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The Mineralogy, Paragenesis and Alteration of the Camino Rojo Deposit, Zacatecas, Mexico

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The Mineralogy, Paragenesis and Alteration of the Camino Rojo Deposit, Zacatecas, Mexico University of Nevada, Reno The Mineralogy, Paragenesis and Alteration of the Camino Rojo Deposit, Zacatecas, Mexico A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology by Saige Sanchez Dr. Greg Arehart/ Thesis Advisor May, 2017 THE GRADUATE SCHOOL We recommend that the thesis prepared under our supervision by SAIGE SANCHEZ Entitled The Mineralogy, Paragenesis And Alteration Of The Camino Rojo Deposit, Zacatecas, Mexico be accepted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Dr. Greg Arehart, Advisor Dr. John Muntean, Committee Member Dr. Scott Mensing, Graduate School Representative David W. Zeh, Ph.D., Dean, Graduate School May, 2017 i ABSTRACT The Camino Rojo Zn-Pb-Ag-Au deposit, located in north-central Mexico, has characteristics of intermediate to low sulfidation epithermal deposits, skarns, and carbonate replacement deposits. Mineralization is hosted in sulfide-rich vein networks and manto-style replacements within calcareous, Cretaceous sedimentary rocks. U-Pb and fission track dating of zircon grains from a mineralized dike constrained the maximum age of the deposit to about 73Ma. Primary sulfides at the deposit include pyrite, sphalerite, arsenopyrite, galena, marcasite, chalcopyrite, electrum, acanthite, pyrargyrite, and tennantite-tetrahedrite. The deposit veins and manto replacement bodies are composed of massive-textured, intergranular sulfides and are concentrated in the carbonaceous siltstone and sandstone of the Caracol Formation. Disseminated sulfides commonly spread outward along bedding planes. Gangue mineralogy includes calcite and potassium feldspar, with lesser amounts of quartz, sericite, and fluorite. Two stages of mineralization were identified using optical petrography. Stage 1 mineralization consists of extensive pyrite-sphalerite-arsenopyrite-galena- chalcopyrite deposited as veins and mantos. Stage 2 mineralization was further divided into two parts: (2a) includes electrum± galena± chalcopyrite± acanthite filling fractures within stage 1 sulfides; and (2b) includes calcite, galena, pyrargyrite, acanthite, and argentiferous tennantite-tetrahedrite within veins which can cross-cut stage 1 mineralization. Over 80% of the electrum at the deposit formed during Stage 2a. Stage 2a and 2b have not been observed to be temporally or spatially associated. Three alteration events were recognized at Camino Rojo. First, early potassic metasomatism and decarbonization bleached and hardened the sedimentary host rocks through K-feldspar flooding of the host matrix. Second, an ore-stage calcite -sericite± pyrite ± quartz alteration overprinted the potassic metasomatism. The final alteration stage is characterized by late calcite dissemination and multiple generations of cross-cutting calcite veins and veinlets. Hornfels texture within the host rocks indicates proximity to a deeper intrusion. Skarn formed within the limestone units underlying the bulk of deposit mineralization. The Camino Rojo deposit is likely part of a larger porphyry system, and the veining and manto replacement which host mineralization may be a transition between distal skarn and the epithermal environment. ii ACKNOWLEDGMENTS The research presented in this Master’s Thesis would not have been possible without the assistance, support, and encouragement of many individuals. I would like to give many thanks to Goldcorp Inc. for their financial contribution. In additional, the individuals involved with Goldcorp’s Camino Rojo project provided assistance and gave access to the property in order to collect hand samples and review core logs, assay data, and cross sections. In particular, Brendan Scorrar provided advice and support in the early drafts of this thesis. I would also like to thank Dr. Greg Arehart, my MS thesis advisor, for his direction, support, and contributions to the manuscript. Dr. Arehart was very patient with my pacing, and never discouraged my research. Acknowledgment and thanks also goes to Dr. John Muntean and Dr. John McCormick, who provided insightful questions which helped direct my study. Special thanks to Steven Weiss for his insights and assistance in writing and editing. Without his guidance and knowledge, this work would not have been possible. As well, to all family and friends who supported me in obtaining my MS in geology: thank you. iii TABLE OF CONTENTS Abstract ............................................................................................................................................. i Acknowledgments............................................................................................................................ ii Table of Contents ............................................................................................................................ iii List of Tables .................................................................................................................................... v List of Figures .................................................................................................................................. vi 1 Introduction ............................................................................................................................. 1 1.1 Rationale for Study .......................................................................................................... 1 1.1.1 Thesis Objectives ...................................................................................................... 1 1.2 Deposit History................................................................................................................. 2 1.3 Methods of Study ............................................................................................................. 3 2 Regional and Deposit Scale Geology ........................................................................................ 5 2.1 Regional Geology ............................................................................................................. 5 2.2 Structural Setting ............................................................................................................. 9 2.3 Geology of Camino Rojo ................................................................................................ 11 2.3.1 Cretaceous Sedimentary Host Lithologies ............................................................. 12 2.3.2 Intermediate Composition Dikes ........................................................................... 15 3 Mineralization at Camino Rojo .............................................................................................. 19 3.1 Mineralization Style (Vein vs Manto) ............................................................................. 20 3.1.1 Vein Sulfides and Textures ..................................................................................... 21 3.1.2 Manto (Replacement) Sulfides and Textures ......................................................... 24 3.2 Mineral and Sulfide Petrography and Paragenesis ........................................................ 27 3.2.1 Calcite Petrography ................................................................................................ 32 3.2.2 Quartz Petrography ................................................................................................ 34 3.2.3 Pyrite and Marcasite Petrography ......................................................................... 37 3.2.4 Sphalerite Petrography .......................................................................................... 39 3.2.5 Arsenopyrite Petrography ...................................................................................... 40 3.2.6 Chalcopyrite and Galena Petrography ................................................................... 42 3.2.7 Tetradymite (Bi2Te2S) ............................................................................................. 46 iv 3.2.8 Electrum Petrography ............................................................................................ 48 3.2.9 Petrography of Silver Minerals .............................................................................. 49 4 Hydrothermal Alteration at Camino Rojo .............................................................................. 54 4.1 Potassic Metasomatism ................................................................................................. 57 4.2 Calcite- Sericite- ±Pyrite ±Quartz Alteration .................................................................. 61 4.2.1 Illite/ Muscovite and smectite as part of ore-related alteration ........................... 65 4.3 Late, Overprinting Calcite ± Pyrite ................................................................................. 67 4.4 Contact Metamorphism, Skarn and Recrystallization at Camino Rojo .......................... 68 5 Assay Correlations and Spatial Trends ................................................................................... 70 5.1 Spatial trends of gold, silver, lead, and arsenic at Camino Rojo .................................... 72 6 Geochronology of a Mineralized (Pre-Ore) Dike ................................................................... 73 7 Summary, Conclusions, and Discussion ................................................................................. 76 7.1 Mineralization at Camino Rojo ...................................................................................... 76 7.2 Hydrothermal
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