GEOLOGY, RADIOACTIVE MINERALOGY AND GEOCHEMISTRY OF THE GRANITEIC ROCKS OF GABAL KHASHM EL-RISHA AREA, NORTH EASTERN DESERT, EGYPT. | ||||
| Delta Journal of Science | ||||
| Article 6, Volume 31, Issue 1, June 2007, Page 30-56 PDF (6.76 MB) | ||||
| Document Type: Research and Reference | ||||
| DOI: 10.21608/djs.2007.152982 | ||||
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| Authors | ||||
| A.A. Omran; S.S. Farag | ||||
| Nuclear Materials Authority, P.O. Box: 530 El-Maadi, Cairo, Egypt | ||||
| Abstract | ||||
| The rocks cropping out in the area are older granitoids (quartz diorites and granodiorites), Dokhan volcanic, younger granites (syenogranites and alkali feldspar granites) and dyke swarms of mafic and felsic composition trending mainly in the NESW and NW-SE directions. Radiometrically, the eU and eTh contents of the studied granitic rocks are systematically increase from the older granitoids passing through syenogranites to alkali feldspar granites. Alkali feldspar granites show wide range of eU contents if compared with syenogranites. The alkali feldspar granites have high eU contents (29.5-63ppm) and high eU/eTh ratios (0.44- 1.25) and the associated pegmatite pockets have eU contents (23.7-112ppm) suggesting a good environment for uranium occurrence. The ferruginated alkali feldspar granites and the associated pegmatite pockets show visible mineralization of metallic back and brown colours as well as waxy yellow colour in addition to disseminations of green and violet fluorite. By means of scanning electron microscope (SEM) the following radioactive minerals were identified in the pegmatite pockets and the ferruginated alkali feldspar granites: lynchodite, euxinite, samarskite, betafite, ashanite and uranophane. The presence of these minerals suggests that the hydrothermal solutions or the residual magma from which these rocks originated are not only rich in uranium and thorium, but also rich in Nb, Ta and REEs. Geochemically, the older granitoids are metaluminous, calc alkaline. Near the syn-collision field and have minimum melting point at moderate to high vapour pressure (>3kb) and crystallizaed at temperature between 850º to 1000°C. on the other hand, the younger granites are metaluminous to peraluminous and calc-alkaline to alkaline in characters with highly fractionated magma essentially with potassic affinities. They could be considered as post orogenic granites (POG), intruded under dry conditions in upper level of the earth crust at water vapour pressure between 1 and 3kb and crystallized at temperatures range from 800 to 850°C. | ||||
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