Pitchblende was discovered in July 1951 in the Alma mining district, Park County, Colo., by the U. S. Geological Survey acting on behalf of the U. S. Atomic Energy Commission.
The pitchblende is associated with Tertiary veins of three different geologic environments: (1) veins in pre-Cambrian rocks, (2) the London vein system in the footwall block of the London fault, and (3) veins in a mineralized area east of the Cooper Gulch fault. Pitchblende is probably not associated with silver-lead replacement deposits in dolomite.
Secondary uranium minerals, as yet undetermined, are associated with pitchblende on two London vein system mine dumps and occur in oxidized vein material from dumps of mines in the other environments.
Although none of the known occurrences is of commercial importance, the Alma district is considered a moderately favorable area in which to prospect for uranium ore because 24 of the 43 localities examined show anomalous radioactivity; samples from anomalously radioactive localities, which include mine dumps and some underground workings, have uranium contents ranging from 0.001 to 1.66 percent.