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By' . Frank C» Armstrong

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By' . Frank C» Armstrong PRELIMINARY REPORT ON THE GEOLOGY OF THE ATLANTIC CITY-SOUTH PASS MINING DISTRICT WYOMING by' . Frank C» Armstrong OPEN FILE^l I CONTENTS Abstract ....................................................................................................................................................................... 1 Introduction ................................................................................................................................................................. 1 Purpose ................................................................................................................................................ 1 Location ............................................................................................_^^ 2 Access .............................................................................................................................. 2 Topography .....................................................................................................^^ 5 Climate and vegetation .................................................................................................................... 6 Previous work .............................................................................................................................................. 7 Acknowledgments........................................................................................................................................... 8 Economic considerations.................................................................................................................. 9 Conclusions ................................................................................................................................................. 10 General geology..................................................................................................................................................... 11 Principal features .............................................................................................................................. 11 Geologic formations ........................................................................................................................ 12 Schist, hornfels and granulite (pre-Cambrian) ............................. 12 Amphibolite (pre-Cambrian) ...................................................................................... 16 Diorite and granodiorite (pre-Cambrian) ................................................ 18 Diorite .................................................................................................................................. 18 Granodiorite ...................................................................................................................19 Aplite (post-Permian).....................................................................................................20 Conglomerate (Tertiary).............................................................................................. 21 Alluvium (Quaternary)....................................................................................................., 22 Structure of the pre-Tertiary rocks .......................................................................... 22 General features ................................................................................................................... 22 Folding ................................................._ 23 Faulting......................................................................_^ 2? Mylcnitized zone...................................................................................................... 30 Amphibolite and granodiorite ................................................................................ 33 Relation to ore deposits ............................................................................................. 3 Geologic history............................................................................................................................... Ore deposits .............................................................................................................................................................. 35 History and production ............................................................................................................... 35 General geologic features ...................................................................................................... 37 Future development....................................................................................................................... l\2 Mine descriptions ...........................................................................................................................Ij-3 Carissa ............................................................................................................................................. ij-3 History, production and development ........................................... U3 Geologic features ................................................................................................. U6 Conclusions .................................................................................................................... 50 Empire State ............................................................................................................................. 5! Duncan ....................................................................................................................................^.M... ^ Mary Ellen ............................................................................................................................. 56 Tabor Grand ............................................................................................................... 59 Carrie Shields..................................................................................................................... 6l Midas ..........., ........................ ....._ 62 Rock Creek Placer................................................................. ...................................63 .II ILLUSTRATIONS Page In folder Plate I Geologic map of South Pass City and vicinity, Fremont County, Wyoming ..................................................................." ^'""^ " II Explanation to accompany Carissa mine maps, Fremont County, Wyoming ...................................................................." £-- '- III Topographic and geologic map, Carissa mine, Fremont County, Wyoming ....................................,................................" *-' '-' ' IV Composite map of underground workings, Carissa mine, Fremont County, Wyoming .................................................." V Geology of second level, Carissa mine, Fremont County, Wyoming.................................................................." VI Geology of third level, Carissa mine, Fremont County, Wyoming .......................................^..^................" VII Cross Sections, Carissa mine, Fremont County, VJyoming .......,....................................,..............." VIII Vertical longitudinal projection, Carissa mine, Fremont County, Wyoming ................................................. ..........." IX Photomicrograph of a thin section of a specimen of the schist, hornfels and granulite unit showing graded bedding ...............................................................14 X Photograph of a polished specimen of the schist, hornfels and granulite unit showing bedding and graded bedding .........................................................................15 XI Photograph of a specimen of the schist, hornfels and granulite unit shoTdng cross bedding .. ................15 XII Diagramatio representation of probable regional structure in the Atlantic City-South Pass district.................................................................................................... Sij. XIII Photomicrograph of a thin section of mylonite ...........32 XIV Photomicrograph of thin section of strained vein quartz showing mortar structure, Duncan mine ....................................................................................... i^l Figure 1 Index map, Atlantic City-South Pass district, Fremont County, Wyoming ..............................................................3 2 Geologic sketch map, Atlantic City-South Pass district, Fremont County, Wyoming .....................................I* PRELIMINARY REPORT ON THE GEOLOGY OF THE ATLANTIC CITY-SOUTH f'ASo HITTING DISTRICT VJYOMING ABSTRACT The Atlantic City-South Pass mining district is in the south­ west corner of Fremont County, 7fyo,, in an area of pre-Cambrian metamorphosed sedimentary rocks. The rocks trend northeast, dip steeply, and have been isoclinally folded. The folds are slightly overturned to the northwest and plunge northeast. The area is cut by east-west and north-south faults* Ore-bearing quartz veins are intimately associated with the east-west system of faults. The ore minerals are arsenopyrite and gold. , Future development of the district is dependent upon the recognition of the relationship be- i tween faulting and ore deposition and the proper metallurgical i treatment of the sulfide ore. It is believed that under favorable conditions a few small mines can be developed. INTRODUCTION Purpose In connection with the full development of the resources of the Missouri valley, the Bureau of Reclamation asked the Geological Survey to investigate the Atlantic City-South Pass district, Fremont County, Wyo*,
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