GEOCHRONOLOGY OF INTRUSIVE ROCKS AND HYDROTHERMAL ALTERATION AT THE STRUCTURALLY CONTROLLED STIBNITE AU-SB-W DEPOSIT, IDAHO

The Stibnite Mining District of central Idaho produced nearly a million ounces of gold plus substantial tungsten and antimony from dilatant zones of the Meadow Creek and West End Faults. In the district, Cretaceous-age Idaho Batholith has intruded a metasedimentary roof pendant of uncertain age (Reed et al., 2014), but an Eocene caldera is now adjacent. Recent exploration work by Midas Gold, Inc., has outlined a multi-million ounce gold (plus antimony and tungsten) resource, but the age of intrusives and ores are unknown. Mineralization (Au-W-Sb-Hg) is hosted by batholith, schist, calc-silicates, carbonates, and quartzites, principally within the N-trending Meadow Creek Fault and NE-trending splays. Gold is present in disseminated pyrite and quartz-carbonate veins and breccias with pyrite, stibnite, arsenopyrite and scheelite. We present new U-Pb ages for 3 major intrusive rocks: quartz monzonite, alaskite and granite. Zircons have large inherited cores, but combined CL imaging and LA-ICPMS spot analysis identify zones of new zircon growth suitable for ID-TIMS analysis. Zircons of ~89 Ma define the quartz monzonite as an older batholith phase. Zircon rims in the granitic Stibnite Stock are 85.7 ± 0.1 Ma, similar to an 85.3 ± 0.1 Ma age on nearby metamorphic sphene. Zircon tips in the texturally younger alaskite are 83.6 ± 0.1 Ma. Post-mineral porphyry dikes, similar to the Eocene-age Challis volcanic suite, intrude along faults. ⁴⁰Ar/³⁹Ar step heating analysis ages of 46.8 Ma ± 0.3 (biotite) and 45.9 Ma ± 0.3 Ma (feldspar) in a latite porphyry match typical ages of Challis volcanics. Vein relationships are complex, but potassic alteration and K-feldspar envelopes are locally common. Hydrothermal K-feldspar in 3 veins in schist and granite from the West End area returned flat plateaus and weighted average ages of 51.7 Ma ± 0.3 Ma, 51.0 ± 0.4 Ma and 50.8 ± 0.3 Ma. While not totally definitive of all mineralization in the district, the assemblage of quartz-carbonate-K-feldspar-sulfides is present in at least the historic Yellow Pine pit and West End deposits. The new ages are compatible with observed mineral assemblages in the ores and suggest the bulk of mineralization is early Eocene (~51 Ma), probably associated with the onset of tectonic extension, faulting, and uplift that preceded continental volcanism. Work is ongoing.