2011

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Petrochemical model of uranium-mineralized albitites of the Novokostantynivka uranium deposit (Ukraine)

Emetz A.V.*, Cuney M.**, Yuzlenko A.T. ***

* M.P. Semenenko Institute of geochemistry, mineralogy and ore formation of NASU, Kyiv, Ukraine; alexander_emetz@yahoo.com

** Henri Poincaré University, Nancy, France

*** ShidGZK, Novokostantynivka mine, Olexiyivka, Ukraine

Uranium deposits in Na-metasomatites of the Ukrainian Shield are known over 50 years from 1945 when the first Pervomayske U deposit was discovered in the metasomatites developed in iron ore metaterrigenous beds of the Kryvbas. Since 1945, over 20 U deposits and rich ore showings were discovered. They are dominantly localized in albitites metasomatically developed in granitic and granitic-gneisses complexes of the Ingul megablock of the Ukrainian Shield, in the Novoukrainka granitic-plutonic complex and in the surrounded territories. Actually, uranium resources of the Central Ukraine U Province (CUUP) exceed 200 000 t taking the sixth place among the world countries with the greatest U reserves. The Novokostantynivka deposit with 93,600 t U is the largest U deposit in Europe, was discovered in 1975 during drilling in the positive magnetic geophysical anomaly resulted from increased content of magnetite in albitites developed in the Novoukrainka granites near Oleksiyivka village. Petrological investigations of the Novoukrainka granitoid complex, the rocks which compose significant part of the Ingul megablock, and of U-mineralized albitites developed in the Novoukrainka rocks but dominantly originated due to transformation of granites, given insufficient information about petrochemical and geochemical characteristics of these rocks [Scherbakov (2005), Belevtsev et al. (1995), Koval (1980)].

Rock samples for the present research were systematically selected from underground mines and drillholes in the Novokostantynivka U deposit and from nearby open granite quarries. Mineral composition of the samples was investigated using methods of optical micrscopy, electron microscopy and electron micro probe analysis. Major and trace elements in the rock samples were analyzed with ICP-MS/-AES in CNRS (Nancy, France).

The Novokostantynivka deposit is situated in the northern part of the Novoukrainka plutonic massif, in the vicinity of its contact with the Korsun-Novomyrgorod Pluton.

The host rocks in the Novokostantynivka deposit are dominantly presented by red mid- to coarse-porphyriblastic granites of the Novoukrainka complex, having well developed textural fabric observed as parallel oriented microcline crysts. Such textures were probably resulted from magma crystallization under dynamical compression. Mineral composition of granites (%): microcline-pertite (8-62), quartz (15-35), oligoclase (15-58), biotite (2-16), almandine (0-8). The widespread accessories are presented by apatite, monazite, ilmenite, pyrite and magnetite. In the Novokostantynivka deposit, the host granites are peraluminous calc-alkaline granites. From regional view, such red granites compose 80-85 % of all magmatic rocks of the Novoukrainka complex. In the deposit, among these granites, there are bodies of gray equigranular granites. Mineral composition of the grey granites is nearly identical with mineral composition of the red granites. However, gray granites are often metaluminous granites. In compliance with Golub (1992) which studied petrographical composition of rocks from drillholes penetrated upper levels of the Novoukrainka granites, gray hedenbergite granites dominate among granitic solids at depth. These granites apparently form the field of metaluminous rocks well observed in petrochemical discrimination diagrams and therefore suggest either different type potolites or diverse source of granitic magma. Therefore, this difference in granite composition at least in part does not fit widely suggested opinion about production of the Novoukrainka granitoid rocks by S-type magma [Scherbakov, 2005]. In addition, the host granites in the Novokostantynivka deposit include relic bodies of gneisses.

Development of alkali (sodium) metasomatites in the Novokostantynivka deposits was controlled by the meridionally oriented Novokostantynivka regional-scale shear zone. This tectonic zone had protracted activity, from high temperature ductile regime to lower temperature fragile deformation regime and consists of a system of smaller tectonic faults well observed as a system of strip zones of plastic deformations, up to some 10-th m thick, which are filled with blastomilonites and are surrounded by zones of cataclasis developed later with different intensity in granites. Albitization is connected with the zones of intense fracturation developed in the host granites in the zone of joining extended from SW to NE Secant fault with a system of meridionally striking faults including the most contrastingly developed Underlaying, Meridional, Eastern and Syenite faults. A range of later faults of NW-SE and NE-SW orientation complicates structure of the deposit.

The earliest metasomatic process occurred before albitization was episyenitization of the host granites. Episyenitization was resulted from total dissolution of magmatic quartz and femic mineral transformation: magmatic almandine and biotite were replaced by chlorite and epidote. Chloritization and epidotization was also developed metasomatically before episyenitization as the most distal initial alterations of granites. Destruction of biotite led to negligible depletion of the rocks in K, decreasing concentration of F and local introducing of Ca. Except of total loos of “free” SiO₂, concentration of other elements in the host rocks are identical to those typically observed in fresh granites. New-formed episyenites are highly porous rocks due to numerous vugs after quartz leaching, but actually often highly fractured rocks because of closing of these cavities under high P-T-conditions. U/Th ratio in episyenites and other petrochemical coefficients have the same values characterizing fresh granites.

