Geological Survey Bulletin 1063—G Geology of the Jewel Cave SW Quadrangle, Custer County, South Dakota

The Jewel Cave SW quadrangle is on the northwest fringe of the Edgemont mining district. Several small uranium deposits and a few radioactivity anomalies are known in the quadrangle, but no large economic deposits have been found. The known occurrences of uranium minerals are all within the Inyan Kara Group.

In the Chilson Member, the S₁ sandstone contains secondary yellow uranium minerals at two places: in the NE1/4 sec. 19, T. 6 S., R. 2 E., and in the NE1/4 sec. 9, T. 6 S., R. 1 E. At the first of these localities the S₃ sandstone is about 20 feet thick and consists of thin beds of sandstone separated by carbonaceous mudstone. No ore has been produced from this occurrence. At the second locality the S₁ sandstone is 80 feet thick and consists of massive, intricately channeled sandstone having irregular pods and lenses of carbonaceous siltstone and clay galls. Yellow uranium minerals occur as disseminations in the sandstone, and the sandstone contains irregular areas that are stained purplish red by hematite, which forms a thin coating on the sand grains. This second occurrence has been prospected by rim cuts and drilling, but concentrations of uranium minerals were so small and scattered that no mining has been attempted.

Most of the occurrences of uranium minerals are in the Fall River Formation in the vicinity of Pass Creek. Many of these occurrences are at the top of the S₅ sandstone along its southwest margin in rock that has been mapped as interbedded sandstone and mudstone. The rock consists of 1- to 2-foot-thick beds of sandstone and thin interbeds of shale, and it is believed to represent the lateral transition of the thick S₃ sandstone to dominantly mudstone units. These uranium occurrences have been extensively explored by drilling in the S1/2 sec. 23 and the N1/2 sec. 25, T. 6 S., R. 1 E., but no commercial ore has been found. The underlying massive S₃ sandstone is not mineralized. Several uranium deposits are in a thin sandstone that overlies the carbonaceous siltstone of the lower unit of the Fall River. (See page 244.) The Wicker-Baldwin mine, in the NW1/4 sec. 26, T. 6 S., R. 1 E., and the Dark mine in the NE1/4 sec. 15, T. 6 S., R. 1 E., are the only mines in the quadrangle from which uranium ore has been produced, but the total production is very small. At the Wicker-Baldwin mine the ore-bearing sandstone is 15 feet thick, fine grained, and carbonaceous. Parts of the sandstone are stained purplish red, presumably by hematite. At the Dark mine the ore occurs in the basal part of the 10-foot-thick sandstone and in the upper part of the underlying carbonaceous siltstone.

The writer does not believe that there is much likelihood of finding large uranium deposits in the Inyan Kara rocks east of Pass Creek. Throughout most of this area the Inyan Kara Group has been deeply dissected, but only rare radioactivity anomalies or uranium occurrences are known.

Uranium deposits may exist in the Fall River Formation west of Pass Creek and south of the Dewey fault. In the main part of the Edgemont mining district many commercial uranium deposits occur in the sandstone interbedded with siltstone of the lower unit of the Fall River Formation, along the southwest margin of the S₅ sandstone. The margin of the S₃ sandstone trends diagonally across the southwest corner of the Jewel Cave SW quadrangle (pl. 22), and by analogy with the known stratigraphic occurrences in the main part of the mining district, this region may contain commercial deposits.

Exploration for uranium deposits here will have to be by rather deep drilling (as deep as 300 feet), because the margin of the S₅ sandstone is overlain by appreciable thicknesses of the Fall River Formation and the Skull Creek Shale.