Integrating mineralogy, geochemistry and aeromagnetic data for detecting Fe–Ti ore deposits bearing layered mafic intrusion, Akab El-Negum, Eastern Desert, Egypt

This study delineated the Fe–Ti oxide deposit concurrencies on the layered mafic intrusion in Gabal Akab El-Negum (GAN), South Eastern Desert, Egypt, using aeromagnetic mapping and chemical analysis of the hosted mafic rocks and mineralogical studies. Aeromagnetic data was improved using the enhanced horizontal gradient amplitudeto detect the primary structures (edges/contacts/faults) that control the distribution of Fe–Ti ore deposit. GAN layered gabbros are differentiated into troctolite, olivine–, pyroxene–, and hornblende–gabbros. These mafic rocks primarily comprise plagioclase, olivine, pyroxene, and hornblende with Fe–Ti ores (magnetite and ilmenite). The significant variation in Mg# of clinopyroxene between 0.70 and 0.82 indicates the importance of fractional crystallization in developing layered mafic intrusion. Clinopyroxene and plagioclase thermometry yielded low temperatures similar to the fractionated primary basaltic magma. The pairs of magnetite–ilmenite minerals in gabbros provide equilibrium temperatures of 539.44–815.56, and high fO2, reflecting various cooling and subsolidus reequilibration phases of minerals. The enrichment of GAN gabbros in light rare–earth elements relative to heavy rare–earth elements indicates the interaction between the Fe–Ti rich mantle and the fractionated tholeiitic magmas in the back-arc setting, generating Fe–Ti oxide ores.


Results
Total magnetic intensity map (TMI). The TMI map (Fig. 3) was reduced to the magnetic pole 17 (RTP, Fig. 4). The RTP map (Fig. 4) reveals magnetic variations between-99.548 nT and > 350 nT and varied magnetic (positive and negative) anomalies. The high magnetic (positive) anomalies (red-pink colors) indicate high ferromagnetic material content within the rocks or buried magnetic bodies. The RTP map is characterized by a broad high-intensity anomaly in the northeastern part of the area ENE trending, that is dissected by N-S to NNE trends. The low magnetic (negative) anomalies appeared over the southern and northwestern parts of the area trending, N-S, NW, and NE. EHGA is applied to the study area's RTP grid. The EHGA map reflects that the dominant structures controlling of the study area are N-S, NNE, NW, NE, and NNW (Fig. 5). The EUD approach is applied to the RTP grid to detect the magnetized sources' depth and lateral extent using SI = 0.5 (Fig. 6). The result EUD map (Fig. 6) indicates that the depth of the magnetic sources varies from zero to ~ 2400 m. These sources are trend NW, N-S, NE, and ENE. Moreover, the Tilt Depth (TD) map (Fig. 7) shows the depths of ore magnetized bodies of the study area. The EUD and TD depth solutions are gridded to produce a depth map for magnetized sources (Fig. 8). The applied depth estimator in our study (Figs. 6, 7, and 8) mapped boundaries, depths, and extent of magnetized ore bodies.
Chemistry of silicates and Fe-Ti oxide minerals. GAN major minerals and Fe-Ti oxides analysis were listed in Table1 and Supplementary (from 1 to 6). The plagioclase composition in GAN mafic rocks (Table 1; Supplementary 1) ranges from andesine to labradorite (An 42.06-56.80 ) 18 (Fig. 9a). Andesine (An 42.06-45.20 ) is recorded only in the hornblende gabbros, whereas labradorite (An 52.09- 56.80 ) is observed in the other varieties. Orthopyroxenes (Opxs) are observed in olivine -and pyroxene gabbros. They are of enstatite composition (En 68.95-72.08 ), with a limited range of Mg# (0.81-0.87), low Cr 2 O 3 contents (< 0.1 wt.%), and high TiO 2 (0.22-0.36wt%; Table 1; Supplementary 2) in comparison with Opx in the ophiolitic rocks of Egypt 19 . GAN Opx plots in the transition zone between igneous and metamorphic orthopyroxenes (Fig. 9b) in the 20 diagram. However, they follow a low-pressure differentiation pattern 21,22 (Fig. 9c). Clinopyroxenes (Cpxs) are found in all gabbroic verities varying from augite to diopside compositions 23  Whole-rock geochemistry. GAN Fig. 10b) are significantly concentrated in GAN mafic rocks compared to HREE and high field strength elements (HFSE) (Zr, Nb, Th, U), limiting chemical signatures of the subduction-zone, and adding LREE and LILE from the mantle 29 . Consequently, they effectively indicate mantle compositions that formed mafic magma 30 . The reduction in HFSE in the examined mafic rocks (Fig. 10b) indicates that the GAN intrusion was formed from a mantle source identical to back-arc basin mafics at the final spreading stage of Shikoku, Philippine 29 .

