The Fourmile area, Custer County, S. Dak., is in pre-Cambrian metamorphic rocks that surround the granitic core of the Black Hills. The area is on the upright limb of an overturned anticline that plunges about 30° S. 10° E.
Three new formations of metamorphic rocks are described that have a total thickness of at least 7, 700 feet. The oldest of these formations, the Ruby Creek quartz-mica schist, is more than 6,500 feet thick. The overlying Raver formation, about 200 feet thick, is composed of thin beds of amphibolite and hornblende schist, lime-silicate rock, cordierite-biotite schist, microcline-biotite schist, and other types of rock. The youngest formation, the Ruddock quartz-mica-feldspar schist, is more than 1,000 feet thick. The presence of kyanite, staurolite, cordierite, and sillimanite in the rocks indicate that they have been subjected to high-grade metamorphism.
About 420 pegmatites were mapped in the Ruby Creek quartz-mica--schist and the Ruddock quartz-mica-feldspar schist. A few thin pegmatites in the Raver formation were not mapped. Most of the pegmatites are concordant with the schistosity and relict (?) bedding of the enclosing metamorphic rocks. They are as much as 250 feet thick and range from 10 to 2,600 feet in length. Nine pegmatites are zoned and are classified as heterogeneous pegmatites. The remainder are homogeneous pegmatites that are poorly zoned. The major constituents of the pegmatites are plagioclase, quartz perthite, and muscovite. The accessory minerals are tourmaline, apatite, garnet, and biotite. Beryl was observed in 15 of the pegmatites.
The heterogeneous pegmatites contain commercial deposits of potash feldspar, mica (sheet and scrap), and beryl.
