Fe Ti Oxide Ores Research Articles

Revisiting magnesium isotope constraints on the petrogenesis of the Fe-Ti oxide-bearing mafic–ultramafic intrusions

Stable isotopes of metals, such as magnesium (Mg) and iron (Fe), typically undergo minimal fractionation at magmatic temperatures. However, in Fe-Ti oxide-bearing intrusions, significant Mg isotope fractionation (Δ26Mg values of up to 23 ‰) has been observed between silicates and Fe-Ti oxides. The mechanism responsible for this substantial fractionation remains unclear. This study presents Mg isotopic compositions for bulk rocks and mineral separates—including olivine, clinopyroxene, titanomagnetite, ilmenite—from sixteen Fe-Ti oxide ores in both the lower and the upper zones of the Baima layered mafic–ultramafic intrusion, as well as bulk rocks from three high-Ti basalts of the Emeishan large igneous province, SW China. Oxide ores from the lower zone and the lower section of the upper zone of the Baima intrusion are characterized by a low abundance of volatile-bearing minerals, such as apatite and hornblende. These ores exhibit δ26Mg values ranging from −0.30 to −0.14 ‰ for olivine, −0.26 to −0.06 ‰ for clinopyroxene, 0.16 to 0.60 ‰ for titanomagnetite and −0.33 to −0.21 ‰ for ilmenite, indicative of near-equilibrium fractionation. In contrast, oxide ores from the upper section of the upper zone exhibit a high abundance of volatile-bearing minerals and significantly different δ26Mg values, ranging from −0.19 to −0.05 ‰ for olivine, −0.25 to −0.10 ‰ for clinopyroxene, 0.57 to 0.60 ‰ for titanomagnetite and 0.86 to 2.34 ‰ for ilmenite, indicative of disequilibrium fractionation. A negative correlation between Fo content and δ26Mg value in olivine suggests that Mg isotopes fractionate as olivine undergoes fractional crystallization. Trapped liquid fractions, calculated based on Ce contents in apatite, suggest that rocks with high volatile-bearing mineral content have undergone re-equilibration with trapped liquids during cooling. The large disequilibrium Mg isotopic fractionation between silicates and Fe-Ti oxides is likely due to sub-solidus re-equilibration between minerals and trapped liquids. We propose that the extent of sub-solidus, diffusion-driven kinetic Mg isotopic fractionation in Fe-Ti oxide-bearing intrusions likely correlates with the volume of trapped liquids, which may serve as a significant medium accelerating ion exchange between silicates and Fe-Ti oxides. Moreover, mass balance calculations indicate that data of Mg isotopes on their own are insufficient to determine the petrogenesis of Fe-Ti oxides in layered intrusions. This insufficiency is primarily due to the low MgO content in titanomagnetite and ilmenite, where their separation causes minimal Mg isotopic fractionation of the melts.

Mineralogy and Geochemistry of Titaniferous Iron Ores in El-Baroud Layered Gabbros: Fe-Ti Ore Genesis and Tectono-Metallogenetic Setting

The Neoproterozoic pyroxene gabbros and gabbronorites in the El-Baroud mafic intrusion in the Northern Eastern Desert (NED) of Egypt host Fe-Ti oxide ore deposits. This study discusses the major and trace elements of both titaniferous iron ores and their host rocks, along with the mineral chemistry (major and in situ trace elements) of interstitial clinopyroxene (Cpx), to gain a deeper understanding of the Fe-Ti oxide genesis. These ores occur as disseminated (55–60 vol.% of Fe-Ti oxides) and massive types (85–95 vol.%) in the form of the dyke, layer, and lens. They are composed of titanomagnetite (80–87 vol.%) with subordinate ilmenite (10–15 vol.%) and magnetite (3–5 vol.%), in accordance with their high Fe2O3 (75.66 wt.% on average) and TiO2 contents (16.30–17.60 wt.%). The Cpx in the investigated ores is diopside composition (Mg#; 0.72–0.83) and exhibits a nearly convex upward REE pattern, similar to Cpxs in the ferropicrite that originated from the primitive mantle. Melts in equilibrium with this Cpx resemble Greenstone ferropicrite melts; the parent melt of El-Baroud gabbros is possibly a ferropicritic melt that was derived from the lithospheric mantle during plume interaction. The El-Baroud gabbroic rocks were generated during the arc rifting and crystallized under a high oxygen fugacity at a temperature of 800–1000 °C and a pressure of 3 kbar with a depth of 12 km. The Fe-Ti oxide ores have been formed from ferropicritic parent melts by two processes, including in situ crystallization that leads to the formation of disseminated Fe-Ti oxides in the iron-rich gabbros at the bottom and liquid immiscibility that is responsible for the formation of thick Fe-Ti ore lenses and layers at the top of the gabbroic intrusion. Initially, titanomagnetite crystallized from the primary Ti-rich oxide melt. As cooling progressed, some of the excess titanium in this melt was exsolved in the form of the exsolution ilmenite lamellae within the titanomagnetite. The Fe-Ti oxide layers in the NED follow the trend of NW-SE (Najd trend), where their distribution is possibly controlled by the composition of parent melts (rich in Ti and Fe), high oxygen fugacity, and the structure related to the Najd fault system. The distribution of Fe-Ti oxide ores increases from the NED to the Southern Eastern Desert (SED), suggesting the dominant mantle plumes and/or shear zones in the SED relative to the NED.

