Abstract of "The metallogenic potential of Australian proterozoic granites"

Record 2001/12

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

Record abstract

This Geoscience Australia Record 2001/12 is the result of ‘The Metallogeny of Australian Proterozoic Granites’ project, undertaken by the then Australian Geological Survey Organisation in collaboration with 20 minerals exploration companies. The project was a data driven exercise that asked the simple question:

‘Where hydrothermal Au, Cu, Zn, Pb, Sn, W, and Mo mineralisation occurs within 5 kms of the boundaries of Proterozoic Granites, are there any specific characteristics of either the granites and/or their host rocks?’

The project compiled data 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 Proterozoic granite boundaries for 20 provinces. The project investigated a spatial association only, and hence it was not considered of major importance if the metals came from the granite or had been leached from the country rock by processes related to granite emplacement. Further, the compilation made every effort to focus on factual data. As the determination of the tectonic setting operating at the time of emplacement of the granites, particularly in rocks as old as Proterozoic, was considered to be highly interpretative, this parameter was not included, due to the data-driven approach of the project.

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Approach

Having compiled the data, a forensic data mining approach was applied. That is, rather than developing a computer derived ‘best-fit model’ from the data, or taking an existing model and finding areas that matched the prescribed model, a more descriptive approach was tried. Our goal was to investigate the spatial relationship to gain understanding by uncovering patterns and relationships. Nine major associations were identified based on granite type and spatially related mineralisation.

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Results

Our results showed that, overall, there is a strong spatial relationship with specific granite types for many commodities. 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.

An unexpected pattern emerged within the five larger I-type associations. The lower temperature granites were always early in the evolution of a province and that with time the temperatures of the magmas increased. As the magma temperatures increased, a pattern emerged of a progression of early barren granites (e.g. the restite-rich Kalkadoon Association) followed by the weakly mineralised Nicholson Association where late vein mineralisation was associated with late fractionationation over a narrow range of silica content (~72-77 wt %). The next association was the predominantly Au ± base metal endowed Cullen Association. This suite is low in Ca and is believed to have been formed by breakdown of biotite in the source region. The Sybella Association generally followed next and is characterised by high concentrations of high field strength elements and fluorine. This association is believed to be formed by dehydration melting of hornblende- and biotite-bearing tonalites and granodiorites at very shallow depths (P ≤ 4 kbar), and is only weakly mineralised. The Hiltaba Association was the generally youngest major Itype association in this progression. It is characterised by spatially related major Au-Cu mineralisation and was formed at temperatures of ~1000°C by breakdown of amphibole in the source region.

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