Abstract

The Aliabad-Khanchy epithermal base metal deposit is located in the Tarom-Hashtjin metallogenic belt (THMB) of northwest Iran. The mineralization occurs as Cu-bearing brecciated quartz veins hosted by Eocene volcanic and volcaniclastic rocks of the Karaj Formation. Ore formation can be divided intofive stages, with most ore minerals, such as pyrite and chalcopyrite being formed in the early stages. The main wall-rock alteration is silicification, and chlorite, argillic and propylitic alteration. Microthermometric measurements offluid inclusion assemblages show that the ore-formingfluids have eutectic temperatures between−30 and−52 °C, trapping temperatures of 150–290 °C, and salinities of 6.6–12.4 wt% NaCl equiv. These data demonstrate that the oreforming fluids were medium- to high-temperature, medium- to low-salinity, and low-density H2O–NaCl–CaCl2 fluids. Calculated δ18O values indicate that ore-forming hydrothermalfluids had δ18Owater ranging from +3.6‰ to +0.8‰, confirming that the ore–fluid system evolved from dominantly magmatic to dominantly meteoric. The calculated 34SH2S values range from−8.1‰to−5.0‰, consistent with derivation of the sulfur from either magma or possibly from local volcanic wall-rock. Combined, thefluid inclusion and stable isotope data indicate that the Aliabad-Khanchy deposit formed from magmatic-hydrothermal fluids. After rising to a depth of between 790 and 500 m, thefluid boiled and subsequent hydraulic fracturing may have led to inflow and/or mixing of early magmaticfluids with circulating groundwater causing deposition of base metals due to dilution and/or cooling. The Aliabad-Khanchy deposit is interpreted as an intermediate-sulfidation style of epithermal mineralization. Our data suggest that the mineralization at Aliabad-Khanchy and other epithermal deposits of the THMB formed by hydrothermal activity related to shallow late Eocene magmatism. The altered Eocene volcanic and volcaniclastic rocks, especially at the intersection of subvolcanic stocks with faults were the most favorable sites for epithermal ore bodies in the THMB.