Palaeozoic geology and resources of Victoria

Victoria provides the only well-exposed section across the southern part of the Lachlan Fold Belt and the easternmost Delamerides. Although the plate tectonic setting of this region in the Palaeozoic is still uncertain, the exposure provides important insights into its depositional, magmatic and structural evolution from the Cambrian to the end of the Devonian.  The oldest known rocks are Late Proterozoic or Cambrian and either calc-alkaline arc-type volcanics (mainly in the west) or MORB-types (mainly in central Victoria). The MORB-types host small copper-gold deposits and may be the source for the turbidite-hosted gold.  The arc-type volcanics have potential for major VHMS deposits.  The volcanics are overlain by an extensive turbidite sheet, which is Cambrian in the west and Early Ordovician in central and eastern Victoria; in east-central Victoria the turbidites may be absent. The Delamerian Deformation affected rocks in the far west; the Benambran Deformation affected the east and the Bendigo and Stawell Zones in west-central Victoria. In the Bendigo and Stawell Zones, late phases of the Benambran Deformation coincided with the formation of the world-famous turbidite-hosted gold deposits. There was no Benambran Deformation in the Melbourne Zone in central Victoria, where marine deposition continued without interruption from the Cambrian to the early Middle Devonian.  In the Silurian, the partly fluvial, partly shoreline facies Grampians Group was laid down on cratonic crust in western Victoria, at what may have been the western shoreline of the Melbourne  Trough in central Victoria. This was followed in the Late Silurian by eruption of the subaerial Rocklands Rhyolite. In eastern Victoria, there are two cycles of rifting in transtensional rift-like grabens into which voluminous silicic volcanics and marine sediments were deposited. The first, Silurian, cycle formed the Cowombat Rift, host to significant base-metal deposits. Away from this rift, there seems to have been a shallow sea during most of the Silurian in eastern Victoria, into which a very condensed limestone seems to have been deposited; however, the limestone is mainly known from olistoliths in the rift sequence. A second cycle of transtension in the Early Devonian formed the Buchan Rift and smaller basins, into which volcanics and/or marine sediments were deposited. In western Victoria, numerous I-type granites were intruded.  In eastern Victoria, mixed I-, S- and A-type granites were intruded; several broke through to the surface and formed calderas. The Middle Devonian Tabberabberan Deformation affected most rocks in central and eastern Victoria. It was rapidly followed by intrusion of several granites and the Woods Point Dyke Swarm in central Victoria, which hosts the large Woods Point-Walhalla gold province in the Melbourne Zone. In the Late Devonian, a molasse-type sequence, mainly fluvial redbed sediments and silicic pyroclastics and lavas, was deposited in the Howitt Province in east-central Victoria, from where the rivers probably flowed to the east coast via Gippsland. The Howitt Province overlaps with the Central Victorian Magmatic Province, where many more granites were intruded with, again, some rising to the surface to form calderas into which thick sequences of silicic volcanics were erupted. The last significant Palaeozoic deformation was the Kanimblan, in the Carboniferous, during which the Upper Devonian and Lower Carboniferous redbeds were mildly to strongly folded and faulted.

Tags: Earth Sciences, GA Publication, Journal, Published_External, VIC
Organization: Geoscience Australia Data