An Occurrence of Pyroxmangite in the NYF Granitic Pegmatite of the Gabal El-Bakriya Intrusion, Arabian–Nubian Shield

Abstract

We report here, for the first time on the Nubian Shield, the western half of the Arabian–Nubian Shield (ANS), pegmatite-hosted pockets with a unique mineralogy, including pyroxmangite. It represents the second discovery on the ANS, where the first one was at Jabal Aja on the Arabian Shield, the eastern half of the ANS. One of the most remarkable aspects of pyroxmangite is its rarity and the potential economic value of its use in jewelry and decorative applications. Pegmatites are associated with A-type granites of the Gabal El-Bakriya intrusion (GEBI), Eastern Desert, Egypt. Mineralized pegmatites occur at the margin of the alkali-feldspar granite and exhibit gradational contacts with the host rocks. The pegmatites were emplaced as plugs and dikes within the intrusion and along its periphery. Pyroxmangite appears as coarse-grained, massive black aggregates or as disseminated crystals. The pegmatites are composed of K-feldspars and quartz, with subordinate amounts of albite, micas, and mafic minerals. Accessory phases include monazite-(Ce), zircon, fergusonite, xenotime, fluorite, pyrochlore, allanite, thorite, bastnäsite, samarskite, cassiterite, beryl, and pyrochlore. Pyroxmangite-bearing assemblages consist essentially of pyroxmangite and garnet, with accessory pyrochroite, quartz, zircon, magnetite, and fluorite. Geochemically, the pegmatites are highly evolved, with elevated SiO₂ content (76.51–80.69 wt.%) and variable concentrations of trace elements. They show significant enrichment in Nb (Nb > Ta), Y, REE, Zr, Th, U, and F, consistent with NYF-type pegmatites. REE contents range from 173.94 to 518.21 ppm, reflecting diverse accessory mineral assemblages. Tectonically, the pegmatites crystallized in a post-collisional setting, representing a late-stage differentiate of the A-type GEBI magma. Mineralization is concentrated in the apical and marginal zones of the granitic cupola and is dominated by barite, fluorite, Nb-Ta oxides, REE minerals, and uranium-bearing phases. The highly evolved granites, greisens, pegmatites, and quartz-fluorite veins of the GEBI have a high economic potential, deserving further exploration.

1. Introduction

The Arabian–Nubian Shield (ANS), is one of the most significant, Neoproterozoic juvenile continental crust provinces on Earth. The ANS extends along both flanks of the Red Sea and was tectonically separated by Red Sea rifting ~25 Ma ago [1,2]. The ANS corresponds to the northern segment of the East African Orogen (EAO) and is largely composed of Neoproterozoic crystalline basement rocks that were assembled through arc–arc and arc–continent accretion between ca. 1000 and 640 Ma [3,4,5,6,7,8]. One of the most striking features of the ANS is the abundance of the post-collisional granites and associated pegmatites [9,10,11,12].

Post-collisional magmatism on the ANS is marked by widespread emplacement of evolved granitoids and associated pegmatites during the waning stage of the Pan-African orogeny (610–580 Ma). This magmatic transition reflects a shift from subduction-related calc-alkaline activity to anorogenic alkaline magmatism [13,14,15,16]. A temporal overlap between the final stages of arc magmatism and the onset of post-collisional, extensional activity is evident in many parts of the shield [17,18,19,20]. These late-stage granitoids commonly exhibit signs of post-magmatic alteration—such as albitization, greisenization, and silicification—which frequently result in enrichment in high-field strength elements (HFSEs) and rare metals of substantial economic significance (REEs; Nb, Ta, Zr, U, and Th) [21,22,23,24,25,26,27].

We investigated the petrology and geodynamic evolution of post-collisional Neoproterozoic pegmatites associated with A-type granites of the Gabal El-Bakriya intrusion (GEBI), Eastern Desert, Egypt. Although numerous studies have addressed the granitoids and associated rocks of the GEBI [28,29,30,31,32,33,34], their genetic interpretation remains controversial.

The investigated pegmatites are notable for containing pyroxmangite-rich pockets, reported here for the first time on the Nubian Shield, and host rare metal-bearing minerals similar to those recognized in other post-collisional systems. This is a report of detailed field observations, petrography, mineral chemistry, and whole-rock geochemical data for the pegmatite and associated pyroxmangite, based on classical methods and advanced techniques such as electron probe microanalyzer (EPMA) and scanning electron microscopy (SEM). The study, thus, sheds light on the magmatic sources and petrogenetic processes responsible for formation of the pegmatites and associated pyroxmangite. Further, it advances our understanding of the mechanisms responsible for rare-metal enrichment in pegmatites. We propose that late-stage magmatic evolution, involving crystallization of a residual melt and pegmatite formation, played a critical role for rare-metal mineralization.

2. Geologic Setting

The GEBI is among the most significant granitic complexes in the Eastern Desert of Egypt. It has garnered significant attention due to its unique geological features and potential economic mineralization. The intrusion is located in the southwestern part of the Central Eastern Desert (Figure 1), about 15 km north of the El-Barramiya gold mine. Geographically, it lies between longitudes 33°41′ and 33°43′ E and latitudes 25°15′ and 25°17′ N. The area is dissected by numerous structurally controlled dry valleys (wadis) filled with alluvial deposits such as the El-Bakriya wadi and its tributaries, which connect with the El-Miyah wadi at the southern part of the study area.

The Gabal El-Bakriya region mostly consists of Neoproterozoic basement rocks, which are unconformably overlain by Cretaceous Nubian sandstone. The Neoproterozoic basement comprises granitoids, gabbros, and post-granitic dikes of diverse composition (Figure 2). The area displays a complex structural history characterized by faulting, joint formation, and subsequent exfoliation. Faults of various ages and orientations intersect the region, primarily going north–south, northwest–southeast, and northeast–southwest. The N–S faults are the most prevalent and are considered the youngest, whereas the NW–SE faults constitute the oldest fault series [36].

The granitoids of the Gabal El-Bakriya can be broadly categorized into syn-collisional and post-collisional varieties. The syn-collisional assemblage comprises diorite to granodiorite, whereas the post-collisional suite includes calc-alkaline monzogranite and A-type granites. The latter defines the GEBI, forming a ring complex with an inner core of alkali-feldspar granite and an outer rim of syenogranite. Although geochronological data are not available for the GEBI, relative age relationships among the rock units have been established from crosscutting relationships and are shown in the geological cross-section (Figure 3).

In the field, the A-type granites are readily distinguished by their color and texture and intrude surrounding country rocks with sharp contacts (Figure 4a). Both sharp and gradational contacts between syenogranite and alkali-feldspar granite are observed (Figure 4b). Locally, shear zones transect the granites, particularly affecting the syenogranite and alkali-feldspar granite. These zones are fine-grained and exhibit a reddish to pinkish color. The outer zone of the alkali-feldspar granites is highly tectonized, displaying pervasive alteration and a distinctly dissected appearance. Alteration is characterized by intense sericitization and silicification. Evidence for brittle deformation, including mylonitization and cataclasis, is widespread, producing sheared and fractured granites. Conjugate fractures are common and often injected with thin quartz veinlets.

