Radiological Risk Parameters of the Phosphorite Deposits, Gebel Qulu El Sabaya: Natural Radioactivity and Geochemical Characteristics

Abstract

This study investigates the distribution of natural radioactivity and geological, geochemical, and environmental risk assessments of phosphorite deposits to determine their suitability for international applications (such as phosphoric acid and phosphatic fertilizers). The examined Late Cretaceous phosphorite deposits belong to the Duwi Formation, which is well exposed on the southern scarp boundary at the central part of Abu Tartur Plateau, Gebel Qulu El Sabaya, East Dakhla Oasis. This formation is classified into lower phosphorite, middle shale, and upper phosphorite members. The lower phosphorite ranges in thickness from 2 to 3.5 m and mainly comprises apatite (possibly francolite), dolomite, calcite, quartz, hematite, anhydrite, and kaolinite. They contain an average concentration of CaO (38.35 wt.%), P₂O₅ (24.92 wt.%), SiO₂ (7.19 wt.%), Fe₂O₃ (4.18 wt.%), MgO (3.99 wt.%), F (1.59 wt.%), Al₂O₃ (1.84 wt.%), Na₂O (1.33 wt.%), and K₂O (0.22 wt.%). Natural radioactivity and radiological parameters were investigated for fifteen samples of phosphorites using a NaI (Tl) scintillation detector. Absorbed dose rates, outdoor and indoor annual effective dose, radium equivalent activity, external and internal hazard, and excess cancer risk values are higher than the recommended levels, reflecting that exposure to these deposits for a long time may lead to health risks to human organs.

1. Introduction

The sedimentary, igneous, and metamorphic rocks are widely distributed and cover vast areas of the Egyptian Sinai, Eastern, and Western Deserts [1,2,3,4,5,6,7]. Egyptian phosphorite deposits belong to the Duwi Formation as a part of the Mediterranean phosphorite belt, which extends from the African (west) to the Asian (east) Arab countries. Egyptian phosphorite occurrences are subdivided by [8] into three belts, east-west trending. The economic occurrences are restricted to the central facies belt, confined to the following localities, the Red Sea coast from Safaga to Quseir; the Nile Valley between Idfu and Qena; and the Western Desert between Kharga and Dakhla oases (Abu Tartur area). The Phosphorite (Duwi) Formation is conformably overlain by the Dakhla Formation and is assigned to Upper Campanian to Early Maastrichtian age [8,9,10,11]. In the present study, the Gebel Qulu El Sabaya sector is extended east-west for about 13 km along the southern scarp boundary at the central part of Abu Tartur Plateau, located about 15 km north of the Dakhla–Kharga road (Figure 1).

In general, the lithostratigraphic section of Gebel Qulu El Sabaya is based on the Quseir shale Member of the Nubia Formation, overlain by the Duwi Formation and Dakhla Formation at the top. The Duwi Formation comprises three members; the lower one is the productive phosphorite. The lower member of the Duwi Formation in the Abu Tartur area represents a productive phosphorite bed with a thickness ranging from 0.7 up to 10 m, while the middle member is composed of black shale and siltstone with a thickness ranging from 12 to 28 m [12]. On the other hand, the upper member has three thin phosphatic bands intercalated with black shale with a total thickness ranging from 0.7 to 1 m. The Duwi Formation is unconformably overlying the Quseir shale member with an undulating surface where the non-oxidized phosphorite rocks were deposited in a reducing environment and changed to a yellowish-brown color by subsequent chemical oxidation processes [13,14]. The geochemistry of phosphorites and their main mineral constituent, francolite, have been widely studied due to their economic importance. Variability in the chemical composition of francolite may reflect the difference in original composition and modification during diagenesis [15]. All sedimentary phosphorites, everywhere in the world, are mainly composed of carbonate fluorapatite (traditionally known as francolite), but the fluorine content increases with age.