Genesis of the Devonian Habaqin ultramafic arc complex and associated low-grade Fe–Ti oxide mineralization in the northern North China Craton

In the northern North China Craton (NCC), abundant non-layered ultramafic complexes and associated low-grade Fe–Ti oxide ores (over 1000 million tons) were uncovered. However, their petrogenesis and metallogeny have remained unclear. The Habaqin complex is one of the larger intrusions of the region and hosts the largest low-grade magnetite occurrence. Here we present petrography, zircon U–Pb geochronology, mineral and whole-rock major and trace element compositions, and whole-rock Sr–Nd and zircon Lu–Hf isotopes of the complex to constrain its genesis. The complex includes early pyroxenite (∼75% clinopyroxene, ∼15% amphibole, ∼10% magnetite) and late hornblendite (∼80% amphibole, 15–20% magnetite, 0–5% apatite). Zircon grains from the hornblendite yielded a weighted mean 206Pb/238U age of 395 ± 2 Ma. The increasing whole-rock FeOtot and TiO2 contents from pyroxenite (FeOtot = 18.9 wt%, TiO2 = 1.31 wt%) to hornblendite (FeOtot = 19.1–30.4 wt%, TiO2 = 2.63–3.12 wt%) correlate with higher modal proportions of amphibole and magnetite. The pyroxenite and hornblendite display similar arc-like trace element characteristics of enrichment in LILEs and LREEs, and depletion in HFSEs. The complex formed by fractional crystallization of clinopyroxene, amphibole, magnetite, and apatite from a hydrous parent magma. Early removal of clinopyroxene caused H2O enrichment in evolved residual magmas and crystallization of magnetite and amphibole, forming low-grade mineralization with Ti-V-poor magnetite in hornblendite. The complex displays high initial 87Sr/86Sr ratios of 0.706413–0.707341, negative εNd(t) values of −16.0 to −14.2 and negative εHf(t) value of −30.6 to −12.0, suggesting a parent magma derived from an enriched lithospheric mantle, with possible existence of amphibole-bearing pyroxenite veins within the source. During the Early Devonian, the retreat of the subducted Paleo-Asian oceanic slab as well as arc-continental collision along the northern NCC induced partial melting of the veins and eventually formation of the complex.

Microstructural constraints for the formation of net-textured Fe-Ti oxide ores of the Baima layered intrusion in the Emeishan large igneous province, SW China

The mafic–ultramafic layered intrusions in the Emeishan large igneous province (LIP) host significant amounts of Fe, Ti and V resources. The Fe-Ti oxide ore layers hosted in these layered intrusions are variable in thickness and it is enigmatic how the large amounts of Fe-Ti oxides are accumulated to form the Fe-Ti oxide ore layers. The Fe-Ti oxide ore layers are mainly composed of net-textured ores, however, the processes to form the net-textured ores have not yet been well constrained due to lack of diagnostic microstructural evidence of Fe-Ti oxides in the ores. Here we use the electron back-scattered diffraction (EBSD) technique to characterize the microstructures of Fe-Ti oxides in the net-textured ores of the Baima layered intrusion in the Emeishan LIP, e.g., crystallographic orientation and neighbor-/random-pair misorientation, and to assess the formation model of net-textured ores. The net-textured ores are composed of silicate crystals (olivine, and minor plagioclase and clinopyroxene) isolated by interconnected matrix composed of Fe-Ti oxides (titanomagnetite and ilmenite) and minor sulfides. It is likely that the Fe-Ti oxides simultaneously crystallized from a common Fe–Ti–(P)-rich melt, but postdated olivine and other cumulus minerals. The compaction induced by the gravity of overlying cumulus minerals only caused weak intragrain deformation of Fe-Ti oxides so that the Fe-Ti oxides did not experience intense dynamic recrystallization. Instead, the relatively high temperature and water-rich condition facilitated the operation of static recrystallization, resulting in the straight or smoothly curved grain boundaries of Fe-Ti oxides. The neighbor-pair misorientation patterns of Fe-Ti oxides show the preference for low-angle misorientation, supporting the idea that adjacent crystallized Fe-Ti oxides were rotated to coincident orientation in a melt. This study provides new microstructural evidence for the presence of immiscible Fe-rich melt from which the Fe-Ti oxides crystallized to form the net-textured ores.

