Two different categories of experiments were undertaken to determine the significance of humic substances in the chemical transport of gold. The experiments of the first category were designed to study Au(Cl)4 reduction by humic acid at pH 3.5, which is consistent with the formation of this ion in sulfide oxidation zones. These experiments were repeated with denaturated humic acid (obtained by heating in an acid medium). The results show that Au(Cl)4 is reduced by humic acid to an unstable colloid, but not by denaturated humic acid. The purpose of the second category of experiments was to study gold solubility in various organic materials: humic acid, fulvic acid, and potassium humate. Solutions of these substances and demineralized water (the latter served as the reference point) were placed in contact with metallic gold for 1 month at pH 7, the common pH in surface environments. The data indicate an increase in gold solubility according to the following order: humic acid < fulvic acid < potassium humate. A two-step gold chemical-transport mechanism for surface environments is proposed on the basis of our results. In the first step, unstable colloidal gold is formed from inorganic complexes produced in the sulfide oxidation zones (zones with acidic pH). After full oxidization, the pH moves to the normal value, causing the dissociation of functional groups in humic substances. In the second step, these humic substances, through the dissociated functional groups, could form organo-metallic complexes with gold, ensuring gold dispersion in surface environments. [Journal Translation]

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