Discussion
Fe-Ti oxide ore deposits distribution. The RTP data (Fig. 4) show that the Fe-Ti oxide deposits are associated with high magnetic responses produced by mafic gabbros. Moreover, RTP and EHGA maps (Figs. 4 and 5, respectively) show that the Fe-Ti oxide deposits are primarily within the layered gabbros, with minor occurrences along the contact with the granites to the north and the amphibolites to the south. The correlation of collected rock samples with the RTP and EHGA data showed that the Fe-Ti oxide deposits are ENE trending and lie along the intersection zones of various fault directions. Furthermore, Figs. 5 and 6 reflect that the ENE steeply dipping and flat-lying ductile shear zones, N-S, E-W, and NW are the main tectonic frameworks controlling the study area in concordance with the N-S strike-slip shear zones 6 . www.nature.com/scientificreports/ Pressure-temperature conditions of crystallization. The clinopyroxene and plagioclase thermometers 31,32 ( Fig. 11a,b) yielded crystallization temperatures from ~ 1150 °C to 1200 °C and ~ 1050 °C to 1150 °C, respectively, close to pyroxene temperature in a fractionated basaltic magma 33 . The crystallization temperature shows higher temperature ranges for troctolite (̴ 1200°C) than hornblende gabbro ( ̴ 1050 °C), reflecting variations in magma compositions and fractional crystallization sequence near the layered intrusion temperature of Grader, Quebec, Canada 34 ( ̴ 1080 °C). The crystallization pressures using XPT and YPT parameters of clinopyroxene are 2-5 kb for troctolite, pyroxene gabbro, and olivine gabbro, whereas hornblende gabbro is < 2kb 31 (Fig. 11c). This is supported using an Al vi versus Al iv diagram 35 , where the analyzed clinopyroxene is plotted in mediumpressure fields for troctolite, pyroxene gabbro, and olivine gabbro and a low-pressure field for hornblende gabbro (Fig. 11d). Also, Opxs plotted in the transition zone in Fig. 9d reflect subsolidus reequilibration during cooling and magmatic crystallization under lower pressure in these rocks.
Equilibrium temperatures and oxygen fugacity. Equilibrium temperatures and oxygen fugacity of magnetiteilmenite pairs 36 were estimated using the ILMAT excel worksheet 37 . The ilmenite-magnetite pairs from olivine, pyroxene, and hornblende gabbros provide equilibration temperatures from 539.44 °C to 815.56 °C and oxygen fugacities from ΔNNO 0.68 to ΔNNO 2.13, indicating various stages of cooling history ( Fig. 11e; Supplementary  6). However, their oxygen fugacity values lay between NiNiO and MH, and each group of samples follows a parallel line trend above the NNO buffer reflecting Fe-Ti oxide crystallization (Fig. 11e).
Magmatic fractionation and contamination processes. GAN 39 . The negative correlation between the An content of plagioclase and Mg# of clinopyroxene reflect the preferred Ti from the melt phase during plagioclase and pyroxene crystallization 40 (Fig. 11f). GAN Fe-Ti rich mafic rocks are similar in Chondrite-normalized REE patterns to the Damiao complex in North China (Fig. 10a), indicating that the GAN mafic represents mixtures of cumulus minerals and trapped liquids 28 . They have positive Eu anomalies in all samples, are the weakest in hornblende gabbro and the strongest in other