In support of an invitation to tender for exploration licenses over the Walhalla-Woods Point-Tallangallook project area, opening 30 June 2006, GeoScience Victoria undertook work to update the quality of geoscience datasets available.
The Walhalla-Woods Point-Tallangallook project area lies in the eastern Melbourne Zone of the Lachlan Fold Belt. The oldest exposed rocks are Late Cambrian Licola Volcanic Group, in a series of small windows eroded through overthrust younger rocks along the eastern edge of the project area. These are the crest of a basement high that underlies the eastern edge of the Melbourne Zone, a structurally complex area known as the Mount Useful Fault Zone. Part of this fault zone occurs in the eastern part of the project area.
The Licola Volcanics are possible correlates of the Tasmanian Mount Read Volcanics and contain greenstone-hosted gold deposits. The two better known examples of greenstone-hosted mineralisation lie just outside the project area –the Hill 800 and Rhyolite Creek prospects. The Hill 800 prospect contains disseminated and stockwork/stringer chalcopyrite, sphalerite, galena ± native gold and has been described as a hybrid between a volcanic-hosted massive sulphide and porphyry copper-style deposit. The Rhyolite Creek Au–Ag prospect consists of vein stockworks and massive sulphide veins. Anomalous gold has also been located in the Whiskey Knob Window, which extends into the project area. Upper Ordovician to Early Devonian deep marine sedimentary rocks – the Mount Easton Shale and conformably overlying Murrindindi Supergroup – overlie this basement. The Murrindindi Supergroup consists of the Silurian Jordan River Group and the Early Devonian Walhalla Group, which hosts the Walhalla and Woods Point goldfields. Faults formed in the Early Devonian Tabberabberan Orogeny separate these folded rocks from the Cambrian basement. Dykes and granitoids intruded 20 million years later, including the famous Woods Point Dyke Swarm. Gold was introduced into the sequence at this time.
The Walhalla–Woods Point–Tallangallook gold district is historically one of the most productive in Victoria. It is the largest east of the Bendigo Zone and includes the highly productive Walhalla and Woods Point–Gaffneys Creek gold fields, which rank as the sixth and eighth largest gold producers in Victoria, respectively. The district contains one of the largest single mineralised structures in the State — Cohens Reef at Walhalla, which is also the district’s most productive mine. The A1 mine near Gaffneys Creek, which operated almost continuously from 1881 to 1992, and the Morning Star mine at Woods Point, are the other major gold producers.
Most gold deposits, whether dyke- or sedimentary rock-hosted, lie within Walhalla Group sedimentary rocks — very few deposits lie within Jordon River Group sedimentary rocks. However, the composition of the Walhalla and Jordon River groups are similar, so the main control over the regional distribution of gold deposits appears to be structural — most deposits are clustered in the Walhalla Synclinorium between the eastern margin of the Mount Easton Anticlinorium (Enochs Point Fault) and the Fiddlers Green or Howes Creek faults. An analysis of the distribution of deposits types shows that dyke-hosted and dyke-affiliated quartz-vein deposits are clustered along the western margin of the Walhalla Synclinorium. Sedimentary rock-hosted quartz vein deposits have a broader distribution, spanning the full width of the Walhalla Synclinorium. Disseminated/stockwork deposits occur mainly in the northern part of the Walhalla Synclinorium in the aureole of the Strathbogie Granite. The regional-scale structural control on the gold deposits may relate to the shape of the upper surface of the underlying basement, with the largest and most productive gold deposits lying along the western half of the Walhalla Synclinorium, above the western flank of inferred basement highs. Dykes and gold were focussed by pre-existing structures developed in the host rocks adjacent to these basement highs–this is part of a new mineralisation model developed by the GeoScience Victoria project team and detailed in Geological Survey of Victoria Report 127.
Apart from gold, a range of other commodities have been mined or prospected in the project area. These include copper and platinum at Coopers Creek, phosphate near Mansfield, ‘red-bed’ copper near Mansfield, and limestone at a number of quarries but mainly in the southern portion of the area. Stibnite has been mined at the Heyfield Reef northwest of Bonnie Doon and also occurs as a minor constituent in a few gold mines. Cassiterite has been found in alluvium near Maindample. Small mercury deposits are located near the junction of Quicksilver Creek and the Jamieson River 15 km east of Jamieson.
(Source: Geological Survey of Victoria, Report 127, 2006)