Sedimentary host rocks, uranium

Uranium is classified as a lithophile element, and its abundance in granitic rocks is about double its average crustal abundance. In fact, the weathering of granites is probably the major primary source for the uranium that presently occurs in sedimentary host rocks. Although reported to have all valence states from 2 + to 6 +, only the 4 + and 6 + states are important in minerals. There is some evidence for a 5 -t state both as a solution species and in some of the uranium oxides, but its role in mineral structures is not confirmed. 

Jones C. A. A classification of uranium deposits in sedimentary host rocks. In NURE uranium geology symposium (Grand Junction, Colo. Bendix Engineering Corporation, 1978), 23-8. 

Vein-like deposits were described as uranium deposits of uncertain genesis by Mathews etal Except for a small number of these deposits in sedimentary rocks, the group is subdivided into unconformity-related deposits and deposits of metamorphic rocks. In a more genetic classification of the major (Proterozoic) vein-like deposits of the world, Dahlkamp and Adams deciphered the relationship of diagcncsis, metamorphism, metasomatism and weathering on uranium concentrations in various host rocks. The result is a logical genetic explanation for the spectrum of vein-like deposits observed today. 

Much of the world s currently minable uranium is found in vein-like deposits of uncertain origin. They consist of three general types of unequal importance. Most important are unconformity-related deposits and vein-like deposits in meta-morphic rocks. Least important and probably of different origin are vein-like deposits in sedimentary rocks. The origin of all three types is a matter of much conjecture. Major unsolved genetic questions are the source of the uranium and its mode of transport, the source of the mineralizing solutions, the nature and role o f reductants and the control exerted on uranium deposition by structural and lithologic features of the host rocks. Unconformity-related deposits and vein-like deposits in metamorphics commonly occur in brecciated and foliated metamorphic rocks in stable Precambrian Shield areas. They contain about 24.4% of the Western world s reasonably assured 30 uranium resources. 

The Witwatersrand Basin The best known late Archaean sedimentary basin is the Witwatersrand Basin, for it hosts the world s richest gold province and is also an important source of uranium. For these reasons the geology of this basin is known in considerable detail. The Witwatersrand basin contains a 7 km-thick succession of terrigenous sediments and volcanic rocks, formed between 3.074 and 2.714 Ga ago. 

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