An Extended Investigation of High-Level Natural Radioactivity and Geochemistry of Neoproterozoic Dokhan Volcanics: A Case Study of Wadi Gebeiy, Southwestern Sinai, Egypt

Abstract

High-level natural radioactivity, geochemical, geological, and radiological hazard assessment of the poorly investigated Wadi Gebeiy Dokhan volcanics rocks are discussed. Wadi Gebeiy Dokhan volcanics are located in Southwestern Sinai, Egypt, covering an area of ~1.3 km². Dokhan volcanics rocks are represented by porphyritic dacite. Geochemically, they have medium-k characters and originate from calc-alkaline magma within a volcanics arc environment. Along the fault plane striking NNE-SSW, and at its intersection with the NW-SE fault plane, altered Dokhan volcanics occur with high radioactive anomalies. Radiological parameters (absorbed dose rate, radium equivalent, activity annual effective dose, external and internal hazard indices) are used to evaluate their suitability as an ornamental stone. Except for the absorbed dose rate, all the radiological hazard indices show that unaltered Dokhan volcanics can be used as an ornamental stone. Controversially, the applied radiological indices reveal that altered Dokhan volcanics have a higher content than the recommended values of UNSCEAR, reflecting their risk on human organs.

1. Introduction

Egyptian Neoproterozoic crystalline rocks include dismembered ophiolitic suites, island arc assemblages that are followed by syn-post orogenic granites, and volcanic activity [1,2,3]. Volcanic rocks can be discriminated into three types: (a) older metavolcanics, (b) younger metavolcanics, and (c) Dokhan volcanics [3]. The former is commonly associated with ophiolitic rocks of the basaltic type, which can be formed in the Mid Ocean Ridge or in the suprasubduction zone [2]. In contrast, the younger metavolcanic range in composition from rhyolitic to basaltic is related to island arc setting. Both older and younger range are abundant in Egypt’s Central and South-Eastern Desert. On the other hand, the medium- to high-K calc-alkaline Dokhan volcanics are widely found in the North Eastern Desert as well as in South Sinai [3]. They can be further classified into older (630–623 Ma) and younger (618–592 Ma) volcanic activity [4]. The former ranges from deformed basaltic andesite and andesitic type, while the latter is rhyolitic to dacites in composition [5].

Dokhan volcanics are a thick sequence of stratified lava flows with an intermediate to acid composition, with subordinate sheets of ignimbrite and a few pyroclastic intercalations formed by fractional crystallization of basaltic magma generated by partial melting of the upper mantle under relatively high oxygen fugacity [6]. The Dokhan volcanics and Hammamat sediments are penecontemporaneous according to [7], and the Dokhan volcanics constitute the surface manifestation of the syn- to late-tectonic calc-alkaline granite series. Dokhan volcanics’ tectonic regime includes active continental margins [8], volcanic arc [9], and post-collisional setting [10].

On the other hand, natural radionuclides, including ²³⁸U, ²²⁶Ra, ²³²Th, and ⁴⁰K, are widely distributed in recent rocks, such as granites and post-collision rocks [11,12,13]. The radioactive-bearing minerals are commonly essentially ascribed to alteration processes (e.g., kaolinitization, seritization, silicification, and silicification), [12,13]. Radioactive minerals include primary uranium (e.g., pitchblende) and secondary (e.g., uranophane) mineralizations.

Here, detailed geological, mineralogical, geochemical, and natural radioactivity of Gebeiy Dokhan volcanics are discussed. In addition, radiological hazards of Dokhan volcanics are estimated in order to assess if they can be used in industrial applications.

2. Geological Setting

Egyptian Neoproterozoic basement complex constitutes the northern sector of the Arabian Nubian Shield (ANS), about 10% of the total area of Egypt, which crops out along the Red Sea (Eastern Desert), Uwainate area, and Southern Sinai [1,3,11,12,13]. ANS represents one of the best-preserved juvenile Neoproterozoic rocks (550–900 Ma), which developed during the closing of the Mozambique Ocean [1]. The Wadi Gebeiy area is located between latitudes of 28°32′–28°34′45″ N and longitudes of 33°34′–33°39′10″ E.

