Hydrothermal evolution in the Maher-Abad porphyry Cu–Au deposit, SW Birjand, Eastern Iran: Evidence from fluid inclusions

Abstract

Copper and gold mineralization in the Maher-Abad deposit, eastern Iran, is closely related to the multiple emplacement episodes of Upper Eocene granodiorite porphyries within andesitic volcaniclastic and coeval quartz monzonite stocks. The magmatic hydrothermal fluids thereafter hydrofracturing provided appropriate conduits, formed a stockwork of quartz and quartz–sulfide veinlets within the porphyritic host rocks which were extensively altered into potassic, propylitic, phyllic, and argillic assemblages. Four main vein/veinlet groups have been identified: (A) quartz±K-feldspar±biotite±anhydrite with pyrite, chalcopyrite, sporadic magnetite and pyrrhotite; (B) quartz, pyrite and minor chalcopyrite; (C) chalcopyrite, quartz and/or minor bornite, pyrite, digenite and sporadic pyrrhotite; and (D) quartz, calcite and/or clay minerals±pyrite±hematite±galena. The Cu–Au mineralization is mainly associated with the early potassic (biotite) alteration zone in the deep central part of the Madanha stock.Based on the phase contents at room temperature, three types of fluid inclusions are recognized at Maher-Abad: (1) liquid-rich two-phase (LV), (2) vapor-rich (VL±H) and (3) halite-bearing multiphase (LVH). Homogenization experiments revealed a temperature range of 150–488°C for the studied inclusions. Ice-melting of aqueous two-phase inclusions and melting of halite in the aqueous multiphase inclusions provide salinity of 9.5 to 47.9wt.% NaCl equivalent. Fluid-inclusion studies reveal that the single-phase low-salinity magmatic fluid, exsolved from a granodioritic magma, was separated into vapor and hypersaline brine (~45wt.% NaCl eq.) phases. The coexisting phases were trapped at temperature 460°C and lithostatic pressure of ~350bars (corresponding to a depth of 1.3km) as VL±H and LVH fluid inclusions, respectively. Such magmatic-derived fluids underwent phase separation episodically under lithostatic pressure conditions and formed the early copper sulfide-bearing quartz veinlets intimately related to the potassic alteration zone. Copper–iron sulfide precipitation and phase separation concomitant of boiling in this zone led to rapid depletion of the fluid in reduced sulfur. As a result, Au(HS)2− complexes are destabilized forcing most of the gold to precipitate above 400°C.A positive trend of the homogenization temperature and salinity during the formation of the phyllic alteration zone is attributed to the cooling of a moderately saline fluid (produced dominantly by magmatic-vapor condensation) and its subsequent boiling and mixing with meteoric waters at a hydrostatic pressure of ~100–125bars corresponding to a depth of ~1–1.2km. The argillic alteration zone formed at temperature of 210–230°C from fluids with salinity of 16–18 NaClwt.% eq. at a hydrostatic pressure of ~80bars and a depth of 800m from paleo-surface. The fluid density is typically lower (<1g/cm3) than that of the potassic zone (>1g/cm3), indicating that the late trapped fluid was more dilute.