Mineralisation and Prospectivity
Historical Overview - Victoria’s Mineral Endowment
Discoveries of gold at Clunes, Castlemaine and Warrandyte in mid-1851 marked the first recognition of Victoria’s earth resources and significantly influenced the social and political evolution of the new colony. Gold raised the perceived importance of the geological sciences and led to the establishment of the Geological Survey of Victoria. The ensuing gold rush over the next 60 years and production of approximately 2500 t of gold places Victoria amongst the world’s major gold provinces. Total Victorian gold production still represents 32% of all gold mined in Australia, and 2% of all gold mined in the world. About 40% of gold production, as well as small quantities of antimony and silver, came from Palaeozoic quartz veins, while the remainder came from Cainozoic placer deposits.
Brown coal resources in the Latrobe Valley are amongst the largest in the world. They have provided most of the feedstock for Victoria’s electricity generation industry since the 1920s and are the basis for much of the State’s industrial development. Likewise, the major discoveries of Cainozoic oil and natural gas in Bass Strait in the 1960s have provided a large part of Australia’s oil requirements and natural gas for Victoria’s domestic and industrial markets. Downstream industries such as petrochemicals have been established in the State as a direct result of the ready availability of oil and gas supplies.
Gold mineralisation in Victoria is chiefly associated with structurally controlled quartz veins in deformed lower Palaeozoic sedimentary rocks. The bulk of gold production has been from the west of the state across the Stawell, Bendigo and Melbourne Zones. The most prospective gold mining and exploration targets occur in these areas and are all structurally controlled. They include:
- slate belt/turbidite hosted lode gold (e.g. Bendigo, Ballarat),
- volcanic associated gold (e.g. Stawell), and
- sedimentary hosted disseminated gold (e.g. Fosterville).
Bendigo goldfield. Bedding-parallel quartz vein in Castlemaine Group.
Bendigo is the largest goldfield in Victoria with orogenic or ‘reef’ mines producing 17 million ounces (529 t) of gold and placer mines producing 5 million ounces (155 t) of gold. It has a strike length of approximately 17 km, is up to 4 km wide and is remarkable for its regular trains of chevron folds that have controlled the distribution and geometry of mineralisation. Most gold came from the hinge zones and east-dipping limbs of three anticlines, the Garden Gully, New Chum and Hustlers ‘lines’, however, significant gold was also mined from other adjacent anticlines. Domal culminations along the main anticlines tend to correspond with well-mineralised portions of the folds. Mineralisation is mostly in west- and east-dipping reverse faults which truncate fold hinges. The famous saddle reefs are composite fault structures controlled by thick, competent sandstone beds. They range from simply folded bedding-concordant veins through to complex structures involving the interaction of limb thrusts, fold hinges and bedding-concordant veins.
The gold endowment of the Bendigo Goldfield has attracted numerous mining companies to the area. Exploration programs aimed at discovering similar styles of mineralisation have focused on either extensions to known mineralisation at depth or along strike to the north beneath shallow transported cover of the MurrayBasin. Bendigo Mining have been successful in identifying repetitions of the rich Deborah and Sheepshead lines at depth and are currently in development. To date limited exploration under cover has failed to locate any significant new gold occurrences. The use of geophysical and sophisticated geochemical techniques are now viewed as essential for locating another Bendigo-style deposit. Such techniques might be used to target favourable structural settings or zones of alteration.
The Ballarat Goldfield is the State's second largest goldfield with a historic production of approximately 3.2 million ounces (98 t) of orogenic gold and 7.8 million ounces (244 t) of placer gold. Orogenic gold mineralisation is controlled by stacked west-dipping reverse faults that generally follow the western limb of regional anticlines defining narrow mineralised corridors. Mineralisation occurs in poorly defined faults along west dipping anticlinal limbs and changes to brecciated quartz bodies with sub-horizontal extension veins (known as leather jackets) when traversing fold hinges and east-dipping limbs.
At Ballarat there are three regional anticlinoria each of which coincide with a goldfield-Ballarat East, Ballarat West and Little Bendigo. The most productive line, the First Chance Anticline at Ballarat East, has in recent times been the focus of several mining developments both at surface and at depth. In the mid 1990s William Australia NL mined several small near surface supergene deposits for a total of 346 kg of gold. Ballarat Goldfields NL defined an inferred resource of 3.3 Mt at 9.5 g/t gold beneath old workings to depths between 350-700 m. An access decline was commenced and had traversed 1 km (140 m deep) prior to operations being suspended in early 2000. Exploration potential exists for discovering new orogenic gold mineralisation beneath historic production centres and beneath shallow Cainozoic basaltic cover to the north and south of Ballarat.
