Gold solubility enhanced by H2O in sulfur-bearing magma: Implications for gold partitioning and mineralization

Most endogenic gold deposits are associated with H₂O-rich magmatism or their associated hydrothermal systems. However, the role that H₂O plays in the transfer and enrichment of gold remains elusive. Here we conducted piston cylinder experiments at 1 GPa and 850 or 950 °C on sulfur-bearing dacitic compositions with variable H₂O contents and under different controlled oxygen fugacity. Our results show that increasing the H₂O contents of the melts from 2 to 15 wt% enhances gold solubilities by up to 37-fold at moderate oxidizing conditions (approximately 2 log units above the fayalite-magnetite-quartz buffer). This dramatic increase results from H₂O-induced elevation of melt FeO content, which increases reduced sulfur concentrations, thereby promoting Au-S complexation in the melt. Under these conditions, H₂O-rich, near fluid-saturated melts exhibit gold partition coefficients between sulfide and silicate melt that are one order of magnitude lower than in H₂O-poor melts. Thus, H₂O content, combined with moderate oxygen fugacity, exerts substantial control on the gold fertility of primary magmas. These findings indicate that both partial melting of H₂O-rich sources and crystallization of H₂O-rich magmas can effectively mobilize and concentrate gold. The process is further enhanced when fluid exsolution occurs before gold sequestration into sulfides. In contrast, copper partitioning shows stronger dependence on oxygen fugacity than H₂O content, explaining the differing behavior of gold and copper during magmatic-hydrothermal evolution. These results provide new insights into the formation of gold-rich magmatic-hydrothermal ore deposits.