Direct detection of gold bearing structures at St Ives, WA ? DHEM vs DHMMR

Abstract

The St Ives terrain is covered by a thick regolith that is saturated with hyper-saline groundwater. The regolith forms a major barrier for electrical and EM surveys designed to detect gold-bearing structures in bedrock. The down-hole magnetometric resistivity (DHMMR) method was trialled at St Ives and detected only small responses from known structures. DHMMR does not detect strong anomalies because the structures are often less than 10 m thick, and have small cross-sectional areas for channelling current. The resistive bedrock does not support large current densities for channelling into targets. Down-hole electromagnetic (DHEM) surveys were trialled at St Ives and detected a strong well-defined anomaly at the Junction gold mine. The anomaly was modelled as a plate in layered earth, and the plate corresponded to a major gold-bearing shear. Petrophysical tests suggest that the shear is conductive because of brine-saturated porosity. DHEM surveys did not detect strong anomalies from a gold-bearing shear at the Argo gold mine, despite the shear having low resistivity relative to host rock in downhole resistivity logging. The absence of DHEM response may be because the shear is not conductive enough to support an anomalous current system, or because the conductivity is not connected on a large scale.