Geology of the Fourmile pegmatite area, Custer County, South Dakota

The Fournile pegmatite area covers 32 sq. mi. in the western part of the Custer pegmatite mining district, Custer County, South Dakota. This area has been one of the major sources of sheet mica in the United States, especially between 1906 and 1911, and it has been a significant source of potash-feldspar, beryl and spodumene.

The area is part of the much larger area of pegmatites and metamorphic rocks to the north and east that surround Harney Peak and extend throughout the southern part of the pre-Cambrian core of the Black Hills. Paleozoic rocks underlie the western part of the Fourmile area.