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Chemical alteration processes for uranium migration and their significance on uranium isotopic ratio changes

ABSTRACT Gabal Abu Garadi area is covered by metasediments, alkali feldspar granites and stream sediments. Geochemically, silicification, desilicification, sericitisation, fluoritisation and Na-metasomatism are the main alteration features of alkali feldspar granites. The activity concentration of 238U in alkali feldspar granites ranged between 1.7 ± 0.1 Bq g−1 and 3.9 ± 0.3 Bq g−1; between 1.7 ± 0.2 Bq g−1 and 9.4 ± 1.4 Bq g−1 for 234U and between 0.012 ± 0.0002 Bq g−1 and 0.09 ± 0.0001 Bq g−1 for 235U. The activity concentration of 228Th ranges between 1.3 ± 0.1 Bq g−1 and 3.6 ± 0.8 Bq g−1; between 1.5 ± 0.2 Bq g−1 and 2.7 ± 0.7 Bq g−1 for 230Th and between 1.8 ± 0.3 Bq g−1 and 4.1 ± 0.4 Bq g−1 for 232Th. The variability in Chemical Index of Alteration (CIA) with U, Th, 230Th/238U and 230Th/234U for the studied altered granite indicates that U and Th concentrations increased with increasing alteration especially in the silicified and desilicified samples. Changes in U, Th, 230Th/238U and 230Th/234U suggest the role of alteration processes in uranium and thorium mobilisation though thorium is geochemically less mobile or immobile. Thorium mobilisation could be controlled by adsorption in the alteration products of Abu Garadi granites such as clay minerals, manganese and iron oxides. In contrast, 230Th/238U and230Th/234U decreased with increasing CIA values suggesting uranium accumulation. 234U/238U ratios were in equilibrium in most samples; this might suggest the effect of severe chemical weathering or alteration processes that leads to the leachability of the two radionuclides (234U, 238U) with the same ratio probably during kaolinisation processes. In contrast, 230Th/238U and230Th/234U>1 were in disequilibrium, suggesting the changes in the physicochemical conditions accompanying the different alteration processes.