2011

News	Registration	Abstract submission	Deadlines	Excursions	Accommodation	Organizing committee
First circular	Second circular	Abstracts	Seminar History	Program	Travel	Contact us

Petrology of ultrabasic rocks of the Devladovo regional-scale fault zone (Mid Fore-Dnieper)

Velikanova O.Y., Velikanov Y.F.

M.P. Semenenko Institute of geochemistry, mineralogy and ore formation of NASU, Kyiv, Ukraine

In the Devladovo regional-scale fault zone which is extended in latitudinal direction eastward of the Kryvy Rig – Kremenchug syncline zone, there are ultrabasic massifs which contain the largest deposits of silicate Ni ores in the Mid Fore-Dnieper region.

Actually, 9 massifs are known in the Devladovo fault zone, all are composed by ultrabasic rocks: Ternivka, Veseloternivsky, Promijny, Pryvorotnensky, Kodaksky, Vodansky, Devladovo, Krasnoyarsky and Gulaypolsky massifs.

The ultrabasic bodies were injected through the Demurinka plagioclase-microcline granites of the Archaean age and Saxagan grey plagiogranites and thus are of latter origin. Real isotopical-geochronological investigations of the ultramafites were not performed. From geological position, the ultramafic rocks are believably of Middle Paleoproterozoic age.

Comprehensive investigations of these badly investigated rocks were executed to reconstruct the composition of the primary rocks, and to establish their mineralogical-petrographical and petrochemical-geochemical parameters.

The massifs consist of differently altered peridotites, gabbro-peridotites, and pyroxenites.

The ultrabasic rocks are greatly variable from chemical composition, mineralogical characteristics and intensity of metamorphic transformations.

Five massifs of ultrabasic rocks are located in the western part of the fault zone: Ternivka, Veseloternivsky, Promijny, Pryvorotnensky, Kodaksky (Ternivka area). Ultrabasic rocks of these massifs were intensively altered due to superimposed postmagmatic processes and are represented by serpentinites, talc-calcareous, serpentine-talc-calcareous, actinolite-tremolite and chlorite schists. Serpentinites include antigorite and antigorite-chrysolithe varieties.

In the eastern part of the fault zone (Devladovo area) 4 massifs are located: Vodansky, Devladovo, Krasnoyarsky and Gulaypolsky. Ultrabasic rocks of these massifs were less altered than in the western area and in the upper part of the column consist of serpentinitized peridotites, gabbro-peridotites, pyroxenites but rarely preferentially in their peripheral parts they include actinolite-tremolite and rarely chlorite schists.

Since different composition and ratio of major petrographical groups every massif is characterized by irregularity resulted from diverse distribution of major rock-forming minerals. Dependently on their quantitative ratio peridotites were divided in: dunites, olivinites, harzburgites, lerzolites, verlites and their plagioclase-bearing varieties.

Pyroxenites are located dominantly in the massifs of the Devladovo area. From mineralogical composition pyroxenites were divided in two varieties: olivine pyroxenites and plagioclase-bearing pyroxenites.

In ultrabasites of the Devladovo and Vodansky massifs, ultrabasic rocks form zoning – dunites and olivinites are located in central part of the massifs whereas pyroxenites form peripheral parts of the massifs. Thus, a magmatic differentiation is considered during solidification of these massifs.

Characteristics of geological nature of ultrabasic rocks intensively altered by secondary processes were interpreted from calculated petrochemical indices 2c, x, y, z, b, s, h, M/F using method of Zavaritsky A.N. [1] with corrections of Sobolev N.D. for ultrabasic rocks [2] and Hess H.H. [3]. In addition, coefficients of magnesia (K_M) and iron (K_F) and alkalinity (К_A) were calculated using method of Chernyshov [4]. Also, classification diagrams: MgO-Al₂O₃-CaO, MgO-FeO+Fe₂O₃-CaO, Al₂O₃-CaO-Na₂O+K₂O and TiO₂-MgO were used.

