Geology, geochemistry, and genesis of Gheshlagh Cu deposit, Tarom-Hashtjin zone, Zanjan, northwestern Iran

The Gheshlagh Cu deposit is located in the Tarom-Hashtjin metallogenic belt (THMB) of northwest Iran. The mineralization is hosted by Eocene volcanic and volcaniclastic rocks of the Karaj Formation at the subduction-related magmatic arc position. Wall rock alterations include propilitization, silicification, and carbonatization. The major copper sulfides are pyrite, chalcopyrite, chalcocite, idaite, bornite, covellite, and digenite associated mainly with hydroxide carbonate (malachite), oxide (hematite, magnetite, and goethite), and native copper that show open space filling, disseminated, vein-veinlet, and replacement textures. Commonly mineralization and alteration are structurally controlled by favorable structures (faults and fracture). Based on crosscutting, the relationship of vein, textural relationship, mineral assemblage, and fluid inclusion microthermometry, it was found that alteration and mineralization in the Gheshlagh ore deposit occurred in the fourth main stage. The first stage (stage I) is accompanied by the formation of pyrite (discontinuous) under reducing conditions in the seafloor and very shallow burial. During the increase of the burial depth (stage II), sediments have hardened, and also tectonic and lithostatic stress, dykes, and floods cause intraformational and magmatic fluid circulation by compaction and heat recharging. These high-temperature fluids are enriched in copper during circulation among volcanic units. Copper-rich fluids migrate upward and are reduced by entering the pyrite-rich andesitic-basaltic unit, and copper is deposited in sulfide forms such as chalcopyrite and chalcocite in favorable site. In deep burial (stage III), the copper mineral formed in the previous stage would be replaced by. At the supergene condition (stage IV), due to oxidizing conditions, weathering, and leaching, copper sulfides mostly appear as malachite in the joints and voids.