The in-vitro growth of gold grains

Gold colloids, octahedral platelets, and foils, directly and indirectly formed from the reduction of soluble Au(I) thiosulfate and Au(III) chloride complexes by iron-oxidizing bacteria, cyanobacteria, and sulfate-reducing bacteria, were combined in an experimental system. This system represented simplified biogeochemical conditions occurring within a fluvial environment in which placer Au could occur. In this study, biofilm formation and physical aggregation (i.e., sedimentation processes) were critical for the accumulation of nanometer- to micrometer-sized Au particles into grains 4-5 mm in size. Characterization of grain surface textures by scanning electron microscopy in association with monitoring soluble Au concentrations over time suggested that dissolution and reprecipitation processes were occurring at the Au grain-fluid interface. This laboratory model demonstrates that the biogeochemical cycling of Au can contribute to the formation of anomalous enrichments such as placer Au deposits.