GEOLOGY AND RADIOACTIVITY OF PERALUMINOUS GRANITE AND ASSOCIATED PEGMATITE HOSTING MAGNETITE MINERALIZATION AT UM REGEBA AREA, SOUTHEASTERN DESERT, EGYPT

The study area represents the southeastern part of Wadi El-Gemal-Hafaﬁt culmination in the south Eastern Desert of Egypt. Magnetite mineralization occurs in Wadi El-Gemal area in many localities, Um Regeba area is the case study. Petrographically, the studied peraluminous granites (PG) are medium- to coarse-grained and mainly composed of plagioclase (An 5-10 ), K-feldspar, quartz, biotite and muscovite. Sericite and chlorite are secondary minerals, while allanite and zircon are the common accessories . The peraluminous pegmatites (PP) are coarse-grained and composed essentially of K-feldspar, quartz, plagioclase (An 6-10 ), muscovite and biotite. Titanite, allanite, zircon and magnetite are common accessories . Geochemically, the studied peraluminous granites and peraluminous pegmatites are monzogranite and syeno- to alkali feldspar granite respectively. They have peraluminous character, calc alkaline and alkaline afﬁnities respectively, emplaced in within plate setting, crystallized under water–vapor pressure (2-3 Kb), temperature from 760 o to 800 o C, pertaining to the I-type granite originated by highly differentiated magma generated from upper mantle contaminated with some crustal materials. Magnetite usually occurs as small lumps, concentrated by to accumulation in magmatic segregations that developed in response to fractional crystallization. Magnetite showed lamellar intergrowth with ilmenite. The latter can be seen with a hand lens in most magnetite samples. The magnetite of Um Regeba area represents a late stage magmatic product. From the microprobe results, magnetite is characterized by high content of SiO 2 , Al 2 O 3 , CoO and Cr 2 O 3 , whereas ilmenite is characterized by high content of MnO and ZnO. The average eU content in the peraluminous granites is 1.96 ppm, while it reaches 5.65 ppm for eTh. The average eU content in the peraluminous pegmatites is 9.12 ppm, while the average eTh content is 21.76 ppm.


INTRODUCTION
Peraluminous granitic segregations are commonly associated with regionally metamorphosed terrains (Clemens and Wall, 1981), Debon et. al. (1986), Inger and Harris (1993). Numerous mechanisms have been proposed to explain the derivation of these segregations from the metamorphosed host rock. Partial melting of metapelites is still the most widely accepted model for the generation of these peraluminous granites (Holtez and Barbey, 1991). The breakdown of the hydrous silicates in these pelites (such as muscovite and biotite minerals) produces most of the water required for this partial melting process (Fyfe, 1969). Four principal mechanisms for the formation of peraluminous granites have previously been advocated (Mohamed and Hassanen, 1997): (1) the peraluminous granite is directly linked to peraluminous source rocks, (2) the peraluminous granite may, at least in part, be the result of reaction with host rocks, (3) the peraluminous granite has been derived from meta-aluminous magmas by fractional crystallization and (4) the peraluminous granite is at least in part if not wholly, the result of interaction between late stage magmas or subsolidus rocks and hydrothermal fluids.
In Egypt, the peraluminous granites represent phases of orogenic to late orogenic. Their quartz veins are rich in -Mo, Sn, W, U, Nb and -Ta mineralization within the granitic rocks (Takla and Nowier, 1980).
Most of the pegmatites are usually associated with granitoid rocks, but little is incorporated with mafic-ultramafic rocks. The pegmatites can be roughly categorized according to their alkali concentration ratios, as follows: primitive pegmatites, intermediate pegmatites and evolved pegmatite (Jolliff et al., 1992).
Granitic pegmatite magma is peraluminous and characterized by enrichment in volatiles such as F, P, B and/or H 2 O. It has been demonstrated that such volatiles have a significant influence on the evolution of the pegmatite magma, the temperatures of the solidus and liquids of the magma, the viscosity of the silicate melt, the crystallization sequence of minerals and also on the partition behavior of trace elements between fluid and melt (Černý and Meintzer 1985;Webster and Rebbert, 1998;London, 1987;Ding-well et al., 1998, Bai and Van Groas, 1999, Keppler and Wyllie, 1991and Keppler, 1993. Magnetite occurs in the peraluminous pegmatites and granites in many localities of W. El Gemal area, especially in El Mokhattata area. The Um Regeba area is the case study for this mineralization and this is the first record of magnetite mineralization in the area. Magnetite forms under a wide variety of con-ditions, crystallizing at high temperature from silicate, sulfide and carbonatite magmas or it can precipitate at lower temperatures from hydrothermal fluids. These different conditions may lead to distinctive trace element signatures for the magnetite. It is possible to use the trace element signature of magnetite as petrogenetic and provenance indicators (Dare et al., 2013).
The main target of this work is to study the magnetite mineralization as well as the hosting peraluminous granites and pegmatites of Um Regeba area geologically, geochemically, mineralogically and radiometrically.

