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Mineral Paragenesis and Ore Fluids at the Turquoise Ridge Gold Deposit, Nevada Michiko Shigehiro University of Nevada, Las Vegas

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Mineral Paragenesis and Ore Fluids at the Turquoise Ridge Gold Deposit, Nevada Michiko Shigehiro University of Nevada, Las Vegas UNLV Theses/Dissertations/Professional Papers/Capstones 12-1-1999 Mineral paragenesis and ore fluids at the Turquoise Ridge gold deposit, Nevada Michiko Shigehiro University of Nevada, Las Vegas Follow this and additional works at: http://digitalscholarship.unlv.edu/thesesdissertations Part of the Geochemistry Commons, and the Geology Commons Repository Citation Shigehiro, Michiko, "Mineral paragenesis and ore fluids at the Turquoise Ridge gold deposit, Nevada" (1999). UNLV Theses/ Dissertations/Professional Papers/Capstones. Paper 1449. This Thesis is brought to you for free and open access by Digital Scholarship@UNLV. It has been accepted for inclusion in UNLV Theses/ Dissertations/Professional Papers/Capstones by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. MINERAL PARAGENESIS AND ORE FLUIDS AT THE TURQUOISE RJDGE GOLD DEPOSIT, NEVADA by Michiko Shigehiro Bachelor of Science Temple University 1996 A thesis submitted in partial fulfillment of the requirements for the Master of Science Degree Department of Geoscience College of Scicnrrs Graduate College University of Nevada, Las Vegas December 1999 Copyright by Michiko Shigehiro 2000 All Rights Reserved Thesis Approval *!16b& f""!!F'Wi6MIN!LNki The Graduate College Univers1ty ot Nevada. Las Vegas ~O~c~tuoubueLr~l~3~-------·19~ The Thesis prepared bv Entitled ~ineral Paragenesis and Ore Fluids at the Turguoise Ridge 1-.. Jpprn\ cd m parti.JI tulfi!!rnent l't the requirements for the degree or· ience ii ABSTRACT Mineral Paragenesis and Ore Fluids at the Turquoise Ridge Gold Deposit, Nevada hy Michiko Shigehiro Dr. JeanS. Cline, Examination Comminee Chair Associate Professor of Geoscience University of Nevada, Las Vegas In this study, mineral assemblages and related fluid inclusions at the Turquoise Ridge Carlin-type gold deposit were examined to answer the following questions: (!)What is the ore-stage mineral paragenesis? (2) At what pressures and temperatures did the deposits form? (3) What are the sources of ore fluids? Pyrite with gold, jasperoid quartz, stibnitc, orpimcnt, realgar, and calcite were successively deposited at Turquoise Ridge. Microthermometric data and isotopic ratios of inclusion fluids in ore-related minerals indicate entrapment of multiple generations of fluids in pre-ore. ore-. and late-ore stage minerals. Primary fluid inclusions in an ore-stage jasperoid quartz crystal have salinities of 12.6 to 19 wt. % NaCI equivalent, and most inclusions homogenized between 130' and 193 'C. Isotopic signatures of inclusion fluids in ore- state jasperoid quartz, 80 = -33 ro. and o18 0 =+ 7.1 to+ 12.2 roo, and oD = -46 roo and 8 180 =+5.4 to+ I 0.5 roo, are consistent with a magmatic or metamorphic source, or both, while inclusion fluids in late-ore stage 1 calcite yielded o '0 = 10 to 14.5 roo and oD = -110 roo, indicating trapping of atl evolved meteoric fluid. iii TABLE OF CONTENTS ABSTRACT ................................................................................................................................................ iii LIST OF FIGURES" ................. " .......... ' .... " "''' .......................................................................................... Vl LIST OF TABLES ............................................................................................................................. viii LIST OF ABBREVIATION AND TERMS ................................................................................................. ix ACKNOWLEDGMENTS ........ ....................... , ........................................................ ,,.,,,,,X CHAPTER 1 INTRODUCTION ....... ...... ·················· ........................................... I CHAPTER 2 GENERAL GEOLOGY....................... .. ................................................................. 5 Investigation of the Turquoise Ridge Deposit ........................................................................................... 5 Stratigraphy ............................................................................................................................................... 5 Discussion of Stratigraphy ...................................................................................................................... 11 Structure .................................................................................................................................................. 12 Discussion of Structure .......................................................................................................................... 13 Alteration .............................................................................................................................................. 15 Previous Studies of Ore and Non-ore Mineral Assemblages .................................................................. 16 Pressures and Temperatures during Gold Mineralization ....................................................................... 18 Origins of Ore Fluids Indicated by Oxygen and Hydrogen Isotopic Signatures ..................................... 19 CHAPTER 3 METHODS OF STUDY .... .................. .......... ............................................................ 21 Sample Collection ............................................................................................................................... 21 X-ray Diffraction and Inductively Coupled Plasma Analyses .............................................................. 21 Petrography and Ore Microscopy ......................................................................................................... 22 Electron Microprobe Analyses....... ........... ........................................................ .. ............ 23 Scanning Electron Microscope Analyses .............................................................................................. 23 Fluid Inclusion Studies ............................................................................................................................ 23 Freezing and Heating Studies .................................................................................................................. 24 Oxygen and Hydrogen Isotope Analysis ................................................................................................. 25 CHAPTER 4 PETROGRA.PHY AND ORE MICROSCOPY............................. ........... ....................... 26 Host Rocks .............................................................................................................................................. 26 Discussion of Host Rocks ....................................................................................................................... 28 Mineral Paragenesis ................................................................................................................................ 29 Pre-Ore Stage Mineral Assemblage: Quartz Pyrite· Marcasite ............................................................ 29 The Ore-Stage Mineral Assemblage: Pyrite- Marcasite Gold- Jasperoid Quartz- K-spar (?) ............ 32 The Late-Ore Stage Mineral Assemblage: Pyrite/Marcasite - Jasperoid Quartz- Stibnite -Fluorite- Orpiment- Realgar- Calcite.. .. ............................................................................................. 34 Summary of Mineral Paragenesis ............................................................................................ 41 CHAPTER 5 FLUID INCLUSION STUDIES...... .. ....................................................................... 43 Petrography and Microthermomerry .... ...... ................................................................................. 43 Fluid Inclusions in Pre-Ore Stage Quartz ................................................................................................ 43 Pre-Ore Stage Vein Quartz............ .. .................................................................................................... 44 Pre-Ore Stage Quartz in Gouge ........................................................................................................... 52 IV Pre"Ore Stage Quartz Crosscut by Ore-Stage Minerals .......................................................................... 57 Discussion of Pre-Ore Stage Quartz Microthennometric Data ............................................................... 57 Fluid Inclusions in Ore-Stage Jasperoid Quartz ...................................................................................... 61 Ore-Stage Jasperoid Quartz ..................................................................................................................... 61 Ore-stage Jasperoid Quartz in Gouge ...................................................................................................... 70 Discussion of Ore-Stage Jasperoid Quartz Microthermometric Data ..................................................... 74 Fluid Inclusions in Late Ore-Stage Minerals ........................................................................................... 76 CHAPTER 6 STABLE ISOTOPE ANALYSIS ........................................................................................ 78 Discussion of Stable Isotope Analysis ................................................................................................... 79 CHAPTER 7 SUMMARY AND DISCUSSION ...................................................................................... 83 CHAPTER 8 CONCLUSIONS
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