Mineralized, coarse-grained granitic pegmatites and greisens are distributed along the margin of the alkali-feldspar granite, generally with gradational contacts. Pegmatites occur as plugs and dikes within the pluton and along its margin intruding the alkali-feldspar granite (Figure 4c,d). For the first time, pyroxmangite-rich pockets (5–15 cm in width) have been identified in the Gabal El-Bakriya pegmatites, representing a new component of the Neoproterozoic basement rocks of the Egyptian Nubian Shield. Pyroxmangite appears as massive, coarse-grained black segregations (Figure 5a), or as disseminated crystals within pegmatites (Figure 5b). Some pyroxmangite pockets also enclose clasts of pegmatite (Figure 5c) and quartz (Figure 5d). Greisenization is observed irregularly along the margin of the alkali-feldspar granite, where greisens occur as pockets, veins filling fractures, or selvages around quartz veins, occasionally extending into adjacent country rocks. Numerous veins and pods of fluorite are found cutting through the greisen bodies, further supporting the post-magmatic hydrothermal activity in the area.

3. Petrography

This section provides a petrographic description of the alkali-feldspar granite, pegmatites, and pyroxmangite-bearing pockets associated with the Gabal El-Bakriya intrusion. Multiple mineral species were identified using SEM and EPMA techniques.

3.1. Alkali-Feldspar Granite

Alkali-feldspar granite is predominantly composed of K-feldspar and quartz with subordinate albite, amphibole, and micas. Accessory minerals include allanite, bastnäsite, thorite, fluorite, zircon, garnet, topaz, monazite-(Ce), fergusonite, Nb-Ta oxides, rutile, and opaques. K-feldspar occurs mainly as subhedral, tabular crystals, primarily of orthoclase perthite, displaying Carlsbad and simple twinning. Microcline occurs as anhedral to subhedral crystals showing cross-hatching twinning. Quartz appears as subhedral crystals, commonly containing muscovite inclusions. Albite is present as subhedral crystals intergrown with other phases. Amphibole forms short, prismatic crystals and is strongly pleochroic, ranging from pale yellow and green to brownish green. The mica assemblage includes biotite and white mica (muscovite and zinnwaldite). Biotite is found as subhedral to anhedral tabular crystals, occupying interstitial spaces between feldspars and quartz or as fine anhedral flakes. Muscovite occurs as deformed crystals (Figure 6a) or also as fine, anhedral flakes between feldspars and quartz. Zinnwaldite is represented by anhedral, corroded crystals that contain albite inclusions and are associated with topaz.

Opaques are mainly Fe-Ti oxides such as magnetite and ilmenite. Zircon appears as prismatic and bipyramidal crystals, both homogeneous and zoned. Allanite occurs as subhedral to euhedral, metamict crystals with a deep brown and black color, corroded and in contact with silicate minerals (Figure 6b). Thorite is present as anhedral crystals that contain monazite inclusions (Figure 6c). Garnet occurs in various forms, including euhedral to subhedral, irregular, and rounded interstitial crystals (Figure 6d). Quartz inclusions are common within garnet. Topaz forms small anhedral interstitial crystals, associated with muscovite (Figure 6e). Bastnäsite is present as anhedral to euhedral tabular crystals ranging from colorless to pale greenish to yellow. Apatite occurs as small subhedral to euhedral crystals, commonly associated with topaz and muscovite. Titanite is rare and was found as a few anhedral grains. Monazite-(Ce) is disseminated throughout the rock and occurs as inclusions in amphibole and allanite. Fluorite is present as interstitial crystals and as veinlets filling fractures. Fergusonite forms dark-brown subhedral prismatic crystals, occasionally zoned and locally altered to pyrochlore. Columbite and rare tantalite occur as small subhedral to anhedral crystals with color varying from black to dark brown. Some columbite crystals are partially replaced along their margins by pyrochlore and tantalite (Figure 6f).

3.2. Pegmatite

The pegmatites are coarse-grained, reddish rocks primarily composed of perthitic K-feldspar and quartz, with minor albite, micas, and mafic minerals. Accessory minerals include opaque phases, monazite-(Ce), zircon, fergusonite, xenotime, fluorite, pyrochlore, allanite, thorite, bastnäsite, samarskite, cassiterite, beryl, and pyrochlore. K-feldspar, orthoclase and microcline, occurs as coarse subhedral to anhedral crystals. Some crystals enclose albite, iron oxide, and zircon inclusions. Orthoclase intergrown with albite forms perthitic texture (Figure 7a), whereas microcline shows tartan twinning. Quartz occurs in the following two generations: an early phase of coarse-grained crystals and a later phase forming fine-grained veinlets, crosscutting earlier developed minerals. Large quartz crystals commonly show undulose extinction and contain inclusions of K-feldspar and muscovite (Figure 7b). Some quartz crystals display graphic intergrowths with K-feldspar. Albite appears as euhedral to anhedral crystals with characteristic lamellar twinning. Some show deformation features such as deformed or broken twin planes, and others are altered to sericite. Muscovite occurs as large crystals with subhedral to anhedral crystals with perfect cleavage and high interference color (Figure 7c). Zinnwaldite forms large, fractured subhedral crystals, corroded and in contact with K-feldspar and quartz (Figure 7d). The corroded zinnwaldite crystals indicate successive replacement and a complex crystallization history.

Opaque phases mainly include Fe-Ti and Nb-Ta oxides. The Nb-Ta oxides exhibit color variation from black to dark brown and often show oscillatory zoning. Some crystals show patchy texture or resorption appearance due to disequilibrium crystallization. Partial replacement of columbite by secondary pyrochlore is also observed. Zircon appears as isolated, reddish brown anhedral to subhedral crystals, as well as inclusions in the other minerals. Monazite-(Ce) is found as an anhedral to euhedral prismatic crystals (Figure 7e). Fergusonite occurs as anhedral to subhedral crystals, partially replaced by pyrochlore. Xenotime is present as an anhedral to subhedral crystals scattered throughout the rock and usually has zircon inclusions (Figure 7f). Bastnäsite is found as euhedral to subhedral crystals associated with allanite. Some bastnäsite crystals show zoning or patchy texture, attributed to minor decarbonation or hydration along fractures due to the circulation of deuteric fluids during late-stage cooling.

Thorite is present as subhedral to anhedral, cracked crystals often associated with muscovite (Figure 7g). Allanite appears as anhedral reddish brown crystals (Figure 7h). Cassiterite forms subhedral crystals characterized by oscillatory zoning. Pyrochlore appears as subhedral to rounded, yellowish-brown crystals, replacing fergusonite. Fluorite occurs as interstitial anhedral crystals or as veinlets.