METHODS OF STUDY
Detailed petrographic examination was carried out to study the mineral constituents and textural patterns of the rock. Fourteen samples (14) were chemically analyzed for major and trace elements from the peraluminous granites (9 samples) and peraluminous pegmatites (5 samples). The heavy minerals were separated using heavy liquid (bromoform), followed by magnetic separation using Frantz Isodynamic Separator. The obtained fractions magnetic and non-magnetic were investigated under binocular microscope. All analyses were carried out in the Laboratories of Nuclear Materials Authority (NMA), Cairo, Egypt. Some individual grains of magnetite were picked and analyzed by X-ray diffraction (XRD) method (in the Laboratories of NMA), and the backscattered electron images were performed at the Microscopy and Microanalyses Facility, University of New Brunswick (UNB), Fredericton, New Brunswick, Canada (model JEOL 6400 SEM).

GEOLOGIC SETTING
The studied area is located about 80 km SW of Marsa Alam city on the Red Sea Coast (Fig. 1)
The Um Regeba peraluminous granites are characterized by white colour, mediumto coarse-grained and occasionally pegmatitic. The former were emplaced into the older rocks in NNW-SSE trend (striking 350 o ) (Figs. 3 and 4) close to the major Nugrus thrust. The peraluminous granites contain -in parts -xenoliths from the older rocks; these enclaves form lenses usually exceeding 10 cm in length. The peraluminous pegmatites occur as dyke-like bodies of variable  Greiling, 1990) 72 FARRAGE M. KHALEAL and MOHAMED S. KAMAR dimensions intruding the granodiorite of the studied area with sharp contact (Fig. 3). They vary from few centimeters to few meters in width and length, and mostly concordant with the common NW-SE trend of the enclosing granites as well as the other country rocks all -over the area (Fig. 4). The magnetite mineralization is associated with the peraluminous pegmatites (Figs. 5 and 6).

Peraluminous Granites
The peraluminous granites are mediumto coarse-grained, mainly composed of plagioclase, K-feldspar, quartz, biotite and muscovite. Sericite and chlorite are the secondary minerals, while allanite and zircon are the common accessories.
Plagioclase is presented by albite (An 5-10 ), which occurs as twinned subhedral to euhedral crystals exhibiting wide range of grain sizes, enclosing sometimes fine crystals of quartz (Fig. 7). The former shows varied degrees of alteration to clay minerals (Fig. 8). Sometimes, it shows corrosion boundaries with quartz ( Fig. 9). K-feldspars are represented by microcline and microcline microperthite. Microcline microperthite is observed in subordinate amount as anhedral to subhedral crystal showing the characteristic cross-hatching or tartan pattern (Fig. 10), as a  (Deer et. al., 1992). Myrmekitic texture is occasionally present due to the replacement of plagioclase by K-feldspar (Fig. 11). Quartz occurs as subhedral to anhedral crystals interstitial between the feldspar crystals. Drop like inclusions of quartz are frequently poikilitically enclosed in the feldspar crystals. Biotite is the dominant mafic mineral, occurs as euhedral to subhedral crystals. It occurs as brown colour flakes (Fig. 12) variably altered to chlorite. Muscovite occurs as euhedral to subhedral colourless flakes. Allanite occurs as brown euhedral yellowish crystals associated with zircon and iron oxides. Zircon occurs as euhedral to subhedral prismatic crystals, associated with quartz and iron oxides.

Peraluminous Pegmatites
The pegmatite pockets are composed essentially of K-feldspar which is mostly represented by orthoclase. Orthoclase occurs as euhedral to subhedral crystals showing simple twinning and occasionally kaolinized. Quartz occurs as subhedral to anhedral crystals, some crystals show undulose extinction and irregular boundaries. Plagioclase is acidic in composition (An 6-10 ) exists as euhedral to subheral crystals. It exhibits albite and Carlsbad twinning and occasionally antiperthitic.

