Nanoforms of gold from abandoned placer deposits of Wądroże Wielkie, Lower Silesia, Poland – The evidence of authigenic gold mineralization

Being the most recently exposed, gold grains from historical tailings of underground and surface workings offer the opportunity to study supergene transformations occurring over historic periods. In this study, gold grains were collected from the abandoned, historically mined deposits of Wądroże Wielkie at Lower Silesia. All three distinct gold sub-types of detrital gold display surface morphotypes and internal textures of Au and Ag dissolution indicative of supergene gold modification, as well as authigenic Au nanoparticle formation and aggregation. The indicative morphotypes include nano-particulate semispherical or bubble-like gold and aggregates of bacteriomorphic gold that could be evidence for microbial gold biomineralization. SEM investigation of the microtopography and internal texture of gold grains revealed indications of post-depositional dissolution of Au-Ag alloys and redistribution of these components within the sediment. Some of seed and bubble-like growths are also embedded in clayey (kaolinite) masses within crevices suggesting that dissolution/re-precipitation processes occur at the gold grain interface. Authigenic gold occurs as overgrowths and aggregates of nearly pure gold on a detrital Au-Ag alloy that are readily removed during transport, but are replaced by re-precipitation occurring over historic rather than geological periods.