Regional Geology sand Mineral Deposits In and Mear the Oentral Part of the Lemhl Range, Lemhi County, Itlaho tJ.S, GEOLOGICAL SURVEY PROFESSIONAL PAPER Regional Geology and Mineral Deposits In and Near the Central Part of the Lemhi Range, Lemhi County, Idaho By EDWARD T. RUPPEL and DAVID A. LOPEZ U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1480 A descriptive summary of the rocks and structure in part of east-central Idaho UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Any use of trade names in this report is for descriptive purposes only and does not imply endorse­ ment by the U.S. Geological Survey. Library of Congress Cataloging-in-Publication Data Ruppel, Edward Thompson, 1925- Regional geology and mineral deposits in and near the central part of the Lemhi Range, Lemhi County, Idaho. (U.S. Geological Survey professional paper ; 1480) Bibliography: p. Supt. of Docs, no.: I 19.16:1480 1. Geology Idaho Lemhi Range. 2. Ore-deposits Idaho Lemhi Range. I. Lopez, David A. II. Title. III. Series. QE104.L43R86 1988 557.9678 87-600492 For sale by the Books and Open-File Reports Section, U.S. Geological Survey Federal Center, Box 25425, Denver, CO 80225 CONTENTS Page Page Abstract ........................................... 1 Granitic intrusive rocks Continued Introduction ....................................... 2 Petrography Continued Topography and drainage ......................... 2 Early Paleozoic granite in the Beaverhead Mountains 59 Present work and acknowledgments ................ 4 Eocene granitic rocks in the Lemhi Range ....... 62 Summary of earlier geologic studies ................ 7 Descriptions of intrusive masses ................... 64 Geology ........................................... 7 Early Paleozoic granite in the Beaverhead Mountains 64 Middle Proterozoic to Lower Cambrian rocks ........ 10 Ima stock and Patterson Creek and Falls Creek Yellowjacket Formation ....................... 11 intrusives .................................. 65 Reasons for interpreting the Yellowjacket Forma­ Big Eightmile (Blue Jay) stock ................. 66 tion to be autochthonous ................. 12 Park Fork intrusive .......................... 67 Lemhi Group ................................ 13 Big Timber stock ........................... 68 Inyo Creek Formation ..................... 13 Gilmore stock ............................... 68 West Fork Formation ..................... 13 Sawmill Canyon sheet ........................ 69 Big Creek Formation ...................... 13 Alder Creek intrusive ......................... 70 Apple Creek Formation .................... 14 Age of granitic rocks ............................. 71 Gunsight Formation ...................... 15 Early Paleozoic granite in the Beaverhead Mountains 71 Swauger Formation ........................... 15 Tertiary granitic rocks in the Lemhi Range ...... 72 Lawson Creek and Wilbert Formations .......... 16 Structural controls on the emplacement of Eocene granitic Regional relations and correlation ............... 17 rocks ......................................... 72 Paleozoic rocks .................................. 18 Structure .......................................... 73 Ordovician and Ordovician-Silurian ............. 18 Precambrian and early Paleozoic folding, faulting, and Summerhouse Formation ................... 19 uplift ........................................ 74 Kinnikinic Quartzite ....................... 20 Thrust faults and related folds .................... 75 Saturday Mountain Formation .............. 21 Steep faults, block uplifts, and monoclinal folds ...... 77 Silurian ..................................... 24 Late Cenozoic regional warping and young normal faults 80 Laketown Dolomite ....................... 24 Some regional consequences of Cenozoic block uplifting, Devonian ................................... 24 warping, and faulting ........................... 81 Channel sandstone and Jefferson Formation . 25 Mineral deposits .................................... 82 Three Forks Formation .................... 26 History of mining ............................... 84 Mississippian and Mississippian-Pennsylvanian ... 27 Structural and stratigraphic controls on mineralization 87 McGowan Creek Formation ................ 28 An alternative hypothesis: stratabound lead, silver, and Middle Canyon and Scott Peak Formations ... 29 zinc in the Jefferson Formation ............... 88 Railroad Canyon Formation ................ 30 Descriptions of mineral deposits ................... 89 Bluebird Mountain Formation .............. 31 Base- and precious-metal replacement veins and bed­ Pennsylvanian ............................... 31 ding replacement deposits ................... 