Geology, Geochemistry and Radioactivity of Gabal Shendib Alkaline Ring Complex, South Eastern Desert, Egypt

Gabal Shendib alkaline ring complex forms high positive topographic rock units of concentric circular outcrop. The igneous complex has a diameter of 12 km emplaced within the meta volcanics and tonalite- granodiorite host rocks. The complex consists of olivine gabbro, alkaline granite, alkaline syenite and remnants of the volcanic rocks represented by porphyritic trachyte, porphyritic rhyolite, rhyolite and tuffs. These alkaline rocks show a strong enrichment in some rare metal contents (Zr = 4527, Nb= 717, Y= 806 ppm) and rare earth elements (La = 553, Ce = 1581, Pr = 214, Nd = 1145, Sm = 299, Gd = 55, Dy = 85 ppm). The highest Rb/Sr ratio and lower Ba/Rb ratio of the studied alkaline rocks of Gabal Shendib ring complex indicating a high degree of fractionation of the melt associated with mineralization. REE abundance patterns of the alkaline granite and alkaline syenite display M-type and W-type tetrad effect with strong Eu depletion in both whole rock samples. Fluid-melt interaction in the late stage of fractional crystallization is suggested to be the most important factor controlling the formation of REE tetrad effects in these alkaline rocks. The most important REE-bearing minerals are zircon, xenotime, monazite, allanite and apatite. Other accessory minerals include columbite, uranophane, uranothorite and titanite are identified. Field radiometric measurements and chemical analysis for uranium content of the alkaline granite and alkaline syenite revealed low uranium content. Radioactivity level reaches up to 13.70 ppm eU in alkaline granite and 4.90 ppm eU in the alkaline syenite. Chemical analysis of uranium revealed that U content reaches up to 35 ppm in the alkaline granite and 15 ppm in the alkaline syenite