Late Neoproterozoic Rare-Metal Pegmatites with Mixed NYF-LCT Features: A Case Study from the Egyptian Nubian Shield

Abstract

The current work records for the first time the rare-metal pegmatites with mixed NYF-LCT located at Wadi Sikait, south Eastern Desert of the Egyptian Nubian Shield. Most of the Sikait pegmatites are associated with sheared granite and are surrounded by an alteration zone cross-cutting through greisen bodies. Sikait pegmatites show zoned and complex types, where the outer wall zones are highly mineralized (Nb, Ta, Y, Th, Hf, REE, U) than the barren cores. They consist essentially of K-feldspar, quartz, micas (muscovite, lepidolite, and zinnwaldite), and less albite. They contain a wide range of accessory minerals, including garnet, columbite, fergusonite-(Y), cassiterite, allanite, monazite, bastnaesite (Y, Ce, Nd), thorite, zircon, beryl, topaz, apatite, and Fe-Ti oxides. In the present work, the discovery of Li-bearing minerals for the first time in the Wadi Sikait pegmatite is highly significant. Sikait pegmatites are highly mineralized and yield higher maximum concentrations of several metals than the associated sheared granite. They are strongly enriched in Li (900–1791 ppm), Nb (1181–1771 ppm), Ta (138–191 ppm), Y (626–998 ppm), Hf (201–303 ppm), Th (413–685 ppm), Zr (2592–4429 ppm), U (224–699 ppm), and ∑REE (830–1711 ppm). The pegmatites and associated sheared granite represent highly differentiated peraluminous rocks that are typical of post-collisional rare-metal bearing granites. They show parallel chondrite-normalized REE patterns, enriched in HREE relative to LREE [(La/Lu)_n = 0.04–0.12] and strongly negative Eu anomalies [(Eu/Eu*) = 0.03–0.10]. The REE patterns show an M-type tetrad effect, usually observed in granites that are strongly differentiated and ascribed to hydrothermal fluid exchange. The pegmatite has mineralogical and geochemical characteristics of the mixed NYF-LCT family and shows non-CHARAC behavior due to a hydrothermal effect. Late-stage metasomatism processes caused redistribution, concentrated on the primary rare metals, and drove the development of greisen and quartz veins along the fracture systems. The genetic relationship between the Sikait pegmatite and the surrounding sheared granite was demonstrated by the similarities in their geochemical properties. The source magmas were mostly derived from the juvenile continental crust of the Nubian Shield through partial melting and subsequently subjected to a high fractional crystallization degree. During the late hydrothermal stage, the exsolution of F-rich fluids transported some elements and locally increased their concentrations to the economic grades. The investigated pegmatite and sheared granite should be considered as a potential resource to warrant exploration for REEs and other rare metals.

1. Introduction

Minerals, especially those containing rare metals, are of critical importance to human life. Some pegmatites contain important rare metals such as tantalum, niobium, zirconium, tin, tungsten, titanium, and beryllium. They are utilized in many important strategic and electronic industries of our time, which are indispensable for mankind. Rare metal pegmatites constitute a distinct category of igneous rocks and have become the focus of extensive geological research due to their potential to host valuable mineral deposits, including REE, Li, Be, Ta, Nb, Zr, Y, W, and Sn (e.g., [1,2,3,4,5,6,7,8,9]). The intriguing relationship between the occurrence of rare metal mineralization with pegmatites has been extensively documented worldwide [10,11,12,13]. However, the genetic relationships between rare metal mineralization and their host pegmatites are complex and remain unclear due to heterogeneity in the source region, complexity of the tectonic setting, multi-stage formational processes, and overprinting metasomatic events (e.g., [2,7,14,15,16,17]).

The Arabian–Nubian Shield (ANS) contains geological settings that are appropriate for the concentration of rare metals to economic grades. The ANS was formed during the Pan-African orogeny through the collision of East and West Gondwana [18,19,20]. The Late Neoproterozoic rocks of Egypt are a part of the Nubian Shield, which was an adjoining part of the Arabian–Nubian Shield (ANS) before the opening of the Red Sea. A notable characteristic of the Nubian Shield is the widespread occurrence of granitoids in the Eastern Desert and Sinai. These granitoids exhibit a range of ages and geochemical characteristics (e.g., [21,22,23]). Azer and Asimow [24] classified the Egyptian granitoids into three petrological categories, including synorogenic calc-alkaline granitoids, late to post-orogenic calc-alkaline granitoids, and post-orogenic alkaline granites.

In the ANS, the post-collisional granites and the associated pegmatites have attracted the attention of many researchers because they host a lot of strategic metals such as Zr, Nb, Ta, Li, Be, Sn, W, Cs, U, Th, Y, Pb, Zn, and REEs that can be used in recent technological applications (e.g., [2,25,26,27,28]). On the other hand, in the Egyptian Nubian Shield, the geological environments suitable as a target for rare metals exploration are available, especially granitoids and associated pegmatites. Some of the post-collisional granitic intrusions and associated pegmatites are known to have high concentrations of many of the rare metals [7,29,30,31,32]. However, despite the evident presence of these rare metals, mineralizations in some pegmatites and granite plutons, their assessment and the understanding of the genetic processes that contributed to their formation remain poorly understood.

Wadi Sikait, as a part of the Egyptian Nubian Shield, is considered a promising area for exploration surveys, especially for rare metal deposits. This work aims to characterize the geology, mineral chemistry, geochemistry, and petrology of the rare metal pegmatites and their host granites to understand the petrogenetic modeling leading to the formation of mineralized post-collisional pegmatites.

2. Geologic Setting

The studied rare-metal pegmatite (RMP) bodies are located in the Sikait–Nugrus–Abu Rusheid area, which lies to the southwest of Mersa Alam in the Eastern Desert of Egypt (Figure 1a). This area is a part of Neoproterozoic rocks of the Egyptian Nubian Shield and represents one of the most important areas in the south Eastern Desert, especially for rare-metal mineralization. Many ancient emerald mines are distributed in several localities at the Sikait area, which represent the oldest emerald mines in the world, since the Pharaonic times [33].

The geology of the Wadi Sikait area has been previously studied by many authors (e.g., [34,35,36,37], etc.). The study area is located in the huge fault systems of the Sikait–Nugrus shear zone, which exhibits varying degrees of deformation [38,39,40,41]. It is cut by various wadis such as Wadi Nugrus, Wadi Abu Rusheid, and Wadi Sikait. Also, there are several thrusts and shear zones following an NW–SE trend caused by the Najd Shear System [42,43]. The Neoproterozoic rocks cropping out in the study area are gneiss, ophiolitic metagabbro, ophiolitic mélange, and syn- to post-collisional granitoids (Figure 1b). Gneisses occur as unmappable small masses (<100 m long) along Wadi Abu Rusheid and Wadi Nugrus as well as blocks of various sizes in the ophiolitic mélange. They seem to be thrust over the ophiolitic mélange, with a well-defined tectonic contact. The present field observations indicate that gneiss may be older than the ophiolitic mélange. Along the WNW-ESE direction (Nugrus thrust fault), the ophiolitic metagabbros are thrust over the ophiolitic mélange from the south and southwest at low to high angles (30°). Some outcrops of the ophiolitic mélange occur as xenoliths or relicts within the granitoids. The ophiolitic mélange consists of masses and fragments of different sizes and types embedded in a sheared and schistose volcaniclastic and sedimentary matrix. The main blocks are represented by metapelitic schists, gneisses, amphibolite, and metagabbro.

Granitoid rocks in the study area include syntectonic granitoids and younger post-tectonic leucogranites. The syntectonic granitoids are represented by tonalite and granodiorite; they are less common and did not map because their outcrops are less than the map scale. The post-tectonic leucogranites include sheared granite, muscovite granite, and Li-rich granite as well as pegmatites. They are intruded by lamprophyre dykes, besides pegmatite and quartz veins. As a result of the tectonic activity in the study area, most of the marginal parts of the post-tectonic granites are sheared and affected by different styles of alteration, such as hematitization and silicification, as well as argillic and phyllic alteration. Most of these alterations are apparently trending in NW–SE directions comparable to the Najd fault.

The sheared granite is fine to medium-grained rock and contains blocks as well as roof pendant of the ophiolitic mélanges (Figure 2a). It is light grey to grey in color, highly sheared, and exhibits titled apparent layering (Figure 2b). The highly altered sheared granite has alternative bands and batches of reddish and yellowish color due to staining using iron solutions (Figure 2c). The contacts between the sheared granite and its country rocks host many ore minerals (sulphides, radioactive minerals, and Nb-Ti oxides) that are visible on the macroscopic scale at the surface and along the fractures. Few lamprophyre dykes are intruded into the sheared granite with vertical dip. They are fine-grained with dark grey to brownish grey colors and highly mineralized [44].

Muscovite granite occupies most of the mapped area. It has sharp intrusive contacts with the sheared granite and the older rocks (Figure 2d). This rock is medium to coarse-grained and varies from massive to weakly deformed. It is highly jointed and contains few xenoliths of older rocks. Along shear zones, the muscovite granite exhibits gneissose texture and contains many pegmatite segregations of leucogranite. Li-rich granite has coarse to pegmatitic grain sizes and occurs as large white masses that are intruded by the muscovite granite and the ophiolitic mélange with sharp contacts (Figure 2e). The largest exposure of Li-rich granite is found at W. Um Soleimat with a NW–SE trending outcrop. It also occurs as apophysis and offshoots protruded into the muscovite granite and the ophiolitic mélange (Figure 2f). This granite is cut by numerous pegmatite dikes and quartz veins.

Figure 1. (a) General geological map of the central and southern sectors of the Eastern Desert of Egypt (dividing line adapted from [45]), showing the location of the study area. (b) Detail geologic map of the Wadi Sikait area (modified after [46]).

Figure 1. (a) General geological map of the central and southern sectors of the Eastern Desert of Egypt (dividing line adapted from [45]), showing the location of the study area. (b) Detail geologic map of the Wadi Sikait area (modified after [46]).

Figure 2. (a) Roof pendant of the ophiolitic mélange above sheared granite, (b) titled apparent layering in sheared granite, (c) alternative bands of reddish and yellowish color in sheared granite, (d) intrusive contact between muscovite granite and ophiolitic mélange, (e) intrusive contact between Li-rich granite and its country rocks, and (f) offshoots of Li-rich granite in ophiolitic mélange.

Figure 2. (a) Roof pendant of the ophiolitic mélange above sheared granite, (b) titled apparent layering in sheared granite, (c) alternative bands of reddish and yellowish color in sheared granite, (d) intrusive contact between muscovite granite and ophiolitic mélange, (e) intrusive contact between Li-rich granite and its country rocks, and (f) offshoots of Li-rich granite in ophiolitic mélange.

