Uranium What is Uranium Uranium Geology Uranium Deposits Uranium Resources Uranium Mining Uranium Logs

Uranium Deposits

Uranium ore deposits are economically recoverable concentrations of uranium within the earth’s crust. Such deposits in Wyoming are found in porous and permeable sedimentary rocks in the Powder River, Great Divide, Wind River, and Shirley basins. Additionally, there are significant uranium deposits in the western Black Hills area in northeastern Wyoming, the Little Mountain area of the northern Bighorn Range, and the southeastern Greater Green River Basin.

►Uranium Districts

1) Elkhorn Creek
2) Hulett Creek
3) Barlow Canyon
4) Carlile
5) Aladdin
6) Pumpkin Buttes
7) Southern Powder Basin
8) Silver Cliff
9) Copper Mountain
10) Gas Hills
11) Crooks Gap-Green Mountain
12) Great Divide Basin
13) Shirley Basin
14) Ketchum Buttes
15) Poison Basin

How Deposits Form

Uranium is found in most igneous rocks. It is one of the last elements to be incorporated into minerals, such as feldspar and zircon, that crystallize out of magma during cooling and solidification. These pockets of crystallized molten rock are often enriched in uranium and can produce rich deposits. Factors contributing to how uranium deposits form include weathering, erosion, and time (millions and even billions of years), all of which lead to the oxidization and relocation of the uranium into mineable concentrations of ore deposits. Many of these deposits are found in Wyoming's sedimentary basins.

Mining in Wyoming usually occurs near basin margins, depending on the structure of the basin. This is because uranium is initially mobilized by surface weathering and transported by a combination of surface water and groundwater into porous and permeable sediments in the basins. Neutral waters or those with slightly higher pH can mobilize uranium upstream from the basin and transport it until the oxygenated water encounters a reducing environment (acidic, lower pH), once it enters the subsurface of the basin. If oxygenated uranium-bearing water doesn’t encounter a reducing environment, the uranium will continue moving and ultimately end up in the seas and oceans.

Roll-Front Deposits

In the early 1960s, when uranium mining in Wyoming represented a large portion of the state’s extractive energy industry, geologists discovered what has become an industry standard used worldwide for locating sandstone-type uranium ore deposits. The “roll-front” model, which represents the underground shape and orientation of uranium ore deposits within sedimentary rocks, was found to be an effective way to pinpoint where the best ore was located, particularly in sandstone deposits.

Several geologic factors are necessary in order for an economic ore deposit to form. In the case of roll-front uranium deposits, aside from the presence of uranium itself, a key requirement is the existence of a reducing (acidic) environment in the groundwater system. Whereas oxidizing water can mobilize and transport uranium compounds, a reducing environment consumes oxygen (reduces the oxygen content), which means that the water no longer contains enough oxygen to maintain uranium in solution. The boundary between inflowing oxidizing water and the existing reducing environment is called a chemical front or a redox (reduction-oxidation) front, and is where uranium minerals begin to precipitate out of subsurface water, resulting in the precipitation of minerals such as uraninite and coffinite on the surfaces of sediment grains, in interstitial pore spaces, and other voids.

Roll-front deposits are ideal for in-situ recovery mining because they occur in sandstones that are bounded above and below by impermeable rock layers such as shales or mudstones. Roll-front deposits are typically C-shaped in cross section and follow a sinuous trend in plan view (see illustrations).

Roll-front deposits are arcuate, crescent-shaped uranium ore deposits that occur down gradient (downstream) from the redox front in sandstones and arkoses. They are also bounded above and below by impermeable rock layers such as shales or mudstones. As oxidizing groundwater continually flows into the reducing environment, the front gradually migrates in the direction of groundwater flow. Some of the uranium is remobilized and re-precipitated farther down the hydrologic gradient as the front migrates. However, some of the uranium mineralization remains, typically toward the top and bottom of the permeable host rock, because the overall water flow direction is toward the front and not toward the impermeable rock layers above and below. This phenomenon results in the crescent shape of the roll front deposit, along with extended, relatively thin limbs of mineralization above and below the oxidized zone (see photo below, at right).

This configuration of the orebody was initially called a “roll” by Colorado and Utah miners in the early 1950s. It was a name to distinguish them from tabular uranium occurrences that were also found in sandstones bounded by impermeable rock layers. In Wyoming, these deposits are found in fluvial (stream deposited) and eolian (wind deposited) rock formations of Cretaceous and Tertiary age.

Other Types of Uranium Deposits in Wyoming

Tabular deposits

Tabular uranium deposits are typically sandstone-hosted and chemically and genetically similar to roll-fronts. The main difference is their morphology, in that tabular ore bodies are generally parallel to the bedding of the host rock. Like roll-fronts, tabular ore bodies also form as a result of groundwater flow, but occur when the reducing environment occurs over a widespread area, usually at the base of the host rock unit. Most of these deposits occur in rocks of lower Cretaceous age (between approximately 138 million to 96 million years old). Nearly 700,000 tons of ore were produced between 1952 and 1968 from the various mining districts in northeastern Wyoming from this type of deposit.

Unconformity deposits

Unconformity deposits occur where older Precambrian rocks are overlain by significantly younger rocks. The contact between the rock formations represents an extended period of erosion and non-deposition. One example of this type of deposit occurs in the Copper Mountain district of northeast Fremont County where Tertiary rocks overlie the Precambrian basement rocks. A few small mines of this type operated in the 1950s and produced less than 50 tons of ore. In the Silver Cliff district of Niobrara County the Cambrian-aged Flathead Formation hosts uranium at the contact with Precambrian rocks of the Hartville Uplift. That mine produced about 920 tons of ore in the 1950s, but production there actually dates back to 1918.

Paleokarst carbonate hosted deposits

Paleokarst is a term that refers to rocks that developed caves, sinkholes, breccia pipes, etc., at some point in their burial history and were subsequently overlain by other sediments. This type of deposit occurs in the Little Mountain area in northeast Big Horn County. Uranium generally occurs within layers of secondary deposits in caves, fractures, and other openings in the original carbonate (e.g. limestone) rocks. Between 1955 and 1970, approximately 23,500 tons of uranium ore were extracted from mines in this district.

 

►References

Dahl, A.R., and Hagmaier, J.L., 1976, Genesis and characteristics of the southern Powder River Basin uranium deposits, Wyoming: Wyoming Geological Association 28th Annual Field Conference Guidebook, p. 243-252.

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, p. 898-916.

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