Uranium Geology

Origin of Wyoming’s Uranium

Most of Wyoming’s uranium deposits are hosted in medium- to coarse-grained sandstones and similar rocks of Paleocene and Eocene age. They mostly occur in the Wasatch, Wind River, and Battle Spring formations. These host rocks are about 40 million to 55 million years old, but the uranium ore deposits contained in them are much younger.

In northeastern Wyoming, uranium host rocks are also found in the lower Cretaceous Lakota and Fall River formations (also known as the Inyan Kara Group), which are approximately 100 million to 138 million years old. Host rock lithologies include sandstone, arkose, and conglomerate.

The uranium found in the host rock was leached from the original source rock and deposited into the host rock as minerals that precipitated out of solution. The solvent, as well as the transport mechanism, was oxygen-rich surface and groundwater. One proposed source for uranium ore deposits in Wyoming is Precambrian granitic rocks such as those of the Granite Mountains in the central part of the state. Uranium occurs as a minor element in minerals within these igneous rocks. Erosion has removed substantial amounts of igneous material from the Granite Mountains, and to such an extent that many geologists believe enough uranium has been removed from those mountains to account for the ore deposits discovered in the nearby basins.

Another potential source for uranium in Wyoming is Eocene, Oligocene, and younger tuffs (volcanic ash-rich material). The tuffaceous beds were deposited beginning about 50 million years ago, forming such rock units as the Wagon Bed, and White River formations and their age equivalents. Volcanism, resulting from molten rock or magma near the surface of the earth, was widespread throughout much of the western United States as well as northwestern Wyoming, and occurred periodically for some 40 million years.

At times, volcanic ash blanketed all but the highest peaks and highlands of the state. That volcanism was the most likely source of tuffaceous beds such as those in the White River Formation. Erosion that occurred over millions of years since has removed most of that material, leaving behind characteristic bluffs such as those in the photo above of Shirley Basin in southeastern Wyoming and at numerous other locations in the state, including Pumpkin Buttes in northeastern Wyoming.

The White River Formation is exposed in several Wyoming basins and forms broad, near-horizontal surfaces. As it erodes, it often forms prominent ridges and badland topography, which is characterized by gullies, steep ridges, and sparse vegetation resulting from severe erosion.

►References

Harris, R.E., and King, J.K., 1993, Geological classification and origin of radioactive mineralization in Wyoming, in Snoke, A.W., Steidtman, J.R., and Roberts, S.M., eds., Geology of Wyoming: Geological Survey of Wyoming [Geological Survey of Wyoming] Memoir 5, p. 898-916.

Harris, R.E., 1984, Alteration and mineralization associated with sandstone uranium occurrences, Morton Ranch area, Wyoming: Geological Survey of Wyoming [Wyoming State Geological Survey] Report of Investigations 25, 29 p.

Contact:
Kelsey Kehoe, kelsey.kehoe@wyo.gov