Geology and gold mineralization of the Richardson district, East-Central Alaska

Abstract

The Richardson district contains multiple granitic units intruding gneiss and schist. The Bald Knob prospect contains gold-bearing quartz veins with the assemblage Bi-̊ Au-̊ hedleyite and high methane fluid inclusions. These features indicate very low fs₂-f₀₂ conditions, lower than any reported for 90 Ma interior Alaska gold systems, including the nearby Democrat and Buckeye prospects. The Bald Knob and Democrat Lode prospects returned ⁴⁰Ar/³⁹Ar ages of 1̃04 and 9̃0 Ma, respectively. Peraluminous dikes possess collisional tectonic signatures and interpreted age of 114 Ma. Younger dikes and 3 km² granite pluton possess arc-type compositions and yield ⁴⁰Ar/³⁹Ar ages of 9̃0 Ma. Garnet-biotite geothermometry on metamorphic rocks indicates low-P regional metamorphism (550-600C; 3-4 kb) and vertical movement between adjacent fault blocks. Highest temperatures are in the fault block hosting the Bald Knob prospect, suggesting it represents the deepest mineralization exposure in the area and is most proximal to a causative pluton.