Geology and Ore Deposits of the Eureka and Adjoining Districts San Juan Mountains, Colorado By WILBUR S. BURBANK and ROBERT G. LUEDKE GEOLOGICAL SURVEY PROFESSIONAL PAPER 535 Prepared in cooperation with the Colorado State Mining Industrial Development Board A study of part of the western San Juan Mountains eruptive center, its related cauldron subsidence structures, altered volcanic rocks, and ore deposits 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. 73-602389 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 CONTENTS Page Page Abstract------------------------------------------- 1 Altered volcanic rocks __________________ - ___________ _ 23 Introduction and acknowledgments ____ - _____________ _ 2 Propylitized volcanic rocks ____ -- ____ --------- __ -- 24 GeographY----------------------------------------- 3 Mineral and chemical nature ____ --- __ ---- ___ - 24 History and production _____________________________ _ 4 Origin and timing of propylitization ___ ---- ___ _ 27 Altered rocks of the solfataric environment ________ _ Regional geology ____ - _____________________________ _ 5 29 Mineral and chemical effects of solfataric ac- Stratigraphy_- ____ ---- __ - ________ ,. ________________ _ 6 Precambrian rocks _____________________________ _ tivity ______________________ --- __ ---- __ -- 30 6 Chemical and physical problems of solfataric Paleozoic sedimentary rocks _____________________ _ 6 alteration ___ - ___ - _____ - _____ ---------_--- 31 Tertiary sedimentary rocks ______________________ _ 7 Altered wallrocks of vein deposits ________________ _ 33 Tertiary volcanic rocks_- ___ - ___________________ _ 7 Mineral deposits ___ ----------- __ ~ -- ____ ------------- 35 San Juan Formation ________________ -------- 7 Vein systems ________ ---- ____ --- ___ ------------- ~6 Silverton Volcanic ·Group ___________________ _ 8 Controls of ore shoots ____ - _____ ---_-----_--- 36 Picayune Formation _______ ------ ______ _ 8 Vein structure _____________________________ _ 39 Eureka Tuff ___________________________ _ 9 Structural control of chimneys and breccia bodies __ _ 41 Burns Formation ______________________ _ 9 Mineralogy and mineral paragenesis __ --- _----- _--- 42 Lower member ______ - _____________ _ 10 Physical and chemical properties_--____ -_- __ -- 43 Upper member ____________________ _ 11 Paragenetic and textural relations ______ -_- __ -- 44 Henson Formation _____________________ _ 12 Origin of ores and gangues ______________________ _ 49 Potosi Volcanic Group ______________________ _ 14 Vein deposits _____________ ----_------------- 49 Quaternary deposits ____________________________ _ 14 Intrusive rocks_------ ___ - _________________________ _ Comparative evolution of vein and chimney 15 54 Volcanic pipes __ ------ ___ -_- _______________________ _ deposits ____ ----------------------------- 16 Supergene and hypogene enrichment_ -- __ ----- _--- 54 Structure------------------------------------------ 16 Exploration and mining ____________________________ _ 55 Regional setting ___________ :.. ___________________ _ 16 Lake Emma-Ross Basin area ____________________ _ 56 Volcanic structural evolution ____________________ _ 17 Cement Creek-Bonita Peak area _____ ---_--------- 59 Descriptions of structural features _______________ _ 19 Engineer Mountain area __________________ --- ___ - 61 Attitudes of the volcanic rocks ______________ _ 19 Poughkeepsie Gulch area ___________ ------------- 61 Major structural elements ___________________ _ 19 Mineral Point area _____________________________ _ 62 Cauldron ring-fault belt ________________ _ 20 California Gulch-Animas Forks-Wood Mountain area ________________________________________ _ Eureka graben ________________________ _ 20 62 Fault systems of Poughkeepsie Gulch ____ _ 21 Treasure Mountain-Eureka Mountain area ____ -_--- 65 Downfaulted Red Mountain block _______ _ 22 Brown Mountain area ___________________ --_- ___ - 67 Conjectural structural features- _____________ _ 22 References cited ___________ -_-_-_------------------- 68 Basement structure ________________________ _ 23 Index--------------------------------------------- 71 ILLUSTRATIONS [Plates 1-8 are in pocket] PLA'I'E 1. Geologic map showing structure of part of the Silverton cauldron and areas nearby in Ouray, San Juan, San Miguel, and Hinsdale Counties. 2. Geologic map of the Eureka district and adjoining areas. 3. Geologic sections of the Eureka district and adjoining areas, San Juan, Ouray, and Hinsdale Counties, and explanation of the geologic map. 4. Generalized geologic map showing principal structural features of the western San Juan region. 