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 (No rthern Territory, Australia) has been characterised to a depth of 10 m in t erms of the total uranium, U-238, U-234, Th-232, Th-230 and Th-228 distribu tions, and measurements of other geochemically important elements. The char acterisation was limited to the < 106 mu m material, where effects of diffe rent 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 re sistate material. It was concluded that the observed uranium concentration and isotopic ratio structures are an image of the outline of the ore body a t the surface. These structures are the result of several physical and chem ical processes which dominate at the Foot wall and at the ore body surface. The processes involve the formation of amorphous iron oxides, alpha-recoil enhanced mobilisation and transport of U-234, Th-230 and Th-228, precipita tion due to evaporation of ground- and interstitial water, and the adsorpti on of uranium and thorium within a time frame less than that required for U -234/U-238 and T-230/U-234 to achieve secular radioactive equilibrium. The mineralogical composition of the soil is to some extent responsible for loc al variations in the measured properties. The differences in chemical speci ation and behaviour of uranium and thorium enhance the variability in the T h-230/U-234 activity ratios. The monsoonal climatic and hydrological variat ions intensify some of the observed effect. (C) 1999 Elsevier Science B.V. All rights reserved.