EXPLORATION ’92 — Lac de Gras compares with Siberian diamond

Several diamond exploration experts have drawn parallels between the diamond-producing regions of Siberia and the Slave province of the Northwest Territories, where exploration is still in its early stages.

Both are underlain by a large Archean craton, or “archon” as it is called in the diamond business. Both have been pulled apart by extensional forces, creating structures favorable for kimberlite emplacement.

And kimberlites on either side of the Bering Strait appear to share the same level of erosion although data from the Northwest Territories remain scanty. “The mineral chemistry is also beginning to look very similar,” says Christopher Jennings, president of SouthernEra Resources (TSE) and a well-known diamond pundit. “There is an unusual amount of chromites in both Canada and Russia.”

The two regions have more than geology in common. Inaccessibility, harsh conditions and a thin population base also characterize the northern neighbors.

“We are at the same stage Russia was at in the 1950s,” says Jennings. “The Russians had nothing in 1950 and now they have about 650 pipes.” Diamond exploration began in earnest in the Soviet Union just after the Second World War, when Russian geologist Vladimir Sobolev recognized the similarities between the central Siberian shield and the diamond-producing Kaapvaal craton of southern Africa.

Teams of prospectors were sent to the mouths of rivers in central Siberia and to the Arctic coast to search for diamonds and their indicator minerals. Years later, the first diamond was found in a tributary of the Vilyui River at the heart of the Siberian shield.

Reports of pyrope garnets found in association with the diamond piqued the interest of geologist Larisa Popugaieva. She joined one of the prospecting crews and in 1954, found the first kimberlite in Yakutia, a semi-autonomous region in north-eastern Siberia.

It would be yet another year before a commercial kimberlite, the Mir pipe, was discovered. But by 1956, 40 occurrences had been found in the vicinity of the Arctic Circle.

The town of Mirny was established in 1957 to service the fabulously rich Mir pipe, where grades average more than 60 carats per ton and about 20% of the diamonds are of gem quality. Now reaching the end of its life, the open-pit mine has produced millions of dollars worth of gems.

Canada’s Popugaieva is Charles Fipke, whose determined search for diamonds took him 750 km from the Mackenzie Mountains in the west to Baker Lake in the east. After years of sampling in the barrens, his company Dia Met Minerals (TSE), along with partner BHP Minerals Canada, discovered the diamond-bearing Point Lake pipe in the Northwest Territories that sparked the exploration frenzy now under way.

As kimberlite discoveries in postwar Siberia became frequent, a definite pattern emerged that mimicked the layout of pipes in South Africa. In general, the kimberlites occurring at the heart of the shield were diamondiferous whereas those at the margins were barren.

This phenomenon, known as Clifford’s Rule, has been used to establish prospective regions throughout the world. The “Archon” is a stable part of the Earth’s crust that has experienced little deformation over long periods of time.

“Canada has the largest expanse of Archean basement in the world,” A.J. Janse, a diamond exploration consultant in Australia, told delegates at the Prospectors and Developers of Canada conference last March. “Therefore, Canadians have the best opportunity to find a diamond mine.” Because of its tectonic environment — including a number of reactivated basement structures — Janse favors the Hudson Bay Platform as an exploration target.

But geologists working in the Lac de Gras area of the Northwest Territories point to the Mackenzie dyke swarm trending north-northwest from Great Slave Lake to the Coronation Gulf as another significant structure that may be related to kimberlite emplacement.

Dubbed “The Corridor of Hope,” the swarm runs through Dia Met’s claim block on Lac de Gras and encompasses the kimberlite pipes recently found by the joint venture of Aber Resources (TSE), SouthernEra and Commonwealth Gold (VSE).

Many of the 12 kimberlite regions of Siberia are also associated with distinct structural trends. The Middle Olonek field, for instance, includes numerous clusters along 250 km of a northwest trending alignment. Studies completed on the Russian pipes suggest that the Lac de Gras pipes were formed over several intrusive episodes ranging in age from 75 to 450 million years.

Jennings says the Royal Ontario Museum has agreed to perform age-dating tests on core from the Lac de Gras pipes.