Petrology and hydrothermal alteration of Jurassic intrusive rocks associated with gold deposits in the Bald Mountain mining district, White Pine County, Nevada

Late Jurassic intrusive rocks in the Bald Mountain mining district of northern
Nevada are spatially and genetically associated with significant coeval gold deposits.A
volumetrically dominant monzogranite, the 159 Ma Bald Mountain pluton, is petrographically
distinct but geochemically similar to other northern Nevada intrusive
rocks. The monzogranite has features typical of reduced intrusions, including a very
lowabundance of opaque oxide minerals, the virtual absence of magnetite, and practically
no aeromagnetic expression. The monzogranite contains distinctive 1-cm-long
K-feldspar, plagioclase, and round quartz phenocrysts, as well as biotite, surrounded
by a fine-grained groundmass. A set of quartz-feldspar porphyry dikes,
compositionally indistinguishable from the monzogranite, and a set of basaltic andesite
dikes intrude, but are essentially coeval with, the monzogranite pluton. The porphyry
dikes contain phenocrysts that are identical in composition to those of the
monzogranite, though less abundant, in an aphanitic groundmass. The porphyritic
basaltic andesite dikes intrude mineralized rock and consist of an intersertal intergrowth
of plagioclase, hornblende, and/or biotite, also in an aphanitic groundmass.
Jurassic lamprophyre dikes and aplite sills areminor components of the igneous rock
suite in the Bald Mountain district. Geochemical data suggest that the Bald Mountain
monzogranite, quartz-feldspar porphyry dikes, and basaltic andesite dikes are
petrogenetically related and associated with back-arc, subduction-related processes
along the west margin of JurassicNorth America. Petrographic features, geochemical
data and limited geochronology suggest that the aplite sills are unrelated to other intrusive
rocks and mineral deposits in the district. The petrogenesis of the lamprophyre
dikes, as indicated by their petrographic and geochemical features, is unrelated to that
of other intrusive rocks in the district, although the associated magma reservoir may
have contributed volatile constituents to mineralizing processes.