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VICTORIA'S GEOLOGY until the Carboniferous

This section provides a brief overview of Victoria's geological history between Late Proterozoic and mid-Carboniferous time. This reflects its source: the Tasman Fold Belt System in Victoria by VandenBerg et al. (2000), a Geological Survey of Victoria Special Publication that deals with the geology of the Tasman Fold Belt, whose history ceases in the mid-Carboniferous in Victoria. References to the source material have been omitted here as they are fully cited in the Special Publication.

An overview of Victorian mineralisation follows the general discussion.

Some images relevant to various parts of the discussion can be found on this page, and have been crosslinked to the text - underlining denotes links.

The subsequent history of Victoria is summarised in a companion page here. A splendid report of the entire history of Victorian rocks is found in The Geology of Victoria (3rd Edition), available from the Society.

The Tasman Fold Belt System in Victoria records a clear sequence of events associated with the building of southeastern Australia. It reveals that mineralisation and magmatic processes are intimately linked with the tectonic development of the region. The history is dominated by east–west shortening of predominantly oceanic sedimentary and volcanic rocks and their resultant folding, faulting and uplift. Recently, it has become increasingly apparent that major north–south movements have also been involved in constructing eastern Australia. The Palaeozoic basement is 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 subdivide Victoria into three main structural rankings consisting of two fold belts—Delamerian and Lachlan, two terranes in the Lachlan Fold Belt (Whitelaw and Benambra), and ten structural zones (Glenelg, Grampians–Stavely, Stawell, Bendigo, Melbourne, Tabberabbera, Omeo, Deddick, Kuark, Mallacoota).
The Moyston Fault is the most important fault as it forms the terrane boundary between the Delamerian and Lachlan fold belts. These two show important differences. The Delamerian Fold Belt is mainly composed of Neoproterozoic–Cambrian rocks and was deformed in the Late Cambrian Delamerian Orogeny whereas the Lachlan Fold Belt contains mainly Cambrian–Devonian rocks with main deformations occurring in the Late Ordovician–Carboniferous interval. The first regional deformation to affect the Lachlan Fold Belt was the Benambran Orogeny, about 50 M.y. after the Delamerian Orogeny. Granites comprise 20% of the total exposed area of the Lachlan Fold Belt and fall within an age range of 440 to 350 Ma. Volcanics associated with the granites are also widespread and cover an additional 5%. Blocks of older crust consisting of Neoproterozoic–Cambrian rocks, such as the Selwyn Block in central Victoria, were deformed during the Late Cambrian Tyennan Orogeny prior to being incorporated into the Lachlan Fold Belt. The second major structural break in Victoria is the Governor Fault, which occurs along the eastern side of the Selwyn Block. This fault divides the Lachlan Fold Belt into two terranes, the Whitelaw Terrane to the west and the Benambra Terrane to the east. The main difference between these is that orogen-parallel (north–south) transport was more prevalent in the Benambra Terrane, whereas convergent east–west transport orthogonal to the orogen was dominant in the Whitelaw Terrane.
Neoproterozoic and Cambrian rocks and the Delamerian and Tyennan orogenies (550–495 Ma)
The oldest rocks of mainland southeastern Australia are Late Proterozoic or Cambrian volcanics, volcaniclastics and turbidites. Only the igneous rocks are exposed in central Victoria, in isolated belts at Waratah Bay, Philip Island and the Barrabool Hills and in a series of structural windows in the east of the Melbourne Zone. The volcanics, along with the Delamerian Fold Belt rocks of the Glenelg and Grampians–Stavely zones, have been affected by an early deformation that is not present in any of the Lachlan Fold Belt rocks. Images of total magnetic intensity data clearly show that the Central Victorian rocks are northern extensions of the Tasmanian crust. The structural windows in the Melbourne Zone imply that this older crust occurs extensively beneath central Victoria.
