GEOLOGY, PETROLOGY AND RADIOACTIVITY OF THE OLDER GRANITOIDS AND YOUNGER GRANITES OF GABAL EL-UMRAH AREA, CENTRAL EASTERN DESERT, EGYPT

Gabal El-Umrah area (about 93 km2) is located between latitudes 25° 16´ – 25° 21´ N and longitudes 34° 15´– 34° 21´ E in the Central Eastern Desert of Egypt. The older granitoids are characterized by their low to moderate topography, exfoliation and caverneous weathering. Their hand specimens are coarse-grained with light to dark grey colour. More than one type of the younger granites are easily noticed in the field because of the variability in grain size ( from fine to coarse) and the difference in colour (greyish pink, pink, buff and light red), which reflect the variability in the type and intensity of mafic minerals as well as the plagioclase/k-feldspars ratios. The contact between these types is irregular and difficult to be traced.
The older granitoids (quartz diorites and granodiorites) originated from upper mantle materials. The younger granites are classified as monzogranites and syenogranites. Monzogranites originated from felsic crust material contaminated with upper mantle. Syenogranites originated from highly fractionated, K-rich, crust materials.
The older granitoids show eU/U ratios greater than one indicating U-loss. The monzogranites show ratios ranging between 0.92 and 1.10, suggesting restricted uranium mobilization or may suggest approximately similar uranium leaching and addition. The syenogranites show ratios lower than one, indicting uranium addition especially to fractured zircon and apatite as well as hematite. The petrographic, geochemical and radiometric data suggest that the studied syenogranites of Gabal El-Umrah are uraniferous granites. Along the northern peripheries of Gabal El-Umrah, some zoned pegmatite pockets possess abnormal values of uranium contents ranging between 67.4 and 92.3 ppm. The associated radioactive minerals occurred either as filling fractures of feldspars or as dissemination. They are represented by betafite, samarskite, euxenite and ashanite.