Factors controlling copper solubility and chalcopyrite deposition in the Sungun porphyry copper deposit, Iran

The Sungun porphyry copper deposit is hosted in a Diorite/granodioritic to quartz-monzonitic stock that intruded Eocene volcanosedimentary and Cretace ous carbonate rocks. Copper mineralization is associated mainly with potass ic alteration and to a lesser extent with sericitic alteration. Based on pr eviously published fluid inclusion and isotopic data by Hezarkhani and Will iams-Jones most of the copper is interpreted to have deposited during the w aning stages of orthomagmatic hydrothermal activity at temperatures of 400 to 300 degrees C. These data also indicate that the hydrothermal system inv olved meteoric waters, and boiled extensively. In this work, thermodynamic data are used to delineate the stability fields of alteration and ore assem blages as a function of fS(2), fO(2) and pH. The solubility of chalcopyrite was evaluated in this range of conditions using recently published experim ental data. During early potassic alteration (>450 degrees C): Copper solub ility is calculated to have been >50 000 ppm, whereas the copper content of the initial fluid responsible for ore deposition is estimated, from fluid inclusion data, to have been 1200-3800 ppm. This indicates that initially t he fluid was highly undersaturated with respect to chalcopyrite, which agre es with the observation that veins formed at T > 400 degrees C contain moly bdenite but rarely chalcopyrite. Copper solubility drops rapidly with decre asing temperature, and at 400 OC is approximately 1000 ppm, within the rang e estimated from fluid inclusion data, whereas at 350 degrees C it is only 25 ppm. These calculations are consistent with observations that the bulk o f the chalcopyrite deposited at Sungun is hosted by veins formed at tempera tures of 360 +/- 60 degrees C. Other factors that, in principle, may reduce chalcopyrite solubility are increases in pH, and decreases in fO(2) and aC l(-). Our analysis shows; however, that most of the change in pH occurred a t high temperature when chalcopyrite was grossly undersaturated in the flui d, and that the direction of change in fO(2) increased chalcopyrite solubil ity. We propose that the Sungun deposit formed mainly in response to the sh arp temperature decrease that accompanied boiling, and partly as a result o f the additional heat loss and decrease in aCl(-), which occurred as a resu lt of mixing of acidic Cu-bearing magmatic waters with cooler meteoric wate rs of lower salinity.