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

The sources and formation conditions of unconventional Zr–Nb–REE mineralisation (REE = rare earth elements) presently found in increasing number worldwide are still poorly constrained. One particular problem is the specific role of magmatic and hydrothermal processes active in various geological settings. Investigation of Zr–Nb–REE mineralisation at Khalzan Buregte and Tsakhir, Western Mongolia, enables to evaluate magmatic processes preceding economic mineralisation and, in a second step, to compare similar ore-forming processes developing in host rocks of contrasting rock composition (low- vs. high-silica rocks). The genesis of the Zr–Nb–REE mineralisation is re-assessed using field observations, whole rock analysis (chemical composition, quantitative modal analysis by X-ray diffraction) and by the application of various transmitted light and electron microscopic techniques. Coarse-grained intrusive bodies, dikes and volcanic rocks of alkaline, silica-saturated composition were found to be contemporarily emplaced at subvolcanic to volcanic levels forming four alkaline massifs within the Khalzan Buregte area. The whole rock composition of weakly altered magmatic rocks ranges from syenite to quartz monzonite and alkaline granite (alkali feldspar syenite to alkali feldspar granite according to their modal composition). Magmatic and at least two subsequent hydrothermal processes contributed significantly to the formation of economic concentrations of high field strength elements (HFSE) such as Zr, Hf, Nb, Ta, REE and Y in the Khalzan Buregte deposit and in the nearby Tsakhir prospect. Mixing of magma from at least three sources and the formation of potassium feldspar cumulates resulted in local enrichment of Zr, Nb and light rare earth elements (LREE) in the rocks up to sub-economic levels. There was no significant increase in Y and heavy rare earth elements (HREE) during magmatism. tage metasomatic alteration resulted in a pronounced chemical and mineralogical heterogeneity of associated alteration assemblages. The main hosts of Zr and Hf in the ores are zircon and other zirconium silicates (gittinsite, catapleiite-(Ca) and elpidite). The rare metals Nb and Ta are mainly contained in various types of pyrochlore (Khalzan Buregte) and, to a lesser extent, in fergusonite and other minerals (Tsakhir). A large variety of REE- and Y-bearing minerals have been identified, including oxides, fluorocarbonates and silicates. Early hydrothermal alteration by silica- and carbonate-rich fluids yielded extreme concentrations of Zr, Nb and LREE. Later alteration resulted in enrichment of Y and HREE. In the latter case, fluids were very rich in fluorine. Our preliminary genetic model assumes a carbonatite-related fluid system responsible for the early alteration that occurred late during or postdating the intrusion/extrusion of the silica-saturated magmas. A “Li-F granite-type” fluid system was active during the late alteration. The interplay of all these processes resulted in the formation of a complex, economic Zr–Nb–REE mineralisation at Khalzan Buregte.