3.3. Pyroxmangite

Pyroxmangite-rich pockets, identified for the first time within the Gabal El-Bakriya pegmatites, are composed mainly of pyroxmangite and garnet, with accessory pyrochroite, quartz, zircon, magnetite, rutile, and fluorite. Pyroxmangite occurs as large, anhedral crystals or as fine, needle-like crystals (Figure 8a). The large crystals exhibit fractures along perpendicular cleavage planes, filled with quartz and magnetite (Figure 8b). Garnet, represented by spessartine, occurs as anhedral crystals with internal cracks filled with secondary minerals (Figure 8c). Magnetite is present as anhedral crystals enclosed within pyroxmangite. Fluorite occurs as both large crystals and veinlets. Pyrochroite is present as anhedral crystals enclosed within fluorite or as veinlets (Figure 8d). Rutile forms fine clusters of reddish color.

4. Analytical Method

Mineral chemical compositions and backscattered electron (BSE) images of both major and accessory minerals were obtained on polished thin sections using a CAMECA SX100 electron microprobe (Agilent Technologies, Oslo, Norway) at the Department of Geosciences, University of Oslo, Norway. The sections were carbon-coated using a vacuum coater to ensure conductivity. Operating conditions were a 15 kV accelerating voltage, 20 nA beam current, peak counting time of 20 s, and a focused beam with a diameter of 2 μm. Analyses were calibrated using natural and synthetic mineral standards, and data were corrected using a ZAF matrix correction protocol. Mineral formula calculations were performed using both custom-built Excel spreadsheets and the Minpet software (Version 2.02, 1988–1997) package and were normalized on the basis of a fixed number of oxygen atoms specific to each mineral (as atom per formula unit, apfu).

Whole-rock, major, and trace element compositions of representative samples of the mineralized pegmatite of the Gabal El-Bakriya area were determined at the GeoAnalytical Laboratory, Washington State University, USA. Concentrations of major oxides and selected trace elements were measured using a ThermoARL XRF Spectrometer (Agilent Technologies, Bellevue, WA, USA). Calibration was performed using the USGS GSP2 standard. Loss on ignition (LOI) was calculated from weight loss after ignition at 1000 °C. Analytical precision was better than 1% (2σ) for most major elements and better than 5% (2σ) for most trace elements, except for Ni, Cr, Sc, V, and Cs.

Rare earth elements (REEs) and additional trace elements were determined by inductively coupled plasma mass spectrometry (ICP-MS) using an Agilent 7700 instrument (Bellevue, WA, USA). Approximately 50 mg of powdered sample was digested in acid-washed Teflon vessel with a 1:3 hydrofluoric and nitric acids at 250 °C for at least 8 h. Instrument sensitivity was calibrated using a blank fused bead from the same flux batch as the samples, along with USGS standards RGM-2 and AGV-2. Additional standards (DTS-2, BCR-1, and G-2) were analyzed as unknowns for quality control. Analytical precision was better than ±5% (2σ) for most trace elements and REEs.

5. Mineral Composition

We present data on composition of both essential and accessory minerals in the pegmatite and associated pyroxmangite from the Gabal El-Bakriya area. The complete dataset is provided in Supplementary Tables S1–S22.

5.1. Silicate Minerals

5.1.1. Feldspars

The feldspars (84 analyses) are compositionally indicative of crystallization under near-equilibrium conditions (Tables S1 and S2). The dominant K-feldspar has near-end member composition (KAlSi₃O₈), with a K₂O content of 15.72–15.5 wt.%, low Na₂O (0.21–0.45 wt.%), and negligible CaO (0.01–0.07 wt.%). The orthoclase content ranges between 95.68 and 98.0 mol.%. The albite analyses display high Na₂O (10.79–11.98 wt.%) and low CaO (0.01–0.46 wt.%), with albite content ranging between 96.10 and 99.34 mol.% and anorthite content <2 mol.%.

5.1.2. Micas

Mica compositions (23 of zinnwaldite and 27 of muscovite analyses; Tables S3 and S4) plot in the zinnwaldite, lithian muscovite, and lithian phengite fields of Tischendorf et al. [37] (Figure 9a). Based on the classification scheme of Miller et al. [38], muscovite is primarily of magmatic origin, with few compositions plotting in the secondary muscovite field (Figure 9b). Enrichment in Ti supports a magmatic origin [39], as Ti absent in secondary muscovite.

5.1.3. Pyroxmangite

Pyroxmangite, recorded here for the first time on the Nubian Shield, is a Mn-rich pyroxenoid (MnSiO₃), with gem-quality potential. It contains 27.19–39.22 wt.% MnO, 11.99–23.84 wt.% total iron (FeO^t) and 45.50–46.66 wt.% SiO₂, with minor CaO (0.88–1.06 wt.%) (Table S5).

5.1.4. Zircon

Zircon (20 analyses; Table S6) has low total major element content, occasionally <98 wt.%, likely due to metamictization. Contributions of TiO₂, Al₂O₃, and FeO* are minimal. Low ThO₂ (0.12–0.36 wt.%) and UO₂ (0.19–0.49 wt.%) contents are typical of magmatic zircon in the Eastern Desert [23,40,41]. The ThO₂/UO₂ (0.4–0.89) and position on SiO₂ vs. ThO₂/UO₂ diagram [42,43] support their magmatic origin (Figure 9c).

Figure 9. (a) Classification diagram of micas [37]; (b) Mg−Ti−Na ternary diagram of muscovite [38]; (c) ThO₂/UO₂ vs. SiO₂ diagram for zircon [42,43].

Figure 9. (a) Classification diagram of micas [37]; (b) Mg−Ti−Na ternary diagram of muscovite [38]; (c) ThO₂/UO₂ vs. SiO₂ diagram for zircon [42,43].

5.1.5. Thorite

Thorite (18 analyses; Table S7) exhibits total major element contents between 95.56 and 99.69 wt.% due to radiation damage. UO₂ contents (9.07–15.63 wt.%) suggest uranothorite composition. CaO content (0.41–1.26 wt.%) variations may reflect fluid composition or alteration [12]. Contents of P₂O₅ (0.92–2.01 wt.%) and trace REEs (e.g., Ce₂O₃, La₂O₃, Nd₂O₃, and Y₂O₃) suggest inclusions or minor substitution.

5.1.6. Topaz

Topaz (17 analyses; Table S8) shows limited compositional variations: the major oxide concentrations are Al₂O₃ (52.21–54.8 wt.%), SiO₂ (9.12–31.46 wt.%), and F (13.42–15.89 wt.%). Minor constituents include FeO, MnO, MgO, CaO, Na₂O, K₂O, P₂O₅, and TiO₂.

5.1.7. Chlorite

Chlorite (19 analyses; Table S9) shows a consistent composition, as follows: SiO₂ (27.02–28.56 wt.%), Al₂O₃ (17.06–20.21 wt.%), MgO (4.45–6.48 wt.%), and FeO (30.13–33.02 wt.%). The analyzed chlorite is chamosite and likely formed under reducing conditions [44,45]. Temperatures calculated using the Kranidiotis and MacLean [46] geothermometer yield 261–293 °C (avg. 271 °C).

