Metamorphic differentiates in the blackbird mining district, Lemhi County, Idaho

Quartz pods and quartz veins devoid of economic minerals occur in the cobalt-copper mineralized Blackbird mining district in Lemhi County, Idaho. These barren quartz bodies are found in regionally and thermally metamorphosed rocks of the Yellowjacket formation (Belt series) of Pre-cambrian age and are believed to be derived from the enclosing quartz-biotite and quartz-biotite-garnet-chloritoid schists by metamorphic differentiation. Other rocks that possibly are products of metamorphic differentiation are discussed. Two zones of regional metamorphism are recognized, a biotite zone and a garnet zone. Contact metamorphism associated with the granitic intrusive rocks of the Idaho batholith is recognized adjacent to the batholith.