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For more than one hundred years it was commonly believed that the mother, or parent rock, of diamonds was blue earth - kimberlite. In this country, however, specialists in the field had come forward even back in the 1980s with theoretical predictions suggesting that diamond deposits can also be produced by other igneous, or plutonic rocks. This assumption was later fully confirmed by practical experience.
by Felix KAMINSKY, Dr. Sc. (Geol. & Min.);
Sergei SABLUKOV , Cand. Sc. (Geol. & Min.);
Central Scientific Research Institute of Geological Prospecting of Nonferrous and Noble Metals, RF Ministry of Natural Resources
LIFTING DIAMOND FROM GREAT DEPTHS
When back in the hoary antiquity our ancestors first stumbled upon pieces of diamonds in river sands they were immediately impressed with their truly remarkable properties. Diamonds were the hardest, most shiny, most beautiful and mysterious stones they ever came across and with the passage of time people naturally wanted to know where from and how these wonder gems came about. Over the ages people found diamonds in river bank deposits in India, on the Island of Kalimantan (Borneo) and also in Brazil. But their "source" remained unknown, and it was only in 1872 that geological experts found in South Africa for the first time bedrock outcroppings of eruptive magmatic rocks - diamond ores, which were called kimberlites. They filled the mouths of ancient volcanoes and contained what was then believed fabulous amounts of the precious crystals. Suffice it to say that whole cartloads of these stones were dug out from the first few known deposits.
The vent funnels of extinct kimberlite volcanoes have been preserved to this day in the form of diatremes, or volcanic pipes, which accompany plate-like magmatic bodies filling up the subvertical (dykes or veins) and sub-horizontal interstratal fissures in sedimentary rocks. It was through these upward canals that kimberlite magma carried up to the surface diamond crystals, with the biggest of them found so far being the size of a clenched fist. These crystals originated at great depths (from 120-180 to 670 km) at temperatures of more than 900 0C and pressures over 40 kbar and it was the rapid upward flow of magma that "saved" them from oxidation and turning into graphite. And kimberlite rocks were distinguished by their peculiar appearance
and features of the composition - a high content of magnesium, granular, or porphyric structure with numerous impregnations of magnesium silicate - olivine, and also fragments of deep mantle rocks and diamond-associated rocks - pyrope (Bohemian garnet), chrome-spinel-lide, picroilmenite, chromodiopside, etc.
Later on kimberlites of this kind began to be found on all continents with the exception of Antarctica. And specialists studied these ores very well, determined the conditions of formation of the precious crystals and the accompanying minerals and formulated methods of geological prospecting for relevant deposits.
NOT ONLY IN KIMBERLITES
Meanwhile in other regions, where such wonder stones were found in river sediments, specialists were unable to trace their original sources.
"Unattached" diamonds of this kind were found, for example, in the Urals, Northern Siberia and also in Brazil and on Kalimantan. And the obvious conclusion was that diamonds come not only with kimberlites, but also in other kinds of rock. But what kind of rocks could these be? In the course of geological prospecting specialists run across isolated and fine precious crystals in various ferro- magnesial magmatic rocks, such as picrites, lamprophyres and peridotites. These, however, were but isolated accidental finds, and there were no grounds to expect for sure diamond deposits in any such geological structures.
Still and all, such "lucky strikes" generated certain conclusions. The theoretical foundations of the formation and preservation of "royal" gems in different types of magmatic rocks were formulated in the late 1970s-early 1980s by one of the authors of the present article - Felix Kaminsky. He demonstrated that the occurrence of diamonds in high-magnesial ultra-basic and magnesial-ferruginous alkaline-basic (non-kimberlite) and magmatic rocks is a natural phenomenon. Such natural aggregates (picrites, basaltoids, various lamprophyres and ultramafites) can be born deep within the bowels of the earth and at high pressures (over 37-45 kbar) and can carry upwards diamond mineralization processes while preserving them intact during the rapid uplift, which also means forming deposits in favorable conditions.
These theoretical considerations have been confirmed by geological prospecting. At about that time diamonds were discovered in lamproital (and we stress - non-kimberlite) rocks in Western Australia. Lamp-roites are volcanic formations which,
together with a high content of magnesium and magnesium silicate-olivine (which is also typical of kimberlites) contain unusually large levels of potassium, titanium, rare and rare-earth elements; also present in them are such minerals - nontypical of kimberlites - as potassium alumosilicate-leucite, and rare minerals of titanium, potassium, barium and zirconium. Today about one third of the world output of diamonds comes from the lamproites of the Argile diamond pipe in Australia.
As it turned out later, however, these rocks are really not the same: those with a high content of the precious crystals approached kimberlites by their composition, while others, and sharply different by their chemical and mineral characteristics, contained but isolated diamonds, or no diamonds at all. This made it possible to speak about one differentiated series of lamproital rocks in which only the border, highly magnesial ultrabasic varieties are diamond-bearing. And one can also speak of the existence of a broader "kimber-lite-lamproite" magmatic type of diamond deposits.
