Geochemical characteristics and petrogenesis of granitic pegmatite rocks in the Um Solimate area, south Eastern Desert, Egypt

doi:10.21203/rs.3.rs-3904444/v1

The Um Solimate granitic pegmatite is an outstanding example of rare-metal mineralization in the Arabian Nubian Shield. In this contribution, new bulkrock compositions (major, trace and rare earth element (REE)) are presented to study and constrain the origin and processes that controlling the formation of the Um Solimate granitic pegmatite and associated rare metal mineralization. The granitic pegmatite rocks exhibit calc-alkaline and peraluminous properties, which are common features observed in A-type granites. The area under study reveals the significant presence of high field strength elements within the granitic pegmatite. Conversely, there is a noticeable deficiency in TiO₂, K₂O, and P₂O₅. The similarity in the geochemical characteristics of the granitic pegmatite and the muscovite granite provpes their genetic relationship. Mineralogical studies have identified a diverse range of minerals in granitic pegmatite, including columbite, bismoclite, xenotime, monazite, pyrite and garnet. Chondrite-normalized REE diagrams show a pronounced flat LREE, slight enrichment of HREEs, and significant negative Eu anomalies. The formation of granitic pegmatite in the study area involved a combination of processes, including partial melting and fractional crystallization. Pegmatite-forming melts can be produced through continuous fractional crystallization of granitic melts, resulting in the emplacement of A1-type rocks in an intraplate setting.

Earth and environmental sciences/Solid earth sciences/Geochemistry

Earth and environmental sciences/Solid earth sciences/Geology

Earth and environmental sciences/Solid earth sciences/Petrology

Granitic pegmatite is a type of igneous rock characterized by a coarse texture, large interlocking crystals, and a composition similar to that of granite. Pegmatite mineralization is important because it represents a unique magmatic state (magmatic-hydrothermal transition) and has significant potential for rare metal mineralization; examples include columbite [(Fe, Mn) Nb₂O₆], tantalite [(Fe, Mn)Ta₂O₆], pyrochlore [(Na, Ca)₂Nb₂O₆(OH,F)], fergusonite [REENbO₄], cassiterite [SnO₂], thorite [ThSiO₄], uranothorite [Th,U)SiO₄], fluorite [CaF₂], euxenite [Y, Ca, Ce, U, Th) (Nb, Ta, Ti)₂O₆], xenotime [YPO₄], monazite [(Ce,La,Nd,Th)PO₄], zircon [ZrSiO₄], tantalumite-[(Mn) (MnTa₂O₆], ishikawaite [(U,Fe,Ca,Th) (Nb,Ta,Ti)₂O₆], and samarskite [(Y,Ce,U,Fe)₃ (Nb,Ta,Ti)₅O₁₆] and parasite-[Ce) Ca(Ce,La)₂ (CO₃) (Elsagheer et al., 2023). In general, rare metals and their corresponding minerals are used in many applications, such as nuclear energy fuel, electronics, deep drilling devices, aerospace devices, and gemstones, such as beryl and tourmaline (Černý et al., 2012; Skublov et al., 2021; Sami et al, 2023; Mahdy et al., 2023; Mushkin et al., 2003).

The Arabian–Nubian Shield (ANS) is known to host rare element-bearing granitic pegmatite, which is an important source of strategic metals such as Nb, Ta, Sn, W, Zr, Hf, Li, and rare earth elements (REEs). These pegmatites are typically associated with highly fractionated rare-metal granites, which are believed to be the primary source of rare elements. Studies have shown that the rare metal granites and associated pegmatites in the Arabian–Nubian Shield (ANS) are postorogenic and related to the late stages of crustal evolution (Fig. 1a). The granites are typically peraluminous and highly evolved, with high concentrations of REEs, Ta, Hf, Nb, and other rare elements. The pegmatites are generally coarser-grained and more enriched in rare elements than the associated granites. The origin and evolution of the ANS rare-metal-bearing granites and pegmatites are still a matter of debate. Some studies suggest that these rocks were derived from partial melting of the lower crust, whereas others propose a hybrid origin involving both crustal and mantle sources. The rare metal enrichment in these rocks has been attributed to a combination of fractional crystallization, fluid–rock interactions, and late-stage magmatic–hydrothermal processes (El Gharbawy et al., 2011; Fritz et al., 2013; Eliwa et al., 2014; Kamar et al., 2020; Ghoneim et al., 2022; Dawoud and Abd El-Naby, 2022; Ghoneim et al., 2023 a; Ghoneim et al., 2023b; Mahdy et al., 2023).

In the Eastern Desert and Sinai Peninsula of Egypt, rare metal mineralization is strongly linked to granites or granitic pegmatite (Saleh, 2007; Ghoneim and Panova, 2018; Ghoneim and Abdel Gawad, 2018; Ali et al., 2021; Saleh et al., 2022; Khaleal et al., 2023; Sami et al., 2023).

The age of the postorogenic rare metal-bearing pegmatite in Aswan, Egypt, is 578 ± 5 million years, which is younger than that of the surrounding granite, which dates back to approximately 600 to 620 million years (Abdeldeladil et al., 2022).

Granitic pegmatites from the Um Solimate area in the southern part of the Eastern Desert of Egypt exhibit potential for use as significant strategic and economic metals, notably niobium– tantalum (Nb-Ta), uranium (U), and thorium (Th) (Ali et al. 2021; Abdel Gawad et al., 2022). Despite its economic significance, insufficient detailed research has been performed in this area. In this contribution, whole-rock geochemical data for granitic pegmatite in the Um Solimate area of the SED and the geochemical characteristics of the granitic pegmatite are clarified. Using these data, we interpret the sources, genesis, and tectonic environment of these rocks. The aim of this study is to provide a comprehensive understanding of the geologic, petrographic, mineralogical, geochemical and petrogenetic characteristics of granitic pegmatite rocks in the Um Solimate area to decipher a geodynamic or geotectonic model for this part of the ANS to develop effective sustainable exploration strategies and to provide insights into the processes that led to the formation of these valuable mineral deposits.

