Epithermal Gold-Silver Deposits Related to the Northern Nevada Rift

ABSTRACT
The northern Nevada rift is a name applied to a prominent,
north-northwest-trending aeromagnetic anomaly that extends about
500 km from southeast Nevada to near the Nevada-Oregon border.
Its surface trace is marked in places by an alignment of middle
Miocene volcanic and hypabyssal rocks and epithermal gold-silver
and mercury deposits that formed during west-southwestÐeastnortheast
extension.
Numerous epithermal gold-silver deposits are present in
igneous rocks along the northern Nevada rift. Three distinct groups
of middle Miocene igneous rocks are associated with the rift: an
early group with mafic (basalt to andesite) compositions and later
groups with trachydacite and rhyolite compositions. The early mafic
magmas may be related to the Yellowstone hot spot and were
derived from upper mantle magmas. The more silicic compositions
probably represent lower crustal melts that may have formed as a
result of hot-spot-related deep crustal heating. All known epithermal
gold-silver deposits along the riftÑincluding Midas, Ivanhoe, Mule
Canyon, and BuckhornÑare low-sulfidation deposits that formed
near the end of the early period of mafic magmatism at 15.6 to 15.0
Ma. All deposits appear to be spatially, if not always genetically,
related to andesite or basaltic andesite magmatism. The deposits are
characterized by Ag/Au ratios generally ²10, high As, Hg, Mo, Sb,
and Se contents, and low Cu, Pb, and Zn contents. Marcasite and silver
selenides are ubiquitous ore minerals, and arsenopyrite and Asrich
rims on pyrite/marcasite are common in some deposits.
Temperatures of ore deposition were between 150-250¡C. Ore fluids
had low salinities and near-neutral pHs, with low sulfur activities.
Evidence for fluid boiling is present in most deposits. Similarities in
ore-fluid chemistry and rock geochemistry of the epithermal
deposits along the northern Nevada rift to Carlin-type deposits suggest
that these epithermal deposits might represent the upper parts of
undiscovered middle Miocene Carlin-type systems. Exploration for
these Carlin-type deposits might focus on the southern part of the rift
where suitable sedimentary host rocks may be present at shallow
depths.