Remote sensing techniques and geochemical constraints on the formation of the Wadi El-Hima mineralized granites, Egypt: new insights into the genesis and accumulation of garnets

The Wadi El-Hima Neoproterozoic I- and A-type granites in the Southern Eastern Desert of Egypt are rich in garnets (up to 30 vol%) and are cut by NW–SE strike-slip faults, as confirmed from structure lineament extraction maps. These mineralized granites and garnet mineralization zones can be successfully discriminated using remote sensing techniques. Spectral angle mapper and matched filtering techniques are highly effective for mapping garnet-rich zones and show that the highest garnet concentrations occur along the intrusive contact zone of NW–SE striking faults. El-Hima granites have high SiO2 (73.5–75.1 wt%), Al2O3 (13.4–15.3 wt%) and total alkali (6.7–8.7 wt%) contents, suggesting that they were sourced from peraluminous (A/CNK > 1) parental magmas. Garnet-bearing trondhjemites are metasomatic in origin and formed after I-type tonalite-granodiorites, which originated in a volcanic arc tectonic setting. Garnet-rich syenogranites and alkali-feldspar granites are both post-collisional A-type granites: the syenogranites formed from peraluminous magmas generated by partial melting of lower crustal tonalite and metasedimentary protoliths during lithospheric delamination, and the alkali-feldspar granites crystallized from highly fractionated, felsic and alkali-rich peraluminous magmas in the upper crust. Garnets in El-Hima mineralized granites occur in three forms: (1) subhedral disseminated crystals, (2) vein-type crystals, and (3) aggregated subhedral crystals, reflecting different mechanisms of accumulation. All are dominantly almandine in composition (Alm76Sps10 Prp7Grs6Adr1) and have high average concentrations of heavy rare earth elements (HREE) (ΣHREE = 1636 ppm), Y = (3394 ppm), Zn (325 ppm), Li (39.17 ppm) and Ga (34.94 ppm). Garnet REE patterns show strong negative Eu anomalies with HREE enriched relative to LREE, indicating a magmatic origin. These magmatic garnets are late-stage crystallization products of Al-rich hydrous magmas, and formed at low temperature (680–730 °C) and pressure (2.1–2.93 kbar) conditions in the upper continental crust. Peculiar garnet concentrations in syenogranites near and along contact zones with alkali feldspar granites are related to peraluminous parent hydrous magma compositions. These garnets formed by in situ crystallization from A-type granite melts, alongside accumulation of residual garnets left behind after partial melting of the host garnet-rich granites along the intrusive contact. Magmatic-fluid flow along the NW–SE striking fault of Najd system enhanced garnet accumulation in melts, which formed clots and veins of garnet.


Introduction
The Wadi El-Hima area is located in the Southern Eastern Desert of Egypt, and comprises mineralized I-and A-type granites associated with ophiolitic mélange and gneisses. These Neoproterozoic rocks are part of the Arabian-Nubian Shield (ANS). The Neoproterozoic (950-550 Ma; Stern 2002) ANS contains voluminous granites that are mostly free of garnets, except for a small number of localities, Remote sensing satellite data, such as Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery, can provide complementary data for rock discrimination and mineral exploration (e.g., Aboelkhair et al. 2010;Amer et al. 2010;Asran et al. 2017;Izawa et al. 2018;Shawky et al. 2019). Although silicates can be detected using Thermal Infrared (TIR), some silicate mineral groups, such as garnet, can be detected in the region of visible-near infrared (VNIR) and short wave infrared (SWIR) of ASTER satellite sensor (Izawa et al. 2018), due to a distinctive absorption characteristic in the Si-O bond that can be detected in the thermal emission region of the electromagnetic wavelength. However, TIR bands in the ASTER sensor have a smaller radiometric and spatial resolution (90 m) compared to VNIR-SWIR bands (i.e. 30 m). The spectral reflectance properties of garnets in the wavelength range (0.35-2.5 μm) suggest that garnets can be detected using ASTER sensor data in VNIR + SWIR bands based on their composition and structure (Izawa et al. 2018). In this study, spectral remote sensing mapping techniques such as automatic lineaments extraction, matched filtering (MF) and spectral angle mapper (SAM) have been used to detect garnet mineralization in host granites and stream sediments from the Wadi El-Hima area in the South Eastern Desert. Using these techniques, Wadi El-Hima mineralized granites have been discovered along intrusive contacts associated with a NW-SE striking tensional fault zone related to the Najd fault system in the Eastern Desert, similar to Abu Dabbab mineralized granites that occur along other NW-SE striking tensional fault zones (Heikal et al. 2019).
We present an integrated study involving field observations, different remote sensing spectral mapping techniques for discrimination of both rock units and garnet-rich zones, mineral chemistry (major, trace and REE) and whole-rock chemistry for Neoproterozoic mineralized granites and some stream sediments. This work aims to shed light on the timing of garnet genesis, its mode of occurrence, chemistry, and mechanisms of accumulation, as well as the factors controlling garnet mineralization in the Wadi El-Hima I-and A-type granites. This study also discusses the petrogenesis and geodynamic evolution of Wadi El-Hima granitic plutons in the context of tectonic and structural trends in the Eastern Desert of Egypt. As such, we emphasize the usefulness of remote sensing techniques for discrimination of mineralized granites and garnet mapping using ASTER satellite sensor bands of VNIR and SWIR.

Fig. 1
Remote sensing and geological map of the Wadi El-Hima area. a Location map of the study area (Stern and Hedge 1985). b ASTER false colour composite image (RGB = 981). c Geological map of the Wadi El-Hima area produced from integrated remote sens-ing techniques and field study, modified from Saleh et al. (2014). Gn, gneisses; OP, ophiolitic rocks; TG, tonalites-granodiorites; GTR, garnet-bearing trondhjemites; GSG, garnet-rich syenogranites; AFG, alkali feldspar granites Wadis, including Wadi El-Hima (Fig. 1c). We present a new geological map of the Wadi El-Hima area (Fig. 1c) based on the combined results of remote sensing processing, and field investigation. The main structure strike in the Wadi El-Hima area is NW-SE, which relates to the Wadi El-Gemal strike-slip fault and the Najd fault system (Sultan et al. 1988;Fig. 1c). This is confirmed from the dominant NW-SE striking faults based on the structure lineaments map, whereas minor structure strikes are E-W and N-S that match other Precambrian deformation structures elsewhere in Egypt. Several rock types in the Wadi El-Hima area can be distinguished from oldest to youngest, including: gneisses, ophiolitic assemblages, tonalites-granodiorites, garnet-bearing trondhjemites, garnet-rich syenogranites, alkali feldspar granites, post granitic dykes, and quartz veins (Fig. 1c). Syenogranites and alkali feldspar granites intrude into ophiolitic rocks and tonalites-granodiorites (Figs. 1c,2a,b), and show intrusive contacts. The ophiolitic belt (Fig. 1b, c) and scattered garnet-bearing trondhjemite pockets have a NW-SE trend, which is parallel to the Najd fault system that controls ore deposits and mineralization in the Central Eastern Desert of Egypt (Heikal et al. 2019).
Garnet-bearing trondhjemites occur in the northwestern part of the mapped area as several large elongate pockets within the garnet-rich syenogranites (Fig. 1b, c). They are very low in relief, up to ~ 15 m in height, but each pluton can extend 20-50 m in width and 500-800 m in length at the surface. They are medium-to-coarse grained and are white in colour (Fig. 2c) due to conversion of Ca-Na plagioclase to Na-rich plagioclase during metasomatism of the precursor tonalites. Garnet-bearing trondhjemites also exhibit a gradational contact with garnet-rich syenogranites. Garnets can be observed with the naked eye as disseminated grains within the trondhjemites ( Fig. 2c; Supplementary 1a), and become more abundant toward the intrusive contact with other granites.
