by Sergei SABLUKOV, Cand. Sc. (Geol. & Mineral), Central Scientific Research Institute of Geological Explorations, RF Ministry of Natural Resources

Russia's North has long been famous for its proverbial wealth of fur and wood, salmon and river pearl, beast and fowl, berries and mushrooms. Only, as if on purpose, has Mother Nature hidden from people mineral deposits deep down there. Advancing and melting time and again, the glaciers after the last hundreds of thousands of years have left behind massive layers of sand and mud, which have locked the underground treasures as if for good. This is why the way to the discovery of diamond fields in the 1980s and 1990s of the 20th century was so long and thorny.

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FROM DIAMOND FIND TO LUCKY STRIKE

This precious stone was first found at the village of Orletsy on the North Dvina in mid 18th century. Academician Mikhail Lomonosov, a native of the Archangel region, believed that "the Orlets mountains can bear diamonds". And his prophecy has come true: one of the first kimberlite pipes of our time has been named after Lomonosov as has the local diamond field-the first one in Europe.

All attempts by lone volunteers in the past to identify any signs of diamonds on the Kola Peninsula, in Timan, and on the North Dvina always proved futile. And no wonder, for such complex prospecting conditions are unknown anywhere in the world. Crater deposits overlapping Paleozoic rock and the massive glacier sediments destroyed all traces of the presence of kimberlites in the surface layer accessible for probe. Only in 1936, when water wells were drilled, fancy looking fragmentary rock samples were uncovered near the village of Nyonoksy on the Letny Shore of the Onega Peninsula. After long arguments they were identified as volcanic rock, which some times occur next to diamond-bearing kimberlites.

The industrial geologic association (PGO) ARKHANGELSKGEOLOGIA, represented by Yurassk, Novodvin and Belomor expeditions, began purposeful work in cooperation with various Moscow and Leningrad organizations on the White Sea's Onega peninsula in the 1960s. Later, ground magnetic surveys revealed there anomalies caused, as turned out, by volcanic pipes of an ultra basic make-up-first, eight were found, then another 30. Small grains ofpyrope, chrome-diopside, and chrome-shpinelide, the minerals "accompanying" diamonds, were discovered in the concentrate samples from river and in particular from the beach sediments on the sea coast. This finally confirmed the presence of real kimberlites in the region.

Exploration was also started on the neighboring territory - the Zimny Shore, and the results proved even more promising than on the Onega peninsula. By geological surveying on a 1:200,000 scale, intrusive sediments, relatives to kimberlites, were found on the Mela river (1975). In addition, it was found that the mineral "satellites" of diamonds spread all over the Zimny Shore. Finally, two crystals of a precious stone were found in the sandstones of medium

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carbon, which prompted direct prospecting for diamonds through aero-magnetic surveys. The very first well drilled in the summer of 1980 revealed a kimberlite pipe, although unusual in composition but diamond bearing. It was called Pomorskaya.

In the next four years, yet another eight pipes were discovered, six of them diamond bearing (actually the only method to discover them in the area was to find out the nature of magnetic anomalies by drilling, compared with Yakutia where direct prospecting for pipes on the concentrated halos of mineral satellites in river sediments yielded splendid results). Then, over 12 years about 50 volcanic pipes of various composition were found but none of them was diamond bearing. However, a Belomor expedition and Nevsky PGO expeditions explored at length seven kimberlite pipes of the Zolotitsky cluster, forming the Lomonosov deposit. In February 1996, prospectors from Russia's company Almazny Bereg and Canada's Archangel Diamond Corporation discovered a new diamond-bearing pipe in the Verkhotinsk area on the Zimny Shore. It was named in honour of Vladimir Grib, the chief geologist of the PGO ARKHANGELSKGEOLOGIYA. The exploration of this pipe, which has become a new diamond field in the Russian North, is planned to be completed in 2001.

LAND OF ANCIENT VOLCANISM

The Archangel diamond bearing province is situated in the north of Russia's European part-within the boundaries of the Archangel and Murmansk regions and partially on the territories of the autonomous republics of Karelia and Komi. It spreads in the north of the East European platform - a large firm block of the ancient earth's crust that was consolidated at the dawn of the Proterozoic period (3.5-2 billion years ago). It includes the Baltic Shield and the northern part of the Russian Platform with the Timan Bulge.