The phosphorite rocks were considered a natural source of uranium, where the uranium-bearing minerals were formed in the sedimentary phosphorite during deposition and digenesis by the seawater. Apatite is a common mineral that contains high concentrations of uranium, which range from 30 to 300 ppm, and may reach up to 500 ppm in phosphorite reworked in the marine environment. The main uranium components in phosphorite are U⁺⁴ and U⁺⁶. The tetravalent form is partially replaced by Ca⁺² in the apatite lattice because of their similar ionic radii, while the hexavalent one is enriched in the ore by adsorption onto the mineral surface [16]. Abu Tartur phosphorite contains the highest recorded concentration of REEs (up to 2000 ppm) [17]. They stated that the Abu Tartur phosphorite has the lowest uranium content at about 24 ppm, relative to El Sibaiya phosphorite at 85 ppm and the Red Sea phosphorite at 68 ppm. The rare earth elements (REEs) patterns of marine phosphorite are useful indicators of marine depositional environments [18]. However, REEs signatures can be altered by burial digenesis or by surface weathering [19].

This study deals with stratigraphical, petrographical, mineralogical, and geochemical investigations of the lower phosphorite member of the Duwi Formation at Gebel Qulu El Sabaya in order to identify the thickness and principal constituents of this phosphorite member. In addition, natural radioactivity and radiological parameters were applied to deduce their impact on human organs.

2. Materials and Methods

2.1. Geological Setting

The Western Desert covers more than 65% of Egypt, spreading from the Nile Valley to the Libyan border. It is a plateau desert with several oases, including Kharga, Farafra, Dakhla, and Beris. It initially developed during the Neotethys opening in the Late Triassic and Early Jurassic periods.

Phosphorite beds are exposed in the stratigraphic section of Gebel Qulu El Sabaya. Herein, the Duwi Formation unconformably overlies the Campanian variegated Quseir shale member of the Nubia Formation with undulating erosion contacts and is conformably overlain by the marine laminated Maestrichtian grey to black shale and marls of the Dakhla Formation (Figure 2). The Duwi Formation is made up of coarse-fine phosphorite grains intercalated with dolomitic phosphorite, glauconitic shale, and black shale lenses in the lower part and glauconitic shale in the middle, with hard dolomitic phosphorite beds intercalated with black shale bands in the upper part. An undulating surface and conglomerate bed characterize the unconformable boundary between the Quseir Member and the overlying Duwi Formation, like the Abu Tartur area [13,14]. The Duwi Formation in the studied area is classified after Issawi et al. [9,10] and [14] into lower phosphorite (I), middle shale (II), and upper phosphorite (III) members (Figure 2).

The lower phosphorite member consists of moderately hard oxidized phosphorite of yellowish-brown color intercalated with black shale lenses with an average thickness of 0.4 m. The total thickness of this member ranges from 2 to 3.5 m with an average of 2.5 m, which could be considered the economic phosphorite beds in the studied area (Figure 2).

The middle shale member is mainly composed of glauconitic shale, siltstone, and grey shale, characterized by cracking filled with gypsum and ferruginous staining. This member has a thickness range of 0.3 to 1.1 m with an average of 0.8 m (Figure 2).

The upper phosphorite member comprises hard dolomitic phosphate beds intercalated with grey-to-black shale lenses. Its thickness varies from 0.2 to 0.3 m, with an average of 0.25 m (Figure 2).

2.2. Petrography

The investigated phosphorite samples of the Gebel Qulu El Sabaya area microscopically show two principal constituents: phosphatic components (apatite, fish bone fragments, and shark teeth) and non-phosphatic components (dolomite, calcite, and iron oxides) as cement materials (Figure 3). The phosphate grains are generally angular to sub-rounded, elongated in shape, and yellowish brown in color (Figure 3a,b,d), which could be reflective of the effect of oxidation of pyrite mineral and the formation of iron oxyhydroxides [14].

Fishbone fragments are generally elongated to sub-angular in shape (Figure 3b). Bone fragments and teeth of different sizes and shapes are common as clastic allelochemical constituents. On the other hand, the non-phosphate cementing components are mostly composed of microcrystalline dolomite, glauconite, calcite grains, and iron oxides (Figure 3b,c). The replacement of phosphate grains by cementing material (dolomite rhombus) reflects the digenetic process effect (Figure 3c).

2.3. Sampling and Analytical Techniques

Fifteen representative samples were collected from Gebel Qulu El Sabaya sections to cover most of the lower phosphorite members at Gebel Qulu El Sabaya. The petrographic characteristics were microscopically investigated for five representative samples. These samples were prepared and subjected to complete wet chemical analysis to estimate their major oxides. At the same time, the trace elements were measured using an XRF (X-ray fluorescence) Philips X-ray spectrometer in the laboratories of the Nuclear Materials Authority. Rare earth elements (REEs) were analyzed using induced Couple Plasma (ICP). The concentrations of uranium (ppm), thorium (ppm), radium (ppm), and potassium (%) in different rock types were detected γ-spectrometric by using a multi-channel analyzer with a 76 × 76 mm NaI detector [20]. These samples are grinded and preserved in plastic containers for at least 21 days to accumulate free radon and other isotope emissions. The stability spectrometer channels change due to variations in power supply and amplification of the photomultiplier tube.

