Abstract
Macraes Flat occupies a shallow upland tectonic basin that contains only Late Pleistocene sediments. The gold-bearing Hyde-Macraes Shear Zone is hosted by schist basement that underlies the basin. Supergene alteration of the mineralised shear zone has been occurring periodically since the Eocene, with both oxidised and unoxidised alteration of gold-bearing sulphides and associated silicates. This alteration led to liberation of gold from encapsulation in sulphides and enhancement of gold particle size into millimetre-scale irregular particles by supergene processes. Erosion of the supergene zone during faulting and tilting of basin margins in the Pleistocene has transferred gold particles into proximal placers on the basin floor. Nanoparticulate and microparticulate authigenic gold has formed on the surface of the supergene gold particles, intergrown with secondary minerals that include arsenolite, Fe sulphates and Fe oxyhydroxide. Chemical gold mobilisation and reprecipitation was facilitated by fluctuating redox conditions and pH variations near to the sulphide-sulphate redox boundary over the last 40 Ma.