Evidence of IOA-IOCG transition ore systems in the Tarom-Hashtjin metallogenic province, NW Iran.

Kiruna-type iron oxide-apatite (IOA) and iron oxide copper-gold (IOCG) deposits are commonly spatially and temporally associated with one another and coeval host magmatism. The Zanjan area in the Tarom-Hashtjin metallogenic province (THMP) in NW Iran hosts several iron oxide deposits that exhibit a close spatial and temporal relationship with Upper Eocene granitoids. Here, using fluid inclusion, mineralogy, and hydrothermal alteration data from the Chore-Nab iron oxide deposit in combination with other deposits, a new genetic model is developed to explain the formation of the iron oxide mineralization. Furthermore, evidence of an IOA-IOCG transition ore system (IO ± A) in this province is presented. The Chore-Nab orebodies have undergone sodic (albite) alteration associated with an early mineralization stage followed by calcic-sodic (actinolite-rich) alteration related to the formation of a second mineralization stage. The process was finally completed by late veinlets of chalcopyrite, which are related to potassic alteration. Fluid inclusion data show that the initial oreforming fluids (liquid-vapor- and liquid-vapor-solid)-type inclusions of disseminated ore) have an average temperature and salinity of 403 °C, 14.2 wt.% NaCl equiv., and 371 °C, 44.9 wt.% NaCl equiv., respectively. The temperature and salinity of fluids decreased to an average of 277 °C and 8.9 wt.% NaCl equiv. during the formation of magnetite and magnetite-actinolite veins (LV-type inclusions). Subsequently, the values dropped to an average of 266 °C and 5.3 wt.% NaCl equiv. from late chalcopyrite veinlets (LV-type inclusions). These temperature and salinity values indicate a magmatic-hydrothermal ore system. Dilution and cooling of the hot magmatic-hydrothermal metalliferous fluid by mixing with meteoric fluids may have been the main mechanisms responsible for the deposition of metals transported as metal chloride complexes. A decrease in temperature and apatite content, as well as an increase in Cu content is observed from southeastern Sorkhe-Dizaj deposit (massive magnetite + apatite) to the northwest Chore-Nab deposit in the Zanjan area. The dominant element association in the Chore-Nab deposit (Fe) differs from IOCG (Fe-Cu-Au) and IOA (Fe-P) deposits. These results reveal that IOA deposits (e.g., Sorkhe-Dizaj) were formed along deep-seated crustal faults and ascending fluids, which transported Fe and minor Cu to intermediate levels of the system. Therefore, magnetite and sulfides precipitated to form the Chore-Nab IO ± A deposit. This model explains the transition of IOA-IOCG deposits within the same area, providing a valuable conceptual framework to define further exploration strategies in the Tarom-Hashtjin metallogenic province and elsewhere. [ABSTRACT FROM AUTHOR]