Götzenite in the Nepheline Syenites of the Pokrovo-Kyriyivo Massif (Azov Area, Ukraine)

Abstract

The nepheline syenites of the Pokrovo-Kyriyivo massif in the junction zone of the Dnieper-Donets Rift and the Priazoviаn block of the Ukrainian Shield contain numerous of rare-metal minerals, among them götzenite is the most common. The nepheline syenites are divided into malignites (melanocratic, early) and juvites (leucocratic, late) based on their mineralogical characteristics. In both varieties, the götzenite is presented as euhedral or subhedral grains with a poikilitic texture (numerous rounded inclusions of nepheline). Among rock-forming minerals are observed feldspar, annite, aegirine (± alkaline amphibole), titanite, strontianite ± sodalite ± cancrinite ± Sr-apatite ± calcite ± natrolite. Usually götzenite in malignites has a 'patchy' zonation due to the presence of metasomatically altered areas in the central parts, which have a lower content of Ca, Na, Sr and F, and a higher Si, relative to the outer parts. In comparation to juvite, the götzenite of malignite is more enriched in Zr, Nb and REE, with a low Y content in both types of rocks. A notable feature of the götzenite is high Sr content (up to 3 wt. % SrO), but low Nb (often up to 1.3 wt. % Nb2O5) and REE (often up to 2.4, rarely up to 5.6 wt. % REE2O3). The occurrence of götzenite in nepheline syenites of the Pokrovo-Kyriyivo massif is related with the early stage of the alkaline silicate melt differentiation with increasing activity of Na and volatile components, especially F. During götzenite crystallization the majority of REE and HFSE remained in the fluid as soluble complexes and precipitate own minerals at the postmagmatic stage. This is confirmed by presence of fine inclusions of fluorobritholite-(Ce), strontianite and burbankite in cleavage fractures, in miarole-like inclusions — aeschynite-(Ce), pyrochlore group mineral, bastnaesite, burbankite, etc. The Sr enrichment of götzenite can be explained by early crystallization from an Sr-enriched alkaline silicate melt and the absence of another Ca-rich rock-forming minerals or by external introduction of CO2-F fluids enriched in Sr and REE separated from a carbonatite melt.