PLACERS: Geophysical surveys of the Eneabba heavy-mineral sand field, Eneabba, Western Australia

Abstract

Magnetic petrophysical studies of ilmenite concentrates from the Eneabba area have shown that ilmenite-rich placer deposits can be detected with high-resolution magnetic surveys and that some of them can be remanently magnetised. Airborne magnetic surveys have been of limited use as the magnetic response of widespread surface laterite and cultural sources such as roads and buildings can mask the weak target anomalies. However, routine use of low-cost, high-resolution ground magnetics on east-west survey lines of up to 20 km length and spaced 1 km apart has proved successful in detecting the very weak anomalies which are typically less than 10 nT in amplitude. The results have been used to target exploratory drilling. Airborne radiometrics has failed to detect buried thorium-rich monazite placers except where exposed by mining. Although induced polarisation and resistivity surveys have been used to detect ilmenite elsewhere, relatively high costs have precluded their use at Eneabba. Ground-penetrating radar failed to detect the buried mineralised placers. Rutile-, zircon- and monazite-rich placers, which have minor concentrations of ilmenite, do not produce an observable magnetic response so extensive drilling is a necessary exploration technique. Because drilling is necessary and inexpensive, the routine application of geophysical techniques is restricted to ground magnetics, which is also inexpensive and able to quickly locate ilmenite-rich placers.