Metasediment−metavolcanic associations, older granites, Dokhan volcanics, and younger granites are among the Precambrian rock strata found in this area (Figure 1) [14,15]. Wadi Wirqa exposes the metasediments−metavolcanic association, which consists of hornblende schist and biotite schist intercalated with subordinate metaandesite, which forms moderate terrains. These rocks are weathered, jointed, cut by acidic and basic dykes, and dissected by strike-slip faults. Between Wadi Ramuz and Wadi Wirqa, older granites with gneissose tonalite and granodiorite are exposed. Gneissose tonalite is a hard, massive grey hue that is foliated mainly at the edges, is medium to coarse-grained, and forms moderate to high topographic relief. Acidic and basic dykes exfoliate, weather, connect, fault, and dissect granodiorite rock. Enclaves are found in abundance along with the metasediments−metavolcanic relationship.

The volcanic of Dokhan encompasses an area of around 1.3 km². The intrusion of sye-nogranite has ripped the rocks apart. The rocks are grey to pinkish-grey in tone, and sinistral and dextral slip faults have joined, crushed, faulted, and dissected them. Two conjugate fault planes strike NNE-SSW and NW-SE, respectively, and cut the rocks. Radioactive anomalies have been discovered along the NNE-SSW fault plane and at the intersection with the NW-SE trend (Figure 2a,b). Wadi Ramuz and Gebeiy have younger granites exposed. It has sharp intrusive contacts with granodiorite and metasediments−metavolcanic association and is medium to coarse-grained with a pink tint. It forms medium to high relief mountain terrains and has sharp intrusive contact with granodiorite.

3. Material and Methods

The major elements were identified using analytical methods [16]. The identity and concentration of the trace elements were determined using an X-ray fluorescence (XRF) instrument under the conditions of W-target tube, LIF-220 crystal, gas flow proportional counter, and scintillation counter 70 kV and 15 mA with a detection limit of 2 ppm. The analytical precision was ±2% and ±3% for the measured major and trace elements, respectively.

X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) were used to identify the minerals and their relative amounts. The XRD and EDX spectrometer units (used at 25–30 kV, accelerating voltage, 1–2 mm beam diameter, and 60–120 s as the counting time) were attached to an Environmental Scanning Electron Microscope (ESEM, model Philips XL30) to obtain several images of the analyzed grains.

To determine the concentrations of radioelements, 350 gm of the examined samples were packed in a plastic container and measured using a NaI (Tl) scintillation gamma-ray spectrometer with a crystal size of 76 × 76 mm (²²⁶Ra, ²³²Th and⁴⁰K). Following this, each sample was placed in a standard size plastic container containing the appropriate weight (300–350 g) (cylindrical in shape with a volume of 212.63 cm³, with an average diameter of 9.5 cm and a height of 3 cm). These canisters were meticulously sealed to keep the spectrometer clean. The radon in the sample, as well as the majority of its gamma radiation, were liberated during the homogenization process. To compensate for radon losses, samples were stored in sealed containers for at least 21 days before radon was restored to its original condition and a radioactive equilibrium between U, Th, and their short-lived daughter products was attained [17,18]. The relationship between the ²²²Rn accumulation percentage and the increase in time to reach steady-state was observed after about 38 days. The instrument was sealed with a photomultiplier tube in an Al-housing. The tube is protected via a 6 mm thick Cu-cylinder against X-rays and a Pb-brick shield against environmental radiation. The detector was enclosed in a cylindrical lead shield with a diameter of 15.7 cm, a length of 20.5 cm, and a thickness of 3.7 cm, with an attenuation factor of 0.16 (stopping at around 84 percent of input photons) for 2.6 MeV gamma rays, resulting in a low background measurement environment. The detection array was energy calibrated using ⁶⁰Co (1173.2 and 1332.5 keV), ¹³³Ba (356.1 keV), and ¹³⁷Cs (661.9 keV) standard sources. The efficiency calibration curve was made using IAEA-314 [19,20,21,22,23]. The background spectra were used to correct the net peak area of the gamma rays of the measured isotopes. ²²⁶Ra, ²³²Th, and ⁴⁰K had gamma energies of E_γ = 1764 keV (with I = 15.30%) from ²¹⁴Bi, E_γ = 2614 keV (with I = 99.754%) from ²²⁸Ac, and E_γ = 1460 keV (with I = 10.66%) from ⁴⁰K, respectively. For ²²⁶Ra, ²³²Th, and ⁴⁰K, the samples were counted for 2000 s, with MDAs of 2, 4, and 12 Bq/ kg, respectively. The error propagation law of systematic and random measurement errors was used to calculate the radiation uncertainty levels. The efficiency calibration had systematic errors of 0.5 to 2%, and the radioactivity readings had random errors of up to 5%. All laboratory work was completed at the Nuclear Materials Authority’s Central Laboratories (NMA).

