Petrology and Structure of Precambrian Rocks Central City Quadrangle Colorado By P. K. SIMS and D. J. GABLE SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER SS4-E A study of high-grade metamorphic and igneous rocks within the Colorado mineral belt UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page Abstract_________________________________________ El Intrusive rocks Continued Introduction. ____________________________________ 2 Emplacement and origin of the intrusive rocks_____ E41 Geologic setting._________________________________ 3 Metamorphic fades. ___ _____________________________ 43 Rock units_________________________________________ 6 Structure______ _______ __ ____ _____ 44 Metamorphic rocks.________________________________ 6 Terminology ___________________________ ______ 45 Lithologic succession__________________________ 6 Folds __________________ .-- _ -_-_ 45 Microcline-quartz-plagioclase-biotite gneiss_ _ 8 Major folds__ _ ______ __ ______ 47 Amphibolite______________________________ 12 Central City anticline. _ ______ ________ 47 Cor dierite-amphibole gneiss ______________________ 13 Idaho Springs anticline. _________________ 47 Cordierite-gedrite rocks__________________ 14 Lawson syncUne. _______________________ 47 Cordierite-biotite rocks______________________ 16 Intermediate-scale folds. ___________ ______ 48 Hornblende-cummingtonite rocks.___________ 16 Pewabic Mountain syncline_----_____-___ 48 Chemical composition.____________________ 17 Bald Mountain syncline___ ____________ 48 Calc-silicate gneiss and related rocks ___________ _ 17 Dumont anticline ___ _________ ______ 48 Biotite gneiss______________________________ 18 Syncline at Mount Pisgah- ______ __ 48 Biotite-quartz-plagioclase gneiss._____________ 18 Pecks Flat anticline and adjacent overturned Sillimanitic biotite gneiss.___________________ 20 folds____ ____________________________ 48 Cordierite- and garnet-bearing sillimanitic bio­ Linear elements related to folds __ 49 tite gneiss_______________________ 21 Idaho Springs-Ralston shear zone___ ______ _ __ 49 Chemical composition of the biotite gneisses__ 24 Shearing and associated alteration in Dakota Hill Origin of metamorpbic rocks.___________________ 26 area 50 Granite gneiss and pegmatite______________________ 28 Intrusive rocks.______________________________ 29 Faults 51 Granodiorite and associated rocks_____________ 30 51 Gabbro and related rocks________________________ 35 Character and environment of deformation_ 52 Quartz diorite and hornblendite______________ 38 Geologic history ________________________ 54 Biotite-muscovite quartz monzonite___________ 39 References cited. __________.__--_______-_--- 55 ILLUSTRATIONS Page PLATE 1. Geologic map and sections of the Central City quadrangle, Colorado. ____________ ___________ _ In pocket FIGTTBE 1. Map showing location of the Central City quadrangle in the Front Range, Colo E2 2. Generalized geologic map of the central part of the Front Range and adjacent areas.________________ ______ 4 3. Triangular diagram showing variation hi composition of microcline gneiss but excluding Quartz Hill layer_______ 9 4. Sketch showing layering hi a body of cordierite-amphibole gneiss, Quartz Hill______ _______________ __ 15 5. Diagram showing Na20:K20 ratios of biotite gneisses___________-___________ ____________________ 26 6. Diagram showing FeO:MgO ratios of biotite gneisses__________________________ _____________ __ 27 7. Triangular diagram showing variation in composition of granodiorite and associated rocks.... ... _. 32 8. Contour diagrams of lineations_______________________________________________________________________ 46 9. Contour diagrams of joints______________________________________________________ 52 TABLES Page TABLE 1. Lithologic succession of Precambrian metamorpbic rocks, Central City quadrangle___________ E7 2. Modes of microcline-quartz-plagioclase-biotite gneiss______-_______-_-_-_-_______---_-_-___---_-_-_---_-_- 10 3. Modes of amphibolite-______________________________________________________________________ 12 4. Chemical and spectrochemical analyses and norms of amphibolite________________________________ 13 5. Modes of representative varieties of cordierite-gedrite gneiss and associated rocks_________________ 14 IV CONTENTS Page TABLE 6. Chemical analyses and modes of cordierite-gedrite gneiss and associated rocks.----.