The bulk of recent Victorian gold production has come from the Stawell Goldfield, the source of 2.5 million ounces (78 t) of orogenic gold and 0.7 million ounces (24 t) of placer gold with reserves in the order of 4.6 million ounces. The main (Magdala-style) mineralisation is in ductile–brittle reverse shear zones related to movement on the Stawell Fault. Mineralisation at the Magdala deposit has been divided into two systems:
- Central Lode system where gold is concentrated in high-strain zones at the contact between competent Magdala Volcanics (tholeiitic basalt lava) and less competent metasedimentary rocks, and
- Scotchmans (or Hangingwall) lodes which overprint the Central Lode along master faults and duplex faults defined by laminated reefs and flat ‘makes’.
Mineralised Central Lode shear zone.Magdala ore body, Stawell.
The subordinate Wonga deposit is located within the aureole of the Early Devonian Stawell Granite. Here mineralisation is controlled by east-dipping reverse shears within more massive quartz reefs. The orientation and timing of the Wonga lodes is now believed to relate to a syn-intrusive phase of mineralisation.
Important elements in a simplified exploration model of the Magdala mineralisation include:
- Tholeiitic basalt domes,
- Sulphidic volcanogenic rocks,
- D4 fault structures (northwest trending, west-dipping reverse faults), and
- Pyrrhotite-stilpnomelane-biotite alteration.
The close relationship between tholeiitic basalt domes and Magdala-style gold mineralisation has enabled Stawell Gold Mines to target similar styles of mineralisation up to 100 km north along the Stawell Corridor (the area bounded by the Moyston and Coongee Faults) using geophysics. Detailed magnetics combined with follow-up aircore drilling has been successfully utilised in exploring for similar gold targets beneath shallow Murray Basin cover sediments.
Mineralisation at Fosterville is distinct from Bendigo and Stawell style mineralisation in that gold is hosted within sulphides disseminated in sandstone. Although not a major producer by Victorian standards, the bulk of the 0.25 million ounces (8 t) of gold production from the goldfield has occurred from recent post-1982 discoveries and highlights the potential of smaller ‘forgotten fields’ for the modern explorer.
At Fosterville fine grained gold occurs as inclusions in arsenopyrite and pyrite closely associated with unmineralised quartz–carbonate stockwork veins. The stockwork veins have developed along a steep west-dipping reverse fault set which has interacted with fold hinges and favourable rock types to produce dilatant structures surrounded by mineralisation.
Disseminated sulphide ore - Fosterville mine.Field of view 6 cm.
Two main fault lines are recognised - Fosterville Fault, mineralised over 7 km, and O’Dwyers Fault mineralised over 3 km. Recent work on the controls of mineralisation have emphasised the importance of:
- Steeply west-dipping rock sequence containing black shale on the eastern limb of a syncline;
- Bedding parallel faulting (Fosterville Fault) along the black shale unit;
- Splay faults which propagate into the footwall across the smaller synclinal fold hinge;
- Vertical repetitions of the splays at depth; and
- Lithological contrast between sandstones which provide a porous brittle host and siltstones which restrict or cap the mineralising fluid.
The position of the Fosterville and O’Dwyers Faults (the latter intruded by rhyolite dykes) can be traced using bedrock geochemistry. This approach has enabled the faults to be located both at surface and beneath cover to the north and south. Threshold values of 100 ppb gold and 100 ppm arsenic currently outline known deposits with antimony also used as a pathfinder element. Induced polarisation surveys have also been used to define the position of the faults and associated sulphide concentrations. Potential exists for discoveries both along strike under cover and in parallel structures to the east and west.
Brown coal mining in the Latrobe Valley
In Victoria economic quantities of brown coal have accumulated in slowly subsiding Cainozoic basins. The largest deposits are located in the east of the state within the Latrobe Depression of the Gippsland Basin, although smaller deposits have also been worked in the southwest.
The Latrobe Valley contains some 160 000 Mt of brown coal of which approximately 35 000 Mt has been estimated as economically winnable. The region annually produces more than 60 Mt of coal for conversion to briquettes, char and electricity within a privatised industry. Although the Latrobe Valley brown coal is considered relatively clean with low ash, low sulphur and low trace elements, it’s high moisture content (48 to 70%) and consequently low specific energy, makes the coal a low-grade fuel.
