Granite-related Ore Systems - Abstract 2

The Metallogenic Potential of Australian Proterozoic Granites, Record 2001/12

Lesley A.I. Wyborn, Anthony R. Budd, Irina V. Bastrakova

• Download PDF
• Australian Proterozoic Granite-related Ore Systems
• Key Results by Granite Type
• Key Results by Region
• Related links

Download PDF

• Download this file as a PDF [14k]

[back to top]

Australian Proterozoic Granite-related Ore Systems

AGSO, in collaboration with Commonwealth, State and Territory Geoscience organisations, has completed a project on the ‘Metallogeny of Australian Proterozoic Granites’ (which was sponsored by 20 mineral companies). Using GIS techniques, the project investigated spatial relationships between specific granite types, host rock compositions and hydrothermal Au, Cu, Zn, Pb, Sn, W and Mo mineralisation. Data were collated on the mineralogy, geochemistry (~7500 analyses), and age of Proterozoic granites and felsic volcanics, as well as the age and mineralogical composition of sediments within 5 kms of granite boundaries for 20 provinces.

[back to top]

Key Results by Granite Type

Overall although there is a strong spatial relationship with specific granite types for many commodities, much of the mineralisation is hosted in the country rocks and particularly for Au, up to 5 kms from the nearest granite contact. I-(granodioritic) compositions dominate and these are inevitably Sr-depleted and Y-undepleted which contrasts with igneous rocks normally found associated with porphyry-style mineralisation. Some significant differences were found between Proterozoic granites and their Archaean and Phanerozoic counterparts (e.g., Proterozoic granites have elevated K, Th U, and High Field Strength Elements contents). Hence classifications for A- and S-types, and some metallogenic indicators developed from Palaeozoic granites were not meaningful. The mineralogy of the country rock was found to play a crucial role not only in determining the site of mineralisation, but also in affecting the redox state of the granite. More specific relationships are as follows:

• I or S-type granites designated as unfractionated were restite-rich and consistently unmineralised.
• Fractionated I-type granites can be divided into 2 groups: those that were either F-poor or F-rich throughout most of their fractionation history. The F-rich granites are often the true rapakivi types and have little mineralisation presumably because Cl had been partitioned into the granite melt early.
• Fractionated F-poor I-type granites can be further divided into 2 classes: Oxidised and Reduced. The Oxidised granites are most commonly associated with Cu-Au deposits and crystallised at higher temperatures than the Reduced class. Although it is commonly stated that Au mineralisation is only associated with oxidised granites, an unequivocal spatial association occurs between the Reduced class and  Au-dominant mineralisation which also has variable amounts of Cu, Sn, W, and Bi. Reduction is due to several causes including increasing ASI, interaction with country rock and perhaps magma cooling.
• Rare fractionated S-types are commonly associated with Sn mineralisation.

[back to top]

Key Results by Region

Mount Isa: The most prospective areas for Cu-Au are those surrounding the Williams Supersuite. The Burstall Suite in the eastern Wonga Belt has some Cu-Au potential including potential epithermal style deposits.

Georgetown: The Esmeralda Supersuite has strong similarities to the Hiltaba Supersuite of the Gawler Craton.

Pine Creek: The Allia Suite and Cullen Supersuite have proven potential: other suites are not highly rated.

Kimberleys: Major suites are essentially unfractionated (restite-rich?) and are not considered prospective. The small Koongie Park Suite (related to the Koongie Park VHMS mineralisation) shows evidence of fractionation.

Granites-Tanami: Although data are limited, many granites are reduced, but some become oxidised at shallower level of emplacement: the two contrasting types may require different conditions for precipitation.

Tennant Creek/Davenport: Au is most likely to be related to the 1820 Ma Treasure Suite.

Gawler Craton: The Hiltaba Supersuite comprises the oxidised Roxby Downs Suite which has Cu-Au-U potential and the more widespread, reduced Kokatha Suite which is associated with Au_Sn mineralisation.

Paterson: Granites closest to the Telfer deposit are reduced, in contrast to the oxidised Mount Crofton Granite.

Gascoyne: 1800 Ma granite suites are fractionated, although the metal potential may be limited by the high metamorphic grade of many hosts. Prospective targets are likely to be small and focussed as at Tennant Creek.

Arunta: Many suites present: the most prospective are the 1710 Ma Alarinjela and Barrow Creek Suites, the 1640 Ma Mount Webb Suite, and the 1567 Ma Southwark Suite. High metamorphic grade may be a limitation.

Broken Hill: Dominated by restite-S-types that are unlikely to be related to the Broken Hill Pb-Zn-Ag deposit.

Olary: If the 1590 Ma suites are true analogues of the Hiltaba Supersuite, then the potential for Cu/Au is high, although by analogy with the Cloncurry district the presence of calc-silicate rocks may cause limitations.

Northhampton, Rocky Cape, Albany Fraser, Leewin, Musgrave: Insufficient data available for confident predictions of metallogenic potential.

[back to top]

Related links

• The Metallogenic Potential of Australian Proterozoic Granites
• Original Proposal for Project
• Project Methodology