Application of a modified GEOTEM® system to reconnaissance exploration for kimberlites in the Point Lake area, NWT, Canada

Abstract

Airborne geophysical surveying with electromagnetic (EM) and magnetic methods is an effective reconnaissance exploration tool for kimberlite pipes because the target can have an associated EM and magnetic anomaly. The EM response of kimberlite pipes is most often attributed to weathering alteration in a near‐surface layer, whereas the magnetic response is attributed to magnetite and ilmenite within the deeper unweathered kimberlite pipe. The discrete shape of kimberlite diatremes results in an easily identifiable anomaly pattern. Diamondiferous kimberlites have recently been found in the Northwest Territories (NWT) of Canada, an area glaciated in the Pleistocene and therefore devoid of a strongly weathered zone. By configuring the GEOTEM® airborne EM system to operate at high frequencies (270 Hz) and to take measurements while the transmitter is switched on, weakly conductive bodies may be detected because there is an adequate contrast with the surrounding highly resistive country rock. System modifications also allow the magnetic field to be sampled at an altitude of only 73 m instead of 120 m and ten times per second instead of once a second. This allows much better definition of weak, small magnetic anomalies. Data sets from two test areas (Point Lake and Willy Nilly, near Lac de Gras, NWT) demonstrate the effectiveness of the airborne system for reconnaissance surveying.