Garnet-rich syenogranites occur as large elongate plutons that are medium-to coarse-grained and have a whitish grey (Fig. 2f,h) to pale pink colour (Fig. 2e,g;Supplementary 1a). They intrude into tonalities and granodiorites in the northwest of the Wadi El-Hima area (Fig. 1b, c). Garnet-rich syenogranites have low to moderate relief, up to 18-25 m height, and cover an area of about ~ 8.77 km 2 (Fig. 1c). They form a sharp intrusive contact with alkali feldspar granites (Fig. 2a, b) associated with a structure strike of 5b). Occasionally, they show a gradational contact with garnet-bearing trondhjemites. These syenogranites are occasionally veined and intruded by alkali feldspar granites (Fig. 2d).
Garnets in the investigated area occur in whole rock granitic masses in all granitic types, but with different concentrations (mainly 2-8 vol% garnet) that increase toward the intrusive contact with alkali feldspar granites. They have three modes of occurrence (Fig. 2) including: (1) disseminated small subhedral to rounded crystals (Fig. 2h), (2) small to medium crystals arranged in veins or clusters (Fig. 2d, e), and (3) patches or spot-like garnet aggregates (Fig. 2e, f, g). The vein-type (Fig. 2e) and aggregated garnets (Fig. 2f,g) in syenogranites occur in higher concentrations (up to 25 vol%) along contacts with alkali feldspar granites (Fig. 2a, b) and disseminated garnet can form up to 30 vol%; Fig. 2h). The Wadi El-Gemal NW-SE strike-slip fault cuts some parts of the garnet-bearing trondhjemites and garnet-rich syenogranites (Figs. 1c,5a).

Petrography
Garnet-bearing trondhjemites petrographically resemble metasomatized tonalites in terms of their mineral constituents and textures. They are composed mainly of plagioclase (50-55 vol%), quartz (24-28 vol%), K-feldspar (6-12 vol%), and garnet (2-6 vol%) (Fig. 3a, b) and show myrmekitic textures (Supplementary 2a). Other minor constituents include biotite and chlorite (2-4 vol%, Fig. 3a, b). Apatite, titanite, epidote, zircon, ilmenite, magnetite and titanomagnetite are accessory minerals. Sodic-plagioclase (An 13-18 ) occurs as subhedral columnar crystals, with variable grain sizes. Coarse plagioclase tabular crystals show albite twinning, whereas other crystals show normal zonation. Some Field photographs and hand specimen samples of Wadi El-Hima granites. a Panorama view of alkali feldspar granites and garnet-rich syenogranites with clearly intrusive contacts. b Close up view of the contact between garnet-rich syenogranites and alkali feldspar granites. c Hand specimen of disseminated garnet grains in garnet-bearing trondhjemites. d Close-up view of garnet-bearing syenogranites inter-veined by alkali feldspar granites, forming mixed zone and garnet arranged as veins. e Hand specimen of garnet-rich syenogranites showing high concentrations of fine-grained garnets arranged as veins and coarse aggregated grains or clots on the rim. f Close-up view of garnet-bearing syenogranites with high concentrations of garnet clots in the form of a wedge shape. g Garnet aggregated grains or clots in garnet-rich syenogranites. h Disseminated and garnet aggregates in garnet-rich syenogranites showing high concentrations (up to 30 vol%). Abbreviations as in Fig. 1 ◂ plagioclase grains are altered to sericite. K-metasomatism has led to the rims of zoned plagioclase being partially replaced by coarse interstitial K-feldspar with myrmekite rims, which leaves small plagioclase grains behind. Quartz grains show undulose extinction and form sharp boundaries with plagioclase and garnet crystals (Fig. 3a). K-feldspar (Supplementary 2a) is represented by microcline and perthite intergrowths (band or patchy type). Perthite prismatic crystals are orthoclase or microcline, including albite threads. Garnets occur as interstitial grains with anhedral or ridge-like shapes, and are rimmed and veined by flaky biotite (Fig. 3a, b). They form homogenous, coarse crystals (2-5 mm) and are free of inclusions, except for some quartz blebs and zircon (Fig. 3a, b). Zircon occurs as inclusions in garnet and plagioclase, whereas ilmenite exists as interstitial subhedral crystals. Flaky biotite contains zircon and apatite inclusions and occurs at the rim of garnets (Fig. 3a, b). Some chlorite flakes occur along garnet and biotite rims and in microcracks.

Remote sensing data and techniques
An ASTER sensor is one of five state-of-the-art instrument sensor systems on board the Terra satellite that launched on December 18, 1999 (Abrams 2000). ASTER remote sensing Level 1B (Granule ID: AST L1B00310162004082323) imagery data was acquired on October 16, 2004, with no cloud cover (i.e. 0%). The data were geometrically corrected, and rotated to a north-up Universal Transverse Mercator (UTM) projection (https:// lpdaac. usgs. gov). Images were pre-georeferenced to UTM zone 36 North projection using the WGS-84 datum. In this study, VNIR and SWIR data were stacked and processed using ENVI version 5.3 and ArcGIS version 10.5 software packages. The visible VNIR and SWIR bands were atmospherically corrected using the Fig. 3 Photomicrographs of Wadi El-Hima granites and associated garnets. a, b Anhedral to ridge shape interstitial coarse garnet (Grt) grains between quartz (Qz) crystals rimmed and veined by biotite (Bt) in garnet-bearing trondhjemites. c Coarse garnet crystals between alkali feldspar (Kf) and plagioclase (Pg) in garnet-rich syenogranites, exhibiting myrmekitic textures. d Coarse disseminated subhedral garnet grains in the interstitial space between plagioclase, K-feldspar and quartz in garnet-rich syenogranites. e Subhedral disseminated and homogenous garnet crystals in garnet-rich syenogranites. f, g High concentrations of subhedral garnet crystals occurring as vein type and grain aggregates in the interstitial space between K-feldspar in garnet-rich syenogranites. h Heart-shaped twinned zircon (Zr) crystal with sharp contacts with K-feldspar and quartz in alkali feldspar granites ◂ 1 3 FLAASH algorithm. Remote sensing processes and techniques were used to emphasise and discriminate different granite types, to identify major structures and pathways of hydrothermal solution transport, garnet-rich zones, outcrops of garnet-bearing granites and garnet concentrations in stream sediments in the Wadi El-Hima area (Figs. 4,5,6). A general flowchart for the methodology used in this study is shown in Supplementary 3.
By applying an optimum index factor (OIF) algorithm to the ASTER data (nine reflected bands VNIR-SWIR) to create an RGB composite (Chavez et al. 1982), the best resultant highest OIF ranking values of ~ 74.96% was found to be represented by bands (9, 8, 1) in red, green and blue (RGB) colour channels, respectively (Fig. 1b). Three granitic phases have been distinguished: garnet-bearing trondhjemites and garnet-rich syenogranites are represented by a whitish tone, and alkali feldspar granites of Gabal El-Faliq and Wadi El-Hima are represented by a dark orange tone and a greyish green tone, respectively (Fig. 1b).
