AbstractsEarth & Environmental Science

The Rosebery massive sulphide deposit, with historical production and current resources of ~32.7Mt @ at 14.5% Zn, 4.4% Pb, 0.58% Cu, 145ppm Ag and 2.2ppm Au, is located in western Tasmania. It is hosted within a Middle Cambrian post-collisional succession of predominantly submarine, calc-alkaline volcanic and non-volcanic sedimentary rocks. The ore occurs as single or stacked, sulphide and barite-rich lenses with podiform to sheet-like morphologies, distributed over 3km of strike and >1.5km depth below the current surface. The basal stratigraphic unit of the east-dipping mine sequence (the footwall volcanics) comprises a syn-eruptive succession of thick rhyolitic pumiceous mass flow units that were rapidly emplaced on the seafloor, and succeeded by rhyolitic to dacitic turbiditic mass flow and suspension settled volcaniclastic sediments of the transitional stratified volcaniclastics (TSV). The quartz and lithic content of the TSV increased with time as sediment provenance became more distal. The base of the Hangingwall Volcaniclastics was marked by a reduced volcaniclastic influx and deposition of a carbonaceous mudstone, followed by further volcaniclastic mass flow units with a distal clastic provenance. Intrusion of rhyolitic to dacitic peperite sills occurred prior and subsequent to the onset of mineralisation. The current study has shown that syn-depositional displacement of the sediments occurred as the result of basement faulting and sill emplacement, with cessation of significant fault movement prior to deposition of the Hangingwall Volcaniclastics. The mine sequence is truncated at the upper and lower margins by major Devonian reverse faults. Mineralisation is hosted within sediments at the top of the footwall volcanics and within the TSV. Regional low-temperature diagenesis commenced upon deposition of the volcaniclastic sediments. Hydrothermal circulation through unconsolidated volcaniclastic sediments commenced prior to sediment compaction and lithification, locally preserving uncompacted vitriclastic textures. Hydrothermal alteration proximal to ore comprises a halo of quartz ± sericite ± Mn/Fe-carbonate assemblages, with a thin discontinuous chlorite assemblage located immediately beneath sulphide ore. The current study has identified a transgressive zone of intense quartz-rich alteration beneath P lens that now delineates what was a zone of fluid upflow along a syn-depositional fault. A broad halo of sericite-rich alteration envelops the ore and proximal alteration assemblages, but does not extend into the Hangingwall Volcaniclastics, as the hydrothermal system had waned prior to their emplacement. Isotopic data indicates a hydrothermal fluid derived through the modification of seawater as it circulated through the Cambrian volcanic succession and underlying Precambrian basement, from which sulphur and metals were leached. At the mine scale, diffuse hydrothermal upflow was loosely focussed along syn-sedimentary faults defined during this study, resulting in the …