Geology and Ore Deposits of the Summitville District San Juan Mountains Colorado By THOMAS A. STEVEN and JAMES C. RATTE GEOLOGICAL SURVEY PROFESSIONAL PAPER 343 Prepared in cooperation with the Colorado Metal Mining Fund Board UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1960 UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. SEATON, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library catalog card for this publication appears after page 70. For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. - Price $2 (paper cover) CONTENTS Page Page Abstract_ _______________-------_._- 1 Structure Continued Introduction ___________-__------_-_-- 2 Volcanoes of late Fisher age Continued Acknowledgments__ -. _._---_-__- 3 Younger volcano__--_-._-___----_______-_ 35 Geography..._______-----------__ 3 Faults_ -_____--_-_----_---__________--_-__- 36 History and production__________ 4 Fissures filled by quartz latite dikes_-----__- 36 Previous geologic investigations.__ 7 Early period of fault movement-___--__-_-_ 36 Scope and methods of investigations. 8 Late period of fault movement _____________ 37 Geologic setting._____________________ 9 Altered and mineralized rocks--____________________ 37 10 Rocks altered during Conejos time_-_------_---_ 38 Rock classification and nomenclature__--.-_------- 10 Rocks altered during Fisher time._______-__---_ 38 Conejos formation ___________._______L.______-__ 11 Limits and scope of alteration study. ______i 39 Lithology.___________________-_----_-___-__ 11 Structural setting.________________________ 40 Petrography ____ ________-__------_-_-__ 12 Description of altered rocks._--__________-_ 40 Chemical composition_-_______----_---_-_-_- 13 Quartz-alunite zone._--______-______-_ 41 Fisher quartz latite_______________-_ _---_------- 13 Distribution of rock types.____-_-. 41 Lower member.__________.__.--___.-_-._-__ 13 Quartz-alunite rock_______________ 42 Rhyodacitic lavas...__--_-----_----_---_ 14 Vuggy quartz rock_-______________ 43 Vitrophyre flow_______--_---_.---_ 15 Compact quartz rock____________ 44 Pyroclastic rocks.___-_-_-----__---_ 15 Primary ore deposits-__,._--___---- 44 Massive flows._______--_-_--_---_-_ 16 Illite-kaolinite zone ___________________ 46 Petrography _. _____________________ 17 Montmorillonite-chlorite zone __________ 47 Quartz latitic lavas and intrusive rocks. 18 Supergene alteration._________________ 48 Quartz latites east of Park Creek.____ 18 Chemical changes during rock alteration____. 49 Quartz latites underlying Cropsy Ridge . 18 Solfataric environment-_______________________ 51 Quartz latites near South Mountain.... 19 Course of solfataric alteration at Summitville. 52 Petrography _______________________ 21 Source,of the sulfuric acid.___---____-___-_ 53 Upper member____-_________---_-_-_------- 22 -Deposition of the ores._.___---___________. 55 Lavas capping Cropsy Peak and adjacent Economic geology______:___._______-____________ 57 ridges-__--_---------_----_-.-__---.- 22 Vein zones__---_---_-_-_---._________________ 57 Lavas underlying North Mountain._______ 24 Tewksbury vein zone____________________ 58 Lavas capping the ridge east of Park Greek. 24 Aztec-McDonald-Black Wonder vein zone... 59 Possible upper Fisher dikes on South Moun­ Hidden vein zone...______________________ 60 tain. _____-___________-------_--._-_- 24 Copper Hill-Esmond vein zone_____________ 60 Petrography. ____-______-__--------_-__ 25 Little Annie vein zone._________-__________ 62 Chemical composition._____-_-----_-----_-_- 25 Dexter (Odin) vein zone.__________________ 62 Structure ___________-___-----_-----------------_-_- 29 Minor silicified pipes and vein zones_______ 62 Shield volcano(?) of Conejos age________________ 30 Distribution of metals in the ore._-____--_______ 64 Rhyodacitic flows and domes of early Fisher age____ 31 Vertical continuity..__________________________ 64 Structures underlying Cropsy Ridge______________ 32 Other factors_---_---__---__________.__-___- 65 Quartz latite dome of South Mountain.___________ 33 Suggestions for prospecting.___________________ 65 Volcanoes of late Fisher age__--__-------_----_--_ 34 Literature cited________________________________ 66 Older volcano.________-__-_--_-----_--__--- 35 Index ___----_-_-_-_-_-_-_-_________________.____ 69 ILLUSTRATIONS [Plates 1-5 in pocket] PLATE 1. Reconnaissance geologic map of the Summitville district and surrounding areas. 2. Geologic map of the Summitville district, and section showing restored South Mountain volcanic dome. 3. Geologic map of the main metal-producing area. 4. Composite map of the underground workings. 