The Crooks Gap area is a major uranium producing district in south-central Wyoming. Uranium occurs there as roll-type deposits in Tertiary sandstones. Alteration of the host sandstones related to the formation of the uranium deposits, is marked at the surface by a pronounced red coloration, which has long been used in the area as a prospecting guide. This report presents a characterization of alteration in the Crooks Gap area based upon (1) field observations of the extent and appearance of the alteration at the surface, (2) determination of the mineralogy and spectral reflectance characteristics of representative samples of altered and unaltered material in the laboratory, and (3) analysis of Landsat data of selected areas of altered and unaltered rocks.
Laboratory spectrophot6metric data show that fresh, red-colored, altered rock has a strong absorption at 0.35 and 0.5 micrometers and a marked rise in reflectivity from 0.7 to 1.6 micrometers; weathered, red-colored, altered rock and Triassic red beds both show strong spectral absorption near 0.35 micrometers but have a relatively flat response from 0.7 to 1.6 micrometers. Landsat radiance values, simulated from the laboratory data, show that both altered rock and Triassic red beds are indicated by a high red-to-green ratio. This is in good agreement with actual satellite data over these rock types. These data indicate that the Landsat system is very sensitive to ferric oxides and can map the distribution of these oxides in semiarid environments for reconnaissance purposes.