University of Nevada, Reno METALLOGENESIS OF THE PEÑASQUITO POLYMETALLIC DEPOSIT: A CONTRIBUTION TO THE UNDERSTANDING OF THE MAGMATIC ORE SYSTEM A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geology By Macario Rocha-Rocha Dr. Tommy B. Thompson, Dissertation Advisor May, 2016 Copyright © 2016, by Macario Rocha-Rocha All Rights Reserved THE GRADUATE SCHOOL We recommend that the dissertation prepared under our supervision by MACARIO ROCHA-ROCHA Entitled METALLOGENESIS OF THE PEÑASQUITO POLYMETALLIC DEPOSIT: A CONTRIBUTION TO THE UNDERSTANDING OF THE MAGMATIC ORE SYSTEM be accepted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Tommy B. Thompson, Ph.D., Advisor Peter G. Vikre, Ph. D., Committee Member Thom Seal, Ph. D., Committee Member John McCormack, Ph. D., Committee Member Victor R. Vasquez, Ph.D., Graduate School Representative David W. Zeh, Ph. D., Dean, Graduate School May, 2016 i ABSTRACT In the study area, the Peñasquito ore deposit, there are some polymetallic ore bo- dies with singular physical, chemical, and mineralogical features, such as diatreme bre- ccias, stockwork zones, skarn system, and a porphyry complex. The geotectonic charac- teristics suggest that the ore bodies are associated with a buried Late Eocene plutonic complex and that three sets of faults, which converge in Peñasquito played an im- portant role during the formation of the ore deposit. The mineralization is hosted mainly in the diatreme breccia system, Caracol Formation, skarn system, and, to a lesser degree, the plutonic complex. The ore occurs as disseminations, veinlets, faults, veins, mantos, and irregular bodies. The mineralogy consists of base metal sulfides, sulfosalts, oxides, gold, electrum, bismuthinite, and accessory minerals, such as carbonates, calc- silicates, fluorite, and quartz. The hydrothermal alteration included a potassic core mainly developed from a plutonic complex of orthoclase, quartz, and plagioclase, which is surrounded by an aureole of calc-silicates and marble as well as an external phyllic halo and local peripheral zones of propylitic alteration which are overprinted by a late stage of carbonate. The geochemical statistical analysis of the ore bodies typically exhibits a strong relationship between metallic elements (Au, Ag, Zn, Pb, Cu, Fe, Co, Mo, Cd, Hg) and semi-metallic elements (As, Sb, Bi, V), suggesting the presence of sulfosalt minerals. The metal content of these ore bodies is up to 536 ppm Au, up to 8,280 ppm Ag, up to 496,000 ppm Pb, up to 393,000 ppm Zn, up to 293,000 ppm Cu, and up to 4,880 ppm ii Mo. The major geochemical elements in the plutonic products exhibit the following ranges: 54.1 to 80.2 % SiO2, 0.4 to 18 % TiO2, 10.5 to 17.6 % Al2O3, 0.2 to 2 % Fe2O3, 0.5 to 6.7 % FeO, 0.01 to 1.2 MnO, 0.4 to 4 % MgO, 0.4 to 22.2 % CaO, 0.1 to 4.7 % Na2O, and 1.7 to 12.2 % K2O, and 0.9 to 0.77 P2O5, reflecting both magmatic compositions and hydrothermal overprinting. The REE pattern indicated enrichment in LREE and depletion in HREE that are in agreement with the chemical signature of the upper crust. The bulk of the plutonic rocks exhibit intermediate compositions and the chemical classification ranges from sub-alkaline to alkaline to; however, magmas are calc-alkaline in source. Though the bulk of porphyries may chemically classify as quartz-monzonite and quartz- monzodiorite, some are classified as, granodiorite and others as quartz microdiorite. The trace elements and major oxides corroborated magmatic evolution-fractionation. The major oxides, trace elements, and REE suggested that magmas were generated in the upper crust during a continental epeirogenic uplift and expansive tectonic setting. However, the source could also be related to subduction, indicating possible crustal contamination. This ore deposit is located on the western border of a high magnetic anomaly and two diatreme breccias exhibit a low gravity and resistivity anomaly. Microthermometric studies of fluid inclusions revealed homogenization tempera- tures (Th) in a range from 177°C to 600°C. The melting temperatures (Tm) observed in unsaturated fluid inclusions varied between -7.5°C and -21°C, which corresponds to 11.1 and 23.05 equivalent weight percent NaCl, but saturated fluid inclusions range between 30 to 58.32 equivalent weight percent NaCl, and 20 to 23 equivalent weight percent KCl. iii The most common eutectic temperature (Te) was approximately -21°C, suggesting a predominance of sodium-chloride brine with moderate potassic-chloride brine. The C isotope analysis (¹³C VPDB), in calcite and rhodochrosite from veins related to these ore 18 bodies, showed a range of -6.9 to -1.6 ‰; whereas, results of oxygen isotopes ( O VPDB) 18 showed a range from -17.8 to -11.6 ‰. The range obtained for the standard O VSMOW was from 12.5 to 19 ‰. These results indicate a limestone source for oxygen and suggest a thermal influence on the isotopic fractionation