Magnetic methods may be used to seek specific mineral deposits, map lithologies and structures and detect alteration. The most important magnetic mineral is magnetite, followed by pyrrhotite.

Rocks are magnetised by the presence of the Earth’s magnetic field and often also carry a natural remanent magnetisation (NRM) acquired in the past. The NRM may dominate in some volcanics and chemical sediments.

Magnetic anomalies, mapped in airborne, ground or marine surveys, are caused by local variations in rock magnetism. Their shapes depend on the geomagnetic field orientation as well as the source geometry and magnetisation. Interpretation aims at revealing what is in the ground, from the anomaly patterns.

The magnetic anomaly over the Wallaby gold deposit in W Australia
(below right) is caused by hydrothermal magnetite-actinolite alteration.


Following discovery of gold at Wallaby, drilling to about 450 m defined several almost flat-lying zones of gold. However, analysis of drill core magnetic susceptibility measurements (above) suggested the magnetic zone that contains the gold-bearing structures had a pipe-like geometry. This justified using plunging prism models to interpret the aeromagnetic anomaly.

The modelling results defined the plunge angle and direction and provided deep drill targets, leading to the discovery of additional gold zones all hosted by the magnetic alteration pipe (right).

From Coggon J, 2003, Magnetism – key to the Wallaby gold deposit: Exploration Geophysics v 34 p 125-130.

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