Mafic-hosted zinc mineralisation, High Point, western Tasmania

The High Point prospect is located on the northern slopes of Mount Charter in western Tasmania within the Que-Hellyer Volcanics of the Cambrian Mount Read Volcanic belt. The main lithologies include intrusive sub-ophitic dolerite, pyritic black shales, andesite dykes and sills, coherent basalt units, basalt breccias, dacite lavas and dacite breccias. The prehnitepumpellyite grade of metamorphism recognised in the area is attributed to the Devonian Tabberabberan Orogeny. The structure of the area is dominated by N.E.-plunging synclines and by the Henty Fault zone to the east of the region, and the Mount Charter fault immediately to the southwest of the study area. The previously enigmatic High Point dolerites are now considered to be broadly comagmatic with the upper basalts of the Que Hellyer Volcanics based on geologic relationships and geochemical analyses. Similarly, andesite units in the area are geochemical equivalents of the hornblende andesites of the southern Central Volcanic Complex. Dacites units in the High Point region correlate with the 'mixed sequence' dacites of the Que-Hellyer Volcanics (part of suite Ill of Crawford et al., 1992). Alteration of the High Point lithologies is considered to be a low intensity, low temperature equivalent of VHMS-related alteration at the Que River and Hellyer massive sulphide deposits. Typical alteration assemblages, considered to be Cambrian, include; silicasericite in dacites, chlorite in basalts, sericite-fuchsite in the upper basalts and silica-albite in the andesites. Epidote alteration overprints the Cambrian alteration assemblages and is likely to be related to the Devonian Tabberabberan Orogeny. The High Point prospect contains low grade, disseminated sphalerite mineralisation which occurs as fine-grained anhedral masses associated with minor galena and pyrite, and is predominantly hosted within the basalts and andesites. Lead isotope data suggests that the disseminated mineralisation is Cambrian and has a target signature similar to that of the Que River massive sulphide deposit, Zn ratios also indicate that the mineralisation is Cambrian VHMS style. Vein style sphalerite mineralisation is associated with minor pyrite, galena, iii chalcopyrite and pyrrhotite and is attributed to remobilisation during the Devonian Tabberabberan Orogeny. The heat source for the Cambrian mineralisation and alteration is interpreted to be related to the emplacement of the andesite dykes and sills. Hydrothermal fluids have preferentially precipitated sphalerite within the ba~alts and andesites, particularly below the contact of the Que River Shale. The Que River Shale has behaved as a chemical trap for the mineralised hydrothermal fluids. The mineralisation is currently subeconomic however, potential exists for economic mineralisation deeper within the stratigraphic sequence.