CHANGING URANIUM DISTRIBUTIONS DURING WEATHERING OF GRANITE

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.