Pyrite deformation textures in the massive sulfide ore deposits of the Norwegian Caledonides

The Norwegian Caledonides represent a belt of late Precambrian to early Palaeozoic rocks, emplaced as a series of nappes onto the Fennoscandian basement. Massive sulfide orebodies occur along virtually the whole length of the orogenic belt making it possible to study deposits of a similar origin, chemistry and mineralogy but variably deformed at different metamorphic conditions. Samples from seven pyrite-rich ore deposits deformed at temperatures from ∼ 320 °C to ∼ 610 °C have been investigated using reflected light petrographic observation, orientation contrast (OC) imaging and electron backscatter diffraction (EBSD). While a range of brittle, annealment and growth textures are preserved in the deposits, all preserve evidence for lattice misorientation and low-angle (∼ 2°) sub-grain boundary development indicative of dislocation creep. These results are at odds with those implied from the published pyrite deformation mechanism map, which suggest that pyrite does not deform by plastic mechanisms at natural geological strain-rates and that the brittle–ductile transition is at ∼ 425 °C. Our results imply that pyrite is more ductile than generally inferred and the current deformation mechanism map needs to be revised with plastic deformation mechanisms operating at a much wider range of temperature and strain-rate conditions.