The layered gabbroic intrusion in the Southern Eastern Desert of Egypt: Implications for Fe-Ti-rich magmatism at the end of the East African Orogeny

The Rahaba layered intrusion in the Southern Eastern Desert of Egypt is mainly composed of pyroxene hornblende-, and hornblende gabbros. It hosts thick FeTi oxide ore layers that are composed of titanomagnetite with subordinate ilmenite and magnetite. To understand the petrogenesis of the FeTi rich gabbroic rocks, the nature of their parent high-Fe primitive mantle melts and condition of FeTi oxide layer formation, we obtained new mineral chemistry and whole-rock major and trace element compositions. The geochemical characteristics of the gabbroic rocks resemble those of picro-basalts, suggesting generation from picritic basaltic-like melts of tholeiitic affinity. Their high-Fe-Ti-Mg nature (TiO2, ∼1.91 wt%; MgO, ∼11.0 wt%; Fe2O3, ∼14 wt%) probably indicates elevated mantle potential temperatures due to upwelling of hot asthenospheric mantle. Rare earth elements (REEs) of clinopyroxene show flat patterns with depletion in LREEs, similar to those of the host gabbros and picritic basaltic lava, suggesting a mantle origin for their parental melts. Moreover, clinopyroxene compositions show differentiation trends similar to low-pressure igneous intrusions, and reveal a rift-related origin. Chemistry of plagioclase (An45–62), clinopyroxene (Wo38.4–45.3, En39.9–43, Fs14–19.3; Mg#: 0.71–0.92) and olivine (Fo56–80) shows wide compositional ranges due to variable degrees of crystal fractionation. The calculated equilibrium temperature for the Rahaba Fe-Ti-rich gabbros is mainly 820–1000 °C, at pressure below 5 kbar, reflecting depth of rock solidification of <15 Km. These rocks show both subduction-like and rift-related geochemical signatures comparable to other post-collisional layered intrusions in the Arabian-Nubian Shield. The low average Ce/Pb (8.3) and high Ba/Nb (391) ratios together with the enriched large-ion lithophile elements (Cs, Ba, K and Sr) and depleted high-field strength elements (Th, Nb, Ta and Zr) reflect the contribution from a preceding subduction event. The lineament map, structural lineament and lineament density map reveal main WNW-ESE and NW-SE structural trends of the Najd System in the Rahaba area. The Rahaba FeTi rich gabbroic intrusion was likely formed by initial impact of a mantle plume head with Fe-rich streaks at the base of metasomatized lithospheric mantle during an extensional phase associated with activity of the Najd-related strike-slip faults, at the end of the East African Orogeny. The resultant mantle-derived melt compositions and Najd-related structures possibly controlled the distribution of FeTi ore deposits during this Orogeny. The Rahaba gabbroic intrusion was emplaced during post-collisional extension after slab break-off and lithospheric delamination following collision of accreted island-arcs with the Saharan Metacraton.

Apatite trace elements and O-Sr isotopes reveal different magmatic sources of Fe-Ti oxide deposits in the eastern Tianshan, NW China

Magmatic Fe-Ti oxide deposits are one of the most important iron deposit in eastern Tianshan, NW China, and their ore-forming mechanism remains obscure. Here, we use trace elements and O-Sr isotopes of apatite from two representative magmatic Fe-Ti oxide deposits (Niumaoquan and Weiya) in the eastern Tianshan to constrain the genesis of ore formation. The apatites from the Niumaoquan Fe-Ti oxide ores exhibit higher Eu/Eu* values, lower Mn contents, and larger REE variations than those from the Weiya Fe-Ti oxide ores, indicating a protracted evolution process of the Niumaoquan ore-forming magmas under a relatively reduced condition. Specifically, the apatites from the Niumaoquan Fe-Ti oxide ores can be divided into two groups: Group 1 with relatively low REE contents, low 87Sr/86Sr ratios (0.70574–0.70641) and high δ18O values (5.1–5.6‰), and Group 2 with relatively high REE contents, high 87Sr/86Sr ratios (0.70703–0.70792) and low δ18O values (2.8–3.3‰). The Group 1 apatites formed by early crystallization in the mantle, while the Group 2 apatites formed by assimilation of low δ18O-high 87Sr/86Sr altered rocks during magmas emplacement en route to the continental crust. Unlike the Niumaoquan apatites, the apatites from the Weiya Fe-Ti oxide ores have relatively homogeneous, high δ18O values (8.3‰-9.1‰) and 87Sr/86Sr ratios (0.70798–0.70848), indicating involvement of continental crustal materials in their mantle sources. Combined with previous studies, we suggest that the Niumaoquan and the Weiya Fe-Ti oxide deposits formed in a non-plume tectonic setting but they originated from different magmatic sources. Our study highlights that trace elements and O-Sr isotopes of apatite can be used as powerful tools to constrain the source characteristics and ore-forming processes of magmatic Fe-Ti oxide deposits.

