Gold in pyrite: thermodynamic model of socrystallization in chloride fluids

A thermodynamic description of gold-bearing pyrite as a substitution solid solution is proposed depending on external conditions and gold content in the fluid. The model is based on experimental data of the synthetic pyrite grown in hydrothermal chloride solutions in the presence of metallic gold. The proposed model makes it possible to estimate the upper limit of gold content in pyrite. The thermodynamic model of gold-bearing pyrite was verified by modeling the process of formation of mineral associations of gold ores of the Darasun and Talatui deposits by chloride aqueous fluids in an integrated fluid-magmatic system of the Darasun ore field. Previously it was revealed that the average gold content in pyrite from the Darasun and Talatui deposits, intergrown with native gold, does not exceed 1 ppm, while synthetic pyrite grown in equilibrium with metallic gold under similar conditions contains 10–150 ppm of gold. This contradiction does not allow explaining the formation of gold ores of the Darasun ore field by its direct crystallization from hydrothermal fluid. A possible explanation is the presence of an additional stage of ore transformation, in which recrystallization of sulfides occurred with gold undersaturated solutions. It was shown that a decrease in gold concentration in the mineral-forming fluid below the saturation limit should lead to a synchronous decrease in the gold concentration in the resulting pyrite. The calculated values of gold concentration in model pyrite make it possible to estimate gold concentrations in mineral-forming chloride fluids at different stages of the formation of the Darasun ore field deposits at a known temperature. It has been shown that natural gold-bearing pyrite from various gold deposits was formed mainly from chloride fluids undersaturated to gold.