types, indicating plagioclase accumulation (Fig. 10a). Ilmenite and magnetite's association with primitive rocks, such as trocholite and pyroxene gabbro, indicate that fractional crystallization from Fe-Tirich parental magma had occurred 41,42 . GAN mafic rocks show no changes in chemical and mineralogical compositions, as supported by the low LOI values < 6 (0.46-2.47 wt%; Table 2) 43 , the absence of significant Ce anomalies (Fig. 10a), unvaried Pb contents, and similar LILE distributions 44 indicate the primary geochemical features of magma with no alteration evidence. In addition, low SiO 2 and REE contents, low Th/Nb ratios (0.04-0.23), and negative Zr anomalies, providing good evidence for the absence of crustal magma contamination through emplacement 44 . Genesis of Fe-Tirich magma and tectonic Setting. The studied mafic rocks of GAN intrusion are good indicators to recognize the magma natures and tectonic settings of various magmatic rocks formed during its evolution. The parental magma compositions, trapped liquids, and their oxygen fugacity strongly control the accumulation of good quantities of Fe-Ti oxide ore deposits 10,19,45 . The Fe-Ti rich parental melts were produced from partially melting of Fe-Ti rich mantle sources or because of the fractionation of Fe-Ti rich mantle-derived tholeiitic magmas or a combination of both processes 10 . Based on the whole-rock chemistry, GAN mafic rocks are enriched in FeO t , MgO, and Na 2 O + K 2 O, similar to arc-related mafic accumulated rocks 46 with tholeiitic affinities 47 (Fig. 12a). A high variation in whole-rock composition can be related to the accumulation of Fe-Ti ore deposits 10 . The GAN mafic rocks have major element compositions similar to high Mg-tholeiitic www.nature.com/scientificreports/ basalt, except for the pyroxenegabbro, which are rich in Fe-Ti ore deposits in the high Fe-tholeiite basalt field 48 (Fig. 12b). The GAN tholeiitic parental magma composition is the primary factor controlling the deposition of Fe-Ti ore deposits that crystallized mainly from Fe-Ti rich tholeiitic magma. Moreover, the high oxygen fugacity (ΔNNO 0.68-2.13) and the trapped liquids are crucial for controlling Fe-Ti oxide ore deposits 19,45 . GAN mafic rocks are plotted outside the deep-level arc cumulate field and follow a low-pressure differentiation trend typical for low-pressure igneous intrusions 21 forming in extension environment 42 (Figs. 9c and 12c). The high difference between TiO 2 and Na 2 O oxides 49 indicated a great degree of partial mantle melting and aqueous fluids in the magma, leading to lower contents of incompatible-elements 49,50 . The highand uniform enrichment of LREEs/ HREEs in the studied patterns are considered a back-arc basin environment 51 . This is confirmed using the Th/Nb versus Ce/Nb tectonic discrimination diagram, where all GAN samples plot in the back-arc field, except for hornblende gabbros samples that plot in normal mid-oceanic ridge basalt (NMORB) areas 52,53 (Fig. 12d). The FeO t /MgO-TiO 2 diagram 54 (Fig. 12e) shows that the GAN mafic intrusion is BABB except for troctolite plot out of the field. Moreover, the assemblage of arc-related mafic cumulate and MORB basalts (Fig. 12a,d, respectively) reinforces the back-arc extension environment 38 .