4. Petrography

Petrographic investigations of Dokhan volcanics samples were carried out at NMA using a polarizing microscope (Olympus bx53).

Representative samples that were grabbed from Dokhan volcanics were investigated using a polarizing microscope in order to detect their mineralogical composition. Gebeiy Dokhan volcanics are fine-grained porphyritic dacites composed essentially of plagioclase, quartz, and K-feldspar as the phenocryst crystals, giving a porphyritic texture (Figure 2c,d). Plagioclase exhibit lamellar and zoned twinning. It varies from fine- to medium-grained. Occasionally, plagioclase reveals slight saussuritization, and is fractured and filled by secondary quartz. K-feldspar minerals are represented by sanidine, which reveals a turbid surface as a result of slightly too extensive kaolinitizaion and seritization. Quartz ranges from fine- to medium-grained. It shows normal extension. Zircon, opaques (iron oxides), and titanite (sphene) are accessories, while saussurite, epidote, and kaolinite are the main secondary minerals. Porphyritic and spherulitic textures are abundant in the examined rocks.

5. Results

5.1. Geochemical Characteristics of Unaltered Rocks

Gebeiy Dokhan volcanics were analyzed for major and trace elements (

Table 1. Whole-rock analyses (major (%) and trace elements (ppm) of the examined Dokhan volcanics rocks.

	Unaltered Volcanic Rocks					Altered Volcanic Rocks
SiO2	64.46	65.61	65.90	66.40	65.43	60.29	60.26	60.02
TiO2	0.30	0.35	0.28	0.25	0.29	0.25	0.26	0.26
Al2O3	15.69	14.60	15.70	14.10	15.67	16.95	16.58	16.21
Fe2O3	3.87	3.87	3.97	3.39	3.70	3.69	3.90	4.10
FeO	0.12	0.13	0.13	0.11	0.10	0.11	0.11	0.10
MgO	2.50	2.70	2.40	2.16	2.30	2.10	2.15	2.20
CaO	5.60	5.50	5.40	4.10	5.20	6.20	6.20	6.20
Na2O	3.16	3.06	3.30	3.40	3.26	3.78	3.29	3.59
K2O	2.33	2.13	2.60	2.80	2.50	4.17	4.07	4.37
P2O5	0.12	0.13	0.10	0.10	0.11	0.10	0.11	0.11
L.O.I	1.54	1.13	1.10	1.50	1.50	2.30	2.47	2.64
Cr	36.00	34.00	35.00	33.00	30.00	30.00	31.00	32.00
Ni	6.00	5.00	5.00	4.00	7.00	4.00	4.50	5.00
Cu	35.00	34.00	30.00	28.00	34.00	14.00	15.00	16.00
Zn	46.00	21.00	20.00	18.00	34.00	12,251.00	14,310.00	16,469.00
Zr	138.00	72.00	80.00	99.00	104.00	147.00	140.00	129.00
Rb	8.00	7.00	9.00	18.00	22.00	34.00	34.50	35.00
Y	3.00	2.00	4.00	5.00	5.00	15.00	15.50	16.00
Ba	27.00	44.00	40.00	38.00	67.00	2.00	2.00	2.00
Pb	11.00	8.00	7.00	6.00	6.00	658.00	799.00	938.00
Sr	375.00	578.00	510.00	420.00	201.00	307.00	294.50	282.00
Ga	19.00	26.00	22.00	18.00	21.00	20.00	18.00	16.00
V	22.00	27.00	25.00	20.00	19.00	33.00	19.50	6.00
Nb	9.00	8.00	10.00	12.00	3.00	31.00	30.50	30.00
U	8.00	6.00	4.00	5.00	3.00	161.00	175.50	190.00
Th	24.00	15.00	14.00	15.00	9.00	119.00	122.50	126.00

). The studied Dokhan volcanics rocks were classified as dacite (Figure 3a,b) according to the total alkali versus silica (TAS) diagrams [24,25]. Further constraints are that they straddle the boundary between younger metavolcanic and Dokhan volcanics fields that are established by [26]. The examined porphyritic dacites are medium-K (Figure 3c) volcanic rocks, as indicated by the SiO₂ versus K₂O diagram [27].

