Abra lead-silver-copper-gold deposit, Western Australia: a geophysical case history

Abstract

The Abra base-metal deposit is a large, deeply buried, low-grade mineralised body located in the Jillawarra mineralised belt in the Bangemall Basin, Western Australia. It has no surface geological or geochemical expression. The deposit was discovered in 1981 by drill testing a 270 m deep target, based on the detailed modelling of a 400 nT bullseye magnetic anomaly which had a coincident weak residual gravity anomaly. Follow-up drilling broadly confirmed the original magnetic interpretation, and outlined an estimated 200 Mt of low-grade iron-barium-lead-silver-copper-gold mineralisation. Both moving-loop and large fixed-loop time-domain electromagnetic surveys recorded a broad anomaly over the Abra mineralised system. The anomalous time-domain electromagnetic transients have an exponential decay with a time constant of about 1.3 ms, indicative of low conductance. Downhole time-domain electromagnetic and mise-a-la-masse surveying confirms that the whole of the mineralised system, including both the stratiform zone and the underlying stringer zone, is weakly and uniformly conductive. No specific zones of locally greater conductance, which might be indicative of higher-grade mineralisation, were detected in the surface time-domain electromagnetic work. In the downhole time-domain electromagnetic surveying, the broad response to the bulk mineralised body is complicated by the use of relatively small transmitter loops which have 'selectively' energised portions of the large conductive body, resulting in both 'in-hole' and 'off-hole' responses being recorded, depending upon the relative transmitter loop-conductor-drillhole geometry. Several localised secondary conductor responses (both in-hole and off-hole) were also detected but, to date, this work has not successfully demonstrated the presence of discrete high-grade zones of significant dimensions within the overall system.