Rare-earth minerals in pre-Cambrian rocks of the Lyapinskii anticlinorium (Subpolar Urals)

Research subject. REE-containing minerals in metamorphic pre-Cambrian rocks of the northern part of the Lyapinskii anticlinorium of the Subpolar Urals. Aim. To establish typochemical features and conditions for the formation of rare-earth minerals in the studied rocks. Materials and methods. Chemical compositions and photographs of minerals were obtained using a JSM-6400 scanning electron microscope with a Link energy spectrometer and a Tescan Vega 3 LMH with an Instruments X-Max energy-dispersive attachment. The content of rock-forming oxides was determined by the gravimetric chemical method. The contents of rare and rare-earth elements were determined using an Agilent 7700x inductively coupled plasma mass spectrometer. Results. Rare earth carbonates and titanoniobates were identified in the pre-Cambrian rocks of the Subpolar Urals. In the mica garnet-bearing crystalline schists of the Nyartinskii complex, ankylite-(Ce) and hydroxylbastnäsite-(Ce) were identified for the first time, and hydroxylbastnäsite-(La) was identified in the actinolite-bearing quartz-albite-epidote-chlorite schists of the Puyva Formation. In the metarhyolites of the Sablegorskii Formation, the typochemical features of bastnäsite-(Ce), kainosite-(Y), and аeschynite-(Y) were studied. Ankylite-(Ce), hydroxylbastnaesite-(Ce), and hydroxylbastnaesite-(La) from Lower Proterozoic and Upper Riphean metamorphic rocks were probably formed during metamorphogenic-hydrothermal processes in the presence of minerals containing rare-earth elements and with the participation of carbon dioxide solutions. A scheme for the formation of rare-earth minerals was proposed for the metarhyolites of the Upper Riphean-Vendian Sablegorskii Formation. Aeschynite-(Y) was probably formed as a result of the transformation of rhyolites, due to the presence in the residual melt of increased concentrations of some rare metals, i.e., Zr, Nb, Y, and REE. As a result of metamorphogenic-hydrothermal processes, bastnäsite-(Ce) replaces allanite-(Ce). Kainosite-(Y) in metarhyolites is the latest mineral, formed due to the recrystallization of allanite-(Ce) and bastnäsite-(Ce) during the decomposition of аeschynite-(Y). Conclusions. The study of REE-containing minerals in Precambrian rocks of the Lyapin anticlinorium found that rare-earth titanoniobates were formed during the post-magmatic stage of metarhyolite transformation. The formation of rare-earth carbonates may be associated with metamorphic and hydrothermal processes.

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