The Egyptian Nubian shield basement complex comprises four main tectonostratigraphic units. These units, arranged in ascending order, include high-grade gneisses and migmatites; a volcanic–sedimentary association with dismembered ophiolites; the Hamamat and Dokhan sequences; and granitoids. The Nubian Shield stretches approximately 1000 km from northern Sudan to northern Egypt and is mainly composed of relatively recent Neoproterozoic rock formations. These units were brought together through thrusting associated with accretion, sinistral strike-slip shearing along the Najd, and NW-striking shear. Sheared granitoid gneisses can be found sporadically in Egypt’s Eastern Desert and appear in antiformal "domal" sites. On the borders of these units, low-grade supracrustal assemblages such as Meatiq and Hafafit assemblages are commonly found (Johnson et al. 2011; Mohammad et al., 2019; Abd El-Wahed and Hamimi, 2020; Abdel Gawad et al., 2021a; Abdel Gawad et al., 2021b; Abdel Gawad et al., 2021a; Abdel Gawad et al., 2021c; and Hamimi et al., 2019).

The Eastern Desert (ED) is divided into three tectonic regions: the North Eastern Desert (NED), Central Eastern Desert (CED), and South Eastern Desert (SED). These provinces are distinguished by noticeable variations in lithology and structure in the exposed ED basement and are believed to be located close to each other along significant shear zones (Liégeois and Stern, 2010) (Fig. 1b). The SED Province is primarily composed of migmatite, gneiss, foliated granodiorite, and high-grade schistose metasedimentary rocks. In the SED, granitic pegmatites can be found as intrusions within the gneisses, migmatite, and other high-grade metamorphic rocks. The presence of granitic pegmatite in the SED highlights the diverse geology and mineralogy of this province. (Stern and Hedge, 1985; El-Gaby et al, 1988; Hermina et al., 1989; Hamimi et al., 2019).

In general, the geology of the Um Solimate deposit is complex and variable, reflecting its rich geological history and tectonic activity. It offers a diverse range of rock types and formations, making it an interesting area for geological study and exploration. (Ali et al., 2021) (Fig. 1c).

Biotite gneisses are the oldest rock units in the study area. Biotite gneisses exhibit significant shear deformation. They have alternating bands of black and white minerals composed of quartz, plagioclase K-feldspar, and biotite. Mylonite (cataclastic) rocks overlie metasedimentary rocks through a thrust fault. This thrust fault has a ramp angle of approximately 30 degrees. The fault also features a horizontally oriented fault plane with a dip angle of approximately 60 degrees. Mylonite rock comprises quartz-feldspar and biotite. They display strong fluxion banding with several white porphyroblasts of feldspar. These rocks deform under ductile and brittle conditions. The area is dissected by shear zones in the NNW–SSE, NE–SW, and N–S directions. Mylonites are mineralized and enriched with uraninite, uranophane, kasolite, uranothorite and thorite, xenotime, cassiterite, ferroclumbite, fergusonite, microlite, pyrochlore, pyrochlore and betafite, fluorite, monazite, and zircon, as reported by Ali et al. (2021). The predominant ultramafic rocks found in the study area consist primarily of serpentinite and other related rocks. Talc-carbonate and magnetitic cavernous masses are present. The extensive transformation of serpentinite into talc carbonate is a distinctive property. They may be isolated from serpentinized masses or may be found together. Ophiolitic metagabbros are frequently observed in the northwest–southeast direction. They comprise plagioclase, pyroxene, and hornblende. Metasedimentary rocks (shists) are characterized by intense foliation and primarily consist of various schist types. These schists display different colors because of their diverse compositions, such as hornblende schist, mica schist, graphite schist, and talc carbonate. They have low to moderate topography with NE–SW and NW–SE trends. Muscovite granites are highly jointed and fractured, reddish-pink in color, and medium- to coarse-grained. They are composed of K-feldspar, plagioclase, quartz, muscovite, biotite, and garnet. They occur as an extended belt in the NW–SE direction. The Um Solimate muscovite granites are younger than biotite gneisses, mylonites, and metasedimentary rocks but older than granitic pegmatites and dykes. The muscovite granite in Um Solimate is characterized by pegmatite pockets of different sizes. These pockets have widths ranging from 0.2 to 1.6 m and lengths of approximately 3.7 m. Muscovite granitic pegmatites are very coarse-grained, hard, and massive with light colors. They display distinct to gradual boundaries with the surrounding rocks. In addition, aplitic dikes can occasionally be observed adjacent to pegmatite contacts (Figs. 2a and b). Mineralization that contains Nb and Ta minerals is typically associated with granitic pegmatite (Figs. 2d and 2e). These are dissected quartz veins bearing tourmaline and beryl grains (Fig. 2c and 2f). (Ali et al., 2021).

Twelve granitic pegmatite samples were selected and subsequently crushed into 10-mesh-sized particles. These particles were finely ground to a 200-mesh size. Scanning electron microscopy (SEM) and semiquantitative analyses were performed using a Phillips XL 30 SEM at the Nuclear Materials Authority of Egypt’s laboratory. The composition of the major oxides was determined by X-ray fluorescence analysis using an X-ray spectrometer (ARL 9800 f. ARL. Furthermore, specific trace and rare earth elements were identified using inductively coupled plasma‒mass spectrometry (ICP‒MS ELAN 6100 DRC). The loss on ignition (LOI) of the powdered samples was determined by heating them at approximately 1000°C for approximately 1 h. This methodology adhered to a technique developed and certified by VSEGEI (https://www.vsegei.ru/ru/activity/labanalytics/lab/lab-operations/). The detection limits for this method are provided in Table 1. The ICP‒MS instrument is located at the Karpinsky Russian Geological Research Institute (VSEGEI) in Saint Petersburg. The analysis of major oxides was conducted using X-ray fluorescence silicate analysis at the Central Laboratories of Saint Petersburg State University in the Russian Federation. The procedure involved the following steps:

1. The sample was dried at 105°C for 1.5 h.

2. The sample was annealed at 960°C for 2.5 h.

3. The sample was mixed with a flux consisting of 66.67% lithium tetraborate, 32.83% lithium metaborate and 0.5% lithium bromide at a ratio of 1:9 (the total weight of the mixture was 5 g).