Trace elements of the studied Dokhan volcanics were normalized to a primitive mantle [28], which revealed (Figure 3d) strong Th, Pb, and anomalies. They exhibited an enrichment of significant ion lithophile elements (LILEs) relative to high field strength elements (HFSEs). Negative Ba and Nb anomalies were observed, reflecting feldspar fractionations. In addition, the examined rocks exhibited negative anomalies of Ti and P, suggesting apatite and titanium oxide fractionations.

Figure 3. (a,b) SiO₂ vs. Na₂O + K₂O diagram after [24,25], respectively. The curve that isolates sub alkaline from alkaline is obtained from [29]. Fields of Dokhan volcanics (DV), younger metavolcanics (YMV), and older metavolcanics (OMV) of Egypt are obtained from [26]; (c) SiO₂ vs. K₂O diagram [27] and (d) trace elements normalized to [28].

Figure 3. (a,b) SiO₂ vs. Na₂O + K₂O diagram after [24,25], respectively. The curve that isolates sub alkaline from alkaline is obtained from [29]. Fields of Dokhan volcanics (DV), younger metavolcanics (YMV), and older metavolcanics (OMV) of Egypt are obtained from [26]; (c) SiO₂ vs. K₂O diagram [27] and (d) trace elements normalized to [28].

5.2. Geochemistry of the Altered Rocks

Alteration processes refer to changes in the mineralogical and chemical composition of a rock by the action of hydrothermal solutions [12,13]. The alterations of the examined Dokhan volcanics along the faults led to the precipitation of radioactive minerals that geochemical studies can deduce. The altered Dokhan volcanics samples exhibited the addition of K (K-metasomatism) and the removal of silica (desilicification) according to the Na%-K% (Figure 4a) diagram [30], who classified the alteration types using an AKF ternary diagram into potassic (alteration of mafic mineral to muscovite), propylitic (containing epidote-chlorite alteration), and sericitic (containing k-feldspar converted to sericite). On the AKF ternary [31] diagram where A = Al₂O₃-(Na₂O + K₂O), K = K₂O, and F = Fe₂O₃^t + MgO + MnO, the altered samples occupied a propylitic field (Figure 4b). The chemical index of the alteration (CIA = Al₂O₃-CaO + Na₂O-K₂O) [32] was used to detect the total alteration/weathering of the examined rocks. The analyzed sample plot in the medium weathering field along with the muscovite-illite trend of the ideal feldspar weathering (Figure 4c) line (IWL), which is parallel to the Al₂O₃, CaO, and Na₂O trend towards illite, was due to post-depositional K-enrichment in the clay fraction, suggesting K-metasomatism [33].

From Table 1, it can be seen that the altered Dokhan volcanics samples exhibited depletions in the concentrations of SiO₂ (av. 60.19 wt.%), Cu (av. 15 ppm), Ba (av. 15 ppm), and Sr (av. 294.5 ppm) relative to those of the unaltered rocks. In addition, the altered rocks possessed a noticeable enrichment of K₂O (av. 4.2 wt.%), Zn (av. 1443 ppm), Zr (av. 139 ppm), Pb (av. 798 ppm), U (av. 175 ppm), and Th (av. 122 ppm), relative to those of the unaltered rocks. The enrichment of the Sr (av. 417 ppm) content (unaltered rocks) was related to the abundance of plagioclase minerals. The hydrothermal solution caused the alteration processes that led to the addition and removal of some elements and to the formation of new accessory minerals [33]. In addition, these solutions make changes in the chemical and mineralogical composition of the rock. The enrichment of Zn content (in the altered rocks) led to the formation of willemite and wulfenite minerals and to the formation of gale-na due to the high concentration of Pb.

6. Discussion

6.1. Tectonic Setting

The tectonic setting of Egyptian Dokhan volcanics has been a subject of debate [3,10,19]. The examined rocks have medium-K with Nb negative anomaly, reflecting a volcanic arc regime. The Binary Cr-FeO^t/MgO diagram illustrates that the studied rocks have an arc affinity (Figure 5a). By using the SiO₂-Nb discrimination diagram, the examined samples straddle the volcanic arc field (Figure 5b). From the previous geochemical characteristics, the examined volcanic rocks have volcanic arc signatures that were developed in a subduction-related setting.

