Uranium-thorium-rare earth minerals in pegmatites of the Selyankinsky syenite massif (the South Urals)

Cherednichenko S. V.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 5. P. 80-91

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Language: Russian

Abstract

The article displays data on U-Th-REE minerals in agpaitic pegmatites of the Selyankinsky syenite massif in the South Urals: monazite-(Ce), allanite-(Ce), nioboaeshynite-(Ce), thorianite, and minerals of the betafite subgroup. These minerals were found in the titanite rim and in marginal parts of ilmenite ore aggregates. Their composition was compared with similar minerals from pegmatites and carbonatites of the Ilmeny-Vishnevogorsky complex. Monazite-(Ce) is divided there in three types by the La/Nd ratio. Monazite-(Ce)-I is the more common, with its composition characterized by Ce > Nd >> La. The transition of monazite-(Ce) from I to II type includes an increase in its composition of S and U admixtures and decrease of the REE amount. Significant variation of the U content (from 1.50 to 12.36 wt % UO₂) has been revealed there inside a single monazite-(Ce) grain. Monazite-(Ce)-III differs in composition from above mentioned types by increased La, Th and Si contents. By the La/Nd ratio, it is similar to monazite from carbonatites of the Ilmeny-Vishnevogorsky complex. There has been revealed also the consecutive change of pyrochlore group minerals: pyrochlore—calciobetafite—uranobetafite, during formation of ore aggregates. The degree of fluids saturation by silica and alumina decreased in the process, and alkalinity become increased in the final stages of crystallization of the agpaitic pyroxene-feldspar pegmatites in the Selyankinsky syenite massif. This evolution explains location of Th-REE mineral aggregates in the outer part of ore assemblages.

Key words: monazite-(Ce), allanite-(Ce), nioboaeshynite-(Ce), thorianite, minerals of the betafite subgroup, Selyankinsky syenite massif, the South Urals.