Geochemical and Isotopic Constraints on the Origin of the Early Palaeozoic Chah‐Nar Pb–Zn Deposit, Sanandaj–Sirjan Zone (Iran)

ABSTRACT

The Chah‐Nar Pb–Zn deposit in the southern Sanandaj–Sirjan Zone, Iran, represents Early Palaeozoic stratiform sulfide mineralization hosted by organic‐rich volcano‐sedimentary rocks deposited in a Proto‐Tethyan back‐arc basin. Despite the occurrence of stratiform Pb–Zn mineralization in the Southern Sirjan Basin, the origin, fluid evolution, and regional metallogenic significance of the Chah‐Nar deposit remain insufficiently constrained. This study integrates field observations, ore petrography, lithogeochemistry, fluid inclusion microthermometry, and SCO isotope data to evaluate the ore‐forming processes and genetic setting of the deposit. Mineralization occurs as stringer, massive, and bedded ore facies, which are best interpreted to record a transition from early seafloor to shallow subseafloor sulfide deposition to later hydrothermal replacement. Fine‐grained bedded sulfides are interpreted to represent an early synsedimentary to early diagenetic stage, whereas coarser massive and replacement sulfides are most consistent with later focused hydrothermal flow, possibly guided by synsedimentary extensional faults. Fluid inclusions in quartz associated with the stringer and massive ore facies yield homogenization temperatures of 165°C–268°C and salinities of 3.0–9.9 wt.% NaCl equivalent, consistent with low‐ to moderate‐salinity basinal fluids. Sulfur isotope compositions of sulfides (δ ³⁴ S _CDT  = −5.8‰ to +16.9‰) indicate seawater‐derived sulfur modified by multiple reduction pathways, with bacterial sulfate reduction dominant in the bedded ores and thermochemical sulfate reduction more important in the stringer and massive ores. Carbon and oxygen isotope data further indicate fluid–rock interaction involving seawater‐derived basinal fluids, organic‐rich sediments, and isotopically lighter hydrothermal components. Collectively, the geological, textural, geochemical, fluid inclusion and isotopic evidence indicates that the Chah‐Nar deposit formed by discharge of metal‐bearing basinal fluids into a reduced, organic‐rich sedimentary environment, with fluid flow focused by synsedimentary faulting. The integrated dataset is most consistent with interpretation of Chah‐Nar as a SEDEX‐type Pb–Zn system developed in an Early Palaeozoic back‐arc rift setting. These results refine the metallogenic model for the southern Sanandaj–Sirjan Zone and highlight the roles of basin architecture, focused fluid flow and redox‐controlled sulfur reduction in Palaeozoic Pb–Zn mineralization in Iran.