GEOCHEMISTRY
The data of chemical analyses (major oxides, trace elements and normative values) are given in Table (1). The general characteristics of the granitic rocks depend on the behavior and distribution of the major and trace elements. The average chemical composition of the studied peraluminous granites and peraluminous pegmatites when compared with Group II the younger granites of the Eastern Desert (Greenberg, 1981). They show increasing in Al 2 O 3, MgO, CaO (PG), Na 2 O and P 2 O 5 decreasing in SiO 2 , TiO 2 , Fe 2 O 3 , CaO Sericitization and kaolinization are encountered. Muscovite forms subhedral to anhedral flakey crystals, sometimes interstitially filled the space between the major constituents. Biotite occurs as subhedral flakey crystals stained with iron oxides. Zircon occurs as euhedral to subhedral prismatic crystals, associated with quartz, allanite and iron oxides (Fig. 13). Allanite occurs as euhedral yellowish brown crystals associated with zircon and iron oxides, and sometimes result from the alteration of epidote (Fig. 14). Titanite forms subhedral to anhedral rhombic crystals, associated with biotite. Magnetite occurs as subhedral to anhedral crystals usually associ-  (Table 2). On the Ab-An-Or ternary diagram (O'Connor, 1965), the studied peraluminous granites and peraluminous pegmatites lie in granite field (Fig. 15). On the R 1 -R 2 discrimination diagram of De La-Roche et al. (1980), the peraluminous granites fall in the monzogranite field ( Fig. 16  alkaline, (one sample lie in the alkaline and highly fractionated calc-alkaline field), on the other hand the peraluminous pegmatites lie in the alkaline and highly fractionated calc alkaline (Fig.18).
The studied peraluminous granites and peraluminous pegmatites lie in the I-type granite of Chapell and White (1974) (Fig. 19). On Pearce et al. (1984) discrimination diagram (Fig. 20), the studied peraluminous granites and peraluminous pegmatites lie in the within plate granite field. The studied peraluminous granites and peraluminous pegmatites are restricted to the syn-collision field (Fig. 21) after Batchelor and Bowden (1985). The Ab-Qz-Or normative ternary diagrams (Figs. 22&23) (one sample lies in the syenogranite field), while the peraluminous pegmatites plot in syeno-to alkali feldspar granite. The studied peraluminous granites and peraluminous pegmatites have peraluminous characters according to the alumina saturation of Clarke (1981) on Shand's index diagram (Fig. 17). On the discrimination plot of Sylvester (1989) (Greenberg, 1981). 4 : Average of World granites (Le Maitre, 1976). 5 : Average of granite in G. Nasb Aluba (Saleh, 1992 Clarke (1981) reveal that the studied peraluminous granites and peraluminous pegmatites have water-vapor pressure (2 to 3 kb) and temperature from 760 o to 800 o C suggesting a formation at moderate levels in the crust. The Fe 2 O 3 /FeO ratio varies from1.67 to 3.84 more than 1 ( Table  1), suggesting that the rock originated under higher oxidizing condition (Shalaby, 1995 andAli et al., 1998 . 24), reveals that the studied peraluminous granites and peraluminous pegmatites lie in the field of type III. It indicates that the studied peraluminous granites melts could be originated in the the LILE-enriched mantle wedge and contaminated by crustal melts.
The Rb versus K/Rb diagram (Fig. 25) for the studied peraluminous granites and peraluminous pegmatites, show that the Rb tend to be enriched relative to the K in the strongly differentiated granites (Imeokparia, 1981) and the K/Rb ratio ranges from 214.54 to 434.99 for the peraluminous granites while it is 195.10 to 240.40 for the peraluminous pegmatites. The ratio of Ba/Rb decrease with magmatic differentiation due to the crystallization of the feldspar (Fig. 26). In the studied peraluminous granites and peraluminous pegmatites, the Ba/Rb ratio ranges between 0.77 to 5.13 with an average of 1.98, and from 0.92 to 1.66 with an average of 1.20, indicating derivation from mantle origin contaminated with crustal materials.
Spiderdigram of normalized elements data of peraluminous granites and peraluminous pegmatites relative to Chondrite of Sun and McDonough (1989) (Fig. 27), illustrating Based on X-ray patterns and ESEM techniques, the minerals can be classified according to their anion into the following groups:

