Abstract
Volcanic-hosted massive sulfide (VHMS) deposits from western Tasmania have significantly higher Au grades than other VHMS deposits from eastern Australia and elsewhere. Huston and Large (1989) have suggested that the dominant controls on Au grades in VHMS deposits are probably temperature and pH of the hydrothermal fluids. However, the concentration of Au in the source rocks being leached by the hydrothermal fluids may also affect the final Au grades in the deposit (e.g., Keays, 1987). The major VHMS deposits of western Tasmania are hosted within the Central Volcanic Complex of the Mount Read Volcanics or overlying andesites of the Que-Hellyer sequence. Other potential source rocks for leaching of metals include late Proterozoic basalts and sediments of the Crimson Creek Formation, ultramafic-mafic rocks from a tectonically emplaced ophiolite complex, and Precambrian schists and quartzites.The background Au concentrations in the least altered Mount Read Volcanics (mainly andesites, dacites, and rhyolites) are in the range 0.9 to 1.3 ppb Au, and the mean values are comparable with those of unaltered modern volcanics of similar composition. Hydrothermally altered equivalents of these volcanics generally have somewhat higher Au concentrations. The Tasmanian ultramafic rocks have significantly lower Au (mean 0.5 ppb) than unaltered periodotite xenoliths, but similar concentrations to ophiolitic ultramafic rocks. High Mg, low Ti volcanics from the western Tasmanian ultramafic-mafic complexes have Au contents (mean 1.6 ppb) similar to the Mount Read mafic rocks. In contrast, relatively high Ti basalts of the Crimson Creek Formation are the most Au-enriched primary volcanics in the area (up to 23 ppb Au). These volcanics were probably erupted during an early rifting phase and may underlie much of the Mount Read Volcanics.Source-rock modeling calculations, based on average Zn concentrations of potential source rocks, suggest that about 70 km 3 of andesitic volcanics would have to be leached to provide the metal content of the Hellyer VHMS deposit. Similar calculations based on Au concentrations suggest somewhat larger leach volumes. Both estimates suggest that some leaching of basement rocks may have occurred during the formation of the VHMS deposits. The relatively high Au concentrations in the Crimson Creek basalts indicate the presence of an anomalous Au-enriched source rock beneath the Mount Read Volcanics which may have contributed to the relatively Au-rich character of the Tasmanian VHMS deposits. However, the temperature, composition (e.g., PH, f (sub O 2 ) , a (sub H 2 S) , m Nacl ), and the consequent solubility of metal species in the hydrothermal fluids will be the major factors which determine the final concentrations of base and precious metals in a specific massive sulfide deposit. Source-rock composition will only be important if the hydrothermal fluid has an appropriate chemistry to allow significant Au solubility.
Published Version
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