Constraining Ore-Forming Processes Using Magnetite-Titanomagnetite Chemistry: A Case Study of the Mamuniyeh Cu Mineralization System, Urumieh-Dokhtar Magmatic Arc

The low-sulfidation epithermal copper mineralization in the Mamouniyeh area occurs as silica-sulfide-oxide veins hosted by monzonitic, gabbroic intrusions, and andesite. Magnetite and titanomagnetite are the primary hypogene oxide ore minerals in this system, present as titanomagnetite in intrusions and mainly as magnetite in silica veins. The chemical composition of Mamouniyeh magnetites in the FeO-Fe2O3-TiO2 system indicates a tendency towards wüstite (FeO). Increased Al2O3 and TiO2 content in silica vein magnetites compared to monzonitic intrusions is characteristic of hydrothermal magnetites. The decreased Cr2O3 and V2O3 content in re-equilibrated silica vein magnetites suggests their formation at higher oxygen fugacity than monzonitic titanomagnetites. The Al+Mn vs. Ti+V diagram shows that intrusive titanomagnetites formed at temperatures above 500°C, while silica vein magnetites formed at 200-300°C. The temperature drops in the system, influenced by atmospheric fluid mixing during hydrothermal fluid intrusion, led to magnetite deposition in silica veins at lower temperatures. The Ti vs. Mg+Al+Si diagram indicates the crystallization of intrusive titanomagnetites under conditions of limited hydrothermal fluid-wall rock interaction. An increase in oxygen fugacity from the parent magma towards the mineralized veins is observed, with intrusive magnetites forming at higher temperatures and lower ƒO2. ntroduction Iron oxides are present in many magmatic-hydrothermal mineral deposits, either as primary minerals (e.g., IOCG deposits and banded iron formations) or as secondary minerals (e.g., massive sulphide deposits). The chemical composition of magnetite provides insights into the characteristics of ore-forming fluids during magmatic or hydrothermal processes. Unique features of magnetite, such as its formation under various geological conditions and its ability to host numerous trace elements, have led to its use as an important petrogenetic indicator in recent years. This study investigates the composition of magnetite - titanomagnetite as the main hypogene oxide minerals associated with the low-sulfidation epithermal copper mineralization system in southern Mamuniyeh, within the central part of the Urumieh-Dokhtar magmatic arc (UDMA). The findings offer a better understanding of the evolution of the epithermal mineralization system and magmatic evolution in this area for the first-time using magnetite-titanomagnetite compositions. Despite numerous signs of ancient mining, mineral indices, and copper-gold-silver deposits associated with Eocene magmatism in this region, it has received less attention from researchers compared to other areas of the UDMA. Petrography, Mineralogy and Mineralization The study area features a series of intrusive and volcanic rocks ranging from acidic to basic, including andesite tuff, pyroxene andesite-porphyritic andesite, dacitic-rhyodacitic tuff, acidic lava, basaltic andesite, diabase, gabbro, diorite, monzonite, granodiorite, monzodiorite, and basalt-diabase. Geochemical characteristics show calc-alkaline magmatism related to a subduction zone, with crustal contamination during magma ascent (Goudarzi et al., 2024a). Copper mineralization appears as veins, primarily aligned NW and N40W. Six main types of veins/veinlets exist: quartz + pyrite (Qz+Py); quartz + chalcopyrite + pyrite (Qz+Ccp+Py); quartz + chalcopyrite (Qz+Ccp); quartz + specular hematite + pyrite (Qz+Py+Hem); quartz + chalcopyrite + specular hematite ± pyrite ± bornite (Qz+Ccp+Hem±Py±Bor), and quartz + secondary copper minerals, with magnetite ± titanomagnetite as minor accessory minerals (Fig. 2). During main mineralization, quartz formed with sulfides like chalcopyrite, pyrite, and bornite, and oxides like magnetite-titanomagnetite and specularite (Goudarzi et al., 2024c). In the supergene stage, chalcocite, covellite, minor native copper, and limited magnetite were observed. The oxidation stage saw minerals like malachite, cuprite, azurite, chrysocolla, hematite, goethite, and limonite forming. Syngenetic iron oxide ores include magnetite, titanomagnetite, specular hematite, and ilmenite exsolution lamellae. Magnetite and titanomagnetite, as primary hypogene oxide ores, are found in hypabyssal monzodioritic bodies and silica veins, sometimes associated with copper sulfides. Magnetite occurs as scattered grains, while titanomagnetite forms micro-grains in mineralized veins. Some titanomagnetite crystals intergrow with ilmenite, and hematite blades form during final cooling stages. The transformation of magnetite to hematite due to Fe2+ leaching in acidic environments results in martitic textures. The association of iron and titanium oxides suggests non-equilibrium conditions. Replacement of magnetite and titanomagnetite by hematite indicates alteration under higher oxygen fugacity, likely due to weathering or hydrothermal alteration (Klein, 2005; Makvandi et al., 2016; Riegler et al., 2014). Research methodology After detailed field examinations, 70 polished sections from various ore-bearing sections and veins were prepared for mineralogical studies. The study of oxide minerals in 8 polished sections was conducted using an electronic microscope and SEM-BSE analyses. The samples were analyzed using the CAMECA SX Five Electron Microprobe at the University of Vienna. The analysis was performed on 48 points of primary titanomagnetite-magnetite in intrusive units and 45 points of magnetite associated with mineralized veins. Results and discussion The results show that the FeO and TiO2 contents vary significantly. In intrusive rocks Fe2O3 ranges from 60 to 80 wt.