Some pegmatitic bodies are observed in the post-tectonic granites, especially in the sheared granite. They occur as dikes and pockets of different sizes (Figure 3a,b). The pegmatites and the associated rocks are sheared and brecciated and cross-cut by a number of quartz veins that are enclosed by a few meters of alteration zones and greisens. The pegmatitic dikes trend NNW–SSE with a dip of about 15–30° parallel to the apparent banding of the sheared granite. They are zoned, composed of barren cores (quartz and K-feldspars), and surrounded by mineralized outer zones (emerald, Nb-Ta oxides, and REE-bearing minerals). Emerald crystals with variable sizes (up to 5 cm long) occur at the contact between the sheared granite and the pegmatites as well as the metasediments. Emerald has a green color and materializes as large crystals in the phlogopite schist (Figure 3c) and as disseminated smaller crystals in the outer wall zones of pegmatite (Figure 3d). Greisen is a fine-to-medium-grained rock that has a light color (Figure 3e). Few yellow pockets of uranium-rich rock are recorded in the greisen within the alteration zone around the pegmatite bodies (Figure 3f).

3. Materials and Methods

3.1. Fieldwork and Sampling

The field works were carried out through different field trips to collect fresh samples on a systematic basis and field observations for lithological variation, contacts, and structural elements. Representative sampling was carried out using GPS locations and extensive photo-documentation of sample sites. In order to discern the petrological characteristics of the studied granitoids and the types of alterations, 25 samples of the pegmatites and 20 samples of the country rocks were collected for the present study.

3.2. Petrographic Studies

Petrographic studies of freshest-possible samples along with altered samples were conducted with the intent of characterizing the entire mineralized samples and selecting a subset for microprobe mineral analyses and whole-rock major element compositions. Representative thin and polished sections were prepared for the petrographical studies to examine to mineralogical composition, including alterations and textures. Special care was taken to note the abundance of important ore minerals.

3.3. Analytical Conditions

Electron probe microanalysis and backscattered electron imaging of the essential and the accessory minerals were conducted at the Department of Geosciences, University of Oslo, Norway, using a CAMECA SX100 electron microprobe (CAMECA, Madison, WI, USA). Carbon-coated, polished thin sections were used for this study. Analytical conditions were 15 kV, 15 nA, 2 μm diameter beams, with a counting time of 10 s on-peak and 5 s for the background. Natural and synthetic mineral standards were used, as well as a ZAF matrix correction routine. The standards were orthoclase for K, albite for Na and Al, anorthite for Ca, rutile for Ti, zircon for Si, forsterite for Mg, and fayalite for Fe. The low detection limits (LDL) of all analyzed oxides are ~0.01. The Li content of micas was quantified using laser-ablation inductively coupled plasma mass spectrometer (LA-ICP-MS), using a New Wave Research UP 193 Solid State Laser System coupled to an Agilent 8800 triple-quadrupole ICP-MS (Agilent Technologies, Oslo, Norway).

The XRF and ICP-MS analyses of some selected samples of the mineralized pegmatite (outer zones) as well as adjacent sheared granite from Wadi Sikait were carried out at the GeoAnalytical Lab, Washington State University (WSU), USA. The analyzed samples were crushed using an agate grinding bowl into pebble particles of homogenized size, which were subsequently pulverized. Major oxides and some trace element contents were determined via X-ray fluorescence with a ThermoARL XRF Spectrometer (Agilent Technologies, Oslo, Norway). The powder of each sample was weighed and mingled with two parts of di-lithium tetraborate flux, then fused at 1000 °C in a muffle furnace and cooled. The resulting bead was ground again, re-fused, and then polished on a diamond lap to produce a flat and smooth surface suitable for analysis. The standard used for calibration was the reference material GSP2 standard rock powder from the USGS. Detection limits for the major oxides and trace elements are available online from the GeoAnalytical Lab. The loss on ignition (LOI) was determined by the difference in weight after ignition at 1000 °C.

Concentrations of REE and some trace elements were determined using ICP-mass spectrometry (in particular, an Agilent 7700 ICP-MS instrument, Agilent Technologies, Oslo, Norway). The powder of each sample, approximately 50 mg, is dissolved in an acid-washed Teflon vessel via refluxing in hot (250 °C) at a 1:3 hydrofluoric and nitric acid ratio for at least 8 h. A calibration for the instrument sensitivity was developed using an empty fused bead from the same batch of flux as used to prepare the unknowns, along with RGM-2 and AGV-2 standards from USGS. Additional USGS standards (DTS-2, BCR-1, and G-2) were included as unknowns for quality control.

4. Results

4.1. Petrography

The petrographic features of the studied rocks in the Wadi Sikait area revealed that the granitic rocks consist of different types, including sheared granite, muscovite granite, and Li-rich granite. Also, many pegmatitic dykes cut through the different types of granite. The petrographic descriptions of pegmatites and their host granites, as well as greisen, are described in detail below.

4.1.1. Pegmatite

Wadi Sikait pegmatites are medium to very coarse-grained rocks, consisting essentially of quartz, K-feldspars, albite, and micas of various types, occupying the outer zones. Their compositions are similar to Zareib pegmatite [2] as well as granitic rocks with a higher percentage of K-feldspars. Pegmatites contain a wide range of accessory minerals such as columbite, zircon, apatite, garnet, pyrochlore, monazite, xenotime, fluorite, thorite, allanite, cassiterite, beryl, and opaques. K-feldspars are the essential minerals and occur as subhedral crystals of orthoclase and, less commonly, microcline. Orthoclase exhibits different types of perthitic texture, including patch and flame types, whereas the microcline is characterized by cross-hatched twinning (Figure 4a). Some coarse crystals of K-feldspar contain small inclusions of albite and/or opaques and zircon. Quartz occurs as coarse crystals showing wavy extinction, and some crystals include inclusions of K-feldspars, albite, and zircon (Figure 4b). Albite occurs as anhedral to subhedral columnar crystals with lamellar twinning and is corroded by quartz and perthite.

Micas in the pegmatites include lepidolite, zinnwaldite, and muscovite. Lepidolite occurs as subhedral to anhedral pinkish to violet color crystals, associated with other mica phases or other minerals (Figure 4c,d). In some parts, it is deformed, showing kinks and wavy margins. Zinnwaldite forms large subhedral fractured crystals associated with muscovite and lepidolite, and is locally corroded by K-feldspars and quartz (Figure 4d). Muscovite is present in both primary and secondary phases. The primary muscovite crystals are euhedral to subhedral plates that were corroded by orthoclase and quartz (Figure 4d). Secondary muscovite forms anhedral aggregates associated with sericite or filling fractures.

The most prevalent accessory mineral is zircon, which can be found as inclusions in other minerals or as isolated reddish-brown anhedral to subhedral crystals. Allanite exists as reddish-brown anhedral crystals that are disseminated in the rock (Figure 4e), and they are locally associated with zircon and thorite. Tourmaline forms anhedral crystals among quartz grains (Figure 4f). Garnet is isotropic and occurs as irregular and rounded crystals with high relief. Some garnet crystals are zoned with a core containing an inclusion of rutile and thin rims free from inclusions. Xenotime is found as interstitial grains or as overgrowths on zircon. Beryl occurs as greenish-yellow to pale brown hexagonal crystals associated with muscovite, topaz, and fluorite. Topaz forms small idiomorphic crystals. Fluorite is found as interstitial crystals among other minerals and as veinlets filling fractures.

Other accessory minerals are identified in the backscatter images, such as monazite, thorite, columbite, and pyrochlore. Monazite arises as individual anhedral crystals that are surrounded by an amorphous radiation damage halo (Figure 5a). Thorite exists as subhedral to anhedral cracked crystals that are associated with allanite and zircon (Figure 5b). Locally, thorite is partially metamictized and covered with amorphous blackish brown materials. Nb-Ta oxides occur either as individual crystals or as granular aggregates associated with altered mafic minerals. Also, they are found as small inclusions in quartz and feldspars. They are represented mainly by columbite that is altered along the margins into pyrochlore (Figure 5c) or fergusonite. Cassiterite exists as subhedral to euhedral crystals and is characterized by oscillatory zoning (Figure 5d).

4.1.2. Sheared Granite

Sheared granite consists mainly of quartz, K-feldspars, albite, muscovite, and zinnwaldite, along with many accessory minerals. The accessory minerals are opaques, zircon, allanite, garnet, monazite, thorite, xenotime, and fluorite. The major minerals show subparallel arrangement, forming weak foliation (Figure 6a). Quartz occurs as coarse grains with serrated boundaries and shows wavy extinction; it also exists as recrystallized grains segregated in bands defining the weak foliation of the rock. Larger quartz crystals show undulose and chessboard extinction due to deformation. K-feldspars are present as anhedral to subhedral crystals of microperthite or microcline, which are locally sericitized. Some K-feldspar crystals are affected by cracks filled with microcrystalline silica and muscovite that are displaced later. Albite crystals are subhedral to anhedral tabular and prismatic and are corroded by quartz and K-feldspars. Their lamellar twins show kinking, bending, and displacement resulting from the brittle deformation. Muscovite shows two generations, primary large crystals that are corroded by quartz and K-feldspars (Figure 6b) and secondary fine aggregate. The large crystals of muscovite exhibit the bending of cleavage planes as a result of dynamic loading during deformation. Zinnwaldite occurs as corroded crystals containing inclusions of albite (Figure 6c). Zircon occurs as small prismatic crystals disseminated among other minerals or as inclusions within K-feldspars and quartz. Occasionally, it shows pleochroic haloes due to its radioactive contents. Fluorite exists as an interstitial filling space between essential minerals. Allanite is associated with zircon and opaques, occurring as zoned crystals that are locally altered along margins to bastnaesite (Figure 6d). Garnet shows equant anhedral crystals containing quartz inclusions and crossed by cracks filled with secondary minerals. Monazite is found as euhedral prismatic crystals. Xenotime forms subhedral crystals displaying high-order interference colors. Thorite occurs as anhedral, cracked crystals that are associated with allanite. Opaque minerals comprise Fe-Ti oxides, Nb-Ta oxides, pyrite, chalcopyrite, and galena.

4.1.3. Greisen

Greisen is a fine-to-medium-grained rock, consisting mainly of K-feldspars, quartz, and muscovite. Accessory minerals include cassiterite, fluorite, tourmaline, and topaz. K-feldspars are subhedral to anhedral crystals of microcline and perthite that are occasionally kaolinized, especially along the fractures. Quartz occurs either as coarse anhedral crystals that exhibit wavy extinction, or as fine aggregates, or as fracture fillings. The coarse quartz crystals have microfractures that are occasionally filled with secondary muscovite. Muscovite forms large anhedral crystals embedded in anhedral quartz or as fine aggregates formed at the expense of K-feldspars. Few large crystals of muscovite are slightly altered to secondary muscovite and chlorite. Cassiterite shows two generations, an earlier generation with parallel oscillatory zoning and a later generation showing chaotically zoned crystals. Tourmaline forms anhedral to subhedral tabular crystals among quartz grains (Figure 6e). Fluorite occurs as small anhedral crystals interstitial to other minerals or as veinlets filling fractures. Topaz is found as subhedral crystals with moderate relief and clear cleavage (Figure 6f).

