Foliation boudinage structures in the Mount Isa Cu system

Abstract

Small-scale foliation boudinage structures occur in rocks that were sampled in drill core from the Mount Isa Cu deposit, northwest Queensland. The necks of foliation boudinage structures plunge gently to the north and south as a result of layer normal shortening and layer parallel extension of the steeply west-dipping Urquhart Shale. Detailed petrographic analysis of the foliation boudinage structures has identified an initial rim of quartz and dolomite, followed by infill and replacement by pyrrhotite and minor chalcopyrite. Foliation boudinage structures formed after dolomitisation and silicification of the shale. They occur most commonly in the unaltered Urquhart Shale where the anisotropy and homogeneity provided by the shale layering is still intact. Infilling of the structures occurred during protracted silica-dolomite alteration, pyrrhotite and chalcopyrite mineralisation. The paragenesis of the foliation boudinage structures is consistent with the established paragenesis of the main Cu mineralisation. Foliation boudinage structures formed over the period from shortening during D4a through to the main Cu mineralisation during D4b west-northwest–east-southeast sinistral-reverse shortening. The timing of foliation boudinage is consistent with a current kinematic model for the Mount Isa system. KEY POINTS First record of foliation boudinage structures at Mount Isa. Foliation boudinage structures with sulfide-dominated infills. Foliation boudinage structures formed as a result of progressive deformation from a D4a dextral-reverse through to D4b sinistral-reverse slip. Foliation boudinage structures are associated with the timing and kinematics of Cu mineralisation at Mount Isa.