Geology and Structural Control of Ore Deposition in the Creede District San Juan Mountains Colorado GEOLOGICAL SURVEY PROFESSIONAL PAPER 487 Prepared in cooperation with the Colorado State Metal Mining Fund Board Geology and Structural Control of Ore Deposition in the Creede District San Juan Mountains Colorado , By THOMAS A. STEVEN and JAMES C. RATTE GEOLOGICAL SURVEY PROFESSIONAL PAPER 487 ·Prepared in cooperation with the Colorado State Metal Mining Fund Board UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965 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, n-:-c. 20402 - CONTENTS Page Page Abstract __________________________________________ _ 1 Structure-Continued Introduction ______________________________________ _ 1 Creede caldera ___________ -----_----------------- 58 59 Location and geography ____________________________ _ 2 Subsidence in the caldera core _____ -_--------- Caldera-border faults_-.- ____ ----------------- 60 Previous investigations _________________________ - ___ _ 3 Faulting in the Creede graben _________ ------- 60 Present investigation _______________________________ _ 5 Northwest-trending faults in the Farmers Creek- Acknowledgments _______ --_--- _____________________ _ 5 Wason Park areas _______________________ _ 61 History and production-----------------------------.- 6 Doming of the caldera core ____ -------------- 62 Geologic formations ________________________________ _ 11 Form of dome_________ ----------------- 62 La Garita Quartz Latite ________________________ _ 15 Crestal graben zone __ - _______ ----------- 62 Rhyolite of Miners Creek _______________________ _ 19 Causal force ___________________________ _ 63 Shallow Creek Quartz Latite ____________________ _ 19 Late faulting in the Creede graben __ -------------- 63 Bachelor Mountain Rhyolite ____________________ _ 21 Alpha-Corsair fault zone ___ ------------------ 64 Farmers Creek Rhyolite ________________________ _ 26 Amethyst fault zone ______ ------------------ 65 Mammoth Mountain Rhyolite ___________________ _ 28 Bulldog Mountain fault zone_---------------­ 67 Rhyolite neck and flow near Shallow Creek _______ _ 30 Solomon-Holy Moses fault zone __ ------------ 67 Wason Park Rhyolite __________ ---- ____________ _ 30 Ore deposits _______________ ------------------------- 68 Huerto Formation ______________________________ . 32 Age of mineralization ___________ ----------------- 68 Quartz latite neck and flows north of Nelson Moun- Ore deposits along the Amethyst fault zone_------- 69 tain ________________________________________ _ 34 Amethyst vein _________ -------------------- 69 Rat Creek Quartz Latite ________________________ _ 34 Vein rna terial ________________ ---------- 69 Nelson Mountain Quartz Latite __________________ _ 37 Detailed description _________ ------------ 70 Snowshoe Mountain Quartz Latite _______________ _ 39 Southern segment_________________ -- 70 Mixed breccias in the caldera core _______________ _ 42 Middle segment ________ - - - - _ -- - - - - - 71 Point of Rocks volcano _________________________ _ 43 Northern segment ___ --------------- 72 Fisher Quartz La ti te ___________________________ _ 43 0 H vein ______________________ ------------ 74 Creede Formation _____________________________ _ 46 Vein rna terial ______________ ------------ 74 Surficial deposits _______________________________ _ 49 Detailed description ____________________ _ 75 Structure _________________________________________ _ 50 Oxidation _________________________________ - 77 La Garita cauldron ____________________________ _ 50 Alpha-Corsair vein zone ___________________ -- __ -- 78 Outlet Tunnel and Holy Moses mines area _____ _ 51 Ore deposits in the Creede Formation _______ ------ 78 Phoenix Park area _________________________ _ 51 Monon Hill mine ____________________ --- __ ------ 78 Upper East Willow Creek area ______________ _ 52 Solomon-Holy Moses vein zone ________ -- __ --_---- 79 Wason Park-La Garita Mountain area _______ _ 52 Equity vein ____ ------- __ -----------_-_--------- 81 Bachelor Mountain cauldron _____________________ _ 53 Suggestions for prospecting _________________ ---------- 82 Arcuate faults and brecciated rock ___________ _ 53 Amethyst and related veins ______________________ _ 84 Ancestral Bulldog Mountain fault ___________ _ 55 Bulldog Mountain fault zone ________ --- ____ ------- 84 Ancestral Amethyst fault ___________________ _ 55 Disseminated deposits in the Creede Formation_---- 84 Ancestral Solomon-Holy Moses fault _________ _ 56 Alpha-Corsair fault zone ______________________ -_- 85 Faults in Phoenix Park _____________________ _ 56 Solomon-Holy Moses fault zone ______ --- ___ -.