Denson and Gill reported in 1965 that uranium in carbonaceous shale and lignite occurs at many horizons throughout 2,500 feet or more of Upper Cretaceous and lower Tertiary (Paleocene and Eocene) rocks in the southwestern part of the Williston basin. The uranium occurrences extend northward 150 miles or more in general alignment with the structurally lowest part of the basin from Harding and Perkins Counties in northwestern South Dakota to the western part of Dunn County in southwestern North Dakota. The carbonaceous host rocks containing the higher grade occurrences range from 6 inches to 2 feet or more in thickness and are characterized by lenticularity, relatively high ash contents (35 to 40 percent), and low heating values.
Although small amounts of uranium occur sporadically in carbonaceous rocks over a wide area along the southwestern part of the Williston basin, the important reserves of ore-grade uraniferous lignite and carbonaceous shale containing greater than 0.1 percent uranium are present in the North Cave Hills
and Slim Buttes areas of Harding County, S. Dakota, and in the Little Missouri River escarpment area (Rocky Ridge and Saddle Butte localities) in eastern Billings and northwestern Stark Counties, N. Dakota. In these areas blanket-type deposits, containing in excess of 150,000 tons of ore-grade material, and
lentil-type deposits of higher grade material, containing a few hundred pounds to as much as 1,500 tons, have been reported.
Some of the deposits in the Cave Hills and Little Missouri River escarpment areas are overlain by relatively thin overburden which make them amenable to strip mining. Inferred reserves of ore-grade material in the region exceed 1 million tons. Chemical analyses of 10 large samples of uranium-bearing lignite and carbonaceous shale collected by the Atomic Energy Commission from Billings County, N. Dakota, indicate an average content of 0.18 percent uranium, 0.3 percent molybdenum, 0.09 percent phosphorus, and 0.01 percent vanadium. Only 2 of the 10 large samples analyzed contained less than 0.1 percent
uranium.
Primary structural and stratigraphic controls affecting the localization of uranium appear to be shallow troughs superimposed ·on the flanks of the basin by late Tertiary tectonic movements and by the proximity of the host rocks to the pre-Oligocene erosion surface. The localization of uranium in the Williston basin, as in the Colorado Plateau and Texas Coast Plain, is remarkably influenced by the presence of carbon, which has an affinity for uranium and which is effective as a reducing agent in causing the precipitation of uranium and associated metals from solution. Porosity of the lignitic host materials and the enclosing rocks are important secondary controls of mineralization.
From a study of the spectrographic, chemical, and mineralogic data and of the principal structural and "topographic relations· of the deposits, we conclude that uranium, arsenic, molybdenum, selenium, vanadium, and some other elements were introduced into the lignite by ground water. Mineralization was probably continuous from Miocene to present. The abundance of these elements in. samples of ground water draining the middle and upper Tertiary tuffaceous rocks in the region and their presence in only minor amounts in ground water draining the underlying lignite-bearing sequence in unmineralized areas suggest an origin by leaching from the volcanic constituents composing the tuffaceous rocks.
Other evidence supporting this mode of origin is the fact that all significant occurrences of uranium in the region fall to within about 200 feet stratigraphically of the base of the tuffaceous sequence or to its projected base in areas where erosion has removed the tuffaceous sequence.
Microscopic studies of 135 grab samples from the continental basin-fill sedimentary rocks comprising the Hell Creek, Fort Union, Chadron, Brule, and Arikaree Formations reveal a characteristic heavy-mineral assemblage for each formation that differs from the assemblage in the adjacent formation. No relation was found between any of these assemblages and the occurrence of uranium.
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