2013

ULTRABASIC – ALKALINE MAGMATISM OF THE KOSTOMUKSHA ORE PROVINCE, WEST KARELIA

 Gorkovets V.Ya.¹, Popov М.G.¹, Rudashevsky N.S.², Rudashevsky V.N.², Dudarev А.G.³, Maksimovich L.А.³

¹Institute of Geology, KarRC, RAS, Petrozavodsk, gorkovet@krc.Karelia.ru;

²«OJSC NS+z, St.Petersburg;

Karelsky Okatysh OJSC;

Lamproites and kimberlites commonly occur in old cratons. The relationships of geodynamic igneous and sedimentary geological processes are of great value for the formation of polygenic and polychromous ore deposits in the earth crust. In this respect, the Kostomuksha Ore Province of the Karelian Craton, West Karelia, is undoubtedly of interest as a model province, where various genetic types of iron and gold deposits and ultrabasic-alkaline complexes of diamondiferous lamproites and kimberlites occur [1,2].

Kimberlitic and lamproitic magmatism has been revealed for the Precambrian of the Fennoscandian Shield. Finds of diamonds, accessory minerals, kimberlite pipe fields and abundant lamproite dikes are known from many parts of the shield.

Analysis of the distribution of kimberlite and lamproite fields in the Karelian Craton of the Fennoscandian Shield has shown that active different-aged basaltic and komatiitic volcano-plutonism, whose evolution reflects the consecutive deepening of melt sources, is required for their emergence. Marginal zones above diapirs, which have been subjected to multiple tectonic reworking, are most promising for diamond prospecting. Kimberlitic and lamproitic magmatism is a final event in these zones. The distribution of lamproite dikes and kimberlite dikes and diatremes in the Kostomuksha Ore Province is irregular because they are confined to the zones of intersection of different-aged arcuate and linear deep-seated tectonic structures comparable in rank. The most favourable conditions for the formation of highly permeable zones, capable of provoking the intrusion of mantle matter into the earth crust, emerge here. The spatial combination of the Early Archean (3.5 Ga) Voknavolok block and the Late Archean (2.9–2.7 Ga) Lopian volcanic-sedimentary crystalline rocks of the Kostomuksha Ore Field, which comprise Riphean (1.23 Ga) ultrabasic – alkaline magmatism in the Maanselkä zone [1], are similar to the relationship of Archean cores and mobile zones in Western Australia and South and West Africa that control the structural position of lamproite and kimberlite intrusions [4].

The Kostomuksha alkaline intrusive complex consists of olivine-phlogopitic and phlogopite-leucitic varieties of lamproites and camptonite-montsekite-series alkaline lamprophyres, kersantites and calc-alkaline lamprophyres [5].

The present paper deals with ultrabasic-alkaline lamproites and kimberlites. Over 120 0.5-15 m thick lamproite dike bodies have been reported from the Kostomuksha.Ore Province. Such bodies are most abundant in the Kostomuksha iron deposit. The age of lamproites and kimberlites, estimated by the K-Ar method from phlogopite is 1230 Ma, which is consistent with a Riphean Late Proterozoic stage in protoactivation.

Ultrabasic alkaline lamproite complexes in the Kostomuksha Ore Province make up 0.1- to 10-15 m thick dikes confined to a near-N-S-(15⁰ NE)-trending deep fault zone. The zone is 25 km long and up to 20 km wide [1]. Dikes occur among Lopian volcanic-sedimentary Kontokki- and Gimoly-series rocks. Lamproites display a porphyritic structure and are highly altered: “porphyric phenocrysts” consist of serpentine at the contact with komatiite-series ultrabasic rocks or saponite, 30-500 μm in size, in iron-schist rocks. The matrix of lamproites is composed of phlogopite- and tetriphlogopite-group mica. Other rock-forming minerals, such as feldspar and monoclinic and rhombic amphiboles, namely edenite, grunerite, hypersthene, diopside and forsterite, are present as relics. Saponite, calcite and quartz occur as secondary minerals. Numerous accessory minerals are represented by chrome-spinellids, which exhibit a porous internal structure, and titanium oxides such as anatase and rutile; sulphides such as pyrite, pyrrhotite, pentlandite, chalcopyrite, galena and sphalerite; and barium (barite, priderite, henrymeerite, baotite and atstonite), strontium zirconium and rare-earth minerals that emphasize the rare-metal specialization of the rocks typical of all lamproites.

These lamproites are well correlated in mineral and chemical composition with those of classical lamproite localities in Australia, USA, Spain, Greenland, Africa and the Antarctic [4].

Typical exenocrysts of deep-seated mantle rocks, such as emerald-green “chrome-diopside” and chrome-spinellids (including chromium-rich varieties, 64.3 – 66.5 mas. % Cr₂O₃), have been revealed in lamproite minerals.