6.2. Radiometric Measurements

The equivalent uranium (eU) contents of unaltered Dokhan volcanics range from 5 to 10 ppm (av. 7 ppm), while the equivalent thorium (eTh) contents range between 15 and 27 ppm, with an average of 19.0 ppm. In contrast, their U_ch contents range from 3 to 8 ppm (av. 5.2 ppm), and their Th_ch contents range from 9 to 24 ppm (av. 15.4 ppm), (

Table 2. Radiometric and chemical measurements of the natural uranium and thorium (U and Th) of the Wadi Gebeiy Dokhan volcanics.

	Unaltered Volcanic Rocks					Av.	Altered Volcanic Rocks			Av.
eU (ppm)	10	7	6	7	5	7	95	130	120	115
eTh (ppm)	27	16	16	21	15	19	150	165	158	157.67
(Ra) (ppm)	11	6	5	4	3	5.8	70	82	78	76.67
K%	0.16	0.1	1.2	1.1	1.34	0.78	2.2	2.3	2.24	2.25
Uch (ppm)	8	6	4	5	3	5.2	161	190	175	175.33
Thch (ppm)	24	15	14	15	9	15.4	119	126	122	122.33
eU/eRa	0.91	1.17	1.2	1.75	1.67	1.21	1.36	1.59	1.54	1.5
Uch/eU	0.8	0.86	0.67	0.71	0.6	0.74	1.69	1.46	1.46	1.52
eTh/ eU	2.7	2.29	2.67	3	3	2.71	1.58	1.27	1.32	1.37
eU/ eTh	0.37	0.44	0.38	0.33	0.33	0.37	0.63	0.79	0.76	0.73

). It is noticeable that U and Th are chemically lower than the radiometrically measured U and Th, which may be ascribed to the migration of uranium from these rocks to others.

On the other hand, radioactive anomalies are recorded along fault plane striking NNE-SSW fault planes as a result of alteration processes. We noticed that the chemical uranium (av. 115 ppm) is higher than the radiometric uranium, reflecting the presence of recent uranium deposits [13]. However, Th_ch (av. 122 ppm) is lower than eTh (157 ppm). The eU/eTh ratio is used as an indicator for uranium mineralization [13]. All of the examined samples contain eU/eTh higher than the recommended values of [36], suggesting uranium mobilization, especially in the shear zone. Further constraints are that the unaltered samples have a P-factor (eU/Ra) [13] more than unity in an average (1.21), and D-factor (U_ch/eU) [13,37] less than unity in an average (0.74), whereas the altered Dokhan volcanics contain eU/Ra and Uc/eU higher than one, reflecting the addition of uranium mineralization.

6.3. Radionuclides Activity and Radiological Hazards

Figure 6 shows the activity concentrations of U, Th, Ra, and K in Bq/kg, with 1 ppm U = 12.35 Bq/kg, 1 ppm Ra = 11.1 Bq/kg, 1 ppm Th = 4.06 Bq/kg, and 1% K = 313 Bq/kg [38]. According to [39], the average activity concentrations of ²³⁸U (86.8 Bq/kg), ²³²Th (76.76 Bq/kg), and ²²⁶Ra (64.38 Bq/kg) for the unaltered Dokhan volcanics are higher than the recommended worldwide values (33 Bq/kg for ²³⁸U, 32 Bq/kg for ²²⁶Ra, and 45 Bq/kg for ²³²Th). On the other hand, they contain low concentrations of ⁴⁰K (av. 244 Bq/kg) compared to [39] (412 Bq/kg), causing controversy. In comparison to the recommended worldwide values, the altered Dokhan volcanics contain high concentrations of ²³⁸U (1426 Bq/kg), ²³²Th (637 Bq/kg), ²²⁶Ra (851 Bq/kg), and ⁴⁰K (703 Bq/kg) on average [39]. If these rocks are used in industrial applications such as ornamental stone, and assessment parameters such as the absorbed dose rate (D), annual effective dose (AED), radium equivalent activity (Ra_eq), and external (H_ex) and internal (H_in) hazards are used to evaluate them (Figure 6).

The natural radionuclide contribution to the absorbed dose rate in air (D) is determined by the natural specific activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K. The absorbed gamma dose can be obtained for the radionuclide distribution in the air at 1m above the ground surface [40,41], using the following equation: D (nGy/h) = 0.462 ²²⁶Ra + 0.604²³²Th + 0.0417⁴⁰K. The unaltered samples have absorbed doses ranging from 69 to 124 nGy/h with an average of 86 nGy/h [42]. In contrast, the average absorbed gamma dose (807 nGy/h) value of the altered samples has a high concentration in comparison with unaltered samples and the recommended value.

