Economic Geology of the Idaho Springs District Clear Creek and Gilpin Counties, Colorado GEOLOGICAL SURVEY BULLETIN 1208 (=) Prepared on behalf of the I U.S. Atomic Energy Commission o 00 * Economic Geology of the Idaho Springs District - i Clear Creek and ; i ?i Gilpin Counties, Colorado By ROBERT H. MOENCH and AVERY ALA DRAKE, JR. GEOLOGICAL SURVEY BULLETIN 1208 Prepared on behalf of the U.S. Atomic Energy Commission UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1966 WUTED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director Library of Congress catalog-card No. GS 65-368 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Abstract_ - _______-_________-_-_--________--___-__-________-_'-_.-. 1 Introduction. _ _______________._________________________ _______-___ 3 Geography. __________________.__._________________________ ___ 6 Purpose and scope of report ___________________________________ 6 Mines and prospects in the Idaho Springs district ________________ 7 Previous studies.__________________________________ _ _____ __!__ 13 Fieldwork____.._________.__..__._______________ ____-- 13 Acknowledgments. _________________-_--__-_______-_-_-_--__--- 14 History, production, and future__--___-__-___---_-----------_---l--- 14 General geology._____________________________________ _1 ___________ 18 Precambrian rocks.______________________________________i_____ 18 Gneissic rocks__________________-_-_______--_^----__-___-- 20 Biotite gneiss. _____________ _:__-_____________ __ _ i-__- 20 Granite gneiss and pegmatite.-------------_-_I-__':!_---_ 21 Microcline gneiss. _________________________L___ J.______ 21 Quartz gneiss.____________________________L__-_-_._-__ '22 Amphibolite and associated calc-silicate gneiss..____i___i__' 22 Calc-silicate gneiss-___--__-_-------______-_--____------ 22 - Origin of the gneissic rocks____-----__---_--__-_-----.-_ 23 Granitic rocks_______.______--____--_______._-_______-.__ 23 Granodiorite-__-_------____--------------------------- 24 Quartz diorite_____-_--__________-____-__--__-__-___-__ 24 Biotite-muscovite granite..___------_-_--_--_______----_ 24 Pegmatitic rocks._________________________________________ 24 Tertiary intrusive rocks_____-_______---__--____--_-_---_-_-_--_ 25 Quaternary deposits..___________________________ __________-_^_ 28 Structure____________________________________________ 28 Foliation and lineation_______________--____^_______________ 29 Folda..................... ..... ------ _"____ _____________ 30 Faults ------_-_---------- -.------_-_-------------.--- 33 Faults of Precambrian (?) age____-------_-______.!l___'-___--- 35 Faults of Laramide age___________________________________ 36 Origin of the faults___________-.-____..__.____.___._._.__ 38 Joints.___________________________________________________ 39 Primary igneous joints._______-___-_________-___-__---- 40 Joints related to Precambrian folds______________________ 40 Regional joint system_______-__-__.__._^__._-_____-.-_- 40 Geologic history summarized__________________________________ 42 Ore deposits.______________________________________________________ 43 Mineralogy._ ________-____-_--_____-_--_--_____________-_-_--- 45 Pyrite____________ __________________________________ 45 Sphalerite. _________.__ -_______---_-_-____________'___-_--- 47 Galena. __________________________ ______________ __________ 48 Chalcopyrite.____________.__:_____________________________ 48 Hi IV CONTENTS Ore deposits Continued Mineralogy Continued Page Marcasite_ _____-_____--_---__-_-_____-_______-_-_________ 48 Bornite_ _-___________-___-____-___-_--______-_-___--__-_ 48 Covellite________________________..________ 48 Chalcocite. ..^-.........-.-.-.......--.-......-. __________ 48 Argentite.________________ ____1___________________________ 49 Tennantite.____-__-_---_---_-_--_-___-______------_----_- 49 Enargite_____-_______--_-____-___----__---___-__-------_ 49 Pearceite._ _-_-_________-__-_-_--------__---_-__-_---____- 49 Polybasite.__-_--_-_-_---------------------_------_------- 50 Proustite-pyrargyrite_ ____________________________________ 50 Gold__-_--_-----__-___-_-------------_------------------- 50 Saver............_._._ 1. 50 Copper-___----______-_-------_-_------__--------------__- 50 Pitchblende and coffinite-___-______-___.________--_____-.__ 50 Hematite_-_-------_----------------_----_-----_---------- 51 Hydrous iron and manganese oxides.________________________ 51 Wolframite_-----_-_-__--------_-----------_--------_-.. 51 Telluride minerals.______-----___--___-_______,._____-______ 52 Fluorite.._________________________________ 52 ; Quartz and cryptocrystalline silica.__________________________ 52 Uranophane.