GEOLOGY AND URANIUM, THORIUM DISTRIBUTIONS OF GABAL HUMR AR-RAHA AREA, NORTH EASTERN DESERT, EGYPT

Gabal Humr ar-Raha area, about 150 km2 of crystalline basement rocks, is delineated by Lat. 27o 18’ and 27o 26’ N and Long. 32o 42’ and 32o 48’ E. It is covered by Dokhan volcanics (oldest), younger granites (syenogranite) and post granitic dykes (youngest). Dokhan volcanics are represented by a thick sequence of stratified lava flows of intermediate (andesites) and acidic (rhyolites) composition. The main rock type of Gabal Humr ar-Raha granitoid rocks are classified as syenogranites; their emplacement were structurally controlled by pre-existing fractures trending NNE-


INTRODUCTION
The basement rocks of Egypt cover about 100 000 km 2 (about 10 % of the total area of Egypt). Granitoids constitute an important rock group covering vast areas of the Arabian-Nubian Shield. They cover about 38 % of the basement rocks in Egypt. In the north Eastern Desert of Egypt, the Pre-Cambrian rocks are dominated by Dokhan volcanics (D.V) and granitoids (Basta et al., 1980;Greenberg, 1981;Dardir et al., 1982;Stern and Gottfried, 1986;Wetait, 1997 andAyoub andMoharem, 2008).

AHMED A. ABU STEET
In spite of the accessibility of the present area, few workers had contributed to its regional geology. Hume, (1934) gave a generalized geological map of the Eastern Desert of Egypt. El-Ramly, (1972) compiled a regional geologic map for the basement rocks in the Eastern and south Western Desert of Egypt. Dardir and Abu Zeid, (1972) mapped an area between Lat. 27 o 00 ' and 27 o 30' N. Abu El Leil, (1980) described the geology of the northern part of the Eastern Desert of Egypt and classified the rocks as follows: Syn-orogenic stage (quartz-diorite), Late-orogenic stage (granites) and Post-granitic stage (dyke rocks). Ayoub, (2003) studied the petrogenetic implications of the Umm Tweir granitoid rocks and stated that the analyzed syenogranites fall within the calc-alkaline I-type granitoid field; and is associated with radioactive anomalies concentrated along fault planes and shear zones.
The present paper aims to give detailed geological and radioactive studies of the different rock types exposed at Gabal Humr ar-Raha area.

GEOLOGIC SETTING
Gabal Humr ar-Raha area covers about 150 km 2 of crystalline basement rocks, located west of Gabal Umm Tweir, along Wadi Giala and Wadi Lissan El-Baqara, about 150 km west Hurghada in the northern part of the Eastern Desert, delineated by lat. 27 o 18' and 27 o 26' N and long. 32 o 42' and 32 o 48' E ( Fig.1). The area is covered by Dokhan volcanics (oldest), younger granites (syenogranite) and post granitic dykes (youngest).

