Uranium deposits of the Black Hills, South Dakota and Wyoming

Abstract

port. Uranium ore has been produced only from the Inyan Kara group of early Cretaceous age. Anom,alous radioactivity has been reported locally in the Black Hills in the Precarnbria.n core, in most of the Paleozoic and Mesozoic formations, and in some of the Tertiary intrnsives. Most of the anomalous radioactivity h.as been found in formations of the Inyan Kara group, and all economic uranium deposits in this group are on the southern, western, and northern flanks of the Black Hills. Figure 1 is an index map showing the area of outcrop of the Inyan Kara group, Morrison formation, and Unkpapa sandstone around the Black Hills, and the principle uranium mining areas. STRATIGRAPHY* The Inyan Kara group is composed of the Lakota, Minnewaste, Fuson, and the Fall River forrrm.tinns. This group overlies ,the Morrison formation, or its lateral equivalent in the southern Black Hills, the Unkpapa sandstone, both of Late Jurassic age, and is gradatlonal with the overlying Skull Creek shale of Early Cretaceous age. The Lakota formation, the lower unit of the Inyan Kara group, has the most varied l ithology of any formation in the Black Hills. It consists of tenticular beds of sandstone that locally are conglomeratic and crossbedded, varieg.ated elaystone and sandy daystone, and local seams of coal and carbonaceous shale at the base. The forrnation ranges in thickness from approximately 100 to 350 feet. It is thickest at the southern end of the Black Hills. In Wyoming and northern South Dakota ~ts thickness averages approximately 250 feet. The Minnewaste limestone crops out in the southern Black Hills and separates the Lakota sandstone below from the Fuson formation above.