DIAMOND SPECIAL — The kimberlites of Yakutia

Descriptions of 10 pipes in the Russian diamond province of Yakutia follow:

Mir The Mir kimberlite pipe was discovered in 1955 by geologists of the Amakinskaya expedition. It was the first primary deposit of diamonds found in the Soviet Union. The pipe is in the Ochchuguy Botuobiya River basin within the Malo-Botuobiya diamondiferous area, West Yakutia.

The deposit has been mined by open-cut since 1958; at present, the depth of the pit is more than 340 metres. The town of Mirny, named after the pipe, grew up around it. The pipe produced many well known rough gem diamonds such as “Anton Chekhov”, “Indira Ghandi,” “A Miner,” “Maria” and the 342.5-carat “26th Party Congress” which is the largest gemstone recovered in Yakutia. In plan, the pipe is oval, elongated in a north-west direction and slightly constricted in the middle. In vertical section, it is a typical funnel in its upper part and a cylindrical, slightly thinning body in its lower part. The Mir pipe is in the centre of a pronounced dome-like structure composed of Lower Paleozoic sedimentary rocks.

Kimberlitic rocks crop out and are represented by two varieties, (1) massive porphyritic kimberlite and (2) typical kimberlite breccia that consists of fragments of kimberlite, pyrope, picroilmenite, less frequently olivine, chromite, chrome-diopside, and fragments of various deep-seated rocks entrained en route to the surface.

Sputnik

The Sputnik (Satellite) kimberlite pipe is connected by a kimberlite vein with Mir. Sputnik occurs within the outline of the pit of the Mir pipe and is mined in the same pit. The pipe is oval and elongated in a northwest direction at surface and dips at 70 to the northeast.

Host rocks are Lower Paleozoic carbonate rocks. The pipe is composed of predominant kimberlite breccia and subordinate massive kimberlite. Kimberlite breccia contains grains of olivine, pyrope, picroilmenite, chrome-spinel and sometimes chromediopside set in a fine-grained groundmass of serpentine-carbonate composition. Xenoliths of carbonate rocks are abundant and deep-seated xenoliths occur in lesser amounts. Diamonds from the Sputnik pipe are similar in properties to diamonds from Mir.

International

The International kimberlite pipe was discovered in 1969 by geologists of the Botuobinskaya expedition on the basis of diamond indicator minerals. The pipe is 16 km southwest of Mir, in the Malo-Botuobiya diamondiferous area. It has been mined as a primary deposit since 1971. At present, the depth of the quarry is 280 metres and construction of an underground mine is under way. In plan, the pipe is irregular, oval and elongated in a northwest direction. Vertically, it is a steeply dipping cylindrical body thinning out downward. The pipe intrudes horizontally lying, Lower Paleozoic terrigenous-carbonate rocks and is overlain by a 2-9-metre-thick cover of Lower Jurassic sediments. Two lithological varieties of the rock are present, kimberlite breccia and massive porphyritic kimberlite. Massive kimberlite consists of grains of pyrope, chrome-diopside, picroilmenite, less commonly fresh olivine set in a serpentine-carbonate groundmass. Kimberlite breccia differs from massive kimberlite in the presence of fragmental kimberlite of the earlier phases of emplacement and xenoliths of foreign rocks (diabase and carbonate rocks). Deep-seated xenoliths are relatively scarce in kimberlite breccia. 23rd Party Congress

The 23rd Party Congress pipe was discovered in 1966 by geologists of the Botuobinskaya expedition during investigation of a geophysical anomaly. The pipe sits between the Ochchuguy Botuobiya and Ulakhan Botuobiya rivers in the Malo-Botuobiya diamondiferous area. The deposit was mined in the 1979-1982 period.

The pipe is overlain by a 12-20-metre-thick cover of Lower Jurassic sediments. This is a funnel-shaped body which takes on a dyke-like form at a depth of 150 metres. At surface, the pipe is oval with a long axis orientated in a northwest direction.

The pipe is composed of carbonatized eruptive breccia with a microlitic groundmass. Pyrope and chrome-spinel are characteristic minerals and picroilmenite occurs in far lesser amounts.

Udachny

The Udachny kimberlite pipe was discovered by geologists of the Amakinskaya expedition in 1955 when diamond indicator minerals were found in a heavy concentrate from Quaternary sediments. The pipe is on the left bank of Pyropovy Creek in the Daldyn-Alakit diamondiferous area, West Yakutia. Three settlements — Udachny, Nadyozhnyi and Polarnyi — have grown up in the vicinity of the pipe since the beginning of diamond production in 1968. Many well known rough gem diamonds come from this pipe, among them “Anna Akhmatova” (77.35 carats), “The 60th Anniversary of the YaASSR” (273.7 carats) and “Alexander Pushkin” (320.65 carats), the second largest gemstone found in Yakutia.

The pipe consists of two conjugate bodies (western and eastern) which resemble a distorted “number 8” in plan. The two bodies are in direct contact with each other in their upper parts but separate at depth.

Hosts are massive dolomites, limestones, marls, siltstone and calcareous conglomerates of Lower Ordovician and Middle Cambrian age.