An empty sealed beaker with the same geometry as the samples was used to evaluate the background around the detector. With 12 h of measurement time, the Genie 2000 (Canberra) software was used to analyze the spectrum. The ²²⁶Ra activity was evaluated using γ-peaks of ²¹⁴Pb (0.3519 MeV (36.7%)) and of ²¹⁴Bi (0.6093 MeV (46.1%), 1.1203 MeV (15%), 1.7286 MeV (3.05%), and 1.764 MeV (15.9%)). The ²³²Th activity was estimated using γ-peaks of ²²⁸Ac (0.9112 MeV (29%)), ²¹²Pb (0.2386 MeV (43.6%)), and ²⁰⁸Tl (0.5831 MeV (84.5%). The ⁴⁰K activity was estimated using a self γ-peak of 1.461 MeV (10.7%).

2.4. Energy and Efficiency Calibration

Variations influence the stability of the energy of the spectrometer channels in the power supply, voltage, and photomultiplier tube amplification characteristics. The characteristics of the power supply and the photomultiplier are impacted by temperature. To ensure that the instrument accurately records the gamma radiation energy of the radioactive elements, a permanent calibration is performed using radioactive calibration sources like ¹³⁷C (made in channel 662) and ⁵⁷Co (122.1 keV, manufactured in channel 122): First, the ¹³⁷Cs source is used for gain adjustments before the ⁵⁷Co source is used for zero alterations, and second, the ¹³⁷Cs source is usually used as a minimum technique.

The efficiencies was estimated experimentally using standard sources (IAEA-314) with particular activity for Radium-226 (732 Bqkg⁻¹) and Thorium-232 (17.8 ppm). According to an academic program in MATLAB, the absolute efficiency at any interesting energy varied from 10 to 1764 keV. For Quality Assurance, the uncertainty of activity u(A) was evaluated using the following formula.

$$u\left(A\right)=A\sqrt{[\frac{u\left(N_p\right)}{N_p}{]}^2+[\frac{u\left(\eta \right)}{\eta }{]}^2+[\frac{u\left(m\right)}{m}{]}^2+[\frac{u\left(P\gamma \right)}{P\gamma }{]}^2}$$

where u(Np), u(η), u(m), and u(Pγ), respectively, stand for the absolute efficiency for each gamma line discovered in the same number of channels in the sample, sample mass, and absolute transition probability of decay. Computer “analysis” software calculated the uncertainty of each individual net-peak area.

3. Results and Discussion

3.1. Geochemical Characteristics

3.1.1. Major Oxides

The collected phosphorite samples of the Gebel Qulu El Sabaya area were subjected to chemical analysis to determine the content of the major oxide (

Table 1. Whole rock {major (wt.%), trace (ppm), and REEs (ppm)} analysis of Gebel Qulu El Sabaya phosphorite samples.

