The Gas Hills Project is owned by UColo Exploration Corp. (UColo), a Utah corporation, and a wholly owned subsidiary of URZ Energy Corp. (URZ). URZ is a wholly owned subsidiary of Azarga Uranium Corp. (Azarga) which is a wholly owned subsidiary of enCore. Surface land ownership at the Project is predominantly managed by the U.S. Department of Interior, Bureau of Land Management (BLM) and the minority of the land is owned by the State of Wyoming and private landowners. Mineral ownership at the Project is a combination of federal, state of Wyoming, and private (fee) ownership.
Summary:
In the Gas Hills district, lower Tertiary rocks unconformably overlie folded and faulted Mesozoic and older rocks. The Wind River Formation is conformably overlain by tuffaceous sandstones of the Eocene Wagon Bed Formation.
The Puddle Springs Arkose member of the Wind River Formation is the host rock for the uranium deposits at the Project. It consists of poorly consolidated arkosic sandstone and conglomerate with thin discontinuous interbeds of mudstone. The Puddle Springs arkose was deposited rapidly by northward-flowing braided streams to form coalescing piedmont alluvial fans (Soister, 1968).
The full thickness of the Wind River Formation is present from just north of the base of Beaver Rim Divide southward for a few miles. North of the contact between Wind River Formation and younger rocks, erosion has cut across the formation at a low angle and it progressively thins toward the north, where basal beds lie unconformably on older rocks.
Mineralization
The uranium deposits are present in an arkosic sandstone facies of the Puddle Springs member of the Wind River formation (Strathmore, 2013). Drilling in the west Gas Hills indicates that the favorable arkosic sandstone grades into an 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 sandstone 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” (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.
On the concave side of a crescent-shaped mineralized body, relatively light gray colored altered host rock is present. The contact is a slightly irregular narrow zone, and the change from uranium-bearing to bleached or altered rock normally takes place within a short distance. On the convex side of a crescent shape mineralized body, relatively dark greenish-gray unbleached (unaltered) rock is present. The contact between uranium-bearing and unbleached or unaltered rock is irregular interfingering, mostly gradational feature but the contact between individual fingers of mineralized rock and unbleached host may be moderately sharp. The fingers of mineralized rock point in the direction of unbleached rock.
Upper-limb mineralization dies out away from the body of the crescent in an abrupt manner somewhat similar to that of the contact between uranium-bearing and bleached rock on the concave side of the crescent. In contrast, lower limb mineralization normally terminates gradually in the way that mineralization terminates on the convex side of a roll.
The crescent-shaped contact between bleached rock and uranium mineralization is commonly referred to as a “front”. In mapping a front, the point of maximum advance of the altered rock is indicated. In plan-view, the trace of a front is extremely sinuous.
Rolls may be 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.
Rolls and lower-limb mineralized bodies normally are underlain by a mudstone layer. In many places a mudstone layer also overlies the roll. The upper limbs of some mineralized bodies end in sandstone and the next higher roll rests on a mudstone layer that is separated from the lower roll by un-mineralized sandstone.
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 nonsilicate 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.
Uranium is not distributed uniformly throughout the roll; rather, it is normally concentrated in the body of the crescent close to the concave side. High-grade mineralization locally contains several percent U3O8. The grade progressively decreases away from the high-grade zone. In the direction of bleached rock, the grade decreases abruptly and there is a sharp break between mineralization and waste rock. In the direction of unbleached rock, grade decreases gradually. The high- grade zone in the body of the crescent and the area immediately adjacent to it contains most of the total uranium in the body. Most of the uranium produced from the Gas Hills has come from this location in rolls, and therefore most future production can logically be expected to come from similar positions in other rolls.
Uranium was discovered in the Gas Hills near the center of the district at the north end of what later became known as the Central Gas Hills. As exploration continued, uranium was found at widely scattered localities and after a while it became evident that uranium occurrences were concentrated in three separate areas: the western, central and eastern trends. Each trend was considered to be a separate entity until about 1963, when it was realized that the different trends appear to be parts of a single complex, geologic feature (Armstrong, 1970).
In the Gas Hills, the lateral extent of the host sandstone and favorable environment for uranium mineralization is continuous on the order of miles along trend (direction of solution flow in channels) and hundreds of feet across trend.