5.2. Non-Silicate Minerals

5.2.1. Nb-Ta Oxides

The analyzed Nb-Ta oxide minerals in the pegmatite include columbite-(Mn) (24 analyses), fergusonite-(Y) (16 analyses), and pyrochlore (19 analyses). Chemical compositions and structural formulae of columbite are given in Table S10. Columbite is characterized by high contents of Nb₂O₅ (57.11–68.17 wt.%), Ta₂O₅ (9.27–19.94 wt.%), MnO (11.52–13.43 wt.%), and FeO (7.19–9.42 wt.%). On the classification diagram of Nb-Ta oxides, columbite composition plots within the Mn-dominant field (Figure 10a) [46]. Narrow ranges in FeO and MnO contents suggest lack of compositional zoning.

Fergusonite (YNbO₄) is a complex oxide that incorporates Nb, Y, and various REEs. Analytical data and calculated formulae are listed in Table S11. The fergusonite exhibits elevated Nb₂O₅ (39.77–41.32 wt.%) and Y₂O₃ (22.14–23.7 wt.%), with low TiO₂ (1.69–2.29 wt.%) FeO* (2.69–4.45 wt.%), CaO (0.7–1.77 wt.%), Ta₂O₅ (2.62–4.02 wt.%), UO₂ (4.17–4.67 wt.%), and ThO₂ (2.85–3.97 wt.%). REE content is dominated by HREEs, Dy₂O₃ (3.61–4.29 wt.%), Er₂O₃ (1.38–1.65 wt.%), and Yb₂O₃ (2.14–2.72 wt.%).

Pyrochlore compositions and structural formulae are reported in Table S12. Based on cation distribution at the B-site, the analyzed pyrochlores are Nb-dominant and fall within pyrochlore sensu stricto (Figure 10b) [47].

5.2.2. Niobian Rutile

Niobian rutile, Fe–Nb–Ti oxide, is reported here for the first time from pegmatites in the ANS. Fifteen analyses are presented in Table S13. The primary constituents are TiO₂, FeO*, and Nb₂O₅,with minor contribution of Ta₂O₅, SnO₂, and F.

5.2.3. Pyrochroite

Pyrochroite occurs exclusively in pyroxmangite-rich pockets within Gabal El-Bakriya pegmatite. It is a MnO-bearing phase, with low analytical totals (63.61–68.55 wt.%) due to its hydrous nature. Nineteen analyses and structural formulae are provided Table S14. Pyrochroite is composed mainly of MnO (63.61–68.55 wt.%), with subordinate FeO* (1.36–3.81 wt.%) and CaO (1.13–2.29 wt.%).

5.2.4. Cassiterite

Cassiterite occurs as a disseminated accessory mineral. Seventeen analyses are compiled in Table S15. It is compositionally homogeneous, with SnO₂ ranging between 95.35 and 96.65 wt.%. Other oxides are present in trace amounts, including SiO₂ (0.26–0.36 wt.%), TiO₂ (0.3–0.76 wt.%), FeO (0.06–0.32 wt.%), MgO (0.02–0.08 wt.%), CaO (0.11–0.12 wt.%), Na₂O (0.06–0.11 wt.%), K₂O (0.02–0.06 wt.%), Nb₂O₅ (0.53–1.24 wt.%), and Ta₂O₅ (0.12–0.58 wt.%).

5.2.5. Beryl

Beryl crystals (18 analyses; Table S16) exhibit chemical homogeneity, consistent with crystallization from Be-rich hydrothermal fluids. Major oxides include SiO₂ (62.74–65.31 wt.%), Al₂O₃ (16.43–18.04 wt.%), and BeO (12.11–13.96 wt.%). Minor constituents include FeO (0.73–1.4 wt.%), MgO (0.09–0.17 wt.%), CaO (0.02–0.13 wt.%), Na₂O (0.53–0.9 wt.%), and K₂O (0.01–0.18 wt.%).

5.2.6. Bastnäsite-(Y)

Bastnäsite, a rare earth fluorocarbonate [(Ce, La, Y) CO₃F], occurs as bastnäsite-(Y). Nineteen analyses are presented in Table S17. Bastnäsite-(Y) contains Y₂O₃ (36.92–38.51 wt.%), Gd₂O₃ (3.38–4.81 wt.%), Dy₂O₃ (2.74–3.73 wt.%), La₂O₃ (1.42–2.17 wt.%), Ce₂O₃ (4.41–5.74 wt.%), and ThO₂ (2.14–4.46 wt.%).

5.2.7. Monazite-(Ce)

Monazite, identified as monazite-(Ce), is represented by 23 analyses (Table S18). It contains significant P₂O₅ (23.48–26.93 wt.%), Ce₂O₃ (27.09–34.98 wt.%), La₂O₃ (5.59–7.85 wt.%), Pr₂O₃ (3.16–3.96 wt.%), Nd₂O₃ (8.15–9.74 wt.%), and Sm₂O₃ (2.04–2.79 wt.%), with minor Gd₂O₃ (0.6–0.8 wt.%), Dy₂O₃ (0.34–0.6 wt.%), and ThO₂ (7.55–19.22 wt.%).

5.2.8. Apatite

Some apatite analyses (Table S19) show low total major component content (up to 95.76wt.%) since Cl was not measured. Apatite composition includes CaO (50.26–53.93 wt.%), P₂O₅ (37.28–39.98 wt.%), and F (5.95–6.12 wt.%), along with minor SiO₂ (0.24–0.51 wt.%), total iron FeO^t (0.09–0.32 wt.%), MnO (0.08–0.34 wt.%), Na₂O (0.25–0.99 wt.%), and SO₃ (0.03–0.55 wt.%). High F content (2.64–3.25 wt.%) supports classification as fluorapatite.

Apatite occurs as an accessory phase and could preserve primary geochemical signature of the host magma [48,49,50,51,52,53]. On the discrimination diagrams of Chen et al. [48] and SiO₂ vs. MnO (Figure 10c,d) [48,51], the data indicate a magmatic origin for apatite, corroborated by its occurrence interstitially between feldspars and quartz or as inclusions within them.

Figure 10. (a) Ta/(Ta + Nb) vs. Mn/(Mn + Fe) classification diagram for Nb-Ta oxides [46]; (b) Ti-Nb-Ta ternary classification diagram for pyrochlore group minerals based on B-site cations [47]; (c) FeO* vs. SiO₂ diagram for apatite [48]; (d) SiO₂ vs. MnO diagram for apatite [51].

Figure 10. (a) Ta/(Ta + Nb) vs. Mn/(Mn + Fe) classification diagram for Nb-Ta oxides [46]; (b) Ti-Nb-Ta ternary classification diagram for pyrochlore group minerals based on B-site cations [47]; (c) FeO* vs. SiO₂ diagram for apatite [48]; (d) SiO₂ vs. MnO diagram for apatite [51].

5.2.9. Fluorite

Fluorite (25 analyses; Table S20) is composed predominantly of Ca (80.12–88.17 wt.%) and F (47.79–62.5 wt.%), with subordinate elements such as Si, Fe, Na, K, Mn, and Mg present in trace amounts (<1.0 wt.%).

