Geoprovince-Scale Assessment of Mineral Potential

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Methodology

Introduction

A national-geoprovince-scale (1:2.5 million) assessment of mineral potential was carried out by Geoscience Australia (GA) for 17 significant types of mineral deposits of gold, uranium and base metals (copper, lead, zinc, nickel, cobalt). Broadscale regional geological criteria were used in the assessment to determine the favourability of selected geoprovinces for the presence of specific types of undiscovered mineral deposits.

The 17 types of deposits for which the mineral potential was assessed are:

The geoprovinces are defined on Geoscience Australia's 1995 1:2.5 million scale national geotectonic province map based on W.D.Palfreyman's BMR Bulletin 181 and on the map of major Structural Elements (D'Addario G.W., Palfreyman W.D. & Bultitude J.M., 1979 in BMR Earth Science Atlas of Australia). Boundaries of subdomains of these geoprovinces are derived from more recent datasets defined by past and current Geoscience Australia regional projects. The national framework of geoprovince boundaries will need to be updated with more recent information as time permits.

The assessments incorporate available geoscientific information and data on mineral potential from the States and the Northern Territory geological agencies.

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Methodology

The methodology used is a GIS-based (OZPOT GIS) qualitative assessment of mineral potential. The stages of assessment include:

  • Compilation of the latest geology, mineral occurrences/deposits for the geoprovince being assessed
  • Determination of styles of mineral deposits likely to be present in the area including
    • Types of mineral deposits known to exist in the geoprovince, and
    • Types of deposits not known to be present but for which assessment criteria indicate that a favourable environment exists in the geoprovince
  • Preparation of descriptive mineral deposit models incorporating minerals systems criteria for source of elements/fluid pathways/traps
    • The mineral deposit models used are derived from those developed by Cox and Singer (1986) and have been modified and adapted for Australian conditions
  • Definition of geoprovince-scale assessment criteria for each of the types of deposits being assessed.

Geoprovince-scale assessment criteria are compiled in a matrix format, which is used for the assessment of mineral potential for that particular style of deposit. An example of assessment matrix for porphyry style copper-molybdenum-gold deposit is shown in the table below.

Table 1: Assessment matrix used for the porphyry style copper-molybdenum-gold deposit style

Province:

Potential:

Certainty:

Critical Elements (Assessment Criteria)

Identified

Not Identified, but likely

Unlikely

Weighting

Setting:

  • Convergent plate margin: island arc and/or continental arc environment
  • Magmatism related to rifting/extension in back arc basins
       

Source (fluid, metal, energy):

  • I-type, oxidised calc-alkaline to alkaline intrusives (For Copper, Molybdenum and Gold, and fluid as well as energy)
       

Fluid pathway:

  • Structures along and across the arcs important for the emplacement of volcanic-plutoniccomplexes
  • Structures associated with stratovolcanoes and volcanic- plutonic complexes important as fluid pathways
       

Trap (any of the following; predominantly structural):

  • Calderas and caldera-related structures
  • Breccia pipes and diatremes
  • Faults and shear zones
       

Signs of mineralising process (any of the following, but if occurrences have been identified the level of certainty increases):

  • Wall rock alteration (any of the following):
    • Propylitic
    • Potassic
    • Phyllic
    • Argillic
  • Geochemical anomalies
  • Geophysical: 'bulls-eye' type of magnetic anomaly
  • Known occurrences porphyry copper ± molybdenum ± gold
       

Preservation (any of the following but presence of sinter increases certainty because it is a more reliable criteria for assessing preservation):

  • Age: Mesozoic to Tertiary dominant although deposits known throughout Phanerozoic. Only a few reported in Proterozoic and Archaean
  • Presence of epithermal gold mineralisation could be indicative of preserved nature of the porphyry system
       
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Mineral Potential

The mineral potential for a given geoprovince is the likelihood that specific types of mineral deposits are present in a geoprovince and may be discovered with ongoing exploration.

The level of potential refers to a specific type of mineral deposit. The same geoprovince can have a high potential for one type of mineral potential (eg porphyry style copper gold) and a low level potential for another type of deposit (eg volcanic hosted massive sulphides).

The level of mineral potential for each style of mineral deposit is assessed using the criteria outlined in the matrix, in this case for porphyry copper-molybdenum-gold deposits. The level of mineral potential is determined by the extent to which assessment criteria is met in the geoprovince being assessed and is scored on a scale of 1 (low) to 5 (high).

