Microstructural timing of the Rosebery massive sulphides, Tasmania: evidence for a metamorphic origin through mobilization of disseminated base metals

Abstract

ABSTRACTMicrostructural timing relationships indicate that the Rosebery massive sulphide ore, western Tasmania, Australia, formed by metasomatic replacement of ‘sericite’ schist during a Devonian deformation event (D3). This interpretation is contrary to earlier volcanogenic‐based interpretations, but accords with the discordant position and inferred structurally controlled emplacement of the orebody. The main timing criteria are: overprinting of S3 by the late ore minerals, replacement textures in undeformed mineral parageneses, and a D3 structural control from the microscopic to the macroscopic scales. The consistent observation of these criteria in the orebody and the complete lack of pre‐D3 ore argue against in situ dissolution of a primary orebody and local redeposition of sulphides by replacement.D3 deformation at Rosebery is inferred to have been characterized by heterogeneous cleavage‐parallel extension, which resulted in localized microfracturing and selective replacement of zones of maximum strain rate. Continuous shifts in the strain‐rate distribution pattern during progressive mineralization led to the compositional ore banding.Published Pb‐isotope data point towards a Cambrian source rock for the orebody. This suggests a metamorphogenic origin by regional‐scale dissolution of dispersed volcanogenic metals, focused solution transfer and concentrated redeposition in a structural trap.