Zircon (ZrSiO 4 )
Zircon crystal showed different forms such as bipyramids, short prisms with brown or honey brown colour, may exists as normal or metamict. It is iso-structural with xenotime. It is confirmed by .
the enrichment of Ba, Rb, Pb, K, Nb, Zr, Y and depletion of P and Ti, while Sr distribute around the unity. Spiderdigram of normalized elements data of peraluminous granites and peraluminous pegmatites relative to average continental crust of (Weaver and Tarney, 1984) are given in (Fig. 28). They show enrichment of Rb, K, Y, Nb and Zr, and depletion of Ba, Sr, Ti and P (PP), while P (PG) distribute around the unity.

MINERALOGY
Detailed microscopic investigations supported by X-ray diffraction (XRD) and

Magnetite (Fe 3 O 4 )
Magnetite is a common, highly magnetic, black opaque mineral with a metallic luster and high specific gravity (4.9-5.2). It is one of the important iron ores and is a common constituent of igneous and metamorphic rocks. Due to its black colour, surface chemistry and strong magnetic property, it has found a great number of applications in industry. Magnetite crystals reach 2cm in size (Fig. 6). It is found as inclusions in the essential minerals or as secondary minerals filling cracks and cleavage planes. It is confirmed by XRD-technique (Fig. 32) and by ESEM techniques (Fig. 33) and contains

Phosphate minerals [ Xenotime (YPO 4 )]
Xenotime is a widespread and important rare earth bearing mineral, where Y can substitute by U, Ca and Si. It is considered as iso-structural mineral with zircon. Khomyakov (1970) has considered the potential of coexisting monazite and xenotime as a geotherometer. El-Kammar et al. (1997a) considered xenotime is a good host for the rare earth elements, where it concentrates the heavy rare earths. This mineral is confirmed by XRDtechnique (Fig. 31).

Ilmenite (FeTiO 3 )
Ilmenite occurs as accessory mineral in the studied peraluminous pegmatites associated with magnetite. It has an iron black or asphaltic colour as well as it is metallic with dull luster black streak and confirmed by ESEM (Fig. 34), it composed of 34.48 % FeO, 51.2% TiO 2 , 16.39% MnO and 0.11 Al 2 O 3 .

MICROPROBE ANALYSIS OF MAGNETITE
Mineral compositions were determined on the JEOL JXA-733 Superprobe, operating conditions were 15 kv, with a beam current of  (Tables 3 to 7). The standards used are Hmt (for Fe), Ilm (for Ti), Grtpyr (for Al, Si, Mg), Bust (for Mn, Ca), Vmet (for V), Chr (for Cr), Comet (for Co), Gahnite (for Zn) and NiO (for Ni). Five magnetite grains were systematically selected named A, B, C, D and E. The five grains were chemically analyses by microprobe along profiles as seen in Figs.
In general the magnetite is enrich in FeO, Al 2 O 3 , SiO 2 , CoO and Cr 2 O 3 (Table 8)

SPECTROMETRIC INVESTIGATION
The instrument used in the ground γ-ray spectrometric survey measurements is RS-230. Ground γ-ray spectrometric survey can detect dose rate (D.R.) in unit (nSvh -1 ), potassium (K%), equivalent uranium content (eUppm) and equivalent thorium content (eTh ppm). Uranium mobilization (eUm) in the studied rock types can be calculated as follows; the uranium mobilization is calculated by the difference between the measured eU and the expected original uranium, which is calculated by dividing the measured eTh by the average eTh/eU ratio in the crustal acidic rocks (original uranium = eTh/3.5 according In the grain no. B; the magnetite is enrich in FeO, Al 2 O 3 , SiO 2 , MgO, CoO, V 2 O 3 and Cr 2 O 3 (Table 4), whereas ilmenite in TiO 2 , MnO and ZnO (except spot no. 18).
In the grains no. D and E; the magnetite is   Table 6: M i croprobe analysis of magnetite sample no. D Table 5: Microprobe analysis of magnetite grain no. C to Clark et al., 1966) to give the leaching values of uranium (eUm= eU-eTh/3.5). Positive values indicate uranium addition by mobilization, whereas negative values indicated migration of uranium by leaching.
The eU contents in the studied peraluminous granites range between 0.6 and 3.20 ppm with an average of 1.96 ppm. The eTh contents range between 2.4 and 13.20 ppm with an average of 5.65 ppm, while the average of (eTh/eU) ratios are 3.32 ppm and (eU/ eTh) ratios are 0.42, and average of eUm is 0.35 indicating its magmatic origin (Table 9 and Fig. 40). The eU contents in the studied peraluminous pegmatites range between 6.2 and 11.9 ppm with an average 9.12 ppm. The    (Table 9 and Fig. 40).