------------------------- E15 7-12. Modes of: 7. Biotite-quartz-plagioclase gneiss____--___-__-__-___-_-_-_-------_-_---------_-_-_--------------_ 19 8. Garnetiferous biotite-quartz-plagioclase gneiss_______-_______________-_____-_-___-_-_---_-_-____ 20 9. Sillimanitic biotite-quartz-plagioclase gneiss______-____-___________________-___-_-----_-----___- 21 10. Sillimanitic biotite-quartz gneiss___________-___________-_-_-_-_-_______-_-_-_-_---_-_---_-_-_- 21 11. Garnetiferous Sillimanitic biotite-quartz-plagioclase gneiss___-____________-______-_-_---_---_--___ 22 12. Cordierite-bearing garnet-sillimanite-biotite gneiss--___-_----_-___--_--_-_------__------------_--_ 22 13. Cordierite-biotite-quartz-plagioclase gneiss.------------------------------------------------------ 23 14. Estimated chemical compositions and average modes of principal types of biotite gneiss.______-_-_- -_-____ 25 15. Chemical analyses and modes of cordierite-bearing garnet-sillimanite-biotite gneiss, Sillimanitic biotite-quartz-plagio­ clase gneiss, and garnet-sillimanite-biotite-quartz-plagioclase gneiss.____-_-______-_____-__-_-_-_-______ 26 16. Semiquantitative spectrographic analyses of minor elements in biotite gneisses_____________-_-_-------__-_-_ 27 17. Modes of granite gneiss and pegmatite._________________-____---_____-____-________--_-_----__-_-_-____ 29 18. Modes of granodiorite and associated rocks__________________________________-_____-_--_--__--_-_--___-_ 32 19. Chemical and spectrochemical analyses and norms and modes of intrusive rocks.___-______--_-_-_-_---_-____ 34 20. Chemical and spectrochemical analyses of biotites from intrusive rocks.____________________________________ 35 21-23. Modes of: 21. Gabbro and related rocks___-___________________-_--_-__----_-____---_----_--_-_-_----------_ 37 22. Quartz diorite and hornblendite.__________-___-_-_-_-_---------___-_-____------_--------------- 39 23. Biotite-muscovite quartz monzonite_____-____-_-___-_-_-_--_-------------_--_-_-----------_----- 40 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY PETROLOGY AND STRUCTURE OF PRECAMBRIAN ROCKS, CENTRAL CITY QUADRANGLE COLORADO By P. K. SIMS and D. J. GABLE ABSTRACT Cordierite-amphibole gneiss: The Central City quadrangle, in the east-central part of the Biotite-cordierite-garnet-gedrite-plagioclase-quartz Front Range, is underlain by Precambrian rocks which are Calc-silicate gneiss and related rocks: intruded by abundant porphyritic igneous rocks of Tertiary age. Clinopyroxene-garnet-plagioclase-quartz-sphene The Central City district and parts of the Idaho Springs and Garnet-magnetite-quartz Lawson-Dumont-Fall River mining districts, which are integral Clinopyroxene-epidote-hornblende-plagioclase-quartz parts of the Front Range mineral belt, are located at intrusive The metamorphic rocks are interpreted to be dominantly of centers of the Tertiary igneous rocks in the southern part of the metasedimentary origin. The microcline gneiss is thought to quadrangle. represent arkose, and the biotite gneiss, to represent interlayered The Precambrian rocks are dominantly microcline-quartz- shale and graywacke. Garnet- and cordierite-bearing varieties of plagioclase-biotite gneiss and migmatitic biotite gneiss. These biotite gneiss formed from shale that was somewhat enriched in rocks contain layers and lenses of amphibolite, cordierite- iron and magnesium and deficient in calcium. Probably the amphibole gneiss, and calc-silicate gneiss and related rocks and minor metamorphic rocks also were derived mainly from sedi­ are intruded by generally small bodies of several types of igneous mentary rocks. rocks, some of which have been metamorphosed. The biotite gneisses are migmatized and contain an estimated The metamorphic rocks constitute a well-defined lithologic 15-20 percent by volume of granite gneiss and pegmatite. Other succession that seems to represent a normal stratigraphic rock types contain lesser amounts of similar material, as streaks sequence. Three principal layers of microcline gneiss, each or interlacing veinlets. several hundred to a few thousand feet thick, are interlayered in Four types of Precambrian intrusive rocks granodiorite and alternate sequence with equally thick layers of biotite gneiss. associated rocks, gabbro and related rocks, quartz diorite and An estimated maximum of 15,000-16,000 feet of strata is exposed hornblendite, and biotite-muscovite quartz monzonite each in the quadrangle. The amphibolite forms concordant bodies as with associated pegmatites, intrude the layered rocks. From much as 500 feet wide and 3,000 feet long within and at the oldest to youngest, they are described as follows: margins of major layers of microcline gneiss and local small bodies 1. Granodiorite and associated rocks occur as subconcordant in the biotite gneiss. The cordierite-amphibole gneiss and calc- folded sheets and small plutons, some of which are phaco- silicate