Foliation-boudinage control on the formation of the Rosebery PbZn orebody, Tasmania

Abstract

The Rosebery PbZnAgAu massive-sulphide deposit occurs within the Middle-Cambrian Mount Read Volcanics of western Tasmania. It is hosted by a tuffaceous slate lens that is surrounded by felsic volcanics. The volcanic belt and its Ordovician cover were deformed and metamorphosed during an Early to Middle Devonian orogeny (Tabberabberan orogeny). During this orogeny D 3 E-W shortening was partially accommodated by reverse bedding-parallel shearing within the slate unit and immediately underlying volcanic rocks. This caused reactivation and intensification of an early foliation ( S 2), and the development of asymmetric chocolate-tablet foliation-boudinage structures with boudin necks parallel to the X- and Y-directions. The mineralization occurs along extensional shear-planes that are associated with foliation-boudinage. Mineralogical zoning in the orebody can be directly related to dilational bends in shears associated with foliation-boudin-necks. It is proposed that these structures gaped during the D 3-deformation, allowing infiltration of metalliferous hydrothermal-fluids along shear plane and cleavage directions into the fractured wall-rock. Progressive metasomatic replacement of the wall rock resulted in the formation of a sulphide-sulphate-silicate orebody. The orebody is completely discordant to folds in the host rock; this observation confirms a structurally-controlled origin and refutes earlier interpretations of the deposit as a tightly-folded chemical sediment. Isotope data indicate that the orebody was probaby derived from volcanogenic sulphides within the Mount Read Volcanics. The Rosebery orebody is best regarded as a metamorphogenic deposit that formed by dissolution, upward transport, concentration and re-precipitation of primary sulphides. The proposed macroscopic model complements separate microstructural evidence by the author indicating a syn- D 3-timing of mineralization