Geology and Beryl Deposits of the Peerless Pegmatite Pennington County South Dakota GEOLOGICAL SURVEY PROFESSIONAL PAPER 297-A This report concerns work done partly on behalj of the U. S. Atomic Energy Commission and is published with the permission of the , » Commission Geology and Beryl Deposits of the Peerless Pegmatite Pennington County South Dakota By DOUGLAS M. SHERllDAN, HAL G. STEPHENS, MORTIMER H. STAATZ and JAMES J. NORTON PEGMATITES AND OTHER PRECAMBRIAN ROCKS IN THE SOUTHERN BLACK HILLS GEOLOGICAL SURVEY PROFESSIONAL PAPER 297-A This report concerns work done partly on behalf of the U. S. Atomic Energy Commission and is published with the permission of the Commission UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957 UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. SEATON, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. - Price $1.50 (paper cover) CONTENTS Page Page Abstract.__________________________________________ 1 Geology Continued Introduction _______________________________________ 1 Peerless pegmatite Continued Location_____________________________________ 1 Chemical composition......____________ 17 History and production________________________ 2 Origin________ ____________________ _. 18 Past and present investigations.__________________ 3 Mineral deposits_____________________________ 21 Acknowledgments__________________________ 4 Mica__ _________ _________________________ 21 Mine workings _____________________________________ 4 Lithia mica.______________________________ 21 Geology ___________________________________________ 4 Beryl_ __________________________________ 21 Metamorphic rocks.____________________________ 4 Potash feldspar..._________________________ 22 Quartz-plagioclase-muscovite pegmatite, undiffer- Sada feldspar____________________________ 23 entiated _____________________________________ 5 Amblygonite-montebrasite ______-____.-_--_. 23 Peerless pegmatite ______________________________ 5 Other minerals-_______________________ 24 Size, shape, and structure__________________ 5 Reserves__ __________________________________ 24 Internal units______________________________ 6 Literature cited______________________________ 24 Variation in mica composition._______________ 17 Logs of TJ. S. Bureau of Mines diamond-drill holes- 25 Faults___________________________________ 17 Index. ___________-_-______-_______--__-_----. 45 ILLUSTRATIONS [Plates 4-7 in pocket] PLATE 1. Wall rock, Peerless pegmatite. A, Quartz-mica schist containing staurolite partially changed to muscovite, chlorite (ripidolite), quartz, and biotite. B, Schist in which staurolite has been completely replaced by muscovite, quartz, and biotite. C and D, Tourmalinized schist. E, Pseudomorph of a staurolite crystal______Facing 4 2. Peerless pegmatite. A, The main axis of Peerless pegmatite. B, Layers in the wall aone___________.Facing 5 3. Wall zone, Peerless pegmatite. A, Layers in the wall zone. B, Beryl in the wall zone_______________Facing 12 4. Geologic map, Peerless pegmatite. 5. Geologic map of underground workings, Peerless pegmatite. 6. Geologic sections, Peerless pegmatite. 7. Correlation charts of the Peerless pegmatite. FIGURE 1. Index map showing location of Peerless pegmatite____________________________________-_________--_______ 2. Geologic maps, Peerless pegmatite____________________________________________________________________ TABLES TABLE 1. Production of mica, potash feldspar, soda feldspar, beryl, amblygonite-montebrasite, tantalite-columbite, and cassiterite, Peerless pegmatite, 1907-September 1950_________-------_--_--__________________________ 3 2. Production of punch and sheet mica, Peerless pegmatite, 1943-47__________--_____----_-__________________ 3 3. Approximate mode of internal structural units, Peerless pegmatite._________________---_-__-_--------------- 7 4. Sequence of layers in the border and wall zones, northeastern part of main open pit, Peerless pegmatite._________ 8 5. Mineralogy of quartz-muscovite-plagioclase pegmatite (zone 1, border zone)_-_____________-_----_----------- 8 6. Mineralogy of albite-quartz-muscovite pegmatite (zone 2, wall zone)-________-______---_---_---_-- ________ 9 7. Mineralogy of cleavelandite-quartz-muscovite pegmatite (zone 3, first intermediate zone)____________.---__--_- 12 8. Mineralogy of perthite-cleavelandite-quartz pegmatite (zone 4, second intermediate zone)__________-_---_--__- 13 9. Mineralogy of cleavelandite-quartz pegmatite (zone 5, third intermediate zone)______________________________ 13 10. Spectrographic analyses of mica____________________________________________________-_----___-_----_-_- 16 11. Estimated mineral content of the Peerless pegmatite---_-__-_-__--_-------------------------------------- 17 12. Composition of minerals used in computing chemical composition of the Peerless pegmatite.