___--_____------_---_--______-___-___________ 53 Uranium phosphate minerals______________________________ 53 Primary carbonate minerals_______________________________ 53 Secondary carbonate minerals_______._______________________ 53 Bayleyite_-__-__-_-_--_--_--__________.___________________ 54 Barite I 54 Anglesite.__---____-------_.._--___-___-________--__-______ 54 Hydrous uranium sulfate minerals.__-____..__.____._________ 54 Copper sulfate minerals____________________________________ 54 Epsomite(?)_-__..____----.-_-_-__.________.____._____._ 54 Classification of the veins____-_-_-__________.___________________ 54 Pyrite veins ______________________________________________ 55 Pyritic copper veins--------_------------_-_-__-------___ 58 Pyritic lead-zinc veins_____________________________________ 60 Lead-zinc veins-__________--__-__-________________-____-__ 64 Local variance in the vein ores.__-____-_-___________-__-.___ 65 Paragenetic sequence of primary vein minerals ____________________ 67 Pyrite stage_______________r ______________________________ 69 Base-metal stage_______-___________________.______________ 70 Zonal distribution of vein types _________________________________ 73 Areal zoning______________________________________________ 73 ' : Depth zoning.___________---______^_______________________ 75 Relation of silver-gold ratios to zonal pattern _________________ 76 Ore bodies_____________-- ----__----________^_________. 77 Localization ___________------______----____________-_---_- 78 Ore bodies along deflections in strike or dip of veins.._____ 79 Ore bodies at vein intersections _________________________ 80 Ore bodies related to rock competency.._________________ 80 Ore bodies at the intersections of veins and fold axes_____ 81 Wallrock alteration.__________________-____-____.._-__--___----- 81 Supergene alteration.__________________________________________ 83 CONTENTS Ore deposits Continued Page Genesis of the veins____________ 84 Sequence of ore formation.___. 84 Hypogene zoning.____________ 85 Environment of ore deposition. 87 References cited..___.____.__-____.__. 89 ILLUSTRATIONS [Plates are in pocket] PLATE 1. Map of the Idaho Springs district, Colorado, showing location of mines and distribution of veins and igneous rocks of Tertiary age. 2. Generalized geologic map and sections of the Idaho Springs district. 3. Geologic section along the Big Five tunnel. Page FIGURE 1. Map showing location of the Idaho Springs district._________ 4 2. Index map showing the Idaho Springs district and adjacent mining districts.______________________________________ 5 3. Map showing major veins in the Idaho Springs district---..-- 34 4. Stereodiagram showing average attitudes of joints in the regional joint system-_________________________________________ 41 5. Diagram showing vein classification based on quantitative mineralogy ___________________________________________ 56 6. Map showing distribution of different classes of veins._______ 57 7. Diagram showing the paragenetic sequence of the primary ore and gangue minerals.____---_______-__-____----___----_ 68 8. Map showing hypogene mineral zones, sample localities, and molecular percentage of FeS in sphalerite.____---______-__ 74 TABLES Page TABLE 1. Ore produced from the Idaho Springs district, 1904-59.___-._ 17 2. Petrography of the Tertiary igneous rocks.--___----._______ 27 3. Primary and secondary ore and gangue minerals_______--__ 46 4. Tenor of ore from some pyrite veins.______________________ 58 5. Tenor of ore from some pyritic copper veins_____-_,---_____ 59 6. Assays of selected ore samples from the Phoenix and Donaldson mines. _ ____________________________________________ 60 7. Tenor of ore from some pyritic lead-zinc veins._____________ 62 8. Assays of selected ore samples from the Alma Lincoln and Bald Eagle mines._________________________________________ 63 9. Tenor of ore from some lead-zinc veins._______-__--_____.-_ 65 10. Iron content of sphalerite.______________________________ 88 ECONOMIC GEOLOGY OF THE IDAHO SPRINGS DISTRICT, CLEAR CREEK AND GILPIN COUNTIES, COLORADO By ROBERT H. MOENCH and AVERY ALA DRAKE, Jr. ABSTRACT The Idaho Springs district is in the central part of the Front Range mineral belt, a northeast-trending zone of porphyritic intrusive rocks and hydrothermal veins of early Tertiary age. From 1860 through 1959 about $65 million worth of gold, silver, lead, copper, and zinc was mined from the veins of the district. The bedrock of the district is composed largely of conformably layered gneissic rocks and small bodies of granitic and pegmatitic rocks of Precambrian age. The most abundant gneissic rocks are biotite gneiss, granite gneiss, and microline gneiss, which form five large conformable layers. In addition,