Cambrian volcanism and mineralization, South-West Tasmania

Cambrian volcanic rocks in South-west Tasmania are associated spatially and chronologically with ultramafic-gabbro complexes and volcaniclastic eugeosynclinal sediments. They are composed mainly of rhyolites and dacites, but andesites and basalts also occur, and are locally dominant. The basalts include varieties that resemble present-day ocean floor anecalc-alkaline basalts. The rhyolite-dominated calc-alkaline suite (the Mount Read Volcanics) is similar to Andean-type volcanic suites and is restricted to a narrow, linear, mineralised belt. These volcanics range from acid to basic with acid predominating in an average ratio of 7 rhyolite-dacite : 1 andesite-basalt. Volcanic correlates west of the Mount Read Volcan-ics are largely andesitic in composition, and lateral variations suggest generation above an east-dipping Cambrian Benioff Zone. The Mount Read Volcanics comprise a thick pile of massive lavas, ignimbrites, air-fall tuffs and epipyroclast-ics, with small apparently subvolcanic intrusions. Trace element stability studies have enabled determination of the original composition of the volcanics despite the superimposed effects of metamorphism and hydrothermal alteration. Two major stratigraphic subdivisions, separated at least in part by an unconformity, have been recognised in the Mount Read Volcanics of the southern West Coast Range. Hydrothermal alteration and associated mineralization are developed only in the lower stratigraphic unit (Intercolonial Volcanics). It is believed that an abundance of conduits (permeable pyroclastic horizons and faults) and localised heat sources within the Intercolonial Volcanics caused circulation of sea water. The sea water, and possibly young connate brines from the adjacent geosynclinal sediments (Dundas Group) are thought to have leached metals from volcanics and sediments at shallow crustal levels and deposited them in near-surface and surface submarine conditions. Mineralization in the Intercolonial Volcanics of the southern West Coast Range is an extension of the Mount Lyell type of mineralization, and occurs mainly in four assemblages: 1. pyrite-chalcopyrite in sericitised and chloritised volcanics 2. bornite-neodigenite-chalcopyrite 3. chalcopyrite-pyrite-barite 4. magnetite-hematite-pyrite-chalcopyrite. The Mount Lyell type represents an unusual form of massive sulphide mineralization characterised by lack of zinc and lead. The copper prospects and orebodies are interpreted as sub-surface addition and replacement deposits, eroded pipes, and surface exhalative deposits. Compared to areas of massive sulphide mineralization elsewhere, surface exhalative deposits are rare, apparently as a result of post-mineralization erosion. The present distribution of mineralization as a narrow linear belt results from the Intercolonial Volcanics being confined to the axial zone of a major anticline which was initiated in the Jukesian Orogeny (Late Cambrian) and accentuated in the Tabberabberan Orogeny (Middle Devonian).