Sub-Audio Magnetic Survey Experiments for High-Resolution, Subsurface Mapping of Regolith and Mineralisation at the Songvang Gold Mine near Agnew, Western Australia

Abstract

Experimentation with Sub-Audio Magnetic (SAM) survey parameters over the Songvang mesothermal gold deposit demonstrates that this technology can be effective for identifying conductive, mineralised structures and differential weathering in the regolith 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 equivalent magnetometric resistivity (EQMMR) response produced by horizontal current channelling between transmitter electrodes has a very similar pattern to gradient-array apparent resistivity results using a 50 m or less dipole spacing. However, SAM data are recorded using a magnetic sensor, thus avoiding electrical contact with the ground, and the 2 m along-line sample density provides much greater resolution. The same area was surveyed using the same parameters, except that the transmitter electrode directions were at a right angle, and this produced radically different EQMMR results. EQMMR anomaly trends running N–S were found to correspond to gold-mineralised shears, and dips of these shears estimated using gravity modelling methods agree with drilling results. The Songvang main lode is an elongated, shallow plunging, rod-like zone of gold-pyrite ore that is detected by a gradient-array induced polarization survey. SAM equivalent magnetometric induced polarization (EQMMIP) was ineffective at imaging the main lode using surface transmitter electrodes and 1 Hz transmitter frequency. However, the 4 Hz EQMMIP response of the main lode was later imaged in great detail when transmitter electrodes were placed down boreholes into the orebody. The downhole SAM EQMMIP results were very effective at highlighting zones of intense sulphide-related gold mineralisation ahead of drilling. SAM surveying provided EQMMR data that identified shallow, conductive shears that contained economic gold mineralisation, and the EQMMIP data helped to plan resource definition drilling by prioritising chargeable targets related to increased pyrite alteration and gold grade along the Songvang main lode.