Aeormagnetic data helps in diamond hunt

Diamond exploration activity in the Lac de Gras area in the Northwest Territories and elsewhere in the country has been a considerable boon to Canada’s otherwise sluggish mineral geophysics industry.

The great majority of the world’s known kimberlite pipes possess distinct magnetic and/or electromagnetic signatures. This knowledge has led to a considerable amount of work with airborne geophysical data over Canada’s north, both old and new.

Grassroots diamond exploration often covers large acreage. Where the competition for ground is fierce, the rapid assimilation of available data is necessary to locate the most prospective areas. Once the ground is tied up, there may remain large amounts of data to be analyzed. This is particularly true of aeromagnetic data, which is often the primary tool in the search for kimberlites.

The famous Swiss mathematician, Leonhard Euler (1707-1783), developed a homogeneity relationship which was adapted by Thompson (1982) to locate the sources of magnetic anomalies measured along flightlines. The method, called Euler deconvolution, was further developed into a full 3-D implementation by Reid et al (1990) to work with gridded magnetic data. The latter technique is available in a commercial form known as GRIDEPTH (a trademark of Simon-Robertson, Llandudno, Wales).

The method was initially applied to structural and depth-to-basement mapping in petroleum exploration. However, it was soon adopted by mining geophysicists for delineating contacts, dikes, sills, etc.

Figure 1 shows the magnetic anomaly due to a dike (magnetic inclination = 45 degreesAE). The application of Euler deconvolution to the gridded magnetic data yields a series of solutions that locate the dike (Figure 2). The diameters of the circles are proportional to the depth, which is 1 km in this case. In Figures 3 and 4, we see the anomaly and Euler results for a vertical pipe.

One advantage of Euler deconvolution is that we can target a specific geological model as demonstrated here. This is related to the manner in which the magnetic field dissipates as one moves away from the source. The usefulness of the Euler technique to locate pipe-like bodies was demonstrated in Paterson et al. (1991). Case studies were undertaken in the James Bay Lowlands and the Kirkland Lake, Ont., area, where diatremes had previously been located by aeromagnetics and other exploration techniques. A low level aeromagnetic survey located several pipes for Selco Mining Corp. and ESSO Minerals Canada near James Bay. Euler deconvolution easily confirmed their locations.

Of more significance, the older Geological Survey of Canada (GSC) aeromagnetic data (800 metre line spacing, 300 metre flying height) also yielded confirmed pipe targets when processed with the Euler technique. Near Kirkland Lake, several kimberlites were found by Lac Minerals (TSE) using the Ontario Geological Survey’s KLIP dataset. The application of Euler deconvolution was able to very rapidly locate these targets in a geologically difficult area of dikes, greenstones and intrusives.

Figure 5 shows contours of the total magnetic field in the Lac de Gras area, from a survey flown in 1963 by the GSC (800 metre line spacing, 300 metre flying height). Superimposed are the Euler results for a vertical pipe target. The BHP-Utah/DiaMet discovery is marked by a rather nondescript magnetic low. One would be hard-pressed to interpret such an anomaly as a pipe target under normal circumstances. However, we see that Euler deconvolution does indeed demarcate the area as prospective, albeit not entirely clearly (the more “focused” the circles are over an anomaly, the better). Elsewhere, we do see other potential pipes well delineated.

There is some element of interpretation required. For example, the magnetic anomaly where a dike is delineated. There is some element of interpretation required. For example, the magnetic anomaly where a dike is truncated or offset is similar in form to that of a pipe, often resulting in Euler solutions at these locations as well.

Stephen W. Reford is senior geophysicist with consulting firm Paterson, Grant & Watson Ltd. of Toronto.