S.NO.	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
CaO	37	38	39	41	38	36	40	37	38	39	38	37	38	39	41
P2O5	22	24	26	27	24	23	27	23	26	25	25	24	25	25	28
SiO2	9	8	6	5	8	9	6	9	7	6	7	7	7	7	6
Fe2O3	5	4	4	4	5	5	4	5	4	4	3	5	3	5	3
MgO	6	5	3	3	5	5	3	5	3	4	3	4	3	4	3
F	2	1	2	3	1	2	1	1	1	2	2	1	2	1	2
Al2O3	2	2	2	2	2	2	2	2	1	2	2	2	2	2	2
Na2O	1	1	2	1	1	1	1	1	1	2	1	1	2	1	1
K2O	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
L.O.I.	10	9	11	12	11	7	15	9	10	7	9	16	12	14	11
Cd	12	11	9	10	8	11	7	5	4	8	7	11	9	14	18
Cr	111	113	75	91	153	145	93	76	141	77	98	113	122	157	114
Cu	30	33	32	29	27	28	24	10	26	29	26	27	11	19	24
Ni	41	47	39	40	42	44	43	12	44	39	43	45	18	42	39
Zn	73	198	159	156	158	149	152	69	127	95	157	201	143	179	181
Pb	2	6	10	9	5	7	3	2	6	11	3	6	3	9	12
U	33	29	35	35	30	26	22	27	36	24	27	29	32	30	28
Y	615	611	265	236	228	217	211	255	100	218	268.	548	603	611	605
La	240	208	285	301	276	271	247	161	83	298	280	221	254	250	216
Ce	554	549	543	533	543	555	535	540	540	526	542	546	552	550	549
Pr	45	55	63	55	62	64	45	66	48	55	64	53	54	46	54
Nd	288	273	283	220	298	218	290	215	230	283	280	268	283	285	258
Sm	85	118	90	60	87	56	63	51	54	45	55	42	130	77	84
Eu	18	15	10	18	15	18	10	12	18	17	15	18	15	18	10
Gd	75	78	70	81	78	76	79	75	68	93	88	78	82	73	78
Tb	7	5	9	5	8	5	9	5	8	7	5	7	5	9	8
Dy	65	58	65	78	60	85	53	48	65	65	63	68	73	78	70
Ho	5	5	5	8	5	8	8	5	8	5	5	5	5	8	5
Er	33	25	35	28	30	35	33	38	33	43	27	28	40	37	35
Tm	5	5	3	8	5	8	8	5	8	3	5	10	5	8	5
Yb	41	42	38	35	45	38	33	38	34	33	35	40	37	33	32
Lu	3	5	3	7	3	8	3	9	3	7	3	9	3	8	5
TREEs	1464	1441	1502	1437	1515	1445	1416	1268	1200	1480	1467	1393	1538	1480	1409

and Figure 4). Results show that the studied phosphorite deposits are dominated by CaO (36–41 wt.% with an average of 38.35 wt.%), P₂O₅ (22–27.4 wt.% with an average of 24.92 wt.%), SiO₂ (5.4–9.2 wt.% with an average of 7.19 wt.%), MgO (3–3.4 wt.% with an average of 3.99 wt.%), and Fe₂O₃ (3–4.8 wt.% with an average of 4.18 wt.%).

The average content of Al₂O₃ and F is 1.84 wt.% and 1.59 wt.%, respectively. However, Na₂O and K₂O are minor constituents, showing averages of 1.33 wt.% and 0.22 wt.%, respectively. Traditionally, CaO and P₂O₅ are the main components of apatite minerals, while Fe₂O₃ and MgO content is related to the presence of hematite and dolomite minerals, respectively. On the other hand, the observed variation in the P₂O₅ content from one sample to another (22–27.4 wt.% P₂O₅) could reflect the digenetic and geochemical processes that cause limited replacement of P₂O₅ by other components such as SiO₂, MgO, and Fe₂O₃. The microscopic examination indicated the replacement of phosphate particles by cementing materials. This reduces the grade of the phosphorite. Generally, the P₂O₅, Fe₂O₃, and MgO contents are widely used for quality evaluation in the industry.

Phosphorite ore deposits into three grades: high (with more than 27 wt.% of P₂O₅), medium (27 to 23 wt.% P₂O₅), and low (with less than 23 wt.% of P₂O₅) [21]. According to that classification, the studied phosphorite of Gebel Qulu El Sabaya is considered a medium grade (an average of 24.92 wt.% P₂O₅). This phosphorite, compared with other Egyptian phosphorites, is nearly similar in P₂O₅ relative to Abu Tartur [12] and Sibaiya phosphorites according to [22] and higher in Fe₂O₃ and MgO content (

Table 2. Comparison of the examined major oxides (wt.%) of Qulu El Sabaya phosphorites with some Egyptian phosphorites.

Areas	CaO	P2O5	SiO2	Fe2O3	MgO	Al2O3	Na2O	K2O
Gebel Qulu El Sabaya (present study)	38.35	24.92	7.19	4.18	3.99	1.84	1.33	0.22
El Ziat phosphorite [19]	38.72	24.75	7.3	3.38	3.4	1.7	0.92	0.2
Eldehous phosphorites [10]	35.3	22.7	10.6	4.8	6.9	1.9	1.8	0.2
Abu Tartur [13]	40	25	5	3	3	2	0.6	0.14
Sibaiya Phosphorite [13]	36	25	18	2.23	2.4	1.79	0.88	0.22

). This variation may be due to a slight difference in the depositional environment or geochemical process.

