Origin and tectonic setting of the carbonate-hosted Kopuz Fe skarn Deposit, Sakarya Zone, NE Türkiye

The Kopuz Fe skarn deposit in the eastern Sakarya Zone is genetically associated with the 44–45 Ma Kopuz granitoid. The Fe ores are hosted within the contact zone between this granitoid and Jurassic Lower Cretaceous limestone. The deposit is characterized by both endo- and exoskarn zones, which exhibit a diverse range of minerals including clinopyroxene, plagioclase, amphibole, biotite, garnet, epidote, phlogopite, magnetite, hematite, pyrite, quartz, calcite, actinolite, limonite, and goethite. Magnetite and hematite, accompanied by small amounts of pyrite, are primarily ore minerals. Pyroxene types observed in the mineralization include diopsidic hedenbergite in the endoskarn zone and diopside in the exoskarn zone. Plagioclase in the endoskarn zone vary from oligoclase to Labrador in composition, which transition towards Na-rich plagioclase and orthoclase in the ore-bearing exoskarn zone. Amphiboles, primarily calcic in nature, are classified as Mg-hornblende and actinolite, with low F and Cl contents, indicating minimal F enrichment during Fe-skarn formation. Garnet is characterized by andradite in the exoskarn zone close to the endoskarn and grossular-andradite in the distal exoskarn zone. The increase of Ad/Gr ratios in zoned garnet indicate an increasing ƒO2 value outward from it. Considering that rare pyrite occur together with magnetite and hematite, we suggest that the deposit formed under conditions of low ƒS2 and high ƒO2 conditions. The homogenization temperature and salinity of the fluid inclusions in quartz from the exoskarn zone vary from 224 °C to 349 °C and 4.18 % (wt% NaCl equiv.), respectively. The oxygen isotope composition (5.10 ‰ to 7.86 ‰) of the fluids in equilibrium with magnetite and quartz minerals in the exoskarn zone supports a magmatic fluid origin. The oxygen isotopic signatures of epidote and amphibole further indicate a minor interaction with an additional source, likely meteoric waters. The sulfur isotope composition of pyrite (2.3 ‰) is consistent with a magmatic sulfur source. In summary, the middle Eocene-aged Kopuz Fe skarn deposit occurred a shallow depth within a magmatic arc, where oxidized conditions predominated.