GSV Report 126 - Dargo 1:100 000 map area geological report

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Abstract This report describes the geology of the area covered by the Dargo 1:100 000 map sheet 8323 (DARGO). It contains a summary of previous work and gives a description of all rock units in the map area and a comprehensive account of the physiography, landscape evolution, geological history and structural, metamorphic and economic geology. DARGO contains rocks of mainly Palaeozoic and Cainozoic age. Palaeozoic rocks lie within two structural zones of the Lachlan Fold Belt: the Tabberabbera Zone in the west and the Omeo Zone in the east. Cainozoic rocks generally comprise thin cover sequences scattered across the land surface.

The oldest units are marine metasedimentary rocks of the Pinnak Sandstone (Early and Middle Ordovician), Bendoc Group (Late Ordovician) and Cobbannah Group (Early Silurian) which are considered to be conformable. Pinnak Sandstone and its metamorphic derivatives form the bedrock of much of DARGO. Bendoc Group only occurs as fault slices within the Wonnangatta Fault Zone. The Cobbannah Group occurs to the west of the Wonnangatta Fault Zone, but is also incorporated into that fault zone in places. The Early Silurian Benambran Orogeny is responsible for regional deformation and metamorphism of the Pinnak Sandstone, Bendoc Group and Cobbannah Group. Contrasting structural styles created during this orogeny has enabled DARGO to be subdivided into two structural zones: the Tabberabbera Zone in the southwest and the Omeo Zone in the northeast. Regional low-pressure/high-temperature (Buchanstyle) metamorphism accompanied deformation in the Omeo Zone to form the Omeo Metamorphic Complex. Metamorphic zones in the complex are cordierite, muscovite–sillimanite, andalusite–K-feldspar, sillimanite–K-feldspar and migmatite, and rock types are pelitic to psammitic schist, gneiss, migmatite and S-type granite and rare ultramafic igneous rock.

Regional strike-slip deformation at the end of the Silurian (Bindian Orogeny) generated large dextral strike-slip shear zones and faults (Kiewa, Cassilis, Barmouth, Shanahan, Ensay, Lochiel), refolded Benambran folds and faults into sinuous shapes, and caused greenschist facies metamorphism of Silurian dykes in the metamorphic complex. The western side of the Omeo Metamorphic Complex is truncated by the Kiewa Shear Zone, which separates the metamorphic complex from low-grade Pinnak Sandstone of the Tabberabbera Zone. Further south, this fault breaks into a number of divergent splays, which together mark a transition between the zones. During and just after the Bindian Orogeny, numerous plutons of granite, granodiorite, tonalite and quartz diorite were emplaced, most of them magnetic I-types—these are largely undeformed. Numerous dykes of basalt, dolerite, andesite, diorite, rhyolite and porphyry were also emplaced in the Early Devonian, most prominently in the Omeo Zone. They are undeformed but commonly altered. Early Devonian fluvial and shallow marine sediments of the Wentworth Group occur in southeastern DARGO, intruded by dykes and sills. They were deformed into the tight Mitchell Syncline in the Middle Devonian Tabberabberan Orogeny. Contact-aureole rocks surrounding some of the granites were retrogressed during this event. Late Devonian fluvial sedimentary and subaerial felsic volcanic rocks of the Avon Supergroup rest unconformably on Cobbannah Group in the southwest corner of DARGO. These rocks were gently tilted during the Early Carboniferous Kanimblan Orogeny.

Major uplift in the Mesozoic was associated with the break-up of Gondwana and continues today. The resulting erosion cycles have dominated the Cainozoic geological history in DARGO. Paleogene rocks in DARGO are Eocene alluvial deposits and alkalic basalts of the Older Volcanic Group. Dykes and plugs of basalt and phonolite, probably of the same age, are also common. Dissection has continued and has created an inverted landscape, with former basalt valley flows now forming plateau remnants flanked by deep valleys. At the plateau edges, the basalts are the source of periglacial ‘rock rivers’ and landslide deposits. Erosional basins have formed on some easily eroded plutons.

The main economic commodity in DARGO is gold. Gold mining, both hard rock and alluvial, has been concentrated in three gold fields: Grant, Dargo and Dargo High Plains. These fields were discovered and worked in the late 19th century. Gold in the Grant Goldfield occurs in quartz reefs occupying faults in the Pinnak Sandstone—new radiometric dating shows this to be Early Devonian. Gold in the Dargo High Plains is mostly found in palaeoplacer deposits, recovered from Eocene sub-basaltic gravel on the Dargo High Plains and at Brandy Creek. Occasional hardrock sources were discovered beneath the volcanics in quartz veins. Gold in the Dargo Goldfield is found in reefs within the granitic phase of the Dargo Tonalite, and also in east–west-trending quartz reefs in the adjacent hornfels aureole. Minor gold prospecting has occurred on other plutons such as the Tucker Creek Tonalite, and along the trace of some faults, notably the Kalk Kalk Fault.

Bibliographic reference Willman, C.E., Cayley, R.A., VandenBerg, A.H.M., Haydon, S.J., Osborne, C.R., Seymon, A.R. and Thom, J.L., 2005. Dargo 1:100 000 map area geological report. Geological Survey of Victoria Report 126.