Alteration/Mineralization and Controls of Chalcopyrite Dissolution/Deposition in the Raigan Porphyry System, Bam-Kerman, Iran

Abstract

The Raigan porphyry system that intruded Eocene volcano-sedimentary rocks is hosted in a diorite-granodioritic to quartz-monzonitic stock. Several stages of hydrothermal alteration ("e.g., potassic, phyllic, and propylitic") and associated mineralization occurred within the system. Copper mineralization is associated with potassic and minor sericitic alteration. Based on fluid inclusions, mineral paragenesis, and field relationships, the main Raigan copper mineralization took place at 350-550°C. It is not considered to be an economic Cu deposit. In this research, thermodynamic data were used to delineate the stability fields of alteration and ore assemblages as a function of fS2, fO2, aCl-, and pH (at an average temperature of 460°C and 550 bars) for chalcopyrite dissolution and deposition. During potassic alteration (>550°C), copper solubility is calculated to have been >1,000,000 ppm, whereas the copper content of the initial fluid responsible for ore deposition is estimated from fluid-inclusion data to have been 1200-2000 ppm. Thus the ore-forming fluid was initially undersaturated with respect to chalcopyrite, which agrees with the observation that veins formed at T >500°C contain only rare chalcopyrite. Copper solubility dropped rapidly with decreasing temperature, and at 400°C, was approximately 1000 ppm. In equilibration with the wall rocks, all the copper separated from the fluid before the temperature decreased below 400°C. These calculations are consistent with observations that the traces of chalcopyrite precipitate in the Raigan system are hosted in veins formed at temperatures more than approximately 460°C. Based on the new calculations, it is also proposed that the Raigan hydrothermal system formed partly at high temperature, high pH, and low aCl-. This combination of pH conditions prevented the Raigan system from producing an economic porphyry copper deposit.