Albitization characterizes Na-metasomatism in granite and granite-gneiss complexes. Mineralogially it looks like peudomorphic replacement of microcline and microcline-pertite by albite, decalcification of oligoclase and by total replacement of femic minerals by albite in the most intense zones of Na-metasomatism. Also, albite was crystallized as single crystals and druses in cracks and vugs of episyenites. Due to total albitization, some albitites aggreagates up to 98% are composed by albite. However in the most occurrences femic minerals were replaced by aegirine or reibeckite and/or actinolite metasomatites. Aegirine and amphibole albitites contain different amount of titanite, rutile, ilmenite, magnetite and/or hematite which were crystallized due to destruction of Ti- and Fe-bearing femic minerals of granites and episyenites. Eventually, due to gradual cooling of the hydrothermal system, aegirine is replaced by amphibole often without significant addition or depletion of the rock elements. Mass-balance diagrams (isocon diagrams of Grant (Grant, 1986)) show that Na-metasomatism led to significant ejection of K, Rb, Cs, Ba and F due to destruction of microcline and biotite, and new introduction of Na, V and U, and Ca and Sr. The last 2 elements (Ca and Sr) characterize formation of actinolite and/or calcite during Na-metasomatism.

Before the next stage of hydrothermal activity, Na-metasomatites were locally subjected by ductile and fragile deformations resulting in albite granulation or flexing of albite and titanite crystals. New minerals which were crystallized due to introduction of new portions of hydrothermal fluids both in new cracks and cavities, and metasomatically, compose andradite-diopside-epidote, actinolite-calcite-epidote or calcite-magnesioreibeckite paragenesises with U minerals (davidite, Brannerite and uraninite). These minerals cement cracks in aegirine albitites, corrode and often pseudomorphically replace aegirine. The major elements introduced during Ca-metasomatism: Ca, Sr, Ba and U. Na and Si were ejected.

The later calcite-phlogopite-magnetite paragenesis was formed at the end of Ca-metasomatism with dramatic manifestation of K-metasomatism in the deposit. Zones of K-Ca metasomatism correspond to the richest U ore bodies in the deposit. Calcite-phlogopite-magnetite paragenesis was superimposed on all previously formed metasomatites at different intensity, and includes plentiful U mineralization represented by davidite, brannerite and uraninite, which mostly replace magnetite. The last is plentiful among phlogopite and calcite, was crystallized due to destruction of Fe-minerals: mostly aegirine and amphiboles. Also, newformations of titanite are locally plentiful because of destruction of aegirine-augite or other Ti-rich silicates which were dissolved during carbonate metasomatism. New crystallization of malacon with high Zr/Hf ratio in calcite-phlocopite-magnetite aggregates was common. U content is well correlated with concentration of Fe²⁺/Fe_tot ratio suggesting U deposition in strong dependence on redox barrier which was created by Fe-bearing metasomatites in contact with U-saturated hydrothermal solutions. During Ca-K metasomatic process, the albitites were enriched in Rb, Cs, K, Mg and F which accumulated in phlogopite, U characterizing crystallization of U-minerals, Ca, Sr, Mn and Zn concentrated in carbonates, Zr and Hf accumulated in hydrothermal zircon, but locally Ca-K metasomatites were enriched in Cu, Ni and Co which were concentrated in thin disseminated sulfides. In contrast, Na, V and Si were furnished out due to destruction of Na-metasomatites.

The latest hydrothermal mineral-forming processes in the deposit were apparently connected with progressive cooling of the hydrothermal system and led to total chloritization of femic minerals with ejection of U, K, Rb and Cs. Also, in zones of new tectonic fracturation, the hydrothermal activity was locally intensified with formation of epidote-calcite-barite paragenesis surrounded by zones of chloritization These zones were additionaly enriched in Ca, Sr and Ba.

References:

Belevtsev N.Ya., Koval V.V., Bakarjiyev A.H. et al. Genetic types and regularities in placement of uranium deposits of Ukraine. Kyiv: Naukova dumka, 1995. 400 p.

Golub E.N. Petrology and petrochemistry of basites and metabasites of the Ukrainian Shield // Geochemistry and ore formation. 1992. No 19. P. 70-78.

Koval V.B. Geochemical model of uranium accumulation in alkali-calcareous metasomatites of Precambrian. Kiev: Naukova Dumka.1980. 148 p.

Scherbakov I.B. Petrology of the Ukrainian Shield. Lviv: ZUKC. 2005. 366 p.

Grant J.A. The isocon diagram – a simple solution to Gresens' equation for metasomatic alteration // Economic Geology.1986. Vol. 81. P. 1976-1982.