Principal component analysis (PCA) displays the maximum contrast from several spectral bands with just three primary displaying colours (Vincent 1997;Sadek et al. 2015). Based on ASTER-image composites (PC6, PC4, and PC2) in RGB (Fig. 4c), tonalities and granodiorites were successfully discriminated by soft green to slightly desaturated red colours, while garnet-bearing trondhjemites were distinguished by a cyan colour. Both garnet-rich syenogranites Fig. 4 Remote sensing ASTER data image techniques. a Band ratio composite image of (2/1, 3/4, 4/7) in RGB. b Band ratio composite image of (2/1, 3/4, 4/7) in RGB. c PCA composite image of (PC6, PC4, PC2) in RGB. d PCA composite image of (PC4, PC3, PC2) in RGB. Abbreviations as in Fig. 1 and alkali feldspar granites were discriminated into moderate dark to soft green colours in the mapped area (Fig. 4c). The PCA-image (PC4, PC3, and PC2) in RGB discriminated tonalites and granodiorites as blue and green colours, while the garnet-bearing trondhjemites were distinguished by a very light cyan colour. Garnet-rich syenogranites have a very soft magenta colour, and alkali feldspar granites show soft green to vivid orange colours (Fig. 4d).
An automatic lineaments extraction method is widely used to extract surface structure lineaments to investigate geological structure and tectonic fabrics (Hung et al. 2005). In this study, the GeoAnalyst-PCI software was used to perform an automatic lineament extraction from ASTER sensor data image of VNIR-SWIR. The first principal component represents the direction of largest data variance and largest information of surface structure features (Richards and Xiuping 2006). The PC1 of the eight-bit grayscale is suitable for structure lineament extraction in this study. The lineaments length and number percentages extracted from the ASTER data are shown in the rose diagrams ( Fig. 5a, b), which reveal dominantly NW-SE-trending lineaments and secondary NNE-SW and N-S trending lineaments. The dominant NW-SE trending lineaments (Fig. 5a, b) are parallel to the NW-SE strike of the major Najd fault system in the Eastern Desert of Egypt.
A lineament density map ( Fig. 5c) was created using the interpolation process in ArcMap. In the Wadi El-Hima granites, high-density zones were found in the garnet-bearing trondhjemites (i.e. associated with pervasive metasomatism) and at the contacts between garnet-rich syenogranites and alkali feldspar granites (Fig. 5c). These higher density zones coincide with rock units characterized by faults and fractures, and these zones are generally oriented towards the north and the northwest, corresponding to the dominant strike of lineaments. Because lineaments are considered as potential pathways for fluid migration, areas with a higher density of lineaments (Fig. 5a) should be zones of mineral accumulation. Zones of blue circles on the density map ( Fig. 5c), which have a NW-SE striking fault, are considered the main garnet-rich zones in the Wadi El-Hima area.
Spectral mapping techniques, which are represented by SAM and MF methods, were applied for ASTER (VNIR-SWIR) bands of the study area to map garnet compositional endmembers. ENVI's spectral analyst tool applied algorithms with the aid of the USGS's spectral library (Fig. 6a) after resampling the ASTER data's spectral resolution to facilitate the matching processes (Fig. 6b). The SAM technique (Kruse et al. 1993) depends on angles between image pixel spectra and training data (ROIs) spectra or library spectra. The result of the SAM technique is shown tonalities-granodiorites, stream sediments and pegmatite of Gabal El-Faliq. e Close-up view of decorrelation stretch (7, 3, 1) in RGB to discriminate Wadi El-Hima granitic types and distribution of garnets. f Relation between distance (meter) from the contact with alkali feldspar granites and garnet volume percentage (vol%) in different granite types, showing increasing of garnet concentration in garnet-rich syenogranites toward the contact with alkali feldspar granites in Fig. 6c, with the almandine endmember of garnet selected from the USGS library (at a threshold of 0.46). This technique revealed the highest concentration of garnet in the northwest of the mapped area, especially in trondhjemites and syenogranites (Fig. 6c). Other garnet concentrations are located in the northeastern region in Wadi stream sediments (i.e. Wadi Abu-Qarya) (Fig. 6c).
Finally, a matched filtering (MF) technique was employed to determine the abundance of garnet using a partial unmixing algorithm. MF scores indicate the degree of how well unknown pixels were matched with endmember minerals (Pour and Hashim 2012). The result of MF analysis (Fig. 6d) show the distribution of the almandine endmember of garnet (at a threshold of 0.077) in the northwest (in garnetbearing trondhjemites and garnet-rich syenogranites), in the northeast (in stream sediments of Wadi Abu Qarya and tonalites-granodiorites), and in pegmatite dykes at Gabal El-Faliq (Fig. 6d). These results are consistent with field observations (Fig. 2), petrography (Fig. 3), and EDS analysis of mineral chemistry (Supplementaries 1,4,5), which indicate strong correlations between field and applied techniques. Garnet-rich zones ( Fig. 6c-f) and garnet-rich granites (Fig. 1c) have a NW-SE trend (Fig. 5b) matching the Najd fault system that is dominant in the CED of Egypt and controls mineralization (e.g., Gabal Abu Dabbab after Heikal et al. 2019) in granites, for example garnet-rich zones ( Fig. 6c-f) in granites from the Wadi El-Hima area.

Major element composition
Major element compositions of feldspars, biotite, garnet, chlorite, Fe-Ti oxides, zircon, rutile, and titanite (Tables 1, 2; Supplementary 4) in I-and A-type mineralized granites were determined using an electron probe microanalyzer with a wavelength-dispersive X-ray spectrometry (JEOL JXA-8600SX) housed at Niigata University, Japan. Operating conditions comprised 15 kV accelerating voltage, 13 nA beam current, and ~ 1 µm beam diameter. Data were processed by using an oxide ZAF matrix correction. Fe-Ti-V oxide minerals and accessory minerals were analysed by scanning electron microscopy (SEM) energy-dispersive X-Ray spectroscopy (EDS) at Niigata University, Japan. Operating conditions were 20 kV accelerating voltage, and working distance (WD) was 10 mm. Selected electron probe microanalyzer analyses of mineral separates from stream sediments of Wadi El-Hima in the study area are listed in Supplementary 4. The analyzed minerals were garnet, amphibole, ilmenite, magnetite and titanite. Zircon and other rare minerals (e.g. monazite, thorite and fergusonite) were identified and analyzed by SEM-EDS (Supplement 5).

Trace and rare earth elements
Trace-element concentrations of feldspar, biotite and garnet (Table 3) in garnet-rich syenogranites were determined in situ by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) using a quadrupole ICP-MS (Agilent 7500a) coupled to a 213 nm Nd:YAG laser ablation system (New Wave Research UP213) at Niigata University, Japan. Analyses were carried out by ablating 50 to 80-µm diameter spots at 4 Hz with an energy density of 10 J/cm 2 per pulse. The total time of data acquisition for one spot was 105 s, including laser ablation for 45 s and analysis of the background before and after for 40 s each. CaO and SiO 2 contents of silicates, and Al 2 O 3 , Fe 2 O 3 and TiO 2 contents of oxides determined by electron microprobe were used as an internal standard for quantification of silicates and iron oxides. SRM 612 and SRM 610 (Pearce et al. 1997) were used as external standards for calibration of trace element concentrations.
Subhedral disseminated garnets in syenogranites are rich in heavy rare earth elements (HREE = 1636 ppm on average) contents and Y (3394 ppm), but are depleted in light rare earth elements (LREE = 11 ppm) and Eu (0.5 ppm) with very low LREE/HREE and (La/Yb) N ratios. They have a very low concentration of Nb (≤ 0.5 ppm), and Ta (≤ 0.07 ppm) with slightly low and variable Zr (0.95-36.6 ppm). They have significant amounts of Zn (325 ppm), Li (39 ppm) and Ga (35 ppm). Chondrite-normalized REEs (Sun and McDonough 1989) of El-Hima garnets show strong enrichment in HREE, with slightly negative Eu anomalies (Fig. 7a), similar to magmatic garnet (Zhou et al. 2017;Sami et al. 2020), but different to hydrothermal garnet (Fig. 7a). In general, primitive mantle (PM)-normalized patterns show depletion in large ion lithophile elements (LILE) with low concentrations of Ba, Nb and Sr (Fig. 7b). This is due to the large ionic radii of these elements compared to the octahedral coordination of the garnet crystal structure (Gaspar et al. 2008).

