CONTRIBUTION TO THE GEOLOGY, PETROLOGY AND AIRBORNE GAMMA-RAY SPECTROMETRY OF GABAL EL-MUEILHA GRANITIC ROCKS, SOUTHERN EASTERN DESERT, EGYPT

Gabal (G.) El-Mueilha Granitic rocks are represented by an oval shaped stock that intruding the surrounding older rock units including ophiolitic mélange, metavolcanics, older granitoids and younger gabbros and sending several offshoots into them. The study area is affected by several fault systems in NW-SE, NE-SW and N-S directions. Petrographically, these younger granites are mainly represented by muscovite granite (syenogranite) that shows different degrees of albitization especially at the peripheral parts of the stock beside the ferrugination and silicification. The main accessory minerals are cassiterite, fluorite, zircon, beryl, monazite, thorite and uranothorite which are responsible for the radioactive anomalies in G. El-Mueilha syenogranitic mass. The airborne gamma ray spectrometry shows high radioactive anomaly restricted to the younger granitic mass relative to the other surrounding rocks that have low to moderate radioactivity. The eTh/eU ratio of G. El-Mueilha syenogranite indicates U-enrichment especially during albitization processes. ISSN 2314-5609 Nuclear Sciences Scientific Journal 8, 5978 2019 http:// www.ssnma.com

Gabal El-Mueilha younger granitic mass is considered as a topographic mark in the study area (703 m a.s.l.).It is located in the southern Eastern Desert, between latitudes 24° 51 ' and 24° 56' N and longitudes 33° 58' 10'' and 34° 03' 50'' E (Fig. 1).The early comprehensive petrological studies and geological mapping have been carried out and the granitic mass of G. El-Mueilha was a part of those studies (e.g.El-Ramly and El-Far, 1955;Soliman, 1981;Obeid et al., 2001;Badran, 2009;Samaan, 2009 and others).The present study deals with the geology, petrography, mineralogy and airborne spectrometry of the basement rocks in G. El Mueilha area with special emphases on the granitic rocks to indicate the factors controlling distribution of uranium and thorium.The importance of this study area regarded to the high radioactive anomaly that appears on the airborne spectrometric maps and the presence of rare metals as well as tin and fluorine mineralization in the rock association around G. El-Mueilha younger granites.

GEOLOGIC OUTLINE
A geologic map (scale 1:50000) for the study area is constructed by the aid of landsat-8 image processing as well as field work and the airborne spectrometric data (Fig. 2).G. El-Mueilha is an oval-shaped stock of younger granites that characterized by high and rugged topography (703 m. a.s.l.), medium-to coarsegrained and light yellow to whitish buff or pink colours (Fig. 3).It is characterized by well-developed vertical and nearly horizontal joints as well as the characteristic spheroidal exfoliation and cavernous weathering (Fig. 4) especially at the northern and northeastern roots of the pluton.G. El-Mueilha younger granites intrude the surrounding older rock units, which are mainly metavolcanics and ophiolitic mélange especially at Talat Um Hawd (Fig. 5) and send several offshoots into them.Some huge offshoots from this pluton appears at El-Mueilha tin mine to the northwestern corner of the studied area (Fig. 2).
The shared contacts between G. El-Mueilha granites and the metavolcanics are sharp intrusive and dip gently away from the granitic mass indicating its downward enlargement (Fig. 6).Lithologically, these younger granites are mainly represented by muscovite   granite that shows different degrees of albitization especially at the peripheral parts of the stock.The rock is whitish pink in colour and medium-to coarse-grained with characteristic feldspars phenocrysts.Different stages of alteration in G. El-Mueilha younger granites are encountered especially along fault and shearing planes comprising ferrugination and silicification.The number of the granitic offshoots increases near their contacts (Fig. 7).Generally, El-Mueilha younger granites are intersected with numerous dykes of different thickness and attitudes; they range in composition from mafic to felsic ones (Fig. 8) where the majority of them are mafic and showing lower topographic features.The study area is affected by several fault systems in NW-SE, NE-SW and N-S directions that more or less coincide with most wadis in the study area.Along some faults of them, many quartz and quartz with fluorite mineralization encountered such as at wadi Um Dalalil northeast the granitic mass.

