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.

KEYWORDS:

• Alteration processes
• uranium migration
• isotopes
• Abu Garadi
• Egypt

Disclosure statement

No potential conflict of interest was reported by the authors.