89 Snaky Canyon Formation .................. 32 Tungsten-quartz veins and copper-silver-quartz veins 92 Permian .................................... 33 Disseminated deposits of copper and molybdenum in Grandeur Member of Park City Formation ... 33 granitic rocks .............................. 95 Challis Volcanics and related intrusive rocks ......... 34 Disseminated deposits of lead in granite at the Kimmel Volcanic stratigraphy ......................... 34 mine ...................................... 95 Volcanic rocks in the northern part of the Patterson Deposits of secondary copper minerals .......... 96 quadrangle ................................ 38 Deposits of copper-bearing magnetite ........... 96 Cenozoic sedimentary rocks and surficial deposits .... 39 Suggestions for prospecting ....................... 96 Tertiary tuffaceous sedimentary rocks ........... 39 Descriptions of mines ............................ 98 Other Tertiary conglomerates and relict gravels ... 45 Junction district ............................. 99 Glaciation and glacial deposits ................. 46 Texas and Spring Mountain districts ............ 100 Mass-wasting deposits ........................ 55 Blue Wing district ........................... 110 Alluvial deposits ............................. 57 Other mines and prospects .................... Ill Granitic intrusive rocks .............................. 58 Measured sections .................................. Ill Petrography .................................... 59 References cited .................................... 118 in IV CONTENTS ILLUSTRATIONS Page PLATE 1. Geologic map of the central Lemhi Range, Idaho ................................................... In pocket FIGURE 1. Index map of east-central Idaho and southwest Montana ............................................... 3 2. Photographs of ice-carved mountains of the central Lemhi Range ........................................ 4 3. Generalized section of sedimentary rocks, Lemhi Range ................................................. 8 4. Map showing approximate location of the Lemhi arch and flanking regions of Late Proterozoic and early Paleozoic sedimentation ................................................................................. 9 5. Map showing the distribution of Eocene Challis Volcanics ............................................... 35 6. Stratigraphic column and plots of compositions of Challis Volcanics ...................................... 36 7. Map showing distribution of Tertiary sedimentary rocks in east-central Idaho and southwest Montana ........ 40 8. Photographs of glacial erosional features in the central Lemhi Range ..................................... 47 9. Map showing ice distribution and flow in the central Lemhi Range during the last major glaciation ........... 49 10. Photograph of valley of Big Timber Creek ............................................................ 51 11. Photographs of thrust fault surfaces ................................................................. 52 12. Photograph of glaciated valley and compound terminal moraine, Meadow Lake Creek ....................... 54 13. Photograph of rock glacier in the Middle Fork of Little Timber Creek ..................................... 56 14. Photograph of snow avalanche chute and deposit on south flank of Gunsight Peak ......................... 57 15. Diagrams of nomenclature of intrusive igneous rocks ................................................... 59 16. Plot of comparison of oxides in intrusive rocks, central Lemhi Range and Beaverhead Mountains, east-central Idaho 61 17. Plot of modal proportions of quartz and feldspar in granitic intrusive rocks in east-central Idaho ............. 62 18. Map showing the effects of Pliocene to Holocene arching and faulting north of the Snake River Plain ......... 80 19. Map showing locations of mining districts and principal mines and prospects in the east-central Lemhi Range, Idaho 83 TABLES Page TABLE 1. Revised sequence of Middle Proterozoic sedimentary rocks, Lemhi Range, Idaho 11 2. Tentative regional correlations of Middle Proterozoic sedimentary rocks ........ 17 3. Chemical analyses of Challis Volcanics and related intrusive rocks 37 4. Chemical analyses of granitic intrusive rocks in and near central Lemhi Range, Idaho 60 REGIONAL GEOLOGY AND MINERAL DEPOSITS IN AND NEAR THE CENTRAL PART OF THE LEMHI RANGE, LEMHI COUNTY, IDAHO By EDWARD T. RUPPEL and DAVID A. LOPEZ ABSTRACT continuing to the present, major block uplifts formed the present northwest-trending mountain ranges. The Lemhi Range, in east-central Idaho, is one of the longest moun­ Regional arching, parallel to the Snake River Plain and forming the tain ranges in the state. Its central