DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, DIRECTOR BUIJLETIN 528 GEOLOGY AND ORE DEPOSITS 1 OF LEMHI COUNTY, IDAHO BY JOSEPH B. UMPLEBY WASHINGTON GOVERNMENT PRINTING OFFICE 1913 CONTENTS. Page. Outline of report.......................................................... 11 Introduction.............................................................. 15 Scope of report......................................................... 15 Field work and acknowledgments...................................... 15 Early work............................................................ 16 Geography. .........> ....................................................... 17 Situation and access.........................--.-----------.-..--...-.. 17 Climate, vegetation, and animal life....................----.-----.....- 19 Mining................................................................ 20 General conditions.......... 1..................................... 20 History..............................-..............-..........:... 20 Production.................................,.........'.............. 21 Physiography.............................................................. 22 Existing topography.................................................... 22 Physiographic development............................................. 23 General features...............................................'.... 23 Erosion surface.................................................... 25 Correlation............. 1........................................... 25 Age of the surface................................................. 26 Early Eocene elevation............................................ 28 Bearing of Eocene erosion surface on economic problems.............. 29 Post-Eocene topography................................................ 29 General geology............................................................. 30 Age and succession of the rocks........................................ 30 Sedimentary rocks..................................................... 30 Algonkian system.................................................. 30 Paleozoic rocks................................................... 32 Cambrian system............................................... 32 Ordovician system............................................. 33 Silurian (?) system............................................ 33 Devonian system............................................... 34 Carboniferous system.......................................... 34 Cenozoic deposits................................................... 35 Tertiary system............................................... 35 Miocene lake beds......................................... 35 Distribution and thickness.-............................ 35 Relations and constitution............................. 36 Deformation............................................ 38 Age................................................... 38 Correlation................................;............ 39 Extent of the Miocene lake............................ 40 Quaternary system............................................. 40 Pleistocene deposits....................................... 40 3 4 - CONTENTS. General geology Continued. page. Igneous rocks......................................................... 41 Archean gneiss............................'.................'....... 41 Late Cretaceous or early Eocene granite............................. 42 Dikes............................................................. 43 Distribution and character..................................... 43 Types......................................................... 44 Age........................................................... 46 Tertiary lavas...................................................... 47 Epitome of geologic history................................................ 48 Ore deposits.............................................................. 49 General features....................................................... 49 Character of deposits................................................ 49 Distribution...................................................... 50 Geologic relations................................................. 50 Gold deposits.......................................................... 51 Placers............................................................ 51 Distribution................................................... 51 Production and history......................................... 52 Origin of placer gold........................................... 52 Character of the placer deposits................................. 53 Age of placers.................................................. 53 Late Tertiary gold veins............................................ 54 Distribution........'........................................... 54 Treatment of the ores.......................................... 54 Geologic relations.............................................. 55 ' Character..................................................... 55 Ores.......................................................... 55 Age and genesis............................................... 56 Similar deposits............................................... 57 Late Cretaceous or early Eocene gold veins.......................... 57 Distribution and history....................................... 57 Geologic relations................................................ 58 Nature........................................................ 58 - Ores.......................................................... 59 Character.................................................. 59 Relation to depth.......................................... 59 Relation to granite.......................................... 60 Secondary concentration............................... 7 .... 61 Age and genesis............................................... 62 Similar deposits............................................... 63 Lead-silver deposits.................................................... 63 Distribution...................................................... 63 Geologic relations.................................................. 63 Character.......................................................... 64 Ores.............................................................. 64 Mineral character............................................. 64 Oxidation and ground-water level............................... 66 Relation of ores to ground-water level........................... 66 Age and genesis.................................................... 68 Copper deposits........................................................ 69 Distribution....................................................... 69 Geologic relations................................................. 69 Character..................:......................................- 70 Ores............................................................... 70 Age and genesis.................................................... 70 CONTENTS. 5 Ore deposits Continued. Page. Cobalt-nickel deposits.................................................. 71 Distribution...................................................... 71 Geologic relations................................................. 71 Character.......................................................... 71 Ores..........................................:................... 72 Age and genesis.................................................... 72 Tungsten deposits...................................................... 73 Distribution....................................\................. 73 Character......................................................... 73 Ores.............................................................. 73 Genesis............................................................ 73 Tin deposits.......................................................... 74 Mineralogy of Lemhi County ores....................................... 74 General features................................................... 74 Ore minerals...................................................... 74 Oxidation and ground-water level....................................... 80 Age and genesis of the ores, by groups............................'...... 80 Superficial deposits. .................... f .......................... 80 Inclosed deposits................................................... 81 Late Tertiary gold veins....................................... 81 i Late Cretaceous or early Eocene deposits........................ 81 Absence of contact-metamorphic deposits. ............................... 82 Future of mining in Lemhi County.......................................... 82 Mining districts of Lemhi County.......................................... 83 Scheme of treatment................................................... 83 Nicholia district...................................................... 83 History and production............................................ 83 Geology and ore deposits........................................... 84 Spring Mountain district................................................ 85 Situation and access................................................ 85 History and production............................................ 85 Physiography...................................................... 85 Geology.......................................................... 86 Ore deposits.....................................................