Vancouver — Three large-diameter (24-inch) reverse circulation drill holes have been completed to date on the kimberlite body dubbed 141 at the Fort la Corne joint-venture diamond project in northern Saskatchewan. The material is part of a mini-bulk sampling program to evaluate the potential of a number of high- priority kimberlites.
The Fort la Corne project comprises 255 sq. km of ground east of Prince Albert and contains 69 confirmed kimberlites. At least 49 of the bodies are diamondiferous, and 34 have yielded stones larger than 1 mm in at least one dimension. De Beers and Kensington each hold a 42.25% stake, whereas Cameco and its wholly owned subsidiary, UEM, hold the remaining 15.5%. The 2001 exploration program has been funded by Kensington, De Beers, and Cameco at a cost of $4.79 million.
The three holes completed on the 141 kimberlite total 409 metres and have produced 159.4 tonnes of wet chips, all greater than 1.5 mm in size. The holes were drilled to a depth of between 230 and 255 metres and the material has been shipped to the De Beers preparation facility in Grand Prairie for diamond recovery processing.
In addition, four drill holes were selected by the partners for multi-parameter down-hole geophysical surveys. The surveys are designed to collect natural gamma-ray spectrometry, magnetic susceptibility, density and sonic data. These data will be compared with petrographic descriptions to highlight different types of kimberlite and layering within the body. Recent core examination supports the theory that the 140 and 141 kimberlite bodies form one single entity, which is estimated at 932 million tonnes. The original geophysical signature was interpreted as two separate bodies.
The first phase of this year’s program saw completion of 16 NQ-sized core (diameter of 46.4 mm) for a total of 2,413 metres of kimberlite intersections on the 140, 141 and 150 bodies. The goal was to assess the zonation pattern in the kimberlites and to refine the geological models for each body.
Based on a limited parcel of stones weighing 21.06 carats — recovered from a 251.8-tonne mini-bulk sample taken last year from kimberlite 141 — De Beers modeled a grade of 18 carats per 100 tonnes, with a best-fit value of US$153 per carat, or US$28 per tonne. Kimberlite 150 has a predicted grade of 16 carats per 100 tonnes and is reported to have good potential for larger diamonds.
A decade of work, costing in the order of $22 million, has confirmed the presence of 69 kimberlites. Individual bodies measure up to 184 ha in area. To put this into perspective, the pipes in the Northwest Territories typically cover 1-to-3 hectares. The top three targets — kimberlites, 140-141, 148 and 150 — are situated within a 2-km radius and contain an estimated 1.9 billion tonnes of kimberlitic material.
The Fort la Corne area is different from other kimberlite fields because it is almost entirely intact. When the pipes were first emplaced 100 million years ago the area was at the edge of a vast sea that covered the central portion of North America. The pipes were buried in sediments which became rock. Over time, erosion stripped the overlying rock layers and revealed an almost intact kimberlite field. Kimberlites in the Territories are younger, but have been eroded to deeper levels and have smaller surface expressions.
Southeast of the Fort la Corne joint venture property,
The kimberlite was first discovered in 1996, and was defined over a 4-sq.-km area, with a minimum thickness of 30 metres. It is now known that the actual area of kimberlite exceeds 100 metres in thickness and extends over about 0.5-sq.-km area. The most spectacular intercept drilled to date was hole 20, which cut 539 metres of kimberlite before it was terminated at a depth of 627 metres. This is comparable to the height of the CN Tower in Toronto.
This year’s drill program is designed to test the thickness, geometry, and consistency of the Star kimberlite towards the margins of the body. It will also provide additional geological information on certain areas within the drill-defined kimberlite structure where the drill-hole spacing exceeds 200 metres. At last report, four of seven NQ-sized diamond drill holes have been completed and logged. Preliminary estimates of continuous diamond-bearing kimberlite exceed 400 million tonnes. This figure does not include kimberlite at depths in excess of 150 metres.
Analysis of 16 drill holes drilled last year produced 523 diamonds, including 120 macrodiamonds, from 2,245 kg of sample. The total average grade of all 16 drill holes in last year’s first-phase program resulted in an average grade of 0.28 carats per tonne.
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