3.1.2. Trace Elements

The concentration of trace elements in Gebel Qulu El Sabaya phosphorite samples is shown in (Table 1, Figure 5a). Cd (ranges from 4 ppm to 18 ppm with an average of 10 ppm), Cr (from 75 ppm to 757 ppm, avg.= 112 ppm), Pb (2–12 ppm, avg.= 6 ppm), Cu (10–33 ppm; avg.= 25 ppm), and Ni (18–47 ppm, avg.= 39 ppm). They reveal a wide variation, particularly in Zn content, in the studied phosphorite samples, ranging from 69 ppm to 201 ppm with an average of 146 ppm. Zn content is attributed to the presence of the sulfate phase in phosphorites [23].

3.1.3. Rare Earth Elements (REEs)

The geochemistry and economic importance of REEs in the Western Desert phosphorite were first considered by El-Kammar [24], who reported REEs abnormality in Abu Tartur phosphorite. Then, Dardir and Kobtan [25] reported on the economic potentiality of REEs in Abu Tartur phosphorite (about 1718 gm/ton, on average). Amin and Zidan [17] stated that the average REEs content in Abu Tartur phosphorite is 2000 ppm. In addition, REEs contained in phosphorite may be extracted as a by-product during phosphoric acid production [13]. Gebel Qulu El Sabaya phosphorite samples exhibit total REEs content ranging from 1200 to 1538 ppm with an average of 1430 ppm (Table 1). Furthermore, the increase in REEs concentration from the west to east is most likely related to depositional and diagenetic factors. On the other hand, the average total REEs content recorded in the studied phosphorite is nearly like that of Abu Tartur phosphorite at 1407 ppm [25] and higher than that recorded in Eldehous phosphorite at 616 ppm [26].

Normalization of REEs to the Post Archean Australian Shale (PASS) values [27] is shown in Figure 5b. The normalized REE pattern shows that the Gebel Qulu El Sabaya phosphorite is characterized by a slight negative Eu anomaly (Figure 5b). El-Kammar and El-Kammar [28] stated that the Dakhla phosphorite could generally be described as Ce and Eu deficit. Additionally, the plots show that Gebel Qulu El Sabaya phosphorites have a very similar REEs pattern to Abu Tartur phosphorites (Figure 5b). This non-conventional distribution is not comparable to seawater and many other marine phosphorites, characterized by large negative Ce anomalies [29,30,31,32,33,34].

Accordingly, the authors believe that seawater is not the only ultimate source of REEs in the studied phosphorite. This behavior could be related to the influence of heavy detrital minerals like monazite admixed with the studied phosphorite. Similar behavior was previously described in East Sibaiya phosphorite [35] and Gabal Tarawan phosphorite of Kharga Oasis [26]. Hassan and El-Kammar [36] stated that the shallow nature of the depositional basin of Abu Tartur phosphorite maintained a higher influx of heavy detrital minerals than other Egyptian phosphorites. They added that this case seems to apply to most Western Desert phosphorites.

3.2. Radioactivity Concentrations

Radiometrically, it is found that the equivalent uranium content (eU) in Gebel Qulu El Sabaya phosphorite ranges from 28 ppm to 42 ppm with an average of 36 ppm, which is slightly higher than the chemical uranium (cU), which attains values ranging between 22 ppm and 36 ppm with an average of 30 ppm. However, the Ra content ranges from 33 to 44 ppm with an average of 38.4 ppm (

Table 3. Radiometric measurements (ppm) of the examined phosphorites compared with other phosphorites.

S.NO.	eU	eTh	eRa	eU/eTh	eU/eRa	cU/eU	cU
1	39	1	40	39	1	1	33
2	40	5	41	8	1	1	29
3	41	6	44	7	1	1	35
4	31	4	33	8	1	1	35
5	32	5	38	6	1	1	30
6	32	8	37	4	1	1	26
7	31	2	36	16	1	1	22
8	35	7	36	5	1	1	27
9	42	1	44	42	1	1	36
10	28	2	33	14	1	1	24
11	38	1	39	38	1	1	27
12	36	5	38	7	1	1	29
13	31	4	33	8	1	1	32
14	41	3	43	14	1	1	30
15	40	2	41	20	1	1	28
Min.	28	1	33	4	1	1	22
Max.	42	8	44	42	1	1	36
Average	36	4	38	16	1	1	30
El Ziat phosphorite [19]	27	-	25	-	-	-	-
Eldehous phosphorites [10]	35	-	35	-	-	-	-
Abu Tartur phosphorites [13]	23	-	19	-	-	-	-
Sibaiya phosphorites [13]	85	-	89	-	-	-	-
Red Sea phosphorites [13])	68	-	69	-	-	-	-

