Limestones, uranium deposits

Other Deposits. Those deposits which cannot be classified as one of the previous 14 deposit types are called other. These include the uranium deposits in the Jurassic Todilto Limestone in the Grants district in New Mexico (17). 

Sierra Pena Blanca district. Chihuahua, Mexico The Pena Blanca uranium district is about 50 km northeast of Chihuahua, Mexico, on the east side of a large Basin and Range horst block and near the eastern edge of the Caenozoic Sierra Madre Occidental volcanic province. The bulk of the uranium deposits, which total about 5000 ton of reasonably assured U3O8 resource, occur in extracaldera ash-flow tuffs overlying Cretaceous limestones. The source of the tuffs is unknown, but some appear to be related to a large caldera south of Chihuahua. Other tuffs appear to have a western source. 

Non-metamorphic equivalents of this Th-free mineralization may be sought in some rare limestone-hosted U-occurences, such as the Jurassic Todilto lacustrine formation in the Grant Uranium Belt (Rawson Richard 1980), the Cretaceous Toolebuc marine formation in Eromanga Basin (Ramsden 1982), the Mesoproterozoic Vempale marine formation in Cuddapah Basin (Sinha et al. 1989), and the Cretaceous Probeer marine formation in the Huab deposit (Hartleb 1988). 

Platform phosphorites are generally nodular, rather than bedded, and are associated with sandstone, limestone and glauconite. Most are low in uranium, but an exception is the Bone Valley Formation of the southeastern U.S.A. This unit has been reworked and enriched by re-exposure to sea water during a subsequent transgression. These phosphorites differ from shelf phosphorites in that they are near-shore sub-tidal and shoreline deposits, and they change facies oceanward to carbonate sediments. 

Karstic deposits are those in which secondary uranium minerals occur in large caverns, cave breccias and in bedded cave-fill deposits of clay and silt in karstified limestone. Best known of these deposits are Tyuya-Muyum, U.S.S.R., and Pryor Mountains, U.S.A. Tyuyamunite is the principal uranium mineral at both localities. These deposits are also of minor importance and contribute little to the 30 uranium resources. 

Most uraniferous phosphates are classified as syngenetic types because they acquired their uranium at the time of deposition. One exception appears to be the occurrence at Baukoma, Zaire, where uranium occurs in an Eocene phosphatic clay beneath a shallow lake. The phosphatic clay, with an average content of 3000ppm uranium, rests on a limestone and in places on a sandstone that overlies the limestone. ... 

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