Dislocation-mediated Interfacial Re-Equilibration of Pyrite: an Alternative Model to Interface-Coupled Dissolution-Reprecipitation and Gold Remobilisation
GEOCHIMICA ET COSMOCHIMICA ACTA(2024)
Curtin Univ
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 {1 0 0} oscillatory bands are truncated by Au -As -poor pyrite along {1 0 0} 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 Asrich 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 reequilibration (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.
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Key words
Dislocation,Gold,Remobilisation,Nanoscale,Re-equilibration
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