Whole-rock chemistry
Ten rock samples of Wadi El-Hima granitic varieties were selected for major, trace and rare earth element (REE) analyses.We select fresh and homogenous granite samples with less garnet concentrations (mainly less than 6 vol% of garnets) for whole-rock chemistry. XRF and ICP-MS analyses are carried out at the GeoAnalytical Lab, Washington State University (WSU), USA. An agate-grinding bowl was used to crush each sample into homogenized pebblesized particles, and subsequently pulverized. The concentrations of major elements and some trace elements were determined via X-ray fluorescence using a ThermoARL XRF Spectrometer. Each powdered sample was weighed, mixed with two parts di-lithium tetraborate flux, fused at 1000 °C in a muffle furnace, and cooled. The resulting bead was reground, re-fused, and polished on a diamond lap to produce a smooth, flat surface for analysis. The  (Sun and McDonough 1989) calibration standard was the reference material 650 CC from the USGS standard rock powder GSP2. The detection limit for major oxides and trace elements is available online from the GeoAnalytical Lab. The loss on ignition (LOI) was determined by the weight difference after ignition at 1000 °C.
Concentrations of REE and some trace elements were determined via ICP mass spectrometry (Agilent 7700 ICP-MS). About 50 mg powder of each sample was dissolved in acid-washed Teflon containers by refluxing in hot (250 °C) 3:1 nitric and hydrofluoric acid for at least 8 h. A working curve for instrument sensitivity was developed using a blank fused bead from the same batch of flux as used to prepare the unknowns along with USGS standards AGV-2 and RGM2. Additional USGS standards (DTS-2, BCR-1, G-2) were used as a reference for quality controlling.
On an R1-R2 classification diagram (De la Roche et al. 1980), garnet-bearing trondhjemites lie along the boundary between granodiorites and monzogranites (Supplementary 7a). Garnet-rich syenogranites and alkali feldspar granites lie in the field of syenogranite and alkali feldspar granite, respectively (Supplementary 7a). In addition, El-Hima garnet-bearing trondhjemites plot in the field of trondhjemites, where garnet-rich syenogranites and alkali feldspar granites lie in the granite field (Supplementary 7b) (Barker 1979).
Chondrite-normalized (Sun and McDonough 1989) REE patterns of garnet-bearing trondhjemites (Fig. 9a) show two types of patterns with wide ranges of Eu/Eu* (0.37-1.44; Table 4); one displays enrichment of HREE relative to LREE with a negative Eu anomaly, and the other displays slight enrichment of LREE relative to HREE with a positive Eu anomaly, similar to those of I-type trondhjemites from the ANS (Eliwa et al. 2014). In contrast, garnet-rich syenogranites and alkali feldspar granites (Fig. 9b) exhibit parallel patterns with a negative Eu anomaly (Eu/Eu* = 0.3-0.47) and resemble REE patterns of A-type granites (Sami et al. 2017). The low (La/Yb) N ratio (0.5-2.71; Table 4) of Wadi El-Hima garnet-bearing trondhjemites and garnet-rich syenogranites is due to abundance of garnet (2-30 vol%) that is the main host of HREE (Gromet and Silver 1983). PM-normalized trace elements (Sun and McDonough 1989) for Wadi El-Hima granites exhibit slightly different patterns (Fig. 9c, d): garnet-bearing trondhjemites are rich in LILE (Cs, Rb, Th, U, K and Sr) relative to HFSE (Ta, Nb, Ti), but garnet-rich syenogranites and alkali feldspar granites are rich in HFSE (Nb and Zr), and poor in Ba, Sr, P and Ti (Fig. 9d).

Identification of garnet-rich zones and structure lineaments using remote sensing techniques
Band ratio composites (1/2, 4/7, 3/4; Fig. 4a) and principal component composite images (PC4, PC3, and PC2; Fig. 4d) allow accurate discrimination of garnet-bearing trondhjemites from garnet-rich syenogranites. PCA composite (PC6, PC4, and PC2) successfully discriminated tonalites-granodiorites and alkali feldspar granites from ophiolitic rocks and gneisses (Fig. 4c). Band ratios and PCA techniques simply discriminated gneisses, ophiolitic rocks, alkali feldspar granites, and tonalites-granodiorites from garnet-bearing granites (trondhjemites and syenogranites); therefore, these techniques are suitable to remotely identify mineralized granites, especially garnet-rich zones (Fig. 4a-d). Structure lineament map data extracted from the first PC of ASTER imagery data produced accurate structure trends in the Wadi El-Hima area (Fig. 5a, b). The main structure strike is NW-SE, which is parallel to the Najd fault system in the Central Eastern Desert of Egypt (Sultan et al. 1988), and other minor trends are NNE-SSW, N-S and E-W (Fig. 5a, b). A density lineament map indicates a highly dense set of structure pathways for hydrothermal solutions in trondhjemites and syenogranites that host garnets in the Wadi El-Hima area (Fig. 5c). These structure pathways share a NW-SE strike with the Najd fault system and are thus considered an ancient channel for magmatic-fluid flows, which drove metasomatism and formation of trondhjemites as well as facilitated residual garnet accumulation along the intrusive contact with alkali-feldspar granites (Figs. 2b-g, 3c-g, 5c). This is supported by field evidence showing a high concentration of garnet (up to 30 vol%) along intrusive contacts that strike NW-SE ( Fig. 6c-f). Therefore, we suggest that magmatic-fluid flows along fault zones may play very important roles for collection and accumulation of residual garnets in partially melted garnet-rich granites along these zones.
As discussed previously, garnets in the Wadi El-Hima area are dominantly almandine in composition, and have high average concentrations of FeO (34 wt%) and Al 2 O 3 (21 wt%), and minor amounts of MnO (4 wt%), and MgO (1.8 wt%) ( enrichment in El-Hima garnets, ASTER sensor data in VNIR and SWIR bands can be used to detect and map these garnets. Consequently, remote sensing spectral mapping techniques (SAM and MF) provide reliable constraints on the locations of garnet accumulations and distribution in the Wadi El-Hima area (Fig. 6a-d). The accuracy of these techniques has been confirmed with fieldwork and lab analyses; so they represent an effective tool for economic mineral mapping in regions that may be inaccessible, and should encourage further future exploration of mineralized granites. Besides tracing garnet in host rocks, the results show other examples of garnet in Umm Asheira tonalites-granodiorites, pegmatite dykes in Gabal El-Faliq, and stream sediments ( Fig. 6a-d). Tonalites-granodiorites in the eastern part of Wadi El-Hima are highly enriched in garnet (e.g., Umm Asheira) relative to those in the western part (Fig. 6c, d). False-colour composite images (7, 3, 1, in RGB) with decorrelation stretch enhancements allowed successful lithological discrimination of granitic phases with regards of garnetbearing rocks in the Wadi El-Hima area (Fig. 6e).  (Sun and McDonough 1989). The estimated depth based on the density of continental crust 2.7 g/cm 3

T, P and fO 2 conditions of granite crystallization
El-Hima garnet and subhedral ilmenite crystals (Supplementary 2d) were used to estimate a minimum temperature of garnet growth (Pownceby et al. 1987(Pownceby et al. , 1991 using Mn partitioning in garnet-rich syenogranites and alkali-feldspar granites. This geothermometer produced values of 695 °C and 700 °C (Supplementary 4), respectively, suggesting that garnet equilibrated at the same conditions in both granite types. Equilibrium temperatures during cooling and crystallization can also be calculated by utilizing garnet-biotite Fe-Mg exchange thermometry (Bhattacharya et al. 1992;Holdaway 2000), where calculated temperatures of equilibration (T GB ) range from 680 °C-730 °C for garnet-rich syenogranites (Supplementary 4). Consequently, estimated temperatures of formation of Wadi El-Hima garnets lie within the range ~ 680 to 730 °C, which straddles the wet solidus for granitic and intermediate magmas in the middle to upper continental crust (Palin et al. 2016).
