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AAPG Rocky Mountain Section Meeting

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Paleosols as Source Rocks for Uranium Deposits in Wyoming and Nebraska

Abstract

Tuffaceous sediments of the White River Group may have been the source of uranium found in roll front deposits formed in older Paleogene sediments in Wyoming and Nebraska. Rhyolitic volcanic glass contains relatively high concentrations of uranium, which is released when it is weathered during pedogenesis. Uranium is soluble under oxidizing conditions and will precipitate under reducing conditions. A roll front uranium deposit can form in underlying aquifers at the interface between oxidizing groundwater and reducing groundwater. While the roll front model has been successfully used in the discovery of many uranium deposits, there are significant sedimentary deposits where the roll front model is not applicable. Recent test hole drilling by the Conservation and Survey Division has penetrated the entire White River Group section at a number of sites in Western Nebraska. Detailed sampling of drill cuttings and geophysical logs has shown that the Yellow Mounds Paleosol Series has anomalously low gamma readings compared to the parent material (Pierre Shale), and some geophysical logs also showed a sharp gamma spike at the top of the unweathered shale. There were no noticeable low gamma readings in paleosols in the White River Group in the test holes possibly due to the relative thinness of the White River Group paleosols. The most likely candidates for source rocks are the Interior Paleosol Series, which formed at the unconformity at the top of the Chamberlain Pass Formation, and the paleosols of the lower part of the Chadron Formation. Whole rock chemical analyses quantifying thorium, uranium, and rare earth elements would help identify which paleosols were possible source rocks. Analyses of the lower White River Group paleosols will resolve some of the basic questions concerning the genesis of the deposit at the Crow Butte ISR mine that has recovered 18 million pounds of yellowcake [U3O8]. Identifying uranium source rocks will be important in developing successful exploration strategies for sedimentary uranium deposits that do not fit the classical roll front model.