Abstract

The Chah Zard gold –silver deposit, in the central part of the Urumieh-Dokhtar Magmatic Arc (UDMA) of Iran, is a breccia-hosted low- to intermediate-sul fi dation epithermal deposit with a resource of ~ 2.5 Mt averaging 1.7 g/t Au and 12 .7 g/t Ag. Gold and silver mineralization occurs in breccia and veins associated with a 6.2 ± 0.2 Ma volcanic complex. Microthermometric measurements on quartz - and sphalerite-hosted, two-phase liquid-rich fl uid inclusions indicate that the mineralization may have taken place between 260 and 345 °C, from a moderately saline hydrothermal fluid (8.4 –13.7 wt.% NaCl equiv.). First ice-melting temperatures be-tween − 37 and − 53 °C indicate that the aqueous fluids contained NaCl, CaCl 2 ± MgCl 2 ± FeCl 2. Coexisting liquid-rich and vapor-rich fl uid inclusions in quartz and sphalerite provide evidence for boiling in ore-stage breccia and veins. Additionally, the occurrence of adularia and bladed calcite in high-grade ore zones and the presence of hydrothermal breccias and chalcedonic quartz are consistent with boiling. Calculated δ18 O values of water in equilibrium with quartz (+ 3.4 to + 13.1‰ ) suggest that the fl uid may have had a magmatic source, but was 18 O-depleted by mixing wi th meteoric water. The average calculat ed δ34 S H2S values are −0.2 ‰ for pyrite, + 0.2 ‰ for chalcopyrite, − 1.0 ‰ for sphalerite and − 0.2 ‰ for galena. The δ34 S H2S values are consistent with a magmatic source for sulfur. Gold deposition at Chah Zard is inferred to have been largely caused by boiling, although fl uid mixing and/or wall rock reactions may also have occurred. After rising to a depth of between 970 and 440 m, the fl uid boiled, causing deposition of fi ne-grained quartz, and sealing of the hydrothermal conduit. Episodic boiling in response to alternating silica sealing and hydraulic breccia-tion was responsible for ore deposition. Gold and silver may have precipitated due to the destabilization of HS − complexes, caused by the boiling-off of H 2S to vapor, whereas the dilution and/or cooling of hydrother-mal fl uids led to the precipitation of base metals.