The 2003 Gawler Craton Seismic Survey: notes from the Seismic Workshop held at Gawler Craton State of Play 2004

Iron oxide-copper-gold (IOCG) systems, and their related magnetite-apatite systems, are suggested to form in orogenic belts (e.g., Cloncurry, Andean magmatic arc, Urals) and extensional settings (e.g., St François Terrane, Missouri). Even so, the tectonic setting of arguably the most important of the IOCG class of the deposits, the giant world-class Olympic Dam deposit in South Australia, has been uncertain to date. It has previously been used as an example of the IOCG in an anorogenic intracontinental setting. The region has no outcrop as it is covered by Neoproterozoic sedimentary successions exceeding 5 km thick, in places. Given its economic significance and importance in defining the IOCG mineral deposit class, resolving the tectonic setting of Olympic Dam is crucially important. To address this, two orthogonal deep seismic reflection traverses, centred on the Olympic Dam deposit, were recorded to 18 s TWT. The nearly north-south line, 193 km long, oriented as near to regional dip-direction, defined by potential-field data, as land access would allow, sounded units of the Archaean-Proterozoic Gawler Craton and a possible allochthanous Proterozoic terrane. The shorter east-west cross-line (57 km) provides control on the three dimensional geometry of the major structures and gives information about some out-of-plane structures imaged by the north-south traverse. The seismic data show that the Olympic Dam deposit occurs in a fold- and thrust-belt, formed over a number of periods of orogeny, which was probably subject to compressional tectonics at the time of IOCG mineralisation. In the north of the study area, the upper crust, of the crystalline basement, shows south-dipping reflectors, interpreted as shear zones that cut through crust of sub-horizontal reflections. To the south, the upper crust and lower crust have reflectivity indicating thrusting to the south and, or, southwest, toward the interior of the Gawler Craton. However, the upper crustal deformation is decoupled from lower crustal deformation at a layer of sub-horizontal reflectors interpreted, by their reflection-amplitudes, to contain more mafic rocks than the regions above and below. This layer is up to 5 km thick and mostly un-deformed, except where it is duplexed. The duplexes appear to form the loci of strain partitioning in the upper crust. The lower crust of the centre of the study area contains fewer reflections, and appears anomalous but homogenised compared with the crust to the north and south. Imaging of the cover-successions shows a previously unknown Adelaidean rift, which has been partially inverted.