Thermodynamic properties of ruthenium (IV) chloride complex and the transport of ruthenium in magmatic-hydrothermal fluids

Abstract

Thermodynamic properties of platinum-group element (PGE) coordination complexes are significant for our understanding of PGE mobilization, transport, and deposition in hydrothermal fluids. This study conducted hydrolysis experiments of ruthenium (IV) chloride complex (K2RuCl6) at temperatures of 200–600 °C and water pressure of 100 MPa, combined with related geochemical modeling, to quantitatively investigate the thermodynamic behavior of the Ru-Cl complex in magmatic-hydrothermal fluids. The results show that the hydrolysis reactions of [RuCl6]2– get equilibrium within 24 hours at 200 °C and have positive temperature dependence. The cumulative hydrolysis constants (K) calculated for the reactions fit well on a linear correlation with temperatures (T, Kelvin):lnK = (42.95 ± 4.13) – (56205 ± 2594)/T.Based on the cumulative hydrolysis constants obtained, we established a preliminary model to decipher the quantitative relationship among the temperature, pH, total chlorine concentration, and the mobility of the Ru-Cl complex in natural Cl-bearing hydrothermal fluids. The calculation results demonstrate that Ru could be mobile in the Cl-bearing (0.05–0.5 wt%), low to moderate temperature (≤550 °C), and acidic (pH ≤ 4) fluids, and be highly mobile in the Cl-rich (0.5–20 wt%), acidic - weakly alkaline fluids. The fluid’s pH and total chlorine concentration dominate the Ru-Cl complex’s stability and the Ru transport. Therefore, dilution of ambient seawater on the hydrothermal fluids and plumes related to the magmatic-hydrothermal vent systems is the geochemical barrier, which is advantageous to PGE precipitation and mineralization. Furthermore, this study highlights the importance of metal source for the PGE mobilization and deposition and suggests that the source’s deficiency substantially limits the PGE mobilization, transport, and deposition, resulting in the scarcity of hydrothermal PGE deposits in the crust.