Radiological risk, mineralogical and geochemical signatures of trace and rare earth elements (REEs) in calc-alkaline...

The investigated area reveal that the Precambrian tectono-magmatic history includes gneisses, ophiolitic mélange, metavolcanics, and a variety of granitoids, one of which is alkali feldspar granite. The main objective of this work is to investigate the natural radioactivity, mineralogy, geochemistry and radiological implications of the alkali feldspar granite. About fifty granitic rock samples were systematically collected and analyzed by high-purity germanium (HPGe) gamma spectrometry, resulting in activity concentrations of ²³⁸U, ²³²Th, and ⁴⁰K. The average activity concentrations of ²³⁸U, ²³²Th, and ⁴⁰K are 89, 98, and 1427 Bq kg⁻¹, respectively, all statistically significantly higher than global averages. The activity values are elevated primarily due to the presence of U- and Th-bearing accessory minerals such as thorite, uranothorite, monazite and zircon in addition to allanite and xenotime, which are known to host these radionuclides in granitic systems. Radiological indices radium equivalent activity (Ra_eq), hazard indices (H_ex and H_in) and the gamma index (Iγ) and the annual effective dose (AED) were estimated and the results indicate they pose some risk for indoor radiological exposure, especially from inhalation of radon. Statistical analyses using (Q-Q plots, Pearson correlation, PCA and HCA) confirmed that ²³⁸U and ²³²Th contribute most to radiological risk, while ⁴⁰K, despite high activity, supported negligible risk. The analyzed alkali feldspar granite is distinguished by silica-rich igneous rock that has undergone significant differentiation. It displays unique geochemical traits, characterized by high concentrations of silica content reaching 75.33 wt% and total alkalis 8.88 wt%, alongside preferential enrichment of large-ion lithophile elements (LILE; e.g., Ba, Rb, Sr) relative to high-field-strength elements (HFSE; e.g., Zr, Y, Nb, Ta). Geochemical signatures reveal high-K calc-alkaline affinity, and peraluminous characteristics for the alkali feldspar granite. The tectonic settings indicate that the granite formed during syn-collisional to post-collisional phases, volcanic arc granite with magma differentiation processes influenced by crustal components, as evidenced by Ba/Rb ratios and rare earth elements (REEs) trend. The study concludes that, in comparison to other similar granites, the alkali feldspar granite of Wadi Rahaba is enriched in radionuclides and has evolved geochemically. This has the potential for radiological concerns regarding their use in the environment and/or construction applications.