Abstract

The low- to intermediate-sulfidation epithermal Au-Ag deposit of Chah Zard is located in central part of the Urumieh-Dokhtar magmatic arc, west-central Iran. The host rocks are late Miocene calc-alkaline to high-K calc-alkaline intermediate to felsic volcanic rocks with arc magma geochemical affinities. Gold and silver are hosted by hydrothermally cemented breccia bodies, veins, and disseminated sulfides and sulfosalts. The mineralogy of the veins and breccias is dominated by pyrite, marcasite, arsenian pyrite, arsenopyrite, chalcopyrite, galena, sphalerite and silver sulfosalts together with native gold and electrum. The gangue mineralogy includes quartz, adularia and clay minerals with minor carbonate. Fluid inclusion microthermometry with petrographic data on ore minerals constrains the stages of ore deposition in Chah Zard. At room temperature, the entrapped fluids are of four compositional types: (1) three-phase, with vapour bubble, liquid, and cubic NaCl solid; (2) liquid-rich liquid-vapour aqueous homogenized into liquid phase; (3) one-phase vapour inclusions; and (4) one-phase liquid inclusions. All fluid types are present in quartz from the cement of the Au-Ag-bearing polymictic breccia. Type (2) and (3) or (4) fluids also occur within sphalerite from late base metal-rich veins and show consistent phase proportions. At Chah Zard, type (2) fluid is the main ore-forming fluid. Microthermometric measurements show that quartz-hosted type (2) fluid inclusions homogenized within 197 to 345°C. The corresponding bulk salinities vary between 10 and 14 wt percent NaCl equiv. Laser Raman spectroscopy confirms that the vapour phase in these inclusions is dominated by CO₂. Sphalerite-hosted type (2) fluids show a distribution of T_h(total) and bulk salinities between 262 and 311°C and 8 and 12 wt percent NaCl equiv., respectively. Based on the T_h of inclusions trapped from type (2) fluids, a minimum hydrostatic depth of 970 to 440 m is estimated for the depth of mineralisation at Chah Zard. The salinities and homogenization temperatures of the main-stage ores indicate that the hydrothermal fluids responsible for the epithermal gold–silver deposit at Chah Zard may have been derived from an interaction of magmatic fluids with meteoric water under epithermal conditions.