Moore, and two co-authors, reported in 1954 that uranium-bearing lignite was mapped and sampled in the Bullion Butte &Sentinel Butte 9 HT Butte 9 and Chalky Buttes areas in southwestern North Dakota. The uraniferous lignite occurs at several stratigraphic positions in the Sentinel Butte member of the Fort Union formation of Paleocene age. A total of 261 samples were collected for uranium analysis from 85 localities.
Lignite containing as much as 0.045 percent uranium, 10.0 percent ash, and 0.45 percent uranium in the ash was found although the average is lower.
Inferred reserves for the four areas examined are estimated to be about 27 million tons of lignite in beds about 2 feet thick and containing more than
3,000 tons of uranium. The lignite averages more than 30 percent ash in the surface samples. The principal factor that seems to influence the uranium
content of lignite beds is their stratigraphic position below the overlying rocks of the White River group of Oligocene age. All of the uranium-bearing
beds closely underlie the base of the White River group. Although this relationship seems to be the controlling factor, the relative concentration of
uranium may be modified by other conditions.
Beds enclosed in permeable rocks are more uraniferous than beds in impermeable rocks, and thin beds have a higher content of uranium than thick beds. In addition, thick lignite beds commonly have a top-preferential distribution of uranium. These and other factors suggest that the uranium is secondary and that it was introduced by ground water which had leached uranium from volcanic ash in the overlying rocks of the White River group. It is thought that the.uranium is held in the lignite as part of a metallo-organic compound.
Deposits of uranium""bearing lignite were examined in the Bullion Butte, Sentinel Butte, HT Butte. and Chalky Buttes areas in southwestern North Dakota during 1953 by the u.s. Geological Survey on behalf of the Division of Raw Materials of the U.S., Atomic Energy Commission (fig. 1) · The common association of uranium with carbonaceous material has long been recognized, and at least one deposit of uranium-bearing coal has been known for 80 years (Berthoud, 1875).
Other deposits of uranium-bearing coal were discovered in 1946 in the Red Desert, Sweetwater County, Wyoming, by Slaughter and Nelson. Later work by Wyant and Beroni (1950) resulted in the discovery of additional uraniferous lignite beds in North and South Dakota.
Each of the three areas investigated is dominated by one or more high buttes. Sentinel, Bullion, and HT (Black) Buttes each rise more than 500 feet above surrounding gently rolling plains. HT Butte, with an altitude of 3,468 feet, is the highest point in North Dakota. The uranium-bearing lignite lies at most places near the tops of the buttes. All of the areas are fairly accessible. U.S. Highway 85 passes through the Chalky Buttes area
and U.S. Highway 10 passes within 3 miles of Sentinel Butte.
The nearest rail shipping points are 12 miles south of the Chalky Buttes area at Bowman and 3 miles north of Sentinel Butte at the town of Sentinel Butte. Bullion Butte is about 30 miles by county road maintained from the town of Sentinel Butte.
Previous geologic mapping in these areas was done by Leonard (1908)., Leonard and Smith (1909), and Hares (1928). Later Beroni and Bauer (1952) sampled and described uranium-bearing lignite in the Sentinel and Bullion Butte areas. Deposits of uraniferous lignite in nearby parts of South Dakota have a similar geologic setting and are described by Denson and others (1950).
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