Constraints on the ore fluids in the Chah Zard breccia-hosted epithermal Au–Ag deposit, Iran: Fluid inclusions and stable isotope studies

• Constrain the source of fluids and sulfur in an Au-Ag epithermal deposit.
• Strong magmatic signature of ore fluids in a low- to intermediate sulfidation deposit.
• Involving two fluids with different salinities in ore-grade zones.
• Boiling and mixing as the major Au-Ag deposition processes in an epithermal system.

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-sulfidation 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 fluid 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 between − 37 and − 53 °C indicate that the aqueous fluids contained NaCl, CaCl2 ± MgCl2 ± FeCl2. Coexisting liquid-rich and vapor-rich fluid 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 δ18O values of water in equilibrium with quartz (+ 3.4 to + 13.1‰) suggest that the fluid may have had a magmatic source, but was 18O-depleted by mixing with meteoric water. The average calculated δ34SH2S values are − 0.2‰ for pyrite, + 0.2‰ for chalcopyrite, − 1.0‰ for sphalerite and − 0.2‰ for galena. The δ34SH2S values are consistent with a magmatic source for sulfur. Gold deposition at Chah Zard is inferred to have been largely caused by boiling, although fluid mixing and/or wall rock reactions may also have occurred. After rising to a depth of between 970 and 440 m, the fluid boiled, causing deposition of fine-grained quartz, and sealing of the hydrothermal conduit. Episodic boiling in response to alternating silica sealing and hydraulic brecciation was responsible for ore deposition. Gold and silver may have precipitated due to the destabilization of HS− complexes, caused by the boiling-off of H2S to vapor, whereas the dilution and/or cooling of hydrothermal fluids led to the precipitation of base metals.