PETROGRAPHY
Gabal El-Mueilha younger granites are mainly represented by muscovite granite that exhibits holocrystalline hypidiomorphic granular texture.The modal analysis (Table 1 and Fig. 9) indicates that these younger granites fall in the syenogranite field of Streckeisen (1976).They enclosed several fresh plagioclase and quartz crystals in the form of poikilitic texture (Fig. 11).Quartz ranks second in abundance.It occurs as interstitial subhedral to anhedral crystals ranging in size from 0.2 mm to 3.5 mm across.Cracked quartz sometimes contains apatite, zircon, iron oxides and smaller plagioclase and quartz crystals as inclusions and is frequently dusted with iron oxides and clay minerals especially along peripheries and cracks (Fig. 12).Only few crystals show undulose extinction indicating that this rock is affected by stresses much lower than the other albitized younger granites.The cracked quartz crystals are very rare and are restricted to the fault planes.Plagioclase (An 15 -An 20 ) occurs as subhedral to anhedral prismatic twinned crystals that sometimes show deformed twin lamellae (Fig. 13).They are generally fresh clase, quartz and K-feldspars, muscovite and lepidolite.Cassiterite, beryl, fluorite, apatite, zircon, monazite, thorite and uranothorite are accessories.
Plagioclase is the most dominant mineral.It occurs as anhedral to subhedral prismatic crystals ranging in size from 0.3 mm to 2.6 mm across.Some plagioclase crystals corrode each other indicating more than one plagioclase generation.This could be proved by the cracking and glide twinning and the core altered as well as the overgrowth zoning of some plagioclase crystals, while others show clear simple and lamellar twinning without and sometimes are slightly to moderately altered especially in their cores and along cracks.Muscovite occurs as medium, yellow, elongated, fan-shaped or leaf-like anhedral to subhedral interstitial flakes between other silicates.In addition, it includes subhedral prisms of plagioclase that dusted with fine inclusions along cleavage planes.Muscovite flakes also corrode and embay plagioclase and perthite crystals.Some muscovite flakes show pleochroic haloes (Figs. 14 and 15).They appear to be squeezed and deformed due to tectonic stresses or secondary growths of feldspars and quartz crystals reflecting the primary nature of muscovite.In some cases, minute muscovite flakes appear to be of secondary nature that filling the cracks.The presence of primary muscovite as inclusions in quartz or as large elongated flakes may reflect the peraluminous nature of this granite (Read,1984).Biotite occurs as subhedral to anhedral flakes.They are generally bleached and show one set of cleavages that traced with opaques due to alteration.
Opaques, titanite, apatite, fluorite and zircon are encountered as inclusions in silicates especially perthites, micas and quartz.Subhedral fine fluorite crystals are scattered all over the rock where the violet fluorite is engulfing anhedral grains of monazite and/or zircon.Zircon occurs in a very minor amount as subhedral to euhedral prismatic crystals as inclusions in muscovite and quartz.Titanite appears as anhedral to subhedral brown crystals, usually altered to magnetite and ilmenite along peripheries and cracks.
Along the shearing planes, the granite components are highly altered and fractured where quartz grains exhibit wavy extinction and plagioclase shows kinking and bending of the twinning lamellae.

Albitized Syenogranites
The albitized syenogranites are similar in their mineral composition to the normal variety.They are mainly composed of plagio- any sign of alteration.Some crystals represent a younger phase corroding other minerals, especially altered potash feldspars and plagioclase.Other crystals show zoning (Fig. 16), indicating local albitization (Peterson and Eliasson,1997).
Quartz exhibits undulose extinction and is highly cracked and mylonitized (Fig. 17) indicating that they were subjected to high stresses (Read, 1984).The cracks are usually dusted with iron oxides and muscovite.Some quartz crystals show myrmekitic and graphic textures (Fig. 18) with feldspars, suggesting reaction during crystallization (Read, 1984).The alkali feldspar crystals and plagioclases are usually serecitized (Figs.19 and 20) and replaced with albite due to albitization processes.Brown cassiterite crystals are also recorded especially in the sheared and greisenized granite (Fig. 21).
All over the peripheral zones of the granitic mass especially at El Mueilha tin mine as well as the main fault planes, the granite shows greisenization (El-Mansi,1996) that being partly or completely transformed into yellowish green rock composed of quartz, muscovite, yellow lepidolite, cassiterite and fluorite.Relics of biotite are also found in the muscovite flakes.Shearing effect is indicated from the granitic rocks or the quartz veins from El-Mueilha Tin m i ne as translucent subhedral prismatic crystals with brown to reddish brown colour.Zoning is common in most of the detected crystals.Some bipyramidal crystals found in some thin sections.Cassiterite is also identified by ESEM technique.EDAX shows 80 % or more Sn-content and some sort of chemical variation of cassiterite crystals from different samples (Fig. 22).

