Significant Enrichment of Rare Earth Element Concentrations in Stream Sediments of Sharm El-Sheikh Area, Southern Sinai-Egypt: Geochemical Prospecting and Heavy Mineral Survey

Abstract


Introduction
As industrialized economies worldwide shift away from carbon-based fossil fuels, much alternative technology is reliant on a growing number of scarce yet critical elements.Rare Earth Elements (REEs) represent key elements since they are crucial to modern technologies.The demand for REEs is increasing globally with the advancement in technology and the continuing need for increasingly complex technological equipment.Considering the unique properties of REEs, which may boost efficiency and performance in a wide range of technological products and techniques, they can be securely regarded as the key to a green energy future.REEs in common are critical components in a broad range of modern technologies, including electrical, medical, military, and environmental applications (Balaram et al., 2012;Mez, 2020;Milinovic et al., 2021).
REEs mineralization occur in a wide range of geological settings, but they only become of economic importance when they are concentrated significantly above the background level of about 125 ppm (Rudnick and Gao 2003).Monazite ((Ce,La,Th)PO4), and xenotime (YPO4) are the most widely exploited REE-bearing minerals on a global scale (Sengupta and Van Gosen, 2016).Other REE-bearing minerals such as zircon, apatite, allanite, synchysite (Ce), samarskite (Y), steenstrupine (Ce), rhabdophane-(Ce,La,Nd,Y), florencite (Ce,La,Nd), fergusonite (Ce,Nd,Y), loparite (Ce), and cerianite (Ce) are rarely found in deposits of economic significance (Jordens et al., 2013).Awadalla (2010) reported elevated REE concentrations in the Abu Tartur phosphorites, Egypt, ranging from 518 to 1011 ppm.Abou El-Anwar et al. (2022) studied the REEs enrichment in black shales of Safaga-Qussier sector, Egypt.They found that these shales were extremely, significant, and very highly enriched with some REEs.Ghoneim et al. (2021) pointed out that the post-granitic dikes have higher REEs content than two-mica granite in El Sela area, Eastern Desert, Egypt.Nasr (2021) reported significant content of REEs, reaching up to 5,736 ppm (average 4,204 ppm), in gibbsite mineral samples of the Um Bogma Formation at El Ramsa area, Southwestern Sinai, Egypt.The modern regional heavy mineral survey is a method of geochemical exploration that consists of measuring the variety, abundance, and regional distribution of heavy minerals, mainly detrital grains in stream sediments, and using the results to delineate the most favorable places for a comprehensive search for ore deposits (Overstreet, 1963).
Through the identification of potential sources of abnormal element concentration, active sediments in the channels of streams and rivers play a significant role in geochemical exploration (Landry et al., 2014).Since the active stream sediment composition closely resembles the chemical composition of bedrock and surficial geology within the upstream catchment, it is widely utilized in mineral prospecting, geochemical mapping, and environmental assessment (Abd El Ghaffar, 2002;Cohen et al., 2010;Bern et al., 2016;Elshahat, 2018;Ramadan et al., 2019;Ramadan et al., 2022).Exploration for heavy mineral resources and the related REEs in Egypt has focused predominantly on the Mediterranean coast (Dawood and Abd El-Naby, 2007).However, geochemical exploration in arid zones using stream sediment is challenging in many aspects (El-Kammar et al., 2020).In general, little focus has been given to REE prospection in Egypt using stream sediments.Therefore, the aim of the present work is mainly concerned with a heavy mineral survey in dry stream sediments of Wadi Lethi area, Sharm El-Sheikh, Southern Sinai, Egypt, which is characterized by having one type of bedrock (granites).The mineralogical and geochemical characteristics of these stream sediments were studied in detail.The geochemical data have been statistically treated.In addition, the reserve of economic heavy minerals and REEs were estimated.
The area is mostly covered by various types of Neoproterozoic granite.These granites are recognized as monzogranite, syenogranites, alkali-feldspar granite, and riebeckite bearing granite (Abd El Ghafar et al., 2021).They are related to G.2 granites of Hussein et al. (1982).These granites are crossed by a huge plain namely Wadi Lethi.Monzogranites constitute about 50% of the studied granitic rocks, and covered the great part of the mapped area.They are hard, massive and greyish pink in color.Syenogranite is exposed in the eastern part of the mapped area, covering about 130 km2.These rocks are hard, massive and medium to coarse grained with reddish pink color and have uniformly equigranular texture.The alkali feldspar granite in the studied area covered a wide area of about 250 km 2 .It is medium to coarse grained, massive and equigranular texture.It is generally greyish white in color with minor amount have pink color.Riebeckite granites form the main outcrop Umm Adawi and Sahara plutons.These rocks are felsic, fresh, with visible clots of riebeckite's mafic components.

