Fission-track analysis of uranium distribution in granitic rocks

Abstract

The natural uranium concentration and distribution in three granitoids from Australia has been used to develop an understanding of the geochemical processes which control its behaviour in the natural environment and the potential behaviour of some analogue elements within a crystalline high-level radioactive waste (HLW) repository environment. In the Coles Bay Granite, Olympic Dam granite and the Kambalda Granodiorite, fission-track micromapping has been used to identify primary and secondary distribution between four main mineralogical locales: (1) Background U; (2) Resistate U; (3) Secondary U; and (4) Fracture U. Minor concentrations of total U are present as Background U in the major rock-forming minerals whereas Resistate U, contributed by the accessory minerals, comprises up to 60% of total U. Two modes of redistribution of primary U are evident: (1) in the secondary minerals, particularly Fe- and Ti-hydroxides; and (2) on alteration minerals which infill microfractures. The association of U with these secondary and fracture-infilling minerals indicates that it has been redistributed during post-magmatic hydrothermal alteration and possibly by recent groundwater-rock interaction. The most important geochemical processes involved in this U redistribution include mobilisation and fixation through complex decoupling and redox reactions, and retention by secondary minerals through adsorption and ionic exchange. The effect of these processes upon natural U redistribution may be considered as an analogue of geochemical processes around a HLW repository. This work shows that a degree of post-magmatic alteration of a granite increases the proportion of secondary minerals of high sorption capacity present, which may thus enhance the radionuclide retardation capability of the granitoids.