The province combined motley volcanic manifestations primarily, on the edge of the White Sea, on the Zimny and Letny Onega shores and on the Tersk and Kola peninsulas. All of them are part of the White Sea riftogenic system which shaped in late Proterozoi (about 1.2 billion years ago) as narrow linear flexures and cavities stretching north-west in the old gneissoid crystallitic foundation of the platform. The narrow flexures - Kandalaksha and Onega-Dvina rifts and the Zimnegorsky flexure - are filled with massive, several kilometers thick fragmentary rocks - sandstones, aleurolites and argillites. Then, a long period of relative calm set in the region with the sea either advancing and depositing massive strata of sandstones, aleurolites, and clays, or with the territory rising, washing out and destroying the sediments.

In the late Devonian epoch (360-374 million years ago), the region became active again: numerous volcanoes erupted on the White Sea planes, covered with a dense tropical forest forming volcanic pipes, spewing hard rock up and forming clouds of ash. During the emptions, the trees growing near the volcanoes fell into their craters. The newly found charred fragments of the trees have made it possible to determine the age of the kimberlite and other pipes.

Apparently, eruptions continued for several million years. Many craters the extinct volcanoes still remain as explosion pipes, accompanied by magmatic bodies filling in sub-vertical (dikes, veins) and sub-horizontal inter-layer clefts in sedimentary rock (sills). Further on, volcanic apparatuses (systems of canals by which magma flows up to the surface) were overlapped by the massive depths (50-200 m) of continental and shallow sea water sediments of Carboniberous and Permian periods-sandstones, limestones, dolomites, and gypsum. And in the Glacial Epoch, which is nearer to us in terms of time, the tongues of ice moving

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from the north-east and north-west brought fragmentary rock along, forming powerful, scores of meters thick, layers of clay, loam, and sand.

ZIMNY SHORE: HIGH DIAMOND INCIDENCE

In the Devonian period, the Zimny Shore saw the most intensive eruptions. There, on a territory of 20,000 km ² , in the center of the elevated White Sea-Kuloi plateau, in the upper layers of the Zolotitsa, Kepina, Yerna, Megra, Mela, Chidvia, and Izhma rivers and in the middle flows of the Pinega river, geologists have discovered about 60 old volcanic apparatuses, 48 of them formed by kimberlite rock, the rest by basalt.

The first are represented primarily by explosion pipes; their vent parts are shaped as a cone-like cylinder turned upside-down. At top, some have retained craters - surface bell-shaped expansions. The shape of pipes ranges from isometric to extended, the size across - 60-2,000 meters, area - up to 180 hectares (however, mostly about 30 hectares). Their crater parts are filled with sandstones, tuffstones, seldom - tuffs. The structural diversity of volcanic rock underscores color varieties, typical for the Archangel region, noted for all tones and shades of red and brown as well as violet, green, blue, yellow, grey-black, ordinary, spotted, and striped in design.

Unlike the majority of other diamond-bearing provinces in the world, Archangel's north stands out for the extraordinary diversity of volcanic manifestations; melts of various compositions implanted in surrounding rock almost simultaneously there (by geological standards, of course). Becoming solid, they formed bodies of kimberlites, picrites, melilitites and intermediary composition rock-kimberlite-picrites as well as toleites and sub- alkoline basalts. Even being close to each other, sometimes just a few hundreds of meters within one another, they strongly differ in general mineral composition. The content of deep material and the concentration of some chemical elements may differ scores and even hundreds of times.

Comprehensive analysis of rock has shown that in each of the clusters of Zimny Shore pipes volcanic rock contains a certain part in a single scale of the decreasing degree of fundamental geo- chemical elements - magnesium, nickel, chromium, cobalt and of the increasing degree of "basalt" elements - silicon, aluminum, calcium, sodium, iron, scandium. While fundamental geochemical elements in rock decrease, the content of deep mantel rock (large oval intratelluric embedments of olivine-1, mantle xenoliths and diamond's mineral satellites) - from 50 percent of rock volume in diamond kimberlites to the extremely small amount in picrites and melilitites. And on the contrary, melted crystallization products build up close to the surface (small crystals of olivine-2, phlogopite, melilite, and nepheline).

