In northwestern South Dakota and adjacent areas uranium-bearing lignite beds occur at many horizons in the Hell Creek formation of late Cretaceous age and the overlying Ludlow, Tongue River, and Sentinel Butte members of the Fort Union formation of Paleocene age. Uranium analyses of 275 surface and auger samples and about 1,000 core samples show that many of the lignite beds contain 0. 005 to 0. 02 percent uranium with concentrations of 0. 05 to 0.10 percent uranium in the lignite ash. Analytical data indicate that the region contains an aggregate of at least 47,500, 000 tons of lignite with an average grade of slightly more than .0. 008 percent containing 3, 900 tons of uranium. Almost a fifth of the estimated reserves are adapted to strip mining and are in beds averaging about 4 feet in thickness. Uranium concentrations of this magnitude in lignite indicate that these deposits upon the development of proper utilization techniques and processes may be an important future source of uranium. Recent discoveries of ore-grade deposits of autunite-bearing lignite and secondary uranium minerals in carbonaceous sandstone at Cave Hills and Slim Buttes indicate that northwestern South Dakota and adjacent areas may containimportant reserves of uranium-ore. The stratigraphic units containing the uraniferous lignite beds have a combined thickness of about 1, 500 feet and are unconformably overlapped by 300 feet or more of tuffaceous sandstone and bentonitic claystone of the White River and Arikaree formations of Oligocene and Miocene age. The stratigraphically highest lignite beds in the local sequence have the greatest concentration of uranium,, and the uranium content is greatest at the top of thick lignite beds, diminishing progressively downward to a vanishing point in their lower parts. Variations in permeability of the rock overlying the mineralized lignite beds seem to be reflected in the intensity of uranium mineralization. Most of the known uranium-bearing lignite deposits in the region are closely overlain by the White River and Arikaree formations. Field evidence indicates that the uranium in the lignite is independent of the age of the formation in which the lignite occurs and that the uranium has been concentrated by downward and laterally moving ground water from the overlying mildly radioactive, tuffaceous rocks. The White River and Arikaree formations have about 1.2 times more uranium than the average sedimentary rock. The uranium content of spring water from these formations is 30 times as great or greater than that of normal ground water. Field relations suggest that the uranium is of secondary origin and has been introduced subsequent to the accumulation and marked regional warping of the lignite beds and associated rocks. During the transportation of the uranium-bearing volcanic materials from their place of origin to the site of deposition in South Dakota, the uranium is believed to have been held as a finely disseminated constituent in volcanic ash. Subsequent release or displacement of the uranium is thought to have resulted from weathering and chemical breakdown, thus freeing and allowing the transportation of the uranium, probably in ionic form, to the lignite by downward and laterally moving groundwaters 0 Gar bonaceous material in the path of these uranium-bearing waters is believed to have acted as a receptor that extracted the uranium as a result of an ion exchange mechanism or by the formation of organo-metallic compounds Geologic factors that seem most significant in controlling the distribution and concentration of uranium in Dakota lignites are as follows: 1) stratlgraphic proximity of the lignite to the base of the White River formation; 2) permeability of the rocks overlying the lignites; 3) adsorptive properties and porosities of the lignitic constituents; 4) present and past position of the groundwater table; and 5) the amount of uranium in
the original White River and Arikaree sediments. Individual maps showing the extent, thickness, and variations in mineral content of the important deposits in the Table Mountain, Cave Hills, Slim Buttes, Lodgepole, and Medicine Pole areas are included. Conditions controlling the concentration of uranium are described and their application as guides to finding additional reserves by the presently held concepts are explained and illustrated.