Materials and Methods
The studied sediment is a loose mixture of gravel (trace amounts) and sand.The gravel content was taken off after sieving the samples in the field with 1 mm sieve.Twenty-three stream sediment samples (below 1mm size) were mechanically analyzed by taking a kilogram of each sample after quartering the bulk sample and screened in a set of stainless steel sieves after washing and panning.Then separate the heavy and light minerals of these samples by using heavy liquid bromoform (2.89 gm/cm3).The abundance of rare earth elements was determined in 23 selective samples of heavy concentrates.13 rare earth elements were analyzed, namely; La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Lu, Yb, and Tm.All REE were performed by Inductively Coupled Plasma Mass Spectrometry (1CP-MS) at Acme Analytical Laboratory LTD., Vancouver, Canada.
Fine and very fine fractions (0.125-0.25mm and 0.125-0.063mm)are the most promising fractions for the heavy mineral and rare earth content (Abd El Ghaffar, 2002).

Grain Size Analysis and Mineralogy
Table 1 shows percentages of the various size fractions in the samples and the percentages of the separated heavy and light minerals from fine and very fine sand size fractions.Fine fraction is selected in mineral separation to recognize the different minerals (Fig. 5).The obtained concentrates of these sediments result in a very important mineral assemblage (monazite, sphene, apatite, garnet, xenotime, magnetite, ilmenite, hematite, with subordinate riebeckite, epidote and chlorite).The grains are mostly fresh, without alteration.Some show slight alteration of iron oxides at the periphery.The grains are mostly represented by elongated cleavage fragments with different degrees of roundness, varying from angular to rounded but mainly subangular and they are characterized by fire ends (Fig. 2), they ranged from 0.3 % to 4.83% (Table 2).
4. 1.2. Riebeckite [Na2O. Fe2O3. FeO. 5SiO2.H2O] Riebeckite appears as irregular shapeless plates up to 1.2 mm across of greenish blue to deep blue color, perfect cleavable grains.It is the most abundant mineral in the area.The morphology of the grains varies from short prisms to irregular platy fakes.The grains are usually abraded showing saw teeth marks (Fig. 2).They display a pleochroic formula and ranged from 45.58% to 80.87% (Table 2).

Pyroxenes [XY(Si,Al)2O6]
The pyroxene grains vary from colorless to green in color, fresh to partially altered, subrounded to subangular, and show fractures and inclusions (Fig. 2).Pyroxenes are absent in many samples, they ranged from 0.28% to 1.35% (Table 2).

Xenotime [YPo4]
It grains attain neutral, pink to white colors.Sometimes the grains have a pink nucleus (core) and white boarders (rim).They form prismatic and tetrahedral crystals.Inclusions are frequently observed in some grains (Fig. 2).It ranged from 1.33% to 9.22% (Table 2).
4. 1.6. Biotite [K(Mg,Fe) 3AlSi3O10(F,OH)2] Biotite flakes are brown and green.Biotite is characterized by strong pleochroism (Fig. 2) and ranges in color from pale brown to dark brown.They ranged from 0.36% to 6.95% (Table 2).4.1.7.Apatite [(Ca4 (CaF,Cl) It is a less abundant mineral in the area and is already absent in some of the samples.It ranged from 0.00 % to 2.63% (Table 2).It occurs as euhedral to subhedral colorless grains with high relief.The grains take several shapes prismatic, rectangular, and irregular (Fig. 2).Inclusions are variably recorded in some grains.

