2011

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Geochemistry and ore-bearing ability of alkaline rocks of the Ukrainian and the Aldan Shields

Ponomarenko A.N.*, Vladykin N.V.**, Kryvdik S.G.*, Dubyna A.V.*

*N.P. Semenenko Institute of geochemistry, mineralogy and ore formation NAS of Ukraine, Kyiv, Ukraine

pan@igmof.gov.ua

**A.P. Vinigradov Institute of geochemistry SB RAS, Irkutsk, Russia

vlad@igc.irk.ru

The Ukrainian Shield (USh) is one of the classical province of Proterozoic (1.8-2.0 Ga) alkaline magmatism. Phanerozoic (Devonian) alkaline and subalkaline rocks are only known in marginal South-Eastern part (East Azov area) where there are both Proterozoic (mainly) and Devonian alkaline rocks. Among Proterozoic massifs Oktysabrsky, Chernigivka (Novo-Poltavka), South-Kalchyk and Mala Tersa (located in suture zone connecting Azov and Middle Dniper megablocks of USh) are most known. Devonian alkaline and subalkaline massifs and occurrences of alkaline rocks mainly are concentrated in joint zone of USh and folded Donbass system (Pokrovo-Kyryevo and Khomutov) or westward of this zone (Prymorsk, Mariupol). One of them (Zirka) is located among the Proterozoic South-Kalchyk massif of subalkaline rocks.

Both Precambrian and Mesozoic magmatism is developed in the Aldan Shield (ASh). To the first type belong: Khani massif, outcrop Ukduska – consist of biotite pyroxenites (with related apatite deposit), and shonkinite dikes, syenites, granites and carbonatites. In addition all of these rocks are cros-cutted with olivine lamproite dike (age 2.7 Ga) (Vladykin, Lepekhina, 2009). Selegdar carbonatitic massif (1.8 Ga) contains apatite deposits. Arbarastakh massif (670 Ma) consists of pyroxenites, shonkinite dikes, syenites, phoscorites and carbonatites, to the last Nb-Ta deposit is related. Ingili massif (650 Ma) consists of amphibole-biotite pyroxenites and ijolite dikes, syenites and carbonatites. Katugin massif of rare metal arfvedsonite alkaline granites (2.2 Ga) represents the second intrusive phase of Kolar granite rapakivi massif (1000 km²). The large Nb, Ta, REE, Zr, Y deposits are related to alkaline granite of this massif.

Mesozoic period of alkaline magmatism is connected with Mz-rifting (180-110 Ma). Rift zones are spread in Northern part of the shield and strike from Western to Eastern part. Alkaline magmatism is presented by potassium and ultrapotassium varieties. There are known more than 30 massifs. The largest massifs are Murun, Bilibinsky, Inagli, Konder, Ryabinoviy, Yakokutsky, Tommotsky, Ilimakh and lamproitic Kayla and Hatastir diatremes. In first two of mentioned massifs complete sequence of K-alkaline rocks differentiates of from ultrabasites to granites is revealed (Vladykin, 1997). With these potassium massifs occurrences of carbonatites, lamproites, K-Al raw materials (synnyrite, industrial stones) and ore deposits are conected. Paleozoic stage of magmatism is presented by Sakun massif with synnyrite deposit.

In spite of such varieties and distinctive peculiarities of alkaline magmatism in these two shields, their similarity is that only or predominantly Proterozoic alkaline massifs are enriched or ore-bearing in such rare metals as Zr, REE, Y, Nb. For examples in the USh Oktyabrsky (Mazurivka deposit), South-Kalchyk (Azov deposit), Chernigivka (with carbonatite deposit of the same name),Yastrebecky (of the same name deposit) massif are known. Among Devonian massifs of alkaline rocks of this region only Pokrovo-Kyryevo massif is distinguished by high contents and mineralization of REE, Y, Nb, partly Be, but deposits of these rare metals are not found here yet. The other massifs and occurrences of Devonian magmatism in this region (Khomutov, Mariupol, Zirka) have in general low (as for kind of these rocks) concentration of Zr, Nb, REE, Y. Such depleted in these rare metals is alkaline-ultrabasic rocks of Western part of USh (Proskurivka, Antonivka, Gorodnica, Glumcha, Bolyarka).

