, and secondary according to the relative time of their entrapment in the host mineral, using the criteria of Roedder (1984). Primary and pseudosecondary assemblages were preferred because they can be directly related to a hydrothermal stage. Secondary assemblages where analyzed additionally, particularly where primary and pseudosecondary inclusions were missing or not measurable. Primary assemblages commonly show a larger range of Th and Tm compared to secondary assemblages. The distinction between primary and secondary and the definition of assemb