Thorite and Uranothorite (Th,U)(SiO 4 )
Thorite and uranothorite are found as subhedral prismatic crystals of yellow to brownish yellow colour, translucent to opaque.Sometimes, uranothorite is found as minute grains and as inclusions in other minerals especially muscovite, fluorite, and quartz (Fig. 23).

Fluorite (CaF 2 )
Fluorite is found as irregular crystals of colourless to purple colour in thin sections.In the hand specimens, green and violet grains can be detected by hand lens.The variable colouration of fluorite was a matter of previous intensive studies that attribute this phenomenon to many reasons such as; impurities, REE contents, Y and Sr contents or/and radiation that cause physical distortion in crystal structure (Deer et al.,1992;El-Mansi, 2000;Raslan, 2009).Inclusions inside some fluorite grains show high U and Th contents.These inclusions are mainly of monazite, uranothorite and thorite (Fig. 24).This is confirmed by ESEM (Fig. 25).

Beryl [Be 3 Al 2 (SiO 3 ) 6 ]
Beryl is found as subhedral to euhedral colourless prismatic to tabular crystals (Fig. 26).Most of the defined beryl grains were found in the albitized samples and rarely detected in the non albitized syenogranites.Some equant crystals were also recorded in the albitized granites.
Fig. 21: Brown cassiterite crystals in greisenized granite, Gabal El Mueilha albitized granite, PPL by the bending of the twinning planes of plagioclase.Also, the crushing effect is mainly demonstrated along the crystal boundaries and by the undulose extinction of quartz.This indicates that the rock was subjected to shearing effects after the crystallization because the normal syenogranites do not suffer any valuable stresses.The micro-cracks are usually filled with quartz, iron oxides, muscovite and epidote.

Cassiterite (SnO 2 )
Cassiterite is found in thin sections either

Monazite (Ce, La, Nd, Th)PO 4
Monazite occurs in the studied granites as an accessory mineral associated with zircon and uranothorite.Here it is found as inclusions in muscovite flakes (Fig. 27).

Zircon (ZrSiO 4 )
Zircon occurs as subhedral to euhedral colourless prismatic crystals in thin sections.Some grains are metamicted due to radiation damage.It contains significant amount of Th due to the replacement of Zr by Th forming thorite (ThSiO 4 ).There is a clear intergrowth between zircon and thorite due to the close-up similarity of the ionic radii of Zr and Th (Om-Fig.27: Scanning electron microscope image and its EDAX table for zircon and monazite inclusions in muscovite flakes of Gabal El Mueilha granites

AIRBORNE GAMMA-RAY SPECTROMETRY
Airborne gamma-ray spectrometry originally developed as a tool for uranium exploration but this tool usage has been extended to include mineral exploration and geologic mapping (Grasty and Shives, 1997;Elsirafy et al., 1999: Fouad et al., 2004;Hyvönen et al., 2005;Khamies and Terras, 2009;Aboelkhair et al., 2014;Badr, 2017;Dawoud et al., 2017).Airborne gamma-ray spectrometry measures the abundance of potassium, thorium and uranium in rocks and weathered materials by detecting gamma-ray emitted due to the natural radioelement decay of these elements.The airborne survey of the study area was carried out by Aero-Service Division of the Western Geophysical Company of America in 1984.Spectral radiometric measurements were made using high-sensitivity 256-channel airborne gamma-ray spectrometer with a primary 50.3 liters sodium iodide, thallium activated (NaI "TI") detector array, (Aero- Service, 1984).
The data generated from the flight line profiles were corrected for background radiation, and for Compton scattered gamma-rays in the potassium and uranium energy window.The corrected data are processed using Geosoft Oasis Montaj software (version 8.4) resulting in filled coloured maps that are subjected to qualitative and quantitative interpretations.All the aero-spectrometric data (TC, K, eU, and eTh values) are multiplied by 10.However, the studied area shows a very high positive anomaly which attracted the attention of many researchers.