__________________ 18 13. Estimated chemical composition of the Peerless pegmatite_-_____-_-____--___.._---_----_------------------ 18 14. Distribution of the principal chemical constituents______________________-____-__-_______--_----_--------- 18 15. Sequence of layers in border and wall zones-_________________________--__--____-----_------------------- 19 16. Measurements of beryl in seven exposures of zone 2, albite-quartz-muscovite pegmatite (wall zone) ______________ 22 17. Measurements of beryl in five exposures of zone 3, cleavelandite-quartz-muscovite pegmatite (first intermediate zone)______--__-__-____--_-______________-_____-___-__----_--___------_ -___----___- --_---- 22 18. Mineralogy of beryl--_______-________-_________________________-_-__-_-_____--_-__-__---------------- 23 in PEGMATITES AND OTHER PRECAMBRIAN ROCKS IN THE SOUTHERN BLACK HILLS GEOLOGY AND BERYL DEPOSITS OF THE PEERLESS PEGMATITE, PENNINGTON COUNTY, SOUTH DAKOTA By DOUGLAS M. SHEKIDAN, HAL G. STEPHENS, MOETIMEE H. STAATZ, and JAMES J, NORTON ABSTRACT The chemical composition of the pegmatite has been deter­ The Peerless pegmatite, half a mile south of Keystone, Pen- mined by estimating the mineral constituents of the internal nington County, S. Dak., has been a major source of scrap mica units and by calculating the tonnage of these units by use of and beryl. Feldspar, amblygonite-montebrasite, tantalite-co- successive geologic sections. The principal constituents are: lumbite, and cassiterite also have been recovered. Si02 (77.0 percent), A1203 (13.7 percent), Na^O (5.0 percent), The pegmatite intrudes Precambrian quartz-mica schist. and K20 (1. 7 percent). Much of the schist contains staurolite and chlorite. Staurolite Chemical composition has also been determined for four sub­ has been, in part, altered to mica, quartz, and chlorite, especially divisions of the pegmatite: zones 1 aad 2; zones 3 and 4; zone 5; near pegmatite contacts. The pegmatite is generally discordant and zones 6 and 7 and the replacement units. The content of with the schist, but in many places a secondary schistosity has Si02 increases, and the content of Al20g decreases inward from been formed parallel to the contact. Tourmaline and muscovite, the outer part of the pegmatite. NaaO forms only 0.4 percent presumably introduced by pegmatitic solutions, are character­ of the inner subdivision, but 4.7 to 6.5 percent of the other sub­ istic of the wall rock near discordant contacts. divisions. K20 forms 4.0 percent of zones 3 and 4, but only At the surface the pegmatite has a tadpole shape, and is 580 0.7 to 1.3 percent-of the other subdivisions. feet long and 360 feet wide. In cross section the pegmatite has The structural, textural, and mineralogic data confirm pre­ an anticlinal shape that indicates control of the intrusion by viously published evidence from other Black Hills pegmatites fractures bearing N. 30° W. and dipping 45° NE. and SW. that indicates crystallization of a magmalike fluid from the wall Dikelike apophyses extending from the main pegmatite have inward. Repetition of layers in zones 1 and 2 indicates changes several attitudes. in composition of the fluid at the crystallizing faee. These The Peerless is a complex pegmatite consisting of 7 zones, 2 changes may have been caused by addition of new material from replacement units, and 2 types of fracture fillings. The zones below, by loss of material to the wall rocks, or by failure of con­ are as follows: 1, quartz-muscovite-plagioclase pegmatite (border vection to maintain equilibrium throughout the fluid in the zone); 2, albite-quartz-muscovite pegmatite (wall zone); 3, pegmatite chamber. Zones 3 to 7 are hi the normal sequence of cleavelandite-quartz-muscovite pegmatite (first intermediate zoned pegmatites that indicates crystallization from a restricted zone); 4, perthite-cleavelandite-quartz pegmatite (second inter­ or nearly closed system. The lithia mica-cleavelaftdite-quartz mediate zone); 5, cleavelandite-quartz pegmatite (third inter­ replacement unit, which extends outward from the core, shows mediate zone); 6a and 6b, quartz-microcline pegmatite and that in the very late stages of crystallization a pneumajolytic or quartz pegmatite (fourth intermediate zone); and 7, lithia mica- hydrothermal fluid escaped outward and replaced previously cleavelandite pegmatite (core). The other units are: lithia inica- crystallized pegmatite. cleavelandite-quartz replacement unit; muscovite-cleavelandite Zone 3, the principal mmable unit, contains 1.7 percent beryl replacement unit; quartz fracture fillings; and tourmaline- and 28