Fe-Ti Oxides

The identified Fe-Ti oxides include magnetite (25 analyses) and ilmenite (14 analyses), with compositions and structural formulae reported in Tables S21 and S22. Magnetite is FeO*-rich (90–94.18 wt.%) and TiO₂-poor (0.0–0.44 wt.%). Ulvöspinel content, calculated following [54], is low (0.0–1.30 mol.%). Ilmenite exhibits compositional variation between FeTiO₃ and MnTiO₃ end-members, reflecting solid solution formation [55], as reflected by an elevated MnO content (3.07–10.14 wt.%). Calculated ilmenite composition ranges from 96.38 to 100.36 mol.% in the pegmatites.

6. Geochemical Characteristics

The results of major oxide compositions and calculated normative values of the pegmatite samples are represented in

Table 1. Major oxides and normative compositions of pegmatite of Gabal El-Bakriya area.

Rock Type	Pegmatite
Sample No	BK52	BK53	BK54	BK103	BK104	BK107	BK108	BK132	BK135
SiO2	80.69	79.35	78.07	78.83	80.05	79.22	78.47	77.54	76.51
TiO2	0.06	0.06	0.05	0.09	0.03	0.12	0.08	0.06	0.16
Al2O3	7.69	8.75	7.62	9.69	9.72	8.18	9.2	9.46	10.76
Fe2O3	1.28	1.32	1.24	1.18	1.39	1.62	1.93	1.43	1.64
MnO	0.01	0.01	0.01	0.01	0.03	0.02	0.02	0.3	0
MgO	0.07	0.08	0.09	0.04	0.05	0.03	0.06	0.14	0.17
CaO	0.24	0.16	0.2	0.15	0.3	0.24	0.23	0.28	0.63
Na2O	3.08	3.04	1.96	2.94	2.98	3.94	3.18	3.19	3.12
K2O	4.42	4.47	6.76	4.4	4.09	4.77	4.75	5.01	5.04
P2O5	0.01	0.01	0.01	0.02	0.02	0.01	0.03	0.02	0.04
LOI	0.95	1.09	1.03	0.78	0.82	0.54	0.61	0.71	0.83
Total	98.5	98.34	97.04	98.13	99.48	98.69	98.56	98.14	98.9
Normative composition
Quartz	45.28	46.34	47.68	44.42	42.67	38.07	52.22	47.83	50.15
Corundum	-	-	-	-	-	-	-	-	-
Orthoclase	26.74	24.53	28.76	28.7	30.42	30.41	26.8	27.19	41.65
Albite	25.58	25.59	15.82	21.33	21.32	26.95	15.32	20.69	1.61
Anorthite	0.24	1.07	-	-	-	0.47	-	-	-
Acmite	-	-	0.78	0.9	0.81	-	0.6	0.62	0.59
Na-Metasilicate	-	-	4.03	1.2	1.28	-	2.5	1.18	3.49
Diopside	0.34	0.27	1.02	0.85	1.13	2.12	1.01	0.66	0.84
Hypersthene	1.3	1.77	1.66	2.35	2.2	1.16	1.39	1.67	1.52
Magnetite	0.29	0.32	-	-	-	0.4	-	-	-
Ilmenite	0.18	0.06	0.23	0.16	0.12	0.31	0.12	0.12	0.1
Apatite	0.04	0.04	0.02	0.07	0.04	0.09	0.02	0.02	0.02
Some chemical parameters
R1	3212	3125	2890	3145	3280	2717	2935	2822	2763
R2	180	193	175	208	225	188	208	222	287
AI	1.28	1.12	1.38	0.99	0.96	1.42	1.13	1.13	0.98
ASI	0.75	0.86	0.70	0.98	0.98	0.68	0.85	0.89	0.92
Ti	360	360	300	540	180	719	480	360	959
K	36,692	37,107	56,117	36,526	33,952	39,597	39,431	41,590	41,839
P	44	44	44	87	87	44	131	87	175
Mg#	9.78	10.72	12.57	6.29	6.65	3.54	5.80	16.24	17.04
Colour Index	2.11	2.42	2.91	3.36	3.44	4	2.52	2.44	2.47
Diff. Index	97.59	96.45	92.25	94.46	94.41	95.43	94.34	95.72	93.42
ANOR	0.89	4.18	0.00	0.00	0.00	1.52	0.00	0.00	0.00
Q/	46.28	47.51	51.68	47.03	45.20	39.70	55.35	49.97	53.69

, while concentrations of trace elements and REEs are given in

Table 2. Concentrations of trace-elements and REE in pegmatites of Gabal El-Bakriya area.

