EBSD mapping of Cu-Fe-sulfides reveal microstructures enriched in critical/precious metals and resolve deformation histories

Abstract Chalcopyrite (CuFeS2) and bornite (Cu5FeS4) from the Olympic Dam Cu-U-Au-Ag deposit (South Australia) are characterized using electron backscatter diffraction (EBSD) to identify microstructures and their correlations with trace element concentrations measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Natural chalcopyrite is shown to be a rich source of micro-structural and -textural information, preserving <110> and <001> crystallographic preferred orientations (CPO), {110} and {112} twin systems, grain boundary migration, foam textures and subgrain boundaries. Selected examples of chalcopyrite illustrate different aspects of its behavior and relationship with bornite across the mineralogically zoned deposit. The oriented stress imposed by brecciation and/or fluid fluxes at Olympic Dam, alongside temperature, pressure and strain rate, are shown to induce a variety of microstructures preserved in chalcopyrite. Microstructures can therefore be used to elucidate sequential stages of low to medium temperature (<300 °C) ore evolution. Although pyrite microstructures are already routinely used to understand higher temperature ore evolution, complementary microstructural study of chalcopyrite coexisting with pyrite has the potential to reveal deformational events across a more complete range of temperatures. In ore systems that lack obvious evidence for post-mineralization deformation, chalcopyrite is particularly well suited to unravel episodes of low-to medium-temperature overprinting. EBSD mapping reveals what appears as single grains of chalcopyrite in reflected light is in fact aggregates composed of ~100 individual grains. In contrast, analyzed bornite displays overwhelming crystallographic homogeneity. Rare instances of misorientation in bornite are all associated with replacement and, if correlated with EBSD analysis of coexisting chalcopyrite and its inclusions (e.g., cobaltite), can be used to discern the origin and evolution of different bornite associations. LA-ICP-MS trace element mapping of chalcopyrite aggregates indicate that grain boundaries host concentrations of Pb, Bi, Ag and Sb, with twin boundaries displaying a weaker concentration of the same elements. Bornite grain boundaries are also enriched in Pb. These observations confirm the critical role played by microstructures in Cu-(Fe)-sulfides as traps for Pb, a non-target contaminant in copper concentrates, as well as new evidence for the physical state of Ag and potential value-add critical metals like Bi and Sb. The preferential occurrence of Pb, Bi, Ag and Sb along permeable grain boundaries may incentivize efforts to remove contaminants and/or recover by-products via leaching.