Opaque mineralogy and mafic mineral chemistry of I- and S-type granites of the Lachlan fold belt, Southeast Australia

About one-quarter or 44000 km'z of the Lachlan fold belt of southeast Australia is underlain by granite batholiths of Silurian-Devonian age. Recent work has recognized a major distinction between granites formed from igneous sources (I-type) and those formed from sedimentary sources (S-type). Lachlan I- and S-type granites are generally characterized by the opaque mineral assemblages magnetite + ilmenite f minor sulfide (pyrite > chalcopyrite > pyrrhotite) and ilmenite * sulfide (pyrrhotite > pyrite > chalcopyrite), respectively. There are, however, magnetite-free I-type and magrretite-bearing S-type granites. Texturally earlier opaque phases are generally magnetite + ilmenite in I-type and ilmenite + sulfides in S-type granites. Sulfide minerals, even when abundant (rarely) in I-type granites, are texturally late phases. On the basis of rock chemistry, opaque mineral assemblages, and mafic mineral chemistry, most Lachlan S-type granites formed at lower ,fo, conditions than most Lachlan I-type granites. The higher sulfide content of S-type granites and their reduced nature may reflect the sulfur and carbon content oftheir sedimentary source rocks. The plutonic opaque mineral assemblage is another useful I- and S-type discriminator; however, the Fe-Ti oxide assemblage alone is not. Application of the magnetite- and ilmenite-series classification, developed from work on Japanese granites, has shown that I- and S-type granites may belong to either the magnetite or ilmenite series. However, the bulk of I-type granites are magnetite series, and the bulk of S-type granites are ilmenite series. The fo, in a granitic magma may be more important than the nature of the granitic source rocks in controlling the type of associated mineral deposits.