5. Gains and losses of the major rock-forming constituents during hydrothermal alteration. in IV CONTENTS Page 6. A, Typical rocks of the quartz-alunite zone; B, Photomicrograph of fine-grained quartz alunite-rock; C, photo­ micrograph of quartz-alunite rock showing relict porphyritic texture.___________________________ Facing 44 7. A, Aztec glory hole; B, Photomicrograph of rock from the illite-kaolinite zone, showing altered phenocryst; C, Photomicrograph of an altered feldspar phenocryst in the illite-kaolinite zone_,_____________ Following 44 8. A, Photomicrograph of kaolinitic rock from the illite-kaolinite zone; B, photomicrograph of a rock from the mont- morillonite-chlorite zone, showing partial alteration of plagioclase phenocrysts; C, Photomicrograph of a rock from the montmorillonite chlorite zone showing biotite relicts _______________________________ _ Following 44 9. A, Photomicrograph of chloritic rock from the montmorillonite-chlorite zone; B, Small silicified pipe about 130 feet southeast and along trend of Hidden vein____________________________________________________ Facing 45 FIGURE 1. Index map of the San Juan Mountains, showing the location of the Summitville district._____________________ 2 2. View looking north across Wightman Fork from the top of South Mountain ________ _ __________________ 4 3. South Mountain and Little Annie Mining camp, Summit district, Colorado.____________ __________________ 7 4. Normative compositions of rocks from the Conejos formation.____________________________________________ 14 5. A sketch showing geology of the Cropsy Ridge area______________________________________________________ 19 6. A view of the Summitville district from the west._______________________________________________________ 23 7. Normative compositions of rocks from the Fisher quartz latite in or near the Summitville district__________ 27 8. Trends of composition changes in the Fisher quartz latite__-______________________________________________ 28 9. Normative compositions of rocks from the Fisher quartz latite____________________________________________ 29 10. A view looking north-northeast across Park Creek from a point near the center of the western margin of the Summitville district____________________________ _________________________________________________ 32 11. A view of Lookout Mountain from the west ___________________________________________________________ 37 12. Vertical projection of mine workings and stopes on the Tewksbury vein____________________________________ 58 13. Sketch vertical projection of mine workings and stopes on the Aztec and Black Wonder veins.________________ 59 14. Vertical projection of mine workings and stopes on the Hidden vein______________________________________ 61 15. Sketch vertical projection of the mine workings and stopes on the Little Annie vein__________________________ 63 TABLES TABLE 1. Production of gold, silver, copper, and lead from the Summitville district, Colorado______.___________________ 5 2. Chemical analyses and normative compositions of rocks from the Conejos formation_______..____,______---__ 13 3. Chemical analyses and normative compositions of rocks from the Fisher quartz latite____--______----_---____ 26 4. Analyses of waters from streams tributary to Alamosa Creek________-.._________________-_-____-_-__---- 38 5. Chemical analyses of an impure specimen of alunite from the Summitville district___________________________ 43 6. Quantitative spectrographic analyses of alunite and sanidine from the Summitville district ____ ____________ 43 7. Chemical analyses of rocks from the main metal-producing area of the Summitville district.____---___-_____-- 50 GEOLOGY AND ORE DEPOSITS OF THE SUMMITVILLE DISTRICT, SAN JUAN MOUNTAINS, COLORADO By THOMAS A. STBVEN and JAMES C. RATTE ABSTRACT Two periods of hydrothermal activity altered large masses The Summitville district, in the eastern San Juan Mountains, of rock in the general Summitville region. The earlier of these Colorado, has produced about $7% million worth of metals; periods was late in the Conejos period of vulcanism, and ap­ most of this has been from gold, with lesser amounts in silver, parently was related to the intrusion of the quartz monzonitic copper, and lead. Copper has been a minor byproduct during stocks along the southern margin of the Summitville district. several periods of mining at Summitville, and since 1940 Unaltered lavas in the lower member of the Fisher rest un- copper has averaged between 1 and 5 percent of the annual conformably on these altered rocks. Most of the rocks altered production value for those years the mines were operating. during Conejos time are outside of the area studied in detail All the rocks that crop out in the Summitville district are during the present investigation. of volcanic or related shallow intrusive origin. The oldest The