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

The Kalchuyeh deposit (similar to 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 calcalkaline 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-calcitechlorite 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 Ito III from 235 to 308 degrees C, to 205-257 degrees C, and finally to 174-217 degrees 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 delta D-H2O = -84 parts per thousand to -70 parts per thousand and delta O-18(H2O) = -4.0 parts per thousand to +1.5 parts per thousand. This shows that the ore-forming fluids had a magmatic source but with late addition of meteoric waters. The delta S-34(sulfide) isotope compositions of pyrite and chalcopyrite from stages I and II range between -1.9 parts per thousand and +0.7 parts per thousand (mean = -0.7 parts per thousand) 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 +/- Ag deposit occurred during subduction of the NeoTethys oceanic plate beneath the Central Iran continental plate, consistent with most epithermal gold systems in the UDMA. Mineralogical evidence that includes comb textures in quartz, bladed calcite, and low total sulfide and base metal contents of the mineralized veins, indicate a low-sulfidation epithermal deposit classification for the Kalchuyeh deposit. Importantly, the results of this research show that the Eocene volcanic rocks in the central UDMA have a high exploration potential for targeting epithermal-porphyry deposits in this area.