Geophysical investigations of volcanic terrains: a case history from the Gawler Range Volcanic Province, South Australia

Abstract

Flat-lying magnetic cover rocks, especially when they comprise thick volcanic sequences, present a difficult interpretation problem. Magnetic maps of such regions essentially reflect the exposed volcanic rocks and sources within and beneath the volcanic rocks cannot be easily resolved. Yet such areas are often of economic interest. The Gawler Range Volcanic Province in South Australia represents an example of one such volcanic terrain. The Volcanics are distinctly bimodal in composition: the 'older' units are mafic and the 'younger' units dominantly felsic, with few rocks of intermediate composition. The exposed Volcanics cover approximately 25 000 km2 and drill-hole intersections confirm their existence well beyond the present limits. Magnetite is present in both mafic and felsic volcanic rocks. The province has been identified as a target area for possible epithermal gold mineralisation and volcanogenic-hosted base metal mineralisation. Regional gravity and aeromagnetic data are available over the whole province, while part is covered by a recent high-resolution aeromagnetic and aeroradiometric survey. The Gawler Range Volcanic Province is associated with a large gravity high caused probably by mantle underplating or the emplacement of mantle derivatives prior to the extrusion of the volcanics. This is consistent with isotope data which were previously interpreted to indicate a mantle component for the Volcanics. Two major gravity trends (NE and ESE) intersect within the study area. The ESE-striking trend has been modelled as a deep crustal fault with the southern block down-faulted. These two trends are also reflected in the high-resolution magnetic maps once the high frequencies have been removed from the data. Initial images of the high-resolution aeroradiometric and aeromagnetic data correlated very closely with the outcrop and joint patterns. Areas of radiating fracture patterns may represent volcanic centres. The image of the K:U Ratio reflected minor variations in the otherwise homogeneous geochemistry of the Volcanics. Through image processing and filtering, and combining the interpretation of available geophysical data, an inferred geological map for the area has been suggested. The igneous province is interpreted as extending further eastwards beneath part of the Stuart Shelfing consisting of three well-defined units. The first corresponds to the main Gawler Range Volcanic Province, the second to the south-west gabbroic extension and the third to an eastward extension consisting of a felsic core circled by a mafic ring.