Abstract

The tertiary basins of Wyoming have been the focus of nearly 50 percent of the nationwide exploration effort for uranium in the past few years. Uranium occurrences have been reported in 20 of the 23 counties in the state and from 34 different stratigraphic units ranging from Cambrian to Quaternary in age. Vein-type uranium mineralization in pre-Cambrian rocks also occurs within the state. Most production of uranium in the State comes from major deposits in fluvial sandstone of Paleocene and Eocene age. Major producing districts in the State are the Gas Hills, Shirley Basin, Crooks Gap and southern Powder River Basin. These districts have produced 20 percent of the uranium mined in the United States and contain 36 percent of the nation’s reserves known in 1975. Total potential resources of Wyoming as forecast by ERDA amount to 15% of the nation’s potential.

The majority of exploratory activity in the State is directed toward sandstone-type deposits of early Tertiary age. These deposits are felt to have been deposited by the precipitation of uranium from a slightly alkaline and oxidizing groundwater at a redox interface. The deposit at this interface is roughly “C-shaped” with the limbs of the “C” pointing from the direction of original groundwater flow, generally updip, and is referred to as a “roll-front”. The nature of the deposition of a roll-front uranium deposit is such that certain characteristics are often sufficiently evident to allow their use in exploration. These characteristics are 1) the alteration of the sandstone on the up-dip side of the roll-front and 2) the up-dip remnant gamma-tracks left by the passage of the roll-front through the sandstone.

Development of interest in an area is normally preceded by geologic studies in the field and office to determine the favorability of the area. If the area is felt to have potential for the discovery of ore deposits of uranium, a program for acquiring land over the most favorable area is begun. Drilling is the primary method of exploring for uranium as it is the only definite way to assess the stratigraphic, alteration, and radiological characteristics of a potentially favorable formation in the subsurface. A carefully planned and executed preliminary drilling program will normally provide the necessary data as to whether or not the target sand may contain a roll-front within the boundaries of the property. A project may be dropped at this stage due to lack of potential or a further program may be initiated to evaluate the location and mineralization of a potential roll-front. Should the operator be fortunate enough to have significant mineralization scattered along the roll-front on his property, a more extensive program of delineation drilling will be needed which hopefully will lead to mine planning and a mining and milling operation.