Qualitative Interpretation of the Airborne Gamma-ray Spectrometric Data.
Radioelements contour maps comprising (eU in ppm), (eTh in ppm), (K in %) in addition to the total count (TC in µr) are shown on Figures.28-31.The interpretation of radioelements distribution for mapping the surface geology is based on the fact that each rock type has its own rock-forming minerals with certain amounts, which comprise specific quantities of radioactive elements (Elawadi et al., 2004).The radioelements concentration can be attributed either to primary mineralization events or to secondary processes.The higher degree of weathering may produce a distinctive gamma-ray response compared with the surrounding bedrock (Wilford, 1992).

Radioelements contour maps
The equivalent uranium (eU) content reaches up to 140 ppm, as a maximum value over younger granites, and diminishes to 3 ppm as a minimum value over ophiolitic mélange (serpentinites and metasediments).The eU contour map (Fig. 28) can be classified into three levels.The first level expands from 15 ppm up to 140 ppm and is essentially associated with G. El Mueilha younger granites, some areas of the older granitoids and the metavolcanics where younger granitic offshoots occurred cutting through the older granitoids and metavolcanics.The intermediate level ranges f r om 10 to 15 ppm and is associated mainly with older granitoids, metavolcanics and wadi d e posits.The low levels with values less than 10 ppm, extend over the ophiolitic mélange comprising serpentinites, talc carbonates and metasediments.
The eTh contour map (Fig. 29) shows that both the normal and albitized syenogranites and small parts of the metavolcanics have h i ghest values in the study area, 222, 121 and 120 ppm, respectively.Besides, the eTh c o ntour map can be divided into three levels.The first level has eTh contents more than 35 ppm and is essentially associated with the younger granites and areas with offshoots from them cutting in the metavolcanics.The second level ranges from 25 to 35 ppm and is restricted to wadi deposits and older granitoids.The low-level is lower than 25 ppm, which is mostly associated with ophiolitic mélange and metavolcanics.
The K contour map (Fig. 30) shows the overall spatial distribution of the relative p o tassium concentrations in the study area.It indicates that serpentinites and related rocks represent the low level (less than 7 %); while t h e intermediate level ranges from 7 to 10 percent, which is associated with the older granitoids and metavolcanics.The high level (>10 %) is associated with younger granites The TC map (Fig. 31) shows three radiometric levels, high, intermediate and low.The h i gh level indicates values exceeding 35 µr.This level is observed over younger granites and the areas affected by their offshoots in the surrounding metavolcanics.The intermediate level ranges from 28 to 35 µr and is m a inly associated with the older granitoids and metasediments as well as wadi deposits.The low level, varies from 5 to 28 µr, and extends over ophiolitic mélange rocks especially serpentinites and metasediments rich in blocks and fragments of serpentinites.
From the above interpreted radioelements and TC coloured maps, it is clear that the ophiolitic mélange rocks (serpentinites and metasediments), the metavolcanics and younger gabbros exhibit low radioactivity levels in the study area.Meanwhile, the metavolcanics are of wide range in radioactivity due to their close vicinity to the younger granitic mass and the common presence of many off-shoots of the younger granites inside them as well as the younger granitic debris flown along wadis and tributaries surrounding G. El-Mueilha younger granites.The older granitoids are of intermediate radioactivity relative to the different rock units exposed in the study area.In general, the anomalous rock unit from the radioactivity point of view, is the younger granites of G. El Mueilha.

