Geology and geophysical exploration of base metals in Victoria

Abstract

"Victoria provides the best exposed section across the Lachlan Fold Belt, which extends from eastern Tasmania through central and eastern Victoria into New South Wales. Western Victoria also contains the eastern parts of the Delamerian Fold Belt. The rocks include the late Proterozoic to Lower Carboniferous sequences that were folded in the Palaeozoic up to the Early Carboniferous Kanimblan Orogeny. The rocks are traversed by thrust faults more or less parallel to the north-south structural grain. The largest faults separate rocks with different ages and structural histories, and have been used to subdivide Victoria into seven structural zones. The oldest mapped rocks are Late Proterozoic or Cambrian and are either calc-alkaline island arc-type volcanics (mainly in the west) or mid oceanic ridge basalts (mainly in central Victoria). The island arc-type volcanics have potential for major volcanic-hosted massive sulphide deposits (VHMS) deposits. The Wickliffe and Hill 800 prospects are in this environment. Small copper-gold deposits are hosted by tholeiitic basalts. An example is the Mount Ararat Prospect which contains an inferred resource in excess of 1 million tonnes of 2.7% copper, 10 g/t silver and 0.6 g/t gold. Electrical exploration methods were ineffective due to a conductive surface layer. Magnetic induced polarization defined anomalous responses coincident with the mineralisation. In eastern Victoria, two cycles of transtensional rifting generated and preserved voluminous silicic volcanics and marine sediments. The Silurian cycle formed the Cowombat Rift, host to significant base metal deposits. These include the Wilga and Currawong VHMS deposits near Benambra. Both were geophysical discoveries. The time-domain electromagnetic surveys showed the massive sulphides were associated with conductivity responses and the aeromagnetic data showed discrete magnetic responses over the mineralisation. The total reserves at Wilga were 3.93 million tonnes of 3.64% copper and 5.44% zinc. The unmined Currawong orebody resource is 8.9 million tonnes of 1.9% copper and 4% zinc. Early Devonian transtension formed the Buchan Rift and smaller basins, into which volcanics and/or marine sediments were deposited. These host the possible Irish-style carbonate-hosted lead-zinc mineralisation at Back Creek, and possible Tennant Creek style mineralisation at Nowa Nowa. The Nowa Nowa deposits were identified from intense magnetic responses and led to the discovery of magnetite - haematite +- chalcopyrite mineralisation. The mineralisation within the Cambrian Mount Stavely Volcanic Complex at Thursdays Gossan, while low grade and uneconomic, is significant because it is the first porphyry copper style identified in western Victoria. It offers the possibility of mineralised porphyry systems being formed in a favourable environment potentially equivalent to other high-gold VHMS deposits in southeastern Australia. In eastern Victoria mixed I-, S- and A-type granites were intruded in the Early Devonian; several broke through to the surface and formed calderas. Some of the granites around the Buchan Rift, at Dogwood and Sunday Creek, are host to porphyry mineralisation. Major government initiatives to encourage mineral exploration have led to the collection of regional semi-detailed geophysical data and so improved the quality and speed of geological mapping. For the first time, the explorer now has access to low cost, high-quality regional scale magnetic, radiometric, digital terrain and gravity data to include in a modern exploration program and to help determine mineral exploration targets. For base-metal exploration, they are particularly valuable. In the highlands of eastern Victoria outcrop is generally good but access can be difficult while in western Victoria geophysical techniques are required to focus exploration beneath cover. In both regions, the new data have highlighted new potential."