Uranium in carbonaceous rocks in the Townsend and Helena valleys, Montana

Abstract

Uranium-bearing carbonaceous shale and lignite beds are exposed in five areas in the Townsend and Helena Valleys in western Montana. The greatest number of exposures is in an area of several square miles northeast of Winston in the Townsend Valley. The uranium-bearing beds are in the lower part of a Tertiary unit that consists largely of thin-bedded, white to buff, pure and impure tuffs, locally altered to bentonite. The uranium occurrences, none of which appear to be commercial, have three characteristics in common: (1) they are in and adjacent to carbonaceous shale or lignite interbedded with light-gray of white, fine-grained tuffs and lapilli tuffs, (2) the stratigraphic section in the vicinity of the deposits includes bentonite and partly bentonized tuff, and (3) the distribution of the uranium in the favorable beds is erratic. The uranium was probably leached from the tuffs and lapilli tuffs by meteoric water during bentonization and was concentrated in the carbonaceous shale and lignite. Similar Tertiary rocks are present in many of the major valleys in Western Montana and probably warrant prospecting for uranium. Areas containing white, fine-grained tuff or lapilli tuff, bentonite, and coal, or carbonaceous shale would be particularly favorable for prospecting.