Regional Geologic Framework of Mineral Deposits in the Stibnite-Edwardsburg Area, Central Idaho

Abstract

Abstract Geologic mapping and associated U-Pb geochronologic work in the Stibnite-Edwardsburg area of central Idaho have provided regional geologic context for the gold-antimony-tungsten-mercury mineralization in this area. Roughly 6,000 m of strata that postdate the Mesoproterozoic Belt-Purcell Supergroup are preserved; overall, the strata young to the southwest and are found as roof pendants or septa within the Idaho batholith. Rocks suspected to be lower Paleozoic in age by early workers in the area contain detrital zircons as young as 500 Ma, confirming that age assignment. We recognized four mappable phases of Cretaceous intrusive rocks, ranging in age from about 95 to 85 Ma, but suspect additional dating and detailed mapping would better show the complexity of the intrusive history. Regional metamorphism ranges from greenschist to amphibolite facies and contact metamorphism is conspicuous near Cretaceous plutonic rocks. Lu-Hf garnet geochronology shows that regional metamorphism of the strata northwest of Stibnite occurred at about 113 Ma and thus prior to batholith intrusion. Contact metamorphism likely occurred some 15 to 30 m.y. later, depending on the specific pluton age. Four large-volume Eocene ash-flow deposits (and their hypothesized eruptive centers) were recognized. Important structures in the Stibnite area include a SW-directed thrust fault, now overturned, that repeats part of the section, and N- to NE-striking faults that have localized mineralization.