Beyond bump finding ? airborne electromagnetics for mineral exploration in regolith dominated terrains

Abstract

Airborne Electromagnetic (AEM) methods are very good for detecting conductive ore bodies within a resistive host, but have not been as successful for detecting ore bodies located beneath conductive regolith. We believe that in the effort to improve the ability of AEM systems to function as "bump finders" in regolith dominated terrains, the potential of AEM to contribute to other spheres of exploration activity in these settings has been neglected. The evidence available from Australian surveys suggests that AEM could be an effective alternative to aeromagnetics when mapping geology through cover ? particularly where the rocks are magnetically quiet and the presence of transported materials precludes the use of multispectral remote sensing data or airborne radiometrics. However, the success of AEM as a mapping tool in regolith dominated terrains is dependent on an adequate contrast in the conductance or conductivity-thickness product of saprolite developed over different rock types. An example where this contrast is present is in the vicinity of the Ida Fault in Eastern Goldfields Province of the Archaean Yilgarn Craton, Western Australia, where AEM has successfully mapped variability in the thickness and conductivity of saprolite developed over felsic and mafic rocks. Contrasts in the conductance of saprolite derived from different lithologies may be obscured by saline groundwater. Saline groundwater may also blur differences in the conductivity of in-situ and transported materials. This is the case in the Black Flag area, Eastern Goldfields Province, Yilgarn Craton, where AEM appears to map total regolith thickness but fails to differentiate between the saprolite and palaeochannel clays. On the other hand, if AEM can provide information on groundwater conditions, then the technology may have a role in mapping the dispersion of solutes within the regolith and in identifying possible sites of supergene enrichment. The potential of AEM in this regard is illustrated with reference to data from the Lawlers area, also in the Eastern Goldfields Province.