Eocene Challis Magmatism in the Western Part of the Thunder Mountain Volcanic Field and Adjacent Stibnite-Yellow Pine Au-Sb-W-Hg Mining District, Central Idaho

Abstract The Stibnite-Yellow Pine gold-antimony-tungsten-mercury (Au-Sb-W-Hg) mining district, an important source of antimony (Sb) and tungsten (W) in World War II with a major (~7 Moz) Au resource, is located a few kilometers west of the Thunder Mountain volcanic field on the northwestern edge of the Eocene Challis magmatic complex in west-central Idaho. Using new field studies,40Ar/39Ar and U/Pb geochronology, and major and trace element and isotope geochemistry, we document the stratigraphy of the western part of the Thunder Mountain volcanic field and the ages of related dikes in the Stibnite-Yellow Pine mining district and Pistol Creek dike swarm. The volcanic succession is divisible into a lower section of dacitic tuffs and lavas (ca. 49.5–48.0 Ma) and an upper section of rhyolitic lavas and tuffs (ca. 47.6–46.5 Ma), locally capped by trachyandesite flows (ca. 42.6 Ma). Dikes in the Stibnite-Yellow Pine mining district similarly range from basaltic andesite to rhyolite. Dike ages range from dacitic dikes as old as ca. 49.3 Ma to rhyolitic dikes as young as ca. 46.9 Ma; the emplacement age of basaltic andesite dikes has not been determined. Geochemical and isotopic compositions of igneous rock units of the western Challis magmatic complex indicate they can be clustered into four petrologic groups, which differ in source characteristics and/or in fractionation history. Although Challis magmatism was synchronous with extensional tectonism and detachment faulting, the broader tectonic setting of Challis magmatism is controversial. Despite exhibiting the geochemical hallmarks of subduction-related magmatism, most authors interpret these igneous rocks to have been derived by partial melting of the previously subduction-modified lithospheric mantle that was heated by upwelling asthenosphere in the Eocene. Epithermal Au-Ag mineralization in the Thunder Mountain mining district, located centrally in the Thunder Mountain volcanic field about 15 km northeast of Stibnite, is dated here at 46.0 ± 0.4 Ma. This age postdates the episodes of Au and W mineralization in the nearby Stibnite-Yellow Pine mining district but overlaps in age with its youngest mineralization episode of Sb-Hg ores.