Petrogenesis of ilmenite-bearing mafic intrusions: A case study of Abu Ghalaga area, South Eastern Desert, Egypt

Neoproterozoic Abu Ghalaga ilmenite-bearing mafic rocks were subjected to geological, petrographical and geochemical studies integrated with mineral chemistry of ilmenite, pyrite and magnetite in order to deduce their petrogenesis. Abu Ghalaga mafic intrusions hosting the largest ilmenite reserve in the Egyptian part of the Arabian-Nubian Shield. Field observations revealed that the intrusions impound ilmenite ore, which occurs as interlayer of massive bands or disseminated within gabbroic rocks. Petrographically, Abu Ghalaga mafic intrusions include different kinds of gabbro; olivine pyroxene gabbro, pyroxene gabbro, pyroxene hornblende gabbro, altered gabbro and Fe-Ti-rich gabbro. They have calc-alkaline to tholeiitic magmas, which are depleted in high field strength elements (e.g., Ta, Nb and Th) relative to low field strength elements (e.g., Ba, Sr and Rb), and exhibit light rare earth elements enrichment relative to heavy rare earth elements with positive Eu anomaly (Eu/Eu*= 0.8-2.4). Variable mineralogical and wide variation of bulk rock geochemistry are ascribed to fractional crystallization and hydrothermal overprinting. Abu Ghalaga gabbroic rocks are derived from fractional crystallization of depleted mantle magmas that were modified by ascending fluids from subducted slab in volcanic arc setting. Fe-Ti oxides required high oxygen fugacity (ƒO2) and water contents by crystal settling of Ti-rich mantle which is applicable with subduction setting. It can be inferred that Abu Ghalaga mafic rocks are neither related to ophiolite nor to Alaskan type but to one of the layered intrusions hosting Fe-Ti oxides.


Introduction
Late Proterozoic mafic-ultramafic rocks are widely distributed in Central and Southern Eastern Desert of Egypt in most northwestern part of the Arabian-Nubian Shield (ANS) (Azer and Stern 2007;Lasheen et al. 2021a;Kamar et al. 2022;Saleh et al. 2022). ANS is the northern expansion of Mozambique belt, which developed during Neoproterozoic Era by amalgamation of West and East Gondwana to form the supercontinent Gondwana during the Pan-African orogeny (Stern et al. 2004;Lasheen et al. 2021b;Hamdy et al. 2022;Khaleal et al. 2022). The accretion of arcs and oceanic terrains led to crustal evolution of ANS (Abdelsalam and Stern 1996;Lasheen et al. 2022). The ultramafic exposures constitute 5.3% form the Egyptian crystalline outcrops mostly as allochthonous sheets (Dixon 1979).
Fe-Ti oxides deposits are widely distributed in Eastern Desert including Egyptian black sand deposits, Abu Dahr, Hamra Dome, Umm Effin, Korab Kansi and Abu Ghalaga area. Abu Ghalaga area is considered the first rank that hosts the largest ilmenite reserve ore in Egypt (Makhlouf et al. 2008;Khedr et al. 2020). Amin (1954) studied the ilmenite deposit of Abu Ghalaga area. He concluded that the ilmenite ore is formed by accumulation of ilmenite-rich residual fluid that occur as sheetlike zone on the top of gabbros mass. Abu Ghalaga ilmenite ore represents the largest reserve in Egypt, whereas the second one is recorded in Korab Kansi (Khedr et al. 2020).
The present work aims to study the geology, petrography and geochemistry of the Neoproterozoic Abu Ghalaga mafic intrusions integrated with Fe-Ti oxides chemistry in order to deduce their petrogenesis as well as the tectonic regime of the host rocks.

