Geology of the Hugo Pegmatite Keystone, South Dakota GEOLOGICAL SURVEY PROFESSIONAL PAPER 297-B Geology of the Hugo Pegmatite Keystone, South Dakota By J. J. NORTON, L. R. PAGE, and D. A. BROBST PEGMATITES AND OTHER PRECAMBRIAN ROCKS IN THE SOUTHERN BLACK HILLS GEOLOGICAL SURVEY PROFESSIONAL PAPER 297-P A detailed structural and petrologic study of a pegmatite containing seven zones and two replacement bodies UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1962 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. CONTENTS Page Page Abstract.. _ ________________________________________ 49 Mineral distribution and paragenesis of the entire Introduction. ______________________________________ 49 pegmatite_ _ ______________________-___---------_ 96 General geology. ___________________________________ 52 Comparison of the zonal sequence with that in other Metamorphic rocks_ ____________________________ 52 pegmatites. ______________________________________ 97 Roy and Monte Carlo pegmatites.- _ __---__-______ 53 Replacement features-______________________________ 100 Structure __________________________________________ 53 Review of the evidence for replacement in pegma­ Pegmatite units ____________________________________ 53 tites __ _____________________________________ 100 Zone 1 : Albite-quartz-musco vite pegmatite ________ 56 Replacement in the Hugo pegmatite.____-_____-_- 102 Zone 2: Quartz-albite-muscovite pegmatite. _______ 57 Mineral variations.____________-____--____-__-_-_-_- 103 Zone 3: Perthite-quartz-albite pegmatite, quartz- Evidence for magmatic intrusion.,..____._______---____ 107 perthite-albite pegmatite, albite-quartz-phosphate Evidence for temperatuie of crystallization___________ 109 pegmatite, and quartz-albite pegmatite.- ________ 63 Chemical composition.______________________________ 111 Zone 4: Quartz-cleavelandite-microcline-amblygonite Course of crystallization._______-_____-____--_-_-_--- 118 pegmatite and quartz-cleavelandite* pegmatite-. __ 71 Origin. _______________-_-_____---_--__-_-_-_------- 123 Zone 5: Quartz-microcline-spodumene pegmatite____ 77 Industrial minerals-.._______________________________ 124 Zone 6: Quartz-microcline pegmatite. ____ _________ 81 Potash feldspar_______________--_____--_----_- 124 Zone 7: Cleavelandite-microcline-lithia mica pegma­ Scrap mica and beryl__________________________ 124 tite. ________________________________________ 82 Lithium minerals. _____-_______----__-----__--_- 124 Cleavelandite-microcline-lithia mica replacement Other minerals. -------------------------------- 125 86 References cited-_________-_-___---_--_-------_-_--- 125 Cleavelandite-quartz-lithia mica replacement unit _ 89 Index.______________-____-_-_______--------__----- 127 ILLUSTRATIONS [Plates are in pocket] PLATE 8. Geologic map and section of the area near the Hugo pegmatite, Keystone, S. Dak. 9. Geologic map, underground workings, and sections of the Hugo pegmatite. 10. Horizontal geologic sections at different elevations. 11. Variation diagram showing the content of SiO2, Al2Oa, Na2O, K2O, Li2O, and P2O5 at successive zonal cor- tacts. 12. Variation diagram showing the content of quartz, albite, microcline, mica, amblygonite, and spodumene a,t successive zonal contacts. Page FIGURE 3. Index map.-____-_________--_________-______________..__-________---_------_-__---------_----__ 50 4, Photomicrograph of granular aggregate of albite, zone l______________-__-_--_---_-----__------------ 57 5-12. Photomicrographs of zones 1 and 2_ ______________________________________________________________ 58 13. Paragenesis of the units in the Hugo pegmatite__________-_______-__--_-_--__________----_-------- 60 14. Layer rich in black tourmaline in zone 2__________-_____--____-___-_-__--_---_-_-----_--__------- 62 15. Photomicrograph of perthite, zone 3a_ -__-____-____-____-____-____------_--_----_----_------------ 70 16-23. Photomicrographs of zone 3_ ____-_______________________-___-_-___-_____-________--____---------- 72 24. Exposure of zones 2 and 4_----_---_---_-_-___-_-.-_,_____-______---____--____-___-__-__--_------ 76 25. Exposure of zone 4__________-_-_-__--_____________-_____________-____-_-_-_-__-_-_--__--------- 76 26-33. Photomicrographs of zones 4 and 5_---_--__-______________-___-_--_-----__-____________-___-__-- 78 34. Photomicrograph of aggregate of quartz, zone 5_______-___________________-_----_-__-_--------_---- 80 35. Exposures of zone 5 containing tabular spodumene crystals- ____-__________-___--_-_-_-__--------_--- 80 36-43. Photomicrographs of zones 6 and 7_____________________________________________----_----------_ 84 44. Typical exposure of zone 7--_--______--______________________-_____--___-___---_--------------- 86 45. Cleavelandite-microcline-lithia mica replacement unit where it replaces zone 5.