Zircon saturation temperatures (T Zr ) were calculated based on the whole-rock chemistry of granites using the parameterization of Watson and Harrison (1983). Zircon crystallizes early from granitic magmas and its partition behavior is mainly controlled by temperature. Slightly elevated Zr contents in alkali feldspar granites (Zr: 415-1189 ppm) and garnet-rich syenogranites (Zr: 471-714 ppm) compared to garnet-bearing trondhjemites (Zr: 4-151 ppm) suggest derivation of the former from melts of significantly high temperatures (Watson and Harrison 1983;El-Bialy and Omar 2015;Sami et al. 2020), assuming zirconium saturation in the melt. Therefore, T Zr averages 955 °C, 930 °C, and 715 °C for alkali feldspar granites, garnet-rich syenogranites and garnet-bearing trondhjemites, respectively (Table 4). These temperatures of El-Hima Fig. 9 Whole-rock chemistry of El-Hima mineralized granites. a Chondrite-normalized REE patterns of garnet-bearing trondhjemites. b Chondrite-normalized REE patterns of garnet-rich syenogranites and alkali feldspar granites. c Primitive mantle normalized trace elements of garnet-bearing trondhjemites. d Primitive mantle normalized trace elements of garnet-rich syenogranites and alkali feldspar granites. Fields of ANS metasomatized I-type trondhjemites (Eliwa et al. 2014), and A-type granites (Sami et al. 2017) are used for comparison. REE and trace elements of granites were normalized to chondrite and primitive mantle values, respectively (Sun and McDonough 1989) A-type granites (935 °C on average) resemble those of the northern ANS A-type granites from Wadi Al-Baroud (El-Bialy and Omar 2015). Calculated results for the studied El-Hima garnets (680-730 °C) represent minimum temperatures at which garnet crystallized from the host magmas, but zircon saturation temperatures (715-955 °C) represent a maximum temperature limit. These conditions are similar to magmatic garnets with lower temperatures of formation (590-645 °C) than their host granites (733 ± 28 °C), such as the Abu Diab A-type granites in the Central Eastern Desert of Egypt (Sami et al. 2020 and reference therein).
Garnet geobarometry estimated from coexisting garnet and biotite (Wu 2019) in garnet-rich syenogranites is 2.93 kbar and in alkali feldspar granites is 2.1 kbar (Supplementary 4), reflecting formation of host granites in the upper crust. This is consistent with magmatic garnets (~ 1 vol%) in Madha granodiorites from ANS (du Bray 1988) and garnet-bearing muscovite granites in Abu Diab area (Sami et al. 2020) that are late-stage crystallization products of peraluminous magmas that cooled at low pressures (< 3 kbar). Based on whole-rock chemistry (Table 4), a numerical method can be used to estimate crystallization pressure of granitic intrusions based on two polynomial equations (Yang 2017): (1) P = − 0.2426 × (Qtz) 3 + 26.392 × (Qtz) 2 − 980.74 × (Qtz) + 12,563, where Qtz is the normative quartz and P is pressure in megapascal (MPa). (2) P = 0.2426 × (A b + Or) 3 − 64.397 × (Ab + Or) 2 + 2981.3 × (Ab + Or) − 464, 224, where P is pressure in MPa, and the sum of the CIPW norm (Qtz + Ab + Or) contents is 100%. The calculated pressure (Table 4) of garnet-bearing trondhjemites using these polynomial equations ranges from 1.20 to 2.96 kbar. This is consistent with experimental results showing that garnet stable in silicic liquids at low pressures (~ 3 kbar) have spessartine contents up to 10 mol% (Green 1977), similar to our garnet spessartine (33.3-15 mol.%; Supplementary 4). Assuming a typical lithostatic pressure gradient of 1 kbar ≈ 3.3 km overburden, these data imply crystallization depths of less than 10 km below the Earth's surface. The garnetrich syenogranites yielded variable pressures from 1.3 to 2.2 kbar, equating to depths less than ~ 7 km, whereas alkali feldspar granites crystallized at low pressure (1.42-1.84 kbar) and very shallow depth (< 5.5 km). Additionally, the Qz-Ab-Or diagram of normative compositions (Fig. 11d) shows that Wadi El-Hima granites lie close to the minimum melt composition at ~ 2 kbar to ~ 4 kbar with excess H 2 O fluid containing 0.5-1% F (Manning 1981). Increasing water pressure shifts the position of the granite minimum melting curve towards albite-rich compositions, supporting the importance of hydrothermal fluids during the metasomatic origin of garnet-bearing trondhjemites as opposed to a magmatic origin for garnet-rich syenogranites and alkali feldspar granites.
The oxygen fugacity (fO 2 ) of the Wadi El-Hima granites was also calculated from the equilibrium expression equation [log fO 2 = − 30,930/T + 14.98 + 0.142(P − 1)/T; Wones 1989], where T is temperature (in Kelvin), and P is pressure (bars). Using the previously constrained pressure and temperature (T Zr ) conditions of crystallization, the calculated oxygen fugacity (log fO 2 ) of garnet-bearing trondhjemites ranges from − 13.43 to − 18.1. In addition, garnet-rich syenogranites have log fO 2 values in the range − 10 to − 11.4 and alkali feldspar granites yield − 9.3 to − 11.8 (Table 4), indicating that the Wadi El-Hima granites were strongly oxidized (Helmy et al. 2004 and references therein). Using a T-fO 2 diagram (Supplementary 5b), the Wadi El-Hima granites plot in the field of titanite + magnetite + quartz, within the range from − 9.5 to − 18, suggesting that they were derived from a hydrous and oxidized source (Wones 1989).
