Although the sources of U in sandstone-type ores are in most cases obscure, the formation of the ore deposits can be attributed on the basis of S isotopic data to the precipitating action of hydrogen sulfide of bacteriological origin. In some areas deposition has been related heretofore to natural gas, but the isotopic evidence does not support this origin. Certain fracture-controlled and breccia-pipe deposits also show isotopic evidence of having been formed under conditions similar to those existing for the sandstone-type ores. Geochemical relations suggest that salt-dome structures as well as fractures transecting carbonaceous sediments may be favorable sites for U accumulation. Certain features provide a unique basis for appraising the U potential of salt domes. The S isotopic data reflecting bacteriological involvement in U deposition in no way refute the possibility that the U was initially derived as an igneous emanation, but the tuff-leach mechanism may provide a more favorable model on which to base exploration in some areas of the U.S. The concepts discussed may aid greatly in developing models of U distribution which may be applied in future exploration.