Strain in sulphide filled foliation boudinage structures at the Mount Isa Cu deposit, Australia

Foliation boudinage structures are vein-like boudins forming in homogeneous, anisotropic rocks that can be filled with significant volumes of mineral infilling. At Mount Isa, foliation boudinage structures are associated with and filled by ore-related sulphides, predominantly pyrrhotite. Deformation of syn-tectonic, ore-related sulphides at Mount Isa has generally not been documented, fuelling controversy about ore timing. EBSD techniques, combined with strain analyses show that the ore-related sulphide infill of foliation boudinage structures underwent significant intracrystalline deformation. Pyrrhotite grains show well-developed grain shape and crystallographic preferred orientations, indicating that the foliation boudinage structures have undergone layer normal shortening at a relatively late stage in the deformation history. Deformation of pyrrhotite grains by dislocation creep was dominated by basal slip leading to dynamic recrystallisation of pyrrhotite by subgrain rotation. A change from plane to flattening strain maintained the same shortening direction, and may therefore have occurred continuously, rather than as two separate deformation events. Results of this study are consistent with a current kinematic model for the Mount Isa system and may have implications for future exploration in the area. •Electron backscatter diffraction analysis of pyrrhotite filled foliation boudinage structures at Mount Isa.•The pyrrhotite has shape and crystallographic preferred orientations.•Dislocation creep and dynamic recrystallisation by subgrain rotation were facilitated by basal slip in the pyrrhotite.•Plane strain changed to flattening during deformation of the pyrrhotite infill as the foliation boudinage structures evolved.