Magnetic and electrical signatures of the Granny Smith gold deposits, Western Australia

Abstract

Gold deposits at Granny Smith occur along a shear between a granodiorite intrusion and epiclastic sedimentary units. Three ore zones, Goanna, Granny and Windich, were identified by CSR in 1987, during exploration that included ground magnetic and induced polarisation surveys. Magnetic data were useful in helping to map intrusive rocks, banded iron-formations and less magnetic sedimentary horizons, and some faults. Although magnetite is a minor alteration mineral in the granodiorite, the mineralised zones do not give identifiable magnetic responses. Dipole?dipole induced polarisation surveys also gave valuable, and complementary, mapping information. At Granny, carbonaceous shale in the hangingwall and weakly polarisable footwall granodiorite masked any induced polarisation signal that the mineralisation may have produced. Drillhole measurements showed that oxidised mineralisation is not anomalously polarisable, whereas fresh mineralisation is moderately polarisable (some pyrite accompanies gold). Despite the lack of a direct induced-polarisation anomaly from Granny, the IP surveys delineated the deeply weathered mineralised shear north of Granny, which contributed to the discovery of Goanna. Electromagnetic measurements have also been used effectively for resistivity mapping.