Geochemical signatures of copper redistribution in IOCG-type mineralisation, Gawler Craton, South Australia

The Emmie Bluff iron oxide, copper, gold (IOCG) prospect is located in the Olympic Dam district, South Australia, and hosts sub-economic 150-m-thick Cu–Au mineralisation associated with the hematite–chlorite–sericite alteration with chalcopyrite commonly replacing pre-existing pyrite at a depth of 800 m. With the use of cutting-edge synchrotron X-ray fluorescence microscopy and field emission gun-scanning electron microscopy, it is shown for the first time that sub-economic IOCG mineralisation in the Olympic Dam district was affected by a late fluid event, which resulted in partial dissolution of Cu mineralisation and transport of Cu in the form of chloride complexes. The porous chlorite–sericite matrix associated with the late alteration of chalcopyrite hosts a Cu–Cl–OH phase previously undescribed in IOCG rocks, which was identified as one of the polymorphs of the atacamite group of minerals, Cu 2 Cl(OH) 3 . Thermodynamic modelling shows that “atacamite” is produced during dissolution of chalcopyrite by an oxidised, Cl-bearing fluid. An acidic environment is produced within millimetres of the chalcopyrite grains during oxidation. This process drives chlorite recrystallisation that is recorded by compositional variation of chlorite proximal to chalcopyrite. The existence of the atacamite is discussed in the context of fluid evolution and interaction with IOCG-type mineralisation and its implications to ore preservation versus destruction and remobilisation.