LEAD ISOTOPE DATA FROM PRECIOUS-METAL MINERALS: IMPLICATIONS FOR THE GENESIS OF BONANZA EPITHERMAL AU-AG-SE DEPOSITS OF THE NORTHERN GREAT BASIN, USA

New high-precision Pb-isotope analyses using MC-ICP-MS and ⁴⁰Ar/³⁹Ar geochronologic studies of Mid-Miocene bonanza epithermal Au-Ag(Se) ores of the northern Great Basin (NGB) add further support to the proposal that these ores are genetically related to the emergence of the Yellowstone hotspot (YHS). Our ⁴⁰Ar/³⁹Ar geochronology of adularia intergrown with precious-metal minerals in these ores indicates most of the NGB epithermal ores formed between ~16.5 and 15.5 Ma, approximately coeval with bimodal basaltic volcanism at Steens Mountain (OR) and rhyolites of the McDermitt caldera (NV) that are interpreted by many to have been formed by the initial emergence of the YHS. Lead isotope ratios measured directly on electrum and naumannite (Ag₂Se) grains physically separated from vein samples were very similar in the four deposits investigated in our preliminary investigations (Sleeper, National, Jumbo, Seven Troughs). Pb-isotope signatures of electrum and naumannite did not match that of vein adularia, the latter of which was similar to the signature of closely associated host rocks (rhyolites, metasediments). Instead, Pb-isotopic signatures of precious-metal minerals match closely Mid-Miocene basaltic units in the region, as well as regionally extensive basalts of Steens Mountain and the Columbia River flood basalts. Thus we interpret our results to support the concept that primitive, mantle-derived mafic magma at depth contributed Au, Ag, and Se, (and at least some S) to the shallow epithermal environment dominated by meteoric water that leached principal major elements (e.g., K, Al, Na, silica) from the surrounding country rocks. It has been widely proposed that precious metals in epithermal ores are leached from host rocks, but our data casts serious doubt on this hypothesis. We propose that a mantle plume in conjunction with thin crust, deep structures (Northern Nevada Rift+Basin and Range faulting?), likely were important in the genesis of the Mid-Miocene epithermal deposits of the NGB. If so, then perhaps their genesis is not too different than low-temperature Tertiary Au-Ag-Te deposits associated with Rio Grande Rift (e.g., Cripple Creek, Ortiz) and Colorado Mineral Belt of Colorado.