De Beers sizes up pipes near James Bay

While it awaits results from bulk sampling on the Victor kimberlite pipe in the James Bay Lowlands of northern Ontario, De Beers Consolidated Mines (DBRSY-Q) is anticipating the likelihood of performing additional infill drilling this winter. In addition to Victor, five other targets of interest will be tested.

The Victor pipe occurs in a cluster of 16 kimberlites first discovered in 1988 by Monopros (now De Beers Canada Exploration), 90 km west of the First Nation community of Attawapiskat. The Victor kimberlite is the largest body in the cluster, comprising one large North pipe and one smaller South pipe. The pipes coalesce at surface and comprise a total area of about 15 ha.

Victor North has been drilled to more than 300 metres, whereas Victor South has been tested to shallower depths. A minimum of 6 metres of glacial and marine till cover the kimberlite bodies.

At a public information session in Attawapiskat, De Beers revealed that a 330-tonne mini-bulk sample of the Victor kimberlite, taken last year, yielded 107.9 carats of diamonds, with a value of US$16,590. These results suggest a preliminary grade of 0.33 carat per tonne, a diamond value of US$154 per carat and a kimberlite value of US$50 per tonne.

This past winter, De Beers collected close to 7,000 tonnes of kimberlite from two surface pits and nine large-diameter holes drilled into the Victor body to a maximum depth of 200 metres. From June to August, the bulk-sample was processed on-site at a sophisticated diamond concentrator mill at the daily rate of 240 tonnes. The concentrate material was flown to Johannesburg for final processing. Full results for this year’s work are anticipated by the end of 2000.

At a recent forum of the Toronto Geological Discussion Group, Bradley Wood of De Beers Canada Exploration, spoke about the company’s exploration program in the Attawapiskat area.

Between 1984 and 1986, De Beers first targeted the area between Kenogami River and Ekwan River by carrying out regional stream-sediment sampling. This work revealed a kimberlite indicator mineral train whose chemical composition was distinct from those indicators occurring in the area of the Selco melnoites, 250 km to the south. Moreover, it suggested an origin compatible with diamondiferous kimberlites.

Wood says the garnets were mainly peridotitic, falling in the cluster group G9, with less common G10 varieties. The recovered ilmenites had high Cr2O3 compositions of up to 8% weight, 7-15% weight MgO and low Fe3+/Fe2+ ratios. The spinels had kimberlitic compositions.

Kimberlite boulders were discovered in 1987 along a 10-km stretch of the Attawapiskat River, where stream samples were found to contain “super abundant” concentrations of indicator mineral grains. De Beers followed with an aeromagnetic survey, using both magnetic field and vertical magnetic gradient measurements, to cover 2,900 sq. km of ground. Thirty-three possible targets were defined with ground magnetics. Drilling in 1988 and 1989 confirmed 16 kimberlites.

The Victor pipes are two of at least eight bodies in the Attawapiskat cluster that contain crater-facies kimberlite. The other bodies appear to be composed of hypabyssal kimberlite. There appears to be no diatreme facies in these bodies.

Victor consists mainly of macrocrystic spinel- and perovskite-bearing carbonate kimberlite. Mantle-derived xenocrysts include ilmenite, garnet and chrome diopside. Two-thirds of Victor North and most of Victor South are infilled by poorly bedded, xenolith-poor pyroclastic kimberlite.

The pyroclastic kimberlite within Victor North can be subdivided into two major phases, with contrasting macrodiamond grades, suggesting that the Victor pipes were infilled by multiple phases of eruption. The geology of the northwestern third of Victor North is much more complex; it consists mainly of magmatic kimberlite, with some xenolith-rich volcaniclastic kimberlite.