The Nb-Ti mineralization of calcite-dolomite carbonatites of the Matcha alkaline pluton, Alay range, southern Tien Shan

Development of Early Mesozoic (~ 200-220 Ma) alkaline felsitic magmatism in Turke-stan-Altay segment of Tien-Shan is accompanied by formation of calcite-dolomite carbon-atites. They are usually found in contact zones of intrusive massifs of alkaline and nepheline syenites as fine (n x 10 m, rarely up to 100-400 m) stocks and vein bodies. Especially numerous accumulations of them are concentrated in contact of the Matcha alkaline pluton (~ 30 km²), which is located in Eastern part of Alay Ridge. Dolomite-calcite varieties (with contents of CaO ~ 35-54, MgO ~ 4-17, FeOT ~ 1-3, SiO2 up to 1-5 wt. %) prevail among carbon-atites. Besides prevailing carbonates (~ 80-85 %), the rocks also content magnesium arfved-sonite, richterite, aegirine-augite, phlogopite, albite, microcline and apatite, as well as accessory zircons, monazite, pyrochlore, titanite, barite, magnetite, sulfides (pyrrhotite, pyrite, etc.). In stock-type carbonatite body in Delbek stream (NE exo-contact of the Matcha massif), Nb-Ti mineralization is noticed, which forms fine (~ 0,01-1 mm) scattered impregnation in silica-carbonate matrix. This mineralization contents pyrochlore group niobates, Nb-bearing silicates (fersmanite, baotite), ilmenite and ilmenorutile, whose chemical characteristics were studied using electron probe microanalysis (a Tescan Vega II scanning electron microscope with an INCA Energy 350 system). Common feature for pyrochlore group minerals (calciobetafite, uranpyrochlore, plumbob-etafite) is high concentrations of UO2 up to ~ 26-29 wt. %, absence or insignificant concentrations of Ta2O5 (0,8-3,4 wt. %), and varying concentrations of niobium (Nb₂O5 ~ 28-51 wt. %). Uranpyrochlore demonstrates zoning, which indicates accumulating of uranium and titanium at a later stage of its crystallization. Simultaneously, Na2O (from ~ 6,0 to 2,8 wt. %) and F (from 1,5-2,3 to 0,4-0,7 wt. %) are carried out and number of vacancies in A- position increases (up to 0,579 apfu) in crystal chemical structure of the mineral. Paragenic calciobeta-fite (СаО 6-13 wt. %) contents more TiO2 (12-15,5 wt. %) and corresponds to the condition 2TiB > (Nb + Ta)B, Ua > 20 %. It is replaced by plumbobetafite (PbO 5,5-17,2, SrO 4,4-5,6 wt. %). Nb-bearing oxides (ilmenite, ilmenorutile) and silicates (fersmanite, baotite) presented in the association concentrate №>205 up to ~ 1,5-6,6 wt. %. Ilmenite (TiO2/FeO* 1,2; MnO 1,64 wt. %) does not show any signs of decay of a solid solution and is replaced by ilmenorutile. Concentrations of main components СаО ~ 26-27, ТЮ2 ~ 37,5, SiO2 ~ 29-30 wt. % make fersmanite look similar to an ordinary titanite. However, if comparing it to other car-bonatites, measured baotite (ВаО up to 38-39 wt. %) stands out by enrichment in titanium and poor niobium content Ba3.9Ti7.2Nb0.7Si4.1O28Cl0.9. Such microstructural and chemical signs suggest late magmatic origin for Nb-Ti mineralization. Hydrothermal changes of niobates following the mineralization could promote increase in F, Ca, U, Pb, Ti, Nb mobility and recrystallization of betafite accompanied by formation of a more stable association of uranpyrochlore II + rutile/anatase or extraction of plumbobetafite. Another possible source for titanium is considered to be a post magmatic transformation of Nb-titanite during formation of ilmenorutile and rutile.

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