4.2. Mineral Chemistry

The identification of the essential and accessory minerals under the microscope was confirmed using electron probe microanalysis (EPMA). The analyzed minerals include silicates (feldspars, garnet, micas (lepidolite, zinnwaldite, muscovite, and biotite), and ore minerals (columbite, fergusonite-(Y), cassiterite, allanite, monazite, bastnaesite, thorite, zircon). The obtained data were used to determine the chemical variation and proper nomenclature of the analyzed minerals. Also, the analyzed minerals were used to identify the source magma and tectonic setting of the host rocks. Various software applications were used to process the raw data in order to calculate the provided structural formulas and present them on appropriate classification diagrams. All the microprobe data are given in Supplementary Tables S1 and S2 and discussed here.

4.2.1. Silicate Minerals

Feldspars are analyzed from both pegmatites (43 spots) and sheared granites (26 spots). The chemical composition, structural formulae, and end-member components of all analyzed feldspars are given in Supplementary Table S1 (K-feldspar and albite). The K-feldspar crystals exhibit homogeneous composition close to end-member KAlSi₃O₈ (Or = 92.11–97.64 mol.%). They show enrichment in K₂O with a narrow range (15.39–16.36 wt.%) and low Na₂O (0.21–0.86 wt.%) and CaO (0.0–0.09 wt.%) contents. The only plagioclase mineral in the analyzed samples is albite. It has high Na₂O content (10.98–11.88 wt.%) and low CaO content (0.1–1.34 wt.%). The analyzed albite crystals are pure with high albite contents (92.69–99.33 mol.%) and very low An content (0.05–6.02 mol.%).

Garnet crystals from both pegmatite (41 spots) and sheared granite (47 spots) were analyzed, and their chemical composition, structural formulae (on the basis of 12 oxygens), and end-members are given in Supplementary Table S1. The analyzed garnet crystals are represented mainly by almandine-rich garnets. The analyzed garnet crystals from pegmatite are rich in FeO (28.13–31.27 wt.%) and depleted in the MnO (7.41–9.23 wt.%) than those found in the sheared granite (22.84–30.55 wt.% FeO and 7.46–13.59 wt.% MnO). Garnets in pegmatites are predominantly almandine-rich (55.07–59.74 mol.%), with significant spessartine and pyrope components, and relatively lower grossular content. This composition suggests crystallization under conditions of high Fe and Mn availability, typical of pegmatitic environments. Conversely, garnets from sheared granites contain much lower almandine (40.95–54.95 mol.%) and show a broader compositional range, including elevated grossular and andradite components, indicative of calcium enrichment and potential metasomatic influences during deformation. The presence of katoite in both rock types highlights hydration processes, though its higher abundance in sheared granites may point to more intense fluid interactions.

The analyzed garnets have chemical compositions (low MgO and high MnO) similar to those crystallized from silicic magma at low pressure [47]. Using the Mn-Mg-Fe ternary diagram of garnet discrimination [48,49,50], all the analyzed garnets plot close to the magmatic garnet field (Figure 7a). On the MgO-FeO-MnO ternary discrimination diagram [51], all the analyzed garnet crystals plot close to the I- and S-type fields (Figure 7b).

Micas in the pegmatite and sheared granite are identified as lepidolite, zinnwaldite, muscovite, and biotite. The microprobe analyses of the analyzed micas (228 spots) are represented in Supplementary Table S1. When plotting the microprobe analyses of the analyzed micas on the discrimination diagram of [52], they are plotted in the lepidolite, zinnwaldite, muscovite, and biotite (Figure 7c), supporting petrographic identification.

Lepidolite is recorded only in the pegmatites, and its chemical composition (57 spots) and structural formula are given in Supplementary Table S1. The analyzed lepidolite crystals have high SiO₂ (47.21–51.84 wt.%), Al₂O₃ (21.71–25.47 wt.%), K₂O (9.91–11.26 wt.%), and Li₂O (4.25–5.88 wt.%), with low contents of FeO (0.7–2.88 wt.%), MnO (0.2–1.51 wt.%), and MgO (0.02–0.32 wt.%). The obtained compositions clearly plot in the lepidolite field (Figure 7c) and are most accurately classified as trilithionite (Figure 7d).

Zinnwaldite was analyzed from pegmatites (40 points) and sheared granite (22 points), and its chemical analyses and structural formulae of zinnwaldites are listed in Supplementary Table S1. The zinnwaldite of pegmatites is rich in Al₂O₃ (21.11–25.34 wt.%) and MnO (0.41–3.1 wt.%) and more depleted in FeO (7.88–13.11 wt.%) than the sheared granites (20.65–23.76 wt.% Al₂O₃, 0.39–0.53 wt.% MnO, and 12.85–18.95 wt.% FeO). Using the discrimination diagram of [52], the zinnwaldite analyses of the pegmatites are plotted in the zinnwaldite field, while those of sheared granite straddle the boundary between zinnwaldite and Li-Fe micas (Figure 7c).

Muscovite is the principal Al-rich phase mineral in the pegmatites and sheared granite. The chemical composition (65 spots) and structural formulae of the analyzed muscovites are given in Supplementary Table S1. The analyzed muscovite crystals in the pegmatite are rich in Al₂O₃ (33.62–36.11 wt.%) and Li₂O (1.79–3.29 wt.%) and more depleted in SiO₂ (42.91–46.16 wt.%), FeO (0.78–1.54 wt.%) and F (0.23–0.88 wt.%) than those of sheared granite (28.01–33.25 wt.% Al₂O₃, 1.22–1.49 wt.% Li₂O, 46.09–50.77 wt.% SiO₂, 1.55–3.11 wt.% FeO and 1.21–2.95 wt.% F). On the discrimination diagram of [52], the muscovites of pegmatites are plotted mainly in Li-muscovite, while those of sheared granite represent a solid solution between Li muscovites and Li phengite (Figure 7c). According to the ternary classification diagram of [53], the muscovites of pegmatites plotted mainly in the primary muscovite field, while the muscovites of sheared granite plotted in the field of secondary type or straddle the boundary between the two fields (Figure 7e).

Biotite is detected only in the sheared granite. The microprobe analyses (44 spots) and structural formulae of the analyzed biotite crystals are presented in Supplementary Table S1. The analyzed biotite crystals are rich in iron with high FeO/MgO ratios (3.95 to 5.75). They have high TiO₂ (1.41–3.08 wt.%) and Al₂O₃ (16.16–18.08 wt.%) contents. On the discrimination diagram of [52], they are plotted in the field of Fe-biotite (Figure 7c). Using the classification diagram of biotite [54], they plot distinctly in the annite field (Figure 7f). The ternary plot of 10 × TiO₂-(FeO + MnO)-MgO after [55] suggests a re-equilibrated primary origin for the biotite (Figure 8a). Using the Al₂O₃ vs. FeO^t biotite discrimination diagrams of [56], all biotites plot in the peraluminous field (Figure 8b).

Figure 7. Mineral chemistry plots showing the following: (a) Mn-Mg-Fe ternary diagram of garnet discrimination [53], (b) MgO-FeO-MnO ternary discrimination diagram for garnet [51], (c) classification diagram of micas (after [57]), (d) compositions of micas plotted in the Fe^t + Mn + Ti − Al^VI (apfu) vs. Mg − Li (apfu) diagram [57], abbreviated names of end-member compositions after [57], (e) Mg-Ti-Na ternary diagram for the compositional fields of muscovite after [48], and (f) classification diagram of biotite [54].

Figure 7. Mineral chemistry plots showing the following: (a) Mn-Mg-Fe ternary diagram of garnet discrimination [53], (b) MgO-FeO-MnO ternary discrimination diagram for garnet [51], (c) classification diagram of micas (after [57]), (d) compositions of micas plotted in the Fe^t + Mn + Ti − Al^VI (apfu) vs. Mg − Li (apfu) diagram [57], abbreviated names of end-member compositions after [57], (e) Mg-Ti-Na ternary diagram for the compositional fields of muscovite after [48], and (f) classification diagram of biotite [54].

Chlorite is recorded in the pegmatite and sheared granite as a secondary mineral that was formed at the expense of mafic minerals. The microprobe analyses (50 spots) and structural formulae of chlorites are shown in Supplementary Table S1. They show limited versions in their compositions; however, chlorite in pegmatite is richer in SiO₂ (25.18–28.56 wt.%) and depleted in FeO* (27.49–35.89 wt.%) than chlorite in sheared granite (22.67–24.87 wt.% SiO₂, 31.74–36.52 wt.% FeO). According to the classification diagram of [58], the chlorite in pegmatite is only brunsvigite, whereas the chlorite in sheared granite is brunsvigite and ripidolite (Figure 8c).

4.2.2. Ore Minerals

Columbite and fergusonite are the recorded Nb-Ta oxide phases in the studied rocks. The columbite crystals are analyzed in pegmatite (28 spots) and sheared granite (31 spots), while fergusonite is analyzed only in pegmatite (23 spots). The chemical analyses and structural formulae of the columbite are given in Supplementary Table S2. The analyzed columbite crystals are rich in Nb₂O₅ (60.84–73.82 wt.% in pegmatite and 50.17–72.21 wt.% in sheared granite) and depleted in Ta₂O₅ (3.58–14.89 wt.% in pegmatite and 4.45–23.12 wt.% in sheared granite). FeO and MnO show similar and narrow ranges in both rock types, 14.02–17.19 wt.% and 2.71–5.81 wt.%, respectively. This narrow range in composition can be attributed to the absence of zoning. Columbite has Mn# [Mn/(Mn + Fe)] ratios of 0.14–0.28 in pegmatite and 0.16–0.31 in sheared granite, while Ta# [Ta/(Nb + Ta)] ratios range from 0.03 to 0.13 in pegmatite and from 0.04 to 0.22 in sheared granite. In the classification diagram of Nb-Ta oxides, the analyses are plotted on the Fe-dominant side of columbite (Figure 8d).

