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Mafic-Ultramafic Igneous Rocks and Associated Carbonatites of the Gem Park Complex, Custer and Fremont Counties, Colorado

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Mafic-Ultramafic Igneous Rocks and Associated Carbonatites of the Gem Park Complex, Custer and Fremont Counties, Colorado Mafic-Ultramafic Igneous Rocks And Associated Carbonatites of the Gem Park Complex, Custer and Fremont Counties, Colorado GEOLOGICAL SURVEY PROFESSIONAL PAPER 649 MAFIG-ULTRAMAFIC IGNEOUS ROCKS AND ASSOCIATED CARBONATITES OF THE GEM PARK COMPLEX, CUSTER AND FREMONT COUNTIES, COLORADO S a wa tc h C r i s t o Range S a n g r e d a Range rh p I e x * '*w 5 tp" *,. ' j^.*f\ A- j*s'j»- ' ^«K*£ DEMOB R^TjC MQ^NTAIN. \3 1 /T* :S>- * >." " '"' _J^ ^: '-V*^ Bdj^|*°C,j^B^*' * X* -H'c;"",.,,,,'*^ ,,' -aCr* Gem Park, Colo. View northwestward from Democratic Mounta GEM MOUNTAIN Sawatch Range ark -Cbtrfplex -« "*» +** 4ifa. ving pyroxenite and gabbro of the Gem Park Complex. Mafic-Ultramafic Igneous Rocks And Associated Carbonatites of the Gem Park Complex, Custer and Fremont Counties, Colorado By RAYMOND L. PARKER and WILLIAM N. SHARP GEOLOGICAL SURVEY PROFESSIONAL PAPER 649 The Gem Park Complex its geology and mineralogy, and its niobium distribution UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1970 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HIGKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director Library of Congress catalog-card No. 79-608291 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, B.C. 20402 (Paper cover) CONTENTS Pag-e Abstract __ _-_---__---__-_--..---__-_____-_______-____ Gem Park Complex Continued Introduction __________________________________________ 1 Carbonatite dikes_--______---------_--__-__-___-__- 7 General geology-______________________________________ 1 Attitude, orientation, and distribution of dikes.___ 7 Precambrian rocks.________________________________ 3 Composition of dikes.-______..______..__-_____-__ P Cambrian rocks.__________________________________ 3 Carbonatite in rocks that enclose the complex. _ _ _ _ P Tertiary volcanic rocks and gravel ___________________ 3 Fenite ----_--_--------_-----_------------------_ 10 Quaternary alluvium and colluvium._________________ 3 Mineralogy.___-_____-_-____.---_-_______-___-_-__--__ 11 Structure _________________________________________ 4 Carbonatites ______________________________________ 11 Gem Park Complex._------_-_--____--___._____________ 4 Fenites ____-_-____-_____-_----_.--_-_-_---------__ 14 Pyroxenite. _______________________________________ 4 Geochemistry. ________________________________________ 1? Gabbro and associated rocks._______________________ 5 Summary and conclusions--..__-.-_______--______----_-- 22 Nepheline syenite pegmatite._______________________ 6 Economic geology____________________________________ 2? Lamprophyre and syenite porphyry dikes.____________ 7 References.______________.-_--_-_____---_________-_-__ 2? ILLUSTRATIONS Page FRONTISPIECE. Gem Park, Colo., view northwestward from Democratic Mountain. PLATE 1. Geologic map of Gem Park______________________________________________________________________ _In pod et 2. Map of fenitized rocks at the Vermiculite mine, showing sample localities and radioactivity anomaly, Gem Park, Colo. _____---_---_--_----____-_____-______-_______-_____-______________-_____________ _______ _In pocHt FIGURE 1. Map showing location and geologic setting of the mafic-ultramafic Gem Park Complex and the McClure Mountain Complex.______________________________________________________________________________________ 2 2-5. Photomicrographs of: 2. Pyroxenite__ - -__--__--__-__--___---__-_-____-__--__----___-__-___--____---_-___--__-__-___ 5 3. Gabbro___-___________-_-________-___________-______--______-_-________________________-___ 6 4. Plagioclase-rich rock.________________________________________________________________________ 6 5. Lamprophyre.______________________________________________________________________________ 7 6, 7. Photomicrographs of deformed dolomite-blue amphibole-pyrochlore Carbonatite _____________________________ P. 9 8. Photomicrograph of finely crystalline apatite clot in carbonatite_____-__--_--_-___-__-__------_------___--_ 12 9. Photomicrograph of monazite rosette in carbonatite-_____________________________________________________ 13 10-17. Photographs of: 10. Black lustrous lueshite crystals.._____________________________________________________________ 15 11. Cut section of lavender-gray lueshite twin cluster.-_____________________________________________ 15 12. Fersmite after lueshite in dolomite__________________________________________________________ 15 13. Fersmite in thin section.