5. Diagrammatic sections showing the Tertiary volcanic and structural evolution of the western San Juan Mountains. 6. Generalized structure map and geologic sections of the Eureka, Animas Forks, and Gladstone areas. 7. Map showing principal mine workings in the Eureka and adjoining districts. 8. Plan map and vertical projections of mine workings and stopes in the Sunnyside and Gold King mines. ni IV CONTENTS Page FIGURE 1. Index map of Colorado showing area of this report____________________________________________________ 2 2. Photograph and sketch of south side of Engineer Mountain____________________________________________ 13 · 3. Photograph of Animas River valley_________________________________________________________________ 20 4. Graph showing some major rock constituents plotted against carbon dioxide in propylitized dacites of Burns Formation----------------------------------------------------------------------------------- 27 5. Diagrammatic sketch showing environment and depth zones in fissure and chimney systems of the Silverton cauldron mineralized belts_____________________________________________________________________ 32 6-8. Sketches showing: 6. Types of fault intersections; ore shoots and deflections in fault trend related to fault intersections____ 37 7. Types of vein patterns as illustrated on plate 2------------------------------------------------ 39 8. Structural features of compound veins within and northwest of the Eureka district _________ .________ 40 9. Photographs of ore and gangue textures_-___________________________________________________________ 45 10. Photomicrographs of ore and gangue minerals of the No Name vein_____________________________________ 46 11. Photomicrographs of manganese silicate gangues------------------------------------------------------ 48 12. Photographs and photomicrographs of ores and gangues----------------------------------------------- 50 13. Photograph and sketch of Sunnyside Basin where the Ross Basin and Sunnyside vein systems join__________ 57 14. Plan map and vertical projection of workings, Lead Carbonate mine____________________________________ 60 15. Photograph and sketch of some of the rock units and veins in the Mineral Point area ________._____________ 63 TABLES Page TABLE 1. Eureka district mine production of gold, silver, copper, lead, and zinc in terms of recovered metals, 1926 and 1932 to 1957 inclusive__________________________________________________________________________ 5 2. Nomenclature of Tertiary volcanic units in the western SanJuan Mountains_______________________________ 7 3. Middle(?) Tertiary volcanic and structural evolution of the western San Juan Mountains____________________ 19 4. Estimated volatile contents of volcanic rocks before and after propylitization_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 24 5. Chemical analyses of propylitized and pyritized rocks of the Burns Formation______________________________ 25 6. Minor elements of propylitized and pyritized rocks of the Burns Formation________________________________ 26 7. Composition of gases from volcanic fumaroles, igneous rocks, steam wells, and geysers_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 29 8. Chemical analyses of altered wallrocks at the Polar Star mine, Engineer Mountain__________________________ 31 9. Chemical analyses of altered rocks near Sunnyside Saddle_______________________________________________ 34 10. Minor elements of altered rocks near Sunnyside Saddle__________________________________________________ 35 11. Selected minor elements in sphalerites from vein and chimney deposits____________________________________ 43 12. Optical properties of some manganese silicate minerals from the Sunnyside veins, Eureka Gulch, San Juan CountY-------------------------------------------------------------------------------------- 43 13. Partial chemical analysis of mixed manganese silicate gangue, Sunnyside mine, Eureka Gulch, San Juan County __________________________________ ·_______ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 43 14. Constituents of manganese silicate gangues_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 52 15. Analyses of hot-spring deposits and water and of manganese silicate gangue of upper Tertiary veins, western SanJuan Mountains-------------------------------------------------------------------------- 53 GEOLOGY AND ORE DEPOSITS OF THE EUREKA AND ADJOINING DISTRICTS, SAN JUAN MOUNTAINS, COLORADO By WILBUR S. BuRBANK and RoBERT G. LUEDKE ABSTRACT diaspore, and other minerals that formed in a relatively acid