The latest bulk-sample results from the Gahcho Kue joint-venture project in the Northwest Territories indicate the presence of big-carat diamonds in the Hearne kimberlite pipe.
The Hearne and 5034 pipes are the two highest-value kimberlites occurring in a cluster of five pipes in the Kennady Lake area of the Gahcho Kue project, 275 km northeast of Yellowknife and 150 km southeast of Ekati. The project is owned 51% by the Canadian exploration division of
The aim of the recent bulk-sampling was to get a better handle on the distribution of the larger-carat, high-quality stones, such as the 9.9-carat diamond (valued by De Beers’ Diamond Trading Company at US$60,000) recovered from the east lobe of the 5034 pipe in 2001. The Diamond Trading Company (or DTC) has observed a sub-population of high-quality diamonds among the diamonds recovered to date from the Hearne and 5034 pipes. De Beers and its joint-venture partners believe the recovery of such top-quality stones may be the key to enhancing the economic viability of the project.
In the fall of 2000, De Beers tabled a scoping study of Gahcho Kue; however, the study fell short of the critical rate of return required for the company to proceed to the feasibility stage. The desktop study proposed open-pit mining for the 5034 and Hearne pipes, along with a high-grade zone in the upper part of the Tuzo pipe. The results of the study showed that a 15% increase in diamond revenues would push the project over the minimum threshold.
De Beers spent the winter of 2002 taking additional bulk drill samples from the 5034 and Hearne pipes to increase enhance its understanding of grade, diamond size, frequency distributions, and values. The two kimberlites contain internal zones that vary in grade.
The Hearne kimberlite consists of two kimberlite pipes containing a modeled resource of 7.2 million tonnes grading 1.71 carats per tonne, equivalent to 12.3 million carats worth an estimated US$63.3 carat. The main body, or the north lobe, is an elongated pipe extending more than 250 metres in length in a north-south direction. The south lobe is a smaller elongated pipe, measuring up to 100 metres long in an east-west direction.
Different phases
The two lobes are further sub-divided into several low- and high-grade phases. A lower-grade zone within the north lobe contains 1.6 million tonnes at 0.6 carat per tonne. This zone is 50-80 metres thick and occurs near the surface at the southern end of the north lobe. It dips steeply to the north and extends below a 300-metre depth at the northern end of the pipe.
During the 2002 winter program, De Beers drilled five large-diameter holes in to the north lobe of the Hearne kimberlite. In total, 1,174 carats of diamonds exceeding a bottom-size cutoff of 1.5 mm were recovered from 665.5 tonnes of kimberlite material.
The bigger stones in the parcel included 157 diamonds weighing between 0.5 and 1 carat apiece, 37 diamonds ranging between 1 and 2 carats, and 13 diamonds weighing greater than 2 carats each. The three largest diamonds recovered, weighing 8.7, 6.4 and 4.9 carats, illustrate the general potential for large stones. In the 1999 and 2001 bulk samples, the largest recovered diamond in each case was 3.4 carats. (During this stage of testing, it is worth noting that no diamonds larger than 10 carats can be recovered, owing to the 12-mm crush size De Beers uses to process the bulk samples.)
The implied grade of three closely spaced holes drilled 7-8 metres apart just north of the centre of the lobe ranged from 1.79 to 2.27 carats per tonne. Another hole drilled 40 metres north of this cluster of holes returned a grade of 2.32 carats per tonne. A hole, which primarily tested the lower-grade zone at the southern end of the lobe, delivered a grade of 0.92 carat per tonne. Together, the five holes averaged 1.76 carats per tonne.
The diamonds recovered from the Hearne pipe will be evaluated by the DTC in London, along with a previously reported 1,215-carat parcel recovered from the 5034 pipe. The 5034 contains a modeled kimberlite resource of 12.5 million tonnes grading 1.64 carats per tonne, or 20.5 million carats, to a depth of 300 metres. It is estimated that the 5034 diamonds are worth U$65.50 per carat, based on August 2001 values.
Upgrading
The results of the DTC valuation will be used by De Beers to update grade variation models for the 5034 and Hearne pipes, and generate new revenue models. The new modeling results are expected to be available in early 2003.
Still to come from De Beers are microdiamond results from delineation drilling on the Tuzo pipe and the MZ kimberlite sill. Tuzo contains a 10.2-million-tonne modeled resource averaging 1.22 carats per tonne at US$43 per carat. The top 80 metres of the pipe include a higher-grade 1 million tonnes grading 2.7 carats per tonne at US$47 per carat.
An additional 4.7 million tonnes of kimberlite on the western side of the pipe (from 200 to 360 metres of depth) and in deeper parts of the pipe (from 300 to 360 metres) have not been included in revenue- and grade-modeling. During the winter 2002 program, De Beers drilled an additional seven holes into Tuzo, two to a depth of 300 metres and five to a depth of 130 metres. A total of 8-10 tonnes of kimberlite were recovered for analysis.
Previous drilling in the MZ Lake area uncovered a series of land-based kimberlite dykes and sills, 20 km northwest of Kennady Lake. A 5-kg sample from one kimberlite intercept, dubbed Sill-73, returned 28 microdiamonds, with the largest being 1 mm. Sill-73 was further tested in 2002 with 10 follow-up holes, which cut numerous intervals of kimberlite ranging from 7 cm to 2.19 metres, with the largest combined kimberlite section in one hole totalling 3.49 metres, between 14.66 and 19.25 metres of depth. The sill appears to extend at least 1 km in distance. Samples of Sill-73 were sent to South Africa for microdiamond analysis.
Photo by Stuart McDougall
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