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Geochemical and Geochronological Constraints on Mineralization within the Hilltop, Lewis, and Bullion Mining Districts, Battle Mountain-Eureka Trend, Nevada

Christopher R. Kelson^1,, Douglas E. Crowe² and Holly J. Stein³

¹ Department of Geology, State University of New York, College at Potsdam, Potsdam, New York 13676
² Department of Geology, University of Georgia, Athens, Georgia 30605
³ AIRIE Program, Department of Geosciences, Colorado State University, Ft. Collins, Colorado 80523

Corresponding author: e-mail, kelsoncr{at}potsdam.edu

The Hilltop, Lewis, and Bullion mining districts, located within the northern Shoshone Range, Lander County, Nevada, are part of the greater Battle Mountain-Eureka trend. Historic production of mostly Ag with Au, Cu, Pb, and Zn from all three districts exploited 3-m-wide quartz veins hosted mainly within upper plate Ordovician-Devonian siliceous and siliciclastic rocks of the Roberts Mountains allochthon. Eocene felsic intrusive rocks, located both proximal and distal to the vein deposits, were emplaced throughout the northern Shoshone Range along a west-northwest trend; molybdenite mineralization is associated with some intrusive rocks, and some intrusive rocks host vein-type mineralization. New and previously published radiometric ages for primary igneous minerals from unaltered rocks within and/or proximal to mineralized areas are nearly coincident with published Re-Os ages for several northern Shoshone Range molybdenite occurrences (40.1 ± 0.6 Ma, avg), supporting a relationship between pluton emplacement and molybdenite mineralization. Age differences between quartz vein-hosted gold ± base metal mineralization (e.g., 38.3 ± 0.1 Ma: Gray Eagle mine, Bullion district) and nearby intrusive rocks (e.g., 39.0 ± 0.1 Ma: Granite Mountain) imply that vein mineralization probably formed during prolonged hydrothermal activity, synchronous with or subsequent to igneous rock emplacement, or from heat associated with a second, slightly younger phase of intrusive igneous activity. Primary fluid inclusion salinity (0–6.4 wt % NaCl equiv) and T_h (115°–425°C) data from ore-bearing vein quartz, in conjunction with measured ¹⁸O_VSMOW data (–0.97 to +17.3), indicate that vein-forming fluids contained variable mixtures of magmatic and meteoric components (calculated ¹⁸O_VSMOW values of –16.0 to +13.3) and spanned the epithermal-porphyry temperature continuum. New and previously published stable isotope (C, O, and S) data from vein-hosted ore and gangue minerals, unaltered and altered siliceous (upper plate) host rocks, and unaltered carbonate (lower plate and upper plate) rocks indicate mixed and variable sources of vein-hosted ore and gangue components and variable interaction between some vein-forming fluids and nonsiliceous rock types (carbonates, bedded barite) that underlie the northern Shoshone Range. Large vein widths, high vein densities, and lack of evidence supporting fluid boiling in primary fluid inclusions suggest that vein deposits of the northern Shoshone Range probably formed at shallow depths (1 km) under lithostatic conditions. Geochronology and stable isotope data support a temporal and genetic link between Eocene magmatism, quartz vein-hosted precious and base metal mineralization, and molybdenite (porphyry) mineralization within the Hilltop, Lewis, and Bullion mining districts.