4. The mixture was melted in platinum crucibles in a Lifumat-2.0-Ox induction furnace according to a standard method.

5.1. Petrography of the granitic pegmatite

The granitic pegmatite in the study area is composed essentially of quartz, alkali feldspars, and mica. Quartz occurs as anhedral cracked crystals. Quartz crystals exhibit undulose extinction and are associated with semicite perthitic feldspars, muscovite, biotite, titanite, and xenotime. (Figs. 3 a, b, c, and d). The predominant types of alkali feldspars include orthoclase, potash feldspars, and microcline perthite. They can be found in the form of subhedral to anhedral crystals surrounded by plagioclase, muscovite, biotite, quartz, zircon, and opaque iron oxide minerals. The larger crystals of plagioclase have euhedral to subhedral shapes and are often cracked; these crystals are commonly found alongside microclines and quartz. They also contain biotite, garnet, zircon, quartz, and opaque iron oxide minerals. Mica are found in the form of flakes or platy crystals of biotite and muscovite. Muscovite laths are frequently connected to plagioclase. Perthite is associated with fan-shaped muscovite. Biotite is found within plagioclase minerals and has undergone partial alteration, resulting in the formation of chlorites and iron oxides. In addition, biotite surrounds quartz, zircon, epidote, and apatite. (Fig. 3 a, e, f). Garnet is found in Um Solimate, either in the form of well-formed regular crystals or in the form of irregular crystals and skeletal structures. These crystals and skeletal structures are typically enclosed by quartz and biotite, which act as inclusions in the surrounding rock (Mahmoud, 2009).

5.2. Mineralogy of the granitic pegmatite

Columbite has anhedral to subhedral medium-grained forms. ESEM data indicate that its composition consists primarily of 42.2% Nb and 7.6% Ta (Fig. 4a). Bismoclite is observed as fine-grained anhedral crystals. ESEM analyses revealed that the main composition of Bi is 78.9% (Fig. 4b). Xenotime (Y) features anhedral fine-grained crystals enclosed in plagioclase. ESEM examinations revealed 44% Y and 17.07% P (Fig. 4c). Monazite–(Ce) is observed in medium-grained structures with anhedral to subhedral forms. ESEM analyses indicate a composition of 29.5% Ce and 15.2% La (Fig. 4d). Pyrite occurs in medium- to fine-grained forms. ESEM analyses revealed a main composition of 53.7% S and 41.6% Fe (Fig. 4e). Garnet is found in coarse-grained crystals, with ESEM analyses revealing a composition typical of spessartine, with Mn at 33% and Al at 14.4% (Fig. 4f). The initial phase of mineral formation is known as the magmatic stage and involves intricate internal patterns and a sequence of paragenetic events. Within this stage, K-feldspar–quartz– muscovite pegmatite zones are predominantly observed. Following this, the albite pegmatite stage is characterized by quartz–muscovite–alpite pegmatite accompanied by albitization. The final stage, referred to as the hydrothermal metasomatism stage, is dominated by highly fractionated pegmatite consisting of quartz; native metals such as bismuth and nickel; tourmaline; beryl; argentite; and fluorite. (Ali et al., 2021)

5.3. Whole-Rock Chemistry

5.3.1 Geochemical classification

There are various ways to categorize pegmatite, including its chemical composition, mineralogical composition, metamorphic characteristics, and presence of trace elements. (e.g., Černý and Ercit, 2005). Categorization is commonly used to classify pegmatites into three primary groups: granitic, gabbroic, and syenitic. Among these groups, granitic pegmatites are the most widespread. (London, 2009). Based on the geochemical data, the pegmatite from the study area was classified as granitic. (Table 1 and Fig. 5a).

5.3.2. Geochemical characteristics, magma affinity, and tectonic settings.

The geochemical data for the Um Solimate granitic pegmatite are presented in Table 1 and indicate consistently high levels of SiO₂ and Al₂O₃ with very limited variations (73-74.1 and 13.8-15.7 wt%, respectively). The Na₂O and K₂O levels fall within the ranges of 3.7–6.12 and 0.4–3.9 wt%, respectively. The CIPW test results (Table 1) demonstrated a relatively low normative An content (5.2-10.4 wt%). Furthermore, they exhibit relatively high differentiation ratios, with a Rb/Sr ratio of approximately 2.28 and a Ba/Sr ratio of approximately 1.5, indicating that they originated from highly differentiated magmatic sources.

The granitic pegmatite under investigation demonstrates alkaline affinity, as shown in Figure 5b. In addition, it falls within the peraluminous domain with an A/CNK ratio of 1.11. Another piece of evidence supporting its peraluminous nature is the range of normative corundum, which varies from 0 to 5.38, as presented in Table 1. This further confirms the peraluminous characteristics of the pegmatite, as depicted in Figure 5c.

The distinguishing characteristic of granitic pegmatite is its large amounts of Nb, Zr, U, and Th, which leads to the occurrence of rare metals within the pegmatite. In addition, it has high concentrations of high field strength elements (HFSE), elevated ratios of Fe₂O₃/MgO and Ga/Al, and contains Ba (7.8-122 ppm) and Sr (11-141 ppm). The Rb/Sr ratios of the granitic pegmatite samples range from 0.3 to 8.6.

This study compared the average trace element contents of granitic pegmatite in the studied area with the average trace element contents of granitic pegmatite in the Earth’s crust, as defined by Clarke, and the average element content of muscovite granite (Table 2). The Clarke coefficient was used to determine the ratio between the average trace element content of the granitic pegmatite in the studied area and that of the same rock in the Earth’s crust, as established by (Sklyarov, 2001). Within the studied pegmatite, certain elements displayed significantly greater Clark coefficients. These elements include Ni, Mo, Nb, Hf, Ta, U, Th, Ga, Sn, Pb, Cu, and Zn. A graphical representation of these elements and their corresponding Clark coefficients is shown in Figure 6. Granitic pegmatite and muscovite granite share similar geochemical characteristics, suggesting a close relationship between the two types of rocks (Fig. 6).

