The Geological Architecture of the Iron-Oxide Copper-Gold (IOCG) corridor at the Mt Isa Inlier

Abstract

The iron-oxide copper-gold (IOCG) corridor of the Proterozoic Mt Isa Inlier (NW Queensland, Australia) is a remarkable and sizeable concentration of high-grade Cu-Au, Pb-Zn-Ag and Mo-rare earth element deposits, which can be traced N-S, from Kuridala to Osborne. Shear zones and brittle faults structurally controlled the ore deposition by serving as conduits and providing efficient structural and geochemical traps. The recognition of the evolution of these zones, in the context of the broader geological architecture, will enhance future mineral discoveries. Field mapping, microstructural studies and K-feldspar 40Ar/39Ar geochronology and thermochronometry have allowed us to decipher the essential tectonothermal events that occurred in association with the ore-controlling structures, both during and after the Isan Orogeny. We suggest the Isan Orogeny modified tectonic scenario, as implied by the switching between extensional and compressional modes. Two episodes of extension were confirmed. The first was associated with the exhumation of the high-temperature/low-pressure metamorphic rocks of the Double Crossing Metamorphics at 1650-1600 Ma. The second event is confirmed occurring around 1530-1535 Ma, and was responsible for the exhumation of the medium temperature/medium pressure schist belt of the Mt Cobalt area. The extensional structures, originating from synsedimentary basin faulting, controlled the distribution of the contrasting metamorphic belts and metasomatic alteration. The post-Isan Orogeny tectonothermal events occurred preferentially along the active fault zones. The Mt Dore Fault Zone experienced multiple episodes of relatively rapid cooling (1-11 C/Myr), indicating that episodes of thrusting occurred at ~1350-1300 Ma and ~1250-1200 Ma, reflecting far-field responses to soft and hard collisions between the Northern and Western Australian Cratons and the Mawson Craton at that time. Subsequent isothermal phases were coupled with low-temperature potassium-iron metasomatism at ~1120 Ma and 830 Ma. Thermal pulses took place at ~900 Ma and ~740 Ma, explained by the uplifting of geotherms associated with continental rifting resuling from the break-up of Rodinia. This geological evolution determined the distribution of the IOCG mineralisation, which accumulated in favourable elements of the geological architecture, including: (1) synsedimentary basin structures that experienced repeated inversion-extension cycles; (2) a fault intersection setting that produced local dilatation zones; and (3) later thrusts that preferentially preserved ore pockets. The recognition of post-Isan Orogeny fault tectonics should be considered a new tool for mineral exploration in the region. In addition, the thermochronology results from the Mt Dore Fault Zone highlighted the most favourable level of erosion for the preservation of the mineralisation. Show more