Geology of the Empire Quadrangle Grand, Gilpin, and Clear Creek Counties ColOrado. By WILLIAM A. BRADDOCK .,..,., GEOLOGICAL SURVEY PROFESSIONAL PAPER 616 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director Library of Congress catalog-card No. 78-650 226 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. .20402 CONTENTS Page Tertiary igneous rocks-Continued Page Abstract___________________________________________ 1 Monzonite group ______________________________ _ 22 Introduction_______________________________________ 1 Feldspathoidal hornblende-pyroxene monzonite_ 22 Geologic history______________________________ _ _ _ _ _ _ 2 Hornblende-pyroxene monzonite _____________ _ 26 Petrographic methods ______________________________ - 3 Porphyritic hornblende-pyroxene monzonite ___ _ 31 Precambrian metamorphic rocks _____________ --_-- __ -- 3 Monzonite porphyry _______________________ _ 32 Biotite gneiss___________________________________ 3 Quartz-bearing monzonite porphyry __________ _ 32 Migmatite ________________________ -- __ -- __ - 6 Leucocratic monzonite ______________________ _ 32 Calc-silicate gneiss ________________________ -- ___ - 6 Biotite quartz monzonite porphyry _______________ _ 33 Microcline gneiss (microcline-quartz-plagioclase-bio- Bostonite porphyry ____________________________ _ 33 ti te gneiss) _________ ._ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 6 Sanidinite porphyry ____________________________ _ 35 Hornblende gneiss and amphibolite_______________ 8 Age and sequence of intrusion ___________________ _ 37 Origin of metamorphic rocks_____________________ 10 Petrology _____________________________________ _ 37 Nature of premetamorphic rocks______________ 10 Definition of rock series ____________________ _ 37 Metamorphic facies_________________________ 13 Calc-alkali series ___________________________ ~ 40 Precambrian igneous rocks___________________________ 14 Alkali-calcic series _________________________ _ Qua.rtz diorite of St. Marys Lake__________________ 14 41 Bo.ulder Creek Granite__________________________ 15 Undersaturated series ______________________ _ 43 Quartz diorite and hornblendite ______________ - _ _ _ _ 17 Summary of petrologic interpretations ________________ _ 45 Hornblende diorite and other metamorphosed basic Quaternary deposits ________________________________ _ 45 dikes________________________________________ 18 Structure _________________________________________ _ 45 Granite porphyry __________________________ - _ _ _ _ 18 Older deformation (B o) _________________________ _ 45 Silver Plume Granite____________________________ 19 Younger deformation (B ) ____________________ -- _ 49 Pegmatite_____________________________________ 19 11 Relations of intrusive rocks to deformations _______ _ 49 Tertiary igneous rocks_______________________________ 19 East-northeast-trending folds of unknown origin ___ _ 51 Granodiorite group_____________________________ 20 Faults ________________________________________ _ Hornblende granodiorite porphyry..:.___________ 20 51 References ________________________________________ _ Biotite granodiorite porphyry________________ 22 52 Granite aplite _________________________ - ___ - 22 Index ____________________________________________ _ 55 ILLUSTRATIONS .Page PLATE 1. Geologic map and sections of the Empire quadrangle _________________________________________________ In pocket 2. Index map of Tertiary igneous rocks in the Empire quadrangle, Grand, Gilpin, and Clear Creek Counties, Colo __ In pocket 3. Foliation and lineation diagrams__________________________________________________________________ In pocket FIGVRE 1. Index map showing location of the Empire quadrangle and the areas covered by other published geologic maps_ 2 2. Photomicrographs of muscovite in biotite gneiss________________________________________________________ 5 3. Photographs of migmatite, hornblende gneiss, and amphibolite _____________________________________ ------ 7 4. Triangular plot of quartz, plagioclase, and microcline in the granitic part of migmatite from biotite gneiss and from micro cline gneiss__________________________________________________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 8 5. Triangular plot of quartz, plagioclase, and microcline in microcline gneiss _______________ - _______ - ___ ------- 8 6. Graph showing comparison of the mineral composition of biotite gneiss with norms of quartz-rich igneous rocks and with mesonorms of sedimentary rocks _____________________________________________________ --__ 10 7. Triangular diagrams showing the comparison of modes of biotite gneiss and mesonorms of sedimentary rocks___ 12 8. Equilibrium curves related to metamorphism in the Empire quadrangle___________________________________ 14 9.