Mineralogy, alteration, and sulfur isotope geochemistry of the Zehabad intermediate-sulfidation epithermal deposit, NW Iran

The Zehabad Pb-Zn-Au-Ag (Cu) deposit lies in the Alborz magmatic arc of northwestern Iran. Ore-bearing breccia veins hosted by Eocene tuffs emplaced along the 80-130 degrees trending fault and fracture zone. Mineralization occurs in the contact of the late Eocene igneous bodies and the Eocene volcanic and volcanosedimentary Karaj Formation. Mineralization formed in five stages: 1) disseminated framboidal pyrite and minor chalcopyrite and sphalerite; 2) quartz veins containing chalcopyrite, bornite, pyrite, and sphalerite; 3) deposition of specularite and gold grains hosted in quartz veins that crosscut chalcopyrite; 4) the main stage of mineralization that contains galena, sphalerite, tennantite-tetrahedrite, pyrite, sulfosalts, and gold; 5) barren quartz-calcite veins with sulfide mineral fragments of earlier stages. The hydrothermal alteration from closest to the veins outwards includes: a) silicification; b) phyllic with quartz, pyrite, sericite, and calcite; c) argillic with illite, kaolinite, and montmorillonite; d) propylitic containing epidote, calcite, chlorite, and sericite and; e) carbonatization that crosscuts all previous alteration types. Quartz and calcite are the most important gangue minerals at the deposit and show a close relationship with mineralization. Sulfur isotope compositions (0.8% to -10.1%) suggest that the ore-forming fluids derived from magmatic sources with a temperature range of 276-288 degrees C. According to the field (macroscopic), microscopic, alteration, and sulfur isotope studies, the Zehabad base and precious metal mineralization is considered an intermediate-sulfidation epithermal deposit.