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Certainty

The amount of information available on the assessment criteria determines the certainty of the assessment and is expressed as a score on a scale of 1 (low) to 5 (high). If there is insufficient information to determine the level of potential for a particular type of mineral deposit then the level of mineral for that type of mineral deposit in the geoprovince being assessed is unknown. The lack of information on the potential for a type of mineral deposit in a given geoprovince cannot be used as a basis for labelling the geoprovince as having low mineral potential for that particular type of deposit. Blank areas outside the assessed geoprovinces are either;

  • those where the known geology rules out the possibility of a particular type of deposit being present, that is the geoprovince is 'not permissive' for that particular type of deposit, or
  • where the mineral potential for that type of deposit is unknown because of insufficient data.

The mineral potential of geoprovinces for individual deposit styles can be combined to produce a composite mineral potential map.

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Composite mineral potential map

The composite mineral potential map displays the highest level of mineral potential out of all the significant types of deposits assessed for a particular geoprovince/subprovince in Australia. Mineral potential assessments were made for 15 significant types of mineral deposits for selected geoprovinces/subprovinces. Where a particular geoprovince/subprovince contains potential for several types of mineral deposits, the geoprovince is assigned the highest level of potential of all the types of deposits assessed for the geoprovince. In this approach, the deposit type with the highest level of potential obscures the lower levels of potential for the other types of deposits in the geoprovince/subprovince. The levels of potential for different types of deposits combined in this way form a composite mineral potential map.

The mineral potential maps/GIS layer shows levels of mineral potential and certainty for specific types of mineral deposits for a given geoprovince and the layer would be linked to:

  • Mineral systems based deposit models,
  • Names and locations of major deposits,
  • World-wide or local grade and tonnage data for deposit styles, and to
  • More detailed assessments of mineral potential if available.
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References

D'Addario G.W., Palfreyman W.D. & Bultitude J.M., 1979. Major Structural Elements. (Map) In: BMR Earth Science Atlas of Australia. Bureau of Mineral Resources, Australia, Earth Science Atlas Series.

Cox, D.P. & Singer, D.A. 1986. Mineral deposit models. U.S. Geological Survey Bulletin 1693.

Palfreyman, W.D. 1984. Guide to the geology of Australia. Bureau of Mineral Resources Bulletin 181.

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Gold Potential

Epithermal gold-silver

Examples of world-class epithermal gold-silver deposits are the Round Mountain in the United States, Hishikari in Japan and the Ladolam in Papua New Guinea. A substantial amount of the world's gold production is derived from epithermal gold-silver deposits although only a minor proportion of Australia's gold production comes from epithermal deposits. Australian examples of epithermal gold deposits are Pajingo and Yandan in Queensland.

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Iron oxide copper-gold-uranium (Olympic Dam style)

Olympic Dam is the largest copper-gold-uranium deposit of this type and the largest single known uranium resource in the world. Olympic Dam is considered to belong to a broad suite of iron oxide copper-gold deposits but very few of the other deposits in this suite carry significant uranium resources. Other, much smaller examples are Prominent Hill, Wirrda Well and Acropolis, all of which are in South Australia.

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Orogenic lode gold

World-class examples of lode gold deposits are the giant Muruntau deposit in Uzbekistan, Hollinger-McIntyre-Moneta in Canada, and the Golden Mile at Kalgoorlie in Western Australia. The Bendigo and Ballarat regions in Victoria are other important Australian examples of lode gold deposits. A very substantial proportion of the world's gold production and most of Australia's is derived from lode gold deposits, particularly from the Yilgarn region in Western Australia.

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Porphyry copper-gold

This is an important class of copper and copper-gold deposits. World-class examples are the giant deposits of Chiquicamata and Elteniente in Chile, and Grassberg in Indonesia. Australian examples are much smaller and include Cadia in New South Wales.
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Iron oxide hosted copper-gold-iron (Tennant Creek style)

Significant amounts of gold production has come from this type of deposit in Australia with the deposits in the Northern Territory's Tennant Creek area yielding bismuth as well.
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Base Metals Potential

Felsic extrusive volcanic associated massive sulphide copper-lead-zinc-silver-gold (Kuroko style)

A significant amount of the world's and Australian base metal production is sourced from felsic extrusive volcanic associated massive sulphide (VAMS) base metal deposits. World-class examples of felsic extrusive VAMS deposits are the Kidd Creek in Canada and the Rosebery and Hellyer deposits in western Tasmania.

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Lateritic/saprolitic nickel-cobalt-iron

A major proportion of the worlds nickel resources are held in lateritic nickel deposits in New Caledonia South America and Australia but most of the nickel production comes from other types of nickel deposits. The lateritic nickel deposits form from weathering of olivine-rich ultramafic rocks. One of the very large deposits of this type is the Goro deposit in New Caledonia. Notable Australian examples are Murrin Murrin, Cawse and Bulong in Western Australia.