· Photomicrographs of the quartz diorite of St. Marys Lake_______________________________________________ 16 10. Triangular plot of plagioclase, quartz, and microcline in the Boulder Creek Granite_________________________ 17 11. Triangular plot of plagioclase, quartz, and microcline in the Silver Plume Granite___________________________ 19 12. Diagrams showing modal compositions of rocks of the Empire stock_______________________________________ 23 13. Photomicrographs of feldspathoidal hornblende-pyroxene monzonite of the Empire stock____________________ 24 14. Photomicrographs of hornblende-pyroxene monzonite, Empire stock_______________________________________ 30 III IV CONTENTS Page FIGURE 15. Photomicrographs of leucocratic monzonite, bostonite porphyry, and sanidinite porphyry____________________ 34 16. Diagram showing the known sequence of intrusion of Tertiary igneous rocks in the Empire quadrangle________ 37 17. Diagram showing relation between normative anorthite and normative quartz or nepheline in Tertiary igneous rDcks-------------------------------------------------------------------------------------------- 38 18. Larsen plot of analyses of Tertiary igneous rocks_______________________________________________________ 39 19. Triangular plot of the calc-alkali series_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 40 20. Phase diagrams for the anorthite-albite-orthoclase system compared with the trends of the alkali-calcic and calc-alkali' series__________________________________________________________________________________ 41 21. Triangular plot of aJkali-calcic series~ _______ --- ______ -________________________________________________ 42 22. Triangular plot of the anorthite-poor members of the alkali-calcic series___________________________________ 43 23. Diagram of quaternary nepheline-anorthite-kalsilite-silica system showing the position of the ternary feldspar plane---~-------------------------------------------------------------------------------------- 44 24. Addition and subtraction diagram used in testing a theory about the origin of feldspathoidal hornblende-pyroxene monzonite-------------------------------------------------------------------------------------- 45 25. Index map of folds and faults in the Empire and Central City quadrangles and adjacent areas---------------- 47 26. Synoptic diagram showing major maximums of foliation and lineation______________________________________ 48 27. Photomicrographs of cataclastically deformed rocks_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 50 28. Diagram showing the strike of large faults and small faults and veins_______________________________________ 52 TABLES Page TABLE 1. Mineral assemblages in several varieties of biotite gneiss__________________________________________________ 4 2. Modes of microcline gneiss ________ ~___________________________________________________________________ 9 3. Modes of amphibolite________________________________________________________________________________ 9 4. Chemical and spectrographic analyses of hornblende from amphibolite_____________________________________ 10 5. Mesonorms of sedimentary rocks ________________________________________ ·______________________________ 11 6. Probable sedimentary parents of biotite gneisses_________________________________________________________ 13 7. Modes of quartz diorite of St. Marys Lake______________________________________________________________ 15 8. Modal compoEition of Boulder Creek Granite___________________________________________________________ 17 9. Modal compositions of Silver Plume Granite____________________________________________________________ 19 10. Correiation of Tertiary igneous rock names as used by Wells ( 1960) and in this report______ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 20 11. Chemical and spectrographic analyses and CIPW norms of hornblende granodiorite porphyry, biotite granodiorite porphyry, biotite quartz monzonite porphyry, and granite aplite_______________________________________ 21 12. Chemical and spectrographic analyses and CIPW norms of feldspathoidal hornblende-pyroxene monzonite______ 26 13. Chemical and spectrographic analyses and CIPW nornis of hornblende-pyroxene monzonite, porphyritic horn- blende-pyroxene monzonite, and monzonite porphyry_________________________________________________ 27 14. Chemical and spectrographic analyses of pyroxene and amphibole from hornblende-pyroxene monzonite of the Empire stock ___________ ._ _