Abstract

In the Zajkan Pb–Zn–Cu (Au–Ag) deposit of the Tarom–Hashtjin metallogenic belt (THMB), northwest Iran, mineralization occurs as quartz-base metal sulfide veins hosted by Eocene volcanic and volcaniclastic rocks of the Karaj Formation. Four stages of ore formation are identified at Zajkan: (1) quartz-chalcopyrite-pyrite veins, (2) quartz-galena-sphalerite ± chalcopyrite ± pyrite veins and breccias, (3) quartz-hematite veins and breccias, and (4) barren quartz-carbonate veinlets. The primary metallic minerals are pyrite, chalcopyrite, sphalerite, galena, and trace amounts of hematite; gangue minerals are mainly quartz, sericite, chlorite, and calcite. The main wall-rock alteration is silicification, and intermediate argillic, and propylitic alteration. The coexistence of different types offluid inclusions (LV, VL, and V) in individualfluid inclusion assemblages in quartz and sphalerite suggests boiling and heterogeneous trapping. Excluding the heterogeneously entrappedfluid inclusions, the microthermometric measurements of primary LVfluid inclusion assemblages in quartz and sphalerite indicate that the veins were formed at temperatures between 217 and 273 °C fromfluids with salinities between 1.7 and 6.4 wt% NaCl equiv. The oxygen isotopic compositions (δ 18 Owater-VSMOWof +3.7 to +1.1‰) indicate that the ore-forming fluids were mainly derived from magmatic water with the input of meteoric water in the later ore-stages. Sulfur isotopic values of sulfide minerals vary over a narrow range from−5.0 to−0.9‰VCDT (averaging−2.2‰VCDT), suggesting that sulfur was derived from a homogeneous magmatic source. Thefluid inclusion and stable isotope data indicate that fluid boiling and mixing facilitated hydrothermal alteration and mineralization at Zajkan. Our data suggest that the Zajkan deposit is an intermediate-sulfidation style epithermal mineralization. The estimated ore formation depth of Zajkan indicates minor denudation in the region and may imply a great prospecting potential for other epithermal mineralization in the THMB.