Lithospheric architecture and mantle metasomatism linked to iron oxide Cu-Au ore-forming systems: multidisciplinary evidence from the Olympic Dam region, South Australia

The role of lithospheric architecture and the mantle in the genesis of iron oxide copper-gold (IOCG) deposits is controversial. Using the example of the Precambrian Gawler Craton (South Australia), which hosts the giant Olympic Dam IOCG deposit, we integrate recently acquired geophysical data (passive seismic tomography, magnetotellurics) with geological and geochemical data to develop a new interpretation of the lithospheric setting of these deposits. Spatially, IOCG deposits are located above the margin of a mantle lithospheric zone with anomalously high electrical conductivities (resistivity <10 ohm.m, top at ~100-150 km depth), low seismic shear-wave velocities (horizontal component, Vsh <4.6 km/s), and unusually high ratios of compressional- to shear-wave velocities (Vp/Vsh>1.80). The high conductivity cannot be explained by water-bearing olivine-rich rock alone. Relatively fertile and metasomatised peridotitic mantle with additional high-Vp/Vs phases, e.g., clinohumite, hydrous garnet and/or phlogopite, could explain the anomalous velocity and conductivity. The top of this high-Vp/Vsh zone marks a mid-lithospheric discontinuity at ~100-130 km depth that is interpreted to reflect locally orthopyroxene-rich mantle. A sub-Moho zone with high Vp/Vsh at ~40-80 km depth correlates spatially with primitive Nd isotope signatures and arc-related ~1620-1610 Ma magmatism, and is interpreted as the eclogitic root of a magmatic arc. Mafic volcanics contemporaneous with ~1590 Ma IOCG mineralisation have geochemistry suggesting derivation from subduction-modified lithospheric mantle. We suggest that Olympic Dam formed inboard of a continental margin in a post-subduction setting, related to foundering and partial melting of previously refertilised and metasomatised lithospheric mantle. Deposits formed during the switch from compression to extension, following delamination-related uplift and exhumation.