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Layered mafic-ultramafic intrusive basal nickel-copper-cobalt (Voisey's Bay style)

The Voisey's Bay nickel-copper-cobalt world-class deposit in Northern Labrador is an example of a basal segregation of nickel and copper sulphides at the base or in feeder conduits of small to medium sized mafic-ultramafic intrusions. These deposits are an important source of nickel as well as copper and cobalt. Platinum group elements are present in minor amounts. Australian examples are much smaller but still significant, represented by Sally Malay in the Halls Creek Orogen and Radio Hill in the Pilbara.

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Layered mafic-ultramafic stratabound platinum group elements-nickel-copper (Bushveld style)

The world-class example of this deposit type is the giant Bushveldt intrusion with its Merensky Reef deposit in South Africa. This deposit type is a major source for platinum group elements (platinum, palladium, rhodium) and lesser amounts of nickel and copper. Subtypes of this style of deposit are important sources of chromium, vanadium and titanium. Australian examples are much smaller and include Munni Munni in Pilbara and Panton in the Halls Creek Orogen. The Windimurra deposit in the Yilgarn is an important vanadium deposit.

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Mafic extrusive volcanic associated massive sulphide copper-zinc-silver-gold-cobalt (Besshi style)

World-class examples of mafic extrusive VAMS base metal deposits are the Windy Craggy deposit in Canada and the Besshi in Japan. Only minor deposits and occurrences of this type are known in several regions in Australia, although some geoprovinces in Australia have potential for this type of deposit. The Besshi type VAMS are only a minor source of base metals and there is no production from this type of deposit in Australia.

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Volcanic associated massive sulphide copper-zinc-silver-gold-cobalt (Cyprus style)

Only a minor proportion of the world base metal production is sourced from Cyprus style mafic extrusive VAMS deposits and Australian production from this type of deposit is not significant. World-class examples include the Mavrovouni in Cyprus and Hidden Creek and Chu Chua in Canada. Some small deposits of this type in Australia include Gulf Creek, Mount Browne and Cangai in New South Wales.

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Mafic extrusive flood basalt nickel-copper-platinum group elements

The giant Noril'sk nickel-copper-platinum-palladium deposit in the Taimyr and Kola peninsulas in Russia's polar region is the only known world class deposit of this type. This deposit alone accounts for a major share of the world's palladium and nickel production and is a significant producer of cobalt and copper. The deposits are associated with subvolcanic tholeiitic sills representing feeders to vast basaltic volcanism. There are no known examples of this type of deposit in Australia but there are several extensive regions of flood basalts in Australia which may host Noril'sk type deposits.

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Sediment hosted copper

The classic regions for the sediment hosted copper deposits and one of the major sources of copper are the very large deposits in the 'Kupferschiefer' belt in Germany and Poland, and in the central African Copperbelt in Zaire and Zambia. Relatively small examples of this type in Australia are the Nifty and Mt Gunson deposits, although there is potential in a number of sedimentary basins in Australia for discoveries of this type of deposit.

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Sediment hosted lead-zinc-silver

Australian examples of world-class deposits of this type include well known lead-zinc-silver deposits of Mt Isa, Century, McArthur River and the largely mined out deposit of Broken Hill. The Howards Pass and the Sullivan deposits are overseas world-class examples of this type of deposit in Canada. These types of deposits are major sources of lead and zinc.

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Ultramafic extrusive komatiite nickel-copper-platinum group elements

This type of deposit provides the major source of Australia's nickel production from numerous small and medium sized high grade deposits (Kambalda, Agnew) and a few large low grade deposits (Mt Keith) that are associated with komatiitic lavas in the Yilgarn region of Western Australia. Major commodities are nickel and copper with minor amounts of platinum, palladium, gold and cobalt.

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Unconformity-related uranium

About 20% of the world's current uranium resources and past uranium production is made up of the unconformity uranium deposits. These deposits are the most important producers of uranium today and all of the large unconformity deposits (plus 50,000t U3O8) are located in Australia and Canada. The Australian deposits are generally low grade with less than 1% U3O8 whereas the grades of some of the Canadian deposits exceed 25% U3O8.

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Sandstone uranium

A major proportion of past uranium production has come from the sandstone type uranium deposits, particularly from the United States. These deposits are generally low grade with less than 1% U3O8 with tonnages rarely exceeding 20,000t U3O8. About 5% of Australia's uranium resources are in the sandstone type uranium deposits, mainly in South Australia.

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