Deposition of gold on kaolinite surfaces from AuCl 4 − solution

Abstract

1 The precipitation of gold in response to changes in various intensive and extensive parameters of a hydrothermal system may be an effective means of gold ore deposition but sorption processes may also provide an exceedingly efficient mechanism for accumulating gold from solution. It is generally accepted that gold can be complexed by a comprehensive range of ligands and is relatively soluble in the supergene environment, depending on the activity of the ligand, the Eh, and the pH of the solutions. In acidic solutions, chloride is the ligand most capable of complexing Au, mainly as Au anion, which is extremely stable in hydrothermal fluids over a wide range of pH and temperatures and is believed to be one of the principal ions responsible for leaching and transport of gold [2‐6]. Adsorption of gold on mineral surfaces is an important mechanism for the formation of gold ore deposits [7‐11]. Gold-bearing complexes can be adsorbed onto the surface of sulfides and can be reduced by the sulfur atoms at the mineral surface; i.e., metal cations are leached and sulfur is oxidized on forms polysulfides [12]. However, since the 1970s, “invisible gold” deposits, as well as rare-sulfide gold deposits and supergene invisible gold deposits, were found and the intimate association of gold with clay minerals was noticed. It is generally assumed that clay minerals, such as kaolinite and hydromica, have the capacity of adsorbing goldbearing complexes from solutions and, thus, result in the accumulation of gold [8, 13]. However, none of these studies have provided evidence for the reduction of gold on such nonmetallic minerals. We undertook the experiments on the adsorption and reduction of gold from Au - solution by kaolinite, and the results indicate that kaolinite are also the potential reducing agents in that type of gold deposit.