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Research about Custer County, South Dakota

Source-linked reports with geographic coverage including Custer County, South Dakota.

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USGS1955

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.

South Dakota↗
USGS1953

Geology, of the High Climb Pegmatite, Custer County, South Dakota

The High Climb pegmatite, Custer County, S. Dak., belongs to the series of pegmatitic and granitic rocks that characterize the Harney Peak region of the southern Black Hills. It intrudes pre-Cambrian metamorphic rocks consisting chiefly of quartz-mica schist. The country rock has been altered to a tourmaline-rich schist along part of the pegmatite contact. The structure of the pegmatite, in general is concordant with the westward-dipping schistosity of the country rock, but locally the pegmatite is crosscutting. The main part of the pegmatite is an irregularly shaped pipe that plunges 45° N. 40° W., parallel to the average plunge of rolls in the foot wall. A small northern extension of the pegmatite has a lenticular shape and crosscuts the schist at a low angle. Rolls in this part of the pegmatite have an average plunge of 28° N. 25° W. One large, north-trending crestal roll divides the outcrop of the northern segment of the pegmatite into two parts. The pegmatite has a well- defined internal structure consisting of five zones. A fine-grained wall zone, consisting of albite-quartz pegmatite, and a medium-grained first intermediate zone, consisting of albite-quartz-muscovite pegmatite, form incomplete concentric shells. Perthite-quartz-albite pegmatite (second intermediate zone) forms a hood-shaped unit between the outer units and the third intermediate zone along the crest and hanging-wall of the pegmatite. A concentric shell of quartz-cleavelandite pegmatite (third intermediate zone) surrounds a lenticular core of quartz pegmatite that contains altered spodumene, in addition -to the five zones, a fracture-filling unit of quartz-perthite-muscovite-albite-pegmatite and a possible sixth zone (or replacement unit?) of very fine-grained muscovite pegmatite were recognized. The essential minerals of the pegmatite include microcline-perthite, quartz, albite, and muscovite. Accessory minerals include tourmaline, beryl, amblygonite (variety, montebrasite), apatite, columbite-tantalite, loellingite, altered spodumene, numerous unidentified dark-colored phosphate minerals, a manganese-bearing carbonate, garnet, and chalcopyrite. An explanation of the origin of the pegmatite requires fractional crystallization and incomplete reaction in a restricted system. The concentric zonal structure and the general increase in grain size from the wall zone to the core suggest that the pegmatite units crystallized in order from the walls inward, without any true replacement stages. Industrial minerals that have been produced at the High Climb pegmatite are beryl, amblygonite, potash feldspar, columbite-tantalite, scrap mica, and sheet mica.

South Dakota↗
USGS1952

Geology and pegmatites of part of the Fourmile area, Custer County, South Dakota

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.

South Dakota↗