The Glenelg Zone extends from an undefined margin west of the South Australian border, east to the Yarramyljup Fault. It contains Cambrian volcanics with ultramafic and within-plate basalt and midoceanic-ridge basalt (MORB) associations, and turbidites of the Moralana Supergroup. The Grampians–Stavely Zone extends east from the Yarramyljup Fault to the Moyston Fault. It consists of the largely buried Cambrian tholeiite–boninite rocks, and poorly outcropping Cambrian calc-alkaline volcanics and Glenthompson Sandstone. Both zones represent a poorly understood collage of volcanic and sedimentary rocks accreted to the older craton as a series of northwest-trending fault-bounded strips. Folding, faulting and regional metamorphism of the rocks of the Glenelg and Grampians–Stavely zones and the Selwyn Block occurred in the Cambrian. Although probably the same event, this is termed the Delamerian Orogeny in mainland Australia and the Tyennan Orogeny in Tasmania (and its northern extension—the Selwyn Block—in central Victoria).
East of the Moyston Fault, Cambrian volcanics not affected by Cambrian deformation are exposed in the Stawell, Bendigo and Tabberabbera zones along the largest faults. These faults penetrate to an inferred major mid-crustal décollement lying within the Cambrian volcanics. The rocks are mainly tholeiitic basalts and less commonly boninites and ultramafics that are interpreted to be the basement to much of the Lachlan Fold Belt. Cambrian sedimentary rocks form the bulk of the exposure in the Stawell Zone, where they consist of thick turbidites (St Arnaud Group). Farther east, however, they overlie the Cambrian volcanics in long narrow belts and are a relatively condensed sequence of mudstone and chert.
Cambrian and Ordovician sedimentation (495–440 Ma)
During the Late Cambrian and Ordovician, the Cambrian tholeiitic volcanics were covered by an extensive sheet of quartz–mica turbidites deposited into a deep marine environment along the eastern edge of the Australian craton. In the central west, the St Arnaud and Castlemaine groups comprise the bulk of these rocks in the Stawell and Bendigo zones respectively. Quartz–mica turbidites also dominate the Lower and Middle Ordovician Adaminaby Group which extends over much of the eastern Lachlan Fold Belt. The only area with restricted Lower and Middle Ordovician sedimentation was in the Melbourne Zone, especially in the east where sediments are extremely condensed and include shale, sedimentary phosphate and, on the Waratah Bay Platform, chert and a marginal marine marlstone. The Waratah Bay Platform contains no record of sedimentation between Late Ordovician and mid-Early Devonian time, although redeposited Silurian limestone derived from it occurs in basinal sediments farther north.
Upper Ordovician sedimentary rocks show a very clear eastward-fining trend. The Sunbury Group, conformably overlying the Castlemaine Group, crops out at the southern transition between the Bendigo and Melbourne zones. This group contains abundant turbidites, whereas farther east in central Victoria, and across much of the eastern Lachlan Fold Belt, the relatively condensed Bendoc Group (and Mount Easton Shale) is dominated by siltstone and black shale that accumulated on a deep anoxic seafloor. An opposite trend, however, is shown in the uppermost Ordovician rocks across the Governor Fault, with a thick sandstone unit, the New Country Sandstone, occurring east of the fault but only black shales occurring to its west. Contribution of volcanic turbidites from the Molong Volcanic Arc was limited to the Kiandra Group, which crops out in a small area in eastern Victoria
Early Silurian (440–ca 425 Ma)
The interval beginning in the Early Silurian and lasting to the end of the Early Devonian was one of great complexity in the Lachlan Fold Belt. Much of the protracted Benambran Orogeny occurred early in the Silurian, and in the Benambra Terrane it was a period of extension and basin formation alternating with strong deformation during the Bindian and Tabberabberan orogenies. In strong contrast with the regions both to the west and east, the Melbourne Zone remained unaffected by the Benambran and Bindian orogenies, a factor which is very important in reconstructing the Palaeozoic history of the Lachlan Fold Belt.
In the Grampians–Stavely Zone, deposition of the Grampians Group platform sequence on low-relief Delamerian crust may have begun at this time. In eastern Victoria, the Ordovician–Silurian boundary coincides with widespread facies changes from euxinic Bendoc Group, dominated by shale, to the sandier Cobbannah and Yalmy groups deposited in aerated sea-water. The eastern Victorian units were deformed at the end of the Early Silurian and underlie the widespread Benambran Orogeny unconformity. During the Benambran Orogeny three metamorphic complexes were generated in Victoria:
  • the Moornambool Metamorphic Complex in the Stawell Zone;
  • the Omeo Metamorphic Complex in the Omeo and Deddick zones; and
  • the Kuark Metamorphic Complex in the Kuark Zone.