Materials and methods
Data. The aeromagnetic data of the surveyed area were collected using an Aero-Service aircraft (Cessna-Titan, Type-404), with a line separation of 1 km and 10 km tie traverse line separation at an altitude of 120 m (topography clearance). The traverse lines were instructed NE-SW with a perpendicular tie to the traverse direction 55 . The aeromagnetic data were corrected and processed by applying diurnal aircraft altitudes and removing the earth's magnetic field corrections 55 . The obtained data are in the form of total (magnetic) intensity (TMI) (Fig. 3). www.nature.com/scientificreports/ Enhanced horizontal gradient amplitude (EHGA). 56 Presented the EHGA as: where the amplitude of the horizontal gradient (HG) is given by 57 as: where p is a constant greater than or equal to 2 56 . In our study, p = 3 was employed to sharply delineate the study area's edges/contacts/faults.

Euler deconvolution (EUD)
. 58 Presented the EUD as an automated method to trace the position and depth of magnetic origins for realistic magnetic data and profiles. 59 molded it for magnetic-grid data. The EUD method runs solution for respective or wholly structural indexes (SIs), dips, strikes, and physical properties (density or magnetization) and is generally stable. The locations and depths (× 0, y0, z0) of source bodies are calculated using the following formula: www.nature.com/scientificreports/ where the observed field is ƒ at location (x, y, z). B is the field's base [regional value at (x, y, z)]. SI is the structural index 59 .

Tilt depth (TD).
A continually operated enhancement approach for the magnetic data is the Tilt-derivative (T) 60 , which calculates the vertical-derivative amplitude of the field employing its horizontal derivatives. 61 explained that when the numerical formulations of the horizontal and vertical gradients over a steep contact were entered into Eq. (4), they are written as: where ∆x and ∆z are the horizontal and vertical distances from the prevalent approximation pinpoint to the center of the boundary top.  (Table 1). Eight samples were investigated for trace and rare earth element (REE) analysis. Major and trace elements were analyzed using a PW 2400 series spectrometer at Vienna University, Austria. Each powdered sample was heated to its exact weight (5 g) for 1 h at 1050 °C to determine loss on ignition (LOI). The analytical accuracy was more than 1% and 2-5%, for major and trace elements, respectively. The analytical precision and accuracy of the tested blanks, samples, and duplicates were confirmed using international standards such as African Mineral Standards (AMIS 0007). REE analysis was determined by a VG Elemental PQ3 Quadru pole inductively at the Institute of Inorganic Chemistry, Vienna University, Austria.
Mineral analyses were conducted at the Vienna University, Austria (Mineralogy and Crystallography Institute) using a Jeol JSM-6400 SEM with an EDX unit. The analytical settings were 20 eV channel width, 20 keV accelerating voltage, and cobalt as an internal gain calibration. The values of Si, K, Al, Fe, Mg, Mn, Ca, Ti, Cr, and Na were determined and calibrated on the standards: garnet, titanite, chromite, and jadeite respectively. Total of  www.nature.com/scientificreports/ 134 spots from different minerals were studied to determine their chemical compositions (29 in plagioclase; 28 in pyroxene; 26 in amphibole; 9 in olivine and 42 in Fe-Ti oxide minerals).

Conclusion
Our aeromagnetic dataset represents the importance of such data enhancement to map the Fe-Ti oxide deposits, which they found mainly within the layered gabbros and minor occurrences at contact with the granites and amphibolites. Furthermore, the abundance of occurrences detected primarily surrounds the strike-slip shear zones N-S.GAN magnetite and ilmenite ores are disseminated ores or layers of 2.5-4 m in width and extend approximately 60 m, concordant with the dominant aeromagnetic structures (N-S, NNE, NW, NE, and NNW) and a high intensity anomaly trending ENE (Fig. 4). These ores originated from fractionating a Fe-Ti rich basaltic magma at reequilibration temperatures from 539.44 to 815.56 °C and high fO2(ΔNNO, 0.68-2.13), indicting a variety of cooling history of ore deposits from the parental magma. Finally, GAN mafic intrusion crystallized at lower pressures and temperature (~ 1050 °C to 1200 °C), formed in a back-arc tectonic regime.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.