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The AAPG/Datapages Combined Publications Database
Wyoming Geological Association
Abstract
Geology and Mineralogy of the Devil Canyon/Little Mountain Area, Northern Big Horn Mountains, Wyoming
Abstract
The Devil Canyon / Little Mountain area of the northwestern Big Horn Mountains in Wyoming was the subject of a recent geological study and surface mapping project conducted by the Worland District Office of the Bureau of Land Management. Primary objectives of the study included assessment of existing and potential mineral resources, particularly uranium; a detailed stratigraphic and structural analysis; assessment of past mineral development activites in the area; and construction of a regional geologic map. Because uranium minerals (chiefly tyuyamunite) were deposited by paleo-groundwater systems, an attempt was made to reconstruct the paleohydrogeology of the area from Mississippian through Plio-Pleistocene time.
The study area lies along three north to northwest-trending and plunging monoclinal structures or drape folds of Laramide age which comprise the northernmost Big Horn Mountains in Wyoming. Lands adjacent to Devil Canyon lie on the near-horizontal east limb of the middle monocline in the series, whereas the Little Mountain area west of Devil Canyon lies along the flexure of the same monocline. The 700 feet thick Madison Limestone (Mississippian), exposed in outcrop throughout the area is typified by karst, paleokarst and solution-brecciated zones in the upper 150 feet (Unit A). Laramide deformation and drape folding caused axial flexure and extension of the sedimentary stack over Precambrian blocks which were uplifted along nearly vertical northeast-dipping reverse faults. At Little Mountain, Unit A paleokarst zones were folded, extended and fractured, and tyuyamunite was later deposited in commercial quantities as coatings, layers, crusts, pockets and pods by groundwater moving along trend through the unit. Tyuyamunite was mined from Little Mountain between 1956 and 1970, but lands adjacent to Devil Canyon have never been mined.
Uranium occurrence and potential is high only in areas subjected to Laramide folding and extension, e.g. Little Mountain. The Devil Canyon area, having been uplifted but essentially undeformed, has a very low potential for uranium mineralization, regardless of the presence of karst, paleokarst and solution-brecciated zones. Analysis of stream sediment (placer) samples from Porcupine Creek by emission spectrographic techniques revealed no anomalous mineral or metal concentrations.
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