GEOLOGY AND BERYL DEPOSITS OF THE PEERLESS PEGMATITE, PENNINGTON COUNTY, SOUTH DAKOTA. PEGMATITES AND OTHER PRECAMBRIAN ROCKS IN THE SOUTHERN BLACK HILLS

Technical Report · Tue Jan 01 04:00:00 EST 1957

OSTI ID:4340959

Sheridan, D M; Stephens, H G; Staatz, M H; Norton, J J

The Peerless pegmatite, half a mile south of Keystone, Pennington County, S. Dak., has been a major source of scrap mica and beryl. Feldspar, amblygonite-montebrasite, tantalite-columbite, and cassiterite also have been recovered. The pegmatite intrudes Precambrian quartzmica schist. Much of the schist coninins staurolite and chlorite. Staurolite has been, in part, altered to mica, quartz, and chlorite, especially near pegmatite contacts. The pegmatite is generally discordant with the schist, but in many places a secondary schistosity has been formed parallel to the contact. Tourmaline and muscovite, presumably introduced by pegmatitic solutions, are characteristic of the wall rock near discordant contacts. At the surface the pegmatite has a tadpole shape, and is 580 feet long and 360 ft wide. In cross section the pegmatite has an anticlinal shape that indicates control of the intrusion by frnctures bearing N. 30 deg W. and dipping 45 deg NE. and SW. Dikelike apophyses extending from the main pegmatite have several attitudes. The Peerless is a complex pegmatite consisting of 7 zones, 2 replacement units, and 2 types of fracture fillings. The zones are as follows: 1, quartz-muscovite-plagioclase pegmatite (border zone); 2, albite-quartz-muscovite pegmatite (wall zone); 3, cleavelandite-quartz- muscovite pegmatite (first intermediate zone); 4 perthite-cleavelandite-quartz pegmatite (second intermediate zone); 5, cleavelandite-qunrtz pegmatite (third intermediate zone); 6a and 6b, quartzmicrocline pegmatite and quartz pegmatite (fourth intermediate zone); and 7, lithia mica-cleavelandite pegmatite (core). The other units are: lithia mica-cleavelanditequartz replacement unit; muscovite- cleavelandite replacement unit; quartz fracture fillings; and tourmnline-quartz fracture fillings. The structural, textural, and mineralogic data confirm previously published evidence from other Black Hills pegmatites that indicates crystallization of a magmalike fluid from the wall inward. Repetition of layers in zones 1 and 2 indicates changes in composition of the fluid at the crystallizing face. These changes may have been caused by addition of new material from below, by loss of material to the wall rocks, or by failure of convection to maintain equilibrium throughout the fluid in the pegmatite chamber. Zones 3 to 7 are in the normal sequence of zoned pegmatites that indicates crystallization from a restricted or nearly closed system. The lithia mica- cleavelandite-quartz replacement unit, which extends outward from the core, shows that in the very late stages of crystallization a pneumatolytic or hydrothermal fluid escaped outward and replaced previously crystallized pegmatite. Zone 3, the principal minable unit, contains 1.7% beryl and 28% scrap mica. Beryl also constitutes more than 1% of parts of the wall zone, especially albiterich layers of the inner part of the unit in the upper part of the pegmatite. Beryl is a less important constituent of other units of the pegmatite. Potash feldspar is mined chiefly from zone 4. Cleavelandite thnt can be hand cobbed and sold as soda feldspar occurs in zones 3, 4, and 5. Amblygonite-montebrasite constitutes between 0.5 and 1.0% of zone 5. Reserves of beryl, scrap mica, potash feldspar, and amblygonite-montebrasite are one to six times past production. (auth)

• Research Organization: Geological Survey
• NSA Number: NSA-12-004817
• OSTI ID: 4340959
• Report Number(s): GS-P-297-A
• Country of Publication: Country unknown/Code not available
• Language: English

Related Subjects

ALBITE
ALUMINUM SILICATES
AMBLYGONITE
BERYL
BERYLLIUM COMPOUNDS
BERYLLIUM SILICATES
BORON SILICATES
CALCIUM SILICATES
CASSITERITE
COLUMBITE
CRYSTALS
DEPOSITS
FELDSPARS
GASES
GEOLOGY
HIGH TEMPERATURE
HYDROLOGY
IGNEOUS ROCKS
LAYERS
LITHIUM COMPOUNDS
LITHIUM PHOSPHATES
MAGNESIUM SILICATES
MICA
MICROCLINE
MINERALOGY, METALLURGY, AND CERAMICS
MINERALS
NIOBIUM OXIDES
PERTHITE
PLAGIOCLASE
POTASSIUM SILICATES
QUARTZ
ROCKS
SCHISTS
SILICATES
SODIUM SILICATES
SOLUTIONS
STABILITY
STAUROLITE
TANTALITE
TANTALUM OXIDES
TIN OXIDES
TOURMALINE
URANIUM MINERALS