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.

To meet the rising global demand for base metals – driven primarily by the transition to cleaner-energy sources – declining rates of discovery of new deposits need to be countered by advances in exploration undercover. Here, we report that 85% of the world’s sediment-hosted base metals, including all giant deposits (>10 Mt of metal), occur within 200 km of the edge of thick lithosphere, irrespective of the age of mineralisation. This implies long-term craton edge stability, forcing a reconsideration of basin dynamics and the sediment-hosted mineral system. We find that the thermochemical structure of thick lithosphere results in increased basin subsidence rates during rifting, coupled with low geothermal gradients, which ensure favourable metal solubility and precipitation. Sediments in such basins generally contain all necessary lithofacies of the mineral system. These considerations allow establishment of the first-ever national prospectus for sediment-hosted base metal discovery. Conservative estimates place the undiscovered resource of sediment-hosted base metals in Australia to be ~50–200 Mt of metal. Importantly, this work suggests that ~15% of Australia is prospective for giant sediment-hosted deposits; we suggest that exploration efforts should be focused in this area. <b>Citation:</b> Czarnota, K., Hoggard, M.J., Richards, F.D., Teh, M., Huston, D.L., Jaques, A.L. and Ghelichkhan, S., 2020. Minerals on the edge: sediment-hosted base metal endowment above steps in lithospheric thickness. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.

Cratonic margins host many of the natural resources upon which our society depends. Despite this, little is known about the dynamic evolution of these regions and the stability of substantial steps in plate thickness that delineate their boundaries with adjacent mantle. Here, we investigate the spatio-temporal evolution of Australian cratonic lithosphere and underlying asthenospheric mantle by using the geochemical composition of mafic volcanic or shallow intrusive rocks preserved throughout the continent’s history. We have collated a large database of mafic samples that were screened to remove data affected by crystal fractionation or assimilation of cumulate material. We use forward and inverse modelling of igneous trace element compositions to calculate the depth and extent of melting for 28 distinct igneous provinces in the North Australian Craton. These results are used to infer mantle potential temperature and lithospheric thickness at the time of eruption. The majority of Paleoproterozoic magmatic events record high mantle potential temperatures of 1350–1450 °C and relatively low lithospheric thicknesses of ≤50 km. In contrast, younger igneous provinces show a gradual decrease in potential temperature and an increase in lithospheric thickness with time. These constraints on the mantle lay the foundation for the development of a quantitative geodynamic understanding of the evolution of the Australian lithosphere and its resources. <b>Citation:</b> Klöcking, M., Czarnota, K., Champion, D.C., Jaques, A.L. and Davies, D. R., 2020. Mapping the cover in northern Australia: towards a unified national 3D geological model. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.