Case Studies - Mineral Exploration

DIGHEM Helicopter Electromagnetic Survey for Kimberlites: Suzie Pipe, Lac de Gras, N.W.T. 

The "Suzie" kimberlite pipe, is located in the Lac de Gras area, Northwest Territories, Canada. The pipe was originally detected by a DIGHEM^V survey conducted for Tanqueray Resources in 1992. Located under the western edge of a lake, the anomaly due to the pipe is obvious on the apparent resistivity maps, and the strength of the response is distinct from the weaker response over the other lakes in the area. The pipe is buried under about 20m of glacial till, and has poor "down-ice" geochemical indicators and no surface indications.

 7200 Hz Apparent Resistivity

Digital Profile

 Digital Line Profile

The anomaly is obvious on the profiles as a strong peak on the 56,000 Hz and 7200 Hz coplanar EM channels (in-phase and quadrature). These are designated as CPI56k and CPQ56k, and CPI7200 and CPQ7200 on the profile plot above. It is also apparent as a weak (quadrature only) peak on the low frequencies (CPQ900), and as a low on the corresponding resistivity profiles (RES56k, RES7200 and RES900). This suggests that the weathered layer is thin enough that the lower frequencies see partially or completely through it, leaving the high frequency as the defining anomaly. The 900Hz frequency resistivity is higher than the 7200Hz or 56kHz resistivity, suggesting that the conductive weathered layer on this pipe is relatively thin.

The coaxial EM data (CX5500) shows the edges of the conductive anomaly, indicating that it is a "flat disk-like conductor" characteristic of the weathered zone at the top of a kimberlite pipe. This geometric information aids the interpreter in rejecting non-pipe anomalies.

The pipe is located under the western edge of a lake. The response from the lake can be seen in the profile data by the wide quadrature-only anomaly in the high frequencies east of the main anomaly on the profile, for example CPQ7200 at fiducial 4545.

56,000 Hz Apparent Resistivity

The location of the pipe is obvious on the apparent resistivity maps, and the strength of the response is distinct from the responses over the other lakes in the area.

 56,000 Hz Apparent Resistivity

Total Magnetic Field

The magnetic anomaly is a low relative to the circular magnetic high caused by the surrounding granodiorites into which the kimberlite pipe intruded. Had the pipe intruded elsewhere, the magnetic anomaly may have been indistinguishable. Where it is now, it may not have been detected as a kimberlite pipe without the EM anomalies.

 Total Field Magnetics