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Geology and beryl deposits of the Peerless pegmatite, Pennington County, South Dakota

May 2, 2015

The Peerless pegmatite, half a mile south of Keystone, Pennington County, S. Dak., has been a large source of scrap mica and beryl. Feldspar, amblygonite, tantalite-columbite, and cassiterite also have been recovered.

The pegmatite is intrusive into Precambrian quartz-mica schist. Much of the schist contains staurolite and chlorite. Staurolite has been partly altered to mica, quartz, and chlorite, especially near pegmatite contacts. The pegmatite is generally discordant with the schist, but in many places secondary schistosity has been developed 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 is tadpole-shaped and is 580 feet long and 360 feet wide. In cross-section the pegmatite has an anticlinal form that suggests control of the intrusion by fractures bearing N. 30°W. and dipping 45°NE. and SW. Dike-like apophyses extending from the main pegmatite have various attitudes.

The Peerless pegmatite is a complex pegmatite consisting of seven zones, two replacement units, and two types of fracture-fillings. These are: Zone 1, quartz-muscovite-plagioclase pegmatite (border zone); Zone 2, albite-quartz-muscovite pegmatite (wall zone); Zone 3, cleavelandite-quartz-muscovite pegmatite (first intermediate zone); Zone 4, perthite-cleavelandite-quartz pegmatite (second intermediate zone); Zone 5, clevelandite-quartz pegmatite (third intermediate zone); Zones 6a and b, quartz-microcline pegmatite and quartz pegmatite (fourth intermediate zone); Zone 7, lithia mica-cleavelandite pegmatite (core); lithia mica-cleavelandite-quartz replacement unit; muscovite-cleavelandite replacement unit; quartz fracture-fillings; and tourmaline-quartz fracture-fillings.

Zones 1 and 2 consist of alternating layers of different texture and mineralogy that are parallel to the contact.  The layers contain differing proportions of quartz, plagioclase, muscovite, perthite, and accessory minerals. Sugary albite-quartz aggregates are important constituents of some layers. Layers of similar composition may occur two, three, or perhaps more times. The overall mineralogic composition of Zones 1 and 2 is similar to the composition of wall zones in many other Black Hills pegmatite, Zones 3 to 7 are in the normal sequence of zones in Black Hills pegmatites.

The structural, textural, and mineralogic data confirm previously published evidence from other Black Hills pegmatites that indicates crystallization of a magma-like 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.  Zone 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 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.

Accessory minerals of the pegmatite include tourmaline, beryl, apatite, amblygonite (variety, montebrasite), lithia mica, cassiterite, tantalite-columbite, garnet, spodumene, svanbergite, loellingite (?), vivianite (?), triploidite (?), dahllite, and vari-colored phosphate minerals of the lithiophilitetriphylite group and their alteration products.

The chemical composition of the pegmatite has been determined by estimating the mineral, constitution of the various units and by calculating the tonnage of these units by use of successive geologic sections.  The principal constituents are: SiO2 (77.0 percent), Al2O3 (13.7 percent), Na2O (5.0 percent), and K2O (1.7 percent).

Chemical composition has also been determined for four subdivisions of the pegmatite: (A) Zones 1 and 2, (B) Zones 3 and 4, (C) Zone 5, and (D) Zones 6 and 7 and the replacement units. The content of Si02 increases and the content of Al2O3 decreases from the outer pan of the pegmatite inward. Na2O forms only 0.4 percent of the inner subdivision (D), but 4 .7 to 6.5 percent of the other subdivisions.  K2O forms 4.0 percent of subdivision (B), but only 0.7 to 1.3 percent of the other subdivisions.

Zone 3, the principal minable unit, contains 1. 7 percent beryl and 28 percent scrap mica. Beryl also constitutes more than 1 percent of parts of the wall zone, especially albite-rich 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.  Clevelandite that can be hand-cobbed and sold as soda-feldspar occurs in Zones 3, 4, and 5. Amblygomlte forms between 0.5 and 1.0 percent of Zone 5.

Reserves of beryl, scrap mica, potash feldspar, and amblygonite are one to six times past production.

 

Citation Information

Publication Year 1955
Title Geology and beryl deposits of the Peerless pegmatite, Pennington County, South Dakota
DOI 10.3133/tei226
Authors Douglas M. Sheridan, Hal G. Stephens, Mortimer H. Staatz, James J. Norton
Publication Type Report
Publication Subtype USGS Numbered Series
Series Title Trace Elements Investigations
Series Number 226
Index ID tei226
Record Source USGS Publications Warehouse
USGS Organization