Redox and compositional parameters for interpreting the granitoid metallogeny of eastern Australia: Implications for gold-rich ore systems

Granitoids and related rocks of eastern Australia can be classified according to their metallogenic potential using a scheme based on compositional character, degree of compositional evolution, degree of fractionation, and oxidation state. The scheme is based on empirical and theoretical considerations and satisfactorily describes the known distribution of granite-related mineralisation. The granitoids range from unevolved, mantle compatible compositions to highly evolved and fractionated. They exhibit age- and region-specific variations in silica content, compositional evolution and oxidation state. The most unevolved intrusive igneous rocks comprise those of the Ordovician of the Lachlan Orogen, and the Devonian of the New England Orogen. Strongly fractionated and evolved I-type granites occur in western Tasmania, the southern New England Orogen, and far north Queensland. Other fractionated suites tend to occur relatively rarely in the Lachlan Orogen and elsewhere. Oxidation states vary markedly. The most consistently oxidised rocks occur in the Ordovician of the central Lachlan Orogen, and the northernmost New England Orogen. The Carboniferous I-types of the northeastern Lachlan Orogen are consistently more oxidised than other Lachlan Orogen I-types. Gold-rich, Cu-poor systems associated with felsic I-types in eastern Australia are associated with W-Mo mineralised suites with gold occurring within a predictable metallogenic zonation. Gold mineralised I-types comprise weakly to moderately oxidised, high-K granitoid suites that, at least in the east Australian context, have low K/Rb ratios and show strong fractionation trends. Gold is readily removed from granitic magmas through the early precipitation of sulfides, or to a lesser extent by magnetite. Crystallisation of Fe-poor, silica-rich granitic magmas in a relatively narrow oxidation window between the FMQ and NNO buffers may provide conditions where retention of An in magmas in felsic granitic magmas is optimised.