Prospecting for such magmatic rocks went on and soon we discovered new varieties on Kamchatka, Taimyr, Timan, in Uzbekistan, Australia and other regions. Unfortunately, diamonds were present in them only as isolated fine grains, mostly fragments, which generally raised doubts about the value of these types of rocks.
QUITE UNLIKE KIMBERLITES
And it was quite by chance, as is often the case in life, that the long-expected rocks, and not just diamond-bearing, but with a high content of diamonds, were finally discovered. In 1994 Canadian geologists, Dr. Neil D. MacRae and Dr. Allan E. Armitage, damaged their disc saw while cutting up one of the samples. And they were studying numerous lamprophyre dykes at Parker Lake (northwestern shore of the Hudson Bay) near the Rankin Inlet village. And it was obvious that the saw could have been damaged by contact with a diamond. And the very first test of the bit of rock proved its diamond content of more than 7 carats per ton. This is nearly an order of magnitude more than in an ordinary kimberlite deposit. The newly found rocks, which differed from diamond- bearing kimberlites or lamproites, turned out to be close to minettes-from the group of calk-alkali lamprophyres (the find became yet another confirmation of the aforesaid theory of the diamond-bearing potential of the eruptive nonkimberlite rock). During one year there were found in Canada three subvertical plate-like formations-dykes of diamond-bearing minette-Thirsty Lake, South Lake and Victory Day Dyke.
In 1995-1996 a research team from our Institute was invited by the Canadian company Cumberland Resource Ltd. for detailed prospecting in the area of the Rankin Inlet settlement during which they studied rocks found there before. As a result we found one more 3-kilometer dyke, which we called Northern (Severnaya); together with the two already known ones - Thirsty Lake and South Lake - it forms a common system of diamond-bearing deposits some 15 km in length which was named Akluilak. We also studied the 300-meter long Victory Day Dyke located a few kilometers to the west of Akluilak (probably a small fragment of a similar system).
All of these deposits are stretching in a general direction close to the meridional, feature considerable rock thickness of 1-3 m which increases from 4 to 5 m in the middle parts and even up to 8 m. These contain intertonguings of interchanging magmatic bodies and nearly parallel minor branches. Dykes contacts with the surrounding rock are even and clear, and the age of these branches themselves approaches 1850 mln years which corresponds to the Early Proterozoic.
COMPOSITION OF DIAMOND-BEARING MINETTES
They consist of hard massive holocrystalline rock of black color with an uneven fracture of a typical homogeneous fine- and medium-grained appearance. The rock itself mostly consists of three minerals: orthoclase (55 percent), black mica scale - biotite (35 percent) and calcium phosphate - apatite (10 percent), whose yellow phenocrysts, big and well faceted, sharply stand out at times against the black background. The main framework of the rock is formed by orthoclase crystals of up to 5 mm in size and present sometimes between them are fine-grain accumulations of biotite, apatite, carbonite and epidote (pistacite). These minerals and the fully-crystalline appearance of the minette are absolutely non-typical either for kimberlites or lamproites. A feature of the minettes consists, practically always, in the presence of oval, and less often angular fragments (xeno-liths) of different metamorphous and magmatic rock of up to 10 cm in size; at some stretches they make up 50 to 80 percent of the dykes.
Judging by the composition, morphology and interplay of mineral grains, dyke rocks are metamorphosed, recrystallized and the only primary, relict mineral here must have been apatite - its impregnations have crystalline facets.
With respect to their geochemistry the rocks of the studied dykes possess ultra-potassic composition (alkali total of 9-13 percent) and are distinguished by a low silica content (40-45 percent) which attests to their intermediate composition - between ultrabasic and basic rocks. Minettes contain very little titanium, niobium and tantalum and much phosphorus, barium, strontium, rubidium and zirconium; at the same time the levels of light rare earths, thorium and uranium are relatively not high for such hyperalkaline rocks.
In general, by the distribution of macro- and microelements, minettes form a group of their own and different from lamproites. Mineralogical analysis has shown that always present in minettes as rare earths are sphene, zircon, rutile and ore varieties (magnetite, pyrite, sometimes ilmenite), and in some of the samples we found single grains of abyssal mantle components of kimberlite-lamproite association, the so-called mineral- complementaries of diamonds - olivine, pyrope, picroilmenite and chromodiopside (it is interesting that the amounts of such complementaries in the rock are one or two orders of magnitude lower than that of diamonds). The presence of these abysmal minerals points to the fact that minette was formed at very high temperatures and pressures.
These diamond-bearing formations possess a very fixed isotopic level of neodymium and broadly varying that of strontium which is typical of magmatic rock whose source was the ancient enriched lithospheric mantle of the Earth.
