The Fort la Corne joint venture will spend $3 million on drilling in Saskatchewan as it tries to learn more about the internal geology and diamond distribution of the kimberlite targets in question. Large-scale bulk sampling will follow.
A drill program of up to 45 HQ-size (2.5-inch-diametre) core holes is targeting kimberlites 140/141, 150, 148 and 122, about 65 km east of Prince Albert. Drilling is also planned for a gravity anomaly discovered last year directly west of the 140/141 kimberlite, and gravity surveys will try to define the size and shape of the 148, 150 and 122 kimberlites.
The 225-sq.-km land holdings of the Fort la Corne joint venture contain a total of 63 kimberlite bodies. In light of substantial improvements in airborne geophysical techniques, the joint venture plans to fly a magnetic gradiometric survey over its entire holdings to provide higher-resolution coverage of the kimberlite field.
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In June, Kensington reported bulk-sample diamond recovery results from the 2002 large-diameter drilling on the 140/141 kimberlite. While the preliminary results revealed several large-carat diamonds (including a 10.23-carat stone described as complex, partially resorbed, with evidence of internal twinning and local concentrations of inclusions), the 140/141 kimberlite appears highly complex, with a lower-than-expected grade.
“The 140/141 kimberlite is now understood as consisting of at least several eruptive centres (vents) generated at different times and which served as conduits for multiple subsequent eruptions,” writes John Kaiser, publisher of the Kaiser Bottom-Fishing Report. “Some of these eruptions were of an explosive Strombolian style that generated ash columns up to 15 km high, which resulted in both lateral and vertical sorting of the kimberlite on the basis of grain size. Other eruptions were quieter, Hawaiian-style lava fountains which could have distributed coarser grained layers of kimberlite. The result is that not only does grade vary as you move sideways from vents and up the vertical column of the eruptive phase, but you also get sorting of diamonds by size, which disrupts the randomness that is critical to the usability of lognormal size distribution statistical concepts for grade and value prediction.”
In total, a 93.06-carat parcel of diamonds greater than a 1.5-mm screen cutoff was recovered from 1,272 tonnes of sample, for an actual overall grade of 7.32 carats per 100 tonnes. Three of the large-diameter holes (36 inches) were positioned on the northwestern centre of 141 in a tight cluster less than 20 metres apart and close to core hole 141-29 and large-diameter hole (24-inch-diameter) 141-04, which returned an actual sample grade of 9.3 carats per 100 tonnes (including two diamonds greater than 1 carat).
In total, 450 diamonds weighing 48.24 carats were recovered from a combined mass of 729 tonnes of excavated “mega-graded bed” material from these three drill holes. Thirty stones larger than 0.25 carat were recovered. The largest stones in each of the three drill holes came in at 0.355, 0.86 and 1.04 carats. The average actual sample grade of the three holes was 6.62 carats per 100 tonnes. The mega-graded bed appears to represent a sizable portion of the 141 sector of the combined body.
A lone hole into an interpreted younger vent at the boundary of 140/ 141 sectors yielded 143 diamonds weighing 16.52 carats from a 176-tonne sample of very-fine-to-coarse-grained pyroclastic kimberlite, for an actual grade of 9.36 carats per 100 tonnes. This sample included the 10.23-carat stone. Six other stones larger than 0.25 carat were recovered, including a 0.835-carat diamond.
Two large-diameter holes drilled into the 140 sector sampled a coarser-grained xenolithic kimberlite breccia, delivering 127 diamonds weighing 23.95 carats from 185 tonnes of sample, for an implied grade of 12.9 carats per 100 tonnes. The south breccia beds held 14 stones larger than 0.25 carat, of which five were greater than 1 carat weighing 1.16, 1.82, 2.57, 2.59 and 3.61 carats apiece.
“These are impressive recoveries and, if not a product of sorting, suggest a very coarse size distribution for the diamond population within the kimberlite phase that produced the south breccia beds,” says Kaiser.
De Beers has completed an updated conceptual study of the Fort la Corne project, which is now in the hands of its joint venture partners. Kensington says it will release projected grade forecasts and valuations based on the 2002 bulk-sampling program.
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