الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 239 |  |  |
| | | from | 239 | | |
Abstract
Gabal Ghurab Ar-Rayyan - Gabal Ghurab Al-Atshan area is represented by older and younger granites that intruded in metasedimentary country rocks Both of them are cut by mafic dykes and pegmatites and capped by Nubian sandstones. The area is dissected by parallel sets of NW-SE and NE-SW left lateral strike-slip faults and E-W to ENE-WSW trending right lateral strikeslip faults. The emplacement of the alkali-feldspar granites is mainly controlled by the NW-SE sinistral fault system, whereas the interaction between the NW-SE and the E-W fault systems as well as their later reactivation play the main role in distributing the different rock types and the development of the faulted bounded basin where the Nubian sandstones have been deposited. The metasedimentary rocks are mainly represented by medium to high grade regionally metamorphosed biotite-chlorite-quartz phyllite and hornblende-biotite phyllite. The older granitoids are classified as quartz diorites and granodiorites with gradational contacts. The younger granites are subdivided into syenogranites and alkali-feldspar granites. The syenogranites are the main rock types of G. Ghurab Al-Atshan and G. Ghurab Ar-Rayyan E), while G. Ghurab Ar-Rayyan (W) is only represented by alkali-feldspar granites. The studied syenogranites is classified as subsolvus-type crystallized under high water pressure. The presence of conspicuous amounts of primary muscovite suggests genesis from peraluminous magma Syenogranites show lower microcline/orthoclase and alkali feldspar/plagioclase ratios relative to the alkali-feldspar granites, suggesting that the studied syenogranites are originated at higher temperature. The previously mentioned granites originated from independent two magmas because the a dyke-like body of alkali-feldspar granites intrudes the syenogranites and possess lower quartz contents than those of syenogranites ii Geochemically, the older granitoids were derived from medium-K, calcalkaline metaluminous magma. The syenogranites were derived from metaluminous-peraluminous magma. While the alkali feldspar granites were derived from metaluminous magma. Both younger granite varieties are K-rich and have the geochemical characteristics of A2-type granites that developed in post orogenic within-plate tectonic regime The gamma radioactivity varies according to rock type. Both Nubian sandstones and metasedimentary rocks show the lowest average of radioactivity level 3.5 μR/h) in the area. They possess eU and eTh concentrations less than 1.6 and 5.0 ppm respectively, suggesting that these rocks are not important from the radioactivity point of view. Thus, they were not studied in detail in the present work The studied older granitoids show slightly higher radioactivity level 4.17 μR/h in average), but with average eTh/eU greater than 3, suggesting uranium leaching. They possess local scattered spots showing eTh/eU ratios less than 3 and sometimes reach 2.3, because these spots are highly fractured and altered to clay minerals and iron oxy-hydroxides which fix some uranium from the circulating solutions. The syenogranites show the highest radioactivity levels in the studied area. Their masses vary in radioactivity and consequently eU and eTh contents with systematic increase from G. Ghurab Al-Atshan to G. Ghurab Ar-Rayyan (E). The average eTh/eU ratios for the syenogranites and alkali-feldspar granites are very close to 3, indicating that these rocks still have their original magmatic concentrations of uranium and
thorium without conspicuous leaching and/or addition The present study showed that the positive anomalies follow pegmatite bodies, albitized zones and hematitized spots. Both equivalent U and Th contents in albitized zones of syenogranites of G. Ghurab Ar-Rayyan (E iii show high values, suggesting that the hydrothermal solutions (hypogene) are
the main reason for albitization and consequently U-Th addition rather than
the meteoric water (supergene), which only adds uranium The minerals separation of albitized zones of the syenogranites revealed the presence of allanite, xenotime, zircon and samarskite which may cause their high radioactivity level The high radioactive pegmatite anomalies are related to presence of some minerals as columbite, zircon, monazite, thorite, uranothorite, xenotime allanite and euxenite. Pegmatites not only possess radioactive and REE minerals, but are also enriched with non-radioactive minerals as garnet titanite and pyrite. X-ray diffractogram and EDX charts using Scanning Electron Microscope (SEM) confirm this result.