Numerous uranium deposits in the sedimentary rocks of the Colorado Plateau contain uraninite. Although this mineral is not restricted to any one formation, the majority of its occurrences are in the Chinle and Shinarump formations of Triassic age. There are two principal modes of occurrence of uraninite, one with sulfides of copper, the other in asphaltic bodies. In both types, the association with fossil plants is the rule. Uraninite generally replaces cell walls of the plants, and the copper sulfides are more apt to fill the cells. Under these conditions the hardness and reflectivity of the uraninite may differ considerably from that of hydrothermal vein uraninite. The paragenesis of the minerals is complex and somewhat obscure, but a formation of the uraninite contemporaneous with, or shortly followed by, copper sulfides is suggested.

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