Botogol type graphite in the Azovof Sea Region of the Ukrainian Shield

Yatsenko V.G.

Institute of environmental geochemistry, Kyiv, Ukraine

Graphite in the world is really extracted from deposits of three commercially-genetic types: (1) - close-grained phanerocrystalline graphite, associated with igneous rocks (mainly syenites) and pegmatites; (2) - flake graphite in ancient regional metamorphosed rock stratas; (3) - close-grained cryptocrystalline graphite formed as a result of a contact metamorphism of coals.

The most important, the top-quality graphite raw materials are obtained from deposits of close-grained phanerocrystalline graphite . "Classical" deposits of the given type are deposits of graphite of Sri Lanka (Ceylon). They are known also in India, Canada, etc. In the USSR graphite ores similar in genesis and quality of deposits were attributed to Botogol commercially-genetic type.

Botogol graphite deposit situated in East Sayan Ridge, Siberia. The deposit is associated with Botogol alkaline massif. According to R.V. Lobzova [Lobzova, 1875], Botogol massif is well differentiated intrusion of early Paleozoic age, mainly composed of nepheline syenites and gabbroids. The host rock belong to Mongoshinska suite. Mongoshinska suite (Neoproterozoic) consist mainly of carbonate - marble, chalkstones with interlayers of shales, quartzites, ets. Regional-metamorphosed rocks of Mongoshinska suite as a rule have black color on account of presence of dispersed syngenetic flake graphite, and high bituminosity. Processes of carbonate rock assimilation by magma, high-temperature magmatic (K-feldspatization, nephelinization, pyroxenization) and postmagmatic (skarn) metasomatic processes are widely developed on the deposit area. Ore bodies are subdivided into column, nests, tabular and lenticular deposits which graphite ores are presented continuous, impregnated, disseminated and banded variety. Phanerocrystalline graphite forming together with cryptocrystalline graphite massive, nodular, somatoid (spherolite), treelike (columnar) and flake units are prevailing in ores. The contents of graphite in ores reaches 98 %.

Despite at more than 150-years history оf exploration, genesis of a Botogol deposit is discussed. As a whole does not raise the doubts, that formation of the deposit is connected with assimilation of metamorphic host rock, which contained significant amounts of both the carbonate and graphite, and also bitumens, by syenite intrusion . It has led to super saturation of the magma, magmatic and post magmatic fluids by carbon, that has caused the formation of graphite ores. Data of the isotope analysis of graphite (a wide range of values δ13С, mainly from -22 up to -15 ‰) confirm the mixed source of carbon (carbonates, graphite, bitumens of a host metamorphic rocks). Nevertheless physical and chemical parameters of environment, a condition of mobilization and form of transport of carbon, the mechanism of graphite crystallization remain not absolutely clear.

The Ukrainian Shield is extremely rich by graphite. However all known deposits belong only to one (zavallia) commercially-genetic type – concentration of flake graphite in metamorphic rocks.

In the Azovof Sea Region of the Ukrainian Shield, in area near Stary Crym village, close-grained graphite deposit occurrence, which is not characteristic for deposits of the Ukrainian Shield, is learned.

Stary Crym close-grained graphite occurrence associated with rocks of Central-Pryazovska series and is localized near to contact (2 km on a map) metamorphic stratas and intrusive rocks of Southern-Kalchsisky massif. In area of study of Central-Pryazovska series (Temriyk suite) is presented by a garnet-biotite, biotite, and graphite-biotite gneisses with interlayers of quartzites, diopside crystalline schists and layers of marbles and calciphyres. Age of the series is Neoarchean (2900-2600 Ma), rocks were affected by granulite metamorphism. Directly near to the investigated area metamorphic strata contains a number of graphite-bearing layers which forming well-known Stary Crym deposit of flake graphite. It is necessary to mention, that in NW direction, the productive strata of Stary Crym graphite deposit is transversely to course cut by contact of Southern-Kalchsisky massif. South-Kalchsisky differentiated massif (Paleoproterozoic, 1800 Ma) consist of subalkaline gabbroids, syenites, granosyenites, granites. Formally it is not alkaline, but genetically, together with October alkaline and other massifs, it comprises a part of alkaline syenite complex of Azovof Sea Region of the Ukrainian Shield.

Within the area of study metamorphic rocks (gneisses, calciphyres) have undergone to intensive metasomatic alteration. Leading process is K-feldspatization, which has led to wide development of massive, coarse-grained, mainly microcline rocks, similar to syenites . In addition to K-feldspatization, development of diopside zones after carbonate rocks, forming of scapolite and chlorite is observed.

