Distribution of uranium- and thorium series radionuclides in mineral phases of a weathered lateritic transect of a uranium ore body

A surficial, weathered 400 m long profile of the Ranger One ore body #3 (Northern Territory, Australia) has been characterised to a depth of 10 m in terms of the total uranium, 238U, 234U, 232Th, 230Th and 228Th distributions, and measurements of other geochemically important elements. The characterisation was limited to the <106 μm material, where effects of different mineralogical composition were found to be small. The distribution was classified in terms of successive extractions for amorphous oxide phases (e.g., ferrihydrites), crystalline oxides (e.g., goethite, hematite), and resistate material. It was concluded that the observed uranium concentration and isotopic ratio structures are an image of the outline of the ore body at the surface. These structures are the result of several physical and chemical processes which dominate at the Foot wall and at the ore body surface. The processes involve the formation of amorphous iron oxides, α-recoil enhanced mobilisation and transport of 234U, 230Th and 228Th, precipitation due to evaporation of ground- and interstitial water, and the adsorption of uranium and thorium within a time frame less than that required for 234U/238U and 230Th/234U to achieve secular radioactive equilibrium. The mineralogical composition of the soil is to some extent responsible for local variations in the measured properties. The differences in chemical speciation and behaviour of uranium and thorium enhance the variability in the 230Th/234U activity ratios. The monsoonal climatic and hydrological variations intensify some of the observed effect.