Rare Earth Minerals in Veined Nepheline Syenites of the Chernihivka Carbonatite Massif of the Azov Region

We studied partially altered veined nepheline syenites at the Chernihivka carbonatite massif of the Azov region, Ukraine. The major minerals of the syenite are albite, altered nepheline, biotite and calcite. Fluorapatite, allanite-(Ce), monazite-(Ce), barite and stronalsite occur as minor accessory phases. The primary minerals, containing REE in the veined rocks, reflect re-equilibration in a mineral/fluid systems and recrystallization of primary rock-forming minerals. The high initial concentration of CaO, CO32–, alkalis and volatile components caused a concentration of REEs in the late differentiated portions of the phonolite melt. The intrusion of the melt into wall-rocks of different composition, together with the excees of Al and Si in solution, caused allanite-(Ce) crystallization. Subsequent postmagmatic fluids contributed to Ba, Sr and REE leaching from major rock-forming apatite and calcite and led to the crystallization of secondary monazite-(Ce), barite and stronalsite.