JUNIOR MINING — Helicopter-borne magnetics help in northern

Helicopter-borne geophysical surveying for kimberlites in the Northwest Territories has been remarkably successful. The thin or non-existent glacial cover and the highly resistive bedrock are ideal conditions for the HEM (helicopter-borne electromagnetic) resistivity and high resolution magnetic mapping techniques.

Electromagnetic surveying has been successful at detecting the conductive clays resulting from the weathering of kimberlite. The exposed Archean bedrock in the Slave Province is fresh, crystalline, and highly resistant to induced electrical currents.

It is the highly resistive nature of the background geology and the lack of cover which allows the electromagnetic resistivity technique to be so successful at outlining kimberlites.

Glaciers have stripped the overlying layers from the Archean basement and spread indicator minerals for hundreds of kilometres. Kimberlite, a relatively soft rock, was more easily eroded by glaciers than the crystalline Archean and therefore most pipes are covered by lakes.

Lake bottom sediments are at least an order of magnitude more conductive than the surrounding Archean bedrock. This sedimentary layer can give a response of similar characteristics to a kimberlite source. Discriminating between kimberlites and conductive sediments requires the identification of a vertical cylinder source as opposed to a horizontal thin plate, a task accomplished by using both the wide range of frequencies and the orthogonal coil configuration of the Dighem V system.

In other kimberlite fields of the world, magnetic surveying has been the primary geophysical method. The magnetic anomalies are due to the magnetic minerals present in the ultramafic kimberlite diatremes (magnetite, ilmenite). The kimberlites of South Africa, Australia and Siberia intrude into hundreds of metres of sedimentary rock with very low magnetic susceptibility, providing a magnetically quiet background for the detection of the often subtle kimberlite magnetic anomaly.

By contrast, the magnetic signature of the background geology in the Lac de Gras area is complex because of several episodes of diabase dyke intrusion and mafic volcanic flows. A low magnetic signature from a kimberlite source is often difficult to separate from the overprinting of these stronger magnetic sources.

Although not all kimberlites have recognizable magnetic signatures, many of the pipes in the Lac de Gras area have negatively polarized remnant magnetism. Both positive and negative magnetic anomalies have been recognized over kimberlites. When a magnetic anomaly is associated with an electromagnetic response it has been a valuable guide for isolating kimberlite sources.

In the Lac de Gras area, the combination of high resolution total field magnetic data and high frequency helicopter-borne resistivity mapping has been the most successful method of pinpointing kimberlites.

— John Buckle is manager, marketing and sales, for Dighem Surveys & Processing Inc., Mississauga, Ont.