Petrogenesis of tin‐bearing granites of the Emuford district, Herberton tinfield, Australia

Abstract

Late Palaeozoic biotite granites from the Emuford district of the Herberton tinfield form part of the Emu Suite, a group of fluorine‐rich, high‐silica, ilmenite series granites associated with extensive tin mineralization in the eastern Tate Batholith. The Emu Suite granites are enriched in F, Rb, U, Th, Y and Sn and depleted in TiO₂, MgO, MnO, CaO, P₂O₅, Ba, Sr and Eu compared with many other silicic granites. Fractionation trends from early coarse grained to later medium and fine grained granites include increases in Al₂O₃, Na₂O, F, Rb, Th, U, Hf, Y, Nb, W, Sn, Ga and HREE, and decreases in TiO₂, MgO, Ba, Sr, Eu, La and Ce. These trends are compatible with an evolution via fractionation of a fluorine‐rich magma involving mainly separation of plagioclase and minor zircon and monazite. The Emu Suite granites form part of a much more extensive group of high‐Si granites associated with tin mineralization in the region. These tin‐bearing granites are amongst the earliest intrusions of a major Late Palaeozoic magmatic episode which extended from approximately 315 to 270 Ma. Modelling the evolution of the Emu Suite granites indicates that the high LREE and Rb and the extremely low Ba, Sr and Eu are not compatible with an origin via partial melting and fractionation of average or depleted crustal sources unless very small degrees of partial melting over a wide area are proposed. The strongly fractionated chemistry of the Emu Suite granites could have resulted from their derivation from a relatively fractionated, crustal source. Heat and volatiles required to promote partial melting were probably supplied by mantle outgassing which homogenized Sr isotope ratios in the region in the early stages of Late Palaeozoie magmatism.