Secondary gold structures: Relics of past biogeochemical transformations and implications for colloidal gold dispersion in subtropical environments

Biogeochemical processes are known to drive the cycling of gold via dissolution/re-precipitation reactions that result in the transformation of gold grain in near-surface environments. In this study, placer gold grains were collected from West Coast Creek, Queensland, Australia and characterized using high-resolution electron microscopy. The outer surface of grains contains 98.1 mol% Au and 1.9 mol% Ag. Crevices occur on the surface of grains and are filled with organics and clay minerals in which nanometer-size gold colloids and micrometer-sized octahedral gold platelets are embedded. The formation of these secondary gold structures is attributed to gold precipitation by the availability of reducing agents such as microbiota, residual organics and clays. Bacterioform gold contains 74.7 mol% Au and 25.3 mol% Ag and is also embedded in clay minerals within crevices. From sonicated gold grains, grain surfaces beneath the clay minerals are striated and contain 68.5 mol% Au and 31.5 mol% Ag. This data suggests that dissolution processes occur at the gold grain interface. Varying sizes of gold colloids on the grains suggest that five ‘episodes’ of gold dissolution/re-precipitation processes occurred; each episode was estimated to be 7.64 ± 4.1 years. Therefore, these grains represent 17.9–58.5 years of gold cycling and mobilization within this subtropical environment. Furthermore, laboratory experiments involving colloidal gold dispersion demonstrated that iron-oxides and organic material from West Coast Creek sediment adsorbed 94.5% of suspended gold colloids. In conclusion, this study highlights the value of nanophase gold characterization for the interpretation of biogeochemical processes affecting gold grain transformation and mobility in near-surface environments. Importantly, this study is the first to estimate the kinetics of biogeochemical gold cycling with regards to colloidal gold dispersion and re-concentration.