Heterodyne Method for Sulphide Mapping. Latest field data

The Heterodyne Method attempts to use the non-linear electrical properties of certain minerals (specifically sulphide minerals) to map these in the subsurface. The method attempts to generate heterodyne frequencies detectable at the surface over known sulphide occurrences. Several field tests of this method have been undertaken over many years. This paper summarises the latest results obtained and difficulties faced in developing a practical field survey technique for this method. A field test of the technique was carried out at the Mallee Bull sulphide deposit near Mt. Hope NSW in August 2023. The Mallee Bull deposit is a steeply dipping massive sulphide lens which is strongly conducting and was discovered using heliborne electromagnetic techniques. East of the deposit is a broad zone of disseminated pyrite and pyrrhotite that is relatively close to the surface. This zone is IP responsive and conducting in parts. The heterodyne survey was designed to cover both the deposit and the zone of disseminated sulphides. For reference a gradient array IP survey was run at the same time using the same electrodes. Complex technical and logistical difficulties arose in the conduct of this survey. The technique produced clear heterodyne signals showing that non-linear conduction is occurring as was found in previous field surveys. However, finding the source of these signals remains problematic. Technical problems during the survey has meant that only one survey line of data can be shown to be completely valid. The data analysis revealed what appears to be a clear correlation between anomalous heterodyne signals and known sulphide occurrences, particularly the area of disseminated sulphides. The survey has highlighted that the recorded data contains significantly more information than was previously considered and work is ongoing to unravel this complicated data and determine its exact geological significance. Valuable insights into the possible uses of this technique have been gathered in this field test and many unforeseen logistical problems have been resolved. Unfortunately, the question of the practicality of the Heterodyne method has not yet been fully resolved.