Mineralogy and radioactivity of pegmatites from

Mineralogy and radioactivity of pegmatites from South Wadi Khuda area, Eastern Desert, Egypt

Abstract: Radioactive minerals in pegmatites associated with granitic rocks are commonly encountered in the south of the Wadi Khuda area and found as dyke-like and small bodies. They are observed within garnet-muscovite granites near the contact with older granitoids. Field surveys indicated that the studied pegmatites vary in dimensions ranging from 2 to 10 m in width and from 10 to 500 m in length. They are composed mainly of intergrowth of milky quartz, reddish-pink K-feldspar and plagioclase together with small pockets of muscovite. Field radiometric measurements indicated that radioactivity in pegmatites is more than twice that of their enclosing
country rocks. Radionuclide measurements revealed that the average contents of U and Th increase gradually from rocks of dioritic to granodioritic composition (1.5×10-6 U and 4.3×10-6 Th) and increase significantly in biotite granites (5.8×10-6 U and 15.2×10-6 Th) but drastically decrease in muscovite granites (2.2×10-6 U and 5.6×10-6 Th).
The average contents of U and Th of anomalous pegmatites are 95.3×10-6 and 116.9×10-6, respectively, indicating their uraniferous nature. In the south of the Wadi Khuda area, pegmatites are low in average Th/U (1.4) and high in average U/K (35.6), which suggests that uranium concentrating processes did not affect the pegmatites, indicating poor source-rocks. Mineralogical investigations of the studied pegmatites revealed the presence of secondary uranium minerals (kasolite and autunite), in addition to zircon, thorite, apatite, garnet and biotite. Primary and secondary radioactive mineralizations indicated that the mineralization is not only magmatic, but also post-magmatic. Electron microprobe analyses showed distinct cryptic chemical zoning within thorite where UO2 decreases from core to rim. This feature in thorite is sporadic, suggesting non-uniform redistributions of UO2 within thorite during magmatic processes.