Abstract

In Monument Valley of Arizona and Utah, on the southern end of the Monument upwarp, exposed sedimentary rocks range in age from Pennsylvanian to Jurassic, and are intruded by Pliocene (?) dikes and volcanic necks. Uranium occurs chiefly in and immediately below paleochannels cut into the Triassic Moenkopi formation and filled with sediments of the Shinarump member of the Triassic Chinle formation. Most ore occurs in local scours along these channel courses. Within the channels and scours, uranium deposits appear to have been controlled largely by permeability. Marked increases in argillaceous content form impermeable barriers that localize a number of ore bodies. These barriers may be either argillaceous material within the Shinarump paleochannel sediments, or the steep channel flanks of Moenkopi mudstone. Carbonaceous material may have been a factor in development of an environment favorable to precipitation.Primary uranium deposits in Monument Valley consisted largely of coffinite, uraninite, montroseite, and copper and iron sulphides. During oxidation, uranium and vanadium combine to form relatively insoluble carnotite-type minerals. Copper is migratory and generally is deposited in carbonates. In a few places copper-carbonates appear in halos surrounding uranium-vanadium ore bodies. Changes of color occur in the upper Moenkopi clastics in and around areas of more intense mineralization. Thickness of the zone of apparent color changes is extremely variable, apparently controlled by both intensity of mineralization and quantity of argillaceous material in superjacent Shinarump sediments.

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