Sub-Audio Magnetic surveying for shallow occurrences of conductive manganese ore, Woodie Woodie area, Western Australia

Abstract

Manganese (Mn) ore in the Woodie Woodie area occurs as podiform ore bodies in Archaean dolomite and chert breccia thought to be 2.5 billion years old. Electromagnetic conductivity mapping using time-domain helicopter electromagnetics (HoistEM) surveying successfully detected six new discoveries under regolith and host rock cover in the area. Sub-Audio Magnetic (SAM) surveys were carried out over known EM-responsive Mn deposits to test the ability of this technology to detect conductive, high-grade manganese deposits as equivalent magnetometric resistivity (EQMMR) anomalies. SAM survey data were compared to other geophysical methods over five EM-responsive manganese deposits, which varied in size and burial depth. The methods compared were high-resolution gravity, HoistEM, gradient-array induced polarization (GAIP), dipole-dipole induced polarization (DDIP), and ground time domain electromagnetic (TEM) surveying. The SAM technique demonstrates that this technology can be effective for identifying conductive, mineralised structures and regolith features at shallow depth, usually less than 40 m. Podiform ore zones below 40 m depth did not produce EQMMR anomalies. The SAM depth of penetration is limited by the ability for current to be channelled into conductive, podiform ore zones sitting in resistive host rocks, below a moderately conductive regolith cover.