Geochemical composition of magnetite from different iron skarn mineralizations in NE Turkey: implication for source of ore-forming fluids

Iron oxide mineralizations in the eastern part of the Pontides (NE Turkey) are hosted in the skarn environments which are the Pontide paleomagmatic arc. These mineralizations always occur in the contacts between granitoid and limestone. Magnetites are hosted in carbonate rocks and generally formed during garnet-magnetite-epidote-quartz phase. Magnetites have high Co (15.8–43.4 ppm for the Kopuz, 11–12.5 ppm for the Eğrikar, and 3.8–1266.7 ppm for the Karadağ) concentrations suggesting that the sulfide decreased from the early to the late phases in the iron skarn mineralization forming systems. The Co/Ni ratios in the magnetites (0.12 to 35.38 for Karadağ, 2.19 to 13.53 for the Kopuz, and 2.39 to 5.21 for the Eğrikar) show the hydrothermal effect on the magmatic source in iron skarns in the study area. Thus, variable Co/Ni ratios reflect interactions between the magma with the host rock during successive alteration stages. Magnetites in this study have Sc-Nb-Mg-Ti depletions and Ta enrichment. In this study, high Co, Ti, and V contents in magnetites suggest the high temperature (300–500 °C) and low ƒO2. The V contents of the magnetite in the Karadağ increase with the decreasing oxygen fugacity of fluid(s) forming magnetites, whereas the V contents of the magnetites in the Kopuz and Eğrikar decrease with increasing oxygen fugacity of fluid(s) forming magnetites. Element patterns and Ni/Cr ratio (< 1) of the magnetite are geochemically similar to those of magnetite in Fe-skarn deposits and partly magmatic accessory magnetite of I-type granites. As a result, the Co, Ni, V, and Ti elements of the magnetite have played an important role in the discriminating and interpreting of skarn mineralizations in the eastern Pontides and support a calcic skarn origin with studies of the mineralization geology.