). Uranium enrichment in phosphorite may be ascribed to apatite lattice and/or adsorbed on the cement [37]. The geochemical relations of U with some major and trace elements of Sinai phosphates mentioned that U is associated with carbonate-apatite and was derived from seawater (i.e., syngenetic origin) [38]. The average radiometric uranium content of the studied phosphorite at 35 ppm is higher than that of El Ziat phosphorite (27 ppm) [19] and Abu Tartur phosphorite (23 ppm) [13]. On the contrary, it is lower than those of Sibaiya phosphorites [13], Red Sea phosphorites [13], and Morocco phosphorites [20]. The eU/eTh ratio is widely used as a geochemical indicator for uranium migration and re-mobilization. This ratio ranges from 4–42, reflecting a high degree of uranium mobilization. In addition, P-factor (eU/Ra) [39] as well as D-factor (Uc/eU) [40] are used for uranium migration and re-mobilization. eU/Ra and Uc/eU ratios of the examined phosphorites are less than unity (avg. = 93 and 0.83, respectively), reflecting the removal of uranium. However, uranium mobilization can be calculated using the [41] equation (eUm = eU-eTh/3.5). The results are more than one (av. 9.81), suggesting U enrichment. ²³⁸U, ²³²Th, ²²⁶Ra, and ⁴⁰K (Bq/kg) activity concentrations for the examined Qulu El Sabaya phosphorites are given in Figure 6. This figure reveals the measured activity concentrations (Bq/kg) of 15 samples that were measured radiometrically using the NaI technique. It is noticeable that the studied rocks show a wide variation in their ²³⁸U (av. 462.93 ± 55.76 Bq/kg), ²³²Th (av. 12.12 ± 3.3 Bq/kg), and ²²⁶Ra (av. 432.9 ± 47.96 Bq/kg) concentrations, which are higher than the recommended values of UNSCAR [42,43].

3.3. Radiological Risk Parameters

From ²³⁸U, ²³²Th, ²²⁶Ra, and ⁴⁰K (Bq/kg) activity concentrations for the examined Qulu El Sabaya phosphorites, we computed the radiological hazards indices given in

Table 4. Variable radiological parameters of Gebel Qulu El Sabaya phosphorites.

	Um	Absorbed	Hin	Hex	AEDout	Raeq (Bq/kg)	ELCR
S.No		nGy/h			(mSv)
1	10.86	223.78	2.69	1.39	0.27	473.88	0.96
2	10	242.51	2.84	1.50	0.30	512.91	1.04
3	10	249.22	2.91	1.54	0.31	549.57	1.07
4	7.71	189.20	2.21	1.17	0.23	413.51	0.81
5	7.71	197.22	2.29	1.22	0.24	474.79	0.85
6	6.86	205.30	2.33	1.26	0.25	481.02	0.88
7	8.29	187.76	2.19	1.15	0.23	442.49	0.81
8	8	218.60	2.52	1.35	0.27	464.14	0.94
9	11.71	239.78	2.90	1.49	0.29	518.28	1.03
10	7.43	167.82	1.97	1.03	0.21	401.96	0.72
11	10.57	218.45	2.63	1.35	0.27	462.78	0.94
12	8.86	221.31	2.57	1.36	0.27	479.85	0.95
13	7.71	189.20	2.21	1.17	0.23	413.51	0.81
14	10.86	239.83	2.86	1.49	0.29	518.73	1.03
15	10.86	231.81	2.78	1.44	0.28	490.76	1.00
Min	7.71	189.20	2.21	1.17	0.23	413.51	0.81
Max	10.86	239.83	2.86	1.49	0.29	518.73	1.03
Av.	9.81	220.28	2.61	1.36	0.27	474.33	0.95
Sd	1.48	22.22	0.29	0.14	0.03	44.50	0.10

Note, Um = Uranium mobilization.

