Mineralogical and radiometric studies of granitic rocks in Wadi Sabbagh area, south Sinai, Egypt

Wadi Sabbagh area is located at southern part of Sinai Peninsula. It is constrained by Latitudes 28° 05´ and 28° 15´ N and Longitudes 34° 00´ and 34° 15´ E. Gneisses, migmatites, metasediments, syn-and post-orogenic granites, in addition to pegmatites and post-granitic dykes are the main rock units encountered in this area. Geological, petrological and geochemical studies indicate that there are two distinct suites of granitoids: older tonalite to granodiorite assemblage and younger two phases (monzo- to syenogranites and alkaline granites). The field radiometric survey and lab investigations indicate that, the anomalous uranium contents are mainly restricted to the pegmatites and altered monzogranites. The eU contents of pegmatites reach up to 168 ppm with an average 117 ppm, while the eTh reach up to 257 ppm with an average 175 ppm. The eTh/eU ratios range from 1.1 to 2.4 with an average 1.6. In the altered monzogranites, eU reachs up to 305 ppm with an average of 126 ppm. The eTh reaches up to 382 ppm with an average of 166 ppm. The average values of Ra and K are 60 ppm and 2.93 wt percentage, respectively. The average values eTh/eU ratios in the examined anomalies monzgranites (1.56) indicating addition of uranium. The high eU and eTh contents may be attributed to the secondary ascending hydrothermal solutions that lead to the accumulation of radioactive mineralizations mostly along fractures, faults and shear districts. The recognized radioactive minerals in the studied pegmatites are thorite, uranothorite, zircon, fluorite, columbite, samarskite, monazite, xenotime and allanite, whereas thorite, uranothorite, zircon and fluorite are the foremost radioactive minerals in the altered younger granites. Additional accessory minerals such as ilmenite, magnetite, hematite, pyrite and rutile were found in the studied pegmatites. Hematite, magnetite and pyrite are the foremost accessories accompanying the radioactive minerals in the altered younger granites.


Introduction
Wadi Sabbagh area occupies a part of south Sinai, Egypt figure 1, that featured by the presence of subparallel ridges and low to moderately elevated hills, mostly formed of older and younger granites. Gneisses, migmatites, metasediments, syn-and post-orogenic granites, in addition to pegmatites and postgranitic dykes are the main rock units encountered in this area. The younger granites, which crop out as elongated belt trending MW-SE and NW-SE directions, occupy about 93% of the total exposed area. Their contacts with all the surrounding rock units are generally sharp and well defined.
The present study deals with field, mineralogy and radioactivity of the younger granites and associated pegmatites in order to identify the radioactive minerals responsible for mineralization.

Figure1.
A) Simplified geologic map of the late Proterozoic rocks in south Sinai [1] inset a location map of the study area, B) Geologic map of Wadi Sabbagh area, southeastern Sinai, Egypt.

Field measurements
In situ gamma ray spectrometry measurements have been carried out using a GS-256 spectrometer (designed by Geofyzika Brno-Czech Republic) with a 3ʺ ×3ʺ sodium iodide (Thalium) [NaI (Tl)] crystal detector. The device, having an automatic dead-time correction and an internal 137 Cs source allows the spectrometer to automatically maintain system gain stability, is measured over a large body of water. Before field measurements, the spectrometer is calibrated on concrete pads containing known concentrations of U, Th and K. This calibration provides the stripping ratios and sensitivities required for correcting the measured eU, eTh and K.
The measurements are based on the detection of γ-radiation emitted in the decay of 214 Bi ( 238 U series) at 1.76 Mev, 208 Tl ( 232 Th series) at 2.41 MeV, and the primary decay of potassium 40 K (1.46) MeV is measured directly.
The determinations of uranium and thorium are based on the assumption that the daughter nuclides are in equilibrium with the parent nuclides that is none of the intermediate steps in the decay series has been disrupted. Consequently, the deduced amounts of uranium and thorium are equivalent to what would be in equilibrium with the measured radioactivity of the bismuth or thallium isotopes. Therefore, the results of our γ-ray analyses are expressed as equivalent uranium (eU) and equivalent thorium (eTh) [2].

