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

Abstract

The deposits are mainly hosted by Eocene tuffs and volcanic rocks and in some places by late Eocene plutonic rocks. The Zehabad deposit is the only active base and precious metal mine in the southern part of the Tarom-Hashtjin subzone. It is located 57 km northwest of Qazvin, in northwestern Iran (Figure 1). The mineralization at the deposit is mainly hosted in breccia veins. There were 30 ore-bearing veins at Zehabad deposit. Mining at Zehabad took place between 1959 and 1977 and then restarted in 2010. The Ghara-Katala and Ghara-Changol veins were operated during 1959-1977. The Ghara-Katala vein is still under exploitation and contains 150,000 tons of Pb and Zn grading 7.7 wt.% Pb+Zn. The galena concentrate contains 8 ppm Au and 600 ppm Ag (Amin Khorramdasht Exploration Company, 2006; unpublished company report). Except for geological mapping studies on the scale of 1:250,000 (Stocklin and Eftekharnezhad, 1969), a few unpublished company exploration reports (Amin Abstract: The Zehabad Pb-Zn-Au-Ag (Cu) deposit lies in the Alborz magmatic arc of northwestern Iran.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° 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 °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.