QUALITY, RESERVES, PERSPECTIVES
The thermochemical analyses of the very first minette samples pointed to a very high general content of diamonds: from 7-8 to 10- 12 carats per ton, including those of commercial size of more than 1 mm and up to 1-3 carats per ton. Few kimberlite deposits can "boast" of such figures. While conducting mineralogical analysis of crushed rock samples of 0.5 to 3.5 kilos we discovered more than 100 diamond crystals with an average size of 0.5 mm. Their morphology is very diverse with predominating rounded dodecahedrons, tetrahexahedrons, cuboids and combination forms; one comes less often across piano-faceted octahedrons and spinel twins of octahedrons. Almost all crystals are of a yellow- green and pistachio-brown colors, sometimes they are green, black and yellow and only some isolated octahedron grains are either colorless or have some weak yellowish tinge.
Because of that these diamonds turned out to be of low value as jewellery.
Apart from such direct finds in smaller samples, there are some indirect signs of high diamonds concentrations in rocks from all the known minettes of the Akluilak type. When we sawed up the first sample from Victory Day Dyke, we saw a deep groove cut upon the saw disc -a clear sign of a very high diamond content. Damage of the same kind was also caused by samples from the Thirsty Lake and South Lake dykes. Later on Canadian geologists even prohibited using expensive saws for cutting these rocks. There appears to be plenty of diamonds in the minette and they are all big enough (incidentally, in sawing up kimberlite samples with large crystals and gneisses with high diamond content with fine grains no such grooves, or furrows, were produced).
Our explorations and assessments in Canada proved that on the northwestern coast of the Hudson Bay alone diamonds reserves in open dykes alone amount to several billions of carats. But they are of practically no commercial value because of their mostly poor quality and remote location. We feel, however, that in the course of further studies new diamond-bearing bodies of a similar (or different) composition will be found with better quality diamonds and of morphological types better suited for commercial mining, such as bigger vertical columns or thicker dykes.
DIAMOND-BEARING NONKIMBERLITES: GROWING DIVERSITY
After Akluilak a similar dyke was discovered in the north of the Province of Ontario near the Wawa settlement and it also possessed an original composition and structure. During the field season of 1998 it was studied by our team on an invitation of the Canadian company Canabrava Diamond Corporation (and at the same time we also came across two kimberlite bodies - the first one in this region). The rock of the dyke is strongly metamorphosed (recrystallized) and consists of biotite and light- green amphibole (actinolite) with small admixtures of feldspar and carbonate. In places it is saturated (up to 30 percent) with big oval fragments of up to 50 cm in size from a homogeneous aggregate of very large crystals of green amphibole, in a fan-like or chaotic pattern and of up to 10 cm in size. According to preliminary assessments, diamond content here amounts to 0.2 carats per ton.
And quite recently one discovered in French Guyana, South America, near the town of Dachin one more unusual diamond deposit. This time around it is in volcanic ultrabasic rocks of massive and detrital, or fragmental, structure. As a result of intensive metamorphism they were strongly recrystallized and turned into albite-carbonate-chlorite-talcose shales with oval fragments of magmatic rock (from 1-3 to 20 cm). Their geochemical features, such as rare-earths distribution, make it possible to assume that the original rocks
here were volcano-clastic ultrabasic comatites (outpoured low- alkaline high-magnesial lavas). Their diamond content is from 2-4 carats per ton, that is almost reaching the level of the richest South African kimberlite deposits. In the winter of 1997 our team visited this region and made an assessment of diamond reserves. In fact, they turned out to be of a very poor quality, although there are crystals of 4-5 carats.
The newly discovered diamond-bearing rocks have some features in common: ancient age (1.85-2.1 bln years and more), strongly metamorphosed character, presence of large quantities of big oval fragments. The latter fact, on one hand, points to their clearly magmatic origin, and on the other-indicates that the formation of dykes must have been accompanied by active splintering, melting and, probably, explosive processes. All these are signs of a rapid uplift of magma to the earth surface as a result of which diamonds escaped oxidation and graphitization. The newly found rocks, like kimberlites, are typical of territories of the greatest (Archean) age of more than 2.5 bln years. It may well be that in areas with such ancient foundations some different rocks can be diamond-bearing, including some very different from kimberlites, or the others mentioned above.
ON THE BRINK OF NEW DISCOVERIES
Summing it up, the theoretical predictions of this country's geologists made two decades ago are being fully confirmed by the present-day discoveries of new diamond-bearing rocks. And although due to the impact of metamorphism and metasomatosis (replacement with secondary minerals) it maybe difficult at times to draw conclusions on their primary-magmatic composition, one can still say with confidence that diamond deposits occur in most diverse magmatic rocks of nonkimberlite type. And diamond mineralization in them is probably of abyssal, or plutonic, mantle origin. So far these have been identified only beyond the confines of this country, but there can be no doubts that sooner or later they will also be discovered here. This optimism rests on the fact that there is no other country on Earth with a greater diversity of geological structures than this country of ours. Decades ago our national geological service discovered other biggest deposits of diamonds of some unique types, unparalleled anywhere else in the world.
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