Geophysical exploration for Archaean gold: a case study from the Southern cross greenstone belt, Western Australia

Abstract

Ground magnetometer and gravity surveys have identified a coincident magnetic (400 nT) and gravity (0.5 mGal) anomaly on the western limb of the Caudan Antiform in the Burbridge area of the Southern Cross Greenstone Belt, Western Australia. The anomaly corresponds with the postulated along-strike continuation of a sulphide-facies iron formation ( if) which locally hosts gold mineralisation. Gold concentrations in surface laterites (up to 100 ppb) occur in the vicinity of the geophysical anomalies. Two-dimensional modelling of the magnetic data, following filtering to remove short wavelength noise originating in the near-surface, predicts that magnetic if (susceptibility contrast 120×10 −3 SI units) occurs at a depth of 100 m, dips sleeply to the west, and is overlain by a non-magnetic “oxidised” iron formation facies in the zone of deep weathering. The stratigraphic thickness of the fresh if is up to 60 m. This model is consistent with the observed gravity anomaly, assuming a density contrast of +0.4 g/cm 3 between the magnetic if and the surrounding lithologies. Magnetic susceptibility measurements of surface laterite failed to reveal anomalous values associated with presence of the if. After completion of the initial geophysical investigation, a reverse circulation drilling program confirmed the inferred position of the if on the western limb of the Caudan Antiform and showed it to be associated with gold mineralisation.