Radioactive mineralization in pegmatites, of Dara-Um Swassi, area North Eastern Desert, Egypt

The Dara-Um Swassi pluton is one of the well-exposed granitic batholiths in the North Eastern Desert. It is cut by pegmatites and dykes of different types running in harmony with the prevailing structural trends. The pegmatites are found as pockets and lenses especially in the peripheral zones and at different topographic levels. They usually have a NE-SW trend, which coincides with the main structural trend. The Dara-Um Swassi pegmatites are both zoned and unzoned. An idealized zonal sequence from outer margin to the center is: (1) plagioclase, quartz, microcline-perthite with or without muscovite and/or biotite (wall zone); (2) orthoclase-perthite, quartz and muscovite (intermediate zone); (3) quartz of monophase type (core zone). In some zoned pegmatites, distinct dark alteration zones are found at contact between quartz core and intermediate zone in the radioactive cataclastic pegmatite. Unzoned pegmatites are of coarse-grained granitic composition. The two types of pegmatites contain fluorite, zircon, allanite, bastneasite and yttrian fluorite, while euxenite, curite, carnotite and thorianite minerals are only found in the zoned pegmatites. Whole rock chemical analyses of the host monzogranites and pegmatite wall zones provide strong evidence that pegmatites in a granitic system are attributed to magmatic differentiation of a volatile-enriched granitic magma of calc-alkaline composition. With respect to radioactivity, in the unzoned pegmatites average U content is 150 ppm; and average Th content is 99 ppm, the zoned pegmatites represent the most important radioactive sites. U and Th contents decrease from intermediate zones toward wall zones (av. = 143 ppm U, 105.5 ppm Th for intermediate zones, and 56.5 ppm U, 47 ppm Th for wall zones). The altered zones represent the most radioactive sites (av. = 233 ppm U, and 205 ppm Th); this is due to the alteration processes caused by late-stage hydrothermal solutions (haematitization, kaolinitization, chloritization and fluoritization).