Compositional features of picroilmenites from kimberlites of East Peri-Azovian.

Tsymbal S.N. *, Sobolev V.B. **, Strekozov S.N. ***, Tsymbal Y.S.*, Litvinenko Y.А.*

* Institute of geochemistry, mineralogy and ore formation of N.P.Semenenko, NAS of Ukraine, Kiev, Ukraine;** Technical centre NAN of Ukraine, Kiev, Ukraine; *** Peri-Azovian complex geological party KP "Ukruzhgeology", Volnovaha, Ukraine

Picroilmenite is one of the most abundant and important deep minerals as to geological prospecting that was found in many kimberlite bodies from different diamond-bearing provinces of the world. And kimberlites of southeast part of the Peari-Azovian megablock of the Ukrainian Shield do not represent an exception. Here Petrovsk, Southern, Novolaspinsk and Nadiya pipes as well as Southern and Novolaspinsk dikes were found. They are comprised by phlogopite kimberlites of eruptive and hypabyssal facies of Devonian age. Both facies varieties of kimberlites belong to high titanium and high potassic types. The contents of TiO₂ in them vary mainly in the limits from 2 to 5 %, and K₂O vary from 1 to 2 % and even more. Main mineral that concentrates TiO₂ is represented by magnesian ilmenite (picroilmenite).

Picroilmenite is usually found in kimberlites in the form of macrocryst and less often as megacrysts and nodules. The size of the macrocrysts varies within 2-5 mm and the megacrysts and nodules show 5-10 mm and even more variations in size. Picroilmenite nodules of more than 6 cm in size are found in kimberlites of Novolaspinsk pipe. Some picroilmenites have monocrystal structure, but other have granoblastic-aggregative one (Smirnov, et. al., 1993; Tsymbal, et.al., 1996). They have rough and sometimes "spine-like" surfaces. Many macrocrysts show the presence of reactionary rims of various width and structure. Wider rims are observed on small fragments of aggregative and monocrystal grains. Internal boundary of rims is strongly sinuous, but it is always clearly defined and confidently enough established by apparent changes in ilmenite composition. Essential lowering of MgO with simultaneous increases of MnO and FeO is observed in a direction to external margin of the rim. At the same time TiO₂ shows little variations in its value. The external part of the rim does not correspond on composition to magnesian variety but is related to manganous variety of ilmenite that does not almost contain MgO.

Numerous determinations of composition of central parts (not altered by reactionary changes) of picroilmenite macro- and megacrysts from Southern and Novolaspinsk pipes and dikes have shown, that they are similar to those kimberlites of the Yakutia province. The contents of the main components in them vary within (%): TiO₂ ‑ 45-55; FeO ‑ 22-30; Fe₂O₃ ‑ 5-17; MgO ‑ 7-14; Cr₂O₃ ‑ 0-5; Al₂O₃ ‑ 0-0,5; MnO ‑ 0,1-0,3. They differ from kimberlites of Yakutia in the absence of low-magnesian (MgO <7 %) ferromagnetic (Fe₂O₃> 20 %) varieties. Based on Cr₂O₃ contents Peri-Azovian picroilmenites can be subdivided into three groups: low-chromium (Cr₂O₃ <0,5 %), middle-chromium (Cr₂O₃ ‑ 1,8-2,4 %) and high-chromium (Cr₂O₃ ‑ 2,7-4,4 %, and rarely up to 5,0 %). They form distinctly separated fields on diagrammes in MgO ‑ Cr₂O₃, TiO₂ ‑ Cr₂O₃, Al₂O₃ ‑ Cr₂O₃ and Fe₂O₃ ‑ Cr₂O₃ coordinates, but any correlations between contents of these oxides are not observed. The major part of low-chromium picroilmenites is more aluminous (Al₂O₃ ‑ 0,2-0,5 %) in comparison with middle-chromium (Al₂O₃ <0,15 %) and high-chromium (Al₂O₃ ‑ 0,05-0,2 %) varieties. Distinct positive correlation between MgO and Al₂O₃established in the studied picroilmenites, with taking into account data published by D.Grin and N.Sobolev (1975) about the fact of raising of Al solubility in ilmenite with more high temperature of its formation, allows us to make a conclusion, that low-chromium picroilmenites with raised contents of Al₂O₃ are more high-temperature varieties in comparison with high-chromium ones. Based on typochemical features establsihed, low-chromium picroilmenites were crystallised at earlier stage than middle- and high-chromium varieties. Among all the picroilmenites analysed (more than 1300 macro- and megacrysts) low-chromium varieties make 46 %, middle-chromium do 32 %, and high-chromium do 22 %.

