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Hossein Kouhestani

 Majid Ghaderi, Hossein Kouhestani, Zhaoshan Chang, Khin Zaw 
Fluid inclusion and stable isotope constraints on the ore fluids in the Chah Zard breccia-hosted epithermal Au-Ag deposit, west central Iran
Abstract


The Chah Zard gold–silver deposit, Urumieh-Dokhtar magmatic arc (UDMA), west-central Iran, is a breccia-hosted low- to intermediate-sulfidation epithermal deposit with a total of ~2.5 million tonnes of ore at 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 took 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 probably contained NaCl, CaCl2, MgCl2, FeCl2, Na2CO3 and/or K2CO3. 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 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 revealed 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 940 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 destruction of HS- complexes, cause by the boiling-off of H2S to vapor, whereas the dilution and/or cooling of hydrothermal fluids led to the precipitation of base metals that are typically carried as Cl complexes.

 

 

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