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

Denis Fougerouse, Steven M. Reddy, Sumail, Joël Brugger, Nicolas Thébaud, William D.A. Rickard, Lin Yang, Zakaria Quadir, Malcolm P. Roberts, Andrew G. Tomkins, Laure Martin, Laura Petrella, Christopher R. Voisey

Research output: Contribution to journalArticleResearchpeer-review

3 Citations (Scopus)

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 <1 0 0>, 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.

Original languageEnglish
Pages (from-to)136-145
Number of pages10
JournalGeochimica et Cosmochimica Acta
Volume374
DOIs
Publication statusPublished - Jun 2024

Keywords

  • Dislocation
  • Gold
  • Nanoscale
  • Re-equilibration
  • Remobilisation

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