Abstract

Syngenetic concentrations of uranium and phosphate occur in thin persistent lacustrine zones of Eocene age in southwestern Wyoming. Uraniferous phosphatic beds known elsewhere are of marine origin; thus, their discovery in lacustrine rocks indicates a new geologic environment in which deposits of both scientific and economic interest might occur, not only in structural basins in the Rocky Mountain region but also in similar basins elsewhere in the world.

In the Green River area, which comprises the southeastern part of the Green River Basin in southwestern Wyoming, the Wilkins Peak Member of the Green River Formation contains more than 35 radioactive zones, of which 25 are known to be uraniferous and phosphatic. The member, which is of late early Eocene and middle Eocene age, is about 1000 ft thick, and is composed of highly tuffaceous lacustrine dolomitic marlstone, limestone, claystone, shale, siltstone, sandstone, trona beds, and oil shale. Maximum uranium content is 0.15% and P₂O₅ is 18.2%. Average for the 25 sampled zones, which range in thickness from 3 in to 6 ft, is about 0.005% uranium and 2.2% P₂O₅.

The unique and varied mineral assemblages include rare-earth minerals and several silicates that are know elsewhere only in igneous and metamorphic rocks. Twenty-three major trona beds, one of which has a maximum thickness of 40 ft, and about a dozen halite beds, one 20 ft thick, are present. Most of the trona beds are closely associated stratigraphically with the uraniferous phosphatic zones, but some zones occur in parts of the section where there is no trona.

The origin of the uraniferous phosphatic zones is unknown. Inasmuch as trona is associated with them only in the Green River Basin, an evaporitic environment is apparently not necessary for the concentration of uranium and phosphate. "Average" shale contains less than 0.0004% uranium and 0.16% phosphate, not a tenth as much as that in most of the uraniferous phosphatic zones. Similarly, the marine Upper Cretaceous Pierre Shale, which was arbitrarily selected as a standard for comparison, contains about 0.0005% uranium and 0.14% phosphate. The Wilkins Peak Member contains abundant dacitic-andesitic volcanic debris, and igneous rocks of this general composition contain about 0.003% uranium and 0.2% phosphate. The strata between the zones have so little uranium and phosphate that the member as a whole contains no more of these elements than the "average" shale. Therefore, the presence of the zones does not necessarily require either sporadic floods of uranium- and phosphate-rich debris from adjacent exposed source rocks or wind- or water-borne volcanic ash abnormally rich in these elements. The zones may well have developed entirely as a result of unique geochemical conditions that were widespread only during parts of Eocene time.