Fergusonite (YNbO₄) is recorded only in pegmatite (23 spots; samples PG-5 and PG-10). Chemical composition and structural formulae of fergusonite are given in Supplementary Table S2. The analyses of fergusonite reveal a dominantly Y- and Nb-rich composition, characteristic of this mineral phase. The Nb₂O₅ content is consistently high (38.7–41.9 wt.%), while Y₂O₃ ranges between 19.8 and 25.4 wt.%, confirming the identity of the mineral as fergusonite-(Y). Fergusonite is a complex oxide that contains various REEs, Y, and Nb as major components. Notably, TiO₂ (up to 2.48 wt.%) and FeO* (up to 6.5 wt.%) show variable enrichment, potentially reflecting redox-controlled substitution and late-stage magmatic differentiation. The presence of moderate amounts of Ta₂O₅ (2.35–4.23 wt.%) indicates advanced fractionation. Elevated levels of CaO and minor Na₂O, K₂O, and P₂O₅ likely reflect accessory substitution or inclusions within fergusonite. The REE distribution is dominated by heavy rare earth elements (HREEs), particularly Dy₂O₃ (up to 4.47 wt.%), Er₂O₃ (up to 1.74 wt.%), and Yb₂O₃ (up to 3.31 wt.%), whereas light REEs such as Nd₂O₃ and Sm₂O₃ occur in lower concentrations. This HREE-enriched signature, along with the moderate fluorine content (up to 0.93 wt.%), is indicative of crystallization from evolved, F-rich pegmatitic fluids. The presence of significant UO₂ (3.3–5.1 wt.%) and ThO₂ (up to 4.23 wt.%) implies partial metamictization due to prolonged radioactive decay.

Cassiterite (SnO₂) is recorded in the pegmatite and sheared granite as a disseminated accessory mineral and as massive pockets. The chemical analyses of the disseminated cassiterites (56 spots in pegmatite and 23 spots in sheared granite) are listed in Supplementary Table S2. Cassiterite shows a consistent dominance of SnO₂ (94.91 and 98.28 wt.%), confirming the high purity and stoichiometric composition of this tin oxide phase. In general, the cassiterite analyses of pegmatite are richer in SnO₂ (96.51–98.28 wt.%) and depleted in Nb₂O₅ (0.11–0.64 wt.%) than cassiterite in the sheared granite (94.91–96.99 wt.% SnO₂ and 0.5–1.92 wt.% Nb₂O₅). Minor substitutions are observed in the form of FeO, which varies slightly but generally remains below 1 wt.%, and trace concentrations of TiO₂, Nb₂O₅, and Ta₂O₅. These compositional variations suggest a limited incorporation of HFSEs into the cassiterite structure, possibly through coupled substitutions involving Fe²⁺ and Nb⁵⁺ or Ta⁵⁺ in place of Sn⁴⁺.

Allanite is the principal LREE-bearing accessory mineral in both the pegmatite and sheared granite samples from Wadi Sikait. Allanite EMPA data (Supplementary Table S2) in the displays nearly identical major element compositions and structural formulas, with no significant variation in major or trace elements, implying uniform magmatic crystallization conditions. The REE oxide contents, especially those of Ce₂O₃ (13.26–15.70 wt.%), La₂O₃ (3.93–5.74 wt.%), and Nd₂O₃ (4.04–4.40 wt.%), are consistently high, reflecting enrichment in light REEs (LREEs) across all samples. The totals of EMPA measurements are consistently high, ranging between ~97 and ~100 wt.%, indicating well-preserved, crystalline allanite with minimal metamictization or alteration. In all samples, Ce₂O₃ is the most abundant REE oxide, classifying the mineral as belonging to the allanite-Ce series [59].

Figure 8. Mineral chemistry plots showing the following: (a) TiO₂-FeO + MnO-MgO ternary classification diagram of biotite [55], (b) FeO^(t) vs. Al₂O₃ discrimination diagram of biotite after [56], (c) classification diagram of chlorite minerals of [58], and (d) chemical composition and nomenclature of the Ta-Nb oxides based on Ta/(Ta + Nb) vs. Mn/(Mn + Fe) ratios.

Figure 8. Mineral chemistry plots showing the following: (a) TiO₂-FeO + MnO-MgO ternary classification diagram of biotite [55], (b) FeO^(t) vs. Al₂O₃ discrimination diagram of biotite after [56], (c) classification diagram of chlorite minerals of [58], and (d) chemical composition and nomenclature of the Ta-Nb oxides based on Ta/(Ta + Nb) vs. Mn/(Mn + Fe) ratios.

Monazite-(Ce) from Wadi Sikait (Supplementary Table S2) displays significant compositional variations. EMPA data reveal distinct chemical signatures between pegmatite and sheared granite samples. Pegmatitic monazites contain substantially higher ThO₂ (20.52–30.48 wt.%) compared to those from sheared granites (7.19–19.2 wt.%), reflected in their structural formulae (Th = 0.78–1.28 apfu in pegmatites vs. 0.27–0.75 apfu in sheared granites). This enrichment is accompanied by elevated SiO₂ (3.68–6.67 wt.%, compared to sheared granite monazites (0.79–3.56 wt.%)), suggesting extensive huttonite substitution (Th⁴⁺ + Si⁴⁺ ↔ REE³⁺ + P⁵⁺) in the pegmatitic environment. Notably, the P₂O₅ content shows an inverse relationship, being higher in sheared granite monazites (23.04–27.53 wt.%) than in pegmatitic varieties (16.42–23.94 wt.%). REE distributions also exhibit systematic differences between the two rock types. Monazites from sheared granites are significantly enriched in LREEs, particularly Ce, La, and Nd, compared to their pegmatitic counterparts. Ce₂O₃ content ranges from 26.3 to 33.96 wt.% in sheared granite monazites versus 19.27 to 26.35 wt.% in pegmatite samples, while La₂O₃ and Nd₂O₃ show similar enrichment patterns (La₂O₃: 5.32–7.48 wt.% vs. 3.74–5.63 wt.%; Nd₂O₃: 7.39–9.28 wt.% vs. 5.65–7.67 wt.%). These compositional variations likely reflect different crystallization conditions, with pegmatitic monazites forming from more evolved, possibly higher-temperature magmatic fluids enriched in Th, while sheared granite monazites may have crystallized under different pressure-temperature conditions or experienced post-crystallization modification during deformation events.

Bastnäsite is a rare earth fluoro-carbonate mineral, typically with the formula (Ce, La, Y) CO₃F; however, the analyzed samples (Supplementary Table S2) from Wadi Sikait show variable compositions. Bastnäsite-Y compositions in the Wadi Sikait samples show variable Y₂O₃ content across different locations. In pegmatite samples, Y₂O₃ ranges from 1.45 to 5.54 wt.%; meanwhile, in sheared granite, it ranges from 0.72 to 4.46 wt.%. Higher Y₂O₃ concentrations are observed in specific pegmatite samples, indicating localized yttrium enrichment during mineral formation. Bastnäsite-Ce from Wadi Sikait is characterized by high Ce content, with Ce₂O₃ ranging from 27.28 to 33.75 wt.% in sheared granite and 27.69 to 30.44 wt.% in pegmatite samples. The relatively consistent Ce₂O₃ concentrations across different samples suggest that cerium was readily available during bastnäsite crystallization, making it the dominant REE in these minerals. Bastnäsite-Nd in the Wadi Sikait samples exhibits Nd content, with Nd₂O₃ ranging from 27.47 to 30.39 wt.% in pegmatites and 10.79 to 13.21 wt.% in specific samples. These Nd₂O₃ concentrations are significant, indicating that Nd is also a substantial component of the REE composition in these bastnäsite samples. The observed compositional diversity indicates variations in crystallization conditions, likely reflecting changes in the chemistry of parental fluids during pegmatite formation or during subsequent alteration processes. The enrichment of Y, Ce, and Nd in different bastnaesite phases suggests distinct partitioning of these REE due to crystal chemical preferences and possibly varying oxygen fugacity and pH conditions during crystallization. Moreover, the presence of minor amounts of Th and U in all types of bastnaesite supports their metamictization, as evidenced by sub-ideal analytical totals. This condition reflects radiation damage in crystal lattices, commonly associated with these mineral phases.

Thorite (Supplementary Table S2) from Wadi Sikait pegmatites and sheared granites exhibit relatively consistent major element compositions. ThO₂ ranges from 60.14 to 63.56 wt.% and SiO2 from 17.2 to 19.26 wt.%. The UO₂ values show more variation, ranging from 8.76 to 17.36 wt.%. CaO content in pegmatite samples is generally between 0.55 and 1.26 wt.%, while sheared granite samples range from 0.64 to 1.34 wt.%. The higher ThO₂ and variable UO₂ contents are typical for thorite. Variations in CaO may indicate differences in fluid composition or alteration processes between the pegmatites and sheared granites. The substantial UO₂ content, ranging widely from about 8.76 to 17.36 wt.%, indicates considerable solid-solution substitution of U⁴⁺ for Th⁴⁺ in the crystal lattice, classifying the mineral as uranothorite in many analyses. REEs such as Ce₂O₃ (up to 1.52 wt.%), La₂O₃ (up to 0.61 wt.%), Nd₂O₃ (up to 0.71 wt.%), and occasionally Sm₂O₃ and Gd₂O₃ are detected in trace concentrations, indicating minor substitutions or microscopic inclusions of REE-bearing phases.

Zircons are recorded in pegmatite (28 spots) and sheared granite (26 spots), and their chemical analysis and structural formulae are shown in Supplementary Table S2. Most of the analyzed zircons yield somewhat low electron probe analytical totals (96.26–98.88 wt.%), likely due to their metamict character. The analyzed zircon crystals are chemically pure (Zr, Hf) SiO₄ and show limited variations in their composition. They have SiO₂ ranging from 31.12 to 33.45 wt.% in pegmatite and from 30.73 to 33.59 wt.% in sheared granite, while ZrO₂ ranges from 57.27 to 61.35 wt.% in pegmatite and from 60.12 to 62.24 wt.% in sheared granite. Zircon in pegmatites contains higher ZrO₂ (57.27–61.35 wt.%) and HfO₂ (3.67–6.47 wt.%) than those from sheared granite (60.12–62.24 wt.% ZrO₂ and 1.12–3.49 wt.% HfO₂). Impurities of TiO₂, Al₂O₃, FeO*, MnO, MgO, CaO, Na₂O, K₂O, and P₂O₅ are typically minimal, generally below 1 wt.% or undetectable, indicating a relatively pure zircon composition with minimal substitution.

4.3. Geochemical Characteristics

Whole-rock geochemical analyses (major, trace, and rare-earth elements) were performed for mineralized pegmatite (10 samples collected from the outer zones) and sheared granite (nine samples). The chemical analyses and calculated normative mineral compositions of the analyzed samples are given in

Table 1. Major oxides and normative compositions of pegmatite in the Wadi Sikait area.

