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METALLOGENESIS OF THE PEÑASQUITO POLYMETALLIC DEPOSIT: A CONTRIBUTION TO THE UNDERSTANDING OF THE MAGMATIC ORE SYSTEM
AdvisorThompson, Tommy B.
Geological Sciences and Engineering
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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 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. 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 bodies, showed a range of -6.9 to -1.6 ‰; whereas, results of oxygen isotopes (18O VPDB) showed a range from -17.8 to -11.6 ‰. The range obtained for the standard 18O 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 S isotope analysis (34S VCDT) varied from -2.1 to 3.2 ‰; for example, analyses of chalco-pyrite ranged from -2.1to 0.2 ‰ 34S VCDT, the galena results ranged between -1.1 and -0.1 ‰ 34SVCDT, those of pyrite were between 0.3 and 2.8 ‰ 34S VCDT, the sphalerite showed a range from -0.5 to 2.3 ‰ 34S VCDT, and a sample of arsenopyrite was 0.8 ‰ 34S 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 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.