Physicochemical modeling of hightemperature stages of gold deposition at the lugokanskoe deposit (eastern Transbaikalia)

The relevance of the study is caused by the need in uptodate data on mineralogical and geochemical features of ores, chemical com position of native gold, sulfide minerals and physicochemical conditions of the Lugokanskoe gold deposit formation. It is possible to get this information due to the ongoing exploration work in Eastern Transbaikalia region. The main aim of the research is to develop thermodynamic model that simulates the formation conditions of three types of orefor ming stages of the Lugokanskoe gold deposit. The PTXparameters of these oreforming stages were determined using fluid inclusion thermobarometry. Based on these data we carried out the calculations. Methods. The calculations were carried out using the «HCh» software within a complex geochemical multicomponent system. Several scenarios of formation of goldmineralization were considered and the possible concentrations of metals and sulfur in the oregenera ting fluid, as well as the species of elements (complexes) during the evolution of the system with a decrease in the temperature and sa linity of the fluids are elucidated. The thermodynamic database UNITHERM was used, supplemented for a number of reference data for minerals. Results. The fluids under consideration are complex reduced multicomponent systems transporting a wide range of siderophile (Fe, Au, Mo), chalcophile (S, As, Cu, Pb, Zn) and lithophile elements (Na, Cl, Al, Si, etc.). Hightemperature fluids at 500 °C, lowsulfide on the assumption of equilibrium with molybdenite, scheelite and quartz, are able to concentrate up to 410^-5 mol/kg H₂O of gold (8 g/ton of fluid). This determines deposition of a significant amount of gold at the AuAsCu stage at 400 °C with a decrease in temperature and H₂S increase. This productive stage is characterized by weakly acidic, reduced and highly sulfide solutions. Model calculations indicate high concentrations of Fe, As, and Cu in them, that results in its turn in formation of midtemperature galena, tennantitetetrahedrite (300 °C) associations. At 200 °C, when the oxidative potential was increased, the fluids were discharged with gold together with bis muthcontaining minerals. The next stage of the model improvement is required after the thermodynamic data adjustment for the Bi, Te, Pb, and Sb minerals.