Rock Type	Pegmatite
Sample No.	PG 1	PG 3	PG 5	PG 6	PG 7	PG 9	PG 10	PG 11	PG 12	PG 14
Major oxides (wt.%)
SiO2	74.69	74.66	75.35	74.21	74.35	75.17	74.8	75.68	74.43	74.37
TiO2	0.06	0.07	0.04	0.06	0.06	0.04	0.06	0.04	0.06	0.04
Al2O3	12.53	12.07	12.75	13.18	12.79	11.91	12.68	11.83	12.5	12.29
Fe2O3	1.16	0.89	0.91	1.07	0.92	1.02	0.89	0.94	1.02	1.14
MnO	0.04	0.38	0.21	0.13	0.31	0.39	0.38	0.02	0.07	0.01
MgO	0.04	0.06	0.04	0.04	0.05	0.06	0.05	0.04	0.03	0.05
CaO	0.21	0.18	0.16	0.26	0.24	0.19	0.23	0.19	0.21	0.24
Na2O	3.62	3.06	3.15	3.69	3.67	3.18	2.68	2.84	2.87	3.31
K2O	4.88	4.93	4.91	4.77	4.6	4.89	5.63	5.39	5.44	5.07
P2O5	0.01	0.01	0.01	0.01	0.02	0.02	0.01	0.02	0.02	0.01
LOI	0.38	1.11	0.66	0.67	1.75	1.02	1.26	0.7	0.98	0.87
Total	97.5	97.9	98.59	97.98	98.88	98.41	98.95	97.82	97.92	97.5
Normative composition
Quartz	34.82	38.28	38.15	34.16	35.26	37.92	37.41	38.79	37.17	35.79
Corundum	0.96	1.45	2	1.53	1.42	1.12	1.83	1.05	1.61	0.97
Orthoclase	29.68	30.27	29.76	28.96	28.04	29.85	34.18	32.87	33.29	31.07
Albite	31.53	26.9	27.34	32.08	32.03	27.8	23.3	24.8	25.15	29.04
Anorthite	1.01	0.86	0.85	1.26	1.09	0.84	1.11	0.84	0.94	1.17
Hypersthene	1.57	1.75	1.52	1.58	1.67	1.99	1.73	1.29	1.41	1.57
Magnetite	0.28	0.31	0.26	0.28	0.3	0.34	0.31	0.22	0.26	0.27
Ilmenite	0.12	0.14	0.08	0.12	0.12	0.08	0.12	0.08	0.12	0.08
Apatite	0.02	0.02	0.02	0.02	0.05	0.05	0.02	0.05	0.05	0.02
Geochemical parameters
R1	2517	2708	2727	2488	2548	2707	2689	2746	2638	2562
R2	270	259	269	288	279	257	276	254	269	269
Ti	360	420	240	360	360	240	360	240	360	240
K	40510	40925	40759	39597	38186	40593	46736	44744	45159	42088
P	44	44	44	44	87	87	44	87	87	44
AI	0.90	0.86	0.82	0.85	0.86	0.88	0.83	0.89	0.85	0.89
ASI	1.08	1.13	1.18	1.12	1.11	1.09	1.16	1.09	1.13	1.08
CIA	0.52	0.53	0.54	0.53	0.53	0.52	0.54	0.52	0.53	0.52
Fe2O3/MgO	29.00	14.83	22.75	26.75	18.40	17.00	17.80	23.50	34.00	22.80
Na2O + K2O	8.50	7.99	8.06	8.46	8.27	8.07	8.31	8.23	8.31	8.38
Color Index	1.97	2.2	1.86	1.98	2.09	2.41	2.15	1.59	1.79	1.92
Diff. Index	96.04	95.45	95.26	95.2	95.34	95.58	94.88	96.45	95.6	95.9
ANOR	3.29	2.76	2.78	4.17	3.74	2.74	3.15	2.49	2.75	3.63
Q/	35.88	39.75	39.70	35.41	36.57	39.33	38.97	39.87	38.50	36.87
Rock Type	Sheared Granite
Sample No.	SG 2	SG 5	SG 7	SG 8	SG 9	SG 11		SG 13	SG 18	SG 22
Major oxides (wt.%)
SiO2	72.07	71.18	70.66	72.87	70.03	71.34		70.47	72.84	72.27
TiO2	0.1	0.14	0.14	0.08	0.13	0.12		0.15	0.08	0.1
Al2O3	13.59	13.96	13.8	13.13	14.6	13.88		14.26	13.48	13.97
Fe2O3	1.14	0.85	1.24	0.98	1.94	1.72		1.32	1.04	1.08
MnO	0.03	0.41	0.16	0.2	0.11	0.11		0.23	0.09	0.22
MgO	0.08	0.07	0.08	0.06	0.09	0.08		0.11	0.07	0.07
CaO	0.35	0.38	0.34	0.29	0.41	0.43		0.49	0.36	0.41
Na2O	4.75	4.75	4.75	4.06	5.02	4.52		4.34	4.46	4.79
K2O	4.57	5.01	4.73	4.64	4.21	4.54		4.98	4.67	4.3
P2O5	0.01	0.01	0.01	0.03	0.02	0.02		0.02	0.03	0.01
LOI	1.14	1.37	1.37	1.51	1.25	1.59		1.67	1.27	1.17
Total	97.7	97.89	97.08	97.7	97.36	98.15		97.92	97.97	98.24
Normative composition
Quartz	26.41	23.63	24.43	31.41	23.52	26.62		25.17	28.48	27.09
Corundum	0.22	0.05	0.28	1.01	1.12	0.82		0.92	0.52	0.73
Orthoclase	27.96	30.62	29.17	28.48	25.81	27.77		30.57	28.44	26.16
Albite	41.61	41.56	41.95	35.69	44.06	39.58		38.15	38.89	41.73
Anorthite	1.73	1.88	1.69	1.29	1.97	2.07		2.39	1.64	2.03
Hypersthene	1.55	1.62	1.81	1.59	2.7	2.4		2.06	1.52	1.72
Magnetite	0.29	0.33	0.35	0.29	0.49	0.44		0.39	0.27	0.32
Ilmenite	0.2	0.27	0.28	0.16	0.26	0.24		0.3	0.16	0.2
Apatite	0.02	0.02	0.02	0.07	0.05	0.05		0.05	0.07	0.02
Geochemical parameters
R1	2013	1857	1878	2299	1845	2038		1951	2147	2077
R2	308	318	311	292	335	322		338	306	321
Ti	600	839	839	480	779	719		899	480	600
K	37,937	41,590	39,265	38,518	34,948	37,688		41,340	38,767	35,696
P	44	44	44	131	87	87		87	131	44
AI	0.94	0.95	0.94	0.89	0.88	0.89		0.88	0.92	0.90
ASI	1.01	1.00	1.02	1.07	1.07	1.05		1.06	1.03	1.05
CIA	0.50	0.50	0.50	0.52	0.52	0.51		0.51	0.51	0.51
Fe2O3/MgO	14.25	12.14	15.50	16.33	21.56	21.50		12.00	14.86	15.43
Na2O + K2O	9.32	9.76	9.48	8.70	9.23	9.06		9.32	9.13	9.09
Color Index	2.04	2.22	2.44	2.03	3.45	3.07		2.75	1.95	2.23
Diff. Index	95.98	95.81	95.55	95.58	93.39	93.97		93.88	95.81	94.97
ANOR	5.83	5.78	5.48	4.33	7.09	6.94		7.25	5.45	7.20
Q/	27.03	24.19	25.12	32.42	24.66	27.72		26.14	29.23	27.92

AI = molar (Na + K)/Al, ASI = molar Al/(Ca + Na + K), CIA = molecular [Al/(Al + Ca + Na + K)] × 100, ANOR = 100 × An/(Or + An), Q/ = 100 × Qz/(Qz + Or + An + Ab).

,

Table 2. Trace element contents (ppm) of pegmatite in the Wadi Sikait area.

