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

摘要

The Sungun porphyry copper deposit is hosted in a Diorite/granodioritic to quartz-monzonitic stock that intruded Eocene volcanosedimentary and Cretaceous carbonate rocks. Copper mineralization is associated mainly with potassic alteration and to a lesser extent with sericitic alteration. Based on previously published fluid inclusion and isotopic data by Hezar-khani and Williams-Jones most of the copper is interpreted to have deposited during the waning stages of orthomagmatic hydrothermal activity at temperatures of 400 to 300 deg C. These data also indicate that the hydrothermal system involved meteoric waters, and boiled extensively. In this work, thermodynamic data are used to delineate the stability fields of alteration and ore assemblages as a function of fS_2, fO_2 and pH. The solubility of chalcopyrite was evaluated in this range of conditions using recently published experimental data. During early potassic alteration (>450 deg C), Copper solubility 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 the fluid was highly undersaturated with respect to chalcopyrite, which agrees with the observation that veins formed at T > 400 deg C contain molybdenite but rarely chalcopyrite. Copper solubility drops rapidly with decreasing temperature, and at 400 deg C is approximately 1000 ppm, within the range estimated from fluid inclusion data, whereas at 350 deg C it is only 25 ppm. These calculations are consistent with observations that the bulk of the chalcopyrite deposited at Sungun is hosted by veins formed at temperatures of 360 +- 60 deg C. Other factors that, in principle, may reduce chalcopyrite solubility are increases in pH, and decreases in fO_2 and aCl~-. Our analysis shows, however, that most of the change in pH occurred at high temperature when chalcopyrite was grossly undersaturated in the fluid, and that the direction of change in fO_2 increased chalcopyrite solubility. We propose that the Sungun deposit formed mainly in response to the sharp temperature decrease that accompanied boiling, and partly as a result of the additional heat loss and decrease in aCl~-, which occurred as a result of mixing of acidic Cu-bearing magmatic waters with cooler meteoric waters of lower salinity.