Macrocrysts of picroilmenite from Southern kimberlite pipe analysed by LA-ICP MS show the presence of su admixture elements as (ppm): Ni ‑ 167-896; Co ‑ 144-207; V ‑ 1476-2847; Sc ‑ 20-53; Ga ‑ 8-22; Zn ‑ 98-249; Cu ‑ 7-78; Zr ‑ 349-932; Hf ‑ 16-72; Nb ‑ 528-2861; Ta ‑ 63-283; Sn ‑ 8-65 (Panov, 2001). High-chromium varieties of picroilmenites are characterized by the highest enrichment in Nb, Ta, Hf and Zr but low-chromium ones show the least concentrations of these element. So there is a basis to believe, that all of them have been influenced by the processes of deep metasomatism.

Studying of a large nodule of picroilmenite has shown, that it is comprised from recrystallised polygonal grains. Thin "flakes" of phlogopite commonly occur at interstitial spaces between these grains and such secondary minerals as ilmenite depleted in MgO and enriched in MnO, titanomagnetite, rutile, barite, calcite and cassite (water oxide of Ca and Ti) are commonly found in cracks and along them. 20 polygonal grains from this nodule have been analysed. It has appeared, that the composition of their central parts varies within (%): TiO₂ ‑ 51,08-55,17; MgO ‑ 9,48-10,63; FeO ‑ 27,35-29,21, Fe₂O₃ ‑ 7,45-9,92, MnO ‑ 0,21-0,56; Cr₂O₃ ‑ 0,16-0,26; Al₂O₃ ‑ 0,35-0,49; BaO ‑ 0,1-0,3. ICP MS analysis of bulk sample of the nodule has shown the presence of following admixture elements (ppm): Zr ‑ 350; Nb ‑ 450; Ta ‑ 59; Hf ‑ 14; Ba ‑ 1060; Ni ‑ 297; Co ‑ 180; V ‑ 1760; Zn ‑ 48; Sc ‑ 25; Ga ‑ 15; Cu ‑ 14; Sr ‑ 12; Sn ‑ 8. By chemical composition the picroilmenite from the nodule is comparable with low-chromium macrocrystic picroilmenite.

Based on the data available on the morphology and compositon of picroilmenite macro- and megacrysts from different kimberlite pipes and dikes of eastern part of the Peri Azovian megablock it is possible to make a conclusion about very close similarity of these picroilmenites. These picroilmenites have common source and are formed as a result of fractional crystallisation of the magmatic melt naturally evolving in direction of low content of magnesia, high content of iron and increased activity of oxygen. On composition some of them show similarity with picroilmenites from xenoliths of diamond-bearing pirope peridotites of kimberlite pipes Mir (Ponomarenko et. al., 1977) and Udachnaya (Pohilenko et. al., 1976), and also show similar features with picroilmenite inclusions in diamond from such kimberlite pipes as Mir (Sobolev et.al., 1976), Premier and Jagersfonrein (Tsai et.al., 1979).

Besides macro- and megacrysts of picroilmenite that are described in olivine microphenocrysts from the great bulk of kimberlites of Southern and Novolaspinsk pipes, for the first time are also found and studied picroilmenite inclusions from kimberlite melt of low deep level.

