The Cambro-Ordovician period has yielded the major development of volcanic-hosted massive sulfide (VHMS) deposits in Australia, resulting in about 12 million metric tons of contained metal (copper, lead, and zinc), concentrated in the Mount Read Volcanics (Tasmania) and the Mount Windsor Volcanics (Queensland). The Archean (4 million metric tons of metal) and the Silurian (3.5 million metric tons of metal) constitute the next important episodes while the Devonian and Permian have produced isolated deposits.The deposits range from Cu type to Zn-Cu type to Zn-Pb-Cu type. The Zn-Cu-type deposits are restricted to the Archean and Silurian, whereas the Cu type and Zn-Pb-Cu type occur sporadically throughout the time span from early Archean to Permian. In general terms, the major VHMS-hearing districts are calc-alkaline in character and display a thick basal pile of rhyolitic volcanics (1-3 km thick) including lavas, epiclastics, and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite, dacite, andesite, basalt, and sediments. The major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence.There is a wide range in variability of styles of Australian VHMS deposits including mounds, pipes, sheets, layered deposits, stacked deposits, stockwork and disseminated deposits, distal reworked deposits, and cyclic layered deposits. Although these various styles show a range of features, there is a consistent theme across the spectrum of metal zonation, alteration mineralogy, alteration chemistry, sulfur isotopes, macrotextures, microtextures, and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit. The classic mound-style deposits, such as Hellyer, have a series of key features, subsets of which are represented in the other deposit styles.The mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent. Growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures, leading to zone refining and lead-zinc-silver-gold enrichment in the outer and upper parts of the mound. Departure from the classic mound style of deposit is related principally to three key factors: the chemistry of the ore fluid (salinity, temperature, f (sub O 2 ) , and a (sub H 2 S) ), the nature (permeability and chemistry) of the volcanic pile, and the sea-floor environment (sea-floor topography and seawater depth). These factors control the aspect ratio of the deposit, the extent of stringer zone development, the degree of subsea-floor replacement mineralization, the nature and spatial development of footwall alteration, and the development and style of related distal mineralization.Sulfur isotope studies of Australian VHMS deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit, whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks. This conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences. The input of metals directly from volcanic magma chambers is not precluded by the available data, and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume, rising to mix with convective seawater fluids which contribute lead, zinc, silver, and gold leached from the footwall volcanics and basement rocks. The relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles and their spatial relationship to volcanic centers or adjacent sedimentary basins.

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