____________________________________________________________________ 16 14. Natroniobite and columbite replacing lueshite__________________________________________________ 16 15. Fragments of natroniobite and columbite._____________________________________________________ 17 16. Grains of fibrous monazite___________________________________________________________________ 17 17. Thorianite crystals.---_--_-_____-___________________--_____--_--____-_----____-_--_-___---- 18 VII VIII CONTENTS TABLES Page TABLE 1. Chemical analyses and norms of some rocks from the Gem Park Complex-_________________________________ 4 2. Modes of mafic and ultramafic rocks from the Gem Park Complex.-____--________________-_____-__-_-_--_- 5 3. Minerals in carbonatites from the Gem Park Complex_ -______________--_-__________--__-_-_-._-__-------- 12 4. Semiquantitative spectrographic analysis of green rare-earth apatite from the Gem Park Complex _____________ 13 5. Chemical composition of light-amber pyrochlore from the Gem Park Complex.-_________-___--___-_-__-__-_- 13 6. Semiquantitative spectrographic analysis of columbite from the Gem Park Complex___.-_-_____-_______-_-__ 14 7. Minerals in fenitized gabbro and pyroxenite from the Gem Park Complex-___--_-__-__--___--____-_-_--_--_ 14 8. Semiquantitative spectrographic analysis of lueshite from the Gem Park Complex___________________________ 16 9. Semiquantitative spectrographic analysis of fersmite from the Gem Park Complex.,_____-______-_--__---___- 16 10. X-ray diffraction data for natroniobite.__-____-_________-__,___________-____-_-_----_--------__-__-_--- 17 11. Analyses of carbonatites from Gem Park.__________________-___________-_-_-________--.-__-__-_-_-__--- 19 12. Analyses of rocks from fenite area, Gem Park Complex-___________-____-______-___---_--------_---_--_- 21 MAFIC-ULTRAMAFIC IGNEOUS ROCKS AND ASSOCIATED CARBONATITES OF THE GEM PARK COMPLEX, CUSTER AND FREMONT COUNTIES, COLORADO By RAYMOND L. PAEKER and WILLIAM N. SHARP ABSTRACT the geologic relationships in the region. With the discov­ The Gem Park Complex (a new name in this report), which ery of the McClure Mountain Complex the senior author lies about 11 miles northwest of Westcliffe, Colo., is a small instigated the study reported here. funnel-shaped composite body related to the McClure Mountain Gem Park has been the site of limited mining sin°.e Complex a few miles to the northeast. The Gem Park Complex the 1880's. The Gem mine, which consists of a group of consists mostly of pyroxenite and gabbro with minor dikes and bodies of lamprophyre, syenite porphyry, and nepheline syenite shallow workings at the north edge of Gem Park, pro­ pegmatite, and abundant dikes and irregular bodies of carbona- duced an unknown but small quantity of nickel-silver tite, all of Cambrian age. A mass of fenite lies near the center ore prior to 1885. The old vermiculite mine in the central of the complex. The whole complex lies discordantly in Precam- part of the complex (called the Vermiculite mine in this brian gneissic terrane and is overlain by Tertiary volcanic rocks. report) and the Niles mine at the west edge of the com­ Large areas in the complex are covered by Quaternary alluvium and colluvium. plex produced a combined total of somewhat less than Some carbonatite dikes and fenite contain concentrations of 3,000 tons of vermiculite during and just prior to World niobium, rare-earth elements, thorium, phosphorus, some other War II (A. L. Bush, oral commun., 1968). A minor ton­ elements, and vermiculite. nage of magnetite ore was scraped from the surface at the The arrangement of carbonatite dikes, the position of the fenite, south border of the complex but the amount recovered and other features suggest that a large carbonatite body lies beneath the surface near the center of the complex. and other data pertaining to that activity are not known. We thank Mr. D. W. Fieldman, of the Congdon and INTRODUCTION Carey Co., and the Cleavenger Land and Cattle Co. for allowing access to properties in the Gem Park area, and The complex of mafic and ultramafic igneous rocks Prof. A. G. Bulakh, of Leningrad University, who gener­ and associated carbonatites, herein formally designated ously furnished information on natroniobite and oth^r the Gem Park Complex, underlies an area of about 2 square miles at Gem Park, a small oval valley in the rare minerals from the Kola Peninsula, U.S.S.R. We also northern Wet Mountains, Fremont and Ouster Counties, thank U.S. Geological Survey colleagues F. A. Hild?- Colo., about 21 miles southwest of Canon City and about brand, who conducted both field and laboratory work 11 miles northwest of Westcliffe which is designated as early in the study, R. B. Taylor, who made some of tH the type area. The complex is similar in composition to photomicrographs used in the report, and J. W. Adams, part of the mafic and ultramafic rock complex at Iron who advised on problems in mineralogy and in the prep­ Mountain, about 9 miles to the northeast in the McClure aration of the manuscript. Mountain Complex (Shawe and Parker, 1967). (See
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