Fluid inclusion and stable isotope constraints (C, O, H) on the Dağbaşı Fe–Cu–Zn skarn mineralization (Trabzon, NE Turkey)

Abstract

The Dağbaşı Fe–Cu–Zn skarn mineralization developed along the contact between the block and lens shaped limestones of the Lower Cretaceous Berdiga Formation and the Upper Cretaceous Dağbaşı Granitoid. The exoskarn-type mineralization is characterized by prograde stage garnet and pyroxene, while the retrograde stage is characterized by epidote, tremolite, actinolite, and chlorite. Quartz and calcites were observed in both stages of the skarn development. The ore minerals mainly consist of magnetite and hematite, with a lesser amount of pyrrhotite, pyrite, chalcopyrite, sphalerite, and minor galena. The homogenization temperatures (Th) and salinity values of the prograde stage halite-bearing fluid inclusions are in the range of 412–514 °C and 48.8–61.8 wt% NaCl equ., respectively. The second stage liquid- and vapor-rich fluid inclusion assemblage reveals that boiling at temperatures of 353–458 °C took place after the formation of halite-bearing fluid inclusions. Final stage liquid-rich fluid inclusions were characterized by low Th (160 and 327 °C) and salinity values (0.5 and 6.2 wt% NaCl equ.). The decreasing salinity trend of the fluid inclusions versus Th indicated that meteoric water was involved in the hydrothermal solutions. Eutectic temperatures (Te) of the prograde stage fluid inclusions were found to be CaCl2 dominated, while retrograde stage inclusions contained different salt combinations rather than a specific salt type. The minimum trapping pressures of the early stage brine fluid inclusions were calculated to be between 710 and 884 bar, while later stage inclusions had much lower trapping pressures between ~195 and 445 bar. The δ18O isotopes of prograde stage quartz, garnet, and pyroxenes are close to the composition of the hydrothermal solutions of magmatic sources. Moreover, retrograde stage quartz, epidote, tremolite-actinolite, and calcite minerals and their equilibrated solutions were found to be highly depleted by δ18O isotopes. Therefore, the fluid inclusion and stable isotope constraints suggest that the hydrothermal solutions of magmatic origin were responsible for the prograde skarn stage, while a mixture of magmatic and meteoric solutions were responsible for the ore formation in a shallow skarn environment.