Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Lithofacies types of Paleoproterozoic meta-kimberlites from the Kimozero area and their geochemical characteristics (Karelia, Russia)

Kargin A.V.^*, Nosova A.A.*, Ruch`ov A.M.**

* - The Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences (IGEM RAS), Moscow, Russia

** - The Institute of Geology KarRC RAS, Petrozavodsk, Russia

kargin@igem.ru

Kimberlites of Kimozero area are unique occurrence of ancient Paleoproterozoic kimberlitic magmatism. Kimozero localized in the central part of the Onega depression among shungite shales, basalts and gabbro-dolerites of Zaonezhskiy complex of Paleoproterozoic age (~2000 Ma). The age of formation of kimberlites of Kimozero is 1986 ±4 Ma (Samsonov et al., 2009). The first discoveries of kimberlite of Kimozero were in 1992 and since this time are known just several publications (Ushkov, 2001; Ustinov et al, 2009).

The main difficulty in the study of the kimberlites of Kimozero is metamorphism in greenschist facies conditions and intensive shear deformation with post-kimberlitic age (~1700 Ma). The presence of these shear deformation have questioned the safekeeping of primary morphological characteristics of the diatreme zone. Modern contours of the kimberlitic structure may not correspond to the source structure; to the same kimberlite output in different parts of the kimberlitic structure have different level of erosion. However, despite the above limits, kimberlites of the one tectonic blocks preserved the original textural and structural characteristics which together with the nature of the secondary alteration and petro- and geochemical characteristics we can distinguish two types: carbonate and magnesium. The kimberlites of these types form different lithofacies types. The presence of several types of kimberlite (Carb- and Mg-kimberlites) as is typical for other kimberlite provinces of the world, for example, Daldyn-Alakit area, Yakutia (Ilupin and 1981).

Lithofacies varieties of kimberlites of Kimozero can be grouped into two zones – diatreme and crater. In addition, there were the thin dikes of Carb- and Mg-kimberlites.

Diatreme zone consist of autolithic kimberlitic breccia (AKB), kimberlitic breccia (KB) and kimberlitic xeno- tuff breccia (KTB). The kimberlitic breccia consists of the xenoliths, megacrysts and groundmass. Among the xenoliths dominated by fragments of dolerite, and found to be subordinate carbonaceous shales. The size of xenoliths in the KB is about a first cm, with their contents up to 15-20 vol. %; the size of the xenoliths in the KTB can reach several decimeters, with their contents up to 50-60 vol. %. Megacrysts represented by olivine, and mica. Olivine is completely replaced by minerals of the serpentine group, which in turn develops chlorite, and carbonates. Mica often forms the deformed leaves, the size of them is 5-6 mm. The micas are chloritized. The groundmass consists of serpentine-chlorite aggregate with a mixture of carbonate material from the first to up to 30-40 vol. %, small isometric grains of completely altered olivine of the second generation, and fine disseminated of ore mineral up to 10-15%. The relics of primary kimberlitic groundmass minerals are not stored. In AKB notes the presence of autoliths up to 20 vol. %. Autoliths have a more fine-grained structure with megacrysts of olivine which are completely replaced by minerals of the serpentine group, and chloritized mica. These minerals immersed in a carbonate-serpentine groundmass with fine ore minerals up to 20 vol. %. Often autoliths have nuclear structure - nuclei are xenoliths of country rocks or large xenocrysts of altered olivine. Among the accessory minerals are garnet, pyroxene, spinel, ilmenite and diamonds.

Crater zone consists of kimberlitic tuffs and tuffites of pyroclastic and epiclastic origin. Tuffites formed by the recycling of pyroclastic material and the kimberlitic material of diatreme zones. The rocks of crater zone consist of disintegrated kimberlitic material (as similar to kimberlites breccia) with the addition of fragments of country rocks represented by dolerites or carbonaceous shales. Sometimes the contents of xenoliths material, for example the carbonaceous shales, may dominate the composition of the rocks. Textures of kimberlites of crater zone vary from fine-grained to medium-grained; in the rocks are vary the proportions of the contents of olivine's fragments, mica and the amount of ore mineral (magnetite). Often there are remaining the relict layered and gradational structures. They are indicating of the recycling of kimberlitic material. For example, there are areas with alternating layers of rich ore mineral (mainly magnetite, up to 80% vol.) and layers of rich kimberlitic material. We may suppose in different parts of kimberlite structures the existence of different sections of the crater area and different level of erosion, because there are variations in size and degree of sorting of mineral aggregates and the distribution of packs which composed of one or another kimberlitic materials.

All the kimberlites were metamorphosed in the greenschist facies. In result of this metamorphic process the primary mineral assemblages have experienced a strong conversion. They were replaced by carbonate-chlorite-actinolite association. Among the tuffs and tuffites, especially near contacts with country rocks, occurrence superimposed processes amphibolization and carbonation.

Carb-kimberlites are enriched in Al₂O₃, CaO, P₂O₅, Li, Sr, Y, Nb, REE, Th, and U and depleted in SiO₂, MgO, Cr и Ni. The Fig. displays the primitive mantle-normalized incompatible element diagram for the Kimozero kimberlites. It shows that Carb-kimberlites do not have positive anomalies of Zr-Hf and have negative anomalies of Ti as opposed to Mg-kimberlites. In addition, Carb-kimberlites have a higher level of fractionation of REE than Mg-kimberlites: relations (La/Yb)_n and (Gd/Yb)_n are ranging in 5.7-7.0 and 71-115 respectively for the Carb-kimberlites; and 20-48, and 3.1-5.2 for the Mg-kimberlites respectively. It also shows that the distribution of trace elements for the majority of representative samples tuffites comparable with the distribution of trace elements for Mg-kimberlites. This evidences their pyroclastic and epiclastic origin.

Fig. The primitive mantle-normalized incompatible element diagram for the Kimozero kimberlites. Primitive mantle values are (McDonough, Sun, 1995).

The resulting petro- and geochemical differences suggest that the Carb-kimberlite had the more geochemically enriched source than the Mg-kimberlites. High content of magnesium component in melt of Mg-kimberlites associated with a higher degree of assimilation of lithospheric mantle peridotite while the generation of Mg-kimberlites melts (Sparks et al., 2008) in contrast to the Carb-kimberlites.

So, the formation of kimberlitic melts occurred in two phases: 1) formation of Mg-kimberlites, their tuffs and products of recycling; 2) Carb-kimberlites and the tuffs.

This study was financially supported by grant of the President of the Russian Federation for state support of young Russian scientists

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