234U/238U signatures associated with uranium ore bodies: part 1 Ranger 3

Abstract

The Ranger 3 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 between 30 and 50 m below the surface. The ground water U concentration and 234U/238U AR signature in the top 10 m of the weathered zone are reported for 357 samples collected over 4 wet seasons, at 5 depths, along a transect in-line with the hydraulic gradient and along the centre line of the ore body and its associated dispersion halo. The results show that the weathered zone displays a general U isotope feature for this type of ore body with the 234U/238U AR for the ground water and amorphous phase of the solid matrix being less than 1. The ground water 234U/238U AR is independent of the annual monsoonal climate and depth within the range surface to 10 m. In the vicinity of the U ore body the ground water 234U/238U AR is 0.75 and is very similar to the 234U/238U AR of the amorphous phase of the solid (0.76). The 234U/238U ARs of the amorphous phase and ground water rise and separate to values of 0.88 and 1.02 at the end of the transect. The rise and separation in 234U/238U AR are interpreted as evidence that the source of the U in the ground water is from the water-soluble sub-phase of the amorphous phase and that the ground water flow is too fast to allow the processes occurring across the solid–water interface to reach chemical equilibrium. The data set is a robust characterisation of the coarse and fine detail of the 234U/238U AR signature in the weathered zone of U ore bodies.