Several uranium deposits are present in the Fall River sandstone of Early Cretaceous age on the northeast flank of the Black Hills, Butte County, South Dakota. The deposits are within a fine-grained, well-sorted, persistent basal sandstone unit that ranges in thickness from 2 to 18 feet and dips about 4° NE. Detailed mapping of about 2 square miles surrounding the deposits have shown that all the uranium occurrences and most of the areas of high radioactivity are where the color changes in the basal sandstone from reddish on the up-dip side of the the occurrences to yellowish-gray or buff down-dip. Radioactivity measurements show that uranium is distributed almost continuously along the sinuous red-buff contact for more than 5 miles. Laboratory work indicates that the red color is caused by the hematite resulting from the alteration of ferrous iron minerals and hydrous ferric oxides. The close association of the red-buff contact and the uranium deposits suggest that the two were formed by the same solutions. The uranium was probably deposited originally from ground water which moved down-dip and gradually changed from an oxidizing solution near the surface to a mildly reducing solution at depth. Concentrations of uranium have resulted from the localization of reducing conditions cause perhaps by structures superimposed on the regional dip, local thinning or decrease in permeability of the sandstone, or concentrations of pyritiferous carbonaceous material. The red alteration is probably the result of pre-Oligocene weathering that has extended downward in the more permeable beds about 200 feet below the ancient erosion surface. Oxidation of the primary uranium during the present weathering cycle has resulted in the formation of carnotite and possibly other secondary uranium minerals.