Geological setting
Abu Ghalaga ilmenite mine lies about 25km Southwest of Abu Ghusun City on the Red Sea Coast, at the intersection of Lat. 24° 21' 00" N and long. 35° 3' 35" E and far about 125km from Marsa Alam city. It is accessible from the Red Sea along Wadi Ranga (Fig. 1). The dominant Neoproterozoic rocks of Abu Ghalaga area are superstructure rocks comprising island arc metavolcaniclastics, gabbroic intrusion (rich in ilmenite bands) and syn-to late-granitic intrusions. The island arc metavolcaniclastics crop out as large belts extending nearly NW-SE (cover about 30 km 2 ) and injected by Abu Ghalaga gabbroic rocks and their contents are observed along Wadi Abu Ghalaga. Late granitic rocks crop out as low to moderate relief of isolated masses in the extreme western part of the mapped area with NNW-SSE trend.
Abu Ghalaga gabbroic rocks are medium-to coarsegrained dark green to black color due to high contents of mafic minerals. They crop out at the central part of the mapped area (Fig. 2a) forming the highest topographic feature herein and decrease to small masses at the western side of Wadi Abu Ghalaga. Generally, the contact between Abu Ghalaga intrusion and the syn-to late-granitic intrusions that are exposed in the southern part of the mapped area is gradational in a zone of few meters. Occasionally, this zone is characterized by hybrid diorites.
They hosted Fe-Ti oxides (ilmenite) with different sizes (Fig. 2a). Ilmenite mine of Abu Ghalaga is considered as one of the oldest and largest ilmenite mines in the ED (Amin 1954;Mahmoud 2021) (Fig. 2b,c). The ilmenite ore occurs either as massive bands or disseminated within the gabbroic rocks. Massive bands occur as sheeted-like body trending NNW-SSE and dipping (20°-60°) to the NNE and having length ranging between 50 m and 600 m, and width ranging from 20 m to 300 m with NW-SE trends, conformable with the primary layering of gabbroic rocks (Figs. 2d,e,f).
Olivine pyroxene gabbros are mainly composed of plagioclase (55-65 vol.%), clinopyroxene (22-35 vol.%) and olivine (6-8 vol.%) minerals. The main accessory minerals are ilmenite, magnetite and pyrite. Plagioclase crystals are the dominant mineral, commonly pristine tabular laths and rarely saussuritized. They occur as euhedral to subhedral ranging in size from 0.4 × 0.5 to 2.5 × 7 mm in length. Occasionally, they show zoning and pericline but lamellar twinning is common, cumulus textures and corroded clinopyroxene crystals. Clinopyroxene (augite) occurs as anhedral crystals and sometimes enclose numerous ilmenite and plagioclase minerals. Therefore subophitic and ophitic textures can be observed. Fresh olivine occurs as subrounded grains with perfect cracks and represents intercumulus phases (Fig. 3a).
Pyroxene gabbros are the dominant rocks type that mainly consist of cumulus of plagioclase (Fig. 3b), and clinopyroxene. Little amounts of hornblende are observed. Ophitic and subophitic textures are common. Corona textures (Cpx is partially transformed into hornblende) are observed (Fig. 3c). Oriented, rod-like of opaques is widely distributed along two intersecting planes (cleavage planes) of clinopyroxene minerals forming schiller structures.
Pyroxene hornblende gabbros are mainly formed of plagioclase, clinopyroxene, brown hornblende and opaques. Brown hornblende (10-15 vol.%) is observed as primary minerals and secondary after clinopyroxene forming corona texture, mostly occurs as brown big oikocrysts enclosing small plagioclase crystals (Fig. 3d).
Altered gabbros are mainly consist of saussuritized plagioclase and intercumulus clinopyroxene that are transformed to hornblende. It is noticed that the opaques are disseminated (Fig. 3e). In addition, pyrite is the main sulfide minerals, which mostly reveals well cubic form and large cluster crystals surrounded by saussuritized plagioclase (Figs. 3f,g).

