The contribution of magnetite to the induced polarization response of the Centenary orebody^*

Abstract

The Centenary gold deposit is a concealed ore body located 110 km north of Leonora, Western Australia. The orebody is associated with sulphides and is hosted in the magnetic portion of the Mount Pickering Dolerite. Due to its sulphidic nature, both gravity and induced polarisation (IP) were trialled soon after discovery.

The gravity survey showed major structures and delineated the host magnetic dolerite, and a trial dipole–dipole IP and resistivity survey detected a significant chargeability anomaly over Centenary. Interestingly, both forward and inverse models showed an IP anomaly that was broader than, and displaced from, mineralisation. Down hole IP and resistivity surveys also showed an elevated chargeability response shallower and broader than the intersected mineralised zone. Pyrite is the main sulphide associated with Centenary and is spatially related to gold mineralisation. These data therefore suggested that pyrite was not the sole contributor to the chargeability response of Centenary.

Petrophysical results, integrated with examination of thin sections, found that the five samples giving the highest chargeability response contained at least 5% pyrite and 5% magnetite, and at least 15% magnetite and pyrite combined. Samples with comparable amounts of pyrite, but less magnetite, gave a lower chargeability response. This supports a hypothesis that rocks containing both magnetite and pyrite at Centenary can generate a larger IP response than rocks containing pyrite or magnetite alone.