Geochemical and geochronological constraints on mineralization within the Hilltop, Lewis, and Bullion mining districts, Battle Mountain-Eureka trend, Nevada
Kelson, Christopher Ronald
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The Hilltop, Lewis, and Bullion mining districts (northern Shoshone Range, Nevada) are part of the Battle Mountain-Eureka trend and contain both vein- and porphyry-type deposits. New geochronology data from igneous rocks, porphyry-style Cu-Mo mineralization, and vein-hosted minerals elucidate the relationship between magmatic activity, hydrothermal fluid flow, and mineralization. Mostly felsic intrusive rocks were emplaced throughout the area between 39.3 ± 0.4 and 38.1 ± 0.4 Ma and weak Cu + Mo porphyry-style mineralization is associated with some of the intrusions. Ages of igneous rocks are nearly coincident with molybdenite ages, supporting a relation between pluton emplacement and porphyry Cu-Mo mineralization. Ages of quartz vein-hosted gold (35.9 ± 0.1 Ma, Hilltop deposit) and base-metal minerals (38.3 ± 0.07 Ma, Gray Eagle mine), established via ages of associated gangue clay minerals, are younger than nearby intrusive igneous rocks and may suggest the vein mineralization formed during prolonged hydrothermal activity related to igneous rock emplacement. Quartz vein-hosted sulfide minerals from the northern Shoshone Range are isotopically 34similar (´SCDT range from -6 to +9 per mil) to sulfide minerals from other Cu-Mo porphyry deposits and Cordilleran vein-type deposits, supporting a mostly magmatic sulfur source. Carbon isotope data from vein gangue carbonate minerals also support a magmatic origin for ore-forming fluids with variable contributions from host rock organic matter or carbonate rocks 13(´CPDB range from -0.2 to 11.6 per mil); carbonate oxygen was derived mainly from 18magmatic fluids (´OVSMOW range from –1.3 to +14.4 per mil). Primary fluid inclusion data (salinity range from 0 to 6.4 equiv. wt. % NaCl; T range from 109-425°C) and measured h18´OVSMOW data (-0.97 to +17.3) suggest the ore-bearing vein quartz formed from variable 18amounts of meteoric and magmatic components (calculated ´OVSMOW -16.2 to +13.3). Depositional temperatures of base metal minerals, calculated using sulfide sulfur isotope geothermometry, range from 249-502°C and agree with vein quartz primary fluid inclusion Th values. Geochronology, stable isotope, and geothermometry data show that vein- and porphyry-type mineralization is genetically related to Eocene magmatism and that some vein mineralization temperatures exceeded relatively low-temperature epithermal conditions and were more closely related to higher temperature porphyry-style processes.