Analytical techniques
Microprobe analyses were carried out at the University of New Brunswick (UNB), Canada. Samples were analyzed with a JEOL JXA-733 Electron Probe Microanalysis, equipped with ds spec and Qant32 automation (Geller Micro Analytical Labs). An accelerating voltage of 15 kV and a probe current of 30 nA. Peak counting times were 30 s (Al and Si), 90 s (Ti, V, Cr and Cs), and 120 s (Na, Mg, K, Ca, Mn, Fe and Ni. Bulk rock analysis (major, trace and rare earth elements) were carried out by (ICP-MS) in ACME Labs, Vancouver, Canada (Some samples were tested in Nuclear Material Authority), following fusion with lithium tetraborate/metaborate with digestion by nitric acid. Calibration of the ICP-MS was carried out using standards (STD OREAS45E and STD OREAS25A-4A). Detection limits for trace elements and major oxides were 0.01-0.5 ppm and 0.001 wt%-0.04 wt%, respectively. The analytical precision, as calculated from replicate analyses, was 0.5% for major oxides and varied from 2% to 20% for trace elements.
Environmental scanning electron microscope (ESEM) investigations were made at the Nuclear Material Authority (NMA). The ESEM is equipped with Link Analytical AN-1000/855 energy dispersive X-ray spectrometer (EDX) calibrated using natural standards to identify elements and  Amin (1947) and Saleh (2012) detect, semi quantitatively, the chemical compositions of minerals, especially Fe-Ti minerals. The accelerating voltage during analysis was 25-30 kV.

Pyrite
Electron probe microanalyses (EPMA) of Abu Ghalaga pyrite show narrow variation in their composition (Table 1). Trace element concentrations are close to the detection limit such as Zn, Ag and Cu or below of it like Sn, Sb, Pb, Bi, Cd and As. Microprobe study coupled with Ore microscopy revealed that no gold grains were detected within this pyrite. EDX such as EPMA analyses of the examined pyrite gave the same results (Fig. 4a). The main element in Abu Ghalaga pyrite is sulfur that ranges from 53.17 to 55.66 with average 54 wt.%. Fe in the examined Abu Ghalaga pyrites ranges from 44.86 to 46.19 with an average value of 45.68 wt.%. It can be inferred that the Abu Ghalaga pyrites are deficient in Fe content and contain excess of S relative to theoretical values of pyrite (Fe = 46.55; S = 53.45 wt.%; Baiwah et al. 1987;and Xu et al. 2020). Fe/S and S/Fe ratios of the investigated Abu Ghalaga pyrites vary from 0.809 to 0.862 and 1.15 to 1.23, respectively.

Major oxides
Chemical analyses of representative samples from Abu Ghalaga gabbroic rocks were carried out for major oxides, trace elements and REEs and are given in Table 4. The examined rocks belong to gabbroic rocks ( Fig. 5b) as indicated by bivariate Zr/TiO 2 *0.0001 versus Nb/Y ratios of  Whitney and Evans (2010). a Granoblastic olivine (Ol) surrounded by zoned and pericline plagioclase. b Cumulus of plagioclase (Pl) associated with amphiboles. c Plagioclase crystals ophitically and subophitically enclosed in clinopyroxene (Cpx)s. d Clinopyroxene oikocryst enclosing apatite (Ap) minerals and rimed by hornblende (Hbl) forming corona structure. e f Partially to completely saussuritized plagioclase with euhedral and cubic forms of pyrite surrounded by clinopyroxene. g Backscattered image (BSI) of cubic and fractured pyrite crystals. h Intercumulus and disseminated ilmenite (Ilm) with clinopyroxene. i BSI reveals disseminated ilmenite associated with clinopyroxene    Winchester and Floyd (1977). They range from clac-alkaline to tholeiitic on AFM ternary diagram of Irvine and Baragar (1971;Fig. 5c). Variable mineralogical contents of Abu Ghalaga gabbroic rocks as well as major, trace and REEs of the present study (as discussed below) are due to fractional crystallization processes.
Primitive-mantle normalized trace elements (Fig. 6a) reveal strong Ni negative anomaly as a result of immobile feature during partial melting (McCulloch and Gamble 1991) and enrichment of LFSEs relative to HFSEs, which is ascribed to mantle melt at arc with low partial melting (Jaques and Green 1980). Furthermore, small negative anomaly of Nb is observed which may be related to the role of crustal contamination of parent gabbro as well as depletion of Nb relative to enriched LFSEs (Taylor and McLennan 1985).