% and TiO2 from 0 to 16.57 wt.%. In mineralized veins Fe2O3 ranges from 80.6 to 91.4 wt.% and TiO2 from 0 to 0.12 wt.%. Al2O3 and TiO2 contents decrease towards siliceous veins, indicating minimal spinel formation, characteristic of hydrothermal magnetites. Fe2O3 in intrusive masses correlates with Cr2O3 and V2O3, whereas in mineralized veins it correlates with MnO and Cr2O3. TiO2 in intrusive masses correlates with Al2O3, V2O3, and MnO, but not in mineralized veins. SiO2 content is generally less than 1 wt.%. Variation diagrams show that in intrusive samples, Al, Cr, and V oxides increase with TiO2, while Fe and Mg decrease. In mineralized veins, Al and Fe oxides decrease with TiO2, while Cr increases slightly, and V, Mn, and Mg initially increase then decrease. Chemical Composition Titanomagnetite (TixFe3-xO4) is a significant Fe-Ti phase in orthomagmatic rocks and oxide deposits (Spencer and Lindsley, 1981). It can undergo reduction or oxidation (O’Reilly, 1984), forming ilmenite lamellae or intergrowths (Saito et al., 2004). Ideal titanomagnetite forms through deuteric oxidation along the magnetite-ulvospinel line. Mamouniyeh titanomagnetites trend towards wüstite (FeO) (Fig. 9). Martitic hematites indicate final oxidation stages with decreasing temperature and increasing oxygen fugacity (Mondal and Baidya, 2015). With rising temperatures, titanomagnetite separates into ulvospinel and magnetite, forming a Widmanstätten texture (Mondal and Baidya, 2015). Ilmenite forms upon cooling and ulvospinel instability, reacting with oxygen and TiO2. Thick ilmenite blades are formed under advanced oxidation conditions and thin ilmenite blades are formed under early oxidation conditions. Martitization intensity varies, with high oxygen fugacity leading to heavily martitized crystals. Hematite lamellae in ilmenites may result from final oxidation and cooling. Petrographic analysis shows disrupted cubic structures in titanomagnetite, with thin lamellae forming due to oxidation and titanium enrichment, and thicker lamellae forming under advanced oxidation (Pasteris, 1985). Origin Comparing magnetite-titanomagnetites from intrusive rocks and mineralized zones reveals element redistribution during iron oxide transformation. Intrusive bodies are enriched in Ti, Al, and V, while mineralized veins are depleted. Higher V and Cr in magnetite from intrusive bodies align with the mafic nature of host rocks (Curtis, 1964). Reduced Cr and V in mineralized veins indicate high oxygen fugacity during formation. In Mamuniyeh, vanadium oxide content in titanomagnetites of intrusive rocks ranges from 0.016 to 1.28 wt.% (average 0.88 wt.%) and in magnetites of mineralized veins from 0.012 to 0.39 wt.% (average 0.12 wt.%). Vanadium content in magnetite reflects oxygen fugacity conditions of the environment, with higher oxygen fugacity leading to less vanadium in magnetite (Canil and Lacourse, 2020). V3+ incorporates into magnetite under low oxygen fugacity, while V5+ is incompatible with iron oxide structures at higher oxygen fugacity. Titanium content in magnetite is temperature dependent, with higher crystallization temperatures resulting in higher titanium contents (Tian et al., 2021). Magnetite appears in primary, secondary replacement, and solid solution forms. Primary magnetite shows no elemental substitution in fractures. Hematite replaces magnetite in fractures, starting from cracks and spreading across the crystal. Magnetite forms solid solutions with ilmenite, indicating limited Ti solubility at low temperatures. In tholeiitic magma, high-temperature liquidus minerals form first, while in calc-alkaline magma, elevated oxygen fugacity leads to earlier crystallization of iron oxide minerals (Mason and Moore, 1966). As magma approaches the surface, increased oxygen fugacity results in fine-grained magnetite and titanomagnetite crystals, with titanomagnetite forming first, followed by magnetite and ilmenite (Wechsler et al., 1984). In Mamuniyeh samples, ilmenite as a solid solution with magnetite indicates similar formation conditions. The V/Ti ratio in magmatic magnetite is generally 1 (Dupuis and Beaudoin, 2011). Vanadium is mobile in low-temperature hydrothermal fluids, while Ti is immobile (Oliver et al., 2004). A V/Ti vs. Fe diagram is used to study re-equilibratio

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