Geophysical characterisation of the Challenger gold deposit, Gawler Craton, South Australia

Abstract

The Challenger Project is located in Archaean rocks of the Gawler Craton and is the most significant gold-only discovery made in South Australia. The deposit was discovered through geochemical sampling of calcrete. It has a measured and indicated resource of 1.7M t at a grade of 8.5 g/t for 465,000 oz of gold. The resource is open down-plunge and has the potential to be significantly greater. Mineralisation occurs as discrete shallowly (30°) plunging shoots hosted by granulite facies paragneiss, which lacks marker horizons to aid in geological and structural mapping. High-grade moderate to steeply dipping auriferous quartz veins occur within the shoots. No major structures are observable in the area in regional (400 m line spacing) aeromagnetic data. However, the deposit lies on the edge of a regional-scale gravity high, as do the majority of significant Archaean lode gold sites in the region. The gravity high is interpreted to indicate a buried intrusive, or crustal underplating. A high-resolution aeromagnetic survey has mapped a subtle, but easily discernible, magnetic high (5-6 nT) that is spatially coincident with the mineralisation. Modelling, constrained by core-magnetisation studies and known mineralised geometries, shows pyrrhotite within the mineralising system is the probable source of the anomaly. However, interpretation is complicated by lateral interference from lamprophyre dikes/sills. During exploration in the Challenger area, radiometrics, digital elevation models and enhanced Landsat data provided a basis for mapping the regolith. The deposit is coincident with a slight topographic high and a pronounced increase in the depth of weathering. The former is mapped by digital terrain data gathered during airborne geophysical surveys. The latter has been mapped by ground electrical resistivity, electromagnetic and gravity surveys, and drilling. A TEMPEST AEM survey has mapped the conductivity structure of the regolith in good agreement with that derived from deposit-scale ground electrical geophysics. The increase in weathering over the Challenger deposit is mapped as a second- or third-order effect. However, the survey was extremely useful because it identified and delineated hitherto unknown 2 km wide, 200 m deep, north-south striking, palaeochannels. Drilling of one of these has shown they are excellent sources of water for the future mine development. A weak bedrock IP response is coincident with the mineralisation, but is not diagnostic. Extreme difficulties were experienced in collecting high-quality IP data due to the highly resistive nature of the surface calcrete and silcrete, and the highly conductive nature of the underlying clays and saprolite material.