COPPER, OXYGEN AND SULFUR STABLE ISOTOPE EVIDENCE FROM BASHKAND IRON DEPOSIT, NORTHWEST IRAN

Bashkand iron skarn deposit is a magnetite-rich skarn occurrence, hosted by Late Proterozoic Kahar Formation in contact-metamorphic aureole of 53.39 ± 0.29 Ma Khorram Darreh tectonized subvolcanic microgranite, southeast of Zanjan in northwestern part of Central Iran structural zone. The mineralizing fluids were probably channelled along foliation planes of meta-dolostone, phyllite and metatuff horizons and possibly along auxiliary faults that are mostly parallel to lothologic contacts.

There are three stages of mineralization: 1) Contact metamorphic occurrence (submicroscopic) including vesuvianite, magnesioferrite, garnet and pyroxene without opaque minerals; 2) Anhydrate bands of silicate skarn zone with a very low width (submicroscopic up to two centimeters) including andradite, diopside and fine disseminated magnetite, metatuff and metasiltstone; 3) Retrograde silicate skarn. The mineralization is dominated by epidote, serpentine (clinochrysotile), talc, garnet and diopside. Mineralization at stages 2 and 3 shows banded structure. The quartz-carbonate veins cut all these stages, carrying some magnetite and pyrite. Major oxides distribution patterns in igneous rocks in the surroundings of the deposit suggest that they were formed in a volcanic arc environment.

Both copper stable isotope data (from chalcopyrite) and sulfur stable isotope data (from pyrite and chalcopyrite) in the exoskarn zone are consistent with a magmatic source for Cu and S. However, oxygen stable isotope data from epidote, magnetite and quartz-carbonate veins in the exoskarn zone imply mixing of magmatic and meteoric sources for O.