Abstract:
Changes in uranium distribution during chemical weathering were investigated in three granitoid types from the New England district, New South Wales, Australia, over a range of samples from unweathered rock to profoundly weathered material. Uranium fission-track micromapping was used to characterise the three principal modes of U abundance in unweathered granitoid rock: low concentrations of Background U in fine fractures in quartz and in deuterically altered feldspar, biotite and hornblende; Interstitial U, in concentrations between 140 and 1500 μg g−1, principally in hydrated Fe-oxide, Mn-oxide and clay minerals; and the greatest proportion as Resistate U (90–4000 μg g−1), located in the accessory minerals.During low-temperature chemical weathering, a significant net U loss occurs, and the remaining U is redistributed amongst the three modes. Background U is depleted, and the mobilised U is adsorbed by interstitial aggregates thereby increasing Interstitial U concentrations. Resistate U is also depleted, as sphene, zircon, monazite and acicular apatite retain their initial U concentrations, but are reduced in modal abundance; prismatic apatite, fluorite and allanite are progressively removed.The significant net loss of U during chemical weathering provides uranium for potential secondary sedimentary mineralisation, and implies that weathered granitoid rock is not an appropriate element in the geological barrier for nuclear waste.