Ore genesis of the Kuhsiah-e-Urmak base-metal epithermal deposit, Iran

This study investigates the mineralization characteristics and ore genesis of the Kuhsiah-e-Urmak area in the central Urumieh-Dokhtar Magmatic Belt, Iran, through fluid inclusion and isotope analyses. The intrusive units in the region, which are hosted within a marginally older volcanic series, reflect a transitional geodynamic environment, with the Kuh-Ghadeh diorite (∼18 Ma) showing arc-related features and the Marfioun tonalite (∼18 Ma) displaying a mix of arc and slab break-off characteristics, indicating an evolution from arc-related to post-arc slab failure regimes. Mineralization at the Kuhsiah-e-Urmak deposit occurs in two stages: Stage 1, marked by chalcopyrite-pyrite-bornite-sphalerite-galena-tetrahedrite with quartz-calcite, and Stage 2, characterized by chalcocite-digenite-covellite-goethite-maghemite-malachite with hematite-limonite ± sericite ± illite. Fluid inclusion studies reveal low- to intermediate-temperature (191–230°C) and low- to medium-salinity (4–8.5 wt%) fluids. Stable isotope results indicate that the strongly negative δ 34 S values of sulfides and the significantly higher δ 34 S values of barite reflect reduction of seawater- or evaporite-derived sulfate, rather than sulfur input from magmatic volatiles. The magnitude of sulfur isotope fractionation is inconsistent with thermochemical sulfate reduction and instead supports microbial sulfate reduction and/or multistage sulfur cycling as the dominant sulfur-generating processes . The δ 18 O and δ 13 C values further record extensive fluid–rock interaction with meteoric waters and limestones in the region. Fluid inclusion salinity–temperature trends exhibit a positive correlation, consistent with dilution of hydrothermal fluids by infiltrating meteoric water. Stage 1 mineralization was driven by high-temperature magmatic hydrothermal fluids, while Stage 2 resulted from progressive mixing with cooler meteoric waters at shallow crustal levels. These findings classify the Kuhsiah-e-Urmak deposit as a low-sulfidation epithermal Cu (Ag–Pb–Zn) deposit, and highlight the dominant role of seawater- or evaporite-derived oxidized sulfur and meteoric water mixing in the evolution of its hydrothermal system.