The Au-SiO2 and Sn-SiO2 complexes have been experimentally calibrated at varying temperature, silica concentration and pH: Open image in new window Compared with Au-Cl, Au-HS and Sn-OH complexes, AuH3SiO4 and Sn(H3SiO4)4 complexes can be recognized as the dominant transport forms in Si-bearing solutions under pH and Eh conditions of general interest. The decrease of SiO2 concentration and oxygen fugacity would reverse the direction of dissolution-complexing reactions, resulting in the precipitation of gold and silica, as well as cassiterite and silica. This study illustrates the significance of SiO2-complexation in hydrothermal solutions for gold, tin and other metallizations.