Textural relationships and mineral compositions of Fe-Ti oxide ores of the Panzhihua layered intrusion, SW China: Insights for magma emplacement and lateral extension of Fe-Ti oxide ore bodies

The Panzhihua gabbroic layered intrusion in the Emeishan large igneous province (SW China) hosts economically important Fe-Ti-(V) oxide ore deposits. The intrusion consists of six sections from northeast to southwest. To constrain the emplacement of parental magma across the sections and the magmatic processes associated with Fe-Ti oxide ore deposits, a combined study on field relations, rock textures, mineral compositions, and late-stage magmatic microstructure was carried out for the Fe-Ti oxide ores from the Zhujiabaobao, Lanjiahuoshan, Jianshan, and Nalaqing sections. A ∼7-meter-thick ultramafic portion comprising peridotite, olivine (Ol)-rich net-textured and massive ores is firstly recognized at the bottom of the Zhujiabaobao section in this investigation. The magnetite grains in the ultramafic portion are Cr-rich with 0.58 to 1.83 wt.% Cr2O3, and may have crystallized from an earlier injection of Fe-Ti-Cr-rich magma. On the other hand, in the predominate clinopyroxene (Cpx)/plagioclase (Pl)-rich net-textured to massive ores of the intrusion, the magnetite grains contain <0.2 wt.% Cr2O3, and may have crystallized from late injections of Fe-Ti-rich, Cr-depleted magma. We proposed that the parental magma may be emplaced into an originally “tilt-spoon” structure and initially unloaded in the Zhujiabaobao section, then infilled the Lanjiahuoshan to Jianshan sections and extended further to the Nalaqing section. The thick, massive Fe-Ti oxide ore layers in the lower part of the intrusion are attributed to the silicate immiscibility of interstitial liquid within the crystal mush, and the large amounts of Fe-Ti oxides likely crystallized from immiscible Fe-rich melt aggregated in the lower part of the magma chamber.

In situ crystallization of Fe-Ti oxides in the Bijigou layered intrusion: Insights from crystal size distributions and compositions of magnetite and ilmenite

The formation of Fe-Ti oxide ore bodies in layered intrusions is attributed to in situ crystallization of basaltic magmas or accumulation after crystal settling regardless of their thicknesses and occurrences in the intrusion. These two key processes are difficult to be distinguished in terms of chemical compositions of minerals or cumulates. However, the crystal size distributions (CSDs) of Fe-Ti oxides in the rocks can be remarkably distinct in these two processes. In this study, we collected samples from a drill core profile across the major Fe-Ti oxide-rich segment of the Bijigou intrusion, a large and differentiated layered intrusion in Central China, and carried out a detailed study on the CSDs and in situ major and trace elements of magnetite and ilmenite to examine how Fe-Ti oxides are accumulated to form Fe-Ti oxide ore bodies in the intrusion. The major Fe-Ti oxide-rich segment of the intrusion consists of, from the base upward, a Fe-Ti oxide ore layer (∼80 m), a troctolite unit (∼153 m), an oxide gabbro unit (∼165 m) and an apatite-oxide gabbronorite unit (∼165 m). Large grains of magnetite and ilmenite in the Fe-Ti oxide ore layer show logarithmic-linear CSD curves, similar to those yielded by in situ crystallization of Fe-Ti oxides. The modes of Fe-Ti oxides in the oxide gabbro unit and apatite-oxide gabbronorite unit fluctuate along the profile, which cannot be attributed to accumulation after crystal settling. In addition, the magnetite of the Fe-Ti oxide ore layer, oxide gabbro unit and apatite-oxide gabbronorite unit shows a gentle increase trend of Cr concentration from the base upward in each unit, inconsistent with the trend for an accumulation after crystal settling. The magnetite grains enclosed within silicates have similar compositions to those interstitial to silicates, indicating that they all crystallized from similarly interstitial Fe-rich melt. The in situ crystallization of Fe-Ti oxides is likely triggered by increasing oxygen fugacity due to dissociation of H2O in highly evolved interstitial liquid that was expelled from underlying crystal mush. The crystallization of low-Cr magnetite at the base of each unit is attributed to suppressed eddy currents by dramatic increase of latent heat of crystallization in the magmas at the base. The eddy currents may gradually become efficient toward the top of the magmas due to rapid dissipation of latent heat of crystallization in the magmas, leading to crystallization of high-Cr magnetite by ion diffusion on the top of each unit. This study also demonstrates that analysis of CSDs for Fe-Ti oxides is applicable to examine the accumulation mechanism of Fe-Ti oxide ore bodies in layered intrusions.

Cryptic compositional variation of olivine between interlayered troctolite and net-textured Fe-Ti oxide ore in the Baima layered intrusion, SW China: Implications for the origin of banded Fe-Ti oxide ores

Cryptic compositional variation of olivine between interlayered troctolite and net-textured Fe-Ti oxide ore in the Baima layered intrusion, SW China: Implications for the origin of banded Fe-Ti oxide ores

Petrology, geochemistry and U–Pb zircon geochronology of Fe–Ti oxide ore‐bearing mafic sill from Saltora‐Mejia area of Chotanagpur Granite Gneissic Complex, eastern India: Implication for late tectonic emplacement of mafic rock of E‐MORB nature in an orogenic belt