Rock Type	Pegmatite
Sample No	BK52	BK53	BK54	BK103	BK104	BK107	BK108	BK132	BK135
Cr	2.33	4.11	2.45	3.09	2.37	3.77	1.89	2.25	1.54
Ni	4.59	2.74	3.32	5.11	3.68	14.02	2.75	3.21	12.63
Co	4.74	5.55	4.44	6.71	8.55	5.43	3.23	8.21	6.83
Sc	0.89	0.75	0.54	0.49	1.03	1.24	0.84	0.47	1.05
V	12.33	15.15	16.63	10.05	3.22	10.05	10.05	16.47	5.93
Cu	7.89	9.14	6.51	11.1	5.63	11.35	9.36	4.68	5.87
Pb	9.07	6.51	11.15	6.58	2.68	6.23	5.16	1.06	7.74
Zn	11.8	7.8	9.16	75.72	158.28	154.12	129.37	10.2	162.94
Rb	224.27	178.05	235.19	176.05	163.77	263.12	267.65	402.3	228.91
Ba	83.56	89.63	40.53	69.35	42.73	74.68	42.59	55.98	64.51
Sr	43.62	52.26	67.49	47.41	30.99	46.07	48.16	54.09	61.59
Ga	18.61	21.17	17.35	16.38	23.11	27.24	24.71	19.54	22.93
Ta	23.54	22.62	21.81	30.2	27.41	25.48	24.36	23.73	25.03
Nb	170.76	143.94	158.95	160.25	135.99	137.96	165.56	333.57	669.93
Hf	10.22	11.16	11.09	13.93	15.5	17.62	15.68	9.51	10.84
Zr	391.41	368.87	361.64	606.06	696.93	763.68	688.89	318.96	537.25
Y	126.36	157.96	130.98	57.62	70.64	63.21	51.32	334.53	464.47
Th	75.66	67.56	56.28	41.36	63.74	57.24	54.78	55.6	112.08
U	42.8	31.05	45.77	23.61	24.95	19.91	26.79	28.22	73.82
Li	39.66	34.46	34.9	35.13	30.58	30.61	34.56	33.41	35.91
Be	35.91	42.66	29.03	47.89	38.2	43.24	40.18	28.51	26.57
Mo	0.96	1.61	1.43	0.42	0.06	0.79	0.42	0.23	0.23
Cs	4.25	3.95	2.55	5.35	10.45	7.35	5.55	10.7	6.95
Sn	16.98	23.27	22.3	10.22	9.56	8.88	7.96	17.09	9.33
La	34.38	40.08	37.35	48.33	58.49	50.36	40.68	19.72	25.98
Ce	69.86	83.48	80.4	131.58	170.39	142.59	119.24	60.5	80.45
Pr	9.5	11.59	10.53	21.44	29.03	23.04	20.07	11	15.05
Nd	40.76	49.81	46.03	100.14	138.47	111.17	94.4	57.08	81.29
Sm	10.74	12.6	12.23	26.46	38.45	30.99	24.5	21.54	32.23
Eu	2.14	1.58	1.8	1.04	1.05	1.1	0.86	1.02	1.04
Gd	12.06	13.38	13.38	23.73	33.36	26.88	21.2	34.98	48.24
Tb	2.28	2.58	2.43	3.12	4.34	3.56	2.75	7.73	10.53
Dy	16.02	18.16	16.78	15.87	22.26	18.42	14.09	57.18	77.99
Ho	3.78	4.34	3.98	2.72	3.72	3.05	2.36	12.46	16.79
Er	12.26	14.76	12.65	6.92	9.05	7.55	5.87	37.59	49.83
Tm	2.1	2.6	2.1	0.96	1.25	1.02	0.81	5.32	6.84
Yb	16.42	20.28	15.95	5.85	7.28	6.29	4.91	31.5	39.45
Lu	2.26	3.46	2.78	0.87	1.07	0.92	0.74	3.96	4.85
Geochemical parameters
K/Rb	163.61	208.41	238.60	207.47	207.32	150.49	147.32	103.38	182.77
K/Ba	439.11	414.00	1384.57	526.69	794.58	530.22	925.83	742.94	648.56
Zr/Rb	1.75	2.07	1.54	3.44	4.26	2.90	2.57	0.79	2.35
Ba/Nb	0.49	0.62	0.25	0.43	0.31	0.54	0.26	0.17	0.10
Rb/Sr	5.14	3.41	3.48	3.71	5.28	5.71	5.56	7.44	3.72
Eu/Eu*	0.57	0.37	0.43	0.13	0.09	0.12	0.12	0.11	0.08
(La/Yb)n	1.42	1.34	1.58	5.59	5.43	5.41	5.60	0.42	0.45
(La/Sm)n	2.02	2.01	1.93	1.15	0.96	1.03	1.05	0.58	0.51
(Gd/Lu)n	0.65	0.47	0.59	3.34	3.82	3.58	3.51	1.08	1.22
(La/Lu)n	1.56	1.19	1.38	5.69	5.60	5.61	5.63	0.51	0.55

. The analyzed pegmatites display considerable compositional variability due to their coarse-grained nature and mineralogical heterogeneity. They are enriched in SiO₂ (76.51–80.69 wt.%), Al₂O₃ (7.62–10.76 wt.%), K₂O (4.09–6.76 wt.%), and Na₂O (1.96–3.94 wt.%), with notably low contents of total iron as FeO (0.24–1.64 wt.%), MgO (<0.7 wt.%), and CaO (<0.63 wt.%). Trace element abundances exhibit wide variation, with elevated concentrations of Nb (135.99–669.93 ppm), Y (51.32–464.47 ppm), Zr (318.96–763.68 ppm), Th (41.36–112.08 ppm), U (19.91–73.82 ppm), and REEs (ƩREE = 224.96–518.21 ppm). In contrast, Ba (40.53–89.63 ppm) and Sr (30.99–67.49 ppm) are relatively depleted. These geochemical characteristics, particularly the enrichment in Nb (Nb > Ta), Y, REE, Zr, Th, U, and F, are consistent with NYF-type pegmatites [56,57,58].

Primitive mantle (PM)-normalized multi-element patterns [59] (Figure 11a) exhibit pronounced enrichment in LILE (Rb, K, Th) and some HFSE (Ta, Zr), along with strong negative anomalies in Sr, Ti, and P. These features are typical of A-type granitoid sources [60]. The enrichment in Ta, Sn, and Cs further supports derivation from highly fractionated, evolved granitic melts.

Figure 11. (a) Primitive mantle-normalized spider diagram, and (b) Chondrite-normalized REE-patterns. Normalization values are from Sun and McDonough [60].

Figure 11. (a) Primitive mantle-normalized spider diagram, and (b) Chondrite-normalized REE-patterns. Normalization values are from Sun and McDonough [60].

REE concentrations are variable (173.94–518.21 ppm), and patterns show diversity in LREE versus HREE ratios [(La/Lu)_n = 0.51–5.69]. The following three REE patterns can be distinguished (Figure 11b): group I (samples BK103, BK104, BK107, BK108) displays LREE-enriched patterns [(La/Lu)_N = 5.69–5.60], group II (samples BK132 and BK135) is HREE-enriched [(La/Lu)_N = 0.51–0.55], and group III (samples BK52, BK53, and BK54) shows V-shaped patterns [(La/Lu)_N = 2.09–2.76]. Negative Eu anomalies (Eu/Eu* = 0.08–0.57) reflect plagioclase fractionation. The variation in REE patterns is interpreted to result from differences in the abundance and composition of accessory minerals such as monazite, zircon, fergusonite, xenotime, fluorite, pyrochlore, allanite, thorite, bastnäsite, samarskite, cassiterite, beryl, and pyrochlore.

Their alumina saturation index (ASI = molar Al₂O₃/(CaO + Na₂O + K₂O)) ranges from 0.68 to 0.98, indicating predominantly metaluminous composition. Agpaitic index [AI = molar (Na + K)/Al] values range between 0.96 and 1.38, suggesting alkaline to peralkaline affinity [61]. The presence of normative acmite (0.59–0.90) and Na-metasilicate (1.18–4.03) further supports the peralkaline character.

Other key geochemical features consistent with A-type granites include high Ga/Al, Nb, Zr, Y, Ta, and Th, along with depletion in MgO, CaO, and P₂O₅ [61,62,63,64,65]. Further discrimination, using Whalen et al. [66] diagrams, confirms their A-type character (Figure 12a,b). Subdivision into A1 and A2 types following Eby [64,65] places the studied pegmatites in the A1 field on the Nb-Y-Ga diagrams (Figure 13a), suggesting that they represent differentiated products of an oceanic island basalt (OIB)-like source, typically emplaced in continental rifts or associated with intraplate magmatism.

This within-plate (anorogenic) setting is further supported by high Rb/Sr (3.41–7.44) [67], and by discrimination diagrams of Pearce et al. [68], where the pegmatite data correspond to the within-plate field (Figure 13b).

Figure 13. (a) Nb-Y-Ce ternary discrimination diagram [66], (b) Y + Nb vs. Rb tectonic discrimination diagram [68].

Figure 13. (a) Nb-Y-Ce ternary discrimination diagram [66], (b) Y + Nb vs. Rb tectonic discrimination diagram [68].

