Case Studies - Mineral Exploration

Ekati Mine, Northwest Territories

The Ekati Mine, jointly owned by BHP Diamonds Inc. and Dia Met Minerals Ltd., is Canada’s first diamond mine, producing diamonds from kimberlite pipes in the Lac de Gras region, Slave geologic province, Northwest Territories, Canada.

Helicopter-borne electromagnetic and magnetic surveys have played an important part in finding kimberlite pipes hidden beneath lakes and overburden, starting from the DIGHEM survey to confirm the suspected Point Lake Pipe for drilling in 1991. (The initial indications of diamond bearing kimberlites are related minerals in the overburden near the deposits.) Based on the Point Lake success, much larger surveys were conducted in the area in following years, and identified the Fox, Koala, and Panda economic pipes shown on these maps, as well as many other kimberlite pipes still in testing. Over 100 pipes have been found by BHP/Dia Met, using HEM and other techniques.

      Total Magnetic Field Data

The geophysical data, apparent resistivity and total magnetic field, clearly map the economic kimberlite pipes (Fox, Koala, and Panda) in this portion of the data from the several DIGHEM surveys flown over the BHP Dia Met claim group. The other pipes in the original Ekati plan, (Sable and Misery) are much farther away. Also apparent are the pipes more recently added to the mine - Koala North and Beartooth.

Kimberlite pipes intruded into the highly resistive crystalline rocks of the Slave geologic province appear as conductive targets to a DIGHEM survey. The pipe itself is conductive relative to the surrounding rocks. Kimberlite is very easily weathered, which reduces it to clay rich "yellow ground" and "blue ground" which are much more conductive than the un-altered kimberlite. The altered kimberlite is also very soft, and was easily eroded by the glaciers, after which the hole was filled in by conductive clay overburden and lakes. (Most North West Territories kimberlites are covered by lakes.)

The apparent resistivity map, calculated from the 7200Hz coplanar data of the DIGHEM survey, clearly shows the economic pipes in this data block. The Fox pipe (south west corner) has the most distinct anomaly, and coincides almost exactly to the overlying lake. The Koala and Panda pipes give clear anomalies also, and are also underneath lakes.

Apparent resistivity map, 7200 Hz

Kimberlite often has more magnetite than typical Slave rocks, so it is common for kimberlite pipes to cause magnetic anomalies. But this is not always so. Some pipes have inverted (low) magnetic anomalies, due to remanent (permanent) magnetisation which is opposite to the earth’s magnetic field. Two non-economic pipes, the Grizzly and Leslie, both have strong magnetic anomalies, but notice that the Leslie anomaly is positive (high) and the Grizzly anomaly is negative (low). The Koala pipe has a very weak anomaly, which could be easily overlooked in many conditions. The Fox pipe may have a weak anomaly, but it’s proximity to a diabase dike (the strong, linear anomalies on the magnetic data) makes it difficult to be certain.

Because kimberlite pipes exhibit variable anomalies on both electromagnetic and magnetic data, the best approach to mapping them is to simultaneously collect EM and magnetic data from a low flying platform. The DIGHEM system collects both data sets from a sensor at 30m altitude, sampling about every 3m (10ft).