Wadi El-Hima garnet-rich syenogranites and alkali feldspar granites are weakly to strongly peraluminous with A/ CNK ratios (> 1) and high 10 4 × Ga/Al values (2.3-8.6) ( Fig. 10a; Table 4). They also have high ΣREE (308 ppm on average), high LILE (e.g., Rb, Cs, K; Table 4) and high HFSE (e.g., Zr, Ta and Nb). Therefore, these granites show an affinity to A-type granites (Fig. 10b), rather than fractionated I-type and S-type granites (e.g., Whalen et al. 1987;Eby 1992;Frost et al. 2001;El-Bialy and Omar 2015;Sami et al. 2020). This is supported by their position on a Zr vs. 10 4 × Ga/Al diagram (Whalen et al. 1987), where garnetrich syenogranites and alkali feldspar granites plot in the A-type granite field, but garnet-bearing trondhjemites lie in the I and S-type granite fields (Fig. 10c). A-type granites are classified into: A1 granites, which form in an intraplate tectonic setting and have a mantle affinity, and A2 granites, which form in a post-collisional setting and have a crustal signature (Eby 1992). The investigated granites have an A2-type affinity, except for one sample of alkali feldspar  (Maniar and Piccoli 1989). A-type granite field is after Whalen et al. (1987). b Chemical classification diagram using SiO 2 versus (Na 2 O + K 2 O)-CaO (Frost et al. 2001). c 10 4 × Ga/Al against Zr for distinguishing between I, S, M and A-type granites (Whalen et al. 1987). d Nb-Y-Ga/3 discrimination diagram for subdivision of A-type granites into A1 and A2 sub-types (Eby 1992). e Y + Nb vs. Rb tectonic discrimination diagram (Pearce et al. 1984). f Na 2 O-K 2 O-CaO ternary diagram of Egyptian granitoids (Hassan and Hashad 1990;Sami et al. 2017). Trondhjemites and calc-alkaline fields are after Barker and Arth (1976) Fig. 11 Model for the origin and thermometry of Wadi El-Hima granites. a Rb versus K/Rb diagram (Akinin et al. 2009). b Zr vs. Th/Nb variation diagrams of Wadi El-Hima I-type granites, showing fractional crystallization (FC), assimilation-fractional crystallization (AFC), and bulk assimilation (BA) trends (Nicolae and Saccani 2003). c Petrogenetic discrimination Al 2 O3/(FeOt + MgO)-3CaO-5(K 2 O/Na 2 O) ternary diagram (Laurent et al. 2014) for Wadi El-Hima I-and A-type granites. d Normative composition of Qz-Ab-Or projection for granite thermometry. Dashed lines show quartz-alkali feld-spar cotectics and trace of water saturated minimum melt compositions at total pressure ranging from 0.5 to 10 kbar (Holtz et al. 1992). Solid line shows trace of minimum melt compositions at 1 kbar with excess H 2 O and increasing fluorine (F, up to 4 wt%) (Manning 1981). e, f Composition of Wadi El-Hima A-type granites compared to melts produced by experimental dehydration melting of metasedimentary rocks. Fields of melt compositions in panel e is after Patino Douce (1999) and in panel f is after Gerdes et al. (2000) granite from Gabal El-Faliq, which resembles an A1-type granites (Fig. 10d). This is consistent with other studies of A-type granites in the Eastern Desert, which are considered as A2-type granites and have been ascribed to formation in a post-collisional setting (e.g., Sami et al. 2017;Azer et al. 2019;Seddik et al. 2020). Finally, Wadi El-Hima A-type granites exhibit nearly flat REE patterns (Fig. 9b), similar to those of Egyptian A-type granites (Sami et al. 2017), except garnet-rich samples that are enriched in HREE relative to LREE. They are poor in Ba, Sr, Ti and P due to fractionation of plagioclase and apatite during formation of early formed I-type granites ( Fig. 9d; Table 4).
Garnet-rich syenogranites and alkali feldspar granites show alkali to calcic-alkalic characteristics and plot in the field of A-type granites on discrimination diagrams (Frost et al. 2001) (Fig. 10b); however, garnet-bearing trondhjemites have calcic characteristics, reflecting an abundance of plagioclase and a relative depletion of potash feldspars (Fig. 10b). On a tectonic discrimination diagram of Rb vs. Y + Nb (Pearce et al. 1984), the Wadi El-Hima A-type garnet-rich syenogranites and alkali feldspar granites appear to be post-collision granites (Fig. 10e). By contrast, garnetbearing trondhjemites lie in the field of volcanic arc granites (Fig. 10e), and are considered as collision-related granites. They are rich in LILE relative to HFSE, and show a strong positive spike of Pb (Fig. 9c), reflecting a subduction-related continental arc setting (Eliwa et al. 2014). In summary, Wadi El-Hima I-type garnet-bearing trondhjemites crystallized from peraluminous magmas in a volcanic-arc tectonic setting during arc-arc collision, while both A-type garnet-rich syenogranites and alkali feldspar granites originated in a post-collision setting (Fig. 12).

Model for the origin and geodynamic evolution of the Wadi El-Hima granites
Mantle materials have very low Rb/Sr ratios (Rb/Sr: < 0.1; Taylor and McLennan 1985), while lower, middle and upper continental crust have Rb/Sr ratios of 0.12, 0.22 and 0.32, respectively (Rudnick and Fountain 1995). The average values of Rb/Sr ratio of Wadi El-Hima I-type and A-type granites are 0.38 and 2.51, respectively (Table 4), similar to that of the continental crustal materials that were involved in generating the El-Hima granites. Mantle-derived rocks have higher Nb/Ta values (> 17.5) than continental crustal materials (Nb/Ta > 17.5) (Rudnick and Fountain 1995;Taylor and McLennan 1985;Green 1995). The average Nb/Ta values of Wadi El-Hima I-type (2.77) and A-type granites (15.17) are consistent with crystallization from crustal-sourced magmas. Mantle sources have a lower Y/Nb ratio (< 1.2) than crustal sources (Y/Nb > 1.2; Eby 1992). The Wadi El-Hima I-type and A-type granites have high Y/Nb ratios of 7.35 and 9.9, respectively, suggesting their parental magmas formed from melting of crustal rocks (Table 4). In addition, mantle derived magmas have a lower K/Rb (~ 100; Akinin et al. 2009) ratio than lower continental crust (K/Rb = 413), middle crust (K/Rb = 270) and upper crust (K/Rb = 250; Rudnick and Fountain 1995). Wadi El-Hima I-type and A-type granites have a high K/Rb ratio of 433 and 344, respectively, reflecting their parental magmas having been derived from melting of lower crustal rocks.
Some diagnostic major oxides and trace element ratios of Wadi El-Hima I-type granites (Fig. 11a, b) have been used to infer their petrogenesis and magmatic processes, such as bulk assimilation, assimilation-fractional crystallization (AFC) and fractional crystallization (FC) (De Paolo 1981). Based on a Rb versus K/Rb diagram (Akinin et al. 2009) and Th/Nb ratios versus Zr contents (Nicolae and Saccani 2003), Wadi El-Hima I-type granites show clear assimilation and fractional crystallization trends (Fig. 11a, b). The investigated granites also exhibit distinct negative Nb anomalies ( Fig. 9d; Table 4), indicating involvement of crustal materials and crustal contamination (Nicolae and Saccani 2003). Therefore, the most suitable differentiation mechanism in this case is AFC (Fig. 11a, b). Throughout this process, lower crustal rocks (e.g., tonalites-metasediments; Fig. 11c) could also partially melt during lithospheric delamination (Fig. 12b). The resultant melts would fractionate during ascent and so crystallize in the upper crust to produce Umm Asheira tonalites and granodiorites (Fig. 1c), where some of these tonalites (Fig. 12c) may then be metasomatized to produce El-Hima garnet-bearing trondhjemites. We observed textural and chemical evidence supporting a metasomatic origin of garnet-bearing trondhjemites, including: (1) replacement of oligoclase by K-feldspar and widespread formation of perthitic and myrmekitic textures ( Fig. 3c; Supplementary 2a); (2) primary biotite being partially or completely replaced by chlorite (Fig. 3a); (3) REE and spider diagrams of garnet-bearing trondhjemites (Fig. 9a, c) resemble those of ANS metasomatized trondhjemites (Eliwa et al. 2014); (4) calculated pressure (1.4-3.0 kbar) and temperature (656-808 °C) conditions (Table 4) of El-Hima trondhjemites are similar to those of ANS I-type tonalites (692-775 °C) (El-Bialy and Omar 2015), suggesting a metasomatic origin after tonalites; (5) the El-Hima trondhjemites show lower temperatures (656-808 °C; Table 4) than associated magmatic A-type granites (930-954 °C), reflecting a metasomatic origin after the parent tonalite-granodiorite bodies that have been affected by A-type magmatic fluids (Fig. 13a). This evolution is similar to the Rockford granites from the Appalachians, which were transformed to trondhjemites by alkali