Lab measurement
Representative grab samples were collected from the studied rocks of Wadi Sabbagh representing the highest values of anomalous field radioactivity. These samples were prepared for gamma-ray spectrometric analysis in order to determine their uranium, thorium, radium and potassium contents by using multichannel analyzer of gamma-ray detector (Gamma-Spectrometer technique). The instrument used in determination of the four radioactive elements consists of a Bicron scintillation detector NaI (Tl) 76x76 mm, hermetically sealed with the photomultiplier tube in aluminum housing. The tube is protected by a copper cylinder protection of thickness 0.6 cm against induced X ray and a chamber of lead bricks against environmental radiation. Uranium, thorium, radium and potassium are measured by using four energy regions representing 234 Th, 212 Pb, 214 Pb and 40 K at 93 kV, 239 kV, 352 kV, and 1460 kV for uranium, thorium, radium and potassium, respectively. The measurements were carried out in sample plastic containers, cylindrical in shape, 212.6 cm 3 volumes with 9.5 cm average diameter and 3 cm height. The rock sample is crushed to about 1 mm grain size, and then the container is filled with about 300-400 gm of the crushed sample sealed well and left for at least 21 days to accumulate free radon to attain radioactive equilibrium. The relation between the percentage of 222 Rn accumulation and time increase till reaching the steady stage after about 38 days [3]. Mineral separation and identification were carried out at the Laboratories of the Nuclear Materials Authority (NMA). The heavy liquids separation technique using bromoform of specific gravity 2.85 gm cm -3 was used to concentrate the heavy minerals. Then, the separation of magnetite was achieved by hand magnet. The heavy mineral fractions were passed through a Frantz isodynamic magnetic to separate the remaining magnetite and produce several magnetic fractions at 0.2, 0.5, 0.7, 1, and 1.5 amperes. Each of these fractions contained its own characteristic minerals. Mineral identification was performed through environmental scanning electron microscope (ESEM) and X-ray diffraction (XRD).

Field radiometric survey
The field radiometric survey stated that the exhibited rock-units of the studied area are commonly different in their radioactivity concentrations, amongst the granitic rocks, Wadi Umm Adawi monzogranites have the highest radioactive levels, varied between 1000 and 2000 cps. This reveals the function of post magmatic alterations, in addition to fracturing, joints and shearing enriched the radioactivity. The pegmatites exhibit high values of radioactivity (600 to 1200 cps.)

Lab radioactive survey 2.4.1 Older granites.
The older granites are relatively depleted in eU, eTh, Ra and K contents as compared with the younger granites, the average contents of eU in the tonalite and granodiorite are 3.4 and 5.2 ppm, respectively, whereas the average contents of eTh in these rocks are 8.2 ppm and 11.4 ppm, respectively table 1. All these averages are lower than twice the Clark value (< 8 ppm U and/or 20 ppm Th). The eU content of the two varieties of older granites rest inside the range of acidic intrusives (1-6 ppm) given by Adams et al., 1956 and the low content of K (1.6 and 1.2%) may be related to the low content of K-feldspar minerals in these rocks [4][5].

Younger granites.
Equivalent uranium content ranges from 4-10 ppm in the monzogranites, 5-14 ppm in syenogranites and from 7-15 ppm in alkaline granites (with an average 6.0 ppm, 9.8 ppm and 11.8 ppm, respectively), which are slightly higher than the average of granitic rocks (U; 4 ppm) [6]  than the average of granitic rocks (Th; 19 ppm), whereas syenogranites and alkaline granites average contents are slightly higher than the average of granitic rocks.

Dykes
The acidic dykes are relatively more enriched in the radionuclides, if compared with the basic dykes. They have eU average content reach up to 9 ppm with an average value 6 ppm, the eTh reaches up to 19 ppm with an average value 12 ppm and the average contents of Ra and K are 5 ppm and 3.7 %, respectively table 3. On the other hand, the average values of eU in the studied acidic dykes show slightly decrease in U contents, but increase in Th contents in comparison with West Dahab area [7] and it is in the range of acidic intrusive and effusive given by Adams et al., 1956 [8]  The field radiometric survey, using γ-gun spectrometry, showed two anomalous zones in the examined pegmatite pockets and veins hosted in syenogranites at the upstream of Wadi Umm Adawi and Wadi Umm Meter, in addition anomalous monzogranites at the downstream of Wadi Umm Adawi figure 1. Thirty representative samples (15 from the pegmatites and 15 from the monzogranites) collected from these regions for the radiometric measurements,

