GARNET LU-HF GEOCHRONOLOGY AND GEOTHERMOBAROMETRY CONSTRAIN PRE-ORE METAMORPHISM IN THE AU-SB-W YELLOW PINE MINING DISTRICT

The largest domestic antimony resource, estimated at 41,000 tonnes, is in a meso- to epithermal gold-antimony-tungsten deposit in the historic Yellow Pine mining district in central Idaho. Ore is hosted by Cretaceous granitic rocks in the Atlanta lobe of the Idaho batholith and by Neoproterozoic and Paleozoic metasedimentary rocks. A first-time Lu-Hf garnet date of 112.8±7.2 Ma from the schist of the Neoproterozoic Moores Station Formation, stratigraphically below and one mile to the northwest of the ore-hosting strata, offers a constraint on metamorphism near the mining area. Metamorphism was coeval with and plausibly related to the last stages of metamorphism and deformation in the Salmon River Suture Zone (SRSZ) roughly 50 miles to the west. Metasedimentary rocks in and around the mining district are folded on the kilometer scale with axial traces that plunge shallowly to the NW and trend NW-SE, with some overturned to the SW, similar to folds in the SRSZ. Geothermobarometric analyses of the schist indicate amphibolite facies metamorphic temperatures and pressures, which is consistent with the sillimanite-garnet assemblage. An undeformed aplitic dike with magmatic garnets that cuts the schist yielded a Lu-Hf garnet date of 99.5±4.0 Ma providing an outside constraint on the end of metamorphism.