Mineralization and Geochemistry of the Metasedimentary Rock-Hosted Au-(Ag-Sb-W) West End Deposit, Stibnite Roof Pendant, Yellow Pine Mining District, Idaho

Abstract The Yellow Pine mining district in central Idaho has been the focus of Au-(Sb-W) mining and exploration for nearly 100 years. Recently, Midas Gold Inc. (now Perpetua Resources) has developed an indicated resource of 170 metric tonnes (t; 5.464 Moz) of Au, 277 t (8.904 Moz) of Ag, and 66.7 Kt (87,000 short tons) of Sb in three deposits. The resources are hosted in the Cretaceous Idaho batholith and a roof pendant of Neoproterozoic to Ordovician metasedimentary rocks that include marble, quartzite, conglomerate, calc-silicate–bearing rock types, and mica schist. Most of the resource hosted in metasedimentary rocks is in the West End deposit. Four stages of mineralization in the metasedimentary rocks have been established by crosscutting relationships between four distinct vein types. Deep early veins are composed of milky white, coarse quartz with pyrite, local molybdenite (86.4 ± 0.4 Ma, Re-Os), and other sulfides, which are associated with mainly propylitic alteration and locally potassic alteration that appears to be temporally associated with the latest phase of the batholith. Deep early veins have the same orientation and similar radiometric dates as late-phase batholith-related leucogranite dikes proximal to the coeval Stibnite stock. Deep early veins are crosscut by main-stage veins composed mainly of quartz, carbonate, arsenopyrite, and arsenian pyrite. Main-stage veins contain more sulfides (up to 7%) and contain the bulk of the gold, which occurs in arsenopyrite and arsenian pyrite. Main-stage veins are strongly spatially and temporally controlled by the West End fault zone. Disseminated mineralization and illite alteration are found proximal to main-stage veins and fault zones. Later epithermal veins with lesser arsenopyrite and pyrite contain gold-rich electrum and are contemporaneous with the Eocene Challis volcanism and subvolcanic intrusions in the district. Epithermal veins formed in fault zones and along joints and lithologic contacts, and locally as stockworks. Unmineralized fine-grained quartz and pyrite microbreccia veins, which locally contain stibnite, crosscut the epithermal veins. The West End deposit shows similarities with the Donlin Creek deposit in Alaska, along with other deposits in the world, where host rocks, mineralogy, and overprinting events provide analogs for West End.