An east–west‐trending medium‐grained mafic sill containing co‐genetic Fe–Ti oxide ore lenses is found disposed within granite gneisses around Saltora‐Mejia area in the eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) of eastern India. CGGC is considered as a Proterozoic mobile belt as it witnessed multiple phases of deformation and high‐ grade metamorphism during 1.8–0.8 Ga. Occurrence of such Fe–Ti oxide ore‐bearing mafic sill is unique in the entire CGGC which is a vast Proterozoic orogenic belt and has witnessed many phases of voluminous mafic and felsic magmatisms. The mafic rock is of gabbronorite composition which contains plagioclase, clinopyroxene, orthopyroxene as major constituent primary minerals and amphibole as late magmatic mineral. The rock shows sub‐ophitic, intergranular, mosaic and poikilitic texture (defined by larger pargasitic grain). The gabbronorite shows iron enriched tholeiitic character, low Mg#, low abundances of Ni and Cr, slight enrichment in LILE, LREE and slight depletion in HFSE like Nb and Ti. The computed melt in equilibrium with the studied gabbronorite shows transitional orogenic to anorogenic, within‐plate and E‐MORB‐like geochemical character. In this study, the U–Pb zircon crystallization age (~960 Ma) of the Saltora‐Mejia gabbronorite is reported for the first time which coincides with the late tectonic stage of the most pervasive orogenic activity in the CGGC around 1.2–0.9 Ga. Transitional orogenic to anorogenic geochemical character, late tectonic evolution and other field and laboratory evidences together suggest evolution of the Saltora‐Mejia gabbronorite sill in a late tectonic extensional environment which might have been facilitated by delamination of a subducted plate and upwelling of asthenospheric mantle during the waning stage of a major orogeny in the CGGC.

Formation of Fe-Ti oxide and Ni-Co sulfide ores by concentric tube flow and hydrous metal-rich melt recharge into the cooling crystal mush: Example from the Hongge intrusion in Panxi region, SW China

The Panxi (ultra-)mafic intrusions are relatively small compared with world-class ore-related (ultra-)mafic intrusions, but they host several super-large Fe-Ti oxide deposits and few small Cu-Ni (PGE) sulfide deposits. The mechanisms that concentrate huge amounts of oxide minerals into these small intrusions and form the distinct oxide and sulfide mineralization remain poorly understood. Here, we investigate the magma dynamics that produced extensive Fe-Ti oxide and coexisting Ni-Co sulfide mineralization in the Hongge intrusion of Panxi region, based on detailed field cross-cutting relations and petrographic observations, bulk-ore/rock geochemistry, and in-situ LA-ICP-MS magnetite trace element analyses. The Fe-Ti oxide ores include magnetite-poor (<60 vol%) disseminated and magnetite-rich (>60 vol%) massive types, with the former further divided into the gabbro-hosted (GH) and (olivine-)pyroxenite-hosted (OPH) subtypes. The GH oxide ores have Cr-poor (median 86.9 ppm) magnetite and abundant apatite interstitial to silicate minerals. In contrast, the OPH and massive oxide ores have Cr-rich (median 13,956 ppm) magnetite that envelops resorbed silicate minerals. Principal component analyses of bulk-ore and magnetite elemental contents reveal genetic links between the OPH and massive oxide ores, but remarkable genetic distinction from the GH oxide ores. Considering the overall lopolith-shape orebody geometry of several branch intrusions, the GH oxide ore formation may have involved spontaneous development of concentric tube flow zones within the cooling crystal mush. However, the lower P but much higher Cr-Cu-Co-Ni contents than the GH oxide ores and the sharp intrusion of massive oxide ores into the OPH oxide ores suggests that the OPH and massive oxide ores were probably formed by upward percolation and later intrusion of hydrous Fe-rich melts from a deep-seated magma reservoir into semi-solidified and fossilized mush, respectively. In addition, sulfide ores occur mainly as irregular/lenticular aggregates in massive oxide ores and are depleted in Cu and platinum group elements (PGEs), indicating early, deep-level sulfide segregation that scavenged the Cu and PGEs from the magmas. Hence, we envisage that the assimilation of S-deficient wall-rocks into basaltic magmas may have induced the early sulfide saturation and retain the majority of Fe for later precipitation as principal oxide and minor PGE-Cu-depleted sulfide ores.

Impact of DEMs for Improvement Sentinel 2 Lithological Mapping Utilizing Support Vector Machine: A Case Study of Mineralized Fe-Ti-Rich Gabbroic Rocks from the South Eastern Desert of Egypt

