Gold in Natural and Synthetic Pyrite: a Case of the Darasun Gold District, Eastern Transbaikal Region, Russia

Gold concentration in natural pyrite from the high-grade sulfide ores of the Darasun gold district and pyrite synthesized from the solution saturated in gold has been studied using laser ablation inductively coupled plasma mass-spectrometry. Parameters and composition of the solution from which pyrite was grown were chosen according to the data obtained in the study of the natural pyrite formation. The experiments were carried out at 350°С/1000 bar and 490°С/1000 bar with synthetic troilite FeS or hydrotroilite FeOHHS as a precursor. Concentration of NaCl was 0, 15, and 35 wt %. During some experiments, As was introduced into the system. It was established that temperature, fluid salinity, and As concentration are the main factors affecting gold solubility in pyrite. The higher temperature results in the lower Au concentration, whereas chloride concentration increasing leads to an increase of the Au content. The low As concentration in pyrite ( C (As) < ~200 ppm) does not influence the Au content: on average, C (Au) increases from a few to ~100 ppm as the temperature decreases from 490 to 350°C. The high As concentration ( C (As) > n × 100 ppm) increases gold solubility in pyrite. At C (As) ~ 0.6 wt %, Au concentration reaches 400 ppm. Hydrothermal recrystallization and coarsening of pyrite grains result in lower Au and As concentrations in the newly formed pyrite as compared to pyrite immediately replacing precursor. Gold enriches pyrrhotite as compared to coexisting pyrite; as temperature increases, its content in pyrrhotite reaches 10 ppm at 490°C and its distribution is close to homogeneous. The comparison of the studied natural and synthetic pyrite samples suggests the following main factors determining the Au behavior in gold-bearing pyrite from the Darasun district: (i) crystallization from fluids undersaturated in Au, (ii) recrystallization and coarsening of initial pyrite with gold realizing as metal, and (iii) different temperature and concentration of dissolved chlorides.