DHEM at Isa Mine

Abstract

Drillhole electromagnetics (DHEM) is extensively used as part of the ongoing exploration program at Mount Isa Mine Limited's copper/lead/zinc mine, the Isa Mine. Exploration targets at Isa Mine are deep, extending to more than 2000m below surface and are tested by both surface and underground drilling. Physical property measurements indicate that although the Urquhart Shale (the host rock) is conductive, the lead/zinc and copper orebodies are approximately two orders of magnitude more conductive. Sirotem axial component, Crone multi-component and the Utem axial component DHEM systems have been used during recent exploration programs. Due to the depth of exploration, large surface loops were needed to generate adequate primary signal levels. However received signal levels were low in the conductive environment at depth and in some cases a total loss of signal resulted from all transmitter loops. Field examples and modelling have shown that significant off-hole economic mineralisation can be detected in the highly conductive environment at Isa Mine. Some responses in the DHEM profiles, that might reasonably be interpreted as being due to off-hole target conductors, have been shown by the modelling to be 'apparent' or 'false' responses produced by deviating drillholes beneath a conductive overburden.