The genesis of Zr-Nb-REE mineralisation at Khalzan Buregte (Western Mongolia) reconsidered

The genesis of the economic Zr-Nb-REE mineralisation at Khalzan Buregte, Western Mongolia was re-explored by field work, whole rock analysis (chemical composition, quantitative XRD mineral analysis) and by application of various transmitted light and electron microscopic techniques. Coarse-grained bodies, dikes, and volcanics of alkaline, silica-saturated rocks were contemporarily emplaced at subvolcanic to volcanic levels forming four alkaline massifs within the Khalzan Buregte area. The whole rock composition of the weakly altered magmatic rocks ranges from syenite to quartz monzonite and alkaline granite (alkali feldspar syenite to alkali feldspar granite according to mineral composition). Magmatic and hydrothermal processes contributed to the formation of economic concentrations of several high field strength elements (HFSE) as Zr, Hf, Nb, Ta, REE, and Y in the Khalzan Buregte deposit and in the nearby Tsakhir prospect. Magmatism with mixing of at least three melt types and, possibly, the formation of K-feldspar cumulates resulted in a local enrichment of Zr, Nb, and of the light rare earth elements (LREE) in the rocks up to a sub-economic level. There was, however, no significant increase in Y and the heavy rare earth elements (HREE). In contrast, hydrothermal alteration by silica- and carbonate-rich fluids yielded extreme concentrations of Zr, Nb, and LREE. Additionally, one of the late alteration events resulted in economic levels of Y and HREE. These fluids were rich in fluorine. There are new indications for two main alteration events causing ore formation: (1) an early, carbonatite-style and (2) a late by F-rich fluids. Multistage metasomatic alteration resulted in a pronounced chemical and mineralogical heterogeneity of the alteration assemblages. The main hosts of Zr (and Hf) in the ores are zircon and other zirconium silicates (gittinsite, catapleiite-(Ca), and elpidite). Nb (and Ta) is mainly contained in various types of pyrochlore (Khalzan Buregte) and – to a lesser extent – in fergusonite and some other minerals (Tsakhir). A large diversity of REE-bearing minerals includes oxides, fluoro-carbonates, and silicates that concentrate LREE, HREE, and Y.