New perspectives on IOCG deposits, Mt Isa Eastern Succession, northwest Queensland

A model for the formation of the iron-oxide-copper-gold (lOCG) deposits in the Mt Isa Eastern Succession involves fluids derived from late orogenic granites mixing with a fluid derived from an external source to fonn iron-rich alteration zones (commonly magnetite-rich) that contain vein stockwork, breccia, dissemination or replacement-style mineralisation. This process is inferred to be spatially and temporally associated with felsic pluton emplacement and cooling at around 1540 to 1500 Ma. This contrasts with an alternative model in which the fluids are entirely intra-basinal and amagmatic in origin. Recent studies at Osborne have highlighted a potential syn-peak metamorphic timing for mineralisation (based on 1595 Ma Re-Os dates on molybdenite and a 1595 ± 6 Ma U-Pb date on hydrothermal titanite), with no apparent proximal major intrusion. There is also a potential link between mineralisation and widespread mafic intrusive activity, which spans the entire range of known mineralisation ages.