The Contribution of Magnetite to the Induced Polarisation Response of the Centenary Orebody

Abstract

The Centenary gold deposit is a concealed ore body containing over 2 million ounces of gold, located 80 km east of Leinster. 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 IP were trialled soon after discovery. The gravity showed major structures and delineated the magnetic dolerite, and a trial IP survey detected a significant chargeability anomaly over Centenary. Interestingly, the anomaly was broader than, and slightly displaced from, mineralisation. An honours study commenced to determine whether magnetite contributed to the chargeability response of the Centenary ore body, as historical petrophysics had suggested magnetic dolerite proximal to the ore body may be chargeable. The petrophysical study, integrated with the 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 lower chargeabilities. This supported a hypothesis that rocks containing both magnetite and pyrite at Centenary generate a larger IP response than rocks containing pyrite or magnetite alone.