GEMOC ARC National Key Centre

Phillip Blevin¹

1GEMOC ANU

Fundamental relationships between ore deposits and certain types of intrusive igneous rocks have long been known. These include the association of chalcophile-dominated mineralisation with diorites and granodiorites, and lithophile-dominated mineralisation with compositionally "specialised" granitic rocks. The relative oxidation state of mineralised intrusives was also known to be important, with Mo and Cu mineralisation associated with oxidised (magnetite-bearing) igneous rocks, and Sn and W with reduced (magnetite-free) rocks.

Integration of these basic relationships with mineral deposit and granite chemical databases for SE Australia have demonstrated the primacy of magmatic compositional parameters in determining ore element ratios in related mineralisation. Furthermore, these studies revealed that ore deposit types change systematically across the compositional spectrum observed within individual supersuites. The range of deposit types also varies according to the degree of the compositional evolution of the suite or supersuite. Thus the compositionally highly evolved Carboniferous I-types of North Queensland are associated with lithophile dominated deposits, while the less evolved Boggy Plain Supersuite are associated with chalcophile dominated deposits. Such relationships also provide a rational basis for the understanding of metal zonation on a district scale.

An enduring theme in the literature has been the concept that high metal inventories in magmas are required for mineralisation to occur. Marked enrichments and depletions in ore metal abundances within granitic magmas occur as a result of fractional crystallisation, and it is this process that is probably responsible for most of the variation in ore element abundances observed in granites (other than processes involving hydrothermal redistribution).

Other magmatic and hydrothermal processes may locally be important, but still require the operation of fractional crystallisation processes to work. One such example is the Mount Leyshon Au-polymetallic system where the timing of the introduction of the Au-Te-Bi ore assemblage along with Fe and S are most closely associated with dykes that contain abundant cognate xenoliths, and have textures indicating disequilibrium crystallisation and resorption. Here, overturning of a subvolcanic zoned magma chamber probably resulted in the re-dissolution of early crystallised sulfides and the liberation of chalcophile elements, Fe and S into an exsolving hydrothermal fluid.

Mineral deposits associated with granitic rocks are fundamentally polymetallic in character. Their metallogeny can however be considered in terms of a continuum in ore element ratios from chalcophile-dominated to lithophile dominated mineralisation in highly compositionally evolved granites. These relationships are evident at all scales from the regional to within individual deposits where variations in metal character can reflect the progress of crystallisation and volatile exsolution within single magma chamber. The polymetallic Mount Leyshon Au deposit (north Queensland) has early Mo mineralisation associated with rhyolitic intrusives cut by later Au-base metal mineralisation associated with andesitic to rhyodacitic dykes. This relationship is also born out regionally in deposits associated with compositionally similar intrusives.

The zoned and polymetallic nature of deposits associated with intermediate to felsic intrusives can therefore be systematised within a scheme that classifies magmas into suites and takes into account such factors as the degree of compositional evolution of the related granite series and its oxidation state.

The use of the suite concept, and an understanding of metal zoning at all scales, enables the regional distribution of mineralisation to be resolved, and for predictions to be made as to the probable distribution of similar mineralisation. The current level of exposure is also critical. Too deeply eroded batholiths lose the mineralised upper portions of magma chambers. Erosional contrasts across the south west USA are reflected in differing W/Cu ratios of ore deposits, for example.