Shortening across the Stawell and Bendigo zones during this orogeny was accommodated by closely spaced tight meridional folds with near-vertical hinges, and by high-angle thrusts spaced at regular intervals, with a consistent reverse displacement. Despite the tight folding, the overall enveloping surface dips gently so that there is only minor variation in the metamorphic grade and the stratigraphic levels exposed. In the Bendigo Zone the rocks range from zeolite to lower greenschist facies, and through most of the Stawell Zone the outcrop is typically lower greenschist facies. Metamorphic grade is much higher in the western 15 km of the Stawell Zone, rising to amphibolite facies in the hanging wall of the Moyston Fault, where there are abundant interleaved fault slices of mafic rock.
In the Melbourne Zone, marine deposition continued without interruption from the Cambrian to the Early Devonian, in the later stages in a foreland basin setting. In eastern Victoria, tectonism in the Benambran Orogeny began in the Late Ordovician in the Narooma Accretionary Complex and was more strongly expressed by a regional facies change, uplift, and local deformation, at about the Ordovician–Silurian boundary at 440 Ma. During continuing deformation Llandovery rocks were folded and faulted together with Ordovician rocks at about 430 Ma (end of the Llandovery) and the Omeo Metamorphic Complex was generated. It was followed by the intrusion of I- and S-type granites with ages of about 425 to 420 Ma.
Late Silurian (425–420 Ma)
In the Late Silurian, deformation of the Grampians Group to form a thrust and fold belt1 commenced, and, together with widespread reactivation of western Lachlan Fold Belt faults, may represent the culmination of the effects of the Benambran Orogeny in western Victoria.
The evolution of eastern Victoria at this time is quite different to western Victoria, and must be seen in the context of prolonged horizontal displacement of eastern Victoria, travelling south with respect to the Gondwana landmass. During and following the Benambran Orogeny, large-scale faults subdivided eastern Victoria into a series of dislocated blocks (structural zones and subzones) that show a prolonged history of differential horizontal displacement during the remainder of Silurian and Early Devonian time. Crustal extension and horizontal displacement in the Late Silurian produced the Cowombat Rift, consisting of the Limestone Creek and Wombat Creek grabens. These grabens contain mainly shallow marine rhyolitic lava and high-level intrusions overlain by marine to subaerial volcaniclastic siltstone, sandstone and conglomerate below fine-grained turbidites with stratiform lenses of altered andesitic lava and limestone.
In the Melbourne Zone, shallow to deep marine sedimentation (Murrindindi Supergroup) continued without interruption through the Silurian and Early Devonian.
Early Devonian Bindian Orogeny (418–410 Ma)
At the end of the Silurian, rocks in the Cowombat Rift were faulted and tightly to isoclinally folded parallel to the rift margins, presumably controlled by the rigid rocks on the rift flanks. The volcanics were metamorphosed to lower greenschist facies and often were strongly cleaved. New fabrics overprinted Benambran fabrics in the Omeo Metamorphic Complex. The most significant structures formed during the Bindian Orogeny include the Kiewa, Kancoona, Ensay and Indi faults.
The deformation occurred in a regime of mainly strike-slip movement. Several hundred kilometres of displacement between the Benambra and Whitelaw terranes was taken up by inferred dextral strike-slip displacement along the Baragwanath Transform. Elsewhere in the western and central parts of the Benambra Terrane, the effects of the Bindian Orogeny were concentrated along smaller strike-slip and thrust faults that accommodated the internal fragmentation of the Benambra Terrane as it moved southward.
No effects of the Bindian Orogeny are recorded in the Whitelaw Terrane of central and western Victoria.
Early Devonian rifting, volcanism and sedimentation (410–385 Ma)
The Bindian Orogeny was followed by a second period of crustal extension that affected much of the eastern Lachlan Fold Belt. Bimodal (but dominantly felsic) volcanism occurred at several scales: in large riftlike basins where the volcanics are overlain by limestone, and in much smaller cauldrons.
The largest rift-like basins in eastern Victoria are the Buchan Rift and the Mitchell Basin, with smaller basins at Errinundra (the Boulder "graben") and Bindi (the Bindi Syncline). The thick but poorly dated Mount Tambo Group may also have been deposited at this time. Cauldrons formed at this time include the Mount Burrowa, Dartella, Besford, Mount Elizabeth and Mount Gelantipy.