Binary diagrams can be used to determine the tectonic setting of the rocks being investigated. Pearce et al. (1984) used diagrams that compare the levels of Y+Nb and Rb to differentiate between various tectonic environments, including Syn-Collision Granites (Syn-COLG), Volcanic Arc Granites (VAG), Ocean Ridge Granites (ORG), and Within-Plate Granites (WPG). The rocks under investigation can be classified as within-plate granites, as shown in Figures 7a and b. The same result can be obtained for the analyzed pegmatite using the Hf-Rb/30-3*Ta ternary diagram based on the categorization by Harris et al. (1986), as depicted in Figure 7c. On the Nb vs. Ga/Al discrimination diagram developed by Whalen et al. in 1987, all the examined samples of granitic pegmatite plot within the A-type category, as illustrated in Figure 7c. The inverse relationship between Rb and Sr indicates the role of K-feldspar fractionation in the development of granitic pegmatite. The Rb–Sr diagram (Condie, 1973) is shown in Fig. 8a. The same result is supported by the good correlations among CaO, Rb/Sr, and Eu/Eu* (Fig. 8b and c).

Additional proof of magmatic fractionation is provided by the Zr–Hf interactions in relation to a fairly strong Y–Ho correlation. The Y/Ho ratios ranged between 15.3 and 535, indicating non-CHARAC behavior, which is entirely consistent with the tetrad effects of the relevant samples (Fig. 8d).

The negative relationship between Zr/Hf and Hf concentrations (Fig. 8e) provides clear evidence that Zr/Hf fractionation occurred because of liquid immiscibility during fractionation, indicating that these granitic pegmatites formed between 20 and 30 km beneath the surface during rather moderate-depth crystallization of zircon minerals. This similarity suggests that the rock source was the mantle (Bau, 1996).

Nb/Ta ratios are modified by internally derived mineralizing ﬂuids during fractionation crystallization (Linnen, 1998). The behavior and increase in the concentrations of niobium (Nb) and tantalum (Ta) are primarily controlled by magmatic phenomena, as evidenced by the fluctuations in Ta vs. Nb (Fig. 8f), which show a linear positive correlation in the studied granitic pegmatite (Charoy and Noronha, 1991).

5.3.3. Geochemistry of the rare earth elements

According to Boynton (1984), REE concentrations normalized by chondrite values exhibit almost flat LREE trends and enrichments in HREEs. The overall concentrations of rare earth elements (ΣREEs) range from 9.6 to 106 ppm, with an average of 57.7 ppm. The light rare earth elements (ΣLREEs) fluctuate between 6 and 57 ppm, averaging 30.7 ppm. Moreover, heavy rare earth element (HREE) levels range from 3.6 to 74.7 ppm, with an average of 27 ppm. Yttrium concentrations varied from 35 to 70 ppm, with an average of 47.2 ppm (Table 4 and Fig. 9a). The samples strongly show negative Eu anomalies (Eu/Eu* =0.01 – 0.4, average= 0.2). The occurrence of a negative Eu anomaly in the melt may arise from the removal of feldspars through processes such as crystal fractionation or partial melting. Alternatively, this could be attributed to the influence of a more volatile-saturated melt with higher oxygen activity and oxidation state during feldspar formation (Cullers and Graf, 1984).

5.4. 1. Zircon saturation temperatures

The solubility of zircon and the composition and temperature of the melt are connected to the Zr geothermometer. This geothermometer is useful for determining the temperature at which zircon formed and the amount of Zr required to saturate the rock from which the zircon was obtained (Watson and Harrison,1983). Based on experimental research by Watson and Harrison (1983) on zircon saturation in hydrous low-temperature, intermediate, and felsic magmas, the zircon saturation temperatures of the studied granitic pegmatite samples are provided in Table 5. The average value of the estimated temperature of the studied granitic pegmatite samples was 735°C for all samples (Table 5). Higher temperatures suggest that the pegmatite crystallized from a hotter magma source, potentially indicating the involvement of more evolved, silica-rich melts and providing valuable information about the conditions under which the pegmatite formed (Eliwa et al., 2014).

5.4.2 Apatite formation temperatures

The temperature at which apatite crystallizes can be determined by comparing natural and experimental systems (Harrison and Watson in 1984). This involves expressing apatite solubility as a function of temperature and identifying the SiO₂ and P₂O₅ concentrations at which apatite crystallizes. The resulting saturation temperatures of apatite appear to have an average of 782.2°C. This information can be incorporated into thermodynamic models to better understand the conditions and processes that lead to apatite formation.

The investigated granitic pegmatite in the study area is distinguished by elevated amounts of Nb (36-323 ppm), Ta (3.5-34 ppm), and Ga (15-37 ppm), as well as an average Rb/Sr > 1. Fluxing materials, such as F, P, and H₂O, significantly contributed to the evolution of pegmatitic magmas by altering the residual melt structure and characteristics, including viscosity and diffusivity (London 2009). The debate surrounding the origin and development of ANS rare-metal-bearing granites and pegmatite remains ongoing. Some research indicates that these rocks derive from the partial melting of the lower crust, whereas others propose a mixed origin that involves both crustal and mantle sources. The enrichment of rare metals in these rocks is believed to be a result of various factors, including the separation of different minerals during cooling, interactions between fluids and rocks, and processes occurring in the late stages of magma and hydrothermal activity (Abdelfadil et al., 2022).

The majority of rare metal-bearing granites and pegmatite are A-type rocks. In the Eastern Desert of Egypt, significant disparities in the concentrations of major and trace elements, as well as isotope ratios, are evident among different A-type granitic rocks within the Arabian–Nubian Shield (ANS). This suggests that a diverse range of processes and sources contributed to their formation (Abdallah et al.2019). The genesis of A-type granites in the ANS is linked to the crystallization of mantle-derived mafic magmas (Mushkin et al., 2003). The Abu Dabab granite and Gabal El Faliq pegmatite in Egypt and the Tawalah and Um Al Suquian albite granites in Saudi Arabia are the most well-known A-type felsic granitic plutons in the Arabian Shield and are comparable to the granitic pegmatite of the study area. These rocks share many of the same chemical traits as the granitic pegmatites of the study area, including HFSEs, Nb enrichment, high Rb/Sr ratios, K₂O depletion, and peraluminous and alkaline affinities within intraplate environments (Heikal and El Amawy, 2013; Heikal et al, 2019; Mahmoud et al., 2019).

