Mineral Inclusions in Zircon from the Velyka Vyska Syenite Massif, Ukrainian Shield

Zircon crystals from crushed syenite samples of individual intrusive bodies in the Velyka Vyska Massif were studied using the SEMEDS (BSE images and chemistry) and SIMS methods. The results obtained were compared with early published data for zircon from the Azov and Yastrubets syenite massifs, Ukrainian Shield that host ZrREE mineralization published earlier. Most of the zircon crystals analyzed are chemically inhomogeneous, but either azonal or show poorlydefined regular zoning. A darker

Trace Element Geochemistry of Zircons from the Velyka Vyska Syenite Massif, Ukrainian Shield

Левашова¹,

Kulchytska²,

Скублов³

et al. 2020

Formation Mechanism of the Velyka Vyska Syenite Massif (Korsun-Novomyrhorod Pluton, Ukrainian Shield) Derived from Melt Inclusions in Zircon

Voznyak¹,

Левашова²,

Скублов³

et al. 2021

The formation of leucosyenites in the Velyka Vyska syenite massif was provoked by the liquation layering of magmatic melt. This assumption is based on the presence of two primary melt inclusions of different chemical composition in zircon crystals from Velyka Vyska leucosyenites. They correspond to two types of silicate melts. Type I is a leucosyenite type that contains high SiO2 concentrations (these inclusions dominate quantitatively); type II is a melanosyenite type that contains elevated Fe and smaller SiO2 concentrations. The liquation layering of magmatic melt was slow because the liquates are similar in density; leucosyenite melt, which is more abundant than melt of melanosyenite composition, displays greater dynamic viscosity; the initial sizes of embryos of melanosyenite composition are microscopic. Sulphide melt, similar in composition to pyrrhotite, was also involved in the formation of the massif. Zircon was crystallized at temperatures over 1300°С, as indicated by the homogenization temperatures of primary melt inclusions. The REE distribution spectra of the main parts (or zones,) of zircon crystals from the Velyka Vyska massif are identical to those of zircon from the Azov and Yastrubets syenite massifs with which high-grade Zr and REE (Azov and Yastrubets) ore deposits are associated. They are characteristic of magmatically generated zircon. Some of the grains analyzed contain rims that are contrasting against the matrix of a crystal, look dark-grey in the BSE image and display flattened REE distribution spectra. Such spectra are also typical of baddeleyite, which formed by the partial replacement of zircon crystals. The formation of a dark-grey rim in zircon and baddeleyite is attributed to the strong effect of high-pressure СО2-fluid on the rock. The formation patterns of the Velyka Vyska and Azov massifs exhibit some common features: (а) silicate melt liquation; (b) high ZrO2 concentrations in glasses from hardened primary melt inclusions; (c) the supply of high-pressure СО2-fluid flows into Velyka Vyska and Azov hard rocks. Similar conditions of formation suggest the occurrence of high-grade Zr and REE ores in the Velyka Vyska syenite massif.

Fluorite Inlaid by Basnäsite From Syenites of the Velykovyska Massif (Ukrainian Shield) as Evidence of the Existence of a Carbonate-Fluoride Melt

Kulchytska¹,

Vyshnevskyi²,

Belskyi³

et al. 2021

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.