FLUORITE INLAID BY BASNÄSITE FROM SYENITES OF THE VELYKOVYSKA MASSIF (UKRAINIAN SHIELD) AS EVIDENCE OF THE EXISTENCE OF A CARBONATE-FLUORIDE MELT

A millimeter-sized fluorite crystal found in leucocratic syenite of the Velykovyska massif on the Ukrainian Shield became the object of study. This is a crystal of irregular shape with smoothed corners, broken by a system of contraction cracks inlaid with basnäsite-(Ce). The latter is characterized by a REE content of 57± 5 wt. %, CaO - of 3.3 ± 0.25 wt. % and Ce2O3 : La2O3 : Nd2O3 ratio of 6 : 4 : 1. A detailed study using microprobe JXA-733 and an electron microscope JSM-6700F, equipped with EDS JED-2300, showed the presence of other mineral phases in the inlays. The middle of the cracks is filled with ferrous aluminosilicate without REE. On the opposite side, a siderite rim was found around the basnäsite and single crystals of calcite were found nearby in the fluorite. Fluorite inlaid with basnäsite is considered as a product of crystallization of the carbonate-fluoride melt drop that separated from silicate melt due to their immiscibility. Subsequent stratification of the melt inside the drop led to formation of fluorite rimmed by carbonates of REE and Fe and displaced to the cracks of Fe-aluminosilicate. Crystallization of the separated carbonate-fluoride melt took place in an isolated volume without any exchange of components with the surrounding Na-K-feldspar. Fluorite, as well as carbonates and mica are less common in the Velykovyska massif than other similar syenite massifs on the Ukrainian Shield. It is assumed that this is caused by the high temperature of the syenite melt, which contributed to evaporation of volatile components (F2, CO2, H2O), and by processes of liquation. The immiscibility between salt and silicate melts contributed to formation of drops of carbonate-fluoride melt and their movement to the apical parts of the massif and outside. The discovery of inlaid fluorite increases the prospects of the Velykovyska massif for the detection of REE ore occurrences in syenites and surrounding rocks. In addition, the finding confirms the existence of carbonate-fluoride melts in nature. These melts are paternal for fluorite-carbonate veins with REE mineralization.