CONTROLS OF ALTERATION AND MINERALIZATION IN THE SUNGUN PORPHYRY COPPER-DEPOSIT, IRAN - EVIDENCE FROM FLUID INCLUSIONS AND STABLE ISOTOPES

The Sungun porphyry copper deposit is located in northwestern Iran (Az arbaijan province) and is associated with diorite-granodiorite to quar tz 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 petrog raphic studies demonstrate that emplacement of the Sungun stock took p lace in several intrusive pulses, each with associated hydrothermal ac tivity. Molybdenum was concentrated at a very early stage in the evolu tion of die hydrothermal system and copper somewhat later. Four main v ein groups have been identified: (I) quartz + molybdenite + anhydrite +/- K feldspar with minor pyrite, chalcopyrite, and bornite; (II) quar tz + chalcopyrite + pyrite +/- molybdenite; (III) quartz + pyrite + ca lcite +/- chalcopyrite +/- anhydrite (gypsum) +/- molybdenite; and (IV ) quartz +/- calcite +/- gypsum +/- pyrite. Early hydrothermal alterat ion produced a potassic assemblage (orthoclase-biotite) in the central part of the stock, propylitic alteration occurred contemporaneously w ith potassic alteration, but in the peripheral parts of the stock, and phyllic alteration occurred later, overprinting the earlier alteratio n. 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 de grees C), low-salinity, vapor-rich inclusions. These fluids are interp reted to represent an orthomagmatic fluid which boiled episodically; t he brines are interpreted to have caused potassic alteration and depos ition of group I and II quartz veins containing molybdenite and chalco pyrite. Propylitic alteration is attributed to a liquid-rich, lower te mperature (240 degrees-330 degrees C), Ca-rich, evolved meteoric fluid . Influx of meteoric water into the central part of the system and mix ing with magmatic fluid produced deep albitization (transition alterat ion) and shallow phyllic alteration. This influx also caused dissoluti on of early formed copper sulfides and remobilization of Cu into the s ericitic zone where it was redeposited in response to a boiling-induce d decrease in temperature. Supergene alteration was minor and restrict ed to a thin blanket of Cu sulfides below an argillic cap.