Sub-audio magnetic survey experiments for high-resolution, subsurface mapping of regolith and mineralisation over a blind gold discovery near Agnew in Western Australia

Abstract

Experimentation with sub-audio magnetic (SAM) survey parameters over a recently discovered Archaean mesothermal gold deposit demonstrates that this technology can be effective for identifying conductive, mineralised structures and regolith features at high resolution to a depth of 100 m, as long as the transmitter electrodes are placed sub-parallel to the strike direction of features to be detected. The horizontal total field magnetometric resistivity (TFMMR) response produced by current channelling during SAM surveying is shown to have a very similar pattern to gradient array apparent resistivity results using 50 m dipole spacing or less. However, SAM data are recorded using a magnetic sensor, thus avoiding electrical contact with the ground, and 2 m along-line sample density provides much greater resolution. TFMMR anomaly trends were found to correspond to gold-mineralised shears, and dips of these shears estimated using gravity modelling methods agree with drilling results. The main ore zone forms an elongated, shallow plunging pod of weakly chargeable ore that shows up in gradient array and dipole-dipole IP surveys. SAM total field magnetometric induced polarisation (TFMMIP) was ineffective at imaging this ore zone using surface transmitter electrodes and 1 Hz transmitter frequency. However, the TFMMIP response of the main ore zone was later imaged in great detail by placing transmitter electrodes down boreholes into a shallow part of the ore body, and then down-plunge 700 m to the south. SAM surveying using optimal survey parameters identified shallow conductive shears and deeper chargeability anomalies within the main ore zone that correlated to economic gold mineralisation, and the SAM results helped to target resource definition drilling.