Olivine forms idiomorphic phenocrysts less than 100 microns in size. It is nearly completely altered and replaced by serpentine or serpentine and calcite. Idiomorphic microcrystals of picroilmenite and their intergrowths are established in pseudomorphs developed after olivine. Faces of many crystals of picroilmenite are parallel to the faces of crystals in olivine-matrix that testify to their syngenetic nature. Most picroilmenites show following variations in their composition (%): TiO₂ ‑ 51,9-55,3; MgO ‑ 11,9-16,8; FeO ‑ 19,6-25,6, Fe₂O₃ ‑ 3,7-8,8, MnO ‑ 0,3-0,6; Cr₂O₃ ‑ 0,3-2,0; Al₂O₃ ‑ 0,05-0,25. It is the most high-magnesian varieties among ilmenite that found in concentrates from kimberlite pipes of Yakutia and Southern Africa and also in xenoliths of mantle rocks. High magnesia content of picroilmenite and low contents of Fe₂O₃ testifies to their formation at high temperature and low fugacity of oxygen. Less magnesian (MgO ‑ 8,7-9,1 %) and less titanium (TiO₂ ‑ 48,2-48,9 %) varieties of picroilmenite with more high contents of FeO + Fe₂O₃ (37,0-38,9 %) and Cr₂O₃ (2,4-2,7 %) are rarely found. Such admixtures as V₂O₅ (0,39-0,68 %) and Nb₂O₅ (0,01-0,31 %) are established in all picroilmenites, but some picroilmenites also show the presence of Ta₂O₅ (<0,1 %). Compositionally most of the studied picroilmenite-inclusions found in olivine microphenocrysts form seperate field on MgO - Cr₂O₃ diagramme. On diagramme this field is plotted between fields of low- and middle chromium megacrystic and macrocrystic picroilmenites and slightly shifted in relation to them into the area corresponding to most highly magnesian (MgO ‑ 12,5-16,8 %) and highly titanium (TiO₂ ‑ 52,8-55,3 %) varieties. Among 14 analyses of picroilmenite-inclusions found in olivine phenocrysts one analysis is located in the field corresponding to low-chromium macrocrystic picroilmenites but two analyses are plotted in the field corresponding to high-chromium macrocrystic varieties.

All crystals of picroilmenite that are included in phenocrysts of serpentinized olivine show the presence of rim. These rims show constant width and are comprised by manganous variety of ilmenite MnO content of which sometimes reaches 8-15 %. If you take into account that content of MnO in picroilmenite makes 0,3-0,6 %, it is not enough for the formation around picroilmenite such large in size reactionary rims with so high contents of MnO and accordingly pyrophanite minale. So based on the above mentioned it is possible to assume, that the rims of Mn-ilmenite are formed at the stage of high-temperature serpentinisation of kimberlites at considerable activity of manganese.

There are two opposite points of view concerning possible origin of macro-and megacrysts of picroilmenite in kimberlites. Some researchers consider them as decomposition products of picroilmenite bearing deep rocks, but others interpret them as phenocrysts formed from kimberlite melt at fractional crystallization of this melt. The data obtained by us while studying of picroilmenites from kimberlites of Peri Azovian gives the bases to assert, that overwhelming majority of picroilmenites should be related to phenocrystic cummulates. Trends of composition and geochemical features of picroilmenites reflect the interrelated changes of thermodynamic and oxidation-redox conditions of mineralization in evolving kimberlite melt. But the melt should be initially enriched in titanium.

Refereces:

Panov Y.B. Typomorphic chemical features of indicator minerals from kimberlites of Peri-Azovian // Scientific papers of Donetsk national technical university // Series mining-geological. 2001. Vol. 32. P. 44-52 (in Russian).

Ponomarenko A.I. First finding of garnet-ilmenite peridotite with diamonds from kimberlite pipe "Mir"// Reports of AS of the USSR. 1977. 235, № 4. P. 914-917. (in Russian).

Pohilenko N.P., Sobolev N.V., Sobolev V.S, Lavrentiev Y.G Xenolith of diamond bearing ilmenite-pirope lherzolite from kimberlite pipe "Udachnaya" (Yakutia)// Reports of AS of the USSR. 1976. 231, № 2. P. 438-441. (in Russian)

Sobolev N.V., Botkunov A.I., Lavrentiev Y.G., Usova L.V. New data about composition of minerals, associated with diamonds of kimberlite pipe "Mir"//Geology and geophysics. 1976. № 12. P. 3-15. (in Russian)

Smirnov G.I, Chashka A.I., Tarasyuk O.N, et.al. Ilmenite from kimberlites of Peri-Azovian //Mineral. journ. (Ukraine) 1993. 15, № 3. P. 33-41. (in Russian)

Tsai H.M., Meyer H.O., Moreau J., Milledge H.J. Mineral inclusion in diamond: Premier, Jagersfonrein and Finest kimberlites, South Africa, and Williamson Mine, Tanzania // Kimberlites diatremes and diamonds: their geology, petrology and geochemistry / Eds F.R. Boyd, H.O.A. Meyer, - Washington: AGU, 1979. P. 16-26.

Tsymbal S.N., Tatarintsev V.I., Kniazkov A.P. The minerals of deep paragenesis from Yuzhnaya kimberlite pipe (East Peri-Azovian) // Mineral. Journ. (Ukraine) 1996. 18. № 5. P. 18-45. (in Russian)