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 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.

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.