Abstract
Orogenic-type deposits are widely developed in the Sanandaj-Sirjan Zone (SSZ) of Iran, but few studies were done on the gold metallogeny in the southern SSZ. Here, we conducted fluid inclusion (FI) studies on the auriferous quartz veins from the Zartorosht deposit, in order to unravel the ore-fluid nature and key controls on ore element transport and deposition. Vein-type gold mineralization at Zartorosht is hosted along E-W-trending normal faults in greenschist-facies metamorphic rocks. Fluid inclusion analysis of the main-ore stage quartz suggests that the ore-forming fluids belong to the H2O-NaCl-CO2 system, with homogenization temperatures of 230–290 °C and salinities of 3–7 wt% NaCl eqv., resembling that of typical orogenic gold deposits. The ore-forming fluids display a mixed metamorphic-mantle source based on multi-isotope evidence: 1) the ore-fluid δDV-SMOW (−69.8 to −45.1‰), δ18OH2O-SMOW (2.3–6.0‰), and δ13CV-PDB (−21.1 to −6.2‰) values are similar to mantle-derived fluids; 2) the ore-fluids have high 3He/4He (2.51–8.00 Ra) and 40Ar/36Ar (296–389) values, suggesting that mantle-derived gases were trapped inside the FIs; 3) calculation suggests that mantle-derived helium in the FIs accounts for (average) 8.34% of the total helium. Consequently, we propose that the Zartorosht gold mineralization is of orogenic-type. The decreasing pressure trend from early- to late-ore stage and the microfracture-filling gold occurrence suggest that temperature and pressure drop is the key trigger for gold deposition. Integrated with previous studies, we demonstrate that the ore-forming fluids were likely produced during the Cenozoic extension and post-peak metamorphism of the Zagros orogeny. Crustal-scale ductile shearing may have allowed the mantle-derived fluids to ascend into their secondary brittle structures, where the ores were precipitated in fracture-filling auriferous sulfide-quartz veins, forming the Zartorosht Au deposit.
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