DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, DIKECTOK BULLETIN 539 SOME ORE DEPOSITS IN NORTHWESTERN OUSTER COUNTY, IDAHO BY JOSEPH B. UMPLEBY WASHINGTON GOVERNMENT FEINTING OFFICE 1913 CONTENTS. Page. Introduction.............................................................. 9 Scope of the report.................................................... 9 Field work and acknowledgments....................................... 9 Earlier work and literature............................................. 11 Geography .............................................................. 12 Situation and access................................................... 12 Settlements............................................................ 12 Lines of travel......................................................... 13 Climate and life........................................................ 13 History of the area.................................................... 14 Physiography.............................................................. 14 General relief......................................................... 14 Drainage............................................................ 14 Summit areas......................................................... 15 Valleys............................................................... 15 Relation to adjacent types............................................. 16 Physiographic history.................................................. 17 General geology.....................................'........'............... 17 Outline............................................................... 17 Algonkian rocks....................................................... 18 Distribution...................................................... 18 Characteristics..................................................... 18 Correlation....................................................... 18 Paleozoic rocks....................................................... 19 Distribution...................................................... 19 Structure........................................................... 19 Lithology.......................................................... 19 Correlation....................................................... 20 Late Cretaceous or early Eocene plutqnic rocks......................... j 21 Distribution................J................................ 1.... 21 Varieties......................................................... 21 Granite........................................................ 21 Quartz monzonite............................................. 22 Quartz diorite................................................. 22 Diorite........................................................ 22 Age and correlation............................................... 22 Early Eocene dikes................................................... 23 Distribution...................................................... 23 Varieties ...........................'............................... 23 Granite........................................................ 23 Granite porphyry.............................................. 24 Diorite porphyry.............................................. 24 Age of the dikes..........................i......................... 24 Miocene volcanic rocks................................................. 24 Distribution and character.......................................... 24 Andesites and latites.............................................. 25 Tuffs.............................................................. 28 Age of the lavas and tuffs........................................... 26 3 4 CONTENTS. General geology Continued. Page. Miocene lacustrine deposits............................................. 27 Distribution...................................................... 27 Character......................................................... 27 Age............................................................... 27 Pleistocene deposits................................................... 27 Morainic material................................................. 27 Outwash material.................................................. 28 Re'sume' of geologic history............................................. 28 Ore deposits.............................................................. 29 General features....................................................... 29 Character of the deposits............................................ 29 Periods of mineralization........................................... 29 Distribution...................................................... 30 Geologic relations................................................. 30 Pre-Oligocene ore deposits.............................................. 31 Classification...................................................... 31 Silver-copper deposits.............................................. 31 Distribution and history....................................... 31 Geologic relations............................................... 32 Form and structure of the deposits............................. 32 The ores...................................................... 32 Pay shoots................................................ 32 Minerals.................................................. 32 Relative abundance.................................... 32 Paragenesis of the vein materials........................ 33 Composition of the tetrahedrite......................... 34 Vertical distribution of minerals........................ 35 Age and genesis................................................ 35 Lead-silver deposits................................................ 38 Distribution and history....................................... 38 Geologic relations..........'................................... 39 Character of the deposits........................................ 39 The ores...................................................... 40 Ore shoots................................................. 40 Minerals of the ore......................................... 40 Tenor of the ore........................................... 41 Age and genesis................................................ 41 Gold-copper deposits.............................................. 42 Geologic relations............................................. 42 Character of the deposits........................................ 42 Ore shoots..................................................... 43 The ores...................................................... 43 General character......................................... 43 Mineralogy of the different shoots........................... 44 Tenor.................................................... 44 Age and genesis................................................ 44 Post-Oligocene ore deposits............................................. 45 Gold-silver veins................................................... 45 Geologic relations:............................................ 46 Metasomatic alteration of the wall rocks......................... 46 Character of the veins......................................... 46 Ore shoots..................................................... 47 CONTENTS. 5 Ore deposits Continued. Post-Oligocene ore deposits Continued. Gold-silver veins Continued. Page. The ores...................................................... 47 Mineralogy................................................ 47 Tenor of the ores.......................................... 48 Age and genesis................................................ 49 Gold placers....................................................... 50 Distribution and history....................................... 50 Character of the deposits........................................ 50 Source of the placer gold....................................... 51 Age of the placers............................................. 51 Minerals of the deposits................................................. 51 Oxidation and ground-water level....................................... 53 Future of mining in northwestern Custer County......................... 54 Mining districts........................................................... 55 Bay Horse district..................................................... 55 Situation and extent............................................... 55 History............................................................ 56 Production........................................................ 56 Mining conditions. ................................................. 57 Topography....................................................... 57 Geology.......................................................... 58 Ore deposits...................................................... 59 Classification.................................................. 59 Lead-silver deposits............................................ 59 Distribution.............................................. 59 Geologic and structural relations............................. 60 Character of the deposits................................... 60 Silver-copper deposits.......................................... 61 Distribution ............................................. 61 Geologic and structural relations .......................... 61 Character of the deposits.................................. 62 The ore................................................... 62 Age and genesis................................................ 63