Rock Type	Pegmatite
Sample No.	PG 1	PG 3		PG 5		PG 6		PG 7		PG 9		PG 10		PG 11		PG 12		PG 14
F	2926.8	2539.1		2904.9		2823.8		2753.2		2608.8		2949.4		2803.8		2794.3		2961.3
Li	1221.7	1658.3		1298.3		989.13		1082.1		1254.2		1791.2		1088.5		987.58		899.87
Be	36.74	23.29		37		31.09		26.63		43.24		31.81		43.96		38.71		40.11
Sn	556.34	767.95		488.8		532.71		420.42		305.4		311.88		493.42		375.54		489.77
Cs	6.67	7.38		8.6		7.42		8.37		6.55		5.58		7.32		7.52		6.38
Rb	715.84	732.88		692.09		764.1		654.97		885.12		681.35		526.27		852.79		672.49
Ba	26.24	38.76		31.22		37.13		42.67		48.43		36.68		33.33		25.89		42
Sr	16.98	18.09		15.59		17.21		19.11		16.04		21.08		15.76		18.43		14.12
Nb	1218.6	1647.2		1181.3		1200.7		1363.9		1701.1		1388.1		1216.3		1383.7		1771.4
Zr	4106.5	4366.7		3864.8		2591.6		4069.7		3057.5		2987.1		3226.1		4428.7		3475.5
Y	845.69	965.24		998.49		798.59		658.41		867.91		945.65		884.97		798.69		625.98
Zn	309.05	1059.9		766.84		693.29		1098.9		1120.2		1569.1		168.54		228.15		342.58
Cu	124.78	542.67		297.98		193.69		450.68		479.27		432.69		174.45		111.59		132.93
Cr	3.76	25.25		12.57		6.63		13.25		10.26		11.25		13.55		5.88		12.5
Co	4.41	25.19		15.32		12.9		15.3		21.35		22.44		10.58		13.63		11.81
V	6.49	5.6		6.92		3.66		3.97		4.05		5.61		5.45		12.66		8.85
Ni	2.71	44.7		26.69		7.84		26.17		43.65		33.7		7.24		4.47		2.51
Sc	6.16	19.98		11.77		8.4		14.25		14.38		8.81		10.32		6.38		9.38
Ga	36.92	43.44		37.45		40.25		47.03		41		36.97		41.07		36.48		27.71
Mo	77.96	79.27		85.54		72.7		135.69		102.77		114.44		101.83		85.96		92.77
Hf	214.55	303.19		240.55		214.54		242.33		204		201.35		232.55		243.97		244.73
Ta	143.83	191.47		146.02		138.18		140.76		151.39		169.82		176.77		170.36		163.95
Pb	879.52	2030.3		1187.5		738.92		1064.3		1363.7		1644.1		747.44		997.44		829.8
Th	480.89	578.96		627.49		444		412.65		537.04		420.47		685.19		564.11		644.12
U	338.68	594.67		415.59		452.07		603.93		501.21		699.46		223.65		300.22		267.67
Geochemical parameters
Ba/Rb	0.04	0.05		0.05		0.05		0.07		0.05		0.05		0.06		0.03		0.06
K/Rb	56.60	55.84		58.87		46.39		58.35		45.89		68.63		84.94		52.92		62.63
Rb/Sr	42.16	40.51		44.39		44.40		34.27		55.18		32.32		33.39		46.27		47.63
Ga/Al	5.57	6.80		5.55		5.77		6.95		6.51		5.51		6.56		5.51		4.26
Zr/Hf	19.14	14.40		16.07		12.08		16.79		14.99		14.84		13.87		18.15		14.20
Y/Ho	19.59	19.50		18.54		24.44		18.29		22.70		22.17		23.18		27.09		23.33
Nb/Ta	8.47	8.60		8.09		8.69		9.69		11.24		8.17		6.88		8.12		10.80
Rb/Ba	27.28	18.91		22.17		20.58		15.35		18.28		18.58		15.79		32.94		16.01
Rock Type	Sheared Granite
Sample No.	SG 2		SG 5		SG 7		SG 8		SG 9		SG 11		SG 13		SG 18		SG 22
F	3130.6		3064.1		2753.7		3185.8		2953.1		1501.9		2090.4		3618.5		2749.2
Li	608.65		523.47		455.08		570.73		547.18		399.07		444.95		459.36		565.47
Be	13.76		16.87		13.73		21.55		13.16		18.72		2.57		16.47		25.75
Sn	227.73		222.16		320.03		437.44		245.13		157.89		387.85		254.46		313.43
Cs	3.05		5.9		3.9		5.7		2.93		4.63		2.61		5.04		4.23
Rb	676.39		936.19		883.38		1079.2		843.91		884.87		702.95		1503.5		820.03
Ba	91.14		118.46		124.53		91.84		142.5		106.74		180.77		141		88.59
Sr	30.9		40.34		38.8		37.01		33.39		32.3		45.37		54.9		37.72
Nb	1014.4		1068.8		1023.8		1085.8		1024.4		1003.4		1085.1		807.64		1194.8
Zr	3875.5		3615.6		3319.6		1985		3828.7		2726		3991.6		1300.3		3112.9
Y	498.24		515.03		451.82		574.1		358.07		529.61		389.77		689.79		558.51
Zn	417.8		3447.2		1797.4		500.79		2051.2		678.27		3573.5		302.63		1084.5
Cu	101.85		269.65		242.01		257.45		177.47		173.06		323.52		164.23		264.56
Cr	8.37		26.83		19.35		13.91		16.76		15.91		30.28		14.58		15.59
Co	2.99		6.09		18.85		15.66		7.8		9.83		13.59		16.02		21.51
V	8.03		8.16		9.07		3.29		6.06		16.29		11.65		2.61		6.91
Ni	5.11		16.6		13.08		13.6		14.83		8.61		20.05		11.02		23.11
Sc	8.32		8.59		7.7		10.89		5.03		8.76		6.64		7.54		10.73
Ga	33.9		42.78		47.76		53.03		46.4		23.84		42.28		59.02		43.98
Mo	54.95		86.05		63.69		63.96		58.13		66.83		56.32		72.23		68.16
Hf	257.93		238.2		217.74		180.64		249.04		194.82		247.52		118.95		216.68
Ta	122.52		165.16		168.75		122.35		173.55		152.69		134.99		103.93		133.91
Pb	589.45		722.87		381.27		515.99		604.22		350.75		662.69		767.65		605.75
Th	554.45		361.62		416.35		329.52		562.99		541.81		430.82		246.39		411.58
U	148.75		150.59		139.99		347.81		129.98		171.59		95.85		91.68		167.99
Geochemical parameters
Ba/Rb	0.13		0.13		0.14		0.09		0.17		0.12		0.26		0.09		0.11
K/Rb	56.06		44.43		44.46		35.69		41.37		42.59		58.84		25.78		43.53
Rb/Sr	21.89		23.21		22.77		29.16		25.27		27.40		15.49		27.39		21.74
Ga/Al	4.72		5.79		6.54		7.63		6.01		3.25		5.60		8.28		5.95
Zr/Hf	15.03		15.18		15.25		10.99		15.37		13.99		16.13		10.93		14.37
Y/Ho	43.36		40.02		45.87		35.99		52.05		40.40		46.79		40.13		33.77
Nb/Ta	8.28		6.47		6.07		8.87		5.90		6.57		8.04		7.77		8.92
Rb/Ba	7.42		7.90		7.09		11.75		5.92		8.29		3.89		10.66		9.26

and

Table 3. REE contents (ppm) of pegmatite in the Wadi Sikait area.

Rock Type	Pegmatite
Sample No.	PG 1		PG 3		PG 5		PG 6		PG 7		PG 9		PG 10		PG 11		PG 12		PG 14
La	55.81		62.79		77.59		38.15		44.38		50.16		49.02		56		49.08		41.69
Ce	250.7		253.93		287.52		170.46		235.02		235.24		216.4		254.16		189.3		156.36
Pr	25.96		27.36		35.73		19.48		23.01		27.66		23.21		32.04		22.74		17.96
Nd	70.67		65.74		106.91		51.74		58.91		70.12		55.76		79.96		57.59		48.68
Sm	33.61		27.18		51.08		24.26		27.16		30.01		23.06		35.92		23.25		18.52
Eu	0.82		0.55		0.82		0.52		0.55		0.57		0.86		0.55		0.29		0.26
Gd	44.54		39.37		49.6		31.25		33.88		33.41		36.18		31.76		23.54		16.98
Tb	16.5		18.32		21.6		12.17		14.52		14.88		16.33		14.29		11.31		8.19
Dy	156.78		184.9		215.04		117.4		139.69		148.05		160.82		145.29		114.48		87.83
Ho	43.17		49.51		53.85		32.67		36		38.24		42.66		38.17		29.48		26.83
Er	178.56		220.85		237.41		133.68		154.15		168.43		189.46		169.67		133.12		107.33
Tm	37.72		50		55.01		29.13		33.96		38.79		44.9		38.68		32.19		27.8
Yb	319.02		398.95		453.48		248.01		272.18		321.87		390.2		315.62		274.71		235.99
Lu	47.3		57.12		64.96		36.97		40.84		47.47		55.79		48.47		42.06		35.72
ƩREE	1281.2		1456.6		1710.6		945.88		1114.3		1224.9		1304.7		1260.6		1003.1		830.14
Geochemical parameters
Eu/Eu*	0.06		0.05		0.05		0.06		0.06		0.05		0.09		0.05		0.04		0.04
(La/Yb)n	0.12		0.11		0.12		0.10		0.11		0.11		0.08		0.12		0.12		0.12
(La/Sm)n	1.05		1.46		0.96		0.99		1.03		1.06		1.34		0.98		1.33		1.42
(Gd/Lu)n	0.12		0.08		0.09		0.10		0.10		0.09		0.08		0.08		0.07		0.06
(La/Lu)n	0.12		0.11		0.12		0.11		0.11		0.11		0.09		0.12		0.12		0.12
T1	1.81		1.83		1.57		1.83		2.03		1.92		1.91		1.90		1.74		1.66
T3	1.26		1.43		1.43		1.29		1.40		1.43		1.42		1.42		1.49		1.37
TE1,3	1.51		1.62		1.50		1.53		1.69		1.66		1.65		1.65		1.61		1.51
Rock Type		Sheared Granite
Sample No.		SG 2		SG 5		SG 7		SG 8		SG 9		SG 11		SG 13		SG 18		SG 22
La		10.78		8.86		6.11		13.37		3.2		10.05		4.33		15.07		13.9
Ce		58.06		51.39		32.56		60.58		20.1		39.78		27.16		69.78		60.78
Pr		7.67		6.39		4.72		7.47		2.85		5.16		3.48		10.17		9.63
Nd		20.96		17.72		13.25		19.24		6.68		14.16		9.03		26.61		23.88
Sm		9.14		9.28		7.66		8.72		4.57		8.03		5.87		11.57		10.72
Eu		0.08		0.17		0.19		0.23		0.13		0.26		0.18		0.23		0.12
Gd		8.6		9.48		7.52		14.27		4.48		11		5.69		13.68		12.57
Tb		4.14		4.69		3.74		5.66		2.43		5.04		3.03		6.44		5.75
Dy		43.73		48.7		38.25		55.53		26.38		49.41		33.06		65.98		57.76
Ho		11.49		12.87		9.85		15.95		6.88		13.11		8.33		17.19		16.54
Er		49.12		57.14		45.51		69.29		29.61		58.75		36.7		76.81		65.72
Tm		10.83		13.1		10.85		15.57		6.63		13.96		8.85		17.94		15.51
Yb		98.76		111.09		94.79		126.93		53.61		121.59		72.12		150.92		134.16
Lu		15.02		15.84		14.13		18.82		7.25		17.98		10.39		21.88		20.25
ƩREE		348.39		366.74		289.13		431.63		174.82		368.28		228.22		504.27		447.3
Geochemical parameters
Eu/Eu*		0.03		0.06		0.07		0.06		0.09		0.08		0.10		0.06		0.03
(La/Yb)n		0.07		0.05		0.04		0.07		0.04		0.06		0.04		0.07		0.07
(La/Sm)n		0.74		0.60		0.50		0.97		0.44		0.79		0.47		0.82		0.82
(Gd/Lu)n		0.07		0.07		0.07		0.09		0.08		0.07		0.07		0.08		0.08
(La/Lu)n		0.07		0.06		0.04		0.07		0.05		0.06		0.04		0.07		0.07
T1		1.98		2.04		1.94		1.87		2.31		1.69		2.19		1.88		1.87
T3		1.47		1.49		1.51		1.28		1.57		1.43		1.58		1.46		1.37
TE1,3		1.71		1.74		1.71		1.55		1.90		1.56		1.86		1.66		1.60

. The analyzed samples have a wide range of SiO₂ (70.03 to 75.68 wt.%) and are highly fractionated, where the differentiation index (DI) ranges from 94.88 to 96.45 in pegmatite and from 93.39 to 95.98 in sheared granite. Pegmatite samples have higher SiO₂ (74.21–75.68 wt.%) and lower Al₂O₃ (11.83–13.18 wt.%) and total alkalis (7.99–8.50 wt.%) than sheared granite (SiO₂ = 70.03–72.87 wt.%; Al₂O₃ = 13.13–14.6 wt.%; Na₂O + K₂O = 8.70–9.76 wt.%) (Table 1). Harker variation diagrams for some major oxides of the analyzed samples exhibit decrease in TiO₂, Al₂O₃, Fe₂O₃, MgO, CaO, and Na₂O, and increase in K₂O with an increase in SiO₂ (Figure 9). On the other hand, trace element concentrations vary widely between pegmatite and sheared granite and show erratic distribution against silica (Table 2).