It is taken for granted that kimberlites and their "relatives", vulcanites, form through the erosion and partial melting of ancient crystal rock, occurring deep enough (50-180 km). The latter came into being under a very high pressure so that nearly all the diamond mineral ingredients assumed the characteristic features of jewels - purple garnet (pyrope), emerald-green chrome- diopside, and olive-yellow olivine (chrysolite) not to mention the king of jewels - diamond itself. Black ore minerals - picroilmenites and chrome-spinelid are also lustrous. Kimberlites and kindred rock contain various amounts of fragments (xenoliths), separate grains

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of minerals and very beautiful stones such as dunites, peridotites, eclogites, etc.

Two contrast groups, or kimberlite series, were spotted on the Zimny Shore - alumina (kimberlites-phiogopite kimmelilitites - pyroxene-free olivine melilitites) and ferrous titanium (kimberlites - kimpicrites - melilitite kimpicrites - melilitite picrites).

The deep rock relicts of the first series are quite rare as compared with the kimberlites of Yakutia and other diamond-bearing regions. They are represented by the small inclusions of chrome-spinel, spinel and less often by pyrope dunites and peridotites. In the ferrous titanium series, deep inclusions are larger and occur far more often (especially in Vladimir Grib's pipe), where pyrope peridotites, phlogopite-ilmenite peridotites and pyroxenites and various eclogites were discovered.

Kimberlites of the first alumina series form the Zolotitsky cluster, comprising ten pipes from north to south - Pervomaiskaya, Belaya, Koltsovskaya, Lomonosovskaya, Pomorskaya, Pionerskaya, Karpin-2, Karpin-1, Arkhangelskaya, Snegurochka). The last seven are the richest and belong to the Lomonosov diamond field. This series includes phlogopite kimmelilitites of the Verkhotinsk cluster (pipes Anomaly-401Yu, 401S, 402, 407, 451, 685, 823A), kimmelilitites of the Chidvinsky cluster (Chidvinskaya and Aprelskaya pipes), Suksoma pipes as well as piroxene-free olivine melilitites of the Vesennya, Izhma-Yuzhnaya, and Izhma- Severnaya pipes.

As for the ferro-titanium series, its diamond kimberlites are represented by only one rich tube Anomaly-441 (Vladimir Grib's deposit) so far, the rest 16 (Anomaly-688, 693, 751, 748, and 734 among them) are much poorer.

The volcanic pipes of basalts are developed in the east of the Zimny Shore region. Tholeit (Anomaly-720, 710, 704, 722, 721, 782, 753, 754, 1026) and sub-alkali (Anomaly-707, 1040, 1042) basalts stand out among them. In age, size, and the morphology of bodies and structural-textural types, they are very close to the rock of the Zimny Shore kimberlite series. However, they do not contain the relict deep rock (even in the form of separate grains) and are formed only by the products of magmatic melting crystallization close to the surface.

Diamonds in the pipes of the Zolotitsky cluster (the Lomonosov deposit) and of Vladimir Grib's cluster have been explored. The first of them is noted for its high share of round stones of "the Ural" or "the Brazilian" type, which is uncommon for the basic kimberlite deposits (except the Argail pipes in Australia and some veins in South Africa). Curve-facet dodecahedrons

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and tetrahedrons prevail among the large crystals of over 1 mm, flat-facet octahedrons, typical for diamonds at the most world deposits, dominate the small stones. However, curve-facet diamonds of a complex shape cannot be processed by the existing automatic cutters: each stone requires an individual approach.

This factor reduces the profitability of the Lomonosov deposit even though its share of jewels is quite high (about 50 percent). Crystals from Vladimir Grib's deposit differ in morphology from those that have just been described and are very similar to Yakutsk stones. Most of them are flat-facet octahedrons, and the crop of round stones is much smaller.

The Zimny Shore ferro-titanium rock series are twins to kimberlites of Yakutia and South Africa's Group-1. As for the alumina series, it comprises a new, independent assemblage, even though, for some properties, it is the closest relative to kimberlites of South Africa's Group-2, Brazil and Venezuela and the newly discovered Nakyn field in Yakutia.

In the Zimny Shore territory, volcanic bodies spread evenly, forming certain zones. Ferro-titanium rock prevails in the central part, alumina series and basalts - on the periphery. Apparently, deep substrata spreads similarly there: the enriched heterogeneous eclogite-peridotite in the center, the exhausted homogeneous dunite on the periphery.

