MULTIPLE MINERALIZATION EVENTS IN THE ZACATECAS Ag-Pb-Zn-Cu- Au DISTRICT, AND THEIR RELATIONSHIP TO THE TECTONOMAGMATIC EVOLUTION OF THE MESA CENTRAL, MEXICO by Osbaldo Zamora Vega A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Earth and Atmospheric Sciences University of Alberta © Osbaldo Zamora Vega, 2018 ii Abstract Mineralization in the Zacatecas district is polymetallic (Ag, Zn, Pb, Cu, and Au) and occurs as skarn-type and epithermal veins formed in different metallogenetic stages. The oldest mineralization in the district is skarn-type, Cu-rich with lesser Zn-Pb-Ag, and is considered to be close in age to felsic dikes and plugs dated at ~51 Ma. Epithermal mineralization is present as both low- and intermediate-sulfidation deposits. Intermediate-sulfidation veins (the Veta Grande, Mala Noche, El Bote, and La Cantera veins) are polymetallic, Ag-rich, hosted in ESE- to SE- striking structures, and were formed at 42.36 ± 0.18 Ma (40Ar/39Ar adularia isochron age; 2; MSWD = 0.76). The low-sulfidation system (El Orito) is Au-rich with minor silver, and lacks base metal sulfides; it is hosted by predominantly N–S-trending structures and was formed at 29.19 ± 0.20 Ma (40Ar/39Ar adularia isochron age; 2; MSWD = 1.8), which indicates that these two styles of epithermal mineralization are temporally unrelated. The skarn-type and intermediate-sulfidation mineralization is broadly coeval with Eocene subduction-related magmatism in the Zacatecas area, which is constrained by zircon U-Pb ages for igneous rocks between 51–42 Ma. The emplacement of these magmas was controlled by the same regional-scale, ESE- to SE-trending, transtensional structures that controlled the skarn-type and intermediate-sulfidation deposits. This mineralization is thus interpreted to be related to the last stages of subduction-related volcanism in central Mexico, under neutral to mildly extensional stress conditions. In contrast, no nearby magmatism is clearly related to the Oligocene low-sulfidation system. However, its age and structural orientation (N–S), combined with a regional change in magma composition from Eocene calc-alkaline to Oligocene bimodal volcanism in central Mexico, suggest that the low-sulfidation mineralization is related to post- ii subduction continental extension processes, reflecting the beginning of Basin and Range tectonics. The mineral paragenesis of the Mala Noche deposit consists of early skarn-type Cu mineralization overprinted by later epithermal Pb-Zn-Ag veins. Skarn-type minerals include relicts of prograde silicate minerals (diopside, hedenbergite, and garnet), retrograde silicate minerals (ilvaite, grunerite, stilpnomelane, epidote, clinochlore), and ore minerals (chalcopyrite, pyrite, sphalerite, galena, magnetite, wolframite, and minor bismuthinite). Epithermal mineralization is characterized by layered to vuggy quartz veins and breccias, with variable amounts of base metal sulfides (sphalerite, galena, pyrite, minor chalcopyrite, and rare acanthite and stromeyerite) intergrown with quartz, calcite, dolomite, and ankerite. Fluid inclusion and stable isotope data, combined with mathematical modelling, indicate that ore formation, was caused by a magmatic fluid (~340°C, ~14 wt.% NaCl eq., 18O = +5 to +8 ‰), which boiled and mixed with isotopically exchanged, moderately saline ground water (~150°C, ~4 wt.% NaCl eq., 18O = +0.9 to +2.9). The Veta Grande mineralization was emplaced in two main stages of Ag-rich quartz veining. Stage I consist of pyrite, followed by sphalerite, galena, and lesser chalcopyrite, acanthite, pyrargyrite, and jamesonite, intergrown with quartz, calcite, and scarce adularia. Fluids from stage I (180°–260°C, 6–10 wt.% NaCl eq., 18O = +2 to +2.8‰) are interpreted to be isotopically exchanged formation waters that boiled during ascent and depressurization. Stage II mineral paragenesis is similar to stage I but contains less sulfides, shows supergene alteration, and is characterized by amethystine quartz. Stage II fluids are of bimodal composition (6–13 and <1 wt.% NaCl eq.) and show a wide range of homogenization temperatures (150°–280°C). Ore iii minerals are interpreted to have precipitated by boiling of the hot and saline, possibly magmatic 18 fluid, end member (~320°C, ~13 wt.% NaCl eq., O = +4.6 to +10.1 ‰). The ore mineral paragenesis of the El Compas vein, which represents the El Orito System, consists of scarce pyrite followed by aguilarite, naumannite, electrum, and native gold with a gangue of quartz, adularia, calcite, and chalcedony. This mineralization was deposited in a single stage from a dominantly meteoric fluid boiling at a temperature of ~210°C, and with a salinity of <1 wt.