In the Buchan Rift there were two stages of rifting producing a mixture of volcanics and minor non-volcanic sediments (Snowy River Volcanics). Prolonged erosion was followed by a final phase of silicic to mafic volcanism and lacustrine sedimentation. Most of the sequence is subaerial, but in the southern half rifting and subsidence outstripped deposition, giving an important interval of submarine turbidites and dark shale. Pyroclastics, especially ignimbrites, make up much of the sequence but most are relatively thin. Sedolithic conglomerate and sandstone occurs at many levels, showing that the rift was a persistent topographic low and that rifting continued through the entire basin history.
In the Melbourne Zone, sedimentation continued without break (Murrindindi Supergroup). Along its western margin, shoreline facies are present. These may be coeval with rare conglomerates preserved in the eastern Bendigo Zone. To the east, the sediments were mostly deep marine except on the Waratah Bay Platform on which very shallow marine limestone accumulated. Not until late in the Early Devonian is there any evidence of sediment supply from the cratons of eastern Victoria. This suggests that either there was a major barrier to the sediment coming west or the cratons were not in their present position with respect to the Melbourne Zone.
In western Victoria, numerous granites, mostly I-types, were intruded. On the Delamerian Craton, A-type volcanics (Rocklands Volcanics) were deposited over the already deformed and eroded Grampians Group.
Middle Devonian Tabberabberan Orogeny (385–380 Ma)
The Tabberabberan Orogeny was the first deformation that affected all parts of the Lachlan Fold Belt in Victoria. The orogeny marks the amalgamation of the Whitelaw and Benambra terranes and cessation of dextral strike-slip movement along the Baragwanath Transform. Its effects were most strongly felt in previously undeformed regions with thick sediment fills such as the Melbourne Zone, Mitchell Syncline, Boulder "graben" and Scrubby Creek Syncline. Deformation was also intense along the Governor Fault, the northern and eastern bounding structure to the Melbourne Zone. Within the Melbourne Zone, sedimentation ceased and the rocks were folded, generally into much more open patterns than the adjacent Bendigo and Tabberabbera zone rocks.
In the areas already substantially cratonised in the Benambran and Bindian orogenies, the Tabberabberan Orogeny mostly produced isolated brittle structures that had little effect on the overall structural framework. The Tabberabbera Zone is probably the only exception to this. More ductile structures, such as minor folds and cleavage, affected the previously deformed bedrock in the south of the zone and strong faulting occurred along its western margin during amalgamation with the Melbourne Zone. In the Kuark and Mallacoota zones the effects of deformation are poorly dated and it is possible that some ductile deformation occurred. Many older faults were reactivated, including the Kiewa, Kancoona, Indi and Ensay faults and the bounding faults to the Buchan Rift.
By the end of the Tabberabberan Orogeny, all of Victoria had been cratonised and the various crustal blocks were in more or less their present-day positions with respect to each other.
Late Devonian volcanism and sedimentation (380–350 Ma)
Volcanism and sedimentation began during the waning phases of the Tabberabberan Orogeny. Upper Devonian rocks are widespread in central Victoria and preserved in several large synclines farther east. The strong palaeogeographic differences that had existed in the various structural zones prior to the Tabberabberan Orogeny are no longer in evidence. They are thus relatively uniform across the fold belt and ignore structural zone boundaries. However, two quite different provinces were formed. The Central Victorian Magmatic Province, which extends from the western edge of the Bendigo Zone across the Melbourne Zone into the westernmost Tabberabbera Zone, consists of granite plutons and silicic cauldron complexes. The Howitt Province, along the margin between the Melbourne and Tabberabbera Zones, contains a thick sequence of red-bed fluvial sediments and silicic volcanics and was originally connected with the East Gippsland Province where fluvial sediments predominate.
Early Carboniferous Kanimblan Orogeny
The Upper Devonian – Lower Carboniferous red-bed sequences were folded and faulted in the Carboniferous—the age control on this event is very poor within Victoria and the deformation is, by convention, correlated with the Kanimblan "Orogeny" in New South Wales where it is considered to have occurred sometime in the Early Carboniferous. In Victoria the deformation was of somewhat different character in the two main outcrop belts, the Howitt and East Gippsland provinces. In the Howitt Province, Kanimblan structures have formed independently of the pre-existing structures and none of the Tabberabberan structures show Kanimblan offset, not even the Governor Fault. The probable reason is that Kanimblan structures originated as extensional structures during Late Devonian basin formation and were not able to employ pre-existing contractional structures. By contrast in the East Gippsland Province, Kanimblan faults are rejuvenated earlier structures, suggesting that there were no Late Devonian extensional structures available.
