Precious Metal Transport and Deposition at the New Savage Mine, Comstock Lode District, Nevada

ABSTRACT
Microthermometry of primary fluid inclusions within preciousmetal
stage quartz at the New Savage mine, located in the central
portion of the Comstock Lode district in western Nevada, documents
homogenization temperatures between 260¡ and 270¡C and
salinities of approximately 1 wt% NaCl equivalent. The alteration
minerals pyrite, adularia, and 2M muscovite occur within the mineralized
zones, suggesting that these minerals were in equilibrium
with the precious-metal stage quartz. The low salinities coupled with
the presence of pyrite, adularia, and 2M muscovite further suggest
that the precious metals were transported as bisulfide complexes
under reduced conditions in near neutral to slightly alkaline solutions.
Under these conditions, gold and silver can be precipitated as
a result of an increase in the activity of oxygen and/or a decrease in
the activity of sulfur species. These changes in solution chemistry
can be achieved by boiling or by mixing of hydrothermal solutions
with cooler, more oxygenated groundwater. No evidence was found
in this study to indicate boiling during the formation of precious-metal
stage quartz. Consequently, mineralization is believed to have
resulted from the mixing of precious metal-rich hydrothermal solutions
with oxygenated groundwater. If, as suggested by other studies,
precious metals in the Comstock Lode system were contributed
by a magmatic fluid, both low salinities and the lack of boiling documented in this study indicate a very high degree of mixing with
meteoric water.