The rocks of the series of lamproite varieties studied from the Kostomuksha Ore Province are generally similar, but they also display substantial differences: some consist of olivine, pyroxene and phlogopite, while others are composed of orthoclase, amphibole and phlogopite. The main secondary mineral of some lamproites is serpentine, while that of others is saponite. The former dike bodies belong to a group of ultrabasic lamproites and the latter to a group of  basic rocks of this family. These differences are due to the contamination of host rocks by parent magma, rather than the different composition of parent melt: some lamproites cross-cut and occur among serpentinized komatiite-series peridotites, while other lamproites are encountered among much more felsic iron-rich quartz-feldspar-biotite schists associated with magnetite quartzites.

A series of kimberlite (orangite)-lamproite diatremes and dike bodies, which cover an area of 3.0 to 3.5 hectares and cross-cut Archean 2.8 Ga Gimoly iron-schist rocks, have been identified in the Kostomuksha deposit. Mineralogical study was carried out in a distreme, up to 200 m in diameter [3].

The diatreme rock shows a brecciform texture (clasts-xenoliths, measuring 1 to 10 cm, make up 50-60 % of total rock volume). The clasts display a rounded or irregular shape. The xenoliths consist of fine-grained aggregates formed of tal-serpentine intergrowths. The matrix of the xenoliths is composed of medium- to coarse-grained mica aggregates, 10-1000 μm in size (phlogopite and tetraferriphlogopite), and fine-grained clusters of secondary minerals such as serpentine, talc, calcite, dolomite and quartz. Scarce relics of K-feldspar and (Ti-K)-richterite have been encountered.

Sulphides (pyrrhotite, pyrite, pentlandite, chalcopyrite, galena and sphalerite) and chrome-spinellids (spinel and chromite) and other accessories, such as apatite (including Sr-apatite), barite, garnets (pyrope and almandine), monoclinic pyroxene (chrome-diopside), ilmenite ( including Mn-, Mg- and Cr-Mg varieties), monozite (Ce), rutile (including Cr-rutile), zircon, Zr-priderite and some others, are the first to accumulate in “heavy” concentrates.

The compositions of chromium-rich chromites of the diatreme (Cr₂O₃ 62-64.5 mas. %) are consistent with those of chromite associated with diamonds.

Ten diamond crystals, measuring 0.8-1.5 mm across, were extracted in the De Beers laboratory, Johannesburg, in 2005.

Fragments of the diatreme of a coarse-clastic vent eruptive breccia, exposed over an area of 14 x 20 m, have been identified in the Kostomuksha open-pit mine, West Karelia. Breccia  occurs in quartz-feldspar-biotite schists associated with Gimoly quartzites.

Fragments of serpentinites or ellipsoidal to rounded units, measuring 7х12 to 20х40 cm, make up over 95% of breccia. The matrix of breccia consists of micaceous material (matrix of breccia). High-Mg serpentine is the dominant mineral of breccia clasts. Only accessory chrome-spinellids (spinel and chromite) and monoclinic Cr-diopside are present as relics of primary mineral paragenesis. Other accessory minerals in serpentinites are magnetite, sulphides (pyrite, pentlandite, violarite, sphalerite and galena), titanite and apatite. Rock-forming minerals in the micaceous matrix are phlogopite and tetraferriphlogopite; secondary minerals are serpentine, talc, calcite and quartz; accessory minerals are sulphides (pyrrhotite and pentlandite, less commonly pyrite, sphalerite, galena and chalcopyrite).

With respect to chemical composition, petrographic characteristics and mineral composition, eruptive breccia occurs as highly altered apolherzolitic, apoharzburgitic and apodunitic serpentinites with an altered olivine lamproite matrix.

Accessories of the minerals present in diamondiferous xenoliths of kimberlites and lamproites, such as chromium monoclinic pyroxene and high-chromium chromite (62.5-68.1 mas. % Cr₂O_3), have been identified in breccia

The authors have drawn the conclusion of the diamond potential of the eruptive vent apoperidotitic breccia with lamproite matrix, based on evidence for the mineralogy of breccia rocks.

References

1. Gorkovets, V.Ya. & Raevskaya, М.B. Mineral genesis of the northwestern Fenno-Karelian Craton // Precambrian mineral genesis. Proceedings of the All-Russian Conference. Petrozavodsk, 2009. P. 63-65.

2. Gorkovets V.Ya., Raevskaya, М.B. and Maksimovich, L.А. Kostomuksha: an integrated ore province of the Republic of Karelia // Gorny zhurnal. 2012, 9/1. P. 19-23.

3. Gorkovets, V.Ya., Rudashevsky, N.S., Rudashevsky, V.N., Popov, M.G. and Antonov, А.V. Diamond accessory minerals in a lamproite diatreme (Kostomuksha District, Karelia). Doklady Akademii nauk. 2013. V. 450, no. 1. P. 62-65.

4. Jakes, А., Louis, J. and Smith, C. Kimberlites and lamproites of Western Australia. М.: Mir. 1989. 430 p.

5. Popov, М.G., Gorkovets, V.Ya., Raevskaya, М.B. Kostomuksha intrusive complex of potassic alkaline and subalkaline rocks, mantle source and geodynamic setting // Relationship of the surface and deep-seated structures of the earth crust. Proceedings of the 14^th International Conference. Part 2. Petrozavodsk. 2008. P. 116-119.