Don, let me try to explain how to interpret these "pics". The airborne magnetic maps are always contoured. The contours represent the relative "magnetic intensity" of the "near surface" rocks. Ultramafic rocks, because of their inherent relatively high magnesium and iron contents which is reflected by their relatively high content of magnetic minerals such as magnetite, are much more "magnetic" than say granodiorite and consequently show up as "magnetic highs", with various shapes. Kimberlite pipes are ultramafics, and because of their frequent characteristic "pipe like X-sections" show up as circular mag high targets. It is the relative magnetic intensity of the various rock types which is being mapped. Some massive sulphide bodies could have high magnetic susceptibilities however.

Electromagnetic surveys (EM surveys) measure the electrical conductivity of bodies. Common examples of highly conductive bodies detected from EM surveys include graphite and massive sulphides. They usually show up as linear "anomalies".

Both types of surveys can be conducted either from an airborne platform or on the ground. Usually, especially in the case of the former, both are conducted simultaneously.

Typical MMS deposits will have coincident high mag and EM positive signatures. Why? Because Ni/Cu sulphides, conductors, are characteristically found associated with ultramafic/mafic rocks, i.e. relatively highly magnetic rocks.

Searching for VMS is different. The host rocks are generally not very magnetic. The sulphides are likewise not magnetic, but are good conductors. Just remember that the "signature" of the Kidd Creek orebody was one of a conductor only, no associated mag high.

Hope this helps.

As always

Cheers

geoprof