U - Th - REE Mobility and Diffusion in Granitic Environments During Alteration of Accessory Minerals and U - Ores: A Geochemical Analogue to Radioactive Waste Disposal

Abstract

ABSTRACTU, Th and REE concentrations and distributions have been studied in granitic rocks, using a multidisciplinary approach involving fission-track micromapping of cracks in oriented samples, together with mineralogical and geochemical studies of the different U-Th-REE bearing phases. The behaviour of U, Th and Nd, considered as chemical analogue elements of the radiotoxic nuclides, was investigated either in the vicinity of microsites (accessory mineral environment) or along plurimetric sections around U-ore bodies. The different granite minerals, especially the accessory minerals (uraninite, monazite, thorite, apatite, xenotime), as well as U-ores, present different initial concentrations of U, Th and REE. Limitations to the analogy between these U-Th-REE concentrations and the radwastes is discussed as a function of their mineralogical features, chemical composition, size and solubilities. These primary concentrations present different behaviour when subjected to hydrothermal alteration, such as propylitization, phyllite type alteration, or clay alteration. Results show that in reduced media, in the temperature range 80–200°C, the rate of mobilization of U, Th, REE is relatively moderate. However, fluids enriched in fluorides, phosphates or carbonates may significantly solubilize and transport U and REE under specific conditions. In addition, the degree of opening of the microcracks and faults, as well as the oxidation-reduction processes, are critical parameters for the efficiency of the granitic geological barrier.