Mineralogy, alteration, and sulfur isotope geochemistry of the Zehabad intermediatesulfidation epithermal deposit, NW Iran

Abstract

The Zehabad Pb-Zn-Au-Ag (Cu) deposit lies in the Alborz magmatic arc of northwestern Iran. Ore-bearing breccia veinshosted by Eocene tuffs emplaced along the 80–130° trending fault and fracture zone. Mineralization occurs in the contact of the lateEocene 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 ofmineralization that contains galena, sphalerite, tennantite-tetrahedrite, pyrite, sulfosalts, and gold; 5) barren quartz-calcite veins withsulfide 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 importantgangue 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 °C. According to the field(macroscopic), microscopic, alteration, and sulfur isotope studies, the Zehabad base and precious metal mineralization is considered anintermediate-sulfidation epithermal deposit.