Niobium mineralization of the Bolshetagninskoye deposit (Eastern Sayan)

The Bolshetagninskoye deposit is one of the largest known endogenous deposits,which is promising for niobium ore development. It is confined to the Ziminsky complex of ultramafic alkaline ultramafic alkaline rocks and carbonatites of Late Riphean age. The purpose of the study is to identify the distribution patterns of niobium in rocks and ores of the Bolshetagninskoye alkaline-ultramafic carbonatite massif at different stages of endogenic and postmagmatic processes using the methods of optical microscopy, scanning electron microscopy, X-ray diffraction electron microprobe analysis (microprobe), and X-ray fluorescence analysis. It has been determined that the primary niobium ores belong to holocrystalline apatite-mica and apatite-pyrochlore rocks. The main minerals concentrating niobium in these ores are fluorocalciumpyrochlore and ferrocolumbite. It is noted that a close paragenesis of apatite and pyrochlore has been observed. The remaining (secondary) ore types are associated with pyrochlore alteration products from primary ores and formed as a result of the redeposition of niobium in new geochemical conditions. Secondary niobium ores are represented by calcite carbonatites, potassium feldspar, and other metasomatites. The main minerals in these rocks with superimposed niobium mineralization – niobium concentrators – are presented by niobium-bearing rutile, ilmenorutile, niobium-bearing hematite, newly formed fluorocalciopyrochlore and ferrocolumbite. Among the studied rocks pyrochlore is represented by fluorocalciopyrochlore and uraniumpyrochlore, significantly enriched in tantalum. Unaltered fluorocalciopyrochlores from apatitite-pyrochlore, apatite-mica, and calcite-feldspar rocks are characterized by a consistent composition of CaO, Na₂O, and SrO. Altered, hydrated varieties (based on the A-site vacancy) are more characteristic of metasomatic rocks and calcite carbonatites with superimposed pyrochlore mineralization. This reflects varying degrees of fluid processing of these pyrochlores. The conducted study revealed that the niobium ores of the massif underwent a multiphase, multistage transformation, which is represented by a decrease in the ore component content and the presence of different oxide forms of niobium in secondary ores compared to primary ores.

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