Crustal contamination and fractionation
Mantle-derived magmas can be contaminated during ascending to shallow levels by crustal sediments (Hora et al. 2009). Some trace elements and their ratios (La/Zr, Nb/Zr and Y/ Nb,) can be used to manifest the possible crustal contamination (Lippard 1973). Low and narrow ranges of these ratios reflect negligible effect of crustal contamination. Crustal contamination played a minor role on the examined rocks, where these ratios are low with narrow variations (0.178-0.146 of La/Zr; 0.026-0.1 of Nb/Zr and 3.62-15 of Y/Nb). Fractional crystallization is indicated by variable factors as indicated by petrographic studies (as cumulate textures) and bulk rock geochemistry (e.g., wide variance of Ni (20-132 ppm) as well as relatively low contents of Cr 2 O 3 ).

Partial melting and oxygen fugacity
REEs abundance can be used to measure the partial melting degree of the mantle sources (Zhu et al. 2008). Enrichment/ depletion mantle degrees can be manifested by using binary diagram of Aldanmaz et al. (2009) and compared with melting curves of Bezard et al. (2011). The examined rocks revealed a horizontal trend on spinel + lherzolite sources with 5 to 30% partial melting degree (Fig. 7a). Fractionation of clinopyroxene and amphiboles has a positive relationship with Zr, while fractionation of olivine and plagioclase is not modified with Zr/Y ratio (Floyd 1993). There are two partial melting curves according to Drury (1983); curve (I) has 60% olivine +20% opx +10% cpx +10% plagioclase, and curve (II) has 60% olivine +20% opx +10% cpx +10% garnet. The examined gabbroic samples show a positive relationship on Zr against Zr/Y diagram and plot near or follow curve II, suggesting medium to high partial melting degree of mafic phases ± plagioclase from clinopyroxene fractionation according to Drury (1983) (Fig. 7b). Fractional crystallization vs partial melting arrows in Ce/Yb-Ce relationship. The examined samples plot follow the fractional crystallization trend (Fig. 7c). The investigated Abu Ghalaga layered    intrusions are modified by fluids from subducted slab that play a significant role in transporting of some elements such as Sr, Nd and Yb (Fig. 7d). High oxygen fugacity (ƒO2) is indicated by massive layers of ilmenite intercalated with the host rocks (Fig. 2), and intercumulus of ilmenite and magnetite (Figs. 3h,i) reflects crystallization under high oxidizing conditions from Ti-rich melts (e.g., Toplis and Carroll 1995).

Tectonic setting
HFSE and transition elements mostly used for discriminating tectonic setting of igneous and metamorphic rocks due to their immobility during low grade of metamorphism (Manya 2014;Lasheen et al. 2021a). Abu Ghalaga gabbroic rocks are enriched in LFSEs than in HFSEs, which is consistent with subduction affinity. In addition, slightly enrichments of LREEs relative to HREEs (Table 4) may be ascribed to the nature of subduction zone since they are added from hydrous fluids to the mantle source during subduction (Wehrmann et al. 2014;Khedr et al. 2020). It is known that the HFSEs and HREEs are immobile; therefore, they can be used as an indicator for magma from which Abu Ghalaga gabbroic rocks were derived.
Volcanic arc affinity can be inferred by some ratios since the examined rocks have average values of Hf/Ta ratios 5.67 (>5), Nb/La ratios 0.74 (≤1), Hf/Th ratios of 2.17 (<8) and Th/Yb ratios 0.65 (>0.1), similar to those of Condie (1989) and Azer et al. (2014). In addition, ARC, MORB and OIB can be distinguished as manifested in Nb/U versus Nb diagram of O'Neill and Jenner (2012). The majority of the studied samples straddling arc field (Fig. 8a). According to Pearce and Gale (1997) diagrams, the examined samples occupy volcanic arc field using SiO 2 vs. Nb binary diagram (Fig. 8b).

