*4.3. Elemental Deportment*

Petrographic and geochemical investigation points out that the major ore mineral classes are Zr-silicates, REE-carbonates, and Ti/Nb oxides that occur within the metasomatic rocks forming the centre of the Khalzan Buregtei deposit. Zircon is the most abundant ore mineral. It can be detected in all samples of ore metasomatites. Further, it is identified as the major carrier for the rare metals Zr, Hf, Y, and HREE due to the high concentrations of Y, Dy, and Er across the analysed zircon crystals, as demonstrated by the representative analyses of different zircon and gittinsite grains in Table 5**.**


**Table 5.** Exemplary analyses of gittinsite and zircon crystals of the metasomatic samples.

Table 6 summarizes EPMA analyses of bastnaesite-(Ce), parisite-(Ce), and synchysite-(Y), which are the most abundant minerals of the REE-carbonate group. It can be seen that the LREEs show a high concentration in the first two minerals, whereas synchysite-Y has go<sup>t</sup> elevated concentrations of Y and is thereby suspected to contain unquantified amounts of HREE as well. Low total sums can be explained by unquantified amounts of HREE incorporated into the analysed minerals. The individual concentration levels of these elements are below the limit of quantification of the applied EPMA.

**Table 6.** Representative EPMA analysis of REE-carbonate minerals bastnaesite-(Ce), parisite-(Ce), and synchysite-(Y).


Pyrochlore occurs as a major carrier for Nb in the Khalzan Buregtei deposit. Eight pyrochlore EPMA analyses are given in Table 7. They show that pyrochlore is enriched in the LREEs Ce, La, and Nd. Heavier REE elements were not detected. Due to the content of Ce, pyrochlore is identified as ceriopyrochlore-Ce, which contains only very small amounts of betafite [Ca2(Ti,Nb)2O6O] and microlite [(Na,Ca)2Ta2O6(O,OH,F)], as indicated by the low concentrations of Ti and Ta, respectively. Pyrochlore also shows a capability to enrich U to concentrations up to 1.16 wt. %. Semi-quantitative analyses indicate that, in addition to pyrochlore, rutile can be another carrier of Nb2O5 with variable concentrations from 2 to 5 wt. %.


**Table 7.** Elemental composition of representative pyrochlore grains from sampled parts of the Khalzan Buregtei deposit.