GEOLOGY AND RADIOACTIVITY OF PERALUMINOUS GRANITE AND
The comparison between the radioelement concentrations in the peraluminous granites and peraluminous pegmatites at Um Regeba area relative to those of the crustal igneous rocks after IAEA (1979) and Boyle (1982) are listed in Table ( 9). It is noticed that the concentration of eU and eTh in the peraluminous granites at Um Regeba area are relatively lower than normal case as the corresponding values in the crustal average, whereas in the peraluminous pegmatites are higher than normal case as the corresponding values in the crustal average.  son et al., 1979). A poor negative relations (r= -0.10) exist between eTh and eTh/eU as shown on Fig. 42 indicating that uranium distribution in these rocks is not only controlled by magmatic processes but also by post-magmatic processes.
The peraluminous granites are emplaced into the older rocks of the area in a NNW-SSE trend-close to the major Nugrus thrust. The peraluminous granites are mainly composed of plagioclase (An 5-10 ), K-feldspar, quartz, biotite and muscovite. Allanite and zircon are common accessories.
The presence of cross-hatching (tartan pattern) as low-temperature transforming twinning in the studied peraluminous granites is consider as a product of a combination between albite and pericline twinning in peculiar relation. The chloritization and sericitization alteration are secondary textures in the granitic rocks arising from deuteric or later hydrothermal activity (Shelley, 1993).
The peraluminous pegmatites occur as pockets of variable dimensions vary from few centimeters to few meters in width and length, and mostly concordant with the common NW-trending of enclosing granites and other country rocks all over the area. The peraluminous pegmatites are coarse grained, composed essentially of K-feldspar, quartz, plagioclase (An 6-10 ), muscovite and biotite. Titanite, zircon, allanite and magnetite are common accessories.
Geochemically, the studied peraluminous granites and peraluminous pegmatites are monzogranite and syenogranite to alkali feldspar granite respectively. They have peraluminous character, calc alkaline and alkaline affinity respectively, emplaced in within plate setting, crystallized under water-vapor pressure (2-3 Kb) and temperature from 760 o to 800 o C, pertaining to the I-type granite originated by highly differentiated magma generated from upper mantle contaminated with some crustal materials. The studied peralu-87 GEOLOGY AND RADIOACTIVITY OF PERALUMINOUS GRANITE AND minous granite originated under high oxidation condition.
The magnetite mineralization was disseminated within the peraluminous pegmatites. The magnetite of Um Regeba area represents a late stage magmatic product. The pegmatite segregates magnetite crystals and form masses of magnetite, concentrated in response to fractional crystallization and this phenomenon is repeated in the peraluminous granites of W. El Gemal, especially in El Mokhattata area.
Magnetite showed lamellar intergrowth with ilmenite. Ilmenite is visible with a hand lens in most magnetite samples. The magnetite is enrich in FeO, Al 2 O 3 , SiO 2 , CoO and Cr 2 O 3 , whereas ilmenite in TiO 2 , MnO and ZnO. The values of V 2 O 3 and MgO fluctuated from one sample to the other, where CaO and NiO contents are negligible. The enrichment of magnetite and ilmenite by the previous elements due to a geochemical behavior (ionic substitution). The depletion in CaO due to Ca +2 ions cannot substitute for Fe +2 in the magnetite and ilmenite lattice because it is considerably larger than Fe +2 and also differ in physical properties. The depletion in NiO is due to the magmatic origin of the magnetite.
Radiometrically, the radioactivity increases from the peraluminous granites to the peraluminous pegmatites. The average eU content in peraluminous granites is less than twice the Clark value (4 ppm). The average eTh/eU ratio is 3.32; indicating the rock is not uraniferous, while the average eU content in the peraluminous pegmatites is more than twice the Clark value (4 ppm) indicating the rock is uraniferous.