7. Discussion

7.1. Petrogenesis

7.1.1. Pegmatite

Granitic pegmatites typically form during the late stages of magma crystallization and are generally composed of quartz, feldspar, ±muscovite, and ±biotite. They exhibit diverse textures and mineral assemblages [12,69,70,71,72,73]. Granitic pegmatites are critical for understanding the processes governing magma differentiation, late-stage fluid evolution, and the geochemical behavior of rare-metal elements. Pegmatites are also of considerable economic interest as important sources of strategic elements such as Nb, Ta, Li, Be, and REEs [74,75,76].

The origin of pegmatites has been the subject of longstanding debate, with the following two primary models proposed: (i) residual pegmatites, which formed by extreme fractional crystallization of granitic magmas [72,76,77,78,79,80,81], and (ii) anatectic pegmatites, generated by low-degree partial melting of crustal rocks [53,76,82,83,84,85,86,87,88]. In this study, multiple pegmatite bodies are found intruding or closely associated with the A-type granite of the GEBI. This spatial relationship implies that the pegmatites formed from a fluid-saturated granitic melt during the final stages of magmatic evolution [89,90,91,92]. The occurrence of fluorite in these pegmatites indicates significant fluorine enrichment in the parental magma. Furthermore, the observed gradational contacts between pegmatites and the host granite support a model of in situ magmatic fractionation and late-stage melt evolution.

Classification of the Gabal El-Bakriya pegmatites, based on field relationships and geochemical signatures, indicates they are simple, small-scale pegmatites with heterogeneous textures and compositions. They generated through late-stage fractional crystallization of the A-type granitic melt. The parental magma was likely derived from partial melting of the juvenile crust of the ANS, itself nearly contemporaneously sourced from the mantle. The hydrothermal fluids that influenced pegmatite crystallization are interpreted as magmatic in origin, as evidenced by the following: (i) the spatial and textural continuity between pegmatites and A-type granites; (ii) their preferential emplacement along the granite margins; and (iii) the presence of fluorine-bearing minerals and rare-metal enrichments. Late-stage fractional crystallization of the Gabal El-Bakriya granites produced volatile-rich, magmatic fluids that migrated toward the apex and periphery of the magma chamber. These fluids interacted with still-hot sub-solidus granite, were emplaced along fractures, and formed pegmatitic veins with gradational boundaries. The residual nature of these pegmatites is thus supported by (1) direct field relationships between pegmatites and hosts A-type granites, (2) the mineralogical and geochemical continuity between them, and (3) spatial proximity between the residual melt and its parental granite source. This genetic model aligns with numerous studies that support pegmatite formation via residual melts from granitic intrusions [57,76,85,93,94,95,96,97].

Based on mineralogical and geochemical criteria, the pegmatites are classified into the following three types: (i) Niobium–Yttrium–Fluorine (NYF) type, (ii) Lithium–Cesium–Tantalum (LCT) type, and (iii) a mixed NYF–LCT type. NYF-type pegmatites are enriched in Nb (Nb > Ta), Y, F, Zr, REE, and U, while LCT-type pegmatites are enriched in Ta (Ta > Nb), Cs, Li, Ga, Sn, Rb, and Be [56,57,58,98,99]. NYF pegmatites are typically small in size and emplaced in anorogenic settings, whereas LCT pegmatites tend to be larger in size and associated with orogenic environments [100,101]. Both types originate from highly evolved, volatile-rich melts that remobilize and concentrate critical metals [12,101]. Notably, the volatile-rich melts of NYF pegmatites are of pure magmatic origin, whereas LCT pegmatites often involve fluids derived from crustal anatexis [76,101]. The Gabal El-Bakriya pegmatites exhibit geochemical signatures, enrichments in Nb (Nb > Ta), Y, Zr, REE, U, and fluorine-bearing phases, and were emplaced in within-plate tectonic setting. These characteristics confirm that they are best classified as NYF-type pegmatites.

7.1.2. Pyroxmangite

This study documents the first reported occurrence of pyroxmangite hosted in pegmatites on the Nubian Shield, the western half of the ANS. It also represents the second reported occurrence on the ANS, with the first discovery being in pegmatites of Jabal Aja on the Arabian Shield, the eastern half of the ANS [102]. Pyroxmangite is the Mn-rich member of the pyroxenoid group, crystallizing in the triclinic system. The mineral typically shows two good cleavages, intersecting at 90 degrees. Pyroxmangite is structurally and compositionally similar to rhodonite. According to Maresch and Mottana [103], pyroxmangite forms under higher pressure and lower temperature conditions than rhodonite, a distinction that can be significant for interpreting the pressure–temperature (P–T) evolution of the host rocks.

The pyroxmangite-bearing pockets at Gabal El-Bakriya consist mainly of pyroxmangite and spessartine, with accessory quartz, pyrochroite, zircon, fluorite, and magnetite. The analyzed pyroxmangite crystals contain MnO (27.19–39.22 wt.%) and SiO₂ (45.50–46.66 wt.%), with very low concentrations of CaO (0.88–1.06 wt.%) and MgO (0.05–0.11 wt.%), suggesting minimal substitution of Mn by Ca and Mg. The compositional dominance of Mn²⁺ indicates Mn-enriched microenvironment that promoted pyroxmangite saturation. The presence of pyroxmangite rather than rhodonite in the Gabal El-Bakriya pegmatite further implies crystallization under elevated pressure conditions, consistent with late-stage crystallization of volatile-rich pegmatitic melts. The mineralogical association, especially with fluorite and pyrochroite, supports a magmatic-hydrothermal origin involving Mn-rich fluids.

7.2. Mineralization of the GEBI

Dynamic Model of Mineralization Enrichment

Preliminary field investigation of the GEBI revealed notable zones of mineralization associated with pegmatites. Integrated geological, petrographic, and mineral chemical data suggest that these mineralized pegmatites formed in a post-collisional tectonic setting, representing the final evolutionary stage of the GEBI. While the main phase of the alkali-feldspar granite is of primarily magmatic origin, hydrothermal overprinting became increasingly significant in the marginal zone, resulting in the formation of pegmatites, greisens, and associated mineralization. The primary mineralization assemblage includes barite, fluorite, Nb-Ta oxides, REE minerals, and a uranium-bearing phase. This mineralization assemblage is interpreted to have formed during the final stages of granitic crystallization. Intense metasomatism associated with these fluids caused widespread fluid–rock interaction and mass transfer in the outer zone of the alkali-feldspar granite. The pegmatites formed first, then while they were still hot and reactive, infiltrating, acidic fluids caused hydrothermal overprint. Mineral replacement, especially with volume decreased, further promoted fluid migration, enhancing mineral deposition along fractures and lithological contacts. The localization of mineralization was restricted to the apical and marginal parts of the alkaline feldspar granite. These zones represent structural and geochemical traps where volatile-rich hydrothermal fluids accumulated during the late magmatic stage. Before complete solidification of the melt, these volatile phases became increasingly concentrated in the upper part of the magma chamber. The progressive build-up of internal pressure from fluid accumulation led to hydraulic fracturing, initiating greisenization and alteration of the surrounding rocks [104]. Pockets in pegmatites might be either primary (due to saturation in a peralkaline magma) or secondary, caused by dissolution with an influx of aggressive, acidic fluids. The mineralized zone extends approximately 20–50 m along the fracture-controlled contact between the alkali-feldspar granite and the surrounding syenogranite, with local extension to the adjacent country rocks. Metasomatism was likely caused by externally derived aqueous fluids. The simultaneous decrease in temperature and increase in pressure during fluid exsolution and magma crystallization facilitated destabilization of early-formed magmatic minerals and their replacement by quartz, mica, topaz, fluorite, cassiterite, REE minerals, and uranium-bearing phases.