The Victorian Government is actively encouraging the economic and environmentally viable utilisation of brown coal resources in the Latrobe Valley. Elsewhere in the state potential exists for defining additional economic brown coal resources in the Otway Basin at Anglesea, Bacchus Marsh and Altona.
Oil and Gas
Refer to Victorian Oil and Gas Overview.
Industrial Minerals and Rocks
Industrial minerals and rocks represent about one third of the non oil and gas mineral resource wealth of Victoria. The sector is dominated by construction materials, which form the bulk of production and are the greatest revenue earner. They are essentially low value materials, such as sand, gravel and crushed rock, and are consumed by markets in fairly close proximity to their sources. The specialised industrial minerals and rocks that have been extracted continuously in Victoria in recent years are limestone, silica sand, gypsum, feldspar, kaolin, dimension stone and mineral sands. These materials support important manufacturing industries, including the producers of ceramics, glass, lime, fillers and cut and polished stone.
Major resources of semi plastic kaolin clays derived from the weathering of granites are mined in the Ballarat area of western Victoria and used in paper filling and coating, in ceramics, and as filler in plastics and rubber. Highly plastic kaolin clays, or ball clays, of alluvial origin are mined from several scattered localities and used in ceramics, whiteware and tableware.
Silica sand resources are widespread and are particularly abundant to the south of the highlands. Apart from their use in construction, filtering and drainage, processed quartz sands and to a minor extent pebbles are used in a variety of industries, including the manufacture of glass, fibreglass, foundry moulds, ceramic products, whitewares, refractories, and as fillers in adhesives, paints, grouts and plastics.
Photo: David Mitchell limestone quarry at Lilydale.
Limestone resources with a wide range in grade are abundant. Relatively low grade Cainozoic limestones of southern Victoria are used mainly in cement manufacture, agriculture and road and track construction. Major, higher grade Devonian limestone deposits are confined to the eastern part of the State and are used in the manufacture of quicklime and hydrated lime, in paper manufacture, as flux stone for steel making and foundry use, and in ground and milled form for agriculture, stock feeds and industrial fillers.
Most of the dimension stone quarried in Victoria is either granite or basalt obtained from a small number of sites. The processing of dimension stone is a significant industry and there are good opportunities for sourcing a wider variety of rock types within the State to supply local plants.
Other minerals of lesser economic importance to the State at present are gypsum (now mainly used for agricultural purposes), bauxite and peat.
The Cainozoic Murray Basin straddles the northwest of Victoria and extends northwards into New South Wales and westwards into South Australia. The presence of heavy mineral sands in the Murray Basin was first noted by the Geological Survey of Victoria in the 1960s, however, it wasn’t until the discovery by CRA Exploration in the early 1980s of giant, fine grained sheet-like deposits (known locally as WIM-style) that the Basin underwent an exploration boom. This exploration phase saw the discovery of numerous WIM-style (4900 Mt at 2.8% heavy minerals) and occasional coarser strandline-style deposits. Despite this exploration success, none of the deposits underwent development principally due to unfavourable metallurgy and a drop in the price of rutile and zircon in 1990. In the mid 1990s, coinciding with an increase in heavy mineral price and the release of new airborne geophysics by the Geological Survey of Victoria, a second exploration boom in the Murray Basin occurred, this time focusing on coarser grained strandline deposits.
Mineral sands concentrator, Wemen.
Since then approximately 200 heavy mineral sand deposits containing more than 80 Mt of coarse grained ilmenite, rutile and zircon have been discovered. Primary strandline deposits occur within a Late Miocene - Pliocene marine sand sequence (Loxton-Parilla Sands). The beach deposits are sub-parallel and range from 200 to 1000 m in width, up to 10 km in length and typically contain 2 to 4% heavy minerals with 15 to 30% rutile, 8 to 24% zircon and 40 to 60% ilmenite. The first mine was established by Murray Basin Titanium at Wemen in 2001 with Iluka’s Douglas project planned to start production in 2004.
Exploration for mineral sands in the Murray Basin has centred on near surface occurrences of the Loxton-Parilla Sand, often preserved as strandlines, and visible as subtle linear topographic highs. Regionally, the identification of favourable trap sites or target areas has involved:
- The use of 200-400 m line spaced regional airborne magnetic data (as some deposits are variably magnetic);
- The use of detailed digital terrain models to highlight the position of strandlines; and
- Basement modelling to broadly identify the location of palaeo-topographic highs and growth faults, which are believed to provide a focus for mineral sand accumulations.
Follow-up, close spaced shallow drilling traverses are typically used to test such targets.