Exploration Geophysics - Volume 27, Issue 1, 1996

Long wavelength magnetic anomalies of moderate amplitude common across large parts of northern Australia are usually assigned to deep sources beneath relatively undisturbed mid Proterozoic or Phanerozoic cover rocks. These can be shown, in the McArthur Basin at least, to be largely due to thick volcanic sequences that either do not outcrop or only outcrop in thinned, onlapped variants near the basin margins. This alternative interpretation of the basin indicates that it might have a scale and formation content comparable with that of the adjacent western Mt Isa sequence and theories about basin formation, mineralising processes and fluid circulation could be reviewed.

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.

Gold mineralisation occurs near granitoid plutons in the Shebandowan granite-greenstone belt of the Superior Province in the Canadian Shield. Conventional structural and petrographic observations show that the Moss Lake Syenite pluton escaped penetrative, regional deformation. Nevertheless, anisotropy of magnetic susceptibility (AMS) reveals a well-developed, consistent, cryptic magnetic fabric, largely due to multidomain magnetite. The magnetic foliation is subparallel to the earlier regional schistosity and reveals the dissipating, regional stress field that was insufficient to produce conventional tectonic fabrics in the syenite. Palaeo-magnetic studies reveal that the characteristic remanence carried by magnetite scatters around the known palaeofield for this region. The feeble strain that imposed the (AMS) magnetic fabric barely affected the palaeomagnetic signal. Gold mineralisation occurs where shear zones intersect the eastern margin of the pluton. Late haematisation concentrates in the auriferous sheared zones. Unlike the palaeomagnetic signature of magnetite, haematite-remanences disperse in a plane parallel to the regional schistosity due to tectonic effects rather than magnetic refraction. We infer that the shear-zones' haematite-remanences developed after the "post tectonic", cryptic magnetic fabric was imposed on the pluton. Since gold is found in the haematised shear zones, we infer that gold mineralisation may postdate the feeble, "regional" deformation of the syenite and is therefore clearly postmagmatic.

In this paper, electrical potential due to a point source in an arbitrarily anisotropic medium is obtained by means of converting the relevant matrix equation into a Poisson's equation through an orthogonal coordinate transformation and a stretching transformation. In a geophysical exploration, depending upon the depth of a mineral deposit (target) from the plane of electrical potential measurements, the effects of electrical anisotropy may result in missing a target.