Metamorphic textures and geochemistry of the Cyprus-type massive sulfide lenses at Zurabad, Khoy, Iran

Abstract

The Upper Cretaceous volcanogenic-exhalative Fe-Cu massive sulfide lenses at Zurabad, Iran occur within spilitized basalts of the Khoy ophiolite. Both the host rocks and the sulfide lenses are metamorphosed to greenschist facies and have been subjected to weak deformation causing textural changes in the ore. The presence of preserved banded pyrite, pyrrhotite, chalcopyrite and sphalerite in a matrix rich in chlorite indicates that the ore was a primary facies of synvolcanic hydrothermal massive sulfide mineralization, similar to those formed in modern seafloor environments and in Cyprus-type ophiolite deposits. Coarse-grained pyrite porphyroblasts or crystalloblasts, recrystallized pyrrhotite and chalcopyrite, together with regional silicate-sulfide foliation were produced by sub-seafloor metamorphism followed by regional metamorphism and ophiolite obduction. Brittle deformation and shearing produced cataclastic textures in pyrite, which were filled by chalcopyrite and pyrrhotite. Post-obduction and or post-metamorphic events produced late veinlets of pyrite-chalcopyrite and gangue minerals. Supergene processes related to gossan formation converted pyrite, pyrrhotite and chalcopyrite to marcasite and birdseye pyrite. Primary sphalerite contains an average of 14 mole % FeS, which shows equilibration at temperatures below 240 °C, and a pressure less than 1 kilobar. Based on its equilibrated textures with other sulfides and silicates in the foliation, pyrrhotite was a primary mineral in the Zurabad volcanogenic massive sulfide lenses. Therefore, pyrrhotite can be primary, and with pyrite forms a buffer that constrains sulphur fugacity during regional metamorphism. On the evidence of textures, mineralogy and geochemistry, pyrite is stable in the greenschist facies, provided that the activity of sulphur remains high. Of prime importance in exploration of the massive sulfide lenses in the Zurabad area has been the recognition of banded gossans and coarse pyrite porphyroblasts, which may prove to be a successful exploration tool in the search for further massive sulfide mineralization in the district.