metasomatism caused by infiltration of Na-rich fluids (Drummond et al. 1986). A-type granites in the Eastern Desert and Sinai have evolved in a transitional post-collisional setting from compression to extension by melting of crustal rocks during lithospheric delamination and slab breakoff (e.g., Moghazi et al. 2004;Eyal et al. 2010;El-Bialy and Omar 2015;Azer et al. 2019). Wadi El-Hima A-type granites are strongly peraluminous (A/CNK: 1.04-1.14; Table 4; Fig. 10a) due to a high proportion of crustal rocks in the source region for magma generation (Moghazi et al. 2004;Eyal et al. 2010;El-Bialy and Omar 2015). They are enriched in total alkalis, Rb, Pb, Th and have total REE (309 ppm), but are depleted in Nb, P, Ti, Sr and have a negative Eu anomaly (Table 4; Fig. 9b, d), also suggesting their derivation from a crustal source (Fig. 11c) coupled with advanced degrees of differentiation (Fig. 11a, b) in a post-collisional setting (Fig. 10e). El-Hima A-type granites lie in the field of tonalites-metasediments (Fig. 11c, e, f) based on parental source diagrams (Laurent et al. 2014;Patino Douce 1999;Gerdes et al. 2000). This suggests that tonalites-metasediments could provide a suitable protolith for Wadi El-Hima A-type granites. This is in agreement with the origin of A-type granitic magmas in Egypt after partial melting of a tonalitic to granodioritic source, which formed during the island-arc collisional stage (Farahat et al. 2007;Eyal et al. 2010). Pelite-derived melts in post-collisional settings are strongly peraluminous and may crystallize granites, similar to the Wadi El-Hima A-type granites (Fig. 11e, f). Therefore, we suggest that Alrich crustal rocks (e.g., tonalites and metasediments) may be sources of the Wadi El-Hima peraluminous A-type granite enriched in garnets. This assumption agrees with tonalite and metasediment being sources for some A-type granites in ANS (Moghazi et al. 2004;Farahat et al. 2007;El-Bialy and Omar 2015;Sami et al. 2017). Figure 12 shows a geodynamic model for the origin of El-Hima mineralized granites. The collisional stage between intraoceanic arc systems or accretion of a volcanic arc system onto the margin of East Gondwana occurred during the evolution of Neoproterozoic juvenile ANS crust. The first collisional stage (Stage I, ~ 650-620 Ma after Kröner et al. 1994;Stern 1994) (Figs. 10c, e, 12a) of Wadi El-Hima was associated with the origin of arc-related calc-alkaline I-type tonalites-granodiorites (Figs. 1c, 12a), which crystallized from magmas derived from assimilation of lower crustal rocks (e.g., tonalites-metasediments) by FC processes (Fig. 11) (Miller et al. 2001;Eliwa et al. 2014;El-Bialy and Omar 2015;Azer et al. 2019). During ascent of these peraluminous magmas, the magmas experienced fractional crystallization to produce the Umm Asheira tonalites-granodiorites (Fig. 1b, c) (protoliths of the Wadi El-Hima trondhjemite; Fig. 11c) with appreciable volumes of garnet (Figs. 2,6,13). Thus, garnets in El-Hima trondhjemites compositionally resemble garnets in Umm Asheira tonalites-granodiorites (Supplementary 6c). Spectral remote sensing signatures (SAM and MF) show high concentration of almandine-rich garnets in tonalites-granodiorites in Gabal Umm Asheira in the east of the Wadi El-Hima region (Fig. 6c, d), similar to garnet signatures in Wadi El-Hima mineralized granites (Fig. 6c, d). El-Hima trondhjemites have similar garnet contents to Sikait garnet-rich granites (4 vol% garnet) that occur close to the El-Hima area (Moghazi et al. 2004) and the garnet-rich (up to 10 vol%) tonalites in NE Iran (Plimer and Moazez-Lesco 1980). In addition, our field observations indicate that El-Hima I-type trondhjemites and syenogranites (Fig. 1b, c) are intruded by A-type alkali feldspar granites. Consequently, we suggest that post-collision A-type granitoids intruded into arc collision I-type trondhjemites (Fig. 10e).
Shearing and extensional stress during the post-collisional period of an orogeny may enable fault planes to propagate to greater depths in the crust and open fault-defined shear zones (Azer et al. 2019;Seddik et al. 2020), such as the NW-SE Najd fault (Fig. 1c, 5a). These shear zones and faults provided a suitable channel for allowing mantle-derived melts and magmatic fluids to ascend through the crust and conduct heat to shallower crustal levels, enhancing partial melting of pre-existing tonalites and granodiorites (Fig. 12b). The remote sensing lineament density map for our study region (Fig. 5c) clearly shows structural pathways of magmatic/ hydrothermal fluids penetrating NW-SE along the Najd fault system. The Najd fault system characterizes the final stages of east and west Gondwana assembly through a complex history of convergence, magmatism, and terrane exhumation in the ANS (e.g., Meyer et al. 2014). Some syenogranites suffered partial melting and alkali metasomatism (Figs. 2a,b,d,3c) along intrusive contacts with the NW-SE striking Najd fault system (Figs. 1c, 6e) during this event, forming accumulations of residual garnet (Fig. 13b).
Magmatic and hydrothermal garnet in granites can sometimes be difficult to identify. Metasomatic garnets show similar compositions to magmatic garnets, but both types generally differ in terms of textures and other silicate mineral chemistry (Kontak and Corey 1988). Hydrothermal garnets are associated with secondary minerals (i.e. metasomatized types) and generally accumulate in open fracture and fissure spaces, forming veins-type ores, ore reticulates, and patches (Kontak and Corey 1988). For instance, they are often associated with metasomatized plagioclase (An 0-7 ) and secondary biotite and muscovite, which is not the case for the El-Hima examples (Table 1, Supplementary 4), ruling out a hydrothermal origin for the studied garnets.
Based on field investigation ( Fig. 2d-h) and petrography ( Fig. 3c-g; Supplementary 2b-d), the Wadi El-Hima syenogranites preserve three garnet occurrences: disseminated subhedral to rounded crystals, vein type and grain aggregates or clots (Figs. 3, 13; Supplementary 2). The vein-type (Fig. 2e) and aggregated garnets (Fig. 2f, g) are accumulation of subhedral to rounded disseminated grains. There is no difference in texture (i.e. they are homogenous and free of inclusions; Fig. 3e) or chemistry (Supplementaries 4, 6c; Fig. 14) between all three garnet occurrences. Garnets in syenogranites and alkali feldspar granites have very similar chemical compositions (Table 2; Supplementary 4) and plot mainly in the magmatic garnet field (Figs. 7,14). They likely crystallize directly from the same Al-rich hydrous magma source (e.g., Hogan 1996;Dahlquist et al. 2007), which had a high FeO content (e.g., Clemens and Wall 1981;Rene and Stelling 2007), during the late stage of upper crustal magmatism, but accumulated by different mechanisms (Fig. 13). Other evidences for the garnets in the Wadi El-Hima A-type granites having a magmatic origin are summarized as follows: (1) subhedral garnets show sharp boundaries with triple junctions among them and against other major mineral constituents (Fig. 3e, f; Supplementary 1b, d), and are evenly distributed, except at intrusive contacts; (2) garnets are homogenous (Fig. 3e) and free of inclusions and replacement textures, excluding them having a secondary origin (Figs. 3; Supplementary 2); (3) REE and spider diagram patterns of garnets in Wadi El-Hima A-type granites exhibit enrichment of HREE (1531-1742 ppm) relative to LREE (10.2-11.6 ppm), with Eu depletion, similar to profiles documented for magmatic garnets (Fig. 7a, b); (4) our garnets are depleted in Nb and LILE (Ba and Sr) relative to the primitive mantle, suggesting a magmatic origin rather than a hydrothermal origin (Zhou et al. 2017;Sami et al. 2020); (5) Phillips et al. 1981) and magmatic almandinerich garnet (FeO > 30 wt%) that formed from hydrous peraluminous granitic melts (~ 1-3 kbar) in the upper crust . They are also similar to magmatic almandine-rich garnets that crystallized at low pressure (~ 3 kbar) in the southern mountain batholith in Canada (Allan and Clarke 1981). Finally, (6) El-Hima garnets are similar in texture and chemical compositions to magmatic almandine-rich garnets in Egypt and worldwide (Allan and Clarke 1981;Moghazi et al. 2004;Emam et al. 2011) (Supplementary 6c) and lie in the magmatic field rather than hydrothermal field in common discrimination diagrams (Fig. 14a-f) (Clarke 1981;Miller and Stoddard 1981). As such, we interpret that they crystallized from highly peraluminous granitic magmas.
