Geophysical exploration for copper-gold mineralisation in the Benagerie Ridge Magnetic Complex, Curnamona Province, South Australia

Abstract

The Benagerie Ridge Magnetic Complex is located in eastern South Australia. The Complex, a component of the Curnamona Craton, comprises highly altered Palaeproterozoic meta-sediments of the Willyama Supergroup, and is concealed by tens of metres of sedimentary cover. The cover includes Neoproterozoic, Cambrian, and particularly, Tertiary strata. Exploration in the region has led to the discovery of Proterozoic iron oxide-style copper-gold mineralisation, plus related elluvial gold in the overlying Tertiary strata. The mineralisation is associated with zones of deeper weathering, resulting from intense feldspathic alteration and sulphidisation of the mineralised stratigraphy. Faulting also influences the distribution of mineralisation. Semi-regional to detailed aeromagnetic and gravity surveys have all been extremely effective for mapping stratigraphy, alteration and structure within the Benagerie Ridge Magnetic Complex. However, airborne EM data did not contain responses from the basement. The intense weathering associated with mineralisation is the cause of residual negative gravity anomalies with amplitudes of 1 to 2 mGal. Consequently, detailed gravity surveys have been very effective for identifying prospects within the region. IP/resistivity and time-domain EM surveys were largely unsuccessful, with conductive overburden preventing responses from the mineralised stratigraphy being detected. However, complex IP/resistivity surveys detected responses that coincide with known areas of mineralisation. A lack of electrical continuity, due to poor development of massive sulphides, means downhole EM methods have been largely ineffective in the area.