Case Studies - Mineral Exploration

Fort à la Corne Kimberlites

Introduction 

The kimberlites at Fort à la Corne, Saskatchewan, have shapes similar to flat discs of about 100 metres thickness and many 100s of metres in diameter. The kimberlites lie beneath 75 to 150 metres of cover, including Cretaceous sediments and overlying glacial overburden. They have no surface expression. The combination of the magnetic and coincident EM data provided by the GEOTEM survey is a powerful exploration tool in this environment.

The kimberlites were discovered through drilling of magnetic anomalies first identified on government airborne magnetic maps. The kimberlites are magnetic, in contrast to their non-magnetic Phanerozoic host rock. The magnetic responses from the crystalline basement, at a depth of 600 metres, are sufficiently longer in wavelength to be clearly differentiated from the sharper signatures of the nearer surface (75 to 150 metres depth) kimberlites.

Total Field Magnetic Intensity

Survey Specifications 

A combined GEOTEM/Magnetic survey was flown over a 12 km by 4 km area in 1996. The area contained 11 kimberlite bodies defined by magnetics (see Total Field Magnetic Intensity Map) and confirmed by drilling. The GEOTEM time-domain EM survey utilised both a 30 Hz and 90 Hz waveform frequency: the former with a 4 ms pulse width; the latter with 2 ms.

30 Hz/4 ms Apparent Conductivity Z-Axis Data 

Results

The GEOTEM data outlined 10 of the 11 kimberlites as high-resistivity anomalies and one as a low-resistivity anomaly. This one low-resistivity anomaly is also one of the strongest magnetic features, supporting other studies that showed a strong correlation between lower kimberlite resistivity and higher magnetic response.

Overall the resistivity background of the survey area is quite active with drill-proven glacial overburden depths of up to 130 metres. The 30 Hz GEOTEM data effectively sees through this cover which could mask the kimberlite signatures.

90 Hz/1 ms Apparent Conductivity Z-Axis Data

Acknowledgement

Airborne data presented with permission of Fort à la Corne Joint Venture (Kensington Resources, Monopros Limited, Cameco Corporation).

Reference

Jellicoe, B.C., Robertshaw, P., Williamson, P. and Murphy, J.D., 1998, Summary of Exploration Activities and Results for the Fort à la Corne Diamond Project, Saskatchewan, in Summary of Investigations, 1998, Saskatchewan Geological Survey.