Abstract

Since 1950 about 50 million tons of U ore has been discovered along the southern margin of the San Juan basin, New Mexico. Here the exposed sequence of sedimentary rocks ranges in age from Permian to Cretaceous, and is associated with intrusive and extrusive rocks of Tertiary and Quaternary age. The U deposits are separable into 3 types - those in sandstones and associated mudstones of the Entrada and Morrison formations of Jurassic age, and Dakota sandstone of Cretaceous age; those in the Todilto limestone of Jurassic age; and one deposit in a pipelike structure in the Morrison formation. The deposits in the clastic sediments are similar to most of the U deposits in other parts of the Colorado Plateau region in type and habit of ore and accessory minerals, in the tabular form of ore bodies, and in their association with some form of carbonaceous material. The deposits in limestone have a somewhat similar mineral assemblage to that of the other deposits in the region but are unique in the type of host rock and their preference for structurally deformed beds. The pipelike deposit is unique. Although igneous activity has been intense in the eastern part of the area from late Tertiary to Recent time, there is no evidence to suggest a genetic relation between the igneous activity and the U deposits. In fact, what are probably the oldest exposed igneous rocks intrude and displace the deposits. Three periods of deformation are recognized. The first was during the time between the accumulation of the Entrada and Dakota sandstones, the second in the early to middle Tertiary, and the third in the middle to late Tertiary. Only structures of the first period show an obvious influence on the distribution and localization of the U deposits. From the pattern or frequency of distribution of the known deposits, and on the basis of interpretations from known geologic relations, the deposits seem to be clustered in a zone at least 20 mi. wide N. of the present outcrop, This zone is called the southern San Juan basin mineral belt. Although the concept of this zone, or belt, restricts the favorable ground geographically, the amount of unexplored ground within the limits of the belt is enough to contain several times as much U resources as are now known.

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