EXPERIMENTAL STUDY ON GOLD DEPOSITION ON HYDROUS METAL OXIDES BY ADSORPTION/REDUCTION: IMPLICATIONS FOR GOLD MINERALIZATION IN SUBSURFACE ENVIRONMENT

Adsorption is recently recognized as a very efficient mechanism for gold precipitation (Widler and Seward, 2002). Gold mineralization often occurs intimately associated with Al-, Fe(III)- and Si (hydr)oxide minerals in subsurface environments. The role of metal (hydr)oxides in the process of gold concentration and reduction, however, has not been completely elucidated. Here I report the experimental results on the behaviors of Au(III) complexes for each metal (hydr)oxide. A series of experiments was carried out as a function of pH and chloride concentration at 30 dgree C. The results of experiments indicate that Au(III) complexes are effectively adsorbed on Al- and Fe(III) hydroxides only when the solutions are near neutral pH and contain low Cl concentrations. Under these conditions, the dominant Au(III) complexes are AuCl₂(OH)₂^- and AuCl(OH)₃^-, and the surface charge of both hydroxides is positive. The adsorption of Au(III) complexes depends on the type of Au(III) species and on the surface charge of metal hydroxides. Importantly, the adsorbed Au(III) complexes are spontaneously reduced to elemental gold without a specific reducing agent. In lateritic environments, gold has often been observed with goethite and bauxite or as gold nugget in iron matrices. Secondary gold also occurs with goethite and hematite in near-surface environments of high-sulfidation epithermal Cu-Au deposits. In addition, gold in low-sulfidation epithermal veins occurs intimately associated with adularia and/or smectite in chalcedonic or fine-grained quartz crystals. These gold deposits were generally formed by ore-forming fluids with neutral pH and low Cl concentrations. The results of this experimental study, therefore, strongly suggest that the surface of metal (hydr)oxides with positive charge plays an important role in gold precipitation in subsurface environments.