------------------------ 86 46. Cleavelandite-microcline-lithia mica replacement unit where it replaces zone 6___________________-_---- 87 47-54. Photomicrographs of the cleavelandite-microcline-lithia mica replacement unit- ______-____-___--------_ 90 55. Optically continuous remnants of quartz in amblygonite matrix.___________________--___--_-------_- 92 56. Exposure of the cleavelandite-quartz-lithia mica replacement unit-_____________________-___--_------ 92 57-64. Photomicrographs of the cleavelandite-quartz-lithia mica replacement unit.____________-__--_--------_ 94 IV CONTENTS Page FIGURE 65. Photomicrograph of streak of nearly untwinned microcline in twinned microcline________________________ 96 66. Paragenesis of the Hugo pegmatite._______________________________________________________________ 97 67. Distribution of minerals in the Hugo pegmatite. _______-__----______--------_____----_______________ 98 68. Indices of refraction of albite andberyL____________________________________________________________ 104 69. Variation diagram for the Hugo and Diamond Mica pegmatites and average values of sor^e igneous rocks. __-_-__-____________________-_____---____----------____----------_---------_----_------ 115 70. Quartz-albite-microcline diagram showing the course of crystallization of the Hugo pegmatite-____________ 120 TABLES Page TABLE 1. Potash feldspar and scrap mica produced from the Hugo mine_ ______________________________________ 51 2. Mineralogy and petrography of zone l____________________-_________-_____________________________ 56 3. Mineralogy and petrography of zone 2_--_-----_-_-_----__-------_-------------------_--____---___ 61 4. Mineralogy and petrography of zones 3a, 3b, and 3c_ _----------____---------____--_____-_____--____ 64 5. Mineralogy of certain minor minerals of the Hugo pegmatite.________________________________________ 66 6. Mineralogy and petrography of zone 4_______--___-_------________---.-____-______--___-______-____ 71 7. Mineralogy and petrography of zone 5_--_-----------------------_---_--------------------__--_--- 80 8. Mineralogy and petrography of zone 6__-_------_----------------_--------_---------__----___----- 81 9. Mineralogy and petrography of zone 7_---------------------__---_------------------_-----_---_--- 82 10. Mineralogy and petrography of the cleavelandite-microcline-lithia mica replacement unit._______________ 87 11. Mineralogy and petrography of the cleavelandite-quartz-lithia mica replacement unit.__________________ 92 12. Comparison of the sequence of mineral assemblages in the Hugo pegmatite with that of other pegmatites __ _ 99 13. Partial chemical analyses, calculated anorthite content, and minimum refractive indices of cleavage frag­ ments of albite from different units of the Hugo pegmatite_________________________________________ 105 14. Spectrochemical analyses of mica from the south segment of the Hugo pegmatite.______________________ 106 15. Chemical analyses of mica from the south segment of the Hugo pegmatite.____________________________ 106 16. Partial chemical analyses of amblygonite from the south segment of the Hugo pegmatite,_______________ 107 17. Minimum temperatures of crystallization of mica from the Hugo pegmatite.___________________________ 109 18. Comparative analyses of a sample of muscovite from zone 2 of the Hugo pegmatite.____________________ 109 19. Estimated composition of the Hugo pegmatite._________-__--___-__-_---_________----________---___ 112 20. Data used to calculate composition and density of the Hugo pegmatite________________________________ 113 21. Mineralogic and chemical changes associated with the formation of the core and replacement units ______ 116 PEGMATITES AND OTHER PRECAMBRIAN ROCKS IN THE SOUTHERN BLACK HILLS GEOLOGY OF THE HUGO PEGMATITE KEYSTONE, SOUTH DAKOTA By J. J. NORTON, L. K. PAGE, and D. A. BROBST ABSTRACT replacement units. Spodumene and amblygonite we^e early minerals of the zones in which they have been found. The Hugo pegmatite, 1 mile south of Keystone, S. Dak., is Evidence that this pegmatite formed by the intrusion of a an unusually well developed zoned pegmatite offering abundant magmatic liquid that crystallized from the contact inward con­ structural and petrographic information that is useful in infer­ sists chiefly of: (1) sharp contacts with the wallroek; (2) ring the behavior during crystallization. This pegmatite is structural relations between zones and erosscutting fracture- also economically important as one of the largest potash feldspar filling and replacement units; (3) systematic