Geophysical proxies for redox gradients in IOCG systems: Cloncurry District, Qld, Australia.

Iron oxide-copper-gold (IOCG) systems, in a broad geophysical sense, include a spectrum of mineralisation styles ranging from reduced, pyrrhotite-dominant examples, sometimes referred to as iron sulphide copper gold (ISCG) systems, to magnetite-pyrite dominant, and more oxidised, hematite dominant Cu and Au deposits. Importantly, the geophysical signatures in and around (IOCG) deposits are frequently associated with mappable changes in redox. Moreover, redox gradients have predictable geophysical signatures, and thus they can be targeted by mineral explorers using integrated petrophysical and geophysical methods. Systematic and scale-integrated petrophysical data ( >1300 samples form 23 deposits and prospects) such as density, magnetic susceptibility, remanent magnetisation, radiometrics and conductivity were obtained and combined with mineralogical and geochemical data measured on the same samples as part of the Cloncurry district as part of the Cloncurry METAL projects. The data show clear relationships between redox gradients, indicated by transitions between pyrrhotite, magnetite and hematite bearing lithologies, and steep gradients in magnetic susceptibility and/or remanent magnetisation. Moreover, the gradients coincide with Uranium radiometric spikes present in rock and breccia intervals with complex mineralogy related to transitional redox. This diagnostic radiometric signature together with the coincident magnetic gradients provide a rigid geophysical proxy for differentiation of IOCG related signatures from false positives. Finally, the data provides new insights into economic IOCG mineralisation and new set of tools for targeting IOCG and related deposits in the surface geophysical and downhole datasets.