Abstract

The Phosphoria Formation, of Permian age, and its close stratigraphic correlatives consist of two overlapping couplets, each composed of a lower carbonaceous phosphatic unit, overlain by a cherty or carbonate-bearing unit. Phosphate deposits are found in the Phosphoria Formation over an area of about 135,000 square miles in Montana, Idaho, Utah, and Nevada, but the richest deposits are confined to a central area in eastern Idaho and adjacent parts of Montana, Wyoming, and Utah. Nearly all the phosphatic beds are uraniferous, but their uranium content ranges from about 0.001 to 0.065 percent. Although some highly phosphatic beds are only weakly uraniferous, the phosphate beds that are more than 3 feet in thickness and that contain more than 31 percent P₂O₅ generally contain 0.01—0.02 percent uranium. Most of the differences in uranium content of the phosphate rocks cannot be correlated with other observable differences in their physical or chemical properties. As a rule, however, beds composed of pellets and oolites are more uraniferous than those composed of fish scales, brachiopod shells, and other organic remains; and highly weathered phosphate beds contain less uranium than their unweathered equivalents.

Most of the uranium seems to occur in carbonate-fluora patite, where it probably substitutes for calcium; but tyuyamunite has been discovered in one area where the rocks are highly weathered.