Mineralogy, geochemistry and oxygen isotope studies of the Gheshlagh Cu-deposit, Tarom-Hashtjin metallogenic 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 setting. The geochemistry and petrogenesis analysis confirmed the subduction setting and mineralogically- dominant plagioclase-pyroxene compositions of the host rocks. A good correlation between Al, Fe, Ca, Mg attests to the minerallogic composition, also a strong correlation between chalcophile elements like Pb, Zn, S, Cu and Ag shows sulfide mineralization in the rocks. Based on cross-cutting, the relationship of vein, textural relationship, mineral assemblage and fluid inclusion microthermometry show that alteration and mineralization in the Gheshlagh ore deposit occurred in the fourth main stages. The first stage (stage I) is accompanied by the propyllitic alteration and formation of pyrite (discontinuous) under reducing conditions in the seafloor and very shallow burial environment. The alteration is attributed to magmatic and seawater mixing, which with assuming of the δ18O ranges from 6-10 ‰ for the magmatic fluids and -0.8 to -0.9 ‰ for the Eocene seawater, supported by the values of δ18O quartz (2.1-6.5 ‰) corrected by homogenization temperature of 389 ºC of the related rocks. As the depth of burial increases (stage II), sediments have hardened and also tectonic and lithostatic stress, dykes and floods cause intraformational and magmatic fluids 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 the favorable sites. In deeper burial (stage III), the copper mineral formed in the previous stage would be replaced by bornite and digenite. At the supergene condition (stage IV), due to oxidizing conditions, weathering, and leaching, copper sulfides mostly appear as malachite in the joints and voids. According to the results, the mineralization model of the copper in the Qeshlaq deposit can be considered as manto type.