Can sulphides be discriminated from conductive slates? A case history at Dugald River

Abstract

If an excellent conductor is located within a conductive host or halo, a counter-intuitive effect may be seen in impulse response results. The presence of the very conductive core can dramatically reduce the fitted time constant of the observed decay at early to mid delay times, and the response due to the slow decay of currents within the core itself may not become dominant until very late delays when the signal may lie beneath the noise level. With an on-time system, although the 'total' response may also lie below observable noise levels, a detectable secondary can easily be inferred since the total field measured is markedly different from the calculated primary field. The Dugald River zinc-lead-silver deposit in Northwest Queensland consists of a massive to brecciated sulphide lode hosted within conductive slates. Modelling of the observed borehole response of Utem and Sirotem data requires the presence of very conductive material with a decay constant of more than 200 ms. This would imply that the conductance of the lode must be well in excess of 500 S. This value may seem very high, but is consistent with a nominal thickness of 20 m and physical property measurements. Interpretation of the surface and borehole Utem data imply that a very conductive target, approximately coincident with the lode horizon, can be discriminated from the slates, and must extend to great depths. Borehole Sirotem data indirectly appear to support the premise that the lode is very conductive, but only with the benefit of careful modelling. By contrast airborne Geotem data using a 75 Hz base frequency is comparatively insensitive to excellent conductors; and all detected anomalies in the survey can be attributed to the response of the shales alone. This is a result of the relatively high frequency transmitter waveform and off-time measurement used in the AEM system.