A prominent characteristic of chemical composition of ultrabasic rocks of the Devladovo zone is high values of K_M,variable alumina content, low content of alkali metals and TiO₂, different values of petrochemical coefficient M/F.

2 genetic groups of the rocks have been divided in the Devladovo regional-scale tectonic zone starting from diverse petrochemical parameters: 1) ultrabasic rocks – derivatives of the deep picritic magma (hyperbasites of N.D. Sobolev’s classification) which compose Ternivka, Veseloternivsky, Promijny, Pryvorotnensky and Kodaksky massifs; 2) ultrabasic rocks – derivatives of basic magma (ultraferbasites), which form Vodansky, Devladovo, Krasnoyarsky and Gulaypolsky massifs.

Rocks of the first group are characterized by high content of magnesia, low concentrations of alkalis and titanium dioxide, low calcareousness and contrasting prevalence of olivine over pyroxenes. M/F petrochemical coefficient exceeds 7.

In the second group of the rocks, average variations of petrochemical coefficient M/F of petrographical varieties of the ultrabasic rocks is below 7. x, y, z parameters suggest pyroxenic composition of the rocks, which is in compliance with petrographical data.

K_M, K_F and K_A indices of the ultrabasic rocks of Devladovo massif are similar to K_M, K_F and K_A indices of the gabbro-peridotitic formation of the Varvarovsky massif of the Verhovtsevo structure, although other parameters are different. Obviously, the ultrabasic rocks of these massifs were formed in different tectonic-magmatic conditions; in addition, they are spatially and temporary different and with different intensity altered during processes of regional metamorphism which provided contrastingly different changes of petrochemical characteristics.

In petrochemical diagrams, a significant part of figurative points of the ultrabasic rocks of the Devladovo zone falls in the contour of derivatives of high-magnesia komatiite series of the Onvervaht group of the Komati suite, a part of the figurative points falls in the field of marianite-boninite island arc series.

Metallogenetic specialization of the rocks was analyzed from formation characteristics, geochemistry and mineralogy of the rocks studied. A special attention was taken in relation to composition of the primary magma, intensity and type of superimposed postmagmatic processes which led to depletion of the primary ores or to reverse enrichment of the ores during later metamorphic and/or metasomatic alterations.

The ultrabasic rocks of Devladovo regional-scale fault zone are characterized by metallogenetic specialization for Cr, Ni, Co and Cu. Dominant elements were incorporated into magnesia-iron silicates (Ni, Co), i.e. in major rock-forming minerals, sulfides (Ni, Co, Cu) and magnetite, but some elements (Cu, Ni, Co, Au) form minerals. PGE-minerals were not detected although their presence in some rich in PGE areas is undoubtful. Increased contents of Cu, Au and PGE were also detected in some zones of quartzification and quartz-sulfide metasomatism.

A range of mineralogical-petrographical, petro- and geochemical characteristics (chemical composition, high content of magnesia, low contents of alkalis and calcium, quite high concentrations of Cr, Ni, Co and Cu) of the ultrabasic rocks of the Devladovo regional-scale fault zone can be compared with ultrabasic rocks of the south-east part of the Voronej massif, which contain significant reserves of copper-nickel ores.

References

1. Zavaritsky A.N. Recalculation of chemical analyses of igneous rocks. // M. – Gosgeoltechizdat, 1960. – 156 p.

2. Sobolev N.D. On petrochemistry of ultrabasic rocks // Geochemistry. – 1959. – No 8. – p. 679 – 695.

3. Hess H.H. Serpentinites, orogenesis and epeirogenesis // Earth core. – M: Foreign literature Press., 1957. – 211 p.

4. Chernyshov N.M. Sulfide copper-nickel deposits of the south-east of the Voronej crystalline massif. – Voronej: Voronej University Press. – 1971. – 234 p.