Fused remote sensing datasets have greatly contributed to enhancing lithological targets and providing significant information for mineral exploration. For instance, multispectral datasets can discriminate rock units through their unique spectral signatures. Digital Elevation Models (DEMs) could be an effective tool boosting lithological discrimination based mainly on their topographic variations. Consequently, the current study applied the power of the support vector machine (SVM) algorithm and the integrated Phased Array L-type band Synthetic Aperture Radar (PALSAR) DEM and multispectral Sentinel 2 data to: 1—Create an updated lithological map of a poorly mapped arid terrain (Khashir-Ras El-Kharit district, Eastern Desert of Egypt), and 2—Compare and assess the distribution of ferrogabbros with the aim of improving the localization of these rock bodies and investigating their mineral content. Our results enhanced the lithological mapping and revealed a better generalization of mineralized ferrogabbros when the input was a fused DEM with Sentinel 2 compared to the salt and pepper effect observed when adopting the Sentinel 2 solely as the input data. Additionally, the current research strongly recommends detailed exploration programs for Fe-Ti oxide ores within the gabbroic rocks delineated through the resultant thematic map. The ferrogabbros were subjected to a comprehensive analysis involving whole rock geochemistry, XRD, EDX, and light-reflecting investigations, which revealed the existence of magnetite, titanomagnetite, and hematite. The titanomagnetite crystals display subhedral morphology and exhibit fine- to medium-grained surfaces with irregular shapes. X-ray diffraction (XRD) analysis revealed the presence of magnetite and hematite in the concentrated Fe-Ti oxides. Additionally, the whole rock geochemistry highlighted the origin of the ferrogabbros and their proposed tectonic setting as an alkaline to calc-alkaline magma type that developed in a continental arc setting.

Ore Genesis of the Abu Ghalaga Ferro-Ilmenite Ore Associated with Neoproterozoic Massive-Type Gabbros, South-Eastern Desert of Egypt: Evidence from Texture and Mineral Chemistry

Massif-type mafic intrusions (gabbro and anorthosite) are known for their considerable resources of vanadium-bearing iron–titanium oxide ores. Massive-type gabbroic and anorthosite rocks are frequently associated with magmatic rocks that have significant quantities of iron, titanium, and vanadium. The most promising intrusions that host Fe-Ti oxide ores are the gabbroic rocks in the south-eastern desert. The ilmenite ore deposits are hosted in arc gabbroic and anorthosite rocks. They are classified into three types, namely black ore, red ore, and disseminated ore. The black ilmenite ore is located at the deeper level, while the oxidized red ore is mainly located at or near the surface. Petrographically, the gabbro and ilmenite ores indicate a crystallization sequence of plagioclase, titaniferous pyroxene, and ilmenite. This reveals that the ilmenite is a magmatic deposit formed by the liquid gravity concentration of ilmenite following the crystallization of feldspar and pyroxene. Meanwhile, quartz, tremolite, zoisite, and opaque minerals are accessory minerals. The Fe-Ti ores are composed of ilmenite hosting exsolved hematite lamellae of variable sizes and shapes, gangue silicate minerals, and some sulfides. The X-ray diffraction (XRD) data reveal the presence of two mineral phases: ilmenite and hematite formed by the unmixing of the ferroilmenite homogeneous phase upon cooling. As a result, the ore is mostly made up of hemo-ilmenite. Using an electron microscope (SEM), as well as by observing the textures seen by the ore microscope, ilmenite is the dominant Fe-Ti oxide and contains voluminous hematite exsolved crystals. Under the scanning electron microscope, ilmenite contained intergrowths of hematite as a thin sandwich and lens shape. The formation of hematite lamellae indicates an oxidation process. Mineral chemistry-based investigations reveal late/post-magmatic activity at high temperatures. The examined ilmenite plots on the ferro-ilmenite line were created by continuous solid solution over 800 °C, whereas the analyzed magnetite and Ti-magnetite plot near the magnetite line and were formed by continuous solid solution exceeding 600 °C.

Origin of the post-collisional younger gabbroic rocks and the associated Fe–Ti oxide ores, Abu Ghalaga area, Southern Eastern Desert, Egypt: mineralogical and geochemical constraints

The Abu Ghalaga pluton in the southern eastern desert of Egypt (SED) consists of undeformed norite and hornblende gabbro intruding island arc metavolcanics, ophiolitic rocks, and syn-tectonic older granitic rocks. The Fe-Ti ore (mainly ilmenite) occurs in the form of lenses at the base of the norite. Based on microtextures, mineralogy, and chemical composition, three types of ilmenites are recognized: massive high-Mg type, disseminated high-Mg type, and disseminated low-Mg ilmenite. The three ilmenite types are chemically distinguished by their MgO, MnO, TiO2, Fe2O3, FeO, and V2O3 contents. The variations in the concentration of these oxides are interpreted as a function of variations in T, fO2, and the composition of the parental magma. Reconstruction of the orthomagmatic conditions shows that T and fO2 of the magma from which the norite and its Fe-Ti deposit formed decrease during the course of magmatic differentiation, whereas the hornblende gabbro and its Fe-Ti mineralization have formed from a more evolved magma under relatively lower T and fO2. The REE patterns and variation diagrams support a model of crystal fractionation as the primary means of formation of the Fe-Ti deposits. Chemical features also suggest that the parent magma of both types of gabbroic rocks was generated in the mantle, but in two different pulses. Subduction zone, MORB, and WPB chemical signatures for these gabbroic rocks suggest a complex tectonic history consistent with compression followed by within plate extension possibly contributing to the development of a back-arc/intra-arc basin.

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.

