The Use of Sub-Audio Magnetics (SAM) in Gold Exploration ? Examples from the Yilgarn Craton, WA

Abstract

The results of Sub-Audio Magnetic (SAM) surveys and the relationship to other geophysical datasets and geological information are presented for three different styles of gold mineralisation within the Archaean Yilgarn Craton, W.A. The Jericho prospect is located to the north of the Butcher Well deposit in the Laverton area and is hosted within a mafic unit consisting of predominately high-magnesium basalt. The source of a broad SAM total field magnetometric resistivity (TFMMR) response initially could not be identified as it did not appear related to the depth of cover or weathering from shallow drilling. Further analysis of the SAM result, following additional drilling and a dipole-dipole induced polarization (IP) survey, indicated that the source of the SAM TFMMR response in the southern portion of the survey area is due to a combination of variation in depth of the regolith and a number of narrow potassic alteration zones within the basaltic unit. The objective of the SAM survey at the Bronco prospect in the Southern Cross region was to map perturbations within the massive pyrrhotite-pyrite horizon that hosts gold mineralisation. In order to maximise current flow within the sulphide horizon, the electrodes were placed within the sulphide units at the base of two open pits located along strike. The TFMMR data shows a response having a width that significantly narrows to both the north and south of the survey area and correlates with the width of the primary sulphide. In addition, a number of flexures and offsets have been delineated. High-intensity magnetic responses, due to maghemite in the regolith, have resulted in noise being introduced in the TFMMR data. Gold mineralisation at the Triple ?O? lode within the Cornishman Deposit is associated with quartz-pyrrhotite-pyrite alteration of banded-iron formation that is located below a thrust in a ?blind? location. The sulphide alteration can be semi-massive and very conductive (up to 3000 S/m), but the use of traditional electromagnetic techniques at similar deposits in the region has not proven particularly successful because of the small spatial footprint of the sulphidic alteration. Hence, the objective of the SAM surveying was to determine whether the technique could successfully map the narrow, sub-horizontal sulphidic alteration zones. The TFMMR data highlighted a broad conductive zone that is comprised of three discrete conductors, two of which have been confirmed by drilling to result from sulphide alteration of the banded-iron formation and to have associated gold mineralisation. The third anomaly is thought to be due to the preferential weathering of a non-mineralised shear zone.