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 Precambrian 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.

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