In the Aldan Shield ore-bearing for these rare metals are Katugin and Arbarastakh massifs. The large apatite deposits are prospected in Khani and Selegdar massifs.

The cause of rare-metal enrichment of Aldanian Precambrian massifs is prolonged differentiation of alkaline melts (for Arbarastakh massifs) and giant volume of granite rapakivi magmas (for Katugin massif) (Vladykin, 1997).

Ukrainian researches supposed various explanations of mentioned geochemical heterogeneity and ore bearing ability of alkaline rocks. In according to one of them (Kryvdik et al., 2003), alkaline-ultrabasic rocks of Western part of the USh have been predominantly forming by partial melting of mantle by absence of metasomatized fluids or without their essential influence. There is an alternative explanation (Kryvdik, Dubyna, 2006) accordingly to that Proterozoic alkaline rocks of the USh with different geochemical specialization in Nb, REE, Zr have been formed in different geodynamic conditions: enriched in Nb (Azov area) are connected with rifting, and depleted in Nb (Western part of the USh) – with pressing conditions of earth crust. As to Devonian alkaline rocks of the USh (Azov area), it was presumed their relations with rifting (Pokrovo-Kyryevo massif) with forming of Fanerozoic folded area of Peleotetis (Scythian plate) (Zirka massif).

Certainly it is necessary to carry out a supplementary geological, geophysical and precision isotope-geochemical investigations for decision of these complicated petrological, geochemical and mineralogical problems of alkaline rocks of various age and their different ore bearing potential from the Aldan and the Ukrainian shields.

This study was financially supported by the project of Ukrainian NAS №1-F/2011 in collaboration with RFBR.

Vladykin N.V. Petrology and ore-bearing ability of K-alkaline complexes of Mongolia-Ohotsk magmatism area. Referat of doctor dissertation. – Irkutsk, 1997. – 80 p. (in Russia)

Vladykin N.V., Lepekhina E.A. The age of unusual xenogenic zircons from kimberlites of Yakutia // Doklady (Reports) RAS, 2009, v. 429, №6, P. 774-778. (in Russia)

Vladykin N.V. Petrology of K-alkaline lamroite-carbonatitic complexes and their genesis and ore-bearing potential // Geology and geophysics, 2009, v.50, №12, P. 1443-1455. (in Russia)

Kryvdik S.G., Dubyna O.V. Geochemical peculiarities of alkaline rocks from Dnister-Bug area of the Ukranian Shield // Mineral. J., 2006, №4, P. 32-42. (in Ukrainian)

Kryvdik S.G., Tsymbal S.N., Geyko Yu.V. Proterozoic alkaline-ultrabasic magmatism of Northern-Western part of the Ukranian Shield as an indicator of kimberlite forming possibility// Mineral. J. (Ukraine), 2003, №5/6, P. 57-69. (in Russia)

Concerning to Mezozoic alkaline intrusions of Aldan Shield it is not improbable their tectonic relation with forming of Mezozoic folded belts that round this massif from the South, East and North.

Though this massif and deposits related with him are quite special and have some likeness with like massifs and deposits of Azov area of USh such as Azov, Yastrebecke and Perga. However Katugin deposits is characterized by clear specialization on Y, Nb and minor role of Zr while Ukrainian deposits – on Zr, REE, Y (Azov), Zr (Yastrebecke) and Be (Perga) but minor Nb (for all types).

The reasons of depleted Fanerozoic alkaline rocks in comparison with Proterozoic in the USh and ASh are not find out finally. Some investigators (Bogatikov et al., ) are supposed that upper mantel in the ASh depleted in the mentioned rare metals after Proterozoic alkaline magmatism (melting of magmas just as were formed Katugin massif). It is supposed (Bogatikov et al.,) that metasomatite fluids during of the Mesozoic lamproite magmatism of ASh had a water composition predominantly (and not alkali-siliceous) what don’t promote of enrichment of alkaline melts in REE, Nb, Zr.