Lithology
The Dokhan volcanics (D.V) are widely distributed through G. Humer ar-Raha area and form an elongated belt of moderate elevation in NE-SW direction, rise up to 588 m above sea level (a.s.l.); as well as small isolated outcrops. They are represented by thick sequence of stratified lava flows of intermediate to acidic composition. They are associated with few intercalations of pyroclastics represented mainly by welded ash flow tuffs, lapilli and crystal tuffs as well as vitric ryholitic tuffs increasing westward. Lapilli and ash tuffs sometimes show graded bedding and lamination. The lava flows are pale grey to dark grey in colour and range in composition from andesite to rhyolite. Porphyritic texture is well developed in most of these rocks, while few of them are non-porphyritic and some are amygdaloidal. The amygdals are filled with quartz and carbonates. These rocks are well represented at Gabal Humr ar-Raha near the syenogranites where they change gradually upward from non-porphyritic andesite to porphyritic varieties. The main alteration processes affected these rocks are chloritization, sericitization and silicification.
Sometimes, they comprise lapillis of different sizes, shapes and composition. The Dokhan volcanics are particularly, unfolded but often tilted, broken and crushed especially near their contact with the younger granites and fault planes.
The younger granites are situated at the southern part of the mapped area. They form few individual bodies as stocks of low to moderately high relief. At south-east of G. Humr ar-Raha, the younger granites form an elongated shaped pluton with low to moderately hilly country of bright reddish pink colour. Their red colour is essentially due to their high contents of potash feldspars and the staining by secondary red hematitic materials particularly along their joints. They are commonly medium-to coarse-grained, hard, massive and compact, rich in quartz and potash feldspars but poor in mafics (they range in composition from varieties that contain no or little mafics to varieties carrying notable amounts of biotite).
These granites intrude the Dokhan volcanics with sharp intrusive contact (Fig. 2) and send few apophyses and tongues in some parts through them. They catch xenoliths of Dokhan volcanics of different shapes and diameters (Fig. 3) and sometimes the Dokhan volcanics overlying them as roof-pendants. They show well marked exfoliation essentially controlled by the topographic surfaces of rocks (Fig. 4).   Generally, these younger granites are characterized by cavernous and bouldery weathering (Fig. 5). They are porphyritic to equigranular and massive. They contain some quartz veins and feldspar filling joints, fault planes and fractured zones (Fig 6). These veins trending mostly in the NE-SW and E-W directions with vertical or steep dips. Moreover, these younger granites are characterized by the common presence of pegmatitic dykes and pockets (Fig. 7). The pegmatites are mainly of unzoned type and composed of milky quartz and reddish pink Kfeldspars with or without mica. Post granitic dykes (felsic and mafic) cutting all the previously described rocks and forming prominent high ridges trending along NE-SW. The felsic dykes are the older one, mainly striking in NE-SW and ENE-WSW directions with vertical or steep dips to NW and SSE and may extend for several kilometers. The mafic dykes strike in NE-SW with similar dips as the felsic dykes.

Structures
Primary and secondary structures are well visible in the study area. Primary structures include volcanic flows, graded bedding, lamination and bedding in Dokhan volcanics. Secondary structures include joints and faults.
Around G. Humr ar-Raha area, the main trend of the bedding plane of Dokhan volcanics varies from NNW-SSE to NE-SW, dipping westwards at angle ranging from 10 o to 15 o . The angle of dip is usually increasing toward the contact with the granite mass; it sometimes reaches up to 25 o at the contact zone due to the emplacement force.
Joints are recorded in all rock types. They are mainly tension and subordinate shear joints. The most common directions of joints in the Dokhan volcanics, in decreasing order of predominance are NW-SE, NE-SW and N-S and dipping 80 o -85 o to SW, 75 o -80 o to NW and 75 o -88 o to E respectively (Fig. 8a). In younger granites, the main joint trends, in   ;Shalaby, (1996) and Ayoub, (2003).

Dokhan Volcanics
The Dokhan volcanics are mainly andesites and rhyolites. Andesites are mainly porphyritic and composed of plagioclase, hornblende, biotite and small amount of quartz phenocrysts embedded in fine-grained groundmass. Apatite and iron oxides are accessory minerals while epidote, chlorite and clay minerals are alteration products. Plagioclase is of andesine composition (An35-45) as anhedral to subhedral phenocrysts exhibiting simple and lamellar twinning (Fig. 11). They are arranged in sub- parallel orientation, suggesting flow texture. These phenocrysts are partly altered to saussurite, chlorite and epidote (Fig.12). Hornblende occurs as subhedral prismatic phenocrysts that are strongly pleochroic from pale to dark green colour. Some hornblende crystals show simple twinning (Fig.13). Biotite flakes and quartz are present in small amounts commonly disseminated in the groundmass. Rhyolite is essentially composed of sanidine, quartz and muscovite. Zircon and iron oxides occur as accessory minerals. Sanidine occurs as phenocrysts usually twinned (Fig.14). Quartz crystals are highly corroded and invaded by the groundmass. Muscovite appears as irregular flakes corroded by quartz and feldspar phenocrysts. The Dokhan volcanic rocks have suffered from low grade regional metamorphism (lower green-schist facies) as indicated from the replacement of hornblende and biotite by chlorite, epidote and iron oxides.

