Drillhole TEM surveys at Thalanga, Queensland

Abstract

Several examples are presented of drillhole transient electromagnetic (DHEM) surveys from the Thalanga area of northern Queensland, where the mineralisation comprises stratiform bodies of zinc, lead and copper volcanogenic massive sulphides. The first example demonstrates that in favourable situations conductive massive sulphides can be detected for a distance of at least 150 m from the drill hole. In the second example discrete DHEM anomalies due to massive sulphides which change in sign from early to late delay times are interpreted to arise either from current channelling or the drive-delay effects of a conductive earth. The third example shows another case of sign reversals but the signs are opposite those expected and although several explanations are proposed, none are entirely satisfactory. The last example illustrates the effects of loop size on DHEM anomalies in an area of high surface conductivity and it is concluded that small loops using relatively high currents are preferable to large loops in maximising the response of off-hole massive sulphide conductors relative to the host rock.