Dislocation-mediated interfacial re-equilibration of pyrite: An alternative model to interface-coupled dissolution-reprecipitation and gold remobilisation

Abstract

Minerals and/or their compositions (substituted minor elements) can become metastable in changing conditions or if formed outside of equilibrium. Unstable minerals undergo chemical and/or structural modifications at rates determined by re-equilibration processes, such as diffusion, coupled dissolution-reprecipitation and recrystallization. However, re-equilibrated domains with sharp contacts that lack porosity or deformation microstructures are difficult to reconcile with previously documented processes. In this study, we investigate the mechanism by which Au-rich pyrite re-equilibrates to Au-poor pyrite. Gold and As-rich {100} oscillatory bands are truncated by Au-As-poor pyrite along {100} re-equilibration interfaces. At the nanoscale, dislocations oriented consistently along <100>, are enriched in Ni, As, Cu, Sb, Pb, and Au. Dislocations are located at the re-equilibration interfaces between the Au-As-rich and Au-As-poor pyrite. Quantitative crystallographic orientation maps do not show the presence of deformation-related boundaries along the re-equilibration interfaces, indicating that the dislocations are not deformation-related but are misfit dislocations to accommodate for lattice stain between As-rich and As-poor pyrite. The co-location of steps along the re-equilibration interfaces and dislocations suggests that pyrite can re-equilibrate by the migration of dislocations. The process is likely driven by lattice strain minimisation induced by As impurities. Element transport is achieved by a two step process with (1) capture of impurities by dislocation-impurity pair diffusion during the migration of dislocations and (2) pipe diffusion along the dislocation network towards the exterior of the crystal. We propose that re-equilibration of Au-rich arsenian pyrite, and the resulting remobilisation of Au, can operate through a dislocation-mediated interfacial re-equilibration (DMIR) process. This new mechanism may be active in a range of mineral reactions, particularly in metamorphic settings where limited fluid availability precludes interface-coupled dissolution-reprecipitation processes.