Compositional Evolution of Interstitial Liquid After Onset of Abundant Fe–Ti Oxide Crystallization in Crystal Mush: Insights from Late-Stage Microstructures and Mineral Compositions of the Bijigou Layered Intrusion, Central China

ABSTRACT Compaction is important to the expulsion and compositional modification of interstitial liquid of crystal mush in the solidification of layered intrusions hosting Fe–Ti oxide mineralization. However, the compaction manner and compositional evolution of interstitial liquid after the onset of abundant Fe–Ti oxide crystallization in crystal mush are still enigmatic. In this study, we collected samples from a drill core profile across the major Fe–Ti oxide-rich segment of the Bijigou intrusion, a large and differentiated layered intrusion in Central China, and carried out a detailed study on the late-stage magmatic microstructures of rocks and crystal size distributions (CSDs) and spatial distribution patterns (SDPs) of clinopyroxene and plagioclase to examine the compaction manner of interstitial liquid. The Fe–Ti oxide-rich segment is composed of, from the base upward, a Fe–Ti oxide ore layer (~80 m), a troctolite unit (~105 m), an oxide gabbro unit (~153 m) and an apatite–oxide–gabbronorite unit (~165 m). The CSD curves for both plagioclase and clinopyroxene grains of the samples from the Fe–Ti oxide ore layer display gentle slopes for small crystals and consistent slopes for large crystals. In addition, there is a positive correlation of the SDP R-value (ratio of observed and predicted nearest neighbor distance of plagioclase/clinopyroxene) and mineral mode except plagioclase/clinopyroxene of the samples. The interstitial liquid was likely expelled by intensive pressure solution concomitant with crystallization of abundant Fe–Ti oxides, evidenced by the positive correlation of aspect ratio and alignment factor of plagioclase and the low fraction of trapped liquid (FTL) of 4–6% for the Fe–Ti oxide ore layer. The cumulus plagioclase of the Fe–Ti oxide ore layer was partially dissolved during pressure-solution compaction, leading to ambient Fe-rich interstitial liquid becoming Al-enriched in local places so that abundant amphibole and hercynite are present in the replacive symplectites adjacent to cumulus plagioclase. Above the Fe–Ti oxide ore layer, plagioclase grains in the samples from the troctolite unit show consistent slopes of the CSD curves and negative correlation of SDP R-value and mineral mode in the SDP plot, indicating that the interstitial liquid was mainly expelled by mechanical compaction, with FTL ranging from 7 to 9%. The CSD curves for clinopyroxene grains of the samples from the oxide gabbro unit have been modified by Ostwald ripening, but the expulsion of interstitial liquid in this unit is also attributed to mechanical compaction due to higher compaction rate than accumulation rate. This study demonstrates that the compaction manner of interstitial liquid can change from pressure solution to mechanical compaction with dramatically diminishing crystallization of Fe–Ti oxides. The pressure-solution compaction accompanying with large amounts of Fe–Ti oxides can remarkably modify the composition of interstitial liquid. This study also demonstrates that a combined measurement of CSDs and SDPs for cumulus minerals can be used to quantify the compaction manner of interstitial liquid during solidification of layered intrusions.

Genesis of magmatic ilmenite ores associated with the Mazua ultramafic intrusion, NE Mozambique

The Mazua ultramafic intrusion is among the most promising intrusions hosting Fe-Ti oxide ores in Mozambique. Fe-Ti oxide ores are mainly composed of ilmenite occurring as lenses, veins and layers, either massive or disseminated in hornblende pyroxenite. This study discusses the nature and origin of the intrusion and related ilmenite ore body on the basis of whole-rock and mineral chemistry. Textural and structural evidences support a cumulate origin for these rocks, including the occurrence of relict cumulate textures, layering of alternating oxides and silicate-rich rocks, and size-grading of silicate-rich layers. Primary magmatic features have, however, largely been overprinted by subsolidus processes. The occurrence of the cumulate assemblage (Fe-Ti oxides and clinopyroxene), without plagioclase and olivine, suggests that the magma underwent differentiation before its emplacement. The evolved magmatic nature of the intrusion is consistent with the calculated Mg-number of approximately 50 for the melt in equilibrium with clinopyroxene crystals. The high TiO2 concentrations (>4 wt%) of the evolved magma, by differentiation, are interpreted as the main factors controlling the crystallization of Fe-Ti oxides. We propose that the ores formed from crystallization and settling of Fe-Ti oxide minerals from an evolved basaltic magma. This model is consistent with (1) the occurrence of Fe-Ti oxides and clinopyroxene as cumulates in relation to the coexisting amphibole, (2) the layering structures of alternating oxides and silicate matrix, (3) the scarcity of apatite supported by the extremely low whole-rock P2O5 concentrations and (4) the restricted occurrence of ore veins, excluding the formation from immiscible oxide melts.