Garnet (spessartine) [3Mn2. Al2O3. 3SiO2]
It is one of the most frequent isotropic mineral.It is already absent in some of the samples, ranging from 0.00 to 1.13% (Table 2).It forms sub-angular colorless grains with high relief (Fig. 2).Grains take subrounded and irregular shapes with corrosion remarks.

Zircon [Zr SiO4]
It occurs either as short or long prismatic grains with bipyramidal terminations (Fig. 2).Short zircon displays evidence of metamictization due to radioactivity.Zircon grains are found as colorless grains of prismatic shape, bipyramidal shape, and rounded to subrounded with a little number of subangular grains.Occasionally they display zoning.It ranged from 0.00% to 4.81% (Table 2).Some minerals are variably recorded in small amounts and easily identified by binocular microscope (monazite, Columbite, pyrite and iron oxides (magnetite, ilmenite and hematite) (Fig. 3).

Geochemical Behaviour
The concentrations of some major and trace elements analyzed in the studied sample are listed in Tables 3 and 4. To recognize the pattern of distribution of any element, it must carry out some univariate statistical analysis on the raw data.The first step in the analysis of data was to compute the minimum (Min), maximum (Max), mean (X) standard deviation (Sd) and the sum of each analyzed element (Tables 3 and 4).
Several statistical procedures can be used to help delineate geochemical anomalies (Govett et al., 1975;Sinclair, 1974 and1991;Stanly and Sinclair, 1989;Cheng, et al., 1994).The preparation of heavy mineral concentrates (panning and heavy liquids) is an effective method of enhancing threshold values in arid zone geochemistry (Fletcher, 1985).As part of the current study, statistical characteristics of the geochemical data were examined by some multivariate analysis procedures, including correlation and factor analyses using SPSS software.
Rare earth element concentrations of heavy minerals are plotted on the hypothetical chondrite-normalized diagram (Boynton, 1984) (Fig. 4).They display more or less parallel trends.The REE trends of these minerals indicate the same source or belonging to similar bedrocks.The diagram displays enrichment of light rare earth elements (LREE) and depletion of heavy rare earth elements (HREE) indicating presence of monazite, zircon and chevkinite minerals (Watt, 1995) with typical negative Eu anomaly which is a common feature of granitic magma.

