Geology of the Mount Wilson Quadrangle Western San Juan Mountains Colorado By CALVIN S. BROMFIELD GEOLOGICAL SURVEY BULLETIN 1227 Prepared on behalf of the U.S. Atomic Energy Commission A bedded sequence of volcanic and sedimentary rocks more than 5,000 feet thick has been intruded by stocks and laccoliths UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director Library of Congress catalog-card No. GS 67-177 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page. Absuact__________________________________________________________ 1 Introduction______________________________________________________ 2 Location, accessibility, and culture_______________________________ 2 Topography__________________________________________________ 3 Climate______________________________________________________ 4 Vegetation____________________________________________________ 6 Previous investigations_________________________________________ 6 Present investigation___________________________________________ 8 Acknowledgments_____________________________________________ 9 General geologic setting_ _ __ _ __ __ __ __ __ _ __ _ __ _ _ _ __ _ _ _ __ __ _ _ __ _ _ _ __ _ _ 9 StratigraphY------------------------------------------------------ 10 Triassic System_______________________________________________ 11 Upper Triassic Series_____ _ __ _ _ __ _ __ _ __ _ _ _ __ __ _ __ __ _ _ _ __ __ _ _ 11 Dolores Formation_____________________________________ 11 Jurassic System_______________________________________________ 12 Upper Jurassic Series_ _ _ _ __ __ __ _ __ _ __ __ __ __ _ _ _ _ _ _ _ _ _ __ _ __ _ _ 12 Enkada Sandstone __________________________ ------~--- 12 VVanakah Formation___________________________________ 13 Pony Express Limestone Member___________________ 13 Bilk Creek Sandstone Member______________________ 14 Marlinember_____________________________________ 14 Morrison Formation___________________________________ 15 Salt VVash Sandstone Member_______________________ 15 Brushy Basin Shale Member__ _ _ _ __ _ _ __ _ __ __ _ _ _ _ _ __ _ 16 Cretaceous System_____________________________________________ 16 Lower Cretaceous Series____________________________________ 16 Burro Canyon Formation_______________________________ 16 Upper Cretaceous Series__ __ _ __ __ _ __ __ _ _ _ _ _ _ _ __ _ _ __ __ __ _ _ __ _ 17 DakotaSandstone_____________________________________ 17 Mancos Shale_________________________________________ 18 Tertiary System___ _ _ __ __ _ _ _ _ __ _ _ _ __ _ _ _ __ _ __ __ _ __ __ _ __ _ __ _ _ _ __ _ 19 Oligocene(?) Series_________________________________________ 19 Telluride Conglomerate________________________________ 19 Middle and upper Tertiary_ _ _ _ __ _ _ _ __ _ _ _ __ _ __ __ _ _ __ _ __ __ _ __ 21 San Juan Formation___________________________________ 21 Silverton Volcanic Group_______________________________ 23 Quaternary System____________________________________________ 24 Pleistocene Series__________________________________________ 24 Glacial deposits_______________________________________ 24 Undifferentiated older drift_________________________ 24 Late VVisconsin drift_ __ _ _ __ _ _ _ _ __ _ _ __ _ _ __ _ _ _ __ __ _ __ 26 Pleistocene and Recent Series_______________________________ 27 Landslide deposits_____________________________________ 27 UI IV CONTENTS Stratigraphy-Continued Page Quaternary System-Continued Recent Series_____________________________________________ 29 Rock glaciers and other glacial deposits__________________ 29 Talus________________________________________________ 30 Alluvium_____________________________________________ 31 Intrusive rocks__ _ __ _ _ _ __ __ __ _ _ __ _ _ _ __ __ _ __ __ __ _ __ _ __ __ __ _ __ __ _ __ __ 31 Flattop laccolith_____ __ __ _ _ _ _ __ __ _ _ _ _ _ _ __ __ _ __ __ _ _ _ _ _ __ __ _ __ _ _ _ 31 ~onzonue porphyry_______________________________________ 31 Black Face-Ames plutons_______________________________________ 32 Porphyritic rhyodacite_____________________________________ 33 Grandiorite porphyry_ _ _ _ _ _ __ __ _ __ _ __ _ _ _ __ _ __ __ _ _ _ __ _ __ _ _ _ _ 33 Inclusions________________________________________________ 35 Wilson Peak and Ophir stocks___________________________________ 36 ~icrogranogabbro_________________________________________ 37 Granodiorite______________________________________________ 41 Porphyritic adamellite_____________________________________ 44 Age of the major intrusive rocks_________________________________ 48 ~in or intrusive rocks__________________________________________ 49 Latite__ _ _ __ _ _ _ __ _ _ _ _ _ __ __ __ _ __ __ _ _ _ __ _ __ __ _ _ _ __ _ _ _ __ _ _ _ __ 49 Dikes and sills of intermediate composition___________________ 50 Lamprophyre dikes ___________ .