Exploring for Roll Front Uranium Deposits and Groundwater Using Airborne Electromagnetic Surveys in the Nebraska Panhandle
Steven Silbray, James Cannia, Bruce Smith, and Jared Abraham
Recent advances in application of airborne electromagnetic [AEM] surveys to groundwater management programs can also be utilized to map aquifers that host uranium deposits. The US Geological Survey and the Nebraska Geological Survey have collected AEM data as part of an ongoing project to define the hydrogeologic framework of the principal shallow aquifer in the Nebraska Panhandle. Additional interpretation of these data sets has identified areas where the deeper confined aquifer of the Tertiary White River Group may contain economic deposits of uranium. Uranium mineralization occurs as roll fronts in the fluvial sediments of the White River Group in the Crawford area. The Crow Butte in situ recovery [ISR] mine has produced approximately 16 million pounds of U3O8 since 1991 from these deposits. The roll front deposits formed along the margins of the paleovalleys cut into the underlying Cretaceous Pierre Shale. Uranium mineralization is present as far south as Cheyenne County approximately 90 miles from Crow Butte. Several mining companies have recently leased additional acreage for uranium exploration across a wide area in the Nebraska Panhandle. The coarse fluvial sediments of the White River Group are electrically resistive when compared to the surrounding material. In 2010, a helicopter time domain AEM system (SkyTEM ) was flown in several areas of the Nebraska Panhandle. The interpreted distribution of deep resistive units correlates with the generalized location of the coarse grained White River Group fluvial sediments mapped from oil and gas borehole geophysical logs. In some areas, the detection of the fluvial sandstones and conglomerates was near the limit of the depth resolution of the SkyTEM system (300 meters) and was masked by thick electrically conductive materials of the overlying sediments. There are fixed wing AEM systems available that can effectively map to depths of 500 meters but with little resolution of shallow aquifers. Deeper mapping AEM systems using carefully designed interpretation (data inversion) would allow mapping of the larger White River Group paleovalleys over most of the Nebraska Panhandle. By using current AEM technologies, both groundwater and uranium resources can be mapped using technologies that are suitable for the depths and the characteristics of the target.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012