The Y–Nb– Ce ternary diagrams, as proposed by Eby in 1992 and capable of distinguishing between different granite types (as depicted in Fig. 9b), indicate that the NYF pegmatites entirely plot within field A1, associated with mantle-derived magma. This magma intruded within the continental plate and underwent extended fractionation processes, ultimately promoting the formation of pegmatite, as illustrated in Fig. 9b.

The dominance of fractional crystallization (FC) and the secondary role of assimilation fractional crystallization (AFC) can be observed in the Rb vs. Rb/Ba plot (Fig. 9c). The decrease in Rb content and the transformation of K-feldspar into albite provide evidence for the dominant influence of fractionation, whereas assimilation plays a secondary role (Fig. 9d).

The samples display the characteristics of an enriched intraplate mantle source influenced by crustal contamination with Th, which has a stronger impact than Ta and Yb during crustal contamination, according to the Ta/Yb vs Th/Yb diagram of Pearce (1983) (Fig. 10a). Consequently, rocks with crustal contamination exhibit high Th/Yb concentrations and function at active continental margins (Wilson 1989). The binary Ce/Yb versus Ce diagram shows that the samples can be derived from the partial melting trend during further crystal fractionation (Figure 10b).

The examined granitic pegmatite displays elevated levels of Zr (with an average of 178.9), Nb (with an average of 84.4), and Y (average: 47.2) and is moderately enriched in total rare earth elements (with an average of 57.7). The concentrations of these elements, whether within the minerals forming the rock or within specific mineral species, primarily depend on the original magmatic composition and the characteristics of the source regions where partial melting occurred. These elements can be incorporated into the crystal lattices of common rock-forming minerals such as mica. The presence of excess alkali-rich fluids is associated with processes that enrich these elements. In addition, these metals are redistributed and concentrated through hydrothermal processes, and an increase in the amount of alkali and fluorine-bearing solution increases the solubility of HFSEs and causes them to form soluble alkali fluoride silicate complexes, especially in the late stages of granitic melt. Plagioclase and mica are the principal carriers of rare metals. The occurrence of volatile substances in fluids plays a role in controlling the extraction of metallic elements from their original positions within the crystalline structures of these minerals, which contributes to the formation of rocks during alteration processes. (Černý et al., 2012; Sakr et al., 2022).

Late magmatic fluids, also known as postmagmatic or residual fluids, play a crucial role in the formation of pegmatite in associated granites. These fluids are derived from residual melts left behind because of fractional crystallization during the late stages of magma evolution. Here, is a more detailed explanation of their role:

1. Extended Fractional Crystallization: As the magma chamber cools and undergoes fractional crystallization, the earlier-formed minerals settle and accumulate, leaving behind a residual melt with a distinct chemical composition. This extended fractional crystallization process led to the formation of late magmatic fluids enriched in incompatible elements and volatile components.

2. Migration toward the Apex: The late magmatic fluids, which are less dense than the surrounding solid granite, started to migrate toward the apex or uppermost parts of the magma chamber. This migration occurred because of buoyancy forces and pressure gradients within the magma chamber. As the fluids rose, they continued to interact with the still hot but subsolidus granite rocks.

3. Reacting with Subsolidus Granite: Late magmatic fluids, with their distinct chemical composition, react with subsolidus granitic rocks located closer to the top of the magma chamber. The process of interaction leads to the exchange and deposition of elements, resulting in the formation of various minerals and mineral assemblages within the granite.

4. Focusing on channels and veins: The upward and outward migration of late magmatic fluids results in their concentration in distinct channels and veins within the subsolidus granite. These channels and veins act as conduits for the ascent and deposition of fluid components, allowing them to concentrate further and form larger mineral crystals.

5. Formation of pegmatites: Concentrated late magmatic fluids, which are now highly enriched in volatiles and incompatible elements, promoted the formation of pegmatite because of the slow crystallization of mineral components from highly enriched late magmatic fluids.

Late magmatic fluids played a vital role in the formation of pegmatite by

Enriching rare elements: The volatile-rich nature of late magmatic fluids allows them to transport rare elements such as lithium, beryllium, tantalum, niobium, and rare earth elements from the surrounding rocks and concentrate them in specific areas where pegmatite forms.
Facilitating crystal growth: The presence of water and other volatiles in late-stage fluid promotes rapid crystal growth. This leads to the formation of large crystals within pegmatite, including minerals such as quartz, feldspar, mica, tourmaline, and spodumene.
Creating mineral diversity: The interaction between late magmatic fluids and surrounding rocks can lead to the formation of a wide variety of minerals within pegmatite. This includes not only rare minerals but also economically valuable minerals, such as gemstones and industrial minerals.

-Large beryl, tourmaline, garnet, xenotime, titanite, flouroapatite and garnet crystals in pegmatite have several significant implications (Figs. 2, 3 and 4f):

1. Indicator of a geologically active environment: The presence of large crystals suggests rapid cooling of the molten rock, indicating a geologically active environment. The large crystals suggest that the cooling process was relatively slow, allowing for the growth of large crystals. 2-Garnet is a high-temperature mineral that forms under conditions of high pressure and temperature. Evidence of fluid-rich environments: The formation of pegmatite is often associated with the presence of fluids that can contribute to the growth of large crystals. These fluids may contain dissolved minerals and elements that can accumulate and contribute to the growth of large garnet crystals. The higher concentrations of rare elements, such as high field strength elements (HFSEs) and large ion lithophile elements (LILEs) and tantalum in pegmatite minerals, also suggest the contribution of late magmatic fluids that transported these elements during the pegmatite formation process.

