An appraisal of the geophysical effects of the Mount Isa ore bodies

Abstract

In past years, regional to detailed gravity and aeromagnetic surveys have been employed to develop a greater understanding of the geology in and around the Mount Isa copper and silver?lead?zinc ore bodies. In recent years, several downhole electromagnetic (DHEM) surveys have been conducted, primarily to target further massive sulphide mineralisation. This paper will present some of the results from these geophysical surveys. Gravity data show a complex but clearly anomalous effect over the mineralisation. A large negative anomaly is correlated with a zone of deep oxidation and leaching in the footwall, with a complex but strong positive anomaly associated with the silica dolomite alteration and silver?lead?zinc mineralisation. Aeromagnetic data over the ore zone are partly obscured by the cultural effects of the city and mine. The removal of the cultural effects has been attempted by measuring the field at a number of terrain clearances and then calculating an actual vertical gradient. These vertical gradients were then used to compute a 'culture' filter. The data, after filtering the cultural effects, suggested that the silver?lead?zinc ore bodies produce an anomaly with a magnitude of approximately 35 nT. Two holes have been probed using DHEM. The results show a complex superposition of responses from many sources and demonstrate the applicablilty of DHEM in exploring for base-metal mineralisation, within the conductive environment at Mount Isa. They also show the heterogeneous nature of the system given the differing responses from both holes.