Mineral paragenesis indicates that cassiterite and beryl crystallized concurrently with the alteration of muscovite to topaz and quartz, pointing to a sustained supply of F-rich fluids during hydrothermal alteration. The occurrence of fluorite within pegmatites further confirms the abundance of fluorine in the residual fluids [105,106]. This style of mineralization is consistent with a magmatic cupola model associated with A-type granitic intrusions [104]. Chemical homogeneity of fluorite, topaz, and cassiterite (see mineral chemistry section) likely reflects stable hydrothermal conditions, potentially maintained by prolonged and focused fluid flow within the apical part of the granite. These observations support a residual magmatic-hydrothermal model for the origin of the mineralization at the GEBI, driven by the late-stage evolution of an evolved, volatile-rich A-type felsic magmatic system [11].

7.3. Ore Minerals

7.3.1. Nb-Ta Oxide Minerals

In the present study, Nb-Ta oxides are recognized in the pegmatites exhibiting notable compositional and textural variations, indicative of the complex evolutionary history of magmatic–hydrothermal system [107,108,109]. The main Nb-Ta oxide phases identified include columbite, fergusonite, and pyrochlore.

Some columbite crystals show oscillatory zoning, patchy textures, and resorption features, which are diagnostic of crystallization under dynamic magmatic conditions. Zoning in columbite reflects differential diffusion rates of Ta and Nb in silicate melts relative to the rate of crystal growth and is characteristic of late-stage magmatic evolution [110]. This interpretation is supported by the frequent association of columbite with magmatic micas [111,112]. Experimental data suggest that fluxing agents and melt composition significantly influence the solubility of Nb-Ta oxides in silicate melts [112,113,114]. Columbite can crystallize early from melts containing >0.05 wt.% of MnO and FeO and Nb concentrations of ~70–100 ppm, at temperatures as low as 600 °C [105]. As temperature decreases, the solubility of Nb and Ta complexes in the melt declines sharply [113], favoring their saturation and crystallization. In the GEBI pegmatites, columbite is interpreted as a primary magmatic phase, later modified by hydrothermal fluids. Overgrowth textures and partial replacement by pyrochlore suggest a multi-stage evolutionary sequence, wherein primary columbite was reworked during late-magmatic to early hydrothermal stages. Patchy textures and resorption rims are consistent with dissolution–reprecipitation processes, attributed to the influx of volatile-rich fluids capable of redissolving and remobilizing Nb and Ta [114,115,116]. The characteristics of Nb–Ta mineralization in the Gabal El-Bakriya pegmatites are comparable to those reported from post-collisional A-type granites and highly fractionated I-type granites in the Eastern Desert of Egypt [12,22,23,117,118,119,120,121,122].

Fergusonite, another major Nb–Ta-bearing phase, is a complex oxide enriched in REEs, Y, and Nb. The HREE-enrichment, along with fluorine content (0.39–0.78 wt.%), suggest crystallization from evolved, F-rich fluids. Minor Na₂O, K₂O, and P₂O₅ may reflect secondary substitution or inclusions.

The presence of significant UO₂ (4.17–4.67 wt.%) and ThO₂ (2.85–3.97 wt.%) in fergusonite resulted in partial metamictization, a common feature in U- and Th-rich minerals due to prolonged radiation damage [123,124,125,126,127]. Together, the Nb–Ta oxide mineral assemblage in the Gabal El-Bakriya pegmatites reflects a magmatic evolution, from high-temperature crystallization of columbite to hydrothermal alteration and metasomatic modification, culminating in the formation of economically significant mineralization in a volatile-rich, peralkaline A-type granite system.

7.3.2. Bastnäsite

In the Gabal El-Bakriya pegmatites, bastnäsite occurs as euhedral to subhedral crystals, consistent with late-stage magmatic crystallization. The absence of replacement textures and the presence of well-developed zoning in bastnäsite crystals support its primary magmatic origin rather than a secondary or metasomatic one. Moreover, its coexistence with allanite, a well-known magmatic REE-bearing phase, further indicates crystallization from an evolved, incompatible-element-rich melt.

Slight patchiness observed in some bastnäsite-(Y) grains may result from minor decarbonation or hydration along crystal boundaries or fractures, likely triggered by interaction with late-stage deuteric fluids during the cooling phase. However, the preservation of zoning and the lack of pervasive alteration textures suggest minimal post-magmatic overprint, unless the bastnäsite-(Y) grew from the post-magmatic fluids.

7.3.3. Cassiterite

Cassiterite is observed within pegmatites, formed during a late-stage hydrothermal episode along newly developed fractures. The presence of oscillatory zoning in cassiterite crystals indicates episodic variations in fluid composition, possibly controlled by fluctuations in pH, redox conditions, or metal availability during crystallization [128].

7.3.4. Beryl

Beryl is identified in both pegmatite and greisen assemblages associated with the alkali-feldspar granite of the GEBI. Its presence reflects a Be-enriched fluid, underscoring the role of fluid-mediated metal transport in evolved A-type systems [129,130].

8. Conclusions

• Pyroxmangite is reported for the first time in the Gabal El-Bakriya pegmatites and on the whole Nubian Shield. The pegmatites are hosted by post-collisional, A-type granites of the Gabal El-Bakriya intrusion (GEBI). They were emplaced as dikes and plugs along the margin of the alkali-feldspar granite.
• The pyroxmangite pockets consist essentially of pyroxmangite mineral alongside garnet, pyrochroite, and fluorite. The composition of pyroxmangite reflects Mn-rich, low-Ca, and high-silica fluids that infiltrated at the hydrothermal stage.
• The pegmatites are NYF-type, characterized by high SiO₂ content and significant enrichment in Nb, Y, REE, Zr, Th, U, and F. These geochemical characteristics indicate advanced magmatic differentiation under oxidizing, fluorine-rich conditions.
• The NYF-pegmatites host a diverse suite of REE- and HFSE-bearing accessory minerals, including monazite-(Ce), xenotime, bastnäsite, fergusonite, and pyrochlore, indicating favorable conditions for the concentration of critical metals. The evolved nature of the GEBI system, combined with its complex mineralogy and REE–Nb–Ta–U–F enrichment, highlights its high potential for mineral exploration for critical minerals, particularly in the greisens, pegmatites, and quartz-fluorite veins. In addition, the studied pyroxmangite has potential economic value for its use in jewelry and decorative applications.