The average levels of U and Th are 20.6 ppm and 38.1 ppm, respectively, as shown in Table 3. The concentrations of U and Th in each mineralized sample are strongly influenced by the abundances of minerals such as uraninite, coffinite, thorianite, uranothorite, and monazite, as discussed previously (Ali et al., 2021). The Th vs. U binary variation diagram in Fig. 10c shows a positive correlation, indicating that Th and U behave similarly during magmatic differentiation. These findings confirm that these two elements were connected and primarily incorporated into uranothorite during mineralization. Th/U ratios in the studied pegmatite range from 1.4 to 2.3, which are lower than the 3.8 ratio of the upper part of the Earth’s crust (Taylor and McClennan 2001). These ratios show that U is postmagnetically enriched compared with Th. Therefore, the partial melting of a metasomatized crustal source can be used to explain how the investigated granitic pegmatite evolved (Fig. 10d). The Y concentration can be used as an indicator of fractional crystallization in the studied granitic pegmatite. The presence of positive correlations among Th, U, ΣREEs, and Y suggest that the increased levels of REEs, Th/U, and U are likely linked to the gradual evolution of the magma through extensive fractional crystallization processes. (Fig. 11a, b and c). The presence of rare metal-bearing pegmatite is commonly associated with the low Y/Nb ratio of 1.2 observed in oceanic island basalts (OIBs) (Eby, 1992). The granitic pegmatite of the study area yields low Y/Nb and Yb/Ta ratios, indicating magma derivation from an enriched OIB mantle source (Eby 1992) (Figure 11d).

The geodynamic model for granitic pegmatites in the Arabian–Nubian Shield is an active research topic with various proposed models. Recent studies have indicated a connection between granitic pegmatite and postcollisional magmatism in this region. For instance, Abuamarah et al. (Abuamarah et al., 2022) proposed a model that suggests the formation of pegmatites through fractional crystallization and volatile exsolution from a primary magma created by partial melting of the juvenile crust of the Arabian–Nubian Shield, specifically in the Homrit Akarem Granitic Intrusion in Egypt’s southern Eastern Desert. Abdelfadil et al. (2022) similarly proposed that the Wadi Muweilha muscovite pegmatite in the Central Eastern Desert of Egypt was formed by fractional crystallization of granitic magma during postcollisional extensional tectonic events in the Arabian–Nubian Shield. These models emphasize the importance of the tectonic setting in the formation of granitic pegmatite in the Arabian–Nubian Shield.

Abdel Gawad et al. (2022) conducted a trace element geochemical analysis of beryl from the Wadi Nugrus area and discovered a connection to muscovite granitic pegmatite that intruded preexisting basement rocks. Their findings indicated that the pegmatite formed through fractional crystallization and volatile exsolution from the primary magma, which was generated by partial melting of the juvenile crust in the Arabian–Nubian Shield. Similarly, Khaleal et al. (2022) studied corundum-bearing pegmatites in the Migif–Hafafit area and found a relationship between late-stage magmatic–hydrothermal activity and postorogenic extensional tectonic processes. They proposed that the pegmatites were formed by partial melting of the lower sections. The Um Solimate granitic pegmatites can be divided into the following multistage procedures, similar to those used in the Kaluan–Azubai–Qiongkuer pegmatite of Zang et al., 2016: (1) within a tectonic setting characterized by postcollision extension, the lower crustal materials underwent heating from the ascending hot asthenosphere, causing partial melting; (2) at deeper levels of the crust, magma originating from the mantle blended with magma derived from the crust; and (3) combined with mixed magma was transported upward through extensional fractures and subsequently stored in a magma chamber located in the shallow crust. Moreover, there was an increase in the presence of molten rock derived from the Earth’s mantle that contained an abundance of substances such as gases, high-field strength elements, and rare metals. These molten rocks followed specific pathways known as shear zones, which exhibited directional patterns of northwest–southeast and northeast–southeast. (Fig. 12). These trends may be linked to the Najd fault system, a geological feature that formed between 620 and 560 million years ago (Stern, 1985). (4) The felsic magma that evolved significantly, which represents the remaining melt, entered the muscovite granite and subsequently underwent crystallization, resulting in the formation of granitic pegmatite after an extended period of fractional crystallization within the magma chamber.

-The granitic pegmatites in the Um Solimate area are highly evolved and display peraluminous and calcalkaline affinities with high SiO₂, Al₂O₃, Zr, Nb, Ga, and Y contents.

-The geochemical signatures of the granitic pegmatite are similar to those of the host muscovite granite rocks, indicating a genetic relationship.

Columbite, bismoclite, garnet, xenotime, monazite, and pyrite rare metal minerals are related to the studied granitic pegmatite.

-The average temperature calculated for the granitic pegmatite samples in the study area using the zircon geothermometer was 735°C. Higher temperatures suggest that the pegmatite originated from a magma source with increased heat, potentially indicating the involvement of more evolved, silica-enriched melts.

-The formation of Um Solimate granitic pegmatite can be attributed to both partial melting and fractional crystallization from the remaining melt of the muscovite granite.

Acknowledgments

The authors would like to express their sincere appreciation and acknowledge Prof. Mohamed A. Ali and Prof. Ahmed E. Abdel Gawad Professors of Geochemistry, Nuclear Materials Authority, Cairo, Egypt, for their invaluable assistance during field work and valuable feedback during the manuscript revision process. The author also extends his appreciation to Prof. Elena G. Panova, Professor of Geochemistry and Doctor of Geological and Mineralogical Sciences at Saint Petersburg University, for her support in conducting research using the XRF and ICP‒MS facilities

Data availability All data generated or analyzed during this study are included in this published article.

Author contributions: Mohamed M. Ghoneim: Conceptualization, sample preparation, data curation, formal analysis, investigation, methodology, software, visualization, writing-original draft, review, editing and supervision.

Competing interests; The author declares no competing interests.

Additional information Correspondence and requests for materials should be addressed to M.M.G.

Data availability statement as '' All data generated or analyzed during this study are included in this published article. ''

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Table 1 Whole-rock major element compositions (wt %) and calculated CIPW norm of granitic pegmatite of Um Solimate area, Eastern Desert, Egypt.