-- ___ _ 85 San Luis Peak cauldron ______________________· ___ _ 57 Minor faults ___________ ---- __ ------------------- 86 Depressed area of Nelson Mountain __________ _ 57 References _________________ -_ ----------------------- 86 The Equity fault __________________________ _ 57 Index ____________________________________________ _ 89 III IV .CONTENTS ILLUSTRATIONS [Plates are in separate volume] PLATE 1. Geologic map and section of the Creede district. 2. Tectonic map of the Creede district. 3. Geologic maps of the Amethyst 5 and Commodore 3 levels, Amethyst vein zone. 4. Geologic map -of the Commodore 5 level, Amethyst vein zone. 5. Geologic map of the Silver Horde and Magnusson levels, Monon Hill mine. 6. Geologic map of the Holy Moses 2, Outlet Tunnel, and Palo Alto levels of the Holy Moses and Phoenix mines. 7. Map of mine workings of the Mexico, Solomon, Ethel, and Ridge mines. FIGURE 1. Map showing location of the Creede district with reference to the area of volcanic rocks of the San Juan Mountains, Page Colo------------------------------------------------------------------------------------------- 3 2. Map showing productive veins and principal mines in the Creede district___________________________________ 4 3. Proportional value of metals produced in the Creede district______________________________________________ 7 4. Diagram showing general relations of major rock units in the Creede district_ ___ _ .:.. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 12 5. Photographs of sheeted rock in the Willow Creek Member of the Bachelor Mountain Rhyolite________________ 54 6. Cross section showing typical structural and stratigraphic control of mineralized rock in the Monon Hill mine___ 79 7. Geologic map of the Equity mine ___________________ .___________________________________________________ 83 TABLES Page TABLE 1. Metal production from the Creede district (Sunnyside-King Solomon districts), Mineral County, Colo_________ 8 2. Comparison of rock units near Creede, Colo., used in earlier reports with those used in this report_____________ 14 GEOLOGY AND STRUCTURAL CONTROL OF ORE DEPOSITION IN THE CREEDE DISTRICT, SAN JUAN MOUNTAINS, COLORADO By THOMAS A. STEVEN and JAMES C. RATTE ABSTRACT Crystal-rich quartz latitic ash was erupted later, during the Sixty million dollars worth of silver, lead, zinc, and gold has time of caldera subsidence, to form the tuffs and welded tuffs been extracted from mines in the Creede district of Colorado. of the Rat Creek Quartz Latite and the densely welded tuffs The principal source of this production has been from veins of the Nelson Mountain Quartz Latite. The last major erup­ along faults that formed during long-continued subsidence con· tions during that time resulted in the accumulation of more current with late Tertiary volcanic activity in the central San ·than 6,000 feet of crystal-rich welded tuff in the core of the Juan Mountains. Most subsidence was in a large volcanic cal­ Creede caldera to form the Snowshoe Mountain Quartz Latite. dera, 10-12 miles in diameter, which we have called the Creede The caldera sank concurrently with accumulation of the Snow­ caldera. The known mineralization was confined largely to a shoe Mountain Quartz Latite, and talus and landslide debris complex graben that extends north-northwest from the caldera. from the exposed walls intertongue with and overlie the pyro­ The rock units in the Creede area record a complex sequence clastic caldera fill. of volcanic events, which can be divided with respect to recur­ Minor eruptions during caldera subsidence resulted in local rent subsidence. The oldest unit is the Outlet Tunnel Member volcanic necks and flows that cut and are intertongued with of the La Garita Quartz Latite; the ash-flow eruptions respon­ the more widespread units deposited then. sible for this unit culminated in collapse along a series of faults Caldera subsidence was followed by pronounced doming of roughly parallel to upper East Willow Creek, and along sev­ the caldera core due, presumably, to magmatic pressure. Local eral northwest-trending normal faults. flows, domes, and pyroclastic breccias of Fisher Quartz Latite The rhyolite of Miners Creek, the Bachelor Mountain Rhyo­ were then erupted along the margin of the caldera or along lite, the Shallow Creek Quartz Latite, and the Phoenix Park fissure zones extending outward from the caldera. Concurrently, Member of the La Garita Quartz Latite were erupted, at least lake and stream deposits, volcanic ash, and travertine from in part, onto the irregular surface formed by the earlier col­ many mineral springs were deposited in the structural moat lapse. The rhyolite of Miners Creek is a local steep-sided accu­ around the margin