Metamorphic host rocks, uranium

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. 

Prato has described the results of geological investigations in the area. Tabular, lens-shaped bodies occur in apatite-rich quartz mica schists composed of alternating muscovite and quartz + feldspar beds interbedded with the acid metavolcanics petrochemical analyses of the host rocks indicate a composition intermediate between feldspathic sandstones and argillites. The mineral association is pitchblende + pyrite-I-chalcopyrite + tetrahedrite -I- hematite, the first two being the most abundant. The uranium -I- sulphide mineralization is parallel both to the schistosity and to the younger tectonic deformations produced by the Alpine dynamo-metamorphism in particular, pitchblende is confined to the apatite-rich micaceous beds. Apatite is clearly replaced by pitchblende. 

---