% NaCl eq., and 18O = -1.4 and +3.1 ‰. iv Preface This thesis is the result of a research conducted by Osbaldo Zamora Vega under the supervision of Professor Jeremy Richards in the Department of Earth and Atmospheric Sciences at the University of Alberta. This an original work except were references are made to previous work. I collected all the samples during field visits to the Zacatecas area in 2009, 2010, and 2015. Dr. Jeremy Richards contributed to data interpretation and edited the manuscript. Terry Spell conducted the 40Ar/39Ar isotopic analysis at the Nevada Isotope Geochronology Laboratory, University of Nevada. Andrew Dufrane conducted the U-Pb analysis at the Radiogenic Isotope Facility, University of Alberta. Richard Stern directed the cathodoluminescence and in-situ oxygen isotope analyses at the Canadian Centre for Isotopic Microanalysis and Karlis Muehlenbachs the BrF5 oxygen isotope analyses at the stable isotopes research laboratory, University of Alberta. Daniel Moncada contributed to RAMAN and LA- ICPMS analyses at the Fluids Research Laboratory, Virginia Tech University. Mark Reed and Jim Palandri provided the software CHIM-XPT, SOLVEQ, GEOCAL, and MINTAB, and contributed to the mathematical modeling of chemical processes. v To the memory of my father, †Jose Refugio Zamora Mendez “Don Cuco” (1927 – 2013). vi Acknowledgments The author would like to thank North Country Gold for financing this research project, and especially to John Williamson for his encouragement to develop and complete this project. Thanks to Adrienne Ross, Peter Kleespies, and Simeon Robinson for their support at the beginning of my program. The Society of Economic Geologist Canada Foundation also provided partial financial support for fieldwork through a student research grant. Thanks to Consejo Nacional de Ciencia y Tecnologia for the scholarship No. 305391 granted to Osbaldo Zamora Vega. I would like to thank Capstone Gold, Oro Silver Resources, and Mr. Jose Parga for access to drillcore and underground sampling. Mineral separation was done with the help of Mark Labbe and at the University of Alberta facilities. Thank you very much to Carlos Linares for his help doing preliminary microprobe analyses at the National University of Mexico. Many thanks to Mark Reed and Jim Palandry for their warm welcoming to the University of Oregon and their training and support using the different software applied for geochemical modeling. I would like to thank the members of my thesis committee, Karlis Muehlenbachs, Sarah Gleeson, and Tom Chacko for trusting on me, and especially to my supervisor Jeremy Richards for his patience on guiding me to conclude this work satisfactorily. Thank you very much to Rob Bodnar for his kind welcoming to the VT Fluid Research Lab. Thanks to Miguel Morales-Gamez, Rares Bistran, Andreas Enggis and Meg, Ali Imer, Mary Borrero, Guadalupe Maldonado, Gideon Lambiv and Nelson Bernal for the memorable times together. Finally, my heart goes to Laura, Nadia, and Monserrat, my wife and daughters. Thanks for walking along with me this long and tough path. “Though the mountains be shaken and the hills be removed, yet my unfailing love for you will not be shaken” (Isaiah, 54:10). vii Table of Contents Abstract ......................................................................................................................................................................................... ii Preface ........................................................................................................................................................................................... v Acknowledgments ................................................................................................................................................................... vii List of Tables ............................................................................................................................................................................. xi List of Figures........................................................................................................................................................................... xii 1. Introduction ....................................................................................................................................................................... 1 2. Regional Geodynamic Setting .......................................................................................................................................... 6 2.1 Late Cretaceous–Early Cenozoic (Laramide Orogeny) .................................................................................... 9 2.2 Eocene Continental Sedimentation and Volcanism (Transitional Tectonics) .........................................