The Origin of Uranium Deposits
Most of Wyoming’s uranium ore deposits occur in the Paleocene and Eocene sandstones of Tertiary basins. Large amounts of uranium have been mined from roll-front deposits in the Gas Hills, Shirley Basin, Crooks Gap, Southern Powder River Basin, and Pumpkin Buttes mining areas. Roll-front uranium deposits develop as ground water migrates through a porous and permeable sandstone or conglomerate (aquifer). Groundwater carries the leached uranium from the source rock – either Precambrian igneous and/or metamorphic basement rock or large-volume volcanic ash fall deposits – and re-deposits it upon migrating into a reducing (acidic) environment within the aquifer. In-situ leach mining (see above) reverses that process to recover uranium.
Reduction and oxidation (known as the redox process) formed the vast majority of Wyoming’s uranium deposits, along with deposits in many other states and countries. However, redox is also responsible for huge deposits of another form, known as unconformity deposits. These occur at or near the boundary between sedimentary and Precambrian rocks. This boundary is an unconformity because the sedimentary rocks were deposited after a period of erosion of the Precambrian rocks. Unconformity uranium deposits in the Athabaska Basin of northern Saskatchewan are among the richest in the world. McArthur River, for example, can produce about 400 million pounds of U3O8, while Cigar Lake can produce more than 200 million pounds. As with sediment-hosted uranium deposits such as roll-fronts, unconformity deposits require the combination of several key components of the system: a source of uranium (sediments or basement rock), a subsurface water system capable of mobilizing and transporting uranium in solution, and a reducing environment to precipitate the uranium back out of solution.
Alternatively, many experts believe that the uranium came from the vast Precambrian basement rocks of Wyoming, such as the Granite Mountains or the Laramie Range. It is likely that the source is some combination of the two hypotheses, as both have merit.
ISR methods use the pre-existing aquifer to reverse the deposition process by bringing oxidizing waters in contact with precipitated uranium to dissolve it so that it can be pumped back to the surface.
Other types of uranium deposits in Wyoming have been mined to various extents. Large amounts of ore have been mined from tabular uranium and vanadium deposits in the Lower Cretaceous rocks of the Black Hills. Ore has also been mined from Tertiary unconformity-related deposits on Copper Mountain, and from paleokarst carbonate deposits in Mississippian rocks on Little Mountain and in the Shirley Mountains.