Composite images
The following composite images are developed by the USGS (Duval, 1983)  with few exception areas (metavolcanics with intensive granitic offshoots and wadis with younger granitic debris) such as the area to the northwest and southwest of G. El-Mueilha (Fig. 32).
The potassium composite image (Fig. 33) exhibits that the highest K-concentration (strong red) is only associated with the fresh granites of G. El-Mueilha whereas the albitized variety is relatively lower in K-content.Also, this image shows a minor spot with high K-concentration to the northwest of G. El-Mueilha younger granites around El-Mueilha tin mine where there are a lot of younger granitic offshoots cutting through the metavolcanics and/or the role of the hydrothermal solutions that responsible for forming tin mineralization.
The uranium composite image (Fig. 34) shows that the lowest eU-concentration (dark black) is associated mainly with mafic and ultramafic rock varieties of the ophiolitic mélange and younger gabbros as well as mafic metavolcanics (scattered all over the area especially north and west of the map) whereas the highest eU-concentration (white) associated with G. El-Mueilha younger granites.
The thorium composite image (Fig. 35) emphases the relative distribution of thorium and highlights area of thorium enrichment.The highest eTh-concentrations (red colour) shown at the southwestern periphery of G. El-Mueilha and associated with the albitized granites.However, the fresh granites are relatively lower in Th-content (magenta colour).corners of the map associating with the ophiolitic mélange rocks.

Quantitative I nterpretation of the Airborne Gamma-ray Spectrometric Data
Th/U ratios for granitic rocks are thought to be normal if they are within the range of 3 to 5, (Nash, 1979;Stuckless, 1979).The minimum, maximum and average of the K%, eU (ppm) and eTh (ppm) as well as their ratios eTh/eU of each rock unit exposed in the study area are shown in Table (2

CONCLUSIONS
ferrugination, silicification and albitization, which are restricted to the fault planes as well as the peripheral parts of the pluton along their contacts with the surrounding metavolcanics.The airborne Gamma-ray spectrometry shows a high anomaly restricted to the syenogranitic mass compared with the other surrounding rocks.Petrographic and mineralogical studies indicated that the high uranium content of G. El-Mueilha syenogranites is attributed to the presence of zircon, monazite, thorite and uranothorite.Beryl is found, for the first time in G. El-Mueilha granitic mass, in both albitized and non-albitized syenogranites.The equant variety of beryl only detected in the albitized syenogranites.Cassiterite and fluorite are not only encountered as discrete crystals in the main syenogranitic mass but also as their own mineralizations away from the mass where cassiterite mineralization occurs at El-Mueilha tin mine about 3 km northwest the main granitic mass and fluorine mineralization at Um Dalalil mine about 2 km northeast of the main granitic mass.The close relation of these mineralizations to the granitic mass suggesting that the magmatic and the subsequent alteration processes play the most important role in formation of these mineralizations.

Fig. 6 :
Fig.6: Sharp intrusive contact, between the albitized younger granites (AYGR) and the metavolcanics (MV), dips gently away from the core of the granitic mass, looking north

Fig. 7 :
Fig.7: Close up view exhibiting a younger granitic offshoot cutting in the metavolcanics

Fig. 30 :
Fig.30: Potassium contour map (K%) of Gabal El Mueilha area, Southern Eastern Desert, Egypt Fig.31: Total-count radiometric contour map (TC in µr) of Gabal El Mueilha area, Southern Eastern Desert, Egypt Fig.32: Radioelements composite image ternary map of Gabal El Mueilha area, Southern Eastern Desert, Egypt Fig.36: Bar diagrams for the minimum, maximum and averages of K%, eU, eTh and eTh/eU in the different rock types of G. El Mueilha area, Southern Eastern Desert, Egypt.(The measured K, eU, and eTh values are multiplied by 10)

eTh/eU Younger granites Albitized Syenogranites
The lowest eTh-concentrations (black colour) shown at the northeastern and northwestern Fig.35: Thorium composite image ternary map of Gabal El Mueilha area, Southern Eastern Desert, Egypt Rock type K % e U (ppm) e T h (ppm)

Table 2 :
K%, eU, eTh and eTh/eU of the different rock units, Gabal El-Mueilha area, Southern Eastern Desert, Egypt "The measured TC, K, eU, and eTh values are multiplied by 10"