Constraints on hydrothermal fluid pathways within Mary Kathleen Group stratigraphy of the Cloncurry iron-oxide–copper–gold District, Australia

Widespread brecciation and Na-(Ca) alteration in the metacarbonate-dominated Mary Kathleen Group characterise some of the district-scale host-rocks to iron-oxide–copper–gold (IOCG) mineralisation in the Proterozoic Cloncurry District of north-eastern Australia. Structural and mineralogical observations combined with stable isotope data indicate that within the Mary Kathleen Group, late- to post-metamorphism brecciation was preferentially developed in calcite-poor lithologies, while marbles and to a lesser degree calc-silicate rocks were not prone to fracturing and acted as impermeable barriers to fluid flow. These impermeable layers helped maintain high fluid pressures promoting brecciation in adjacent lithologies. Only in areas of localised high shear strain did carbonate-rich lithologies allow for appreciable fluid flow. Such carbonate-rich rocks contain isotopically and mineralogically distinct skarn-like assemblages, the recognition of which may be important in identifying high fluid flux corridors in the vicinity of major IOCG deposits.