How low can you go? - Extending downwards the limits of plastic deformation in pyrite

摘要

Understanding the deformation mechanisms that may operate in pyrite (FeS_2) across a range of P-T conditions is important in deciphering the history of deformed ore deposits. Pyrite has frequently been considered a hard mineral, which deforms by cataclastic flow or diffusive processes, if at all, at temperatures <425℃. However, utilizing SEM-based orientation-contrast (OC) imaging and electron-backscatter diffraction (EBSD) techniques, plastic deformation can now be readily identified within pyrite grains. In this study, a series of pyrite-rich polymetallic ore deposits, deformed at low temperature metamorphic conditions (approx 200—420℃), have been investigated. Results indicate that pyrite grains in all the ore deposits preserve internal lattice 'distortion' or 'bending' and therefore plastic deformation mechanisms have operated. Many pyrite grains in the ore deposits also contain low-angle (approx 2 deg ) sub-grain boundaries or 'dislocation walls', indicating that both dislocation glide and creep have been the dominant deformation mechanisms at peak metamorphic conditions within the pyrite grains. These results suggest that the brittle-ductile transition in pyrite occurs at temperatures potentially as low as approx 200℃, far lower than implied from previous studies or the current pyrite deformation-mechanism map.