DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, DIRECTOR BULLETIN 4=78 GEOLOGY AND ORE DEPOSITS NEAR LAKE CITY, COLORADO BY JOHN DUER IRVING AND HOWLAND BANCROFT WASHINGTON QX>,VERNMENT PRINTING OFFICE; 1911 CONTENTS. Page. Field work................................................................. 9 Acknowledgments.......................................................... 9 Geography ................................................................ 10 Location and general features.......................................... * 10 Topography........................................................... 11 Official mining districts................................................. 11 Climate................................................................ 12 Timber............................................................... 12 History.................................................................... 12 Production................................................................ .1.4 Mines. ....................... ^........................................... 16 Geology, by Whitman Cross ................................................. 18 San Juan Mountains................................................... 18 General features................................................... 18 The earliest eruptions.............................................. 19 San Juan tuff...................................................... 19 Silverton volcanic series........................................... 20 Picayune volcanic group....'................................... 20 Eureka rhyolite................................................ 20 Burnslatite................................................... 21 Pyroxene andesite.............................................. 21 Henson tuff.................................................... 21 Potosi volcanic series............................................... 21 Hinsdale volcanic series........................................... 22 Lake City district..................................................... 22 General geology.................................................... 22 Silverton volcanic series................................:.......... 24 Picayune volcanic group....................................... 24 Occurrence ............................................... 24 General character......................................... 24 Typical exposures......................................... 25 Eureka rhyolite. ................................................ 26 Burnslatite tuff............................................... 27 Pyroxene andesite........................................... '... 28 Henson tuff.....l............................................. 28 Potosi volcanic series. .................................... ^......... 29 Hinsdale volcanic series........................................... 29 Intrusive rocks.................................................... 30 Rhyolite....................................................... 30 Quartz latite.................................................. 31 Andesite....................................................... 31 Quartz monzonite porphyry..................................... 31 Decomposed porphyry of Deadmaii Gulch........................ 32 3 4 CONTENTS. Page. Ore deposits.............................................................. 32 San Juan region....................................................... 32 Geographic continuity ............................................. 32 Genesis of the minerals in the San Juan region....................... 33 Mineralogical similarity............................................ 35 Ore deposits of Lake City district....................................... 37 The lodes.......................................................... 37 General character............................................. 37 Mechanical disintegration....................................... 38 Dimensions.................................................... 39 Terminations.................................................. 41 Strike........................................................ 41 Dip........................................................... 42 Intersections................................................... 42 Faults........................................................ 43 Origin of the lodes.............................................. 43 Distribution of the lodes........................................ 43 Fissure filling and metasomatism ............................... 44 Banding...................................................... 45 Mineralization.................................................... 45 Age ......................................................... 45 Effect of country rock.......................................... 4.6 Kelative abundance of minerals................................ 46 Mineral groups............................................. 46 Tetrahedrite-rhodochrosite group........................... 47 Quartz-galena-sphalerite group.............................. 47 Telluride group............................................ 48 Source of mineralization....................................... 48 Mineralogy........................................................ 51 Primary minerals............................................... 52 Minerals formed at shallow and moderate depths............. 52 Tetrahedrite.......................................... 52 Bismuth compounds................................... 53 Barite................................................. 53 Rhodochrosite................................'......... 53 Hinsdalite............................................. 54 Minerals formed at all depths............................... 55 Pyrite................................................ 55 Galena............................................... 56 Sphalerite (zinc blende) ............................... 57 Chalcopyrite.......................................... 58 Tellurides............................................ 59 Quartz................................................ 60 Dolomite and calcite.................................. 60 Secondary minerals............................................. 60 Oxidation products......................................... 61 Secondary sulphide enrichment minerals..................... 62 Pyrargyrite and proustite.............................. 62 Secondary chalcocite.................................. 63 Covellite.............................................. 64 Bornite................................................ 64 Secondary galena....................................... 64 Gold................................................. 64 Precipitation due to sphalerite.............................. 64 CONTENTS. 5 Ore deposits Continued. Page. Ore deposits of Lake City district Continued. Pay shoots........................................................ 65 Value of the ores.................................................. 66 Summary of ore deposits........................................... 67 Future of the district....................................................... 68 Mining conditions.......................................................... 69 Power................................................................. 69 Labor................................................................ 70 Mining methods........................................................ 70 Extraction of metals.................................................... 71 Detailed descriptions of mines............................................... 72 Capitol City group...................................................... 72 Gallic-Vulcan..................................................... 72 History and production........................................ 72 Development and equipment.................................. 73 Country rock................................................... 73 Veins......................................!................... 73 Ores.......................................................... 74 Excelsior......................................................... 74 History and production........................................ 74 Development.................................................. 75 Country rock.................................................... 75 Veins........................................................ 75 Ores.......................................................... 75 Czar.............................................................. 76 Production and development.................................... 76 Country rock................................................... 76 Veins........................................................... 76 Ores.......................................................... 77 Capitol City........................................................ 77 Yellow Medicine................................................... 78 Lilly............................................................. 79 Chord extension................................................... 79 Woodstock......................................................... 79 Ottawa........................................................... 80 Toby............................................................. 80 Silver Chord....................................................... 80 Czarina............................................................ 80 Lucky Strike tunnel.............................................. 80 Henson Creek mines..................................................