Samples	S1	S2	S3	S4	S5	S6	S7	S8	S9	S10	S11	S12	Average	D.L.
SiO₂	73	73.5	73.76	73.52	73.6	73	73.13	73.7	73.28	74.1	73.2	74.1	73.49	0.1
TiO₂	0.178	0.16	0.13	0.05	0.06	0.07	0.07	0.03	0.1	0.07	0.1	0.16	0.10	0.03
Al₂O₃	13.8	14.69	14.5	14.2	14.82	13.8	15.73	14.2	15.3	15.2	14.4	14.45	14.59	0.1
Fe₂O₃	1.873	1.61	1.98	1.3	1.69	2.72	0.84	1.9	1.07	0.42	1.5	1.1	1.50	0.1
MnO	0.041	0.05	0.04	0.02	0.11	0.93	0.05	0.6	0.33	0.01	0.01	0.03	0.19	0.03
MgO	0.767	0.33	0.52	0.76	0.22	0.19	0.27	0.17	0.12	0.19	0.2	0.14	0.32	0.1
CaO	1.82	2.15	1.4	1.23	1.28	1.11	1.54	1.33	1.91	1.3	1.2	1.07	1.45	0.03
Na₂O	4.76	3.72	3.79	5.98	4.76	5.99	6.12	4.12	5.57	4.4	4.7	4.91	4.90	0.2
K₂O	3.1	2.64	0.4	1.55	2.95	1.01	0.9	3.62	2.06	3.92	3.93	3.55	2.47	0.03
P₂O₅	0.01	0.04	0.04	0.03	0.05	0.02	0.06	0.08	0.01	0.03	0.04	0.01	0.04	0.05
LOI	0.3	0.2	0.4	0.4	0.2	0.3	0.2	0.2	0.2	0.2	0.4	0.4	0.28	-
SUM	99.65	99.09	96.96	99.04	99.74	99.14	98.91	99.95	99.95	99.84	99.68	99.92	99.32	-
Q	28.84	36.78	46.53	29.13	31.70	31.70	30.56	33.01	28.76	30.53	28.07	29.49	32.09
C	0.00	1.90	5.38	0.52	1.59	0.88	2.03	1.28	0.46	1.43	0.33	0.61	1.37	-
Or	18.32	15.60	2.36	9.16	17.43	5.97	5.32	21.39	12.17	23.17	23.23	20.98	14.59	-
Ab	40.28	31.48	32.07	50.60	40.28	50.69	51.79	34.86	47.13	37.23	39.77	41.55	41.48	-
An	7.13	10.41	6.68	5.91	6.02	5.38	7.25	6.08	9.41	6.25	5.69	5.24	6.79	-
Di	1.07	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.09	-
Hy	1.42	0.82	1.30	1.89	0.55	0.47	0.67	0.42	0.30	0.47	0.50	0.35	0.76	-
Mt	0.00	0.00	0.00	0.00	0.19	2.83	0.00	1.87	0.79	0.00	0.00	0.00	0.47	-
Il	0.09	0.11	0.09	0.04	0.11	0.13	0.11	0.06	0.19	0.02	0.02	0.06	0.09	-
Hm	1.87	1.61	1.98	1.30	1.56	0.77	0.84	0.61	0.53	0.42	1.50	1.10	1.17	-
Tn	0.32	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.03	-
Ru	0.00	0.10	0.09	0.03	0.00	0.00	0.01	0.00	0.00	0.06	0.09	0.13	0.04	-
Ap	0.02	0.10	0.10	0.07	0.12	0.05	0.14	0.19	0.02	0.07	0.10	0.02	0.08	-
Sum	99.36	98.90	96.57	98.65	99.55	98.86	98.72	99.77	99.76	99.65	99.29	99.53	99.05

Table 2 Average concentration of the studied granitic pegmatite compared with average conc. of the earth crust and Clarke coefficient and muscovite granite (Mahmoud, 2009).

	Clarke (Sklyarov, 2001).	Granitic Pegmatite	Muscovite granite (Mahmoud, 2009)
SiO₂	73.22	73.49	76.23
TiO₂	0.27	0.10	0.03
Al₂O₃	13.74	14.59	13.56
Fe₂O₃	2.23	1.50	0.52
MnO	0.05	0.19	0.05
MgO	0.36	0.32	0.35
CaO	0.99	1.45	1.03
Na₂O	3.52	4.90	3.97
K₂O	4.78	2.47	3.63
P₂O₅	0.14	0.04	0.02
Ni	3.5	87.42	4.2
Cu	10	15.75	4.4
Zn	39	63.25	24.2
Pb	19	25.58	29.6
Sn	3	5.70	13.4
Mo	1.5	21.58	4.8
W	2.2	2.11	14.2
Rb	180	76.31	144
Sr	150	81.16	132
Ba	750	66.49	456.4
Y	50	47.2	94.3
Zr	180	178.8	63
Nb	21	84.4	25.6
Hf	3.9	11.82
Ta	3.6	12.66
Th	18	37.94
U	3	20.61
Ga	18	20.00

Table 3 Trace elements (ppm) of the studied granitic pegmatite, Um Solimate area, Eastern Desert, Egypt.

Sample	S1	S2	S3	S4	S5	S6	S7	S8	S9	S10	S11	S12	Average	Detection Limits
V	13	5.7	14	6.2	2.5	2.7	9	7	5	3.2	5.9	3.3	6	0.05
Co	28	284	240	77	253	6.1	150	240	5.6	35	22	42	115	0.1
Ni	60	103	154	71	91	70	85	130	66	99	60	60	87	0.05
Cu	19	12	9	62	5	8	5	14	10	12	18	15	16	0.05
Zn	106	36	16	32	313	43	2	8	45	32	56	70	63	1
Pb	22	19	16	75	20	21	17	16	18	28	33	22	26	0.3
As	4	3	2	13	5	4	1	2	3	1	3	3	4	0.5
Sn	1.3	2.4	1.6	5.7	2.8	22	4.3	3.4	2.2	9	9.7	4	6	0.2
Mo	6	16	28	17	22	7	18	14	20	9	33	69	22	0.6
W	0.6	2.1	2.5	5.4	0.6	1.8	0.4	1.4	1.4	1.9	4.8	2.5	2	0.15
Rb	52.4	66.5	47	58.7	56.9	155	42	55	111	90	67	114	76	0.1
Sr	104	119	141	122	11	18	15	11	126	117	122	68	81	0.1
Ba	75	122	57	7.8	13	28	113	33	91	111	80	20	63	0.1
Y	36	40	48	70	60.3	48	55	35	49	41	45	39	47	0.1
Zr	56	17	27	1350	66.7	82	38	52	61	140	97	160	179	0.1
Nb	53	36	88	323	38.7	62	46	70	84	92	66	54	84	0.1
Hf	2.2	0.9	1.8	74	1.9	6.9	0.5	12	23	14	2.3	2.3	12	0.05
Ta	4.3	5.9	24	34	3.5	5.5	15	22	6.6	16	9.7	4.4	13	0.1
Th	19	25	30	76	41	24	31	25	29	49	70	38	38	0.1
U	14	16	18	39	20	16	17	16	17	23	30	21	21	0.1
Ga	28	25	37	32	32	15	22	29	21	17	26	15	25	0.1

Table 4 REEs concentrations (ppm) and their chemical ratios of the studied granitic pegmatite, Um Solimate area, Eastern Desert, Egypt.

