Geology of uranium deposits in Triassic rocks of the Colorado Plateau region

Abstract

Important uranium deposits are widely distributed in the Triassic rocks of the Colorado Plateau region. These deposits, which have been the second most important domestic source of uranium in the United States, have also yielded vanadium, copper, and radium during various periods of mining in the past 50 years.Most of the deposits in Triassic rocks are in the Shinarump and Moss Back members of the Chinle formation, but some important deposits are also in other members in the lower part of the Chinle, particularly in beds within 50 feet of the Middle Triassic unconformity. In northeastern Arizona, eastern Utah, and western Colorado three mineral belts have been outlined, each bounded by a pinchout. These belts, which contain about 20 percent of the areas underlain by the Chinle formation, are the Monument Valley belt, the east White Canyon belt, and the Moab belt.The chief unoxidized uranium minerals, uraninite and coffinite, and the oxidized uranium minerals, carnotite and tyuyamunite, impregnate the rocks, forming disseminated ores. Fossil wood replaced by these minerals and the associated iron and copper minerals constitute the high-grade ore. Most of the ore averages between 0.20 and 0.30 percent U 3 0 8 and some ores average either between 1 and 2 percent V 2 0o or between 1 and 2 percent Cu.The ore bodies are irregularly distributed and form uneven tabular and concretionary masses that lie essentially parallel to the bedding of channels and lenses filled with coarse clastic material. They range in content from a few tons to more than a hundred thousand tons.It is believed that in early Tertiary time ground water leached uranium and other ore metals from overlying mudstone beds or from the ore-bearing rocks themselves and redeposited the metals in favorable sedimentary and tectonic structures.Reddish-brown and light-colored siltstone and sandstone, light-colored limestone, horizontally and ripple-laminated; thick• ens westward. ~j P-IS Cutler formation D-1, 500Brown to red sandstone and red to purplish arkose, cross-laminated.~fa faBrown to red and purple sandstone con- ~•a1l~~Rico formation o-600 taining several fossiliferous limestone §ig?Cil<P'-' beds, thin-bedded.