RADIOELEMENTS DISTRIBUTION IN GABAL ABU-MESAID AREA, NORTH EASTERN DESERT, EGYPT

A suite of volcanic and plutonic rocks represent the products of Pan-African orogeny; constitute the major litho-tectonic units of Gabal Abu-Mesaid area, in the northern part of the Eastern Desert. These Petro-tectonic units are the syn-tectonic older granitoids, mature arc to continental arc Dokhan Volcanics and the associated pyroclastics as well as the widely distributed Post- to late-tectonic younger granites and the post-granite dykes. The older granitoid recorded in the study area, is represented by granodiorites that is tectonically described as syn-tectonic plutonites. Dokhan volcanics demonstrated a wide spectrum of varieties. These rocks are andesites associated with rhyolites and andesitic tuffs, agglomerates as well as ignimbrites. Younger grained can be differentiated into two types namely: the fine-grained biotite granites and the medium-to coarse- grained hornblende biotite granites.
Radiometric survey carried out on the rock exposures of G. Abu-Mesaid areareveals that the younger granitic rocks have the highest radioactivity levels. They show average total-gamma radioactivity of Ur for the fine-grained biotite granites and 31 Ur for the medium-to coarse- grained hornblende biotite granites. It is noticed also that, felsic dykes and some pegmatites related to the younger granites have high radioactivity. Generally, felsic associations have total gamma-radioactivity values (15 to 50 Ur) higher than mafic associations (10 to 21 Ur). Granodiorites have average total gamma-radioactivity of 26Ur. Dokhan volcanics have the lowest average of total gamma- radioactivity (13Ur).
Distribution of uranium and thorium in the analyzed samples shows that fine-grained biotite granites have average eU content of 14.7 ppm and eTh content of 26.5 ppm. The average eU-content for the hornblende biotite granites is 6.8 ppm and eTh 20.6 ppm. Granodiortes samples have average eU and eTh contents of 3.8 and 9.2 ppm respectively. Average contents of eU and eTh in andesites are the lowest (1.6 and 5.4 ppm respectively).