or depletion. The results of the 34 S isotope analysis ( S VCDT) varied from -2.1 to 3.2 ‰; for example, analyses of chalco- 34 pyrite ranged from -2.1to 0.2 ‰ S VCDT, the galena results ranged between -1.1 and 34 34 -0.1 ‰ SVCDT, those of pyrite were between 0.3 and 2.8 ‰ S VCDT, the sphalerite 34 showed a range from -0.5 to 2.3 ‰ S VCDT, and a sample of arsenopyrite was 0.8 ‰ 34 S VCDT. These results indicate that the sulfur source was related to the igneous system and linked to a gold-copper-type porphyry system. In addition, several isotopic mineral pairs, Sp-Gn, Sp-Py, and Py-Apy, from the main ore stage suggested temperatures from 264° and 561°C that are in agreement with the homogenization temperatures of fluid inclusions. The U-Pb-Th geochronological data on zircons from the buried porphyries and diatreme breccia system indicate a magmatic pulse throughout ~46 to ~41 Ma and another magmatic stage at ~33.97 Ma (Valencia, 2010). The Re-Os molybdenite ages range from 35.72 ± 0.18 to 34.97 ± 0.17 Ma, indicating coeval mineralization in the Pe- ñasquito ore deposit. Hydrothermal orthoclase and biotite, 40Ar-30Ar geochronological iv data are Early Oligocene about ~33.49 (i.e. biotite ages ranged between 33.95 ± 0.085 and 33.87 ± 0.065, and K-feldspar ages ranged from 32.82 ± 0.12 to 33.32 ± 0.16), suggesting that the bulk of the potassic alteration came slightly after crystallization of the second magmatic pulse, and these ages may be considered as the main mineralization age of the Peñasquito ore deposit. These ages suggest that the Mesozoic marine sedimentary sequence was intruded by several magmatic pulses that prepared and altered the sedi- mentary sequence providing metal ions which formed ore bodies after the Laramide orogeny thrust. v ACKNOWLEDGMENTS This study builds upon more than a decade of exploration by five mining compa- nies in order to better understand the physical and chemical features, shape, mineralo- gical components, and structural control in the Peñasquito ore deposit. This doctoral project would not have been possible without the sponsorship and assistance from the Mexican Geological Survey as well as permission from Goldcorp Inc. (Dr. Charlie Ronkos) and direction from Dr. Tommy B. Thompson of the University of Nevada, Reno. Specifically, the doctoral project was awarded by the Mexican Geological Survey and the dissertation research was supported by Goldcorp Inc. through CREG at UNR, to whom the author is thankful. My deepest gratitude goes to Dr. Tommy B. Thompson for his guidance, fairness, advising, demands for clarity and solid reasoning, assistance, and tolerance during the doctoral project. Thanks also go out to my committee members Drs. Peter Vikre, Thom Seal, John McCormack, and Victor R. Vasquez for their time and thorough reviews. Numerous technical discussions with the Goldcorp exploration staff and other colleagues, that have always been outstanding, proficient, and cordial, helped to place my observations at Peñasquito in the proper tectonic context. Therefore, the author wishes to thank the Geology Exploration staff of Goldcorp Peñasquito and Mine Geology staff of Peñasquito, led by Engr. Dante Aguilar-Casillas for sharing well logs, geochemical and geophysical data, drill cores and cuttings, and local topography data as well as vi providing work space and accommodation during the field work. Furthermore, the au- thor is grateful to the Director of Geological Operation of the Mexican Geological Survey headed by Engr. Hector A. Alba-Infante for providing interactive regional geological data, satellite images, and assistance during the integration of the large data base and 3-D analysis, as well as Engrs. Israel Hernández-Pérez and Ángel Sandoval-Rojas for providing regional geophysical data. The author would like to express his appreciation to the staff of Goldcorp, especially Engrs. Jesús Enrique Cardona-Gutiérrez, Jorge Hum- berto Martínez-Domínguez, Manuel González-Palma, Angélica Ochoa-Barrón, Martín Rocha-Castro, Stan Myers, Sigfrido Robles-Ozuna, Saúl Camacho-Salazar, Omar Alexan- dro Dromundo-Arias, Luis Martín Portugal-Reyna, Pompeyo Valles, and Claudio Patricio Flores-Rivera, as well as Héctor Lorenzo Cardona-Gutiérrez, Jesús Raúl Paz-Castillo, Guillermo González-Vásquez, Mari Carmen Martínez-Gaona for their support and assistance during mapping in the Peñasquito open pit. Furthermore, part of the mi- crothermometric analysis and all SEM work were assisted by Laotse Guerrero-Zurita, Eng. Juan de Dios Pérez-Venzor, and M.Sc. Jorge Gómez-González at the Chihuahua Ex- perimental Center of the Mexican Geological Survey, so I am indebted to them. The au- thor is thankful for the helpful advice and supporting microthermometric analysis by Dr. Eduardo González-Partida, professor at UNAM. The author wishes to acknowledge the help provided by M.Sc.
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