Chemistry, origin, and evolution of mineralized granites in the Lachlan fold belt, Australia: The metallogeny of I- and S-type granites

Granites crop out over 20 percent of the total exposed area of the Lachlan fold belt and comprise subequal proportions of I and S types. Despite the volume of granite present, economic mineralization is sporadic. Major Sn mineralization is related to S- and I-type granites that are both reduced and have undergone extended feldspar-dominated crystal fractionation. Mo showings are common but, as for Cu, large deposits are absent. Scheelite skarn mineralization is represented by the major King Island deposits; however, granite-related W mineralization of a similar scale is not found elsewhere within the belt. The I-type granites of the belt represent the products of partial melting of older tonalitic-granodioritic source rocks in the deep crust which were themselves ultimately derived through fusion of mantle materials (Ed types). This evolution is also mirrored in the nature of related mineralization which changes from Cu dominated at the chemically primitive end of this series through to lithophile dominated in the more evolved I types. Most of the Lachlan fold belt I types have not undergone significant crystal fractionation and are neither strongly oxidized nor reduced. Two prominent mineralized I-type suites, the Boggy Plain Supersuite of the central Lachlan fold belt and the Grassy Suite of King Island, are anomalously mineralized relative to other Lachlan I types. Both are chemically distinct from other I types in the belt by being enriched in the incompatible elements and Ni and Cr. Mafic S types are universally restite rich and unmineralized. S-type magmas are in general more evolved chemically than I types, are all relatively reduced, and when highly fractionated, are related to Sn +/- W mineralization. The ore element ratios (Sn/W/Cu/Mo, etc.) of granite-related mineralization in the Lachlan fold belt is a straightforward function of the relative oxidation state and degree of fractionation within the associated granite suites. The progression from Cu-Au, and W to Mo mineralization related to progressively more fractionated, oxidized I-type magmas can be traced within single supersuites. Such a systematic relationship between magma composition, redox state, and ore element ratios is good evidence for the magmatic source of ore elements in granite-related mineral deposits and for the production of the observed ore element ratios dominantly through magmatic processes. Whether or not associated economic mineralization is developed, however, appears to be determined by factors that are not related in a systematic way to magma composition.