Amphibole geochemistry of the Baima layered intrusion, SW China: Implications for the evolution of interstitial liquid and the origin of Fe-Ti oxide ores

The Baima layered intrusion hosting a giant Fe-Ti oxide ore deposit in the Panxi region (SW China) is mainly composed of troctolite and olivine gabbro in the lower zone (LZ) and isotropic gabbro, olivine gabbro and apatite olivine gabbro in the upper zone (UZ). Amphibole, as a late-stage crystalized phase, occur throughout the Baima intrusion in several forms and can be used to trace the evolution of interstitial melt. The brown amphibole commonly occurs as rims on Fe-Ti oxides, interstitial granular grains, and inclusions in olivine and clinopyroxene. They are abundant in the Fe-Ti oxide ore layers. The different occurrences have similar major and trace element compositions (1.60–4.71 wt% TiO2, 68–160 ppm REE; 2.35–5.25 wt% TiO2, 55–159 ppm REE; and 5.24–5.33 wt% TiO2, 158–177 ppm REE, respectively). They crystallized in the magmatic stage (948–1033 °C, 954–1041 °C, and 983–1015 °C, respectively). The olive amphibole commonly occurs as granular grains at the base of the UZ. These crystals have 0.04–1.17 wt% TiO2 and 24–80 ppm REE, which are lower than those of the brown amphibole, and they crystallized at 634–755 °C. The green amphibole commonly occurs as granular grains, with <1.59 wt% TiO2, 5–144 ppm REE, and a formation temperature of 649–814 °C.The higher REE contents of the brown amphibole compared to the olive amphibole suggest that immiscibility occurred in the interstitial melt at the late magmatic stage of the Baima intrusion. The absence of negative Ti anomalies in the brown amphibole and their textural relationship indicate that the Fe-Ti oxides crystallized at almost the same time as the brown amphibole. Both the brown amphibole and the Fe-Ti oxides crystallized from an interstitial, immiscible Fe-rich liquid and then formed the Fe-Ti oxide ores. The evolved Si-rich melt migrated upward and mixed with the upper melts, which ended the olivine crystallization at the base of the UZ. The olive amphibole crystallized from the mixed melts. The green amphibole exhibits various REE patterns, suggesting secondary alteration.

Large-scale liquid immiscibility in the Hongge layered intrusion hosting a giant Fe-Ti oxide deposit in SW China

The Hongge layered intrusion hosts the largest magmatic Fe-Ti oxide ore deposit in China and provides a valuable opportunity for improving the current understanding of magma differentiation and associated ore-forming processes. In this study, we present geochemical data for apatite- and plagioclase-hosted inclusions in gabbros from the Hongge Upper Zone. The apatite-hosted melt inclusions have a large compositional range with 27.7–60.4 wt% SiO2 and 1.31–25.7 wt% FeOt, which is difficult to be achieved through fractional crystallization but can be well explained by an unmixing process and trapping of immiscible melts along the binodal of a two-liquid field. Only Fe-rich melt inclusions (with 23.8–33.1 wt% SiO2 and 17.7–35.8 wt% FeOt) are observed in plagioclase, which can be attributed to the preferential trapping of immiscible Fe-rich melts by crystallizing plagioclase. The cumulus clinopyroxene and plagioclase display complex zonations with different compositional domains, indicating that disequilibrium crystallization might have happened. We argue that efficient separation of immiscible liquids occurred in the magma chamber and contributed to the concentration of the ore-forming elements Fe and Ti in the remaining immiscible Fe-rich melt. Large amounts of Fe-Ti oxides crystallized from the collected Fe-rich melt forming the major Fe-Ti oxide ore layers of the Hongge intrusion.

Petrogenesis of the gabbronorite sill hosted Fe-Ti oxide ore bodies from the eastern part of Chotanagpur granite Gneissic Complex, India

Magmatic Fe-Ti oxide ore bodies of the eastern Chotanagpur Granite Gneiss Complex (CGGC), India occur as massive conformable lenses within a gabbronorite sill. The occurrence, lithological association, mineralogy, and geochemistry of the Fe-Ti oxide ore bodies are significantly different from the typical Fe-Ti oxide deposits associated with anorthosite complexes and mafic- ultramafic complexes. Textural evidence and mineral chemistry suggest mostly simultaneous fractional crystallization of the comprising minerals e.g. titanomagnetite, ilmenite and magnesio-hercynite. The temperature and fO2 conditions of re-equilibration are calculated using the compositions of ilmenite and pre-exsolved titanomagnetite pairs. Extrapolation of the data along ulvospinel isopleths reveals the probable original magmatic conditions e.g. temperature ~ 900 °C and fO2 ~ ΔFMQ + 2. The ore forming melt calculated by ‘equilibrium distribution method’ shows no crustal contamination and has a trace elemental pattern similar to typical enriched basaltic melts e.g. ferropicrites, alkali basalts, OIB and EMORB. High Ti and Zr contents of the melt suggest a possible within plate tectonic setting and enrichment in HFSEs and LREEs suggest that the parent melt could be generated from an enriched mantle source by low degree partial melting of garnet-peridotite layer of the upper mantle. The ‘quantitative fractional crystallization modelling’ indicates that early crystallization of olivine, pyroxenes, plagioclase and ilmenite from an enriched basaltic melt similar to EMORB (most fit melt) led to a condition of high oxygen fugacity and iron enrichment in the residual melt. The residual Fe-Ti rich melt moved to low pressure zones within the gabbronorite sill during late stage of D2 deformation, and later crystallized in the form of present ore bodies.