Magnetic and gravity interpretation on the Stuart Shelf

Abstract

The Stuart Shelf region is a stable platform of Carpentarian and older crystallisation basement which is overlain by flat lying Adelaidean and Cambrian sediments. It extends east-ward from the exposed crystalline Gawler Block area to the mobile "Torrens Hinge" zone. The Olympic Dam copper-uranium deposit occurs near the eastern margin of the region in a zone of high magnetic relief which covers much of the ANDAMOOKA 1:250 000 sheet. The deposit was discovered by reconnaissance drilling of coincident gravity and magnetic highs by Western Mining Corporation Ltd. in 1975. The mineralisation occurs beneath approximately 350 m of Adelaidean sediments and therefore is an excellent example of the type of concealed ore body which will become a more frequent exploration target in the next decade. Regional gravity and magnetic data in the area are widely spaced and variable in quality. lnterpretation of the magnetic data indicates that the Olympic Dam deposit occurs in a upfaulted basement block, with fault movement controlled by northeast and northwest-trending fractures. Quantitative modelling indicates that the interpreted fault immediately north of the deposit may contribute directly to the magnetic anomaly observed at Olympic Dam. The northwest trend which is prominent in the regional magnetic data is attributed to dolerite dykes which are eroded feeders to the lower Adelaidean Beda Volcanics. Detailed aeromagnetic data from Billakalina improve the resolution of these anomalies in an area where they are not as evident in the regional data. The northwesterly trend is also evident in the region of more intense magnetic relief which contains the Olympic Dam anomaly. lnterpretation of the depth to pre-Adelaidean basement in the region is complicated by the presence of several stratigraphically separate magnetic sources. Anomalies due to the Adelaidean Beda Volcanics and associated dolerite dykes are imposed on basement and intra-basement sources. These are distinguished on the basis of anomaly form, orientation and interpreted susceptibility values, but clear distinctions cannot always be made. The proposed regional interpretation of depth to basement does show some correlation with gravity features, but density variations within basement are also evident. Gravity interpretation is also complicated by the unknown contribution of the Cambrian Andamooka Limestone.