GSV Report 127 - Walhalla-Woods Point-Tallangallook special map area geological report

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Abstract The Walhalla–Woods Point–Tallangallook project area lies entirely in the eastern part of the Melbourne Zone, in the eastern highlands of Victoria about 200 km ENE of Melbourne. It covers the Walhalla, Woods Point and Tallangallook goldfields. These have produced more than 108 tonnes of primary gold and more than 12 tonnes of alluvial gold. This report and accompanying maps document the rock units, geological structure and gold mineralisation.

The oldest rocks are Late Cambrian calc-alkaline Licola Volcanic Group, clastic Wahroonga Breccia and deep-marine Wild Dog Chert. Overlying these is the Early Ordovician (Lancefieldian) deep marine Phosphate Hill Formation. All these rocks appear to form an elongate basement high that underlies the eastern edge of the project area, and are exposed in a series of erosion windows through overthrust younger rocks. At Glen Creek the Cambrian rocks are shown to be a domal culmination of this basement. In contrast, geophysical modelling has estimated the depth to structural basement in the west of the project area at 10–12 km–there is a large and rapid change in basement geometry across the project area. The Cambrian and Early Ordovician sedimentary rocks are overlain, perhaps conformably, by widelydistributed Upper Ordovician Mount Easton Shale, and by siltstone and sandstone of the Murrindindi Supergroup–the Silurian to Early Devonian Jordan River Group and the Early Devonian Walhalla Group. This entire sequence is deep-marine, part of the Mount Easton Province. A key project result is a more detailed stratigraphic subdivision which demonstrates the layer-cake nature of the Palaeozoic rock succession across the entire project area. Several new units have been recognised. Evidence for syndepositional tectonism occurs throughout the area.

Deposition was ended by the Early Devonian Tabberabberan Orogeny, an east–west-directed shortening event that deformed and uplifted the succession to form the Melbourne Zone. Large parts of the stratigraphic succession detached from the Cambrian basement and were thrust towards the east. Structures formed at this time include the Mount Easton Anticlinorium, Walhalla Synclinorium, Mount Useful Fault Zone, and the Fiddlers Green, Ferraris and Thomas faults. Our mapping has refined the position and character of a number of these, and has recognised east-west folds of the Waranga Domain in the northwest corner of the project area. This work has enabled the construction of a series of detailed cross sections, which underpin a new tectonic model of east-directed thrusting of Melbourne Zone rocks over a basement ramp system. Detachment faults that separate the basement from the Melbourne Zone, and related splay structures, have probably acted as conduits for subsequent hydrothermal fluids. These structures controlled the distribution of mineralisation and confined most gold deposits to the Walhalla Synclinorium, between the Mount Easton Anticlinorium and Fiddlers Green and Howes Creek faults. This is part of a new gold mineralisation model presented in this report.

Following deformation, several episodes of magmatic intrusion occurred in the Late Devonian. The Woods Point Dyke Swarm, composed of ultramafic to felsic dykes, appears to be intimately associated with Woods Point and Walhalla goldfields gold mineralisation. Our mapping has confirmed the uneven distribution of the Woods Point Dyke Swarm observed by earlier workers. Primary gold was introduced just after the intrusion of the Woods Point Dyke Swarm. The most significant goldfields are at Walhalla and Woods Point, but others occur near Jamieson and at Tallangallook and many small gold mines are scattered throughout the project area. Most primary gold occurs in structurally controlled quartz veins related to faults, hosted by Walhalla Group sedimentary rocks or dioritic Woods Point dykes. Other intrusive rocks are the Baw Baw Granodiorite and Strathbogie Granite, large, post-tectonic I-and S-type plutons. The Buxton Granodiorite is part of a ring dyke system associated with the Cerberean Cauldron.

The Avon Supergroup is a thick fluvial and subaerial volcanic succession that unconformably overlies the Murrindindi Supergroup. The supergroup comprises the Delatite, Wellington Volcanic and Mansfield groups. Mapping of the Late Devonian–Early Carboniferous Avon Supergroup has been refined and shows that the majority of previously discovered Cu-U concentrations in the Mansfield Basin occur in the newly-recognised Buttercup Mudstone Member. These rocks were folded into the Jamieson Syncline and Mansfield Basin during Late Devonian to Carboniferous deformations, culminating with the Kanimblan Orogeny.

Late Mesozoic sedimentary rocks of the Gippsland Basin in the south were deposited when Australia and Antarctica separated. The stratigraphic nomenclature of the Cretaceous of the Gippsland Basin has been revised. The youngest units are fluvial gravel, sand and mud deposited by present-day river systems, and colluvial deposits that occupy the heads of steep gullies and flank some of the steeper and higher hills. Other commodities mined in small quantities from the project area are copper and platinum at Coopers Creek, phosphate near Mansfield and limestone from a number of quarries. A minor tin occurrence is located at Maindample, platinum occurs with copper ores at Coopers Creek and minor stibnite occurrences have been mined in several locations.

Bibliographic reference VandenBerg, A.H.M., Cayley, R.A., Willman, C.E., Morand, V.J., Seymon, A.R., Osborne, C.R., Taylor, D.H., Haydon, S.J., McLean, M., Quinn, C., Jackson, P. & Sandford, A.C., 2006. Walhalla - Woods Point - Tallangallook special map area geological report. Geological Survey of Victoria Report 127. Geological Survey of Victoria. Department of Primary Industries.