The use of transient electromagnetics in the exploration for conductive sulphides associated with Cu-Au mineralisation, at Moonta, South Australia

Abstract

Three conductive sulphide bodies have been discovered near Moonta, South Australia, using transient electromagnetic (TEM) surveying. The host for the structurally controlled sulphide bodies is the resistive Moonta Porphyry, and these bodies are part of the Moonta lode system. The location of the initial TEM surveying was influenced by anomalous gold assays from mine dump samples, in close proximity to an untested induced polarisation (IP) anomaly. lnterpretation of the TEM data was complicated by cultural effects from past mining and present farming activity. The initial survey, using a coincident loop configuration, led to the discovery of a small uneconomic sulphide body, characterised by low grade Cu and an absence of anomalous gold grades. Further TEM surveying, using a large fixed transmitter loop and a small roving receiver coil, was conducted to explore the immediate area and to evaluate the TEM response of the small sulphide body as a function of loop size. This phase of surveying detected two additional discrete anomalies, both outside of the fixed transmitter loop. A follow-up survey using coincident loop TEM demonstrated that the anomalies were mainly due to vortex currents in discrete conductors and not to current channeling effects. lnterpretation of these responses led to siting of drill holes which intersected the Poona and Wheal Hughes conductive sulphide bodies. The Poona body contains economically recoverable Cu and Au mineralisation, with a conservative in-ground value of $30 million. The Wheal Hughes sulphide body likewise contains economic Cu and Au mineralisation.