Correlation Analysis
The inter-element correlation analysis of the variables CaO, TiO2, MnO, Fe2O3, Ni, Cu, Zn, As, Be, Rb, Sr, Y, Zr, Nb, Mo, Sn, Ba, Pb, Ga, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Lu, Yb and Tm in the heavy concentrates of Wadi Lethi showed a significant correlation between some elements (Table 5), reflecting the type of dispersion of chemical elements and their geochemical behavior.
The significant negative correlation between Fe2O3 and MnO, although they are geochemically associated, reflects the significant independent behavior of the two elements during weathering, transportation, and diagenesis.In other words they were subjected to differential mobilization during diagenesis (Zaghloul et al., 1995).
As, Ba, Br, Ce, Cu, La, Nb, Ni, Sn, Sr, TiO2, Y and Cu showed a significant positive correlation with MnO this due to high negative charge of fresh precipitates of Mn oxides, so these elements may be adsorbed on hydrated colloidal MnO (Rollinson, 1993).The significant positive correlation between Cu and Mo, Ni, MnO and Pb reflects the possibility of the occurrence of sulfide mineralization in the studied stream sediments.The significant positive correlation between Fe2O3 and both Zn and Pb is attributed again to the possible occurrence of sulfide minerals in which Fe replaces Zn (e.g.pyrite or sphalerite).
The positive correlation between Nb and both Y and TiO2 reflected the effects of titanite and xenotime in the chemical composition of the heavy fraction of the stream sediments.The significant positive correlation between Pb, Rb, Ga, and Sr could be explained by their association with K and Al-bearing minerals (e.g., hornblende and pyroxene).The significant positive correlation between elements such as Ba, Br, CaO, Ce, Cu, La, MnO, Ni, Sn, Sr, TiO2, and Y and each other's reflects that the heavy mineral association is mostly titanite, allanite, monazite, garnet, epidote, as well as apatite.These minerals are the common accessory minerals of granitic rocks, which are the major source in the area, i.e., other sources have played a negligible role in the composition of the stream sediments.The significant positive correlation between CaO and TiO2 is related to their association in titanite, which acts as a carrier of Y and other rare earth elements.Also, Ce, Nb, Y, and Zr have a significant positive correlation with La manifesting their essential association in allanite and monazite, which act as a major source of Ce and also the best carrier of REE.Zr -La interrelation could be attributed to the presence of zircon in the heavy concentrates.
It could be concluded that titanite represents a major constituent of the heavy minerals separated from the examined stream sediments.The strong significant positive correlation (r=0.6) between CaO and TiO2 suggests that both elements occur mainly in the titanite structure.Some TiO2 may present as ilmenite, which is recorded in X-ray diffraction patterns.However, the significant negative correlation between TiO2 and Fe2O3 indicates that total iron oxides are essentially magnetite and hematite.

Factor analysis
Factor Analysis is a multivariate statistical technique that can be used to explore the underlying patterns or correlations for a large number of variables and summarize information into a smaller set of factors or components for prediction purposes (Davis, 2002).It is used to find the inter-correlation between measures within the item pool for a developing measure.It uses in measuring the latent commonality between individuals which is called a factor.Factors are an underlying hypothetical, unobservable construct (Mertler and Vannatta, 2010;Williams, 1992).
The extracted varimax R-mode factor matrix is represented in Table 6.Four factors explain why approximately 76.2% of the total variability is extracted.The variances of each factor reflect the amount of the total data contained in it.The factors explain the elemental associations concerning heavy mineral assemblages, which are common results of weathering of the bedrock (granitic source).The factors controlling the interrelationship between elements and heavy minerals are represented by F1, F2, F3, and F4.Factor one (F1) is the major factor and accounts for 10.8 (33.7%) of the data variability.It essentially comprises Fe2O3, TiO2, MnO, CaO, Ni, As, Be, Ce, La, Ba, Y, and Nb.All elements had positive loading, except for Fe2O3.It is noted that CaO and TiO2 are the major components of titanite and they are highly loaded in this factor.Thus, this factor could be considered a titanium-bearing mineral e.g.titanite and, to less extent, columbite (Fe, Mn)(Nb,Ta)O6 and xenotime (YPO4), which are the common carriers of REE.In titanite, Ti may be replaced by Sn and Nb.The negative loading of Fe2O3 to the positive loading of MnO could be attributed to the differential mobilization of them during diagenesis and showed that Mn and Ni can substitute Fe in magnetite (Surour et al., 2003).High loading of Ni, As and Be reflects the adsorption of these elements in hydrate MnO and the presence of pyrite.
Factor two (F2) accounts for 5.3 (16.5%) of the data variability.It is positively correlated with Ga, Rb, Y, Nb, La, Sn, Zr, Ce, Pb, and As.This factor is related to granitic magma differentiation, in which these elements are concentrated.Rb, included in micas, is common lithophile element in granites and is enriched in highly differentiated granites and pegmatites (Smith and Huyck, 1999;Kesler et al., 2012).The presence of Zr (high field strength elements (HFSE)) indicates that the host minerals are exclusively heavy phases derived from the granite source, such as xenotime, columbite, garnet, apatite, monazite, ilmenite, and zircon (Bellehumeur et al., 1994;Embui et al., 2013, Mimba et al., 2014).
Factor three (F3) accounts for 3.7 (11.5%) of the total variability of data, represented by the significant positive loading of Zn, Ag, Pb, MnO, Cu, Ba, Sn, and Sb, CaO has negative loading.It is characterized by ore elements associated essentially sulphide mineralization (Embui et al., 2013).Factor four (F4) is a minor effective factor, which accounts for 2.9 (8.2%) of the data variability.It is positively correlated with Mo, Cu, and Zr.This factor is related to zircon and pyrite occurrences.
These factors indicate the erratic occurrences of phases hosting geochemical signals in the sampling media and allow for the recognition of the geochemical components and the regional phenomena associated with a regional trend (Bellehumeur et al., 1994).The presence of HFSE is also a strong indication of a granitic source (Chandrajith et al., 2001) and their presence, therefore, is related to the weathering of these granitic rocks.

