What is Uranium
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 eastern Greater Green River Basin.
- ►Uranium Districts
- 1) Elkhorn Creek
2) Hulett Creek
3) Barlow Canyon
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 magma bodies. It is one of the last elements to be incorporated into minerals as they crystallize out of the
magma as it cools and solidifies. These pockets of crystallized molten rock are 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. Neutral waters or those with slightly higher pH can then mobilize uranium and transport it until the
oxidized water encounters a reducing (acidic, lower pH) environment, which is usually found deeper in the basin.
Roll-Front Deposits — An Industry Standard
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 sandstone, was discovered as an effective way to
pinpoint where the best ore was located, particularly in sandstone deposits.
Most of Wyoming’s uranium deposits take the form of roll fronts, which are arcuate, crescent-shaped bodies. Tabular-shaped ore bodies are also
common in the lower Cretaceous sandstones of northeastern Wyoming. Mapping efforts originally led to the discovery of the roll-front,
which was then successfully applied to exploration and development drilling, as well as to ore projections ahead of mine workings and to reserve
estimates in the mine.
Roll-front deposits are ideal for in-situ recovery mining because they occur in sandstones that are
bounded above and below by shales or some other impermeable rock layers. These deposits are for the most part C-shaped
in cross-section and follow a sinuous trend in plan view. This configuration of the ore body 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 shales. 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
- Sandstone-hosted and chemically and genetically similar to roll-fronts. The main difference is their morphology.
- The uranium ore typically lies in a zone essentially parallel to the attitude of the sandstone layer (as opposed the “C” shape
of a roll-front).
- Most of these deposits occur in rocks of lower Cretaceous age (between approximately 138 to 96 million years
- Nearly 700,000 tons of ore were produced between 1952 and 1968 from the various mining districts in northeastern Wyoming from such deposits.
Tertiary unconformity deposits
- Occur where Precambrian rocks are overlain by younger Tertiary rocks.
- In the Copper Mountain district, small mines of this type were operated between 1954 and 1958. They produced a little less than 50 tons of ore. Most of the uranium mineralization in these mines occurred as fracture fill in Precambrian rocks and grain coatings on Tertiary arkosic sediments derived from the igneous rocks.
- This type of deposit is known for extremely rich uranium ore zones such as those in the Athabasca Basin of Canada.
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 subsequently overlain by other sediments.
- Uranium generally occurs within layers of secondary deposits in caves, fractures, and other openings in the original carbonate (e.g. limestone) rocks.
- Approximately 23,000 tons of uranium was mined from these types of deposits in the Little Mountain area in north central Wyoming between 1955 and 1970.
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., editors, Geology of Wyoming: Geological Survey of Wyoming Memoir No. 5, pp. 898-916.
Bob Gregory (307) 766-2286 Ext. 237