The Sungun porphyry copper deposit is located in northwestern Iran (Azarbaijan province) and is associated with diorite-granodiorite to quartz monzonite of Miocene age which intruded Eocene volcano-sedimentary and Cretaceous carbonate rocks. Copper mineralization was accompanied by both potassic and phyllic alteration. Field observations and petrographic studies demonstrate that emplacement of the Sungun stock took place in several intrusive pulses, each with associated hydrothermal activity. Molybdenum was concentrated at a very early stage in the evolution of the hydrothermal system and copper somewhat later. Four main vein groups have been identified: (I) quartz + molybdenite + anhydrite + or - K feldspar with minor pyrite, chalcopyrite, and bornite; (II) quartz + chalcopyrite + pyrite + or - molybdenite; (III) quartz + pyrite + calcite + or - chalcopyrite + or - anhydrite (gypsum) + or - molybdenite; and (IV) quartz + or - calcite + or - gypsum + or - pyrite. Early hydrothermal alteration produced a potassic assemblage (orthoclase-biotite) in the central part of the stock, propylitic alteration occurred contemporaneously with potassic alteration, but in the peripheral parts of the stock, and phyllic alteration occurred later, overprinting the earlier alteration. The early hydrothermal fluids are represented by high-temperature (340 degrees - 500 degrees C), high-salinity (up to 60 wt % NaCl equiv) liquid-rich fluid inclusions, and high-temperature (320 degrees -550 degrees C), low-salinity, vapor-rich inclusions. These fluids are interpreted to represent an orthomagmatic fluid which boiled episodically; the brines are interpreted to have caused potassic alteration and deposition of group I and II quartz veins containing molybdenite and chalcopyrite. Propylitic alteration is attributed to a liquid-rich, lower temperature (240 degrees -330 degrees C), Ca-rich, evolved meteoric fluid. Influx of meteoric water into the central part of the system and mixing with magmatic fluid produced deep albitization (transition alteration) and shallow phyllic alteration. This influx also caused dissolution of early formed copper sulfides and remobilization of Cu into the sericitic zone where it was redeposited in response to a boiling-induced decrease in temperature. Supergene alteration was minor and restricted to a thin blanket of Cu sulfides below an argillic cap.