MINERALISATION OVERVIEW
This section provides a brief overview of the earth resources of the Tasman Fold Belt System in Victoria. Metallic mineralisation is subdivided into eight styles based on genesis and setting. Nonmetallics form a large part of the Palaeozoic earth resources of Victoria. The following summary is given in chronological order except for orogenic gold and magmatic– hydrothermal deposits which are grouped.
Cambrian tholeiite–boninite volcanism
Boninite is thought to be the source of metal for Victoria's main orogenic gold deposits, while sulphide-rich seafloor volcanic-associated mineralisation in tholeiitic sequences may have been critical in preconcentrating gold. The richest of these subeconomic deposits, Mount Ararat south of Stawell, contains 1 Mt @ 2.7% copper, 9 g/t silver and 0.6 g/t gold.
Cambrian calc-alkaline volcanism
A few base and precious metal deposits occur within calc-alkaline volcanic sequences—possible equivalents of Tasmania's Mount Read Volcanics. At present these are subeconomic. Preliminary studies indicate a spectrum of deposit styles ranging from seafloor volcanic-associated to magmatic–hydrothermal.
Cambrian Moralana Supergroup
Small gossans in the Moralana Supergroup containing silver–lead ± gold ± copper were worked in the late 1800s on Nolan Creek. Like many deposits in the Kanmantoo Group, mineralisation appears to have early stratabound elements related to lithological boundaries which have been modified by metamorphism and deformation.
Orogenic gold
Most of Victoria's gold has come from orogenic gold deposits in the Lachlan Fold Belt. Total production to June 1998 is about 2500 t. Mineralisation is chiefly associated with structurally controlled quartz veins in deformed lower Palaeozoic sedimentary rocks. Gold production is unevenly distributed and deposits tend to change in structural character and/or mineral assemblage from one zone to the next.
Three main periods of mineralisation have been defined by radiometric dating and geological relationships, broadly correlating with the late stages of the three orogenies—Benambran, Bindian and Tabberabberan. The Bindian Orogeny has not been recognised in western Victoria but mineralisation of this age is present.
Late Silurian Cowombat Rift
The Cowombat Rift contains a complex suite of metallic deposits. The most important are the Wilga and Currawong seafloor volcanic-associated deposits hosted by the Gibsons Folly Formation of the Limestone Creek Graben. Mining of the Wilga deposit produced 34 037 t copper, 16 894 t zinc and 10 136 kg silver between 1992 and 1995.
Early Devonian rifts, basins and cauldrons
Early Devonian extensional tectonic settings in the Benambra Terrane contain the Stateís most diverse suite of syngenetic and epigenetic deposits, although these are mostly small. Intermittent base and precious metal production commenced in 1865. The most important setting is the Buchan Rift where deposits show a strong relationship with extensional faults which appear to have acted as conduits for ore fluids.
Late Devonian Mansfield Group and Combyingbar Formation
The Upper Devonian Mansfield Group and Combyingbar Formation contain small amounts of red-bed copper, uranium, vanadium and silver concentrated at redox and pH boundaries.
Magmatism
While there is no current mining, modest amounts of tin, gold, molybdenum and tungsten have been mined from magmatic–hydrothermal deposits dating back to the late 1800s. Noteworthy molybdenum deposits occur in all zones except the Deddick Zone and the Central Victorian Magmatic Province. Major tin ± gold-bearing granites and dykes are confined to the Tabberabbera and Omeo zones. Granites with tungsten ± molybdenum ± tin mineralisation are widespread, occurring in all Lachlan Fold Belt basement terranes. Small amounts of gold were mined from porphyry deposits in Boggy Plain Supersuite granite in the Benambra Terrane, while porphyry copper ± molybdenum deposits fringing Lower Devonian extensional settings are subeconomic. Granite mineralisation is strongly linked with magma chemistry; and hence composition of the crust and tectonic setting. Minor gold–antimony mineralisation accompanies Middle to Late Devonian high-level magmatism.

last updated: Sunday, 15 October 2006