--> Abstract: Potential Uranium Source Rocks of the White River Group in Western Nebraska and South Dakota, by Steven Sibray; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Potential Uranium Source Rocks of the White River Group in Western Nebraska and South Dakota

Steven Sibray1

(1) Nebraska Geological Survey, University of Nebraska - Lincoln, Scottsbluff, NE.

Recent studies of the paleosols of the White River Group are used to identify two potential source rocks for the uranium mineralization at the Crow Butte mine. Identifying source rocks has important implications for exploration of uranium in the western Nebraska and southwestern South Dakota region. The Crow Butte uranium deposit located near Crawford, Nebraska was discovered in the fall of 1980. In January 1981, the deposit was reported to contain at least 25,000,000 pounds of U3O8 at a grade of 0.25% U3O8 . This deposit has produced over 14,000,000 of U3O8 since 1991.The original lease position was acquired based on a favorable review of the regional geology indicating an extensive fluvial system underlying potential source rocks of the White River Group. In the last 30 years, detailed stratigraphic research has further refined our understanding of the paleosols and the paleohydrogeology of these volcaniclastic sediments. It is now possible for additional geochemical research to identify which paleosol acted as the source of the uranium.

The weakest link in the past exploration model for sandstone uranium deposits was the inability to recognize potential uranium source rocks. Based on the work of the Texas Bureau of Economic Geology in the 1970's and early 1980's, a functional uranium source rock should be a well developed paleosol where a large quantity of rhyolitic ash fall glass has been altered to clay (typically montmorillonite or kaolinite) under oxidizing conditions in the vadose zone. Reviewing the past 30 years of research in the paleopedology and paleohydrogeology of the White River Group, only two stratigraphic units can be considered potential source rocks. The two units are the Interior Paleosol Equivalent found at the top of the Chamberlain Pass Formation and the Peanut Peak Member of the Chadron Formation. If the Interior Paleosol Equivalent is the functional source rock, then only the Chamberlain Pass Formation sandstones in Nebraska and Wyoming are of interest to the uranium exploration geologist. If the Peanut Peak Member is the functional source rock, then the Ahearn Member of the Chadron Formation would also be an attractive exploration target. Whole rock analysis of thorium and uranium of the potential source rocks should be useful as an exploration tool.