Anomalous pegmatites
The anomalous pegmatites are highly enriched in eU, eTh and Ra contents compared to their hosting syenogranites. Their eU content reaches up to 168 with an average of 117 ppm. The eTh content reaches up to 257 with an average of 175 ppm, whereas the Ra content reaches up to 148 with an average of 89 ppm (Table 4). These averages of uranium and thorium contents are higher than those of the world uraniferous pegmatites (av= 28 and 21 ppm for U and Th, respectively) [9]. These averages are also higher than those reported for the Egyptian uraniferous pegmatites (av= 33 and 28 ppm for U and Th, respectively), [10].    Figure 2, shows moderately positive correlation for older, younger granites and acidic dykes, while anomalous monzogranites and anomalous pegmatites show moderately to weak positive correlation between eU and eTh figure 2, indicating that the magmatic processes played an important role in the concentration of the radioelements in these rocks.

eU vs eU/eTh
The eU-eU/eTh variation diagram figure 3 clarifies that the eU/eTh ratios increase with increasing eU content, suggesting post magmatic redistribution of uranium [14], the older, younger granites and acidic dykes display weak positive correlation, while both anomalous pegmatites and monzogranites illustrate moderately positive correlation.

eTh vs eU/eTh
The eTh and eU/eTh binary relation figure 4 exhibit negative correlation, proposes that the distribution of uranium and thorium remained is partly controlled by magmatic processes, [14 and 15].

eU vs K
The eU-K variation diagram shows scatter relationship between the eU and K designates the redistribution and mobilization of uranium [14]. This variety of the inter-elements relationship recommends that the radioactivity could be linked to syngenitic and epigenetic processes.

Mineralogical studies
The mineralogical studies are carried out on some representative samples of the mineralized uraniferous pegmatite and monzogranite. These samples have high uranium and/or thorium.    contents. Twenty-one radioactive samples selected for mineralogical studies. The identification of the heavy minerals in these fractions carried out using X-ray diffraction (XRD) and scaning electronic microscope (SEM) technique The data of both XRD and EDX analyses manifested the presence of significant radioactive minerals in assosciätion with non-radioactive ones. The identified radioactive minerals in the anomalous pegmatites are thorite, uranothorite, zircon, fluorite, columbite, samarskite, monazite, xenotime and allanite. However, thorite, uranothorite, zircon and fluorite are the main radioactive minerals in the anomalous monzogranite samples. Other accessory minerals such as ilmenite, magnetite, hematite, pyrite and rutile were identified in the studied pegmatite. On the other hand, hematite, magnetite and pyrite are the foremost accessories accompanying the radioactive minerals in the anomalous monzogranites.

Radioactive Minerals
Thorite [ThSiO 4 ] mineral grains noted in the pegmatites and in the monzogranites, appears as subhedral to anhedral form varying in colour from brownish black to black with greasy luster figure 6A. figure 6B showed the XRD patttern of picked separated grains of thorite and were also confirmed by EDX analyses which reflecting the chemical composition of thorite figures. 7 and 8.   Uranothorite[(U,Th)SiO 4 ] is noted as subhedral to anhedral grains in the pegmatites, yellowish brown to brown with vitreous and resinous luster figure 9, and found in the monzogranites as inclusions in zircon and flourite. The uranothorite grains are highly metamicted, due to crystal lattice annihilation resulting from the influence of its own radioelement contents. Occasionally, the influence of this metamictization destroy the crystal lattice completely but leave the outward appearance unchanged. The existence of Y and P may assign to the assosciation of xenotime as solid solution with uranothorite, the obtained EDAX spectrum figures 10 and 11 reveals the chemical composition of uranothorite.

Radioelements-bearing minerals
Zircon [ZrSiO 4 ] is the most widespred mineral in pegmatite and anomalous monzogranite, it forms euhedral to anhedral prismatic grains, characterized by bipyramidal ends, varying in colour from yellow, yellowish red to reddish brown and sometimes has colorless to pale yellow colors figure 12. The X-ray diffraction pattern confirms that the picked zircon grains match with the ASTM card No. (6-0266) figure  12E. Also, Scanning Electron Microscope analyses carried out on particular zircon mineral grains picked from pegmatites and alterd younger granites, the back scattered electron microscope images of euhedral zircon grains and the EDX analyses confirm zircon composition [16]