Transport and coarsening of gold nanoparticles in an orogenic deposit by dissolution-reprecipitation and Ostwald ripening

The role of nanoparticle gold in the formation of gold deposits has long been debated. Evidence in support of this process is generally limited to epithermal-, Carlin- and seafloor massive-sulfide-type deposits where gold nanoparticles are associated with primary fluid-related processes. At the Kenty orogenic deposit in Ontario, Canada, gold has been remobilized from early pyrite through secondary coupled dissolution-reprecipitation processes to form high-grade gold external to pyrite. Here we report gold nanoparticles and related textures that help document this gold coarsening process. A combination of focused ion beam technology and transmission electron microscopy provides a rare glimpse of gold coarsening frozen in time, which includes nanoparticles trapped in iron oxides and rutile and non-oriented attachment of gold nanoparticles to bulk gold suggesting coarsening via Ostwald ripening. The processes documented are applicable to orogenic deposits formed through Earth's history, and may explain the formation of ultra high-grade ore zones.