The fluid regime of ore formation in the Balei gold-bearing ore-magmatic system (eastern Transbaikalia,Russia)

Based on comprehensive studies of fluid inclusions in quartz formed at different stages of hydrothermal process, we consider the physicochemical conditions of formation of epithermal ores (K₁) in the Balei ore field. The limiting parameters of hydrothermal process have been established: 353–131 °C, 150–30 bars, and salt concentrations of 7.6–0.5 wt.%-NaCl equiv. A specific feature of the ore-forming process at the Balei deposits is a rapid drop in temperature and pressure, which is typical of open hydrothermal systems. The temperature increase at the beginning of each stage evidences pulse-like ore formation. The productive stage coincides with the initiation of a drastic decrease in temperature (<225 °C) and salt concentration in the solution. The deposits resulted from the functioning of the common Balei ore-magmatic system at shallow depths with a high permeability of the host rocks. High-K calc-alkalic magmas might have been sources of gold mineralization. The ore formation zone is localized above intrusive bodies near their roof. It is not ruled out that the Balei gold was partly borrowed from the products of the early cycles (J_2-3) of gold mineralization and from the host rocks.