GEOCHRONOLOGY AND GEOCHEMISTRY OF THE HILLTOP, LEWIS, AND BULLION MINING DISTRICTS, BATTLE MOUNTAIN-EUREKA TREND, NEVADA

Recent work in the northern Shoshone Range, Lander County, Nevada, provides new insight into the relationship between precious- and base-metal deposits within the Hilltop, Lewis, and Bullion mining districts and to nearby igneous intrusions. Radiogenic and stable isotope data, combined with geochemical analyses, allow us to elucidate the timing and origin of hydrothermal events within the districts.

Five molybdenites from four samples associated with Cu + Mo ± Au porphyry-style mineralization from the Hilltop district yield a weighted mean Re-Os age of 40.2 ± 1.7 Ma (MSWD = 2.4, 95% CL); a single molybdenite sample from porphyry-style mineralization at the Tenabo deposit (Bullion district) provides a 39.0 ± 1.4 Ma age. Subsequent deposition of quartz vein-hosted ore is inferred via younger ⁴⁰Ar/³⁹Ar ages of gangue clay minerals (35.7 ± 0.1 Ma, gold, Hilltop deposit; 37.9 ± 0.07 Ma, base-metal minerals, Gray Eagle mine). ⁴⁰Ar/³⁹Ar ages of biotite and amphibole from unaltered igneous units within and/or proximal to mineralized areas (i.e. Tenabo granite biotite: 38.9 ± 0.07 Ma) are nearly coincident with molybdenite ages and support a relation between pluton emplacement and porphyry mineralization.

Stable isotope data imply a magmatic origin for most sulfide minerals (d ³⁴S_CDT -4 to +4‰); gangue carbonate minerals formed from magmatic (d ¹³C_PDB -0.2 to -11.6‰) and mixed magmatic/meteoric (d ¹⁸O_VSMOW -1.3 to +14.4‰) fluids.

Ore-bearing quartz veins formed from mixed meteoric/magmatic source fluids (d ¹⁸O_VSMOW -16.2 to +13.3‰), as determined by using d ¹⁸O_VSMOW of the vein quartz (-0.97 to +17.3‰) in conjunction with pressure-corrected primary fluid inclusion data (salinity range from 0 to 6.4 equiv. wt. % NaCl; T_h range from 109-425° C). Temperatures of base metal-rich ore-forming fluids calculated using sulfur isotope fractionation between co-existing sulfides range from 249-502° C and correspond with vein quartz T_h values.

Geochronology and stable isotope data suggest base- and precious-metal mineralization within the Hilltop, Lewis, and Bullion mining districts is genetically related to Eocene magmatism. Geothermometry indicates that some depositional temperatures exceeded relatively low-temperature epithermal conditions and more closely resembled higher temperature, porphyry-style processes.