The Koonya Prospect, Rosebery, Tasmania

Koonya is a small lead-zinc prospect located approximately three kilometres south of Rosebery and about one kilometre east of the Rosebery fault. It lies within the basal sequence of the Cambrian Central Volcanic Complex on the western side of the Mount Read Volcanic belt The Koonya prospect occurs in the same stratigraphic sequence that hosts the Rosebery and Hercules ore deposits. The geology of the Koonya area is dominated by variable altered, rhyolitic feldspar-phyric volcanics. These units have flow(?) banding and shard like textures throughout the groundmass and may be either unusual lavas or pyroclastics. Grading observed in several areas suggests an upright facing for the sequence. Three mineralized horizons were recognised at Koonya. The major horizon contains a thin massive sulphide lens and its vein style down dip continuation (zone B). In three dimensions the massive sulphide and zone B are part of a more complex horizon of mineralization. This horizon shows a distinct mineralogical zonation from a chalcopyrite-pyrite-arsenopyrite-tetrahedrite- aikinite-native bismuth assemblage in the south (similar to the copper-rich assemblage seen at Rosebery), to a galena-sphalerite-pyrite-chalcopyrite-pyrrhotite assemblage which dominates the massive sulphide to the north. The mineralization pre-dates the Devonian regional deformation and thus pre-dates the Devonian granite intrusion in the area. Temperatures for deposition of the mineralization, based on the mineral assemblages, ranged from greater than 270°C for the copper rich zone to less than 250°C for the massive sulphide. Zinc ratio distributions indicate that the massive sulphide was formed from lead saturated fluids, but the other types of mineralization were deposited from either lead undersaturated fluids, or formed at temperatures below 200°C. Lead isotope studies indicate the lead in the mineralization is Cambrian in age and sulphur isotopes suggest the sulphur was dominantly derived from seawater. Two major alteration events occurred at Koonya; alteration associated with mineralization and a later stage alteration, often associated with post-cleavage veining. The alteration associated with the main lead-zinc mineralized zone has a very distinctive arrangement directly related to the position of the mineralization. Above the mineralization the alteration is dominated by a quartzsericite- pyrite assemblage but below the mineralization the alteration is dominated by a sericite assemblage. Geochemical studies of the alteration have shown that originally the altered rocks were very similar. The alteration has produced mass and/or volume changes in excess of ±30% of the original mass/volume. Major element changes during alteration included depletion of sodium, calcium and silica and enrichment of potassium, iron, magnesium and manganese. Oxygen isotope studies suggest that the main mineralizing fluid was dominated by heated seawater, supporting the previous conclusion reached from sulphur isotope studies, that seawater was the major component of the mineralizing fluid. Studies of chlorites formed during alteration revealed that the chlorite has been re-equilibrated during later metamorphism. The metamorphic temperature recorded by the chlorites is approximately 310°C to 340°C. The very different composition of chlorites from the quartzsericite- pyrite altered zone and the sericite altered zone suggest that they were formed under different conditions. An unusual feature of the chlorites at Koonya is their very high manganese values, with MnO in excess of 5% in some samples. A three stage model is proposed to explain the genesis of the major mineralized horizon at Koonya. Initially the host rhyolitic sequence was extensively sericitized. The second stage involved deposition of the sulphide mineralization as veins, fracture fillings(?) and disseminations, and co-incident quartz-sericite-pyrite alteration of the hanging wall. In the third stage the host rock sequence is folded and deformed, causing re-orientation of the mineralization.