Uranophane at Silver Cliff mine, Lusk, Wyoming

The uranium deposit at the Silver Cliff mine near Lusk, Wyo., consists primarily of uranophane which occurs as fracture fillings and small replacement pockets in faulted and fractured calcareous sandstone of Cambrian (?) age. The country rock in the vicinity of the mine is schist of pre-Cambrian age intruded by pegmatite dikes and is unconformably overlain by almost horizontal sandstone of Cambrian(?) age.

The mine is on the southern end of the Lusk Dome, a local structure probably
related to the Hartville uplift. In the immediate vicinity of the mine, the dome is cut by the Silver Cliff fault, a north-trending high-angle reverse fault about 1,200 feet in length with a stratigraphic throw of 70 feet. Uranophane, metatorbernite, pitchblende, calcite, native silver, native copper, chalcocite, azurite, malachite, chrysocolla, and cuprite have been deposited in fractured sandstone.

The fault was probably mineralized throughout its length, but because of
erosion, the mineralized zone is discontinuous. The principal ore body is
about 800 feet long. The width and depth of the mineralized zone are not
accurately known but are at least 20 feet and 60 feet respectively.

The uranium content of material sampled in the mine ranges from 0.001 to
0.23 percent uranium, whereas dump samples range from 0.076 to 3.39 percent
uranium.