The SAM EQMMR response of the regolith at East Victory, St Ives Gold Mine, Western Australia

Abstract

The Sub-Audio Magnetics (SAM) equivalent magnetometric resistivity (EQMMR) response has been investigated along a single 2.2 km long transect in the East Victory area at the St Ives Gold mine near Kambalda. The integration of a number of high-definition geophysical surveys, including electrical resistivity imaging (ERI), high-resolution gravity surveying, HeliSAM surveying, drill testing, and downhole logging along the same 2.2 km transect has shown the SAM EQMMR response here is due to variations in the regolith. The thickness of the regolith, predominantly the depth to fresh rock, was accurately determined by inversion of apparent resistivity from the ERI survey. The depth of the regolith along the transect was later confirmed by drilling at six locations, with the base of saprock found to be within 5 m of predicted depth from the ERI profile. From the high-resolution gravity surveying at 10 m station spacing, a residual was created to highlight the contribution of the low-density regolith to the gravity response. This gravity residual was found to have an excellent inverse correlation with the SAM EQMMR anomaly data. The high-amplitude, positive SAM EQMMR responses also show excellent correlation with the depth of regolith interpreted from the ERI profile. HeliSAM showed the same broad EQMMR anomalies as ground-based SAM, but the resolution and amplitude was much lower because of detector height, fewer along-line readings, and helicopter noise. Gravity data were successfully modelled in 2.5D using the ERI inversion to constrain the depth of polygons. Using the gravity model, the SAM EQMMR response along the transect was also successfully modelled by assigning relative current densities to the polygons, as opposed to absolute density values. The SAM EQMMR response was also inverted in 3D from gridded data using the UBC Grav3D program with some degree of success, with the depth of the main conductor in the inversion in agreement with drilling information. Downhole conductivity logging along the East Victory transect shows that the regolith is highly conductive, with peak conductivities ranging from 1200?2700 mS/m. It can be shown that the lateral differences in regolith conductivity do not play a major role in defining the SAM EQMMR response in this environment. The regolith produces SAM EQMMR anomalies that reflect wide zones of deeper weathering, where the conductive regolith contained higher current density compared to the underlying fresh bedrock. It is demonstrated that high-amplitude SAM EQMMR anomalies map out geological structures and rock units that have been preferentially weathered and have both high current densities and large geometric size.