Pyrrhotite-induced aeromagnetic anomalies in western Germany

Abstract Magnetic anomalies from four sites in Germany are described together with their interpretation based on magnetic modelling. The tops of the source bodies are situated several hundred meters below the surface. Boreholes at all the sites enabled the identification of pyrrhotite as the carrier of magnetization. In all four sites the pyrrhotite was found in metamorphic rocks of Palaeozoic age. Peak values of the remanent magnetization are as high as 13 A/m. The ratio of remanent to induced magnetization ranges between 3 and 70. The occurrence of pyrrhotite at the surface is rare due to oxidation. In three of the four sites the shapes of the anomalies correspond to magnetization directions clearly deviating from the present geomagnetic field direction. Computations of the magnetic field of rhombohedric model bodies with inverse directions of magnetization showed that, with increasing depths, the minimum of their anomalies decreases quicker than the maximum. That means that some positive anomalies (indicating a “normal” direction of magnetization) of deep-seated bodies may turn into an anomaly with maximum and minimum at smaller depths (indicating rather “inverse” direction of magnetization).