Uranium geochemistry in peraluminous leucogranites

During anatexis, uranium fractionation into the melt increases with the proportion of U located outside the lattice of weakly soluble accessory minerals. Low degree of partial melting, high fO2 and PCO2 improve U fractionation into the silicate liquid. The study of two different samplings, free of meteoric alteration, allows to characterize the U distribution related to magmatic and late-magmatic processes. During the evolution of peraluminous magma, the proportion of U to be imputed to uraninite increases whereas the proportion to be imputed to monazite decreases. Uraninite has a clear magmatic origin and follows incompatible-element enrichment. A superimposed uraninite distribution occurs in late-magmatic shear zones and is related to the fluid oversaturation of highly evolved endogranitic intrusions. This process strongly enhances the U fertility of a granitic body. The effects of various types of alterations, currently observed in peraluminous granites, on U mobility, are briefly reviewed. Meteoric alteration strongly disturbs previous U distributions, but high U contents are generally partly preserved. Pervasive migration (low water/rock ratio) hardly proceeds in metal mobilization. Besides the nature of the fluids, the efficiency of U mobilization increases with the intensity of the spatial overlap between magmatic (and ductile) shear zones enriched in uraninite and brittle structures favouring hydrothermal circulations. 100 refs.; 16 figs.; 3 tabs