The role of magma sources, oxidation states and fractionation in determining the granite metallogeny of eastern Australia

Abstract

The ore-element associations of granite-related ore deposits in the eastern Australian Palaeozoic fold belts can be related to the inferred relative oxidation state, halogen content and degree of fractional crystallisation within the associated granite suites. Sn mineralisation is associated with both S- and I-type granites that are reduced and have undergone fractional crystallisation. Cu and Au are associated with magnetite- and/or sphene-bearing, oxidised, intermediate I-type suites. Mo is associated with similar granites that are more fractionated and oxidised. W is associated with a variety of granite types and shows little dependence on inferred magma redox state. The observed ore deposit-granite type distribution in eastern Australia, and the behaviour of ore elements during fractionation, is consistent with models of ore element sequestering by sulphides and Fe-Ti phases (e.g. pyrrhotite, ilmenite, sphene, magnetite) whose stability is nominally fO2-dependent. Fractional crystallisation acts to amplify this process through the progressive removal of compatible elements and the concentration of incompatible elements into decreasing melt volumes. The halogen content is also important. S-type granites are poorer in Cl than I-types. Cl decreases and F increases in both S- and I-type granites with fractional crystallisation. Low Cl contents combined with low magma fO2 in themselves seem to provide an adequate explanation for the rarity of Mo, Cu, Pb and Zn type mineralisation with S-type granites. Although such properties of granite suites seem adequately to predict the associated ore-element assemblage to be expected in associated mineral deposits, additional factors determine whether or not there is associated economic mineralisation.

Intrusive metallogenic provinces in eastern Australia based on granite source and composition

“…The regional petrographic and compositional similarities among granites in the Lachlan Fold Belt (LFB) that define nine distinctive petrographic provinces were correlated by with basement terranes, the boundaries between which generally correlate poorly with any tectonostratigraphic boundaries based on geological features exposed at the surface. The distribution of those granite basement terranes was subsequently shown by Blevin and Chappell (1992) to be strongly correlated with the distribution of granite-related mineralisation within the LFB, suggesting that strong genetic links exist between granite source compositions and the ore element ratios of associated mineralisation. If magmatic compositions and processes determine ore element ratios in granite-related mineralisation, then the distribution of those source regions will be one of several important controls on the distribution of such mineralisation.…”

Source and redox controls on metallogenic variations in intrusion-related ore systems, Tombstone-Tungsten Belt, Yukon Territory, Canada

“…The oxygen fugacity of a granitic magma can reflect the redox state of the source region, as well as influence the processes of metal enrichment in the magma (Ishihara 1981;Candela 1989;Carmichael 1991;Blevin & Chappell 1992). Several features Plot of E Nd versus initial Sr ratio for Tombstone, Mayo and Tungsten plutonic suites using data compiled from Godwin et d. (1980), Kuran et d. (1982), Gareau (1986), Abercrombie (1990), Lang (2000) and Farmer et d. (2000).…”

“…Ishihara 1977Ishihara , 1981Blevin & Chappell 1992). Ishihara (1977Ishihara ( , 1981 indicated that the compatibility and partitioning behaviour of metals is strongly influenced by the oxidation state of a magma, which can be estimated based on the dominance of either magnetite or ilmenite.…”

“…Numerous studies have indicated that the lithophile metal association of Mo, W and Sn is also strongly influenced by redox conditions (Ishihara 1981;Blevin & Chappell 1992;Candela 1995). Redox conditions influence the valency of some elements, which, in turn, determines the compatibility of such elements in accessory minerals.…”

“…The tectonic setting, magma composition, degree of fractional crystallisation and redox state all potentially play a role in determining the metallogeny of an intrusion-related mineral deposit district (e.g. Ishihara 1981;Titley 1982Titley , 1991Blevin & Chappell 1992;Barton 1996;Blevin et al 1996;Lang & Titley 1998). Controversies have typically focused on the relative importance of either the source materials or magmatichydrothermal processes in enriching a system in a particular metal suite.…”