Reserve of Rare Earth Elements
A rough estimation of the content of heavy minerals as well as their chemical constituent was tried.The reserve estimation was attempted only for the first surficial 1m layer of the stream sediments that cover the investigated area.The given reserve represents the heavy concentrates in the fine sand fraction.
The average of the apparent density of dry sand was taken as 1.41 gm/cm3 of sand samples, with minimum and values as 1.31 gm/cm3 and 1.42 gm/cm3 respectively.The reserve estimation was achieved throughout successive steps: • Calculation of stream sediment area as 8 km 2 • Calculation of the average bulk volume of the surficial one-meter sand layer, using the measured density (1.41) gm/cm 3 .• Calculation of the weight of that sand body.
• Calculation of the weight of fine sand fraction.
According to this method; the total reserve of heavy minerals in the surficial 1m layer reaches about 0.613 million tons.The expected reserves of total rare earth (TREE) elements are estimated could be estimated according to this reserve and the distribution of such elements according to Tables 4 and 5.

Conclusions
The heavy concentrates contain great amounts of economic minerals such as titanite, apatite, garnet, allanite, hornblende, riebeckite, pyroxene, zircon, and biotite.The chemical analysis of these minerals results in a remarkable enrichment in their elemental content rather than in granitic rocks (source rock).All heavy minerals are marked by a strong negative Eu anomaly and are enriched in light rare earth elements with slight depletion in heavy rare earth elements reflecting typical granitic source.These minerals are the best carriers of rare earth elements, which have future economic importance.These minerals could be classified as light rare earth (LREE) dominant type.
Each estimated value is depended on the first 1m depth, by the application of other procedures (e.g.geophysical) and determination of the total thickness (depth) of the stream sediments these values will be increased.Also, if we need the excess of these reserves of such minerals or elements we could use all size fractions of sand.Application of developmental and detailed methods can utilize these minerals and elements in metallurgical and engineering industries.The estimated reserve of some elements indicates that the heavy concentrates are enriched in their rare earth elements content, which attains 1843.1 tons.The rare earth elements are essentially Ce, La and Nd.Fig. 5 illustrates that Ce reaches 42 %, La reaches 14 %, and Nd reaches 23 % of TREE.Other rare earth elements (Sm, Eu, Gd, Dy, Er, Lu, Yb and Tm) attain 21 % of the TREE.

6 Fig. 5 .
Fig.5.Chart displays percentages of the investigated rare earth element in the studied samples

Table 1 .
The percentages of the examined sand samples' different size fractions and the percentages of the heavy minerals in only two size fractions

Table . 3
. Major and trace elements concentration in heavy minerals of the studied area and their summary statistics Table.4. Rare earth elements concentration (ppm) in heavy minerals of the studied area and their summary statistics

Table 5 .
Correlation analyses of heavy minerals chemical compositions

Table 6 .
The extracted factors matrix from the studied elements

Table 7 .
Economic potentiality of stream sediments in the studied area