Gamma activity concentrations of 226Ra, 232Th, 40K, and health hazard assessments of granites from Wadi El-Nabi’ mining area, Egyptian Nubian Shield

Abstract

Granitic rocks constitute one of the most prevalent and economically significant lithologies, owing to their abundance, mechanical durability, and aesthetic appeal, which render them highly suitable as ornamental stones in architectural and construction applications. In recent years, extensive research efforts have been directed toward quantifying the radiological hazards posed by naturally occurring radioactive materials within these rocks, concerning their potential implications for human health and environmental safety when utilized in building materials. In the present study, a comprehensive radiometric investigation was conducted on 35 granitic rock samples of Wadi El-Nabi' mining area, specifically El-Igl El-Ahmer monzo-syenogranites, to quantify the activity concentrations of principal radionuclides, including ²²⁶Ra, ²³²Th, and ⁴⁰K, utilizing gamma spectrometry with a high-purity germanium (HPGe) detector. Furthermore, an array of radiological hazard indices was systematically calculated to evaluate the potential radiological risks associated with these granitoid specimens. Our findings indicate that the mean activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in the monzogranite samples were 29 (± 6), 34 (± 6), and 883 (± 49) Bq/kg, respectively. In comparison, the syenogranite samples exhibited slightly elevated average values, measured at 31 (± 5), 35 (± 4), and 890 (± 9) Bq/kg for the corresponding radionuclides, reflecting a modest enrichment in radioactivity within the syenogranitic lithology. With respect to the radiological parameters, the results indicate that D_out, D_in, AEDE_in, ELCR_out, ELCR_in, I_γ, and AGDE for both monzogranite and syenogranite samples exceed the internationally recommended reference levels. Conversely, Ra_eq, AEDE_out, H_ex, and H_in remain within acceptable global thresholds. ²³²Th/²²⁶Ra (²³⁸U) ratios for the monzogranite and syenogranite samples range from 0.88 to 1.39 (mean 1.15 ± 0.14) and 0.88 to 1.41 (mean 1.14 ± 0.16), respectively. These values are markedly lower than the canonical crustal Th/Ra ratio of ~ 3.5, indicating post-magmatic hydrothermal alteration and selective uranium enrichment within the host granitoids. This radiological evidence is reinforced by remote sensing observations, which reveal characteristic alteration patterns, including kaolinization, sericitization, fluoritization, and silicification zones, that are spatially associated with the monzo-syenogranitic units, especially in the buffer location. Consequently, the granitic rocks in certain localized areas, particularly where radionuclide concentrations or radiological hazard indices exceed typical thresholds, may be considered unsuitable for use as construction materials.