Fluid inclusion and O-S isotope constraints on the genesis of the Farhadabad Cu prospect, Lorestan province, Iran

The Farhadabad Cu prospect in western Iran is a volcanic-hosted Cu occurrence developed within Eocene andesitic and basaltic-andesitic volcanic rocks. Mineralization occurs as veins, veinlets, and disseminations of chalcopyrite, bornite, chalcocite, covellite, malachite, and azurite, accompanied by magnetite, pyrite, quartz, calcite, clay minerals, hematite, goethite, and Mn oxides. Although volcanic-hosted Cu deposits are commonly associated with complex hydrothermal systems, the sources of ore-forming fluids and metals, as well as the processes controlling ore deposition, remain poorly constrained at the Farhadabad prospect. To constrain the origin and evolution of the mineralizing system, petrographic observations, fluid inclusion microthermometry, and stable isotope (O and S) analyses were conducted. Hydrothermal alteration of the host rocks is characterized by silicification, sericitization, chloritization, carbonatization, and argillization. Fluid inclusion data from sulfide-associated quartz and calcite indicate ore formation at temperatures of 130–280 °C from aqueous fluids with low to moderate salinities of 0.53–16.05 wt% NaCl equivalent. Magnetite δ 18 O values range from +8.6 to +11.3 ‰, corresponding to calculated equilibrium fluid δ 18 O values of +16.9 to +19.6 ‰ at an average homogenization temperature of 160 °C, supporting a significant contribution from evolved basinal/metamorphic fluids. Sulfur isotope compositions of pyrite and chalcopyrite (δ 34 S = −5.0 to +7.0 ‰) yield calculated mineralizing-fluid δ 34 S values of −7.2 to +4.8 ‰. These values are compatible with a possible magmatic sulfur contribution and do not exclude sulfate-bearing basinal fluids; sulfur isotope fractionation during hydrothermal fluid evolution, potentially involving sulfate reduction, may also have contributed to the observed range. Fluid inclusion and oxygen isotope data are consistent with mixing of hot, saline basinal/metamorphic fluids with meteoric water. These fluids, possibly derived in part from Lower Cretaceous strata, are interpreted to have migrated upward along thrust-related faults into the Eocene volcanic succession, where fluid–rock interaction promoted hydrothermal alteration and may have contributed to Cu enrichment through leaching of the volcanic host rocks. Subsequent fluid evolution and mixing likely promoted the destabilization of dissolved metal complexes and precipitation of epigenetic Cu sulfides. The observed geological, mineralogical, and geochemical characteristics are broadly consistent with a Manto-type interpretation; however, in the absence of subsurface and drilling data, the Farhadabad Cu occurrence is considered a Manto-type prospect rather than a definitively established Manto-type deposit.