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.

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