Origin of Abundant Visible Gold in Orogenic Pyrite: New Micro- to Atomic-Scale Insights from the World-Class Jinshan Gold Deposit, China

Orogenic gold deposits commonly host high-grade gold lodes that are rich in visible gold associated with pyrite. The origin of abundant visible gold in pyrite, however, remains unclear. This study uses a novel combination of laser ablation-inductively coupled plasma-mass spectrometry, transmission electron microscopy, and atom probe tomography to reveal the micro- to atomic-scale processes for the formation of visible gold within pyrite from high-grade gold lodes (~3,000 g/t) at the world-class Jinshan orogenic gold deposit, China. These results show that visible gold resulted from multistage processes that include deposition of primary auriferous pyrite, formation of microscopic gold during pyrite replacement, and formation of irregular visible gold. During hydrothermal replacement of pyrite via dissolution and reprecipitation, invisible gold remobilized from primary pyrite formed gold nanoparticles on the replacement interfaces. Aggregation of gold nanoparticles led to the formation of microscopic gold. Interactions between pyrite and another pulse of ore fluid led to the dissolution and remobilization of earlier microscopic gold to produce irregular visible gold surrounding pyrite and within fractures. Mass balance calculations, however, indicate that most of the visible gold was introduced from the ore fluid and not remobilized. This ore fluid preferentially infiltrated into low-stress interfaces between pyrite and chalcopyrite to form Au-rich nanolayers. Dislocation-mediated interfacial diffusion of gold nanolayers contributed to the formation of gold stringers and visible gold. In situ sulfur and iron isotope compositions of pyrite and chalcopyrite indicate that the multiple pulses of ore fluid were mostly derived from the greenschist facies metamorphosed wall rocks. Our results provide significant new insights into the roles of different Au-rich nanophases and their subsequent remobilization and reenrichment in the formation of abundant visible gold in high-grade orogenic gold systems worldwide.