Negative magnetisation contrast in Kimberlite search

Abstract

The magnetic method is the most widely used method in the search for kimberlites. Usually Kimberlites contain magnetite as an accessory mineral. Unweathered Kimberlites usually produce conspicuous magnetic anomalies. But the weathered kimberlite is different. It loses its magnetic property considerably and becomes practically nonmagnetic, depending on the degree of weathering. It may appear that weathering precludes detection by the magnetic method, but such a kimberlite, when the degree and extent of weathering are considerable, in the midst of magnetic country rock, becomes an excellent target for magnetic surveys. The magnetisation contrast between the weathered kimberlite and the magnetic country rock becomes negative. Such a weathered kimberlite manifests itself on the magnetic map giving a picture that is contrary to the normal picture with positive magnetisation contrast. An experimental ground magnetic survey was conducted over a known, highly weathered kimberlite pipe in the Wajrakarur region of Andhra Pradesh, South India. It yielded a conspicuous magnetic response characterised by a smooth broad gradient right over the pipe rock, flanked by 'high' and 'low' closures on the northern and southern margins of the pipe respectively. Modelling was carried out by approximating the pipe to a vertical cylinder-like body with polygonal cross-sections and by assigning a negative magnetisation. The computed and the observed magnetic contour maps show a good correlation. This work demonstrates the efficacy and the usefulness of this approach of ascribing negative magnetisation to the weathered kimberlite to locate hidden kimberlites.