Younger Granites
Modal analyses of the studied younger granites (Table 1) are plotted on Q-A-P diagram of Streckeisen (1976). These younger granites are plotted in the syenogranite field (Fig.15).
The syenogranites are mainly composed of potash feldspars, quartz, plagioclase, biotite and muscovite as essential minerals (Fig.16). Zircon, sphene, allanite and apatite are the main accessory minerals. Clay minerals, epidote, zoisite, iron oxides and muscovite are secondary minerals. Potash feldspars are represented by orthoclase and microcline perthites but orthoclase perthite is the predominant. They are generally of string, patchy and/or flame-like types. They are corroded by quartz and muscovite. Quartz occurs as subhedral to anhedral crystals of variable shapes and sizes. Some quartz crystals show undulose extinction, indicating high strain effects.   Fig.17: Zircon crystal surrounded by pleochroic halo, syenogranites, xpl  Plagioclase is found as subhedral to euhedral albite-oligoclase crystals (An 5-15 ) showing simple and lamellar twinning and partly altered to epidote and zoisite. Biotite occurs as small flakes that may enclose zircon crystals. Zircon is found as prismatic euhedral to subhedral crystals included in quartz and biotite. Some zircon crystals are occasionally surrounded by strong pleochroic halos due to radiogenic effects (Fig. 17). Sphene (titanite) occurs as sphenoid euhedral to anhedral crystals corroded by feldspars and quartz (Fig. 18). Apatite is found as minute euhedral prismatic and needle-like crystals included in feldspars, biotite and quartz. Allanite occurs as euhedral prismatic crystals showing strong pleochroism from buff to deep reddish brown (Fig.19). Iron oxides are found as small irregular patches.

RADIOACTIVITY
The radioactivity of the different rock types in the study area were measured for the equivalent uranium (eU) and equivalent thorium (eTh) in part per million (ppm) using portable gamma ray spectrometer UG-130. The ranges of normal radioactivity values for the different rock types are summarized in Table (2) and graphically represented on Figure (20).
From the field radiometric survey measurements and the distribution of radioactivity values, it is clear that the syenogranites and the acidic Dokhan volcanics possess the highest level of eU and eTh among the various rock types cropping out in the study area. The eU of the fresh coarse-to medium-grained syenogranite of G. Humr ar-Raha ranges between 6 pm and 14 ppm with an average 8.3 ppm and eTh from 22 ppm to 40 ppm with an average 30.7 ppm. The eU of the acidic Dokhan volcanics ranges between 5 ppm and 10 ppm with an average 6.9 ppm and eTh from 20 ppm to 28 ppm with an average 23.6 ppm ( Table 2). This indicates that, the studied syenogranites and acidic Dokhan volcanics are originated from highly fractionated U-rich magma and trapping high concentrations of uranium in accessory minerals.
The syenogranite of G. Humr ar-Raha hosts an interesting radioactive anomaly that is located in the southern part of G. Humr ar-Raha along a small tributary branched from the only accessible narrow wadi traversing this granitic pluton at its contact with the Dokhan volcanic rocks. It is essentially exhibited by a strongly sheared granitic zone (about 2x7m dimensions) following the intersection zone of two major strike-slip faults striking NNW- Table 2: Field gamma-ray radioactivity (ppm) for all rock types exposed in Gabal Humr ar-Raha area using a portable gamma-ray scintillometer (UG-130)

GEOLOGY AND URANIUM, THORIUM DISTRIBUTIONS OF GABAL HUMR
Th are incompatible trace elements with the major rock-forming minerals.
The results of the radiometric analysis of eU and eTh of the studied younger granites are given in Table (3). The relationship between eU and eTh may indicate the enrichment or depletion of U because Th is chemically stable. Normally, thorium is three times as abundant as uranium in all rock types (Darnley, 1982). When this ratio is disturbed, it indicates either depletion or enrichment of uranium (Cambon, 1994). A positive correlation between equivalent uranium and equivalent thorium for each rock type on the constructed diagram (Fig.21) revealing that their behavior was probably controlled by magmatic processes (Simpson et al., 1979).  SSE and NE-SW. The younger granites near this zone show 10 ppm eU and 25 ppm eTh, while at the radioactive zone it rises to 75 ppm eU and 90 ppm eTh reaching up to 90 ppm eU and 110 ppm eTh after digging for 20 cm, where numerous concentrations of dark brown spots of iron oxides are noticed. The granites at this zone are highly sheared, medium-to fine-grained and red to reddish brown colour due to the strong hematitization.
The analyzed grab sample that collected from this sheared granitic zone has average eU content 66.3 ppm and average eTh content 104 ppm, while the average eTh/eU ratio is 1.57 (Table 3). This indicates strong post-magmatic uranium enrichment in this location by hydrothermal solution. The extensive hematitization caused the adsorption of uranium from these circulating solutions.
From Tables (2&3), it is noticed that the Lab. measurements are usually similar to the field measurements suggesting that the UG-130 instrument is well calibrated and the field measurements are dependable.