NO DATA ON OTHER DISTRICTS

Volcanic rock, similar to Zimny Shore kimberlites, are also in other territories of the Archangel diamond-bearing province. In the Nyonoksy district (the Onega Peninsula's Letny Shore), about 40 volcanic pipes formed by tuffisites, tufs, tuffits of feldshpatoid olivine melilitites were discovered. They are 300-600 m in size, isometric or extended in shape, and in some, crater parts have remained. They contain the multitude of fragments of charred vegetative remains of the late Devonian period. Local rock is explicitly enriched with aluminum, sodium, calcium, and iron and is extremely short of incompatible elements, making them similar in this respect to Zimny Shore alumina series. Add to this that Nyonoksy's tubes bear no diamonds.

The Kola Peninsula's Tersk Shore is known for about 60 pipe, dike, and stock-like bodies formed by melilitites, seldom by mica kimberlites. They are 20-200 m in size, extended or heavily extended in shape, very rarely similar to isometric - an indication of a high degree of the bodies' erosion. Local rock form the next petrographic row; mica kimberlites - kimpicrites - pyroxen-free olivine melilitites - olivine melilitites - melilitites. Of deep rock, kimberlites contain olivine-1 and separate grains of chromespinelide and chromdiop-side.

The geochemical characteristics of local rock are quite diverse: too much magnesium, moderate contents of aluminum, titanium, and iron, and enhanced contents of potassium, phosphorus, nickel, and chromium are typical for kimberlites and kimpicrites (local kimberlites differ from mililitites on these characteristics). The concentration of incompatible elements as well as cobalt, chromium, and scandium is moderate, similar to both. According to core extraction tests carried out there, two Tersk Shore kimberlite pipes contain very few diamonds and no mililitites at all.

The Umbinsk district situated on the Volsk-Vym range of the Middle Timan (Komi Republic) is known for three 50-110 m long isometric

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pipes filled with kimberlites, kimmelilitites, kimpicrites, pyroxen-free plogopite-olivine melilitites. Deep rock in kimberlities is featured by olivine-1, grains ofpicroilmenite, pyrope, chrome-spinel, chrome- diopside as well as rare xenolites of pyrope and chrome-spinel periodites and eclogites. No diamonds in the pipes were discovered, but a mine of round crystals, called Ichert-Yu, was found in the sandstones of the middle Devon Age, several kilometers from them, which apparently indicates the presence of diamond-bearing pipes in the area.

Thus, by and large the Archangel diamond-bearing province is noted for the diversity of kimberlite rock, forming its separate series in each of its districts. On the whole, one can see a close link in terms of space, time, and genetics between the kimberlite (diamond-bearing included) vulcanism and that of melilitites and even basalt.

TO NEW DEPOSITS

The discovery of the Archangel diamond-bearing province became an outstanding achievement for the USSR Geological Service in the 1980s. Many of those involved in searches for and the exploration of the Lomonosov deposit have merited various state awards.

Foreign experts value the deposit reserves at $12 billion. To organize diamond mining, POMORALMAZ (later SEVERALMAZ) was set up in 1992. The world's diamond giant De Beers bought a large portion of the Russian company shares in 1998 and began to re-explore pipes at the mine.

A report about diamond reserves at Vladimir Grib's deposit has not been handed in officially yet, nor preparation for mining is under way since the disputable matters concerning the possession of an essential license have not been settled so far between the companies Almaz-ny Bereg and Archangel Diamond Corporation on the one side and ARKHANGELSKGEOLDOBYCHA on the other.

Since 1993, several private companies have been conducting exploration on the licensed areas of Zimny Shore (Almazny Bereg, Arkhangelskiye Almazy, Terra, Kraton), of Letny Shore (Severnaya Gornaya Kompaniya), and in the White Sea (Mineralnye Resursy Arkhangelska). Their effort was crowned with the discovery of the mentioned Vladimir Grib's deposit. The companies have also launched a search across much of the Archangel region: the Vetryanoi Poyas, the lower flows of the Onega river, the Ust- Pinezh areas (the legendary village of Orletsy included), the Shenkur areas, between the North Dvina and Pinega rivers, and also in the neighboring Vologda region (the Ilez area). Big hopes are pinned on the discovery of not only new Devonian Age kimberlite deposits (as on the Zimny Shore) but also on younger Permian and Mesozoic ones. The traits of their existence - the extensive development of diamond's mineral satellites in river sediments-have been spotted in the Archangel region's south. On- going exploration there involves the company Almazy Rossii- Sakha, creating its structures. As work has expanded in the last few years, there is hope that the main discovery of diamond treasures in Russia's North is at hand.

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