Samples	S1	S2	S3	S4	S5	S6	S7	S8	S9	S10	S11	S12	D.L.
La	8.67	3.82	9.48	3.37	1.26	8.18	7.37	2.52	2.07	1.4	8.7	7.3	0.01
Ce	18.6	7.61	24.7	16.4	2.63	23.4	17.3	6.31	7.3	2.5	17.6	17.4	0.01
Pr	2.36	0.88	3.43	2.01	0.37	2.43	2.1	0.7	1.01	0.4	2.2	2.1	0.01
Nd	9.7	3.91	13	6.37	1.09	11.2	8.6	3.7	6.7	1.4	9.4	9.5	0.01
Sm	3.04	2.26	6.36	3.03	0.61	5.6	2.9	2.6	3.02	0.4	3.1	3.03	0.005
Eu	0.3	0.31	0.02	0.12	0.04	0.05	0.2	0.35	0.2	0.02	0.3	0.4	0.005
Gd	3.83	3.71	5.85	2.92	0.5	5.85	3.6	3.71	2.92	0.5	3.6	3.6	0.01
Tb	0.85	0.79	1.25	1.35	0.14	1.5	0.7	0.7	0.8	0.09	0.7	0.9	0.005
Dy	5.65	5.96	7.54	13.2	0.87	6.3	4.4	5.1	10.1	0.7	5.4	5.1	0.01
Ho	1.08	1.15	1.25	3.53	0.18	1.1	0.8	0.9	3.2	0.3	1.3	1.2	0.005
Er	3.11	3.73	4.06	16.4	0.61	3.9	2.9	3.4	15.3	0.8	2.9	3.3	0.01
Tm	0.49	0.59	0.78	3.61	0.14	0.8	0.6	0.7	3.1	0.2	0.3	0.3	0.005
Yb	3.48	3.64	6.37	28.9	1.02	6.9	4.1	4.9	32.5	1.3	3.2	3.1	0.01
Lu	0.5	0.59	0.96	4.81	0.16	1.3	0.7	0.8	5.2	0.2	0.4	0.4	0.005
Eu/Eu*	0.27	0.33	0.01	0.12	0.25	0.03	0.19	0.34	0.21	0.14	0.27	0.37
Ce/Ce _t	0.978	0.936	1.11	1.863	1.037	1.219	1.055	1.043	1.128	0.845	0.933	1.033
Pr/Pr _t	1.145	1.028	1.329	1.769	1.467	1.092	1.166	0.975	1.011	1.296	1.089	1.094
t1	1.058	0.98	1.215	1.815	1.23	1.15	1.11	1.01	1.07	1.05	1.008	1.063
t3	1.18	1.15	1.24	1.44	1.27	1.327	1.13	1.13	1.02	0.71	0.98	1.13
TE	1.12	1.06	1.23	1.62	1.25	1.23	1.12	1.07	1.04	0.862	0.99	1.09

Table 5 Elemental ratio sand calculated crystallization temperatures of zircon and apatite of granitic pegmatite, Southern Eastern Desert, Egypt

Sample	S1	S2	S3	S4	S5	S6	S7	S8	S9	S10	S11	S12	Average
Nb/Ta	12.3	6.1	3.7	9.5	11.1	11.3	3.1	3.2	12.7	5.8	6.8	12.3	8.14
Zr/Hf	25.5	18.9	15	18.2	35.1	11.9	76	4.3	2.7	10	42.2	69.6	27.4
Th/U	1.36	1.56	1.67	1.95	2.05	1.5	1.82	1.56	1.71	2.13	2.33	1.81	1.79
Rb/Sr	0.5	0.56	0.33	0.48	5.17	8.61	2.8	5	0.88	0.77	0.55	1.68	2.28
Y/Ho	33.3	34.8	38.4	19.8	335.1	43.6	68.8	38.9	15.3	136.7	34.6	32.5	69.3
Ba/Rb	1.43	1.83	1.21	0.13	0.23	0.18	2.69	0.6	0.82	1.23	1.19	0.18	0.98
Y/Nb	0.68	1.11	0.55	0.22	1.56	0.77	1.2	0.5	0.58	0.45	0.68	0.72	0.75
Ba/Sr	0.72	1.03	0.4	0.06	1.18	1.56	7.53	3	0.72	0.95	0.66	0.29	1.51
Y/Nd	3.71	10.23	3.69	10.99	55.33	4.29	6.4	9.46	7.31	29.29	4.79	4.11	12.47
Zr/Sr	0.5	0.1	0.2	11.1	6.1	4.6	2.5	4.7	0.5	1.2	0.8	2.4	2.89
M	1.40	1.38	1.27	1.47	1.36	1.34	1.38	1.36	1.40	1.32	1.44	1.39	1.38
T Zr. sat.C	702	622	658	1003	717	735	675	699	708	780	741	786	736
T ap. sat.C	719	819	760	nd	823	736	833	873	709	770	839	724	782

No competing interests reported.

Geochemical characteristics and petrogenesis of granitic pegmatite rocks in the Um Solimate area, south Eastern Desert, Egypt

Status:

Version 1

Abstract

Figures

1 Introduction

2 Regional geology

3 Geology of the Um Solimate area

4 Methods

5. Results

6 Discussion

7 Conclusions

Declarations

References

Tables

Additional Declarations

Status:

Version 1