Magmatic garnets in the studied syenogranites crystallized directly from highly peraluminous magmas in the upper crust, as discussed above (Fig. 13b). Garnets in both trondhjemites and syenogranites ( Fig. 3a-g) have nearly identical compositions (Supplement 4;Fig. 14) and formed at similar conditions (~ 680-730 °C at 2.1-2.9 kbar), suggesting they share the same peraluminous parent magmas. Garnets in syenogranites reach up to ~ 30 vol% proportion near to and along intrusive contacts with alkali feldspar granites (Figs. 2d-h, 3d-g, 6d; Supplements 1b-d). Hogan (1996) attributed late-stage crystallization of garnets to increasing concentrations of Al in the melt, and we find that this mechanism applies here to the Wadi El-Hima garnet crystallizing from highly peraluminous (A/CNK = 1.04-1.14) magmas as coarse subhedral crystals during late-stage magmatism. Therefore, the appearance of garnet in Wadi El-Hima A-type granites is a result of in situ nucleation from highly peraluminous hydrous magmas (e.g., Clarke 1981; du Bray 1988) (Fig. 13). A contribution to high-volume garnet formation in this case is due to mixing of partially melted tonalitic and metasedimentary protoliths (Fig. 11c-f), which would have been rich in Al and Fe and provided these components to promote garnet growth and increase its concentrations in syenogranites .
It is well known that magmatic almandine garnet in peraluminous granites with high silica concentration (~ 4-10 vol% garnet) generally crystallizes at low pressure conditions (e.g. ~ 3 kbar; Speer and Becker 1992;Clemens and Wall 1981;Moghazi et al. 2004;Emam et al. 2011;Gharib 2012), which matches the low pressure (up to 3 kbar) and temperature (680-730 °C) conditions recorded by garnets in the Wadi El-Hima granites. Therefore, low pressure and temperature conditions of parent hydrous magmas likely play an important role in crystallization of garnets in Wadi El-Hima granites (Fig. 13). Abundant garnet in the South Eastern Desert granites (e.g. El-Hudi area and Wadi Sikiat) and worldwide (e.g. south mountain batholith in Canada) is related to parental magmas that were mainly peraluminous type (e.g., Emam 2011; Moghazi et al. 2004;Allan and Clarke 1981;Clemens and Wall 1981) and crystallized at low pressure and temperature.
Garnet veins, disseminated and aggregated crystals ( Fig. 2d-h) all show concentrations near to and along intrusive contacts (mainly 12-30 vol%; Fig. 6f) with alkali feldspar granites, and decrease in concentration away from this contact (Figs. 3c, 6f). In the study region, NW-SE striking fault zones are considered channels for ascending A-type magmas and juvenile fluids, which enhanced partial melting of garnet-rich syenogranites along the intrusive contact zone. This accumulated residual garnets from the host granites, forming the garnet-rich zone (up to 30 vol% garnet; Figs. 2d-h, 6f, 13) with a NW-SE trend (Fig. 5b) that parallels fault traces of the Najd fault system in the Central Eastern Desert of Egypt. The A-type magmas, which crystallized to form alkali feldspar granites, assimilated syenogranites along the intrusive contact zone (Fig. 2a, b) to form a secondary melt (melt B) and residual garnets (Fig. 13). These garnets were transported long distances within the melts (Dorais and Tubrett 2012) and preferentially accumulated as veins or clots 13) by magmatic fluids along intrusive contacts (Creaser et al. 1991) as opposed to areas far away from the contact (Fig. 2d, 6f). This mechanism has been documented via the accumulation of garnets at contact surfaces and shear zones with pegmatites from the Gangdese Orogen in southeastern Tibet (Yu et al. 2021). Therefore, we suggest that the El-Hima garnets in syenogranites and alkali feldspar granites are magmatic origin, but possibly accumulated during a later hydrothermal period. We cannot exclude in situ crystallization of new garnet crystals from melt B, where garnets continued to grow in situ from the melts. Both residual garnets and new garnets with different grain sizes would have been in equilibrium with melt B, and therefore, inherited the same chemical compositions (Supplement 4; Fig. 14). It is well known that garnet may undergo dissolution and reprecipitation, which keeps it in equilibrium with the changing magma compositions (Dorais and Tubrett 2012).
Based on field investigation, structure lineament extraction mapping (Fig. 5a), and other remote sensing techniques, the Wadi El-Hima area is known to have been affected by the Wadi El-Gemal NW-SE strike-slip fault (Fig. 1c) that lies parallel to the NW-SE Najd fault, and which likely played a role in forming residual garnet accumulations. Moreover, the lineament density map shows a high-density zone near and along contacts of garnet-rich syenogranites with alkali feldspar granites (Fig. 5c). This high-density zone coincides with the existence of faults and fractures, which represent structural pathways for magmatic fluid migration and mineral concentration. Remote sensing spectral mapping via SAM and MF (Fig. 6c, d) indicate that there are very high concentrations of garnets in Wadi El-Hima granites that have the same orientation as NW-SE strike-slip faults (e.g. the Najd fault). Furthermore, the SAM and MF techniques show high distribution of almandine-rich garnets in trondhjemites, syenogranites, alkali feldspar granites and tonalites-granodiorites (G. Umm Asheira) east of the study area, stream sediments, and the pegmatite of Gabal El-Faliq (Fig. 6c, d).

Wadi El-Hima I-and A-type mineralized granites in the
Eastern Desert of Egypt contain high concentration of magmatic garnets (2-30 vol%), which occur as disseminated subhedral grains, vein-type and aggregated garnet grains. These reflect different mechanisms of accumulation. 2. Applied spectral remote-sensing mapping techniques, such as SAM and MF using VNIR-SWIR ASTER data, indicate high concentrations of garnets in syenogranites and trondhjemites, which are oriented in the NW-SE direction parallel to strike of the Najd fault system. Wadi El-Hima mineralized granites were likely emplaced along a tensional fault zone with a NW-SE strike related to the Najd fault system. 3. El-Hima garnets in trondhjemites are restitic in origin and have been slightly modified by metasomatism. Garnets in syenogranites and alkali feldspar granites are magmatic in origin and crystallized directly from latestage peraluminous hydrous magmas that were derived from partial melting of tonalitic and metasedimentary protoliths. They are almandine in composition and crystallized at low temperature (~ 680-730 °C) and pressure (~ 3 kbar) conditions. They are rich in HREE, Y, Zn, Ga and Li, but are depleted in LILE, Ba, N Nb, Sr Nb, Ta, U and Th, with strong negative Eu anomalies, suggesting a magmatic origin.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).
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