234U/238U signatures associated with uranium ore bodies: part 3 Koongarra

Abstract

The Koongarra ore body is an early Proterozoic U ore body in the Alligator Rivers U province, Northern Territory, Australia. It has surface expression with a redox front located ∼30 m below the surface. The 234U/238U activity ratios (AR) for the ground water and the amorphous phase of the solid have been analysed for the ore zone and dispersion halo as a function of depth. The results display a 234U/238U AR signature with depth which may be common to all U ore bodies. The 234U/238U AR is depressed below secular equilibrium in the weathered material above the redox front; rises significantly above secular equilibrium in the vicinity of the redox front; and is followed by a gradual decrease with depth below the redox front. The amplitude of the profile is a function of local conditions. A model is proposed for the signature in which oxidising waters preferentially leach the 234U sites at the redox front due to preconditioning of the 234U sites by α recoil during the decay of 238U to 234U. Mass balance requires the solid material left behind the redox front to have a 234U/238U AR reduced below 1. Local second order effects may be superimposed on the signature. The signature may have application to calibrating scenarios for nuclear waste repositories, assisting in understanding historical climates, economic evaluation of U ore bodies and U exploration.