The petrogenesis and metallogenesis of the Kalchuyeh epithermal gold deposit, central Iran: Constraints from geochemistry, fluid inclusion, and H-O-S isotopes

[Display omitted] •Gold precipitation occurred due to destabilization of Au-bisulfide complexes during fluid phase separation.•The Kalchuyeh gold deposit can be considered as a low-sulfidation style epithermal system.•The ore-forming fluids, metals, and sulfur have been made possible due to devolatilization of the volcanic and subvolcanic magmatic rocks.•The Eocene volcanic rocks in the central UDMA have a high exploration potential for targeting epithermal-porphyry deposits in Iran. The Kalchuyeh deposit (∼1 Mt @ 30 g/t Ag and 1.1 g/t Au) is located in the central Urumieh–Dokhtar Magmatic Arc (hereafter UDMA), central Iran. Hosted by Middle Eocene pyroxene andesite lavas, the mineralization occurs as veins and veinlets structurally controlled by NE-trending normal faults. The volcanic host rocks have calc-alkaline compositions with geochemical compositions typical of subduction-related arc magmas, including strong enrichments in LILE (K, Cs, Rb, Sr, and Ba), depletions in HFSE, and distinct negative Ti and Nb anomalies. Three ore-forming stages are recorded based on vein textures and alteration and sulfide mineral assemblages, including early quartz-pyrite-chalcopyrite (stage I), middle quartz-sulfide-gold (stage II), and late quartz-calcite-chlorite veins (stage III). Fluid inclusions in quartz veins are characterized by aqueous LV- and minor VL-types and the absence of any solid mineral- or CO2-bearing inclusions. The ranges of homogenization temperature decreased progressively through stages I to III from 235 to 308 °C, to 205–257 °C, and finally to 174–217 °C, respectively, and salinity ranged from 3.3 to 7.6, 5.5–10.7, and 1.3–4.7 wt% NaCl equiv. Fluid boiling during the intermediate temperature stage II caused a loss of H2S from the hydrothermal solution, reducing Au(HS)2− solubility and triggering gold precipitation. Fluid hydrogen and oxygen stable isotope values, which were measured from fluid inclusion waters from quartz and calculated for hydrothermal quartz, respectively, ranged from δDH2O = −84 ‰ to −70 ‰ and δ18OH2O = −4.0 ‰ to +1.5 ‰. This shows that the ore-forming fluids had a magmatic source but with late addition of meteoric waters. The δ34Ssulfide isotope compositions of pyrite and chalcopyrite from stages I and II range between −1.9 ‰ and +0.7 ‰ (mean = −0.7 ‰) and reveal a magmatic sulfur source. Geochemical data and the regional geological history indicate that Middle Eocene magmatism and coeval mineralization at the Kalchuyeh Au