Eudialyte-Group Minerals in Rare-Metal Nepheline Syenites of the Khashatyn-Khar Massif (Northwestern Mongolia): Indicators of Potential Rare-Metal Ore Mineralization

The Khashatyn-Khar syenite massif is located in Caledonian fold structures of the southern frame of the Tuva–Mongolian Superterrane. Nepheline syenites and numerous dykes consisting of agpaitic varieties of nepheline syenite with aegirine, pectolite–serandite series minerals, fluorite, and eudialyte-group minerals (EGMs), are developed in the apical part of the massif. Compared with other rock varieties of the massif, these rocks are enriched in Zr, Nb, and rare-earth elements, and they enclose EGMs. Less common minerals in the dykes vary widely in terms of quantitative and species composition. This is the result of both variations in the initial melt composition and autometasomatic rock transformations caused by interaction with water-rich intergranular restite melts. Primary EGMs include EGMs of a continuous series from Fe-rich (up to 4.8 wt % FeOtot), Na-poor (up to 3.4 wt %), and water-rich (up to 8.6 wt %), which are similar in composition to ilyukhinite, as well as to Na-rich species (up to 10.9 wt %) similar to georgbarsanovite and davinciite. EGMs in complex polymineral intergrowths with catapleite and other minerals are, presumably, of secondary origin. They are extremely rich in Mn (up to 11 wt %) and are similar in composition to Mn-rich minerals such as zirsilite-Ce or carbokentbrooksite, or can be considered as a Nb-variety of amableite-Ce. Agpaitic dikes of EGM-bearing nepheline syenites in the Khashatyn massif are indicators of potential rare metal mineralization, which is usually associated with agpaitic nepheline syenites, and generally record a new area of occurrence of rocks of this type in the Central Asian region.