The annual effective dose (AED) can be estimated using the absorbed dose values with a conversion factor of 0.7 Sv/Gy and an outdoor occupancy factor of 0.2 [39], as follows: AED (mSv/y) = D(nGy/h)/1000 × 8.76(h) × 0.2 × 0.7 (Sv/Gy). The mean AED value of the unaltered Dokhan volcanics is lower than the recommended value (0.48 mSv/y) of [39]. Controversially, the altered samples have an average AED (1 mSv/y) higher than the safety limit.

The radium equivalent (Ra_eq) index in Bq kg⁻¹ is a widely used radiological hazard index. It is a convenient index to compare the specific activities of samples containing different concentrations of ²²⁶Ra, ²³²Th (²²⁸Ra), and ⁴⁰K. It is defined by the assumption that 10Bqkg⁻¹ of ²²⁶Ra, 7 Bqkg⁻¹ of ²³²Th, and 130Bqkg⁻¹ of ⁴⁰K produce the same gamma dose rate [43]. The radium equivalent activity (Ra_eq) can be obtained using the following equation [42]: Ra_eq (Bqkg⁻¹) = ²²⁶Ra + 1.43²³²Th + 0.077⁴⁰K. The calculated Ra_eq of the unaltered samples (av. 193 Bqkg⁻¹) is less than the limited value (370 Bqkg⁻¹) of [39], whereas the altered samples have Ra_eq values (av. 1816 Bqkg⁻¹) greater than the limited value.

Further indices are used, as follows: The internal hazard index (H_in) is used to control the internal exposure to ²²²Rn and its radioactive progeny. The internal exposure to radon and its daughter products is quantified by the internal hazard index (H_in); the internal hazard index (H_in) can be further applied to determine the effect of dose rate on humans using the following equation: H_in = ²³⁸U/185 + ²³²Th/259 + ⁴⁰K/4810. It is noticeable that the mean values of H_ex and H_in for the examined unaltered rocks are lower than the worldwide average value (1) according to [39,43]. In contrast, the altered samples have Hex and H_in values (av. 6.46 and 10.31, respectively) greater than the limited value.

In addition, the external hazard index (H_ex) is used in order to assess the radiation dose, as follows [22,39,43]: Hex = ²³⁸U/370 + ²³²Th/259 + ⁴⁰K/4810. Among the radiological hazard indices applied (except absorbed dose), we can infer that the unaltered Dokhan volcanics can be used as an ornamental stone. On the other hand, all radiological parameters calculated show that the altered Dokhan volcanics cannot be used as ornamental stone due to their risk to human organs. The radionuclides inhalation, as well as exposure to high radioactive zone (rock) in particular for a long time, can lead to health risks including acute leucopenia, lung diseases, and anemia [39]. In addition, cataracts, anemia, and fracture of teeth are the main negative effects of radium exposure [39,42]. Furthermore, other diseases such as kidney cancers, hepatic bone, and pancreas diseases are due to exposure to thorium [39,42].

6.4. Mineralization

The identified minerals are classified into silicate and non-silicate minerals.

6.4.1. Silicate Minerals

Uranophane (Ca (UO₂) Si₃O₈ 2H₂O) is a secondary uranium mineral that is formed in oxidized zones. The authors of [44] reported that uranophane can occur as a more or less complete pseudomorph after uraninite crystals, and if the alteration is zonal, it constitutes the outermost aureole. It also can be formed chemically in suitable conditions from pH and Eh in hydrothermal solutions. The studied area is pale yellow to yellowish in color with disseminated and micro-fracture filling in patches, as confirmed by ESEM and XRD techniques (Figure 7a,b). It contains 85.56% UO₂, 7.05% CaO, and 6.16% SiO₂.

Zircon (ZrSiO₄) concentrations in the magma will increase gradually until the late stage of magma differentiation to form zircon. It exists as an accessory mineral in most plutonic rocks [26]. Zircon crystal shows different forms such as bipyramids, and short prisms with a brown or honey brown color may exist as normal or metamict. It is iso-structural with xenotime. Minor amounts of the REE are thought to enter the lattice of zircon through the isomorphous substitution of Y³⁺ + P⁵⁺ = Zr⁴⁺ + Si⁴⁺. It is confirmed by ESEM and XRD techniques (Figure 7c,d) and contains 43.13% of ZrO₂, 9.89% of ThO₂, 6.52% of UO_2, and 5.86% of Y₂O₃.