In the western body, three varieties of kimberlite are recognized, each related to an independent stage of kimberlite emplacement. Kimberlite breccias of different stages of emplacement, predominantly grey and green-grey in color, differ from each other in picroilmenite and garnet proportions, diamond morphology, and chemical characteristics of rocks. Kimberlite breccia contains pseudomorphs after olivine, autoliths of sedimentary and metamorphic rocks cemented by a carbonate-serpentine groundmass. In the eastern body, kimberlite breccia is less serpentinized and contains less pyrope and picroilmentite than the western body. Zarnitsa

Zarnitsa was the first kimberlite pipe found in the Soviet Union. It was discovered by geologist L.A. Popugaeva on Aug. 21, 1954. The pipe is 14 km east of the settlement of Udachny, West Yakutia.

During prospecting drilling, a gem-quality diamond weighing 109.35 carats was found in a core sample. This stone was “The 60th Anniversary of the Buryat ASSR”.

The pipe crops out in the area of continuous occurrences of Ordovician carbonate rocks. Zarnitsa is one of the largest pipes of Yakutia. It has a circular outline at surface.

The predominant variety of the rock is eruptive kimberlite breccia if greenish grey color. It consists of pseudomorphs after olivine, grains of pyrope and picroilmenite, chrome-spinel and chrome-diopside, xenoliths of sedimentary and crystalline rocks set in a serpentine-carbonate groundmass. Aikhal

The Aikhal kimberlite pipe was discovered in 1960 by geologists of the Amakinskaya ex-

pedition during stream prospecting in Soksolookh-Markhinsky Creek in the Daldyn-Alakit diamondiferous area.

The pipe has been mined since 1961 and is at a depth of 240 metres. By mode of occurrence and form, the Aikhal pipe is a steeply dipping dyke-like body strongly elongated in a northeast direction in plan. With depth, the pipe thins out and gives way to a dyke. The kimberlite intrudes Ordovician and Lower Silurian limestones, dolomites, marls, and mudstones. The upper levels of the pipe are filled with kimberlite tuffs of sedimentary-volcanogenic origin; downwards, the pipe consists of kimberlite breccia of blue and dark green color.

The kimberlite of the Aikhal pipe is a specific rock in terms of the proportions of primary minerals: very little picroilmenite, but abundant chrome-spinel and pyrope.

Yubileynaya (Jubilee)

The Yubileynaya kimberlite pipe was discovered by geologists of the Amakinskaya expedition in 1975 in the course of aerial prospecting. The pipe is located 15 km northwest of the Aikhal settlement in the Daldyn-Alakit area. The construction of a diamond recovery plant is under way. In plan, the pipe is pear-shaped with long axis orientated in a north-east direction. Vertically, the pipe is funnel-shaped in its upper part and columnar in its lower part. Host rocks are Ordovician and Lower Silurian carbonate-terrigenous sediments. The pipe is covered by 70-110-metre-thick Upper Paleozoic sediments intruded by traps.

The body has a complex internal structure: the upper part (“funnel”) is filled with inequigranular tuffs and the vent facies is kimberlite breccia. The characteristic feature of kimberlite breccia is its relatively uniform composition throughout the stripped section. Kimberlite breccia consists of pseudomorphs after olivine, rare pyrope and picroilmenite grains, fragments of sedimentary rocks and crystalline schists cemented by a

carbonate-serpentine groundmass.

Sytykanskay

The Sytykanskay kimberlite pipe was discovered by geologists of the Amakinskaya expedition in 1955 using a heavy concentrate (pyrope) method. The pipe is located in the Sytykan-Markha interfluve in the Daldyn-Alakit diamondiferous area. Mining activities began in 1979.

The largest rough gem stone recovered weighed 291.7 carats and was named “The 40th Anniversary of Victory.”

The pipe consists of two bodies and is strongly elongated in a southwest-northeast direction. The junction between the pipes is about 100 metres wide at surface and increases with depth. The larger part (85%) of the pipe is overlain by Upper Paleozoic sedimentary and volcanogenic-sedimentary rocks and a trap sill; only the northeastern margin of the pipe is exposed at the surface. The northeastern body is filled with two varieties of rock, tuff-like kimberlite breccia and micro-porphyritic kimberlite breccia, whereas the southwestern body is made up of autolith-bearing kimberlite breccia.

Kimberlite breccia consists of pseudomorphs after olivine, grains of pyrope, picroilmenite, chloritized phlogopite, autoliths and country rock xenoliths set in a carbonate-serpentine groundmass with some admixture of perovskite. Krasnopresnenskaya

The Krasnopresnenskaya kimberlite pipe was discovered by geologists of the Amakinskaya expedition in 1984 during detailed prospecting in the upper Alakit River area (30 km west of the settlement of Aikhal).

The pipe is oval in plan and asymmetrical, funnel-shaped in vertical section. Host rocks are Ordovician and Lower Silurian carbonate-terrigenous sediments. The pipe is completely overlain by Carboniferous-Permian terrigenous sediments, Lower Triassic tuffaceous sediments and dolerite sills. The thickness of overburden varies from 60 metres to 115 metres. The orebody is cut in two by a thick (more than 100 metres), gently dipping trap intrusion. Kimberlitic rocks have experienced a strong metasomatic effect at the contact with traps. Three varieties of kimberlitic rocks occur: porphyritic kimberlite, autolith-bearing kimberlite breccia and kimberlite tuff-breccia. The pipe is largely made up of autolith-bearing kimberlite breccia that consists of xenoliths of sedimentary and crystalline rocks and autoliths cemented by a porphyritic, fine-grained kimberlitic groundmass. Phenocrysts are represented by pseudomorphs after olivine, less frequently by picroilmenite, pyrope and phlogopite.

— From “Diamonds of Yakutia, Russia,” a book of abstracts published by Watts, Griffis & McOuat, Toronto.