Geophysics of the Big Bell gold deposit, Western Australia

Abstract

The Big Bell gold deposit is hosted by a felsic volcanic sequence of Archaean age. The alteration mineral assemblages within the host- and wallrock units of the deposit produce measurable geophysical anomalies. The deposit and altered wallrocks which contain up to 10 vol. % sulphide are chargeable, producing induced polarisation anomalies in dipole?dipole data of up to 30 mV/V at n53. In addition, a strong chargeability of up to 40 mV/V at n52 is evident in the dipole?dipole data, and corresponds to a graphitic and sulphidic horizon in the immediate footwall to the deposit. This unit has been mapped with the gradient array over the entire extent of the leases, providing a useful marker at the top of the felsic volcanic sequence. The strong potassic alteration accompanying the gold mineralisation is delineated in downhole spectral radiometric logs in which highly anomalous potassium values of up to 8 wt % are comparable with those derived by chemical analysis. The downhole logs also indicate that alteration has not enriched or depleted uranium or thorium in the ore zone. There are also ground radiometric potassium anomalies over outcropping lode rocks. Airborne and ground magnetic anomalies adjacent to the lode are due to sources with very high magnetic susceptibility values, measured in pyrrhotite- and magnetite-altered wallrocks by downhole geophysics. The values measured in the logs are 0.14 SI units in the pyrrhotite-altered zone, and range between 0.025 and 0.5 SI units in the magnetite-altered zone. Target generation within the remainder of the Big Bell greenstone belt has relied heavily on geophysics, with the highest ranking being given to magnetic, potassium and induced polarisation anomalies within the felsic volcanic sequence.