____________________________ 51 Age of the minor intrusive rocks_____________________________ 52 Petrologic relations____________________________________________ 53 Chemical composition of major intrusive rocks________________ 53 Variation diagrams_ _ _ _ __ __ __ _ _ __ __ _ __ __ _ __ __ _ __ _ _ _ __ _ __ __ _ 57 Relation of major intrusive rocks to igneous rocks of western San Juan Mountains_____________________________________ 57 Origin of the intrusive rocks________________________________ 61 Contact metamorphism________________________________________ 63 Structural geology_ __ _ _ _ __ __ _ __ _ _ _ _ __ _ _ __ _ __ __ _ __ _ __ __ __ _ __ __ _ _ _ _ __ 64 Geologic setting of Mount Wilson quadrangle______________________ 64 Igneous intrusions and related structural features _______ -- ___ -- __ -- 65 Flattop laccolith___________________________________________ 66 Cross Mountain anticline___________________________________ 66 Black Face pluton_________________________________________ 67 Amespluton______________________________________________ 68 Wilson Peak stock__ __ __ _ __ __ _ _ __ __ _ _ _ __ _ __ __ _ __ _ _ _ __ _ _ _ _ _ _ 69 Depth of intrusion_________________________________________ 72 Comparison of eastern San Miguel Mountains with laccolithic centers of the Colorado Plateau___________________________ 73 Mechanism of emplacement________________________________ 75 Black Face-Ames plutons and Flattop laccocith_ _ _ __ __ _ _ _ _ 7 5 Wilson Peak stock_____________________________________ 76 Some factors influencing mode of emplacement-___________ 78 Ricodome____________________________________________________ 80 Faults________________________________________________________ 80 CONTENTS v Page Mineral resources__________________________________________________ 81 Vein deposits_ _ _ _ _ _ _ __ _ __ __ _ _ _ _ _ _ __ _ _ _ _ __ _ _ _ _ _ __ __ _ _ _ _ _ __ __ _ __ 81 Iron Springs (Ophir or Ames) district________________________ 83 SanBernardomine____________________________________ 83 Butterfly mine________________________________________ 86 San Juan claims_______________________________________ 87 Mount Wilson mining district_______________________________ 88 Morning Star mine_ _ _ __ __ __ _ _ __ _ _ _ _ __ __ _ __ __ _ _ _ __ _ __ _ _ 88 Veins in upper Bilk Basin______________________________ 89 Silver Pick mine_______________________________________ 89 Other mines_ _ _ _ _ _ __ __ _ _ _ __ __ _ __ __ _ _ _ __ _ __ __ _ _ _ __ __ _ __ 93 Coal_________________________________________________________ 93 References cited______ __ _ _ _ __ _ __ __ __ _ _ _ __ __ _ __ __ _ __ _ __ __ _ _ _ __ __ _ __ _ 94 Index____________________________________________________________ 99 ILLUSTRATIONS [Plates are in pocket] PLATE 1. Geologic map and sections of the Mount Wilson quadrangle. 2. Generalized structural map of the western San Juans. 3. Generalized columnar section. 4. Generalized structural map and sections of the eastern San Miguel Mountains. 5. Map showing patented claims in the western part of the Iron Springs district. 6. Map showing patented claims in the Mount Wilson district. Page. FIGURE 1. Index IllaP--------------------------------------------- 3 2. Photograph of north face of San Miguel Mountains___ __ __ _ _ 5 3. Photograph of view southeast across Bilk Basin____________ 20 4. Photomicrographs of granodiorite porphyry________________ 34 5. Diagram showing modal compositions of rocks of the Wilson Peakstock__________________________________________ 38 6. Photomicrographs of microgranogabbro and granodiorite____ 39 7. Photograph of hand specimen showing sharp intrusive contact between microgranogabbro and granodiorite_____________ 41 8. Camera lucida drawings of relations between plagioclase and orthoclase in granodiorite______________________________ 44 9. Camera lucida drawings of ferromagnesian grains in grano- diorite______________________________________________ 45 10. Sketch of contact between porphyritic adamellite and grano- diorite in Bilk Basin__________________________________ 46 11. Photomicrograph of porphyritic adamellite________________ 47 12. Plot of norms of SanMiguel Mountains intrusive rocks_____ 56 13. Variation diagram of eastern San Miguel Mountains intrusive rocks_______________________________________________ 58 VI CONTENTS Page FIGURE 14. Larsen variation diagram of eastern San Miguel Mountains intrusive rocks_______________________________________ 59 15. Larsen variation diagram of major intrusive rocks of the western San Juan Mountains and the Potosi Volcanic Group______________________________________________ 60 16. Differentiation trend of eastern