Willis Hames, Derick Unger, James Saunders, George Kamenov Journal of Volcanology and Geothermal Research, 2009 188:214-224
Abstract: High-grade epithermal gold deposits in the Northern Great Basin have long been associated with regional
Miocene basaltic to rhyolitic volcanism. Previous models for the low-sulfidation epithermal gold ores in this
region have generally portrayed the bimodal magmas as a source of heat to drive large-scale convection of
meteoritic water that leached gold from crustal sources and deposited it in hydrothermal vein systems, or
required that the gold evolve from fractionated silicic magmas. New data of the present study indicate a more
direct genetic link to the plume-related basaltic magmas of the region.
Laser 40Ar/39Ar incremental heating plateau ages for single crystals of adularia from several of these low sulfidation
epithermal gold deposits range from 16.6 Ma to 15.5 Ma. Adularia from the Jumbo deposit yields
three concordant plateau ages with a combined statistical result of 16.54±0.04 Ma (95% confidence level,
MSWD=0.23). Plateau ages for adularia from other deposits in the region, and from gold-bearing veins in the
Owyhee Mountains of southwestern Idaho, yield similar ages up to ~16.5 Ma, however some veins are as young
as ca. 15.5 Ma and the grain-to-grain ages for a given sample can vary by up to ca. 0.5 Ma. Observed variations
in age among the adularia crystals of a given rock sample indicate varying amounts of extraneous argon, and
also loss of radiogenic 40Ar, among the population of grains for a particular sample. The single-crystal results
are interpreted to indicate a 16.5–15.5Ma interval for formation of gold-bearing adularia veins in the region.
The initiation and duration of this gold-forming event appears contemporaneous (within uncertainties) with
the basaltic volcanism at the Steens Mountain section and an ensuing one-million-year episode of basaltic
volcanism from multiple centers in the region (Brueseke et al., 2007).
Trace amounts of lead are alloyed with gold in the deposits studied. The isotopic compositions of this lead are
not compatible with regional crustal units that host the gold ores, or the silicic igneous lithologies of the region,
but have the same lead isotopic composition as basalts of the earliest Yellowstone plume (represented by the
earliest lavas of the Columbia River basalt province, the Steens basalts, and Stonyford Volcanic Complex;
Hanan et al., 2008). We propose that the gold studied and its traces of alloyed lead were derived together from
the mantle, released from basaltic magma chambers of the province, and carried by low-density fluids into
shallow geothermal systems during the earliest stages of Yellowstone hotspot magmatism.