What is Uranium
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 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 to 138 million years old. Host rock lithologies include sandstone, arkose,
The uranium minerals found in the host rock were leached from their original source rock and precipitated out of solution
and deposited into the host rock. 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 comprised in
the Granite Mountains. 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 Middle Eocene and younger tuffs (volcanic ash-rich material).
The tuffaceous beds were deposited beginning about 50 million years ago, forming the White River Formation and its 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, 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 at
Pumpkin Buttes in northeast 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.
- 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 Memoir No. 5, pp. 898-916.
Harris, R.E., 1984, Alteration and mineralization associated with sandstone uranium occurrences, Morton Ranch area, Wyoming: WSGS Report of Investigations No. 25, 29 p.
Bob Gregory (307) 766-2286 Ext. 237