Besides already mentioned dispersed syngenetic flaky graphite, abundant in rocks of Temriyk suite, close-grained graphite mineralization unusual for the Ukrainian Shield was observed on the investigated area. It is possible to mark out three basic forms of segregations of close-grained graphite in Stary Crym occurrence: (1) dispersed impregnations and fine streaks of graphite in K-feldspar areas zones in gneisses and calciphyres, with contents of graphite up to 5-10 %; (2) ample nest impregnation of graphite, to formation of close graphite ores, developing after completely reworked (brecciation, diopsidization, epidotization, scapolitization) carbonate rocks; (3) most interesting feature of Stary Crym occurrence is the presence of unusual, graphite-K-feldspar rocks, filling fractures in width up to 10 sm. This segregations similar in appearance to porphyry plutonic rock where phenocrust are perthitic microcline, and the ground mass has essentially graphitic or microcline (albite)-graphite composition. Dyke-liked form of bodies with distinct, plane contacts and simple wedging along the strike, absence of feature of tectonic crushing, allochthonous, homogeneous composition of rocks, their porphyry appearance, absence of crustified textures and aureoles of hydrothermal alteration indicate, that the way of their forming is not tectonic and they are not metasomatic or hydrothermal formations. It is possible to suggest, that they were formed by an one-act intrusion in fractures of viscous magma-like substance of carbon-silicate composition.

Close-grained graphite forms massive, flake, shelly, dendritic and spherolitic aggregates. Presence of rounded shelly and spherical forms [Kvasnitsa, Yatsenko, 1991] is specially characteristic. Spheroids of graphite of size of 0,15-1,5 mm are located predominantly inside or between K-feldspar grains . They have zonal concentric interior, allowing establishing succession of crystallization of graphite forms. Growth of spheroids took place from the center to periphery. In the beginning (in the center) was deposited close-grained graphite, then fine-grained graphite built up radially-radiant aggregate of dendritic graphite (the predominating form), forming of spheroids was ended by crystallization of tangential dispozed graphite flakes .

Data of isotopic analysis of carbon (The avarage δ13С in flaky graphite of biotite-graphite gneisses from Central-Pryazovska series is -22 ‰, in marbles and calciphyres about 0 ‰, in close-grained graphite of Stary Crym occurrence is varies from -15 to -13 ‰) testifies about the mixed source of carbon and confirms the asimilation and contact-reactionary genesis.

Thus, prominent features of Stary Crym occurrence of close-grained graphite, such as localization near to contact with syenite intrusion on the area where metamorphic strata containing simultaneously both of massive bodies of carbonate rocks and rich in flake graphite layers, wide development of contact-reactionary processes, predominating close-grained graphite with characteristic spheroidal, nest- and shelly-shaped forms, similar isotopic characteristics of carbon allow to approach Stary Crym occurrence to Botogol graphite deposit and allow to assume their analogous genesis.

But there are also differences: (1) Southern-Kalchsisky massif is subalcalinous, instead of alcalinous, as Botogol massif; (2) Stary Crym occurrence is confined to strata of altered metamorphic rocks, immediate contact with intrusive syenites is not observed.

Stary Crym occurrence of close-grained graphite not single in Azov Sea Region. Graphite occurrence, described still by J.Morozewitcsh in the Oktiabrsky alcaline massif is known. On the contrary, it is located inside of nepheline syenite intrusion and has some distance from contact. By same as in Stary Crym characteristic forms of graphite and its isotope composition (δ13С -11,9 ‰) these occurrences have similar genesis.

Stary Crym and Oktiabrsky close-grained graphite occurences taken together supplement each other at their attributing to Botogol commercially-genetic type. Their investigation: (1) gives additional data for understanding of genesis of unique Botogol graphite deposit; (2) present mineralogical interest so them contain unique graphite with abundant spheroidal forms of aggregates; (3) has petrological significance so indicate, that at formation of massifs of alcaline syenite complex of Azov Sea Region, at least in local, processes of assimilation and contact-reactional interacting with carbonate and graphitic host rocks, effect on the composition and redox potential of melts and fluids.

References:

Лобзова R.V. Graphite and alcaline rocks of Botogol massif region. Moscow: Nauka, 1975. 124 p (in Russian)

Kvasnitsa V.N., Yatsenko V.G. Globular graphite from Priazovye. Mineralogicheskii Zhurnal - 1991. - 13, N 1. - P. 95-101.