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The Evolution of Hydrothermal Fluids from the Deep Porphyry

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The Evolution of Hydrothermal Fluids from the Deep Porphyry THE EVOLUTION OF HYDROTHERMAL FLUIDS FROM THE DEEP PORPHYRY ENVIRONMENT TO THE SHALLOW EPITHERMAL ENVIRONMENT by Subaru Tsuruoka Copyright by Subaru Tsuruoka 2017 All Rights Reserved A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Geology). Golden, Colorado Date Signed: Subaru Tsuruoka Signed: Dr. Thomas Monecke Thesis Advisor Golden, Colorado Date Signed: Dr. M. Stephen Enders Research Professor and Interim Department Head of Geology and Geological Engineering ii ABSTRACT The current understanding of magmatic-hydrothermal processes resulting in the formation of porphyry and epithermal deposits is based on case studies that focused on deposits such as Santa Rita porphyry copper deposit in New Mexico, the Refugio porphyry gold deposit in Chile, and the Summitville high-sulfidation epithermal deposit in Colorado. The present study re-examines these classical study sites to constrain the physical nature of the mineralizing hydrothermal fluids and to test recent models suggesting that metal transport can occur in the vapor phase. Careful petrographic investigations involving a combination of microanalytical techniques were performed to unravel paragenetic relationships in the three deposits. Based on fluid inclusion research on quartz closely associated with mineralization, it is shown that ore formation at Santa Rita occurred from a near-critical single-phase hydrothermal fluid under hydrostatic conditions. Observed fluid inclusion assemblages have salinities of ~11 wt% NaCl equiv. and homogenize at ~350–450°C. At Refugio, gold mineralization postdated the formation of banded quartz veins and appears to also have formed from a near-critical single-phase fluid at hydrostatic load. Microthermometric data on a small number of petrographically well-defined fluid inclusion assemblages yielded salinities of ~13 wt% NaCl equiv. and homogenization temperatures of <440°C. At Summitville, enargite precipitated from a hydrothermal liquid. Primary fluid inclusion assemblages have a salinity of ~7.5 wt% NaCl equiv. and homogenize at ~270°C. The paragenetically late gold mineralization formed from a hydrothermal liquid undergoing additional cooling and dilution with ambient water. The research provides new constraints on the formation of porphyry and epithermal deposits, highlighting the importance of near-critical hydrothermal fluids. It is shown that iii mineralization in porphyry deposits takes place late in the paragenesis and is caused by near-critical single-phase hydrothermal fluids derived from an actively degassing magma chamber. Vein formation occurs at the ductile-brittle boundary, which coincides with the transition from lithostatic to hydrostatic conditions. Epithermal mineralization is caused by hydrothermal liquids that originated from the near-critical single-phase hydrothermal fluids through isochemical contraction. The mineralizing hydrothermal liquids undergo cooling and dilution with ambient waters in the shallow subsurface. iv TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii LIST OF FIGURES ....................................................................................................................... ix LIST OF TABLES ......................................................................................................................... xi ACKNOWLEDGEMENTS .......................................................................................................... xii CHAPTER 1 INTRODUCTION .................................................................................................... 1 1.1. Porphyry deposits............................................................................................................. 2 1.2. High-sulfidation epithermal deposits ............................................................................... 3 1.3. Nature of the mineralizing fluids and corresponding fluid inclusion types ..................... 5 1.4. Genetic models ................................................................................................................. 8 1.4.1. Deposit formation from hypersaline liquids ............................................................. 9 1.4.2. Deposit formation from a moderate-salinity liquid phase ....................................... 10 1.4.3. Deposit formation from a vapor phase .................................................................... 13 1.4.4. Deposit formation from a contracted vapor ............................................................ 14 1.5. Thesis layout .................................................................................................................. 15 1.6. References ...................................................................................................................... 20 CHAPTER 2 THE ROLE OF CRITICAL FLUIDS IN THE FORMATION OF PORPHYRY COPPER DEPOSITS: EVIDENCE FROM THE SANTA RITA DEPOSIT, NEW MEXICO .. 27 2.1. Abstract .......................................................................................................................... 27 2.2. Introduction .................................................................................................................... 29 2.3. Geological background .................................................................................................. 32 2.4. Materials and methods ................................................................................................... 36 2.5. Barren vitreous quartz veins .......................................................................................... 39 2.6. Chalcopyrite-pyrite veins ............................................................................................... 42 v 2.7. Quartz-pyrite veins ......................................................................................................... 49 2.8. Whim Hill breccia .......................................................................................................... 51 2.9. Discussion ...................................................................................................................... 53 2.9.1. Distinction of quartz generations ............................................................................ 53 2.9.2. Formation of barren vitreous quartz veins .............................................................. 54 2.9.3. Formation of quartz in chalcopyrite-pyrite veins .................................................... 57 2.9.4. Hypogene ore formation ......................................................................................... 62 2.9.5. Formation of late quartz-pyrite veins ...................................................................... 65 2.9.6. Model of fluid evolution ......................................................................................... 67 2.10. Conclusions ................................................................................................................. 70 2.11. Acknowledgements ..................................................................................................... 72 2.12. References ................................................................................................................... 72 CHAPTER 3 GOLD TRANSPORT IN SHALLOW-LEVEL MAGMATIC-HYDROTHERMAL SYSTEM: EVIDENCE FROM THE REFUGIO PORPHYRY GOLD DISTRICT, CHILE .................................................................................... 78 3.1. Abstract .......................................................................................................................... 78 3.2. Introduction .................................................................................................................... 79 3.3. Geologic background ..................................................................................................... 80 3.3.1. Regional setting ....................................................................................................... 80 3.3.2. Geology of the Refugio gold porphyry deposit ....................................................... 84 3.3.3. Alteration and mineralization .................................................................................. 87 3.3.4. Materials and methods ............................................................................................ 89 3.4. Vein and mineral paragenesis at Verde ......................................................................... 91 3.5. Vein and mineral paragenesis at Pancho ....................................................................... 96 3.6. Discussion ...................................................................................................................... 97 vi 3.6.1. Distinction of quartz generations ............................................................................ 99 3.6.2. Relative timing of the gold mineralization ........................................................... 101 3.6.3. Nature of the mineralizing fluids .......................................................................... 103 3.7. Acknowledgements ...................................................................................................... 104 3.8. References .................................................................................................................... 105 CHAPTER
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