The Maggie Hays and Emily Ann nickel deposits, Western Australia: a geophysical case history

Abstract

Geophysical techniques played a significant role in the discovery of the Emily Ann massive nickel-sulphide deposit and extensions to the Maggie Hays deposit, which are associated with komatiitic olivine cumulate ultramafic rocks, in an Archaean greenstone belt located about 500 km east of Perth, Western Australia. Detailed aeromagnetic surveys were used to outline komatiitic rocks and structures. Physical property measurements on drill core showed the mineralisation to be highly conductive and magnetic. Trial induced polarisation, audio magnetotelluric and time-domain electromagnetic (TEM) surveys indicated that the latter had the most potential for detecting the nickel-sulphide mineralisation. The Maggie Hays deposit comprises both disseminated and massive nickel-sulphides concentrated at the base of an ultramafic unit 200-500 m below the surface. A limited moving-loop TEM survey in 1992 located an anomaly immediately north of the main part of the deposit. Diamond drilling of this anomaly failed to discover extensions to the deposit or explain its source. In 1995, a fixed-loop TEM survey delineated an excellent response confirming the earlier moving-loop anomaly, which when drilled, resulted in the discovery of the Maggie Hays North zone 100 m below the surface. A moving-loop TEM survey resulted in the discovery of the blind, high-grade Emily Ann nickel-sulphide deposit 3 km north of the Maggie Hays deposit, at a depth of 120 m. Downhole TEM surveys aided delineation diamond drilling of the deposits with the location of extensions of mineralisation. High-powered, late-time moving-loop TEM, with fixed-loop TEM follow up, is currently being used routinely to explore for additional deposits. However, the highly conductive overburden response obscures the signal from bedrock conductors, which are often represented only as low-amplitude, late-time anomalies. Geophysical targeting is further complicated by the close proximity of highly conductive barren banded iron formation-hosted massive sulphides. Trial airborne EM surveys have detected Maggie Hays North, but not Emily Ann and probably not the Maggie Hays main zone.