Geology, Petrography and Radioactivity of the Area between Wadi Dib and Wadi Mellaha, North Eastern Desert, Egypt

Abstract

A varieties of rocks including metavolcanics, metagabbro-diorite complex, older granitoids, Dokhan volcanics, Hammamat sediments, younger granites as well as dyke swarms and veins are well exposed and studied in details at the strip enclosed between Wadi (W) Dib and W. Mellaha area. The metavolcanics is represented by the basic variety of metabasalt, intermediate variety of meta-andesite and acidic varities of meta- dacite and meta-rhyolites in addition to meta- pyroclastics. The metagabbro-diorite complex is shown intruding the metavolcanics and enclave rafts of then while this complex is intruded by both the older and younger granites and mainly have the composition of metamorphosed gabbros. The older granitoids in the area are seen extruded by the Dokhan volcanics and are intruded by the younger granites and generaly have the composition of quartz- diorite and granodiorites. The Dokhan volcanics in the study area are present and exposed at W. Dib, W. Abu Had and W. Mellaha while they are represented by successive sequences of lava flows ranging in composition from intermediate to acidic varieties with their related pyroclastics. A wide spectrum of different types of molasses rocks is represented by the Hammamat sediments which have conglomerates, greywakies, purple slate and siltstone. The younger granitic rocks which are considered the most important, from the radioactivity point of view, are classified into syenogranite and alkali feldspar granite varieties and are shown intruding all the rock units exposed in the area. Numerous post- granitic dyke swarms and veins are recorded as basic and acidic dykes invading and cutting all the rock types in the study area. The radioactivity of the investigated rocks show an increase in their values from the older unites to the younger one where the radioactivity is in harmony with the increase of the radioelement content. The younger granitic rocks show the highest values of radioactivity among the other rock units. An anomalous pegmatite body is recorded along a tributary branched from W. Mellaha has 157 ppm uranium content and 4350 ppm thorium content and 4.23 K% and is formed at the pneumatulitic (pegmatite) late stage of the alkali feldspar granites.