Ooldea 3 diamond drillhole is located on the western margin of the Gawler Craton where thin Tertiary (ca. 16 metres) and Palaeozoic (ca. 75 metres) sediments, of the Eucla and Officer Basins respectively, overlie Archaean to Early Proterozoic basement gneisses of the Gawler Craton (Mulgathing Complex). From 101 to 385.85 metres depth Ooldea 3 intersected variably mylonitized quartz + magnetite and quartz + feldspar + mica (+ garnet + sillimanite) gneisses with formerly-intrusive (premylonite) biotite augen gneiss and pink pegmatite. The magnetite-rich basement gneisses define a major ground magnetic and aeromagnetic anomaly which can be traced 300 km to the NE as the Karari Fault. Gravity and ground magnetic modelling suggest that the mylonite zone is steeply dipping to the E-SE, and movement directions deduced from the mylonite fabrics suggest downthrow on the western side. This implies that the fault zone is a steeply inclined reverse fault or thrust.