Why Abu Ghalaga gabbroic rocks are considered as layered intrusion not ophiolitic?
Calc-alkaline magmatism can be subdivided into two groups. The first is older and deformed of syn-/late orogenic, while the second is undeformed post-collision and commonly related to post-collision (Be'eri-Shlevin et al. 2011). Based on the geology, petrography and bulk rock geochemistry, the examined Abu Ghalaga mafic rocks are neither related to ophiolite nor to Alaskan type. Moreover, ophiolitic maficsultramafics are deformed/metamorphosed and hosting chromite, magnesite and talc, when compared with Abu Ghalaga gabbroic rocks (not hosting these deposits). Alaskan mafic rocks are concentrated and enriched in Cu, PGEs and S in contrast to Abu Ghalaga gabbroic rocks host Fe-Ti oxides. Mg# of the Abu Ghalaga rocks [54.82 -64.96 (except pyroxene hornblende gabbro>70)] is lower than that of ophiolitic gabbroic rocks (61-81; Ali et al. 2010) but consistent with the Egyptian layered intrusion (Mg# <68.76;Essawy et al. 1997). In view of that, the examined Abu Ghalaga mafic rocks are neither related to ophiolite nor to Alaskan type. Fig. 5 a FeO-TiO 2 -Fe 2 O 3 discrimination diagram of Fe-Ti oxides. b Zr/TiO 2 *0.0001 against Nb/Y ratios after Winchester and Floyd (1977). c AFM diagram after Irvine and Baragar (1971) Sc  39  42  35  41  29  25  38  40  29  33  30  34  29  V  396  381  310  336  361  450  280  198  360  316  290  265

Genesis of Pyrite
Pyrite is the main sulfide minerals present in various systems, especially hydrothermal one. It is resistant to different physical and chemical conditions and is reliable for gold mineralization (Deditius et al. 2011;Xu et al. 2020). Pyrite genesis can be inferred through using transition elements and their ratios such as Co and Ni (Cook et al. 2009;Zhang et al. 2014;Bierlein et al. 2016;Adam et al. 2020).  (1987) and Brill (1989) There is a good relationship between Co of pyrite and its temperature, since high-temperature pyrite contains Co>1000 ppm, 100-1000 ppm for the medium-temperature one, and less than 100 ppm for the low-temperature one (Li et al. 2017). Furthermore, high Co/Ni ratio reflects hightemperature pyrite Li et al. 2017). Abu Ghalaga pyrite contains Co concentration more than 1000 ppm and ranges from 2114 to 10826 ppm with average value of 7062 ppm, suggesting high-temperature pyrites. In addition, Co/Ni ratio of the examined Abu Ghalaga pyrites varies from 7.86 to 30.84 with corresponding mean 15.14. As in the examined pyrite is below the detection limit since it may replace S in pyrite. As is enriched in low-temperature regime and flushed out in high-temperature pyrites (Yan et al. 2014;Leng 2017).
Depth of pyrite at which it is formed can be inferred from Fe/ (S + As) ratio. Deep depth contains 0.846, medium depth contains 0.863, and the shallower ones contain 0.926 according to Zhou et al. (2005). In Abu Ghalaga pyrite, Fe/(S + As) ratio ranges from 0.809 to 0.862 with an average value 0.848, reflecting deep-depth. Moreover, Ni content of Abu Ghalaga pyrite varies from 0. 0116 to 0.0793 wt.% with average value 0.0514 wt.%, close to that of the mantle (2200 ± 500×10 -6 ), indicating their mantle origin.

