Physicochemical controls on alteration and copper mineralization in the Sungun porphyry copper deposit, Iran

The Sungun porphyry copper deposit is associated with an Andean-type, calc-alkaline diorite/granodiorite to monzonite/quartz-monzonite stock of Miocene age which intruded Eocene volcanosedimentary and Cretaceous carbonate rocks. The intrusive phases are related by fractional crystallization, although surprisingly, the diorite/granodiorite which hosts the mineralization postdates the more evolved quartz monzonite. Copper mineralization was accompanied by both potassic and phyllic alteration. The hydrothermal system involved both magmatic and meteoric waters, and boiled extensively. Molybdenum was concentrated at a very early stage in the evolution of the hydrothermal system and copper later. Early hydrothermal alteration, which was caused by high temperature (340 to >500°C), high salinity ( ∼ 60 wt % NaCl equiv.) orthomagmatic fluid, produced a potassic assemblage characterized by addition of K and Cu and depletions in Na, Ca, Mg and Fe in the central part of the stock. Propylitic alteration, which is attributed to a liquid-rich, lower temperature (240--330°C), Ca-rich, evolved meteoric fluid occurred contemporaneously with potassic alteration, but in the peripheral parts of the stock. Phyllic alteration occurred later, at temperatures in the range from 300 to 360°C, overprinting these earlier alterations, and was accompanied by additions of Si (silicification) at the expense of Na, K and Fe and remobilization of Cu from the potassic zone. It resulted from the inflow of oxidized and acidic meteoric waters with decreasing temperature of the system. During potassic alteration, copper solubility is calculated to have been >100,000 ppm, whereas the copper content of the initial fluid responsible for ore deposition was 1200--3800 ppm. This indicates that the fluid was initially undersaturated with respect to chalcopyrite, which agrees with the observation that veins which formed at T > 400°C contain molybdenite but rarely chalcopyrite. Copper solubility drops rapidly with decreasing temperature, and at 400°C is 1000 ppm. At temperatures <300°C, the solubility drops to <1 ppm. These calculations are consistent with observations that the bulk of the chalcopyrite deposited at temperatures between approximately 300 and 360°C. We propose that the Sungun deposit formed partly in response to the sharp temperature decrease and pH increase that accompanied boiling, and partly as a result of the additional heat loss, increase in pH and decrease in aCl- which occurred as a result of mixing of acidic Cu-bearing magmatic waters with cooler meteoric waters of lower salinity and near-neutral pH.