Does the primary deposit affect the biogeochemical transformation of placer gold and associated biofilms?

Placer gold particles derived from epithermal deposits display distinct morphological and compositional features compared with particles from mesothermal systems. Here, it is hypothesized that the chemical composition of primary gold derived from different deposit types is a principle factor affecting the composition of resident biofilm communities as well as the transformation of placer particles. Gold particles were collected from placers originating from the epithermal system at the Eisenberg, Germany's largest primary gold deposit. For comparison, placer gold from mesothermal sources was studied. Morphological differences due to mechanical transport and physical reshaping were not observed. Biogeochemical gold/silver dissolution and gold re-precipitation were evident on epithermal particles and they accentuate the silver-fabrics and gold-rich clusters. In contrast, on mesothermally derived gold particles these processes led to the development of gold-rich rims via the formation of nano-porous secondary gold. Microprobe- and laser ablation mass spectrometric analyses of polished whole particle mounts confirmed differences in gold/silver content/distribution and trace metal content between particles from epi- and mesothermal sources, respectively. On particles from all sites nano- and micro-particulate gold is associated with polymorphic layers. These are composed of microbial cells, extracellular polymeric substances (EPS) and clay-sized minerals. Multivariate statistical analyses shows a significant difference between biofilm communities from epi- vs. mesothermally derived particles, which is linked to the chemical composition of the primary gold. While a number of key-species capable of gold transformation, e.g., Cupriavidus sp., Geobacter sp. and Rhodoferax sp., were detected on particles from both sources, higher numbers of organisms with the potential for gold solubilization, precipitation and detoxification were associated with particles from the epithermal sources. A range of species involved in gold transformation, i.e., Arthrobacter spp., Delftia sp., Shewanella sp., and Stenotrophomonas spp., were detected only on epithermally derived placer gold. This indicates the communities are sensitive to differences in gold/silver and possibly trace metal-cycling, resulting from differences in their content, distribution and mobilization behaviour in epi- vs. mesothermally derived particles. Ultimately, this study shows that the chemical composition of the primary deposit strongly influences the biogeochemical transformation of placer gold and the composition of associated biofilms, whereas physical transformations appear to be largely unaffected.