The geophysics of the Trough Tank gold?copper prospect, Australia

Abstract

The Trough Tank gold?copper prospect lies approximately 250 kilometres south-east of Mt Isa in Queensland, Australia. The prospect was initially located by airborne magnetics. Reconnaissance drilling located quartz-magnetite rock with anomalous gold beneath 30?40 metres of Mesozoic cover. There is no bedrock or anomalous geochemistry associated with the anomaly and so it represents a true 'blind' target. Ground magnetics, induced polarisation, fixed and moving loop time domain EM and drilling have all been employed to detail the anomaly. Drill core laboratory measurements to determine physical properties revealed minimal remanence but strong demagnetisation. The structural complexity of the prospect plus the demagnetising effects made initial drill siting difficult with high magnetite content producing extremely high magnetic amplitudes. Important modifications to published algorithms and modelling programs were found necessary to calculate theoretical anomalies which correspond to measured anomalies. Dipole?dipole IP over the area defined a broadly anomalous chargeable zone coincident with the postulated sources determined by the ground magnetics. Fixed and moving loop EM using Zonge and SIROTEM equipment permitted greater resolution of the main conductive lithologies and a reasonable structural interpretation was developed. Drilling on EM and magnetic targets to date has intersected up to 41 metres of highly conductive, pyrite-chalcopyrite bearing, quartz-magnetite rocks.