Summary:
Wyoming uranium deposits are roll-front uranium deposits as defined in the “World Distribution of Uranium Deposits (UDEPO) with Uranium Deposit Classification”, (IAEA, 2009). Uranium deposits in the Gas Hills were formed by the classic Wyoming-type roll-fronts. Rollfronts are irregular in shape,roughly tabular and elongated, and range from thin pods and a few feet in width and length, to bodies several hundred or thousands of feet in length. The deposits are roughly parallel to the enclosing beds but may form rolls that cut across bedding. Roll-front deposits are typified by a C-shaped morphology in which the outside of the C extends downgradient in the direction of historic groundwater flow and the tails extend up-gradient of historic groundwater flow. The tails are typically caught up in the finer sand and silt deposits that grade into over and underlying mudstones, whereas the heart of the roll-front (higher grade mineralization) lies within the more porous and permeable sandstones toward the middle of the fluvial deposits.
The Gas Hills uranium deposits are present in an arkosic sandstone facies, the Puddle Springs member of the Wind River formation (e.g. King and Austin, 1966; Armstrong, 1970). Knowledge of the distribution of this member is of great importance in the search for uranium deposits, as permeability determines whether a rock is a favorable or unfavorable host. Fine-grained, only slightly permeable rocks are unfavorable hosts. Highly porous conglomerates, on the other hand, appear to be too permeable to be a good host rock.
Drilling in the west Gas Hills indicates that the favorable arkosic sandstone host passes westward into unfavorable silty facies. A local sandstone facies has been found within the silty facies, and a small area containing uranium (Jeep deposit) has been found in the sandy facies. Thus, the favorable host for mineralization in the above-mentioned deposits is bounded on the north by an erosional pinch out; on the east by a change of facies to an unfavorable silty sandstone host; on the south by a subsurface onlap pinch out; and on the west by change of facies to an unfavorable silty sandstone host.
Uranium mineralization in the Gas Hills is present in bodies usually referred to as “rolls” (e.g. King and Austin, 1966; Armstrong, 1970). In vertical cross section they are irregularly crescent or “C” shaped. Rolls are the result of oxidized and soluble uranium being transported by ground water to a location within a permeable sandstone host where a reaction within a reducing environment occurs and insoluble reduced, uranium minerals are deposited. The contact between oxidized and reduced conditions is the “roll front”.
In the body of the crescent, individual rolls range from a few inches to many feet in vertical thickness. Average thickness of a well mineralized roll is 10 to 15 feet; many rolls thicker than 20 feet have been mined. The upper and lower tails of the crescent thin away from the body of the crescent. In the Gas Hills the lower tail normally is greatly extended and thins gradually, whereas the upper tail is typically short and thins abruptly.
Rolls ordinarily are stacked en echelon, forming multiple mineralized bodies. A series of stacked rolls can be thought of as a frontal system. The number of rolls and vertical separation between them can be large or small, and as a result, mineralization may occur through a large stratigraphic interval. In the Central Gas Hills, uranium mineralization has been found in a stratigraphic interval almost 300 feet thick. Most rolls are stacked so that each successively higher roll is displaced in the direction of convexity and the volume of bleached rock narrows with depth. Each roll in a stack has its own front and each front in plan-view has its own sinuosity. The different fronts occur in the same general area, but the detailed sinuosity of one roll is independent of the sinuosity of other rolls.
Un-oxidized mineralization is dark and usually the darker, the higher the grade. The uranium minerals are very fine grained uraninite and a little coffinite. The only non-silicate gangue minerals present in significant amounts are fine-grained pyrite and marcasite, and they are intimately mixed with uranium minerals. These minerals coat detrital sand grains and fill interstices of the host rock. Oxidized mineralization is present near surface and was mined when production in the district first started. Most production came from un-oxidized mineralization and essentially all present mineralization of potential economic interest is contained in un-oxidized mineralization.