East meets West on the diamond trail — Russians study

He has yet to visit the Lac de Gras diamond exploration camp of the Northwest Territories, but Nikolai Sobolev is not surprised that the primary exploration method used involves tracing indicator minerals back to the kimberlite source.

“It’s a process that works around the world,” the Russian diamond expert told The Northern Miner in a recent interview. He added that indicator minerals are used not only to differentiate between barren and diamondiferous pipes without drilling but to predict diamond grade as well. Sobolev, chief of the Diamond Research Institute in the western Siberian city of Novosibirsk, recently toured western Canada on a lecture tour. “We pay great attention to indicator minerals and we study them grain by grain because they tell a lot of stories,” he said, adding that prospecting techniques today are far more sophisticated than those applied to find the first kimberlite pipes in Russia decades ago.

In the mid-1930s, Russian geologists began investigating how to develop prospecting techniques for diamonds. The initiative was strongly supported by government because of the growing strategic importance of industrial diamonds which were widely used and which were expensive to buy from foreign sources. The natural inclination was to begin searching in the Ural Mountains, in western Russia, where alluvial diamonds were discovered in 1829 and where some mining took place.

“My father (V.S. Sobolev) began working on this problem by researching geological literature, particularly from South Africa,” Sobolev said. The literature described rock relationships in South African diamond mines, and the elder Sobolev (a petrologist and professor at the Leningrad Mining Institute) noted geological, petrological and rock similarities to the Siberian Platform where he had previous field experience.

“By 1940, my father delivered a report to the government stating that kimberlite occurrences could be expected in the northern part of the Siberian Platform. Others continued to recommend checking out the Urals, but my father stuck to his beliefs.”

The first team of geologists began exploring for diamonds in the Siberian Platform in 1947, and diamonds were found in river beds in 1949. The first kimberlite was discovered in 1954 and, although it had only a few diamonds, it provided proof of being in the right hunting ground.

V.S. Sobolev is credited with the discovery of the famous Mir pipe in 1955 while another team found the Udachnaya pipe in the same month and year, using the same prospecting technique. Both became highly successful diamond mines. Over the next few years, hundreds of kimberlites were discovered, but only three became producing mines. After publishing Diamond deposits of Yakutia in 1959, the elder Sobolev went on to organize further studies on diamond exploration. His son Nikolai pursued the same goals and now has more than 30 years experience in all phases of kimberlite research and diamond exploration. Sobolev said early exploration teams paid attention to the geological and tectonic setting as well as to the pyrope mineral group, “even though they didn’t understand the full range of their chemical composition.” In the late 1960s, a Western diamond expert published the first results of microprobe analysis of garnets (and other minerals) included in or associated with diamonds and identified the chemical composition of the key indicator minerals. Sobolev worked at the same problem and subsequently proposed the first techniques aimed at differentiating between diamondiferous and barren pipes. Sobolev published his findings in March, 1971, in On mineralogical criterions of diamond content in kimberlite, which he said was several years ahead of similar reports by South African experts published in 1973. The Russian Geological Survey was contracted to apply the younger Sobolev’s techniques in Yakutia in 1972. Since then, and including earlier discoveries, more than 700 kimberlites have been identified in all of Siberia. Of these, 150 were found to contain diamonds, but only 10 had minable grades. Sobolev said most of the easily detected pipes have already been found, which means ongoing exploration requires an integrated approach. Geophysics is widely used but is obstructed by a 50-metre-thick volcanic layer that covers parts of the Siberian and Arkhangelsk regions. Sobolev said the decision to explore the East European Platform (hosting the Arkhangelsk kimberlites) was made because original, heavy-mineral prospecting revealed just two or three indicator mineral grains (from the few hundred collected) similar to those found in diamondiferous pipes. The first pipe was found in this region by geophysical methods in 1980.

Over the years, the Russian government provided technical support and funding to the development of diamond exploration technology. Teams of physicists and chemists synthesized diamonds as well as the indicator minerals that formed with, and were brought up with, diamonds. Certain aspects of the advanced technology to differentiate between diamond-bearing and barren pipes and predict grade are considered proprietary and, therefore, are not published. But Sobolev did say the Russian technology features a more narrowly defined G10/G9 garnet classification field than is used in the West. The Russians have also been applying the morphology (form and structure) of the grains, which are studied by probe microscopes. This is done to determine whether they are associated with barren or diamondiferous pipes, as well as the distance they have travelled.

Sobolev’s lectures also focused on the morphology and internal variability of kimberlites, and his observations on Group II kimberlites were of special interest to junior companies exploring outside the Lac de Gras region. Sobolev dismissed the prevailing notion that diamond grade always decreases to depth in most mines. “The grade varies, but there is no sharp decrease in diamond grade with depth,” he said of South African and Russian diamond mines. “But the costs do go up.”

Sobolev was recently elected to the U.S. National Academy of Sciences as a foreign associate. Membership is considered one of the highest honors that can be accorded a scientist or engineer. Sobolev will convene the sixth international kimberlite conference in Russia in 1995.