Studies of radioactive minerals and geochemistry of uranium at the granitic rocks of Wadi Um Shillman Area, South Eastern Desert, Egypt

Abstract

The northwestern part of Gabal Um Ara granitic pluton is represents a good example for uranium occurrence in the southeastern desert of Egypt. Among the different lithologies in the study area, the granitic rocks attract attention for more detailed geological, geochemical and radiometrical studies. Um Ara granitic masses cover an oval outline area of about 300 km2, which is characterised by moderate topography. The granitic rocks are classified as monzogranite and alkali feldspar granite. Both of them belong to the post-orogenic younger granite magmatic activity that intruded the Egyptian shield between 620 and 530 Ma. The intrusion of Um Ara monzogranite and alkali feldspar granite appears to have been controlled by deep-seated tectonic zones and block faulting. The studied area lies at southeast of Aswan City between 22ͦ 38′ 12˝–22ͦ 38′ 34˝ N and 33ͦ 47′ 27˝–33ͦ 48′ 14˝ E. Geochemically, the average concentration of the elements in North Um Ara granite shows that the relative abundance decreases gradually with decreasing incompatibility from Rb to Y. The common features are enrichment in more mobile elements Rb, Th and U. The monzogranite shows the lowest U and Th content, whereas the alkali-feldspar shows an enhanced contents of both elements, with average Th/U ratio = 3.0. the alkali-feldspar exhibits an expanded enrichment of Th relative to U. The U of the alkali-feldspar may be included within the structure of the refractory accessory minerals. The northern part of the Um Ara granitoids has subjected to different types of alterations. The radiometric survey of the northwestern part of Gabal Um area granitic pluton revealed the presence of several radioactive anomalies near the contact with the Dokhan volcanic and metavolcanic rocks in the north. Some visible uranium mineralisation of uranium minerals occur mainly along the NW, NE, E-W and N-S joints, of a highly sheared alkali feldspar-granite. Uranophane is visible secondary uranium mineral, characterised by bright yellow lemon colours and occurs mainly as filling fractures and joints associated with violet fluorite, iron and manganese oxides. The radioactive and accessory minerals studying and X-ray diffraction analysis that of the mineralised alkali feldspar-granite granite in the northwestern part of Gabal Um Ara granitic pluton revealed include the presence of fluorite, zircon, columbite, spessartine garnet whereas, the secondary uranium minerals are represented by uranophane, β-uranophane and kasolite. The hydrothermal origin was proposed as two possible alternatives for this uranium mineralisation.