Trace element contents are enriched in the pegmatite samples, with very high abundances of rare earth elements (ƩREE = 830–1711 ppm), Li (900–1791 ppm), Sn (305–768 ppm), Nb (1181–1701 ppm), Ta (138–191 ppm), Y (626–998 ppm), Zr (2592–4429 ppm), Th (413–685 ppm), Hf (201–303 ppm), U (224–699 ppm), and Pb (739–2030 ppm). Also, the sheared granite is somewhat rich in these elements but to a lesser degree than pegmatite (175–504 ppm total REE, 399–609 ppm Li, 158–389 ppm Sn, 808–1195 ppm Nb, 104–174 ppm Ta, 358–690 ppm Y, 1300–3992 ppm Zr, 246–563 ppm Th, 119–258 ppm Hf, 92–348 ppm U, and 351–768 ppm Pb).

On the R₁-R₂ diagram of [60], the analyzed samples plot in the alkali granite field (Figure 10a). This classification is supported by using normative the Q^/-ANOR diagram for the classification of granitoids [61] (Figure 10b). The pegmatite and the sheared granite samples belong to the high-K series, using the SiO₂ versus K₂O diagram (Figure 10c). The chemical index of alteration (CIA = molecular [Al₂O₃/(Al₂O₃ + CaO + Na₂O + K₂O)] × 100) of the pegmatite and sheared granite varies between 0.50 and 0.54 (Table 1), within the range of fresh granites (45–55, [62]). The alumina saturation index [ASI = molar ratio Al₂O₃/(CaO + Na₂O + K₂O)] ranges between 1.08 and 1.18 in pegmatite and from 1 to 1.07 in sheared granite, indicating their peraluminous character (Figure 10d). The peraluminous character of the studied rocks is confirmed by the presence of normative corundum (0.96–2.0% in pegmatite and 0.05–1.12% in sheared granite) as well as the presence of different types of micas (lepidolite, zinnwaldite and muscovite). Also, the composition of biotite in the sheared granite confirms the inferences from whole-rock geochemistry that this granite has peraluminous characteristics (Figure 8b). The calculated agpaitic index values [AI = molar (Na + K)/Al] range between 0.82 and 0.90 (average = 0.86) in pegmatite and 0.88 and 0.95 (average = 0.91) in sheared granite, implying slightly alkaline characters [63]. The Wadi Sikait pegmatite and sheared granite have a slightly peraluminous to slightly peralkaline character.

The studied pegmatite and sheared granite are highly fractionated rocks. The discrimination diagram in [64], established for discriminating among rocks with SiO₂ > 68 wt.%, places the studied rocks in the highly fractionated field (Figure 10e). Sikait pegmatite and sheared granite show several features similar to A-type granites, such as high Ga/Al, Nb, Zr, Y, Ta, and Th and significant depletion in MgO, CaO, and P₂O₅ (e.g., [65,66]). On the Ga/Al vs. FeO/MgO discrimination diagrams of [66], they display clear A-type character (Figure 10f). It is worth mentioning that the extensive fractional crystallization of I-type calc-alkaline granites can produce A-type residual liquids (e.g., [66,67]).

Primitive mantle-normalized spider diagrams of pegmatite and sheared granite, using the normalization values in [68], are presented in Figure 11a. They display uniform spider diagrams with a marked enrichment of LILE (K, Rb, and Th) and some HFSE (Ta, Nb, Zr, Hf) but are depleted in Ba, Sr, P, and Ti. The depletion of these rocks in some elements can be attributed to the fractional crystallization of feldspars (e.g., Ba and Sr), apatite (e.g., P), and Fe-Ti oxides (e.g., Ti).

The concentrations of REE in pegmatite and sheared granite are given in Table 3. The pegmatite samples have a higher abundance of total REE (830.14 to 1710.6 ppm) than sheared granite (174.82 to 504.27 ppm). Their chondrite-normalized REE patterns, using normalization values from [69], are shown in Figure 11b. Both rock types display nearly similar patterns, with enrichment in HREE relative to LREE [(La/Lu)n = 0.04–0.12] being in agreement with their enrichment in zircon, thorite, and fluorite. All samples of pegmatite and sheared granite display strong negative Eu anomalies, with Eu/Eu* values ranging between 0.04 and 0.09 in pegmatite and from 0.03 to 0.10 in sheared granite.

The intensity of the tetrad effect in REE of the studied rocks has been estimated using the equations in [14]. The calculated values of tetrad effect (Table 3) are greater than unity (T₁ = 1.57 to 2.31, T₃ = 1.26 to 1.58, and T_1,3 = 1.50 to 1.90), suggesting an M-type tetrad effect. This type of tetrad effect is characteristic of highly fractionated granite that has undergone significant modification via hydrothermal activity [70].

Figure 10. Geochemical classification plots: (a) R₁-R₂ multicationic classification diagram of granitoids [60]; R₁ = 4Si-11(Na + K) − 2(Fe + Ti), R₂ = 6Ca + 2 Mg + Al, (b) normative Q’-ANOR diagram for classification of granitoids [61], (c) K₂O versus SiO₂ discrimination diagram of [71], (d) molar Al₂O₃/(Na₂O + K₂O) vs. Al₂O₃/(CaO + Na₂O + K₂O) binary diagram [72], (e) discrimination diagram of [64] for rocks with >68 wt.% SiO₂, and (f) Ga/Al vs. FeO^(t)/MgO diagram of [73].

Figure 10. Geochemical classification plots: (a) R₁-R₂ multicationic classification diagram of granitoids [60]; R₁ = 4Si-11(Na + K) − 2(Fe + Ti), R₂ = 6Ca + 2 Mg + Al, (b) normative Q’-ANOR diagram for classification of granitoids [61], (c) K₂O versus SiO₂ discrimination diagram of [71], (d) molar Al₂O₃/(Na₂O + K₂O) vs. Al₂O₃/(CaO + Na₂O + K₂O) binary diagram [72], (e) discrimination diagram of [64] for rocks with >68 wt.% SiO₂, and (f) Ga/Al vs. FeO^(t)/MgO diagram of [73].

Figure 11. Normalized multi-trace element diagrams for whole-rock chemistry: (a) primitive mantle-normalized trace element diagram; normalization values from [68], (b) chondrite-normalized REE patterns; chondrite values from [69].

Figure 11. Normalized multi-trace element diagrams for whole-rock chemistry: (a) primitive mantle-normalized trace element diagram; normalization values from [68], (b) chondrite-normalized REE patterns; chondrite values from [69].

5. Discussion

5.1. Geodynamic Implications

In the present work, we cannot apply the classical discrimination diagrams of magma type and tectonic setting due to the abnormal enrichment in most of the immobile elements such as Y, Nb, Zr, Ta, Hf, and Th. Therefore, the best way to identify the tectonic setting of the studied pegmatite and sheared granite is to combine all the gained information from their field relationships, petrography, mineralogy, as well as their geochemistry. Some inaccuracies that could occur from relying solely on the geochemical data are avoided by using this integrated approach. In this domain, Rollinson [73] believed that the tectonomagmatic discrimination diagrams used to differentiate between tectonic environments seldom provide unequivocal confirmation of a former tectonic environment. The field relationships, petrographic features, mineral chemistry, and bulk-rock geochemistry of the studied granite and pegmatite have been integrated to define their tectonic setting and geodynamic significance. First, the field relations show that the sheared granite and associated pegmatite are post-collisional, being younger than the surrounding subduction-related rocks and intruded by within-plate alkali feldspar granite of Gabal Zabara at the northern end of Wadi Sikait (out of the mapped area). Moreover, the geochemical characteristics of the studied rocks are consistent with a post-collisional tectonic setting; they are markedly depleted in CaO, MgO, and Sr, with high alkali contents and no anomalies in Nb or Ta.

The studied sheared granite of Wadi Sikait was affected by the Nugrus shear zone, which represents one of the major shear zones in the Najd fault system in the Eastern Desert of Egypt [42]. This is reflected by the presence of sub-solidus deformation features observed petrographically in the sheared granite, including undulose and chessboard extinction in quartz, deformation twins of albite, and kinking in muscovite. Generally, shear zones are favorable sites to trap mineralization, as they represent transport channels and precipitation spaces for ore-forming minerals, especially rare metals [74,75,76]. The proposed geotectonic model for the evolution of the pegmatite and associated granite is shown in Figure 12. According to this hypothesis, the hydrothermal processes were mostly connected to regional hydrothermal fluid circulation related to extension, which concentrates certain metals in the sheared granite and related rocks.

Figure 12. Suggested model for the emplacement of pegmatite and granite of Wadi Sikait (modified after [77]).

Figure 12. Suggested model for the emplacement of pegmatite and granite of Wadi Sikait (modified after [77]).

5.2. Source Rocks

The Nubian Shield was characterized by a post-collisional stage that comprises large intrusions of granites and their associated pegmatites. All data of the present work imply that the studied rocks were emplaced in a post-collisional tectonic setting at the final stage of the evolution of the Arabian–Nubian Shield. The investigated rocks (granite and pegmatite) share many features of rare metals-bearing granites as indicated by Rb > 500 ppm (526–1504 ppm), Ba/Rb < 0.5 (0.03–0.26), K/Rb < 100 (26–85), Rb/Sr > 10 (15–55 ppm), and high Fe₂O₃/MgO (12–34) [78,79]. This is confirmed by plotting the ratios of Nb/Ta versus Zr/Hf, where the analyzed samples lie in the field of highly mineralized granites above the field of rare metal-related granites (Figure 13a).

Sheared granite and the pegmatites under study have similar geochemical properties, which suggests a genetic relationship between them. The Rb/Sr and Rb/Ba ratios gradually increase from the sheared granite (15.49 to 29.16 and 3.89 to 11.75, respectively) to the pegmatite (32.32 to 55.18 and 15.35 to 32.94, respectively), providing support for their crystallization from a common parental magma [80]. Several petrogenetic models have been proposed for the origin of Wadi Sikait pegmatite and sheared granite (e.g., [40,81,82,83]). Pegmatite veins and pockets observed in granites indicate that volatiles were abundant in the parent melt [84]. In the previous sections, we have demonstrated that the investigated pegmatite and granite share many features of highly fractionated rocks with A-type character and were emplaced in a post-collisional setting during the final stage of the evolution of the Nubian Shield. The absence of mafic and intermediate intrusive igneous rocks within the studied area rules out the possibility that the investigated granite and pegmatite have originated from a mantle-derived mafic magma through extensive fractional crystallization. Furthermore, it seems unlikely that the examined rocks could have been created by continuous fractional crystallization of mantle-derived magma, as this process would need a significant volume of the original mafic magma, at least nine times the volume of the final felsic result [85].

