Abstract

The Crow Butte in situ recovery uranium mine in Dawes County, Nebraska has produced approximately 15 million pounds (lbs) of U₃O₈ since 1991. In January 1981, Wyoming Fuel Company announced the discovery of at least 25 million lbs of U₃O₈ at a grade of 0.25 percent or greater near Crawford, Nebraska (historic numbers; not 43–101 compliant). At the time of discovery, it was thought that the uranium mineralization was in the basal sands of the Oligocene Chadron Formation in the White River Group. This paper presents evidence that recent revision of the White River Group stratigraphy places the Crow Butte deposit in the Eocene Chamberlain Pass Formation, which is unconformably overlain by the Chadron Formation.

The uranium mineralization at Crow Butte occurs in roll-front deposits similar to the classic roll-front deposits found in the Tertiary basins of Wyoming. These roll-front deposits occur along the margins of the Chamberlain Pass Formation paleovalleys located in western Dawes County, Nebraska. The Chamberlain Pass Formation consists of interbedded subarkosic sandstones and green bentonitic mudstones overlain by the well-developed Interior Paleosol Equivalent. The Interior Paleosol Equivalent is a significant sequence-bounding paleosol associated with the Chamberlain Pass Formation – Chadron Formation unconformity. The overlying Chadron Formation consists of green bentonitic mudstones, and minor lacustrine limestones. The Ahearn Member of the Chadron Formation is an arkosic sandstone and conglomerate found in an incised paleovalley in South Dakota.

The Interior Paleosol Equivalent of the Chamberlain Pass Formation and the paleosols of the Peanut Peak Member of the Chadron Formation are possible uranium source rocks. Determining which stratigraphic units acted as the source of the uranium has implications for the exploration of similar deposits in the Great Plains. Exploration along the margins of the Chamberlain Pass paleovalleys should encounter additional uranium mineralization. If the Peanut Peak Member of the Chadron Formation acted as a source rock, then the Ahearn Member of the Chadron Formation becomes an attractive exploration target.