Source of pyrite
Abu Ghalaga pyrite is depleted in Zn, Ag, Cu, Bi, Cd, Sn, Sb, Mn, Se, Pb, Ni and As that may be attributed to the depletion of these elements in the initial/parent rock or fluids (Reich et al. 2016). Moreover, their concentrations depend on conditions at which pyrite is recrystallized and nature of pyrites refractory (Hazarika et al. 2013;Zhang et al. 2015). Metamorphic pyrite encompasses Co<100 ppm and Co/ Ni<1, while hydrothermal one contains Co>400ppm and Co/Ni>1 according to Carstens (1941). Sedimentary pyrite contains Co/Ni<1, whereas Co/Ni>1 refers to magmatic/ hydrothermal origin according to Clark et al. (2004). Occasionally, Co/Ni ratio is less than unity in hydrothermal pyrite as a result of interaction of fluids and wall rocks (Bajwah et al. 1987;Bierlein et al. 2016). Abu Ghalaga pyrite has Co/Ni ratio ranging from 7.86 to 30.84 with average value 15, suggesting their magmatic/hydrothermal nature, which is consistent with their host rocks (gabbro) (Fig. 8c).
In addition, nature of fluids that precipitate pyrite is indicated by Ni content (Zhao et al. 2011). Ni content in mafic rocks (as in the examined gabbro of Abu Ghalaga) reaches up to 150 ppm (Hofmann 1988), and < 50 ppm in felsic igneous rocks (Rudnick and Gao 2003). In the present study, Ni content of the studied pyrite ranges from 116 to 793 ppm, suggesting that the ascending fluids may have interact with mafic wall rock.
The investigated pyrite contains sulfur range from 53.17 to 55.66 wt.%, which may be related to different sources such as seawater, hydrothermal genesis and syngenetic sulfides (Zoheir et al. 2019). Using the dominant element in the examined pyrites (sulfur), hydrothermal type is depleted in S, while the sedimentary one is enriched in S (Xu and Shao 1980). The deficient character of S/Fe ratio of the examined pyrites (1.159-1.235 with mean 1.178) reflects their hydrothermal origin. From the previous, it can be deduced that the Abu Ghalaga pyrites are of magmatic/hydrothermal nature sources. Schematic model for Fe-Ti oxide-bearing gabbroic intrusions. Oceanic plate collides with a continental one; therefore, the oceanic plate subducted beneath the continental one. Water and fluids enriched in LFSEs were driven from the subducted slab leading to partial melting mantle rocks. Fe-Ti oxides are formed by crystal settling processes associated with fractional crystallization of these magma-forming gabbroic rocks.

Tectonic modelling and petrogenesis of Fe-Ti oxides
Fe-Ti oxides can be developed by fractionation of Fe-Ti mantle magma or by partial melting of Fe-Ti mantle magma. Fe-Ti oxides of Abu Ghalaga gabbroic rocks are widely accepted to be formed by fractional crystallization of Fe-Ti-rich minerals by crystal settling (gravitational separation of the earlier formed minerals (Basta and Takla 1968). This takes place as a result of fluids derived from the subducted slab in lithosphere, which yields molten magma enriched with Fe-Ti oxides. The crystal settling of the earlier minerals leads to accumulation and formation of massive layer of ilmenite intercalated within gabbroic rocks (Fig. 9). In addition, Fe-Ti oxides enrichment in Abu Ghalaga layered intrusions may be related to Ti enrichment in the slab-derived fluids leading to heterogeneity of ANS mantle (Jennings et al. 2016). The formation of Fe-Ti oxides requires high oxygen fugacity (ƒO2) and water contents, which is applicable with subduction signature (Howarth and Prevec 2013).

Conclusions
The current study discusses field, petrographic and bulk rock geochemistry of Abu Ghalaga mafic intrusions coupled with mineral chemistry of Fe-Ti oxides and pyrite. They host significant Fe-Ti oxides as massive layer or disseminated in gabbroic rocks. The examined intrusion exhibits enrichment of LFSEs relative to HFSEs, Nb, Ta negative anomalies and positive Eu anomaly, suggesting arc signature. Moreover, volcanic arc affinity can be inferred by some ratios such as Hf/Ta, Nb/La, Hf/Th and Th/Yb ratios. Crustal contamination played a minor role in the crystallization of the examined Abu Ghalaga gabbros, as indicated by low and narrow variations of La/Zr, Nb/Zr and Y/ Nb. Fractional crystallization of Fe-Ti mantle is a responsible factor as indicated by petrographic study (as cumulate textures) and wide variation in bulk rock geochemistry. Abu Ghalaga pyrite has Co/Ni ratio more than the unity, suggesting their hydrothermal/magmatic nature. Fe oxide is magnetite that contains FeO* (av. 94 wt.%) with less than 0.5 mole% ulvite, while Ti oxide is ilmenite that contains TiO 2 elevated from 45.98 to 50.37 wt.% and FeO* from 40.27 to 45.19wt.%. Abu Ghalaga gabbroic rocks are of layered intrusion rather than those of Alaskan and ophiolitic types.