Magnetic characteristics of some pyrrhotite-bearing rocks in the United Kingdom

Rocks containing the sulphide mineral pyrrhotite are often associated with pronounced aeromagnetic anomalies. The magnetic properties of these rocks have been examined in many laboratory studies. An attempt is made, based on results from SW England, to combine the magnetic anomalies and the magnetic properties in order to improve the understanding of any inter-relationship. It is shown that the pyrrhotite is characterised by high Q-values and well-developed magnetic anisotropies. The pyrrhotite-bearing rocks therefore retain a strong remanence but the direction of this has been strongly influenced by the form and nature of the magnetic bodies. The magnetic anomalies in SW England are extensive and it is suggested that this resulted from the way in which pyrrhotite is readily formed in suitable horizons within sedimentary rock sequences. Pyrrhotite is also easily destroyed by weathering of outcrops so that the availability of borehole cores plays an important part in the study of the area.