New U–Pb age constraints on latest Cretaceous magmatism and associated mineralization in the Fawnie Range, Nechako Plateau, central British Columbia

Abstract

New U–Pb ages and K–Ar dates, primarily for rocks proximal to mineral occurrences in the Fawnie Range of central British Columbia, document latest Cretaceous (ca. 74–66 Ma) continental-arc igneous activity and date associated base and precious metal mineralization. U–Pb ages of ca. 73–69 Ma for the Capoose pluton and hypabyssal to extrusive garnet rhyolites at the Capoose prospect demonstrate a latest Cretaceous age for mineralization and a likely plutonic source for mineralizing fluids. A U–Pb age of ca. 67 Ma for a late mineralized felsic dyke and two K–Ar dates (ca. 70 and 68 Ma) for hornfelsed Jurassic volcanic rocks at the Blackwater–Davidson prospect constrain a latest Cretaceous age for mineralization. A U–Pb age of ca. 74 Ma for a fine grained diorite sill that cuts a significant epithermal gold vein at the Tsacha prospect places a minimum age on mineralization at this probable Jura-Cretaceous deposit and documents latest Cretaceous magmatism. Latest Cretaceous K–Ar dates are reported for an andesite flow adjacent to the Eocene Holy Cross deposit (ca. 66 Ma), about 35 km north of the Fawnie Range, and a Kasalka Group rhyolite (ca. 68 Ma) exposed near the western margin of the Nechako Plateau. Latest Cretaceous magmatism and mineralization in the Fawnie Range represent the waning stages of Bulkley suite magmatism and porphyry-style mineralization, which was concentrated along the western margin of the Nechako Plateau at circa 88–70 Ma. The distribution of latest Cretaceous arc igneous rocks along the North American Cordilleran is reviewed and tectonic implications discussed.