Diamond content of kimberlite pipes governed by zoning

The tendency for kimberlite provinces to show horizontal zoning with respect to age, mineralogy and diamond content has important implications for exploration in the Lac de Gras area of the Northwest Territories, said Russian diamond geologist Galina Kudrjavtseva on a recent visit to Toronto.

“Globally, the transition from central parts to the periphery of kimberlite provinces is marked by a decrease in the size of pipes, an increase in the development of sills, and an increase in mantle metasomatism (mineral replacement),” she concludes in a paper co-authored by Australian diamond expert Bram Janse.

The paper also points out that the older pipes at the centre of a craton (ancient, stable part of the earth’s crust) tend to have a higher diamond content than the younger pipes at the edge of a craton.

“Horizontal zoning is of great genetic and practical importance,” says the report. “It indicates the stage of kimberlite development within the province and therefore helps define the strategy to be used in the search for diamond-bearing kimberlites.”

In the new Arkhangelsk kimberlite province in the northwest corner of the former Soviet Union, for instance, the kimberlite clusters have been divided into three groups on the basis of mineralogy, petrology and geochemical characteristics.

The pipes in the richest cluster, Zolotitskoe, are characterized by an abundance of chrome spinels (an aluminum oxide).

Moving toward the eastern edge of the craton, the diamond content of the kimberlites diminishes, as does the abundance of spinels.

A similar structural pattern holds true for the Yakutian kimberlite province in Siberia, which has a radius of about 1,500 km, and the Kaapvaal province in southern Africa, which has a radius of roughly 750 km.

Recently given the go-ahead for production, the Arkhangelsk province contains about 150 pipes. The five economic pipes are reported to have an average grade of 2-4 carats per ton with a 50% gem-quality content.

On their Lac de Gras claim block, Dia Met Minerals (TSE) and BHP Minerals Canada have intersected 10 kimberlites containing significant quantities of macrodiamonds. The results impressed analysts familiar with the play and left some predicting that a world-class diamond resource would develop in the Northwest Territories.

After completing several years of surface sampling, Dia Met says the best potential for a minable diamond deposit in the Lac de Gras region lies on its 345,000-hectare claim block. The property is in the centre of the Slave craton.

Working on the southern fringe of the Dia Met block, a rival group consisting of Aber Resources (TSE), Commonwealth Gold (VSE) and SouthernEra Resources (TSE), has intersected seven pipes. Three of those pipes contain microdiamonds in minor quantities.

The area staked around Lac de Gras is estimated to be about 300 km long by 500 km wide.