Interpretation and Modelling, Based on Petrophysical Measurements, of the Wirrda Well Potential Field Anomaly, South Australia

The Wirrda Well Prospect is located approximately 20 km south-southeast of Roxby Downs and 500 km north-northwest of Adelaide, South Australia. The Wirrda Well deposit consists of a massive granite breccia. Within the breccia, hematite and magnetite are pervasive, and copper, uranium and gold are associated. Evidence suggests this distinctive deposit may be an analogue of the Olympic Dam orebody.

A variety of petrophysical measurements have been made on drill core samples from the Wirrda Well deposit, including density, gamma activity, compressional velocity and thermal and electrical conductivity. Detailed magnetic studies were also undertaken, comprising magnetic susceptibility, natural remanent magnetisation, partial demagnetisation, anisotropy and saturation magnetisation measurements.

Results of these studies enabled comparisons to be made between the physical properties of the host rocks, the cover sequences and the surrounding country rocks. Generally, the host rocks are of greater density, have a higher magnetic susceptibility and gross gamma ray activity, and are better conductors of thermal and electrical energy than nearby unmineralised rocks. The main carrier of magnetism was found to be multi-domain, coarse-grained magnetite and remanence was a significant component of the total magnetisation.

The Wirrda Well potential field anomaly consists of near-coincident magnetic and gravity highs. Modelling of the ground magnetic data, constrained by geological logs and petrophysical measurements, could not account for the observed anomaly, suggesting the main causative body of this anomaly has not yet been intersected by drilling.

From modelling of the gravity anomaly, it was demonstrated that only half of the magnitude of the anomaly can be resolved, utilising bodies constrained by drillhole data. It has been concluded that the remainder of the anomaly may be explained by the presence of a central vertical pipe with three apophyses radiating to the northwest, southeast and southwest, that are largely untested by drilling.