DEPARTMENT OF THE INTERIOR FRANKLIN K. LANE, Secretary UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, Director Bulletin 626 THE ATLANTIC GOLI) DISTRICT AND THE NORTH LARAMIE MOUNTAINS PAPERS BY ARTHUR C. SPENCER WASHINGTON GOVERNMENT PRINTING OFFICE 1916 CONTENTS. Pago. Preface, by F. L. Eansome ................................................. 7 The Atlantic gold district, Fremont County, Wyo........................... 9 v Introduction.......................................................... 9 Situation and general features.......................................... 9 - Geology^of the general region........................................... 12 Geology of the Atlantic district......................................... 14 . Rocks present..................................................... 14 Schists .......................................................... 14 Schists of the central Atlantic belt............................. 14 Structure of the schists........................................ 15 Schists derived from igneous rocks............................. 16 Magnetite schists in the northern part of the district.............. 16 Distribution and general character......................... 16 The magnetite schists considered as iron ore................. 17 Serpentine and asbestos............................................ 18 Intrusive rocks................................................... 19 Diorite ....................................................... .19 Dikes of porphyry ............................................ 21 Minor intrusions of granite...................................... 22 Inliers of Cambrian sandstone....................................... 23 Tertiary deposits................................................... 23 Historical notes........................................................ 23 Estimated gold production............................................ 27 Veins............................................................... 28 Previous descriptions.............................................. 28 Classification of the veins and other gold deposits................... 30 Cross veins................................................... 30 Strike veins.................................................. 30 Stringer lodes................................................ 31 Gash veins.................................................... 31 Mineralized rock masses....................................... 31 Mineralogy of the veins........................................... 32 Gold content of the ores............................................ 32 Probable persistence of the veins........:.......................... 33 Future development ................................................. 34 General outlook for lode mining.................................... 34 Planning of development work. ...................................... 34 Outlook for railroad transportation .................................. 35 ^ Cost of power...................................................... 36 Milling of the ores. .................................................. 39 Placer deposits......................................................... 42 Economic geology of the North Laramie Mountains, Converse and Albany counties, Wyo........................................................... 47 Introduction....................:::................................... 47 North Laramie Mountains.............................................. 47 Definition.;.....................'.................................. 47 Configuration..................................................... 47 Drainage.................I..:..................................... 48 ' 3 4 CONTENTS. Economic geology of the North Laramie Mountains, Converse and Albany counties, Wyo. Continued. North Laramie Mountains Continued. Page. Climate. ......... ^............................................... 48 Vegetation, stock raising, and agriculture........................... 49 Railroads.......................................................... 50 Wagon roads...................................................... 50 Geology............................................................... 50 General stratigraphy and structure................................. 50 * The geologic map.................................................. 52 Pre-Cambrian rocks..........................................\ ..... 52 Sedimentary rocks, by N. H. Barton................................ 53 General features...............................-.'................ 53 Carboniferous system.......................................... 53 Casper formation.......................................... 53 Satanka (?) shale and Forelle (?) limestone.................. 54 Triassic (?) system.......................,.....:............... 54 Chugwater formation...................................... 54 Jurassic system............................................... 54 Sundance formation....................................... 54 Jurassic or Cretaceous system.................................. 55 Morrison formation......................................... 55 Cretaceous and Tertiary systems................................ 55 Cleverly (?) sandstone.................................. 1... 55 Benton shale.............................................. 55 Niobrara shale............................................ 56 Later Cretaceous and early Tertiary formations.............. 56 White River formation........:........................... 56 Mineral deposits in the North Laramie Mountains. ...................... 56 Development..................................................... 56 North Laramie Peak district....................................... 57 Location and geology............:.............................. 57 General character and occurrence of the mineral deposits......... 58 Maggie Murphy belt........................................... 60 Three Cripples and Tenderfoot belts............................. 62 Maverick prospects............................................ 65 Saul's camp.................................................. 66 Snowbird group................................................ 68 Trail Creek group............................................. 70 Hoosier Boy group................................'............. 71 War Bonnet district................................................ 72 Location and geology........................................... 72 Prospects north of Fortymile ranch............................. 72 Copper King belt............................................. 74 Oriole belt........................ .............................. 75 Brenning copper prospect....................................... 76 Perry claims................................................... 77 Deer Creek district................................................. 77 Location and geology..........................-..........:...... 77 Swede Boy vein.............................................. 77 Chromite in Deer Creek canyon................................. 78 Asbestos prospects east of Deer Creek............................ 79 Mormon Canyon prospects..................................... 79 Martin Smith copper prospect................. i................ 79 CONTENTS. 5 Economic geology of the North Laramie Mountains, Converse and Albany counties, Wyo. Continued. Mineral deposits in the North Laramie Mountains Continued. Page. La Prele district................................................... 79 Location and geology........................................... 79 Copper prospects on Cotton wood Creek......................... 80 Hazenville prospects.......................................... 81 Index...................................................................... 83 ILLUSTRATIONS. PLATE I. Map of Atlantic gold district, Wyo., showing principal geologic features.......................................................... 12 II. Map showing part of the mining claims in the Atlantic gold dis­ trict, Wyo................................................... 22 III. Sketch map of Atlantic and adjacent districts, Wyo., showing dis­ tribution of gold placers in part................................. 42 IV. Geologic map of the North Laramie Mountains, Wyo., and adjacent territory....................................................... 52 V. Sketch map of the North Laramie Mountains, Wyo., showing loca­ tion of principal mineral prospects............................. 56 FIGURE 1. Drainage and railroad map of Wyoming........................... 10 2. Map of part of Fremont County, Wyo., showing situation of the Atlantic gold district and general geologic features of the region. 11 3. Geologic sketch map of part of North Laramie Peak district....... 59 4. Sketch map of Snowbird group, showing schist bands in granite... 69 5. Sketch map of Trail Creek group, showing local geology........... 70 6. Sketch map of Mewis property, showing location of claims and local geology...................................................... 80 PREFACE. By F. L. RANSOME. In response to numerous requests from persons interested in the development of the mineral resources of Wyoming and desirous that reliable information concerning them should be published, A. C. Spencer, geologist, was detailed in 1914 to make an economic recon­ naissance of the region adjacent to South Pass City, Miners Delight, and Atlantic City, in Fremont County, and of parts of the North Lar- amie Mountains, in Converse and Albany counties. The two areas examined by Mr. Spencer lie about 125 miles apart,