Evaluation of the contributions of gold derived from hypogene, supergene and surficial processes in the formation of placer gold deposits

Placer gold particles have traditionally been considered as either detrital products of weathering or authigenic minerals growing within placers. Recent advances in understanding of gold chemistry/bio-geochemistry demonstrate that gold growth in specific environments is plausible, but opinions differ on the importance of ‘new’ gold in the overall placer inventory. Here we draw upon visual inspection over 40,000 polished gold particle sections from locations worldwide to evaluate the implications of gold alloy composition and particle heterogeneity in determining the contributions of detrital and authigenic gold to fluvial placers. We conclude i. the detrital model of placer gold formation is widespread and demonstrable, ii. supergene gold may be a locally important constituent of fluvial placers, iii. gold-rich rims on placer gold particles comprise two distinct components: a surface micron-scale addition of pure Au and a tens- of- micron- scale inner rim formed by Ag depletion, iv. the importance to placer inventories of gold particle formation and modification by biogenic processes is considerably overstated