Inter-Element Relationships
The increase in eU and eTh contents is parallel to the line of differentiation of these granites. This is expected because both U and The poor relationship between eU and eTh/eU (Fig.22) indicates that uranium distribution within these rocks is not only controlled by magmatic processes but also by secondary processes to a great extent, where the anomalous syenogranites show enrichment of uranium while the Dokhan volcanics show variable degrees of U-leaching. The positive correlation between eU and Zr (Fig.23) supports the concept that uranium was trapped in the accessory minerals as zircon. Accordingly, Humr ar-Raha granitic pluton is not only originated from U-rich magma, but also have suffered from secondary processes which added uranium in metamict zircon, allanite and iron oxyhydroxides to adsorb uranium from circulating solutions. eTh/eU ratios for the studied anomalous syenogranites are lower than that given by Tammemagi and Smith (1975) which range from 3 to 4 for the normal granites. This means the enrichment of uranium relative to thorium in the studied granites.   The obtained analysis clarified presence of the radio-elements (U and Th) in the structure of zircon (Fig.26).
The mineralogical studies reveal that the high uranium content in the anomalous syenogranites is related to their zircon and allanite contents. The anomalous syenogranite samples were subjected to heavy liquid separation and picking. The separated zircon forms euhedral to subhedral crystals (Figs.24&25) where their size ranges from about 30 to 140  (Fig.29). The studied allanite exhibits enrichment of light rare earth elements (LREE) indicating syngenetic allanite (El-Balakssy et al., 2011). The presence of Ce-rich allanite indicates much-localized remobilization and concentration of REE during a late hydrothermal alteration (Yuanming and Michael, 1991).

SUMMARY AND CONCLUSIONS
The field radiometric survey and structural studies of joints showed that their radioactivity is normal, whereas the radioactivity readings measured on joints filled with K-feldspars or strongly hematitized are relatively higher than the barren joints. This phenomenon is often attributed to the effect of K 40 isotope of the potash feldspars on the radioactivity of the rocks (Heinrich, 1958). On the other hand, iron oxides are sometimes abnormally radioactive due to their high ability to capture uranium from its circulating fluids (Rogers et al., 1978).
Concerning faults, it is observed that, the distribution of the majority of the radioactive anomalies is controlled by some major faults especially the NE-SW fault sets and their associated shear zones. They play an important role in controlling most of the recorded radioactive anomalies. This trend is considered as the most radioactive direction if compared with the other fault sets recorded in the study area. It is also observed that, the zone of intersection of faults striking NE-SW with those belonging to the NNW-SSE trend are relatively of high gamma radioactivity. The increase of radioactivity at the intersection zone of faults may be due to the brecciated and sheared zones which act as pathway for the circulating hydrothermal solutions bearing radioelements and the localization of radioactive anomalies.
The analyzed grab samples, collected from this sheared granitic zone have average eU content 66.3 ppm and average eTh content 104 ppm, while the average eTh/eU ratio is 1.57. This indicates strong post-magmatic uranium enrichment in this location by hydrothermal solutions. The enrichment of uranium by secondary processes than magmatic crystallization is usually controlled by structural elements. Faults, fractures and joints play an important role as they act as pathways or channels for the circulating hydrothermal solutions.
The poor relationship between eU-eTh/ eU indicates that uranium distribution within these rocks is not only controlled by magmatic processes but also by secondary processes to a great extent. The positive correlation between eU and Zr supports the concept that uranium was trapped in the accessory minerals as zircon. The mineralogical studies reveal that the high uranium content in the anomalous syenogranites is related to their zircon and allanite content.