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 porphyrystyle
mineralization from the Hilltop district yield ages from 40.1± 0.2 to 40.6 ± 1.2 Ma
with a weighted mean of 40.23 ± 1.7 Ma (MSWD = 2.4, 95% CL). Asingle molybdenite
sample from Cu + Mo ± Au porphyry-style mineralization at the Tenabo deposit (Bullion
district) provides a 39.0 ± 1.4 Ma age. 40Ar/39Ar ages for biotite and amphibole
from unaltered igneous units within and/or proximal to mineralized areas (i.e. Tenabo
granodiorite biotite: 38.85 ± 0.07 Ma) are nearly coincident with molybdenite ages,
supporting a relation between pluton emplacement and porphyry mineralization.
Sulfur isotope data suggest a magmatic origin (δ34SCDT range from –4 to +4 per
mil) for most sulfide minerals. Carbon isotope data (δ13CPDB range from –0.2 to –11.6
per mil) from carbonate minerals associated with ore also support a magmatic origin
for the ore-forming fluids; carbonate oxygen isotope data (δ18OVSMOW range from
–1.3 to +14.4 per mil) indicate predominantly magmatic to mixed magmatic/meteoric
Temperatures of base metal-rich ore-forming fluids calculated using sulfur isotope
fractionation between co-existing sulfides are 304–502°C (Gray Eagle mine),
339°C (unnamed prospect), 249°C (Lovie mine), and 434°C (Hilltop deposit).
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 mineralization temperatures
exceeded relatively low-temperature epithermal conditions and were more
closely related to higher temperature porphyry-style processes.