Magmatic-Hydrothermal Evolution at the Kuh-e Janja Porphyry Cu-Au Deposit, Southeastern Iran: Relative Timing of Hypogene Cu Mineralization from Vapor-Like, Near-Critical Density, Single-Phase Fluids

The Kuh-e Janja porphyry Cu-Au deposit is located 60 km southeast of Nehbandan in southern Iran, within the northern part of the Sistan Suture Zone. Several distinct stockwork vein types are recognized, differing in morphology, vein mineralogy, and associated alteration type. Early biotite (EB) veins containing biotite and minor amounts of quartz, magnetite, and anhydrite form the earliest veins. They are crosscut by A veins that are typically several millimeters in thickness and associated with potassic alteration envelopes. The abundant A veins at Kuh-e Janja consist of Q1 quartz that shows textural evidence for extensive recrystallization. Primary oscillatory growth zoning is rarely preserved in Q1 grains. Fluid inclusions in the quartz have been affected by post-entrapment modification. The A veins are interpreted to have formed at high (≳500°C) temperatures at lithostatic pressures. AB veins contain euhedral Q2 quartz crystals that have grown perpendicular to the vein walls within vugs. Molybdenite occurs as ribbons in recrystallized Q1 and Q2 quartz or as an infill in open spaces between the euhedral Q2 quartz crystals. The euhedral Q2 quartz crystals show well-developed sector and oscillatory growth zoning and are characterized by hypersaline liquid-rich and coexisting vapor-rich fluid inclusions that were entrapped at temperatures below ~500°C at the transition from lithostatic to hydrostatic pressure conditions. The AB veins formed at conditions of K-feldspar stability. Zones of high Cu grades at Kuh-e Janja are typified by the presence of abundant hairline fractures coated by chalcopyrite, pyrite, and minor molybdenite. These C veins are surrounded by minor chlorite alteration of the host rocks and crosscut all earlier vein types. The contacts between the early Q1 and Q2 quartz grains and chalcopyrite are irregular in shape. Chalcopyrite crosscuts the primary oscillatory zoning of the Q2 quartz grains suggesting that sulfide deposition occurred under conditions of retrograde quartz solubility. Wallrock or hydrothermal biotite in contact with chalcopyrite is frequently chloritized. Vapor-like single-phase fluid inclusions occur along healed microfractures hosted by euhedral Q2 crystals that are in contact with chalcopyrite in C vein. As many of the fluid inclusions host small triangular opaque phases interpreted to be chalcopyrite, the vapor-like, near-critical density, single-phase fluid likely was the ore-forming fluid. Late D veins containing variable proportions of pyrite and quartz are surrounded by texturally destructive sericite alteration envelopes of the host rocks. Q3 quartz contains primary and secondary liquid-rich fluid inclusions entrapped at hydrostatic pressure conditions. Rare polymetallic E veins at Kuh-e Janja contain Q4 quartz, sphalerite, galena, and minor chalcopyrite. The study of the different vein types suggests that hypogene Cu mineralization at Kuh-e Janja occurred after potassic alteration of the host porphyry at temperatures close to the ductile-brittle transition. The mineralization formed from vapor-like, near-critical density, single-phase fluids along hairline fractures during their escape from lithostatic to hydrostatic conditions.