|1- Nabatian, G., Li, X.-H., Honarmand, M., Melgarejo, J. C., "Geology, mineralogy and evolution of iron skarn deposits in the Zanjan district, NW Iran: Constraints from U-Pb dating, Hf and O isotope analyses of zircons and stable isotope geochemistry", Ore Geology Reviews, 84, 42-66, (2017).|
The Zanjan iron skarn deposits are located in the northwest Central Iranian terrane, along the Soltaniyeh mountains. The deposits are hosted by late Proterozoic to late Cambrian carbonate rocks (Soltaniyeh, and Mila Formations) and by Permian (Ruteh Formations), intruded by the Arjin granite, the Khakriz granodiorite and the Inche-Rahbari quartzmonzonite (53 ± 1 Ma to 55 ± 0.4 Ma, early Eocene). These granitoids show high-K calc-alkaline, metaluminous affinity and have similar geochemical features, including strong enrichment of LILEs and LREEs, and depletion in high field strength elements (HFSEs) and HREE. The Arjin, Khakriz and Inche-Rahbari granitoid samples show zircon εHf(t) values of − 0.7 to + 5.8 (TDM2 = 1052–1640 Ma) and δ18O values of + 5.55‰ to + 7.42‰ suggesting partial melting of older crustal material then mixing with juvenile mantle-derived melts. The thermal anomaly induced by underplating mafic melts resulted in partial melting of continental crust and formation of Arjin, Khakriz and Inche-Rahbari magmas emplaced during several magmatic pulses at 53–55 Ma. The Zanjan iron skarn deposits are magnesian (Arjin and Inche-Rahbari) and calcic (Khakriz) with endoskarn and exoskarn facies. In these skarn facies forsterite, garnet and pyroxene comprise the main prograde skarn minerals, whereas magnetite, serpentine, amphiboles of the tremolite-ferroactinolite series (sometimes asbestiform), epidote, talc and calcite are the principal retrograde skarn minerals. Iron exoskarns formed at the contacts of the intrusions parallel to the fabric of the host rocks and in networks of crosscutting veins and veinlets. The assemblage of ore minerals consists of magnetite, hematite (specularite), pyrite, chalcopyrite and bornite; in addition, minor galena and sphalerite occur in the Khakriz deposit. The sulfur isotopic compositions in the Arjin deposit range from + 15.99 to + 18.35‰ and in the Inche-Rahbari and Khakriz deposits from + 2.96 to 5.30‰. The oxygen and carbon isotope ratios of the fresh or partly altered limestones from Soltaniyeh and Mila Formations reveal δ13C values of − 0.64 to + 0.03‰ and δ18O composition of + 13.12 to + 24.41‰ typical of marine carbonate. The isotopic compositions of the hydrothermal calcite (skarn calcite) range from − 5.32 to − 2.01‰ for carbon isotope and 3.29 to 20.67‰ for oxygen isotope. Complex water-rock interaction between the ore-bearing magmatic fluids and sedimentary wall rocks was responsible for sulfur, oxygen and carbon mixing. Field observations, mineral paragenesis, and stable isotopic signatures in these deposits are consistent with ore mineralization formed by the interaction of magmatic fluids with both cooler meteoric water and carbonate host rocks.