Uranium and granitoids: a tentative interpretation

In recent years petrologic and geochemical research has been carried out, especially in France, on plutonic rocks. Hence, various mechanism, which could control the distribution of uranium in granitoids, have been reviewed. In France, intragranitic mineralization is considered in relationship to the acidic granitoids (leucogranites), which are well developed in the western Massif Central. In the eastern Massif Central the influence of leucogranites is apparently absent. The leucogranitic uranium mineralizations are located in a thickened sialic area, metamorphosed in the Barrovian grade. In the leucogranite bodies, the geochemical content is higher and uranium is mainly within the microcrystals of non-thoriferous uraninite, whereas the veins are strictly located within the leucogranite bodies. In the ante-Hercynian basement of the eastern Massif Central area, covered unconformably with rather thin Devono-Dinantian series, no obvious affinity with a particular granitoid is seen. In that case the uraninite crystals have a notable thorium grade; the vein-type mineralizations are controlled by the basement cover interaction. The concept of 'fertile' granites should only be applied to thickened sialic crusts, where the mineralizing fluids, originated from the anatexis, are correlated with the vein-type fluids. Such a concept must be rejected when thin or normal sialic crusts are concerned. There, the geochemical preconcentration phenomena do not seem related to vein-type mineralization. Moreover, these concepts have been tentatively extended to other uraniferous areas, such as the Rossing deposit in South West Africa and the Spokane district, Western U.S.A. (author)