Abstract

Copper, silver, and zinc occur near or at the top of the Nugget Sandstone (Triassic? and Jurassic?) at six localities in the Lake Alice district and at two localities more than 50 miles to the north, all within the thrust belt of western Wyoming. Maximum values in ppm obtained by chemical analysis (atomic absorption) are: copper, 67,000; silver, 1,200; and zinc, 32,000. Maximum values for some other elements, in ppm, obtained by spectrographic analysis are: arsenic, 7,000; barium, 5,000; cobalt, 700; molybdenum, 150; and lead, 5,000.

Mineralization is in sandstone that was originally red but subsequently was altered to gray, brown, black, and green. The source of elements concentrated in the rock and the time of their introduction are not known. No igneous rocks are present in this part of the thrust belt and there is no direct evidence, other than possibly the mineralization itself, of hydrothermal activity. The wide separation of mineralized localities in several thrust blocks suggests possible downward leaching from Tertiary tuffaceous rocks that formerly may have covered the region. On the other hand, the proximity of these metallic mineral concentrations to bleached red beds and fractures in the brittle Nugget Sandstone and to oil staining and highly soluble gypsum in the basal beds of the Gypsum Spring Member of the Twin Creek Limestone (Jurassic) may be genetically significant. If a similar association can be demonstrated elsewhere, bleaching of red beds and presence of oil staining may be useful guides to prospecting for metallic mineral deposits in certain types of sedimentary rocks.