Gold nuggets: supergene or hypogene?

Native gold in placers and intensely weathered rocks commonly occurs as grains (< 500 mu m) and, more rarely, as large (>4 mm or > 1 g) nuggets. There has long been speculation about whether gold nuggets, especially those found in deeply weathered regolith environments, are supergene or hypogene in origin. An examination of the external and internal characteristics of more than 30 nuggets from different locations in Western Australia, Queensland and Victoria, ranging from 1 g to > 8 kg, was carried out. All of these nuggets are composed of gold-silver alloys, typical of hypogene gold. Furthermore, each has a polycrystalline internal structure that is characteristic of high temperature (>250 degrees C) thermal annealing. All of the nuggets also exhibit corrosion features, including pits, voids and silver depletion that are due to weathering. The silver depletion is strongly controlled by the crystal structure and proceeds along crystallographic grain boundaries by a selective solution process equivalent to depletion gilding. Polycrystalline Au-Ag nuggets with the characteristics presented here are hypogene in origin, concentrated at the surface as the residue of prolonged exposure, weathering and erosion of gold deposits, not by supergene growth.