The geology and available geochemical data of the investigated pegmatite and granite are consistent with formation from parental magma generated through partial melting of a juvenile crustal source, followed by extensive fractional crystallization. The generation of parent magmas was facilitated through the input of heat from a mantle source during asthenospheric upwelling, forming decompression melting, followed by the exhumation of the Nubian Shield. The shear zones can facilitate fluids with volatile phases to ascend along fault planes and enhance the production of magmas, resulting in a melt enriched in rare metals, REE, and elements of polymetallic deposits. Granitic melts are subjected to both extensive magmatic differentiation by fractional crystallization and intense interaction with aqueous hydrothermal fluids. The extensive interactions between granitic melts and halogen-rich fluids were critical to generate extremely negative Eu anomalies in the peraluminous granites and likely contributed to rare-metal mineralization. The role of fractional crystallization in fractionation of the parental magma to produce the granite and pegmatite is supported by a decrease in TiO₂, Al₂O₃, MgO, FeO^t, CaO, MnO and Na₂O, and an increase in K₂O with increasing SiO₂ (Figure 9). In the normalized trace elements patterns, the pegmatite, and the sheared granite are depleted in Ti, Ba, Sr and Eu, which suggest that the fractionation of feldspar and Fe-Ti oxide played a major role in their magmatic evolution. The strongly negative Eu anomalies, along with low contents of Ba and Sr, indicate extensive fractionation of feldspars.

5.3. Magmatic Evolution and Hydrothermal Effect

Despite the effect of hydrothermal processes, numerous magmatic signatures were preserved in the studied rocks, including (1) the presence of sharp intrusive contacts between the pegmatite and sheared granite and their country rocks and (2) the coexistence of primary minerals such as quartz, feldspars, micas, and Nb-Ta oxides. Although the pegmatite and sheared granite are primarily magmatic, the influence of hydrothermal fluids and extensive replacement by secondary minerals becomes dominant in the final stage, resulting in the development of mineralized pegmatite and granite. The isovalent elements ratios such as Y/Ho and Zr/Hf behave coherently and generally remain close to chondritic ratios during magmatic fractionation [14,31,86]. Ionic substitution in mineral lattice sites, which is primarily determined by ionic charge and radius, is the cause of mineral-melt fractionation. This feature has been labeled as CHARAC behavior [14,86,87]. In general, the hydrothermal process leads to leaching or precipitation of some elements by fluids, resulting in non-CHARAC behavior and the loss of coherence between isovalent “twin” elements. The studied rocks have Y/Ho (18.29–52.05) and Zr/Hf (10.93–19.14) ratios plotting farthest away from the CHARAC field (Figure 13b), implying modification of these elements by interaction with the hydrothermal solutions [87,88]. Similar non-CHARAC behavior has been noted in other highly evolved rare-metal granitoids in the Eastern Desert of Egypt that had been affected by later hydrothermal processes, such as Nuweiba and Abu Dabbab granites [31,79]. In the pegmatite and sheared granite of Wadi Sikait, hydrothermal solutions high in F are involved at a late stage of crystallization, as evidenced by the interstitial crystallization of fluorite and topaz among muscovite and albite. [89,90,91,92]. Also, K/Rb in the studied rocks is less than ˂150 (25.78–84.94), which is considered evidence of interaction with an aqueous fluid phase during mineral growth [14,93].

Figure 13. (a) Plot of Nb/Ta vs. Zr-Hf for pegmatite and sheared granite of Wadi Sikait (after [94], and (b) plot of Y/Ho versus Zr/Hf for the pegmatite and sheared granite of Wadi Sikait (after [86]).

Figure 13. (a) Plot of Nb/Ta vs. Zr-Hf for pegmatite and sheared granite of Wadi Sikait (after [94], and (b) plot of Y/Ho versus Zr/Hf for the pegmatite and sheared granite of Wadi Sikait (after [86]).

The role of hydrothermal activity in the evolution of the studied rocks is indicated by several pieces of evidence, including (1) the replacement of primary columbite by pyrochlore and allanite by bastnaesite, (2) the presence of a strong M-type lanthanide tetrad effect, and (3) the non-CHARAC behavior of Y/Ho and Hf/Zr. According to [14], the Zr/Hf ratio varies from 33 to 40 in many igneous rocks; therefore, any deviation from this interval should be related to metasomatism (less than 20) or intense fractionation of accessory minerals. The studied rocks have Zr/Hf ratios ranging from 12.08 to 19.14 (average = 15.45) in pegmatite and from 10.93 to 16.13 (average = 14.14) in sheared granite. These ratios are less than the normal ratio, reflecting the hydrothermal effect.

Rare-metal deposits have been formed through magmatic or hydrothermal processes (e.g., [7,30,95,96,97,98]). The primary zoning in rare-metal minerals from Sikait pegmatite supports the idea that their crystallization occurred from a highly evolved granitic melt [99]. Also, the presence of Nb-Ta oxides as inclusions in quartz and feldspars indicates their magmatic nature. The upward movement of hydrothermal fluids in the later magmatic stages likely played a role in leaching metals from the lower part of the intrusion, precipitating them in the upper part of it. The outer parts of pegmatite bodies contain notably higher modal abundance of rare metals, suggesting that the crystallization of these bodies started from the contacts with granite country, producing mineralized outer zones, and continued inward, producing barren cores. Textural evidence revealed the occurrence of rare metal minerals as inclusions in quartz, and their corrosion via K-feldspars and quartz, indicating an earlier crystallization of rare metal minerals in the pegmatite. Also, the formation of rare metals through the hydrothermal process includes the replacement of primary columbite by pyrochlore and the replacement of allanite by bastnaesite.

5.4. Mixed NYF-LCT Pegmatite

Based on geochemical composition, the mineralized pegmatites are grouped into two families: NYF-type and LCT-type [80,100,101]. NYF-type is characterized by enrichment in Nb (Nb > Ta), Y, REE, Zr, Th, U, and F, whereas the LCT type is typically enriched in Ta (Ta > Nb), Li, Rb, Cs, Be, Ga, and Sn. The NYF pegmatites evolved from a mantle-derived magma, are thought to be formed in an anorogenic setting, and are comparatively small in size. [101]. The LCT pegmatites are large in size, regarded as orogenic, and formed by the melting of metasedimentary crust or by the fractional crystallization of a parent fertile granitic pluton [84,101,102]. Both types of pegmatite were formed from highly evolved volatile-rich melts, where the exsolved fluids altered the magmatic mineral assemblages and remobilized the critical metals [103]. The late fluids in the NYF pegmatites are considered to be entirely magmatic in origin, whereas those of the LCT pegmatites are externally derived fluids due to crustal anatexis and pegmatite formation [102,103].

Sikait pegmatites have peraluminous characteristics, are emplaced in a post-orogenic tectonic setting, and are rich in valuable elements Nb (Nb > Ta), Li, Rb, Ga, Zr, Y, Th, U, F, and REE. The juvenile crust of the Nubian Shield, which had previously derived from a mantle source, partially melted to produce the parental magma of Sikait pegmatites and granites. Adopting the above-mentioned features, the Sikait pegmatites exhibit mineralogical and chemical characteristics of both NYF- and LCT-type pegmatites, i.e., they represent a mixed NYF-LCT type. The studied pegmatites may have been broadly affected by the NYF system that was contaminated at the magmatic stage or altered during the hydrothermal stage by a fluid enriched in the components of LCT-type pegmatites [80]. Consequently, the origin of the studied pegmatites is in good agreement with [2] regarding the Wadi Zareib Li-pegmatite that is located in the central Eastern Desert of Egypt.

6. Summary

The Wadi Sikait study area is a part of the Neoproterozoic rocks of the Egyptian Nubian Shield. It represents one of the promising areas, as it contains various polymetallic minerals of great importance. The studied pegmatite and sheared granite of Wadi Sikait were emplaced into metamorphic country rocks with sharp intrusive contacts. Pegmatite bodies are zoned and composed of barren cores surrounded by mineralized outer zones.

K-feldspars, quartz, albite, and micas (biotite, muscovite, zinnwaldite, lepidolite) are the essential constituents of the studied rocks. The pegmatite and sheared granite contain a wide range of accessory minerals such as columbite, zircon, apatite, garnet, pyrochlore, monazite, xenotime, fluorite, thorite, allanite, cassiterite, beryl, and opaques. The texture and composition of the early crystallized marginal phases of pegmatite and sheared granite were modified by late and post-magmatic fluid–rock reactions; however, the magmatic features are still preserved. A late-stage hydrothermal overprint of the primary rare-metal mineralization is represented by the rims of pyrochlore partly replacing columbite-(Fe), as well as bastnaesite partly replacing allanite.

The pegmatite samples are characterized by extreme enrichment in rare metals than sheared granite, including high concentrations of rare earth elements (REEs), Li, Sn, Ta, Hf, Nb, Zr, Y, and Rb, as well as elevated ratios of Ga/Al and low contents of Sr, CaO, and MgO. They show subparallel chondrite-normalized REE patterns, with an enrichment of HREE relative to LREE [(La/Lu)n = 0.04–0.12] and strongly negative Eu anomalies [(Eu/Eu*) = 0.03–0.10]. The REE patterns show the M-type tetrad effect characteristic of being highly fractionated granite that has undergone significant modification through hydrothermal activity. The ratios of Zr/Hf and Y/Ho indicate non-CHARAC behavior and an exchange in hydrothermal fluid. Sikait pegmatite exhibits the mineralogical and chemical characteristics of both the NYF and LCT groups. Therefore, it is designated as a mixed NYF-LCT type.

The studied rocks have geochemical characteristics similar to post-collisional highly fractionated I-type and A-type granites, including high contents of some trace elements such as Nb, Zr, Hf, Zn, Y, Rb, and REEs with patterns displaying pronounced negative Eu anomalies, as well as low contents of CaO, MgO, and Sr. The extensive interactions between granitic melt and hydrothermal fluids were critical to generating extreme negative Eu anomalies and M-type tetrad effect that likely contributed to rare-metal mineralization.

The petrology and geochemistry of the studied pegmatite and sheared granite are largely consistent with crystallization from an evolved parental magma generated by partial melting of the juvenile crust of the Nubian Shield and emplaced along a regional strike-slip fault system that promoted its ascent. The separation of fluids from the oversaturated melt promoted both the diffuse greisenization and focused segregation of pegmatite and fluorite and quartz veins. The studied rocks provide numerous pieces of evidence that they were formed by extensive fractional crystallization followed by interaction with magmatically derived hydrothermal solutions.

The present work indicates that the pegmatite and sheared granite of Wadi Sikait are a promising source of Li-REE-Nb-Ta-Y-Th-Zr-U minerals in the Nubian Shield. We recommend that further mineral exploration studies be carried out on these rocks, especially on the strongly mineralized marginal zones of pegmatite.