Natural Radioactivity Levels (238U, 232Th, 40K) and Their Geochemical Characteristics of Granitic Rocks from Nuweibi Area, Egyptian Nubian Shield

The Nuweibi area (7.5 km(2)) is located towards NW Marsa Alam City, central Egyptian Nubian Shield. It constitutes ophiolite assemblages (serpentinites, metagabbros), Tuffaceous metasediments, granodiorite-tonalite, and Late Ediacaran albite granite. The present study mainly focuses on determination of the concentrations of specific elements and their interrelationships to U-238, Th-232, and K-40 for the granitic rocks. The radiometric measurements have been performed using a high-purity germanium (HPGe) detector and a portable RS-230 gamma-ray spectrometer in combination with chemical analysis using ICP-MS. The radionuclide values measured with a ground gamma-ray detector are more than twice those recorded using both HPGe spectroscopic and chemical analysis methods. On the other hand, the chemical data given by many correlations between U, Th, and K as well as selected major, trace, and rare earth elements suggest the occurrence of the magmatic process followed by Na-metasomatic albitization in albite granite. This process caused redistribution and remobilization of U, as follows the facts that eTh/eU exceeds 3 and the average eTh/eU ratio of albite granites exceeds the Clark's value (3.5). On the other hand, the higher eTh contents of the albite granites reflect additional contribution from Th-bearing minerals, and in most sampling sites the Th-232/Ra-226 activity ratios are higher than the natural equilibrium ratio, 1.1, indicating possible enrichment process of these radionuclides as a result of alkali metasomatism.