Pyrochlore group minerals are the main raw phases in granitic rocks of the Katugin complex-ore deposit that stores Nb, Ta, Y, REE, U, Th, Zr, and cryolite. They are of three main generations: primary magmatic (I), early postmagmatic (II), and supergene (III) pyrochlores. The primary magmatic phase (generation I) is fluornatropyrochlore with high concentrations of Na2O (to 10.5 wt.%), F (to 5.4 wt.%) and REE2O3 (to 17.1 wt.%) but low CaO (0.6-4.3 wt.%), UO2 (to 2.6 wt.%), ThO2 (to 1.8 wt.%), and PbO (to 1.4 wt.%). Pyrochlore of this type is very rare in nature and limited to a few occurrences, such as rare-metal deposits of Nechalacho in syenite and nepheline syenite (Canada) and Mariupol in nepheline syenite (Ukraine). It may have crystallized synchronously with or slightly later than melanocratic minerals (aegirine, biotite, and arfvedsonite) at the late magmatic stage when Fe from the melt became bound making impossible the formation of columbite. Second generation pyrochlore formed at the early postmagmatic stage of the Katugin deposit. It differs from that of first generation in lower Na2O concentrations (2.8 wt.%), relatively low F (4 wt.%), and les occupancy of the A and Y sites at similar contents of other components. Generation III pyrochlore is a product of supergene alteration processes. It is compositionally heterogeneous and contains K, Ba, Pb, Fe, and significant Si concentrations but low Na and F. Its compositions mostly fall within the filed of hydro- and kenopyrochlore.
pyrochlore-group minerals; fluornatropyrochlore; alkaline granites; Katugin rare-metal deposit; East Transbaikalia
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