. This table reveals the computed radiological hazards, minimum, maximum, mean, and standard deviation of 15 samples that were measured radiometrically using the NaI technique. The distribution frequency histogram of rocks under investigation was determined by analyzing all related radionuclides (²³⁸U, ²²⁶Ra, and ²³²Th; Figure 7). The frequency distributions of activity concentration of all the radionuclides were analyzed, and the histograms are given in Figure 7. The ²²⁶Ra, ²³²Th, and ⁴⁰K graphs show that these radionuclides demonstrate a normal (bell-shape) distribution. Without a degree of multi-modality. This multi-modal feature of the radioelements demonstrates the complexity of Gebel Qulu El Sabaya samples.

Radiological risk parameters were calculated for Qulu El Sabaya in order to infer the radiation effect on human organs during extraction or mining processes. These assessments include absorbed gamma dose, annual (outdoor and indoor) effective dose, radium equivalent activity, external and internal hazard indices, and excess cancer risk (Table 4).

Absorbed gamma-dose rate (D_air)

The absorbed gamma-dose rate was used to assess the gamma rays that were released >1 m from the Earth’s surface [42,43]. ²³⁸U, ²³²Th, and ⁴⁰K activity concentrations are used to estimate the D_air as the following:

D_air (nGy/h) = 0.430 U + 0.666 Th + 0.042 K

(1)

where Th, U, and K are ²³²Th, ²³⁸U, and ⁴⁰K activity concentrations, respectively.

Radium equivalent activity (Ra_eq)

Radium equivalent activity is ascribed to internal as well as external particles of alpha. ²³²Th, ²²⁶Ra, and ⁴⁰K activity limits are used to manifest the Ra_eq index [44,45]:

Ra_eq (Bqkg⁻¹) = Ra + 1.43 Th + 0.077 K

(2)

where Th, Ra, and K are ²³²Th, ²²⁶Ra, and ⁴⁰K activity concentrations.

Annual effective dose (AED)

Absorbed dose (D_air) results, a conversion factor of 0.7 Sv/Gy, indoor and outdoor occupancy factors 0. 8 and 0.2, respectively, are used to estimate AED [46,47]:

AED_out (mSv/y) = D_air (nGy/h) × 8.76 (h) × 0.2 × 0.7 (Sv/Gy) × 10⁻³

(3)

AED_in (mSv/y) = D_air (nGy/h) × 8.76 (h) × 0.8 × 0.7 (Sv/Gy) × 10⁻³

(4)

External and internal radiation indices (H_ex &H_in)

H_in and H_ex can be measured to infer the radiation effect on human organs. They can be estimated using the following equations [45,48,49,50]:

H_in = U/185 + Th/259 + K/4810 ≤1

(5)

H_ex = U/370 + Th/259 + K/4810 ≤1

(6)

Excess lifetime cancer index (ELCR)

ELCR can be calculated using outdoor annual effective dose, over a lifetime (DL = 70 years) and cancer index (RP = 0.05 Sv) as in this equation:

ELCR = AED × DL × RP

(7)

The obtained results of radiological risk parameters (activity concentrations ± standard deviation), including absorbed gamma dose (av. 245 ± 20), annual (outdoor (av. 0.3 ± 0.03) and radium equivalent activity (av. 520 ± 42), external (av. 1.49 ± 0.13) and internal (av. 2.74 ± 28) hazard indices, and excess cancer risk (av. 1.05 ± 0.09) are higher than the acceptable worldwide levels [48], reflecting that the exposure for these deposits for a long time may lead to risk on human organs.

4. Conclusions

The current work discusses stratigraphical, petrographical, radioactivity, and geochemical investigations of the lower phosphorite member of the Gebel Qulu El Sabaya area. It is classified into lower phosphorite (most economical), middle shale, and upper phosphorite members. The former has a total thickness ranging from 2 to 3.5 m and consists of apatite, quartz, hematite, anhydrite, and kaolinite. Geochemically, the lower phosphorite member has high REEs content (av. 1430 ppm), and their patterns reveal a slight negative Eu anomaly. Radiological parameters such as absorbed dose rates, outdoor annual effective dose, radium equivalent activity, external and internal hazard, and excess cancer risk values are higher than the recommended levels, reflecting that exposure to these deposits for a long time may lead to health risks on human organs. Therefore, caution must be taken during the mining of phosphorite or using it as fertilizer.