**6. Conclusion**

Alkaline rock hosted rare metal deposits yield potential as future resource for commodities like Zr, Nb, Hf, Y, and the REE. The economic interest in this deposit type is underlined by various international studies of the last decade [1,7,9,10,30]. The complexity of the ore with respect to texture, composition, and variable types of ore minerals is often caused by post-magmatic alteration of the ore-bearing rock suite. The alteration is identified as one key process of the ore formation, because it enhances formation and concentration of ore minerals. However, post-magmatic alteration also causes rearrangemen<sup>t</sup> of the precursor magmatic rocks. As demonstrated in this study of Khalzan Buregtei, for Zr-REE-Nb deposits traditional used concepts of magmatic rock classification (e.g. TAS, QAPF) fail to properly characterize the altered alkaline rocks.) fail to properly characterize altered alkaline rocks. While not providing common magmatic-related names, multication R1-R2 parameters defined by [21] can serve as numeric values to characterize altered and mineralized rocks, as well as to visualize the chemical effect of alteration processes observed in petrographic inspection. Owing to their quantitative additive property, multication parameters are recommended to be implemented in numerical models to also display and analyse rare metal ore and their alkaline granitoid host rocks in geological 3D models.

The post-magmatic ore mineral assemblage at Khalzan Buregtei is characterized by overall fine grain size and intricate intergrowth. Consequently, in a mining operation, small liberation size, quantified in this study by QEMSCAN© image analysis, would require intensive comminution. High energy demand and lagged masses of finely-ground rock represent a significant obstacle to an economic use of this deposit. Petrographic inspection revealed that, except for the major Nb carrier pyrochlore, all ore minerals of Zr and REE (i.e., zircon, Zr-silicates, and REE-carbonates) occur in pseudomorph-like association mainly replacing precursor magmatic arfvedsonite. This leads to a concentration of ore minerals in clusters characterized by liberation sizes significantly exceeding those of individual ore minerals.

Furthermore, QEMSCAN© image analysis indicated that hematite is a characteristic constituent of clustered ore mineral aggregates. These textural properties allow pre-concentration of ore mineral clusters after clearly less energy consuming comminution to particle sizes < 250 μm and magmatic separation. Further comminution of the pre-concentrate and processing steps to concentrate zircon, Zr-silicates, and REE-carbonates then only have to be applied to 55% of the initial mass of the ore. This implies a significant reduction of energy consumption as well as of water and other supplies (e.g., flotation chemicals). Taking into account that clustering, as well as associations of ore minerals with hematite, are observed in other rare metal deposits formed in alkaline granitoids in different concentrations, the approaches developed for Khalzan Buregtei are considered as important steps towards more e fficient and sustainable extraction of rare metal ores.

**Author Contributions:** Conceptualization, L.H.G. and S.S.; Investigation, L.H.G., J.L.K., and U.B.; Methodology, L.H.G. and S.S.; Project administration, H.W. and F.M.M.; Supervision, H.W. and F.M.M.; Validation, A.H.; Writing—original draft, L.H.G. and S.S.; Writing—review and editing, J.L.K., H.W., and F.M.M.

**Funding:** This publication was financially supported by the Aachen Know-How Center Resource Technology (AKR). The surrounding project OptiWiM is financially supported by the German Federal Ministry of Education and Research (BMBF, OptiWiM; FKZ: 033R162B).

**Acknowledgments:** The authors show gratitude to Chinzorekt of MONNIS International LLC and the sta ff of TD Bank of Mongolia, for granting access to Khalzan Buregtei license area and the permission to collect samples from outcrops. G. Stehr and the sta ff of the German-Mongolian Institute of Resource Technology (GMIT) and M. Bauer of Consulting, Business and Management GmbH (CBM GmbH, Bexbach) are gratefully acknowledged for administration and logistical support during the project. R. Klinghardt of technical sta ff of IML Klockmann Laboratory for Geometallurgy is gratefully thanked for his attendance during EPMA and QEMSCAN © analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