Willemite (Zn₂ SiO₄) is most abundant in the contact deposits; it is recorded along the fault plane and is detected by ESEM techniques (Figure 7e). It contains 26.64% of ZnO and 59.04% of SiO₂.

Fluor-phlogopite K(Mg, Fe)₃AlSi₃O₁₀(OH, F)₂ is reddish brown in color and is confirmed by XRD techniques in association with the fluorite mineral (Figure 7f). The formation of phlogopite could have originated from the parent magmatic biotite, which is destroyed in the deuteric-hydrothermal stage. Later, due to the presence of fluorine in the melt, it is converted into fluorine-rich phlogopite with smaller amounts of Ti, which react with magnetite to form titanomagnetite [45].

6.4.2. Non-Silicate Minerals

Fluorite (CaF₂) crystals have octahedral, cube-octahedral, or cubic habits and have a cubic symmetry. It has a variety of colors, including colorless, light violet, deep violet, and black. The coloration of fluorite can be caused by physical distortion in the crystal structure, radioactive inclusion, and radioactivity from associated radioactive materials or from the presence of trace and/or rare earth elements [46]. It contains 32.75% of F₂O, 58.73% of CaO, as determined by ESEM and XRD techniques (Figure 7f and Figure 8a).

Xenotime (YPO₄) is a widespread and important rare earth-bearing mineral, where Y can be substituted by U, Ca, and Si. It is considered an iso-structural mineral with zircon. Xenotime is the most common mineral in the system YPO₄-YAsO₄ (chernovite); YVO₄ (wakefieldite) all crystallize in the tetragonal system and xenotime is known to form at least a partial solid-solution series with chernovite [47]. Xenotime is a good host for rare earth elements, where it concentrates in the heavy rare earth [48]. It is yellowish-brown in color. It occurs as disseminated crystals and is confirmed by XRD techniques (Figure 8b).

Galena (PbS) has a cubic structure and often a cubic or cube-octahedral morphology and is a perfect cubic cleavage. It often occurs with quartz, calcite, sphalerite, and chalcopyrite. Its color is lead-grey, and its luster varies from metallic to faded metallic. The mineral is a major lead ore, but silver, bismuth, and thallium are also extracted from galena. It is confirmed by XRD techniques (Figure 8c).

Wulfenite (PbMoO₄) crystals appear as orange-yellow in color, and are equant with tabular form and pyramidal faces. Wulfenite is a secondary mineral formed in the ore deposits’ oxidized zone, which contains lead and molybdenum-bearing minerals [49]. It is confirmed by ESEM and XRD techniques (Figure 8d) and contains 48.28% of MoO₃ and 51.72% of PbO₂.

6.5. Origin of Uranium Mineralizations

The magmatic role can be seen in Dokhan volcanics through the positive relation between eU and eTh (not shown). The post-magmatic changes are clear along the fault plane, striking the NNE-SSW with intersection with the NW–SE fault [50], producing radioactive anomalies. The concentration of radioactive elements is attributed to the ascending of hydrothermal solutions and gases, carrying the radioactive elements and depositing them along the fault planes. This is indicated by the presence of secondary uranium minerals (uranophane) and violet fluorite, reflecting their hydrothermal origin [13,51].

7. Conclusions

Dokhan volcanics are extruded along Wadi Gebeiy, southwestern Sinai, Egypt. Porphyritic dacites represent these rocks, with a medium-K and calc-alkaline affinity. They are created by fractional crystallization within the volcanic arc environment. These rocks reveal K-metasomatism, propylitic, and desilisifications along the intersection of NW–SE faults and NNE-SSW faults, yielding radioactive anomalies. The identified minerals in the altered samples include uranophane, zircon, willemite, fluorite, xenotime, wulfenite, and galena. The corresponding radiological hazard results exhibit that the unaltered Dokhan volcanics can be used in industrial applications, whereas the altered one cannot be used as a result of its risk to humans. In contrast, the unaltered rocks can be used as ornamental stones in paving, flooring, statues, and funeral monuments due to their high durability.