Сomposition, evolution and age of Nb-REE mineralization in carbonatite complexes in the Ural Fold Belt: New insight into metallogenesis

The Ilmeno-Vishnevogorsk (IVC) and Buldym carbonatite complexes of the Southern Urals are deformed linear-type carbonatite complexes that underwent tectonic evolution as a result of accretion-collision processes and the Hercynian collision orogeny. The deposits of niobium and rare earth elements are associated with the Ural carbonatite complexes. Nb-REE ore mineralization is represented by the pyrochlore supergroup minerals, aeschynite and monazite. Their compositional evolution and connection with various phases of alkaline magmatism, pegmatite and carbonatite formation, and late postmagmatic (carbothermal) processes were studied. To determine the age and duration of the ore-forming stages, UPb dating of minerals pyrochlore supergroup phases and monazite was carried out. The pyrochlore-group minerals of the Ural carbonatite complexes are represented by calciopyrochlore, rarely natropyrochlore, and kenopyrochlore. Pyrochlore I, rich in U-(Ta), crystallizes in the earliest magmatic phases of the IVC, in the miaskites and carbonatites I of the Central Alkaline Band. In contrast, Ta-(U)-bearing pyrochlore II is formed in the later magmatic phases, in the taxitic miaskites and miaskite-pegmatites, and is present in explosive carbonatite breccias and carbonatites II of the Vishnevogorsk massif. Both varieties of pyrochlore have magmatic characteristics – oscillatory zoning, absence of vacancies in the A-site, and low Nb/Ta <80. Pyrochlore III and Sr-REE-containing pyrochlore IV – with high Nb/Ta >300 and fluorine (4–5 wt%), are formed from fluid-saturated F-containing carbonate systems in carbonatites II of the IVC miaskite intrusions and fenite halos, as well as in carbonatites III and fenites of the Buldym complex. REE minerals – aeschynite-(Ce) and monazite-(Ce) formed in the carbothermalites of the Buldym complex and in fenite halos of miaskite massifs. The morphological features and evolution of the pyrochlore composition indicate the polygenic nature of the ore process, the late low-temperature stage of which is associated not only with Nb but also with REE mineralization (aeschynite, monazite).

The results of U-Pb-dating of the pyrochlore- and monazite-group minerals make it possible to distinguish two stages of ore formation in the studied carbonatite complexes of the Southern Urals. The early stage is recorded by U-(Ta)-rich pyrochlore in carbonatites I, the Potanino deposit (378 ± 5 Ma), and can be correlated with the primary crystallization of the IVС alkaline rocks and carbonatites at the rifting stage (D₃) of the forming continental margins. Whereas, the late stage is dated at ∼255–230 Ma yielded for pyrochlores II-IV of the Vishnevogorsk deposit – in taxitic miaskites, syenite- and miaskite-pegmatites, as well as carbonatites II of various ore zones (P₃-T₂), and pyrochlore and monazite – in carbonatites III and carbothermalites of the Buldym complex (∼ 245–235 Ma, T_1–2). The late ore-forming stage is associated with palingenic-metasomatic processes of transformation of the Paleozoic carbonatite complexes and the formation of ore-bearing fluid-saturated alkaline and carbonatite melts and fluids at the Ural collision stage and post-collision relaxation.