*4.1. Petrography*

Quartz, K-feldspar, and albite are the dominant rock-forming minerals and in total account for more than 70 vol. % in all samples. The mafic minerals arfvedsonite and aegirine are less abundant (<5 vol. %) and can only locally be observed (Figure 2). Despite the variable proportions of the rock forming minerals, mainly showing higher quartz abundances in the KB-S sampling area, all samples, excluding one, can be classified as alkali granites in terms of the terminology of [20] for magmatic rocks. Only the sample KB S10 was classified as quartz rich granitoid rock due to a significantly higher abundance of quartz. All samples show variable proportions of HFSE ore minerals, such as Zr silicates, REE-carbonates, pyrochlore, as well as of hematite, rutile, titanite, or fluorite (Figure 2).

Quartz is present in all samples and shows a variation in grain size. The coarse fraction is represented by quartz grains up to 2 mm in KB G and up to 5 mm in KB S. Some grains show snowball structures enclosing other mineral phases like K-feldspar and albite. Further, big crystals of quartz show evidence of ductile deformation, like bulging and the formation of sub grains (Figure 3A, yellowish). In addition to the coarse fraction, a fraction of fine quartz grains can be observed. The latter is strongly intergrown with HFSE minerals, as well as with hematite and fluorite (Figure 3A).

**Figure 2.** Modal composition (vol. %) of samples from ore bearing metasomatites in sampling areas KB G (**top**) and KB S (**bottom**), calculated from mineralogical phase maps.

K-feldspar shows a size distribution similar to the coarse grained quartz in both sample suites. Irregularly shaped crystal domains inside of K-feldspar grains may represent relict hatch-twin domains formed in earlier microclinization (Figure 3B). Euhedral K-feldspar is in close association to albite, which can be observed parallel to grain boundaries. K-feldspar often shares straight boundaries with coarse grained quartz (Figure 3B).

Albite forms lath shaped crystals varying in grain size from 50 μm to 400 μm (Figure 3C). Coarse laths of albite, however, are found at the edges of coarse grained snowball quartz and K-feldspar grains. Fine albite crystals can be observed in interstitial volumes of quartz and K-feldspar. Evidence of the replacement of quartz and K-feldspar by albite is dominant in all samples from the centre of the Khalzan Buregtei deposit. The close intergrowth of lath-shaped albite crystals with each other leads to the formation of a matrix-like texture observable in all samples containing high amounts of albite.

The mafic minerals arfvedsonite and aegirine occur in the rocks with a grain size up to 1 mm but with low modal abundance. As Figure 2 shows, significant amounts are only observed in the samples KB G1, G2, and G9. Arfvedsonite is euhedral and shows good cleavage. Aegirine forms halos surrounding arfvedsonite as illustrated in Figure 3D. Ongoing replacement reaction from arfvedsonite to aegirine is observed propagating along amphibole cleavage plains. Aegirine is further replaced by albite laths. Albite grains in contact with arfvedsonite and aegirine often show red staining (Figure 3D).

**Figure 3.** (**A**) Image of primary coarse quartz (Qtz) in sample KB G2 (yellowish interference color of quartz is due to section thickness > 30 μm) showing sub grain formation in adjacency to fine grained interstitial quartz intergrown with dark zircon-hematite aggregates. Optical microscopic image with cross-polarized light (xpl, transmitted light). (**B**) K-feldspar grain with irregular internal domains (xpl, transmitted light). (**C**) Lath shaped albite (Alb) crystals in interstitial volume of coarse grained quartz (xpl, transmitted light). (**D**) Arfvedsonite grain showing dark blue pleochroic color and typical cleavage, surrounded by aegirine (green pleochroic color). Lath-shaped albite (white to red) is colored red by fine grained hematite on lath planes. Microscopic image of KB G1 plain polarized light (ppl, transmitted light).

The group of HFSE-bearing minerals comprises Zr-silicates, REE-carbonates, and the Nb mineral pyrochlore. Zircon is the most abundant mineral with concentrations as high as 11 vol. %. The Zr-silicate gittinsite [CaZrSi2O7], in contrast, is only present in samples KB G1 and G2, in which it occurs in aggregates with zircon clustering around relicts of aegirine (green in Figure 4A). According to [14,16] and [6] more Zr-silicate minerals like elpidite [Na2ZrSi6O15·H2O] and armstrongite [CaZrSi6O15·H2O] occur at the Khalzan Buregtei deposit; however, at the studied location, these minerals were found only on a very rare level. Due to the low abundance of these minerals at the sampled parts of the deposit, they are not considered as ore minerals in this study.

The REE-carbonates are represented by bastnaesite-(Ce) as the most abundant phase, as well as by parasite-(Ce) and synchysite-(Y). In total, their modal volume is <5 vol. %. Pyrochlore can be observed in nearly all samples, but with low abundance compared to zircon or REE-carbonates.

**Figure 4.** Representative QEMSCAN© phase maps of Zr-REE-Nb ore. (**A**) Zircon-gittinsite cluster in sample KB G1. The core of a residual aegirine (green) is recognizable within the cluster. (**B**) Zircon-quartz cluster containing fluorite (purple) and hematite (pale red) in sample KB G6. Lines indicate the preserved subhedral shape of precursor amphibole as straight grain boundaries to K-feldspar and coarse grained quartz.

Except for pyrochlore, the HFSE minerals occur concentrated within cluster-like aggregates together with fine grained quartz. It can be recognized in Figure 4B that such clusters form pseudmorphs after amphibole indicated by the subhedral shape of the aggregates, which have straight grain boundaries against coarse grained quartz and K-feldspar. These polymineralic ore clusters, which can be observed through the entire suite of samples from KB G and KB S, have variable sizes from 250 μm up to 2 mm, whereas the minerals within the clusters are mainly anhedral and show smaller diameters. While the REE-carbonates vary in grain-size from 90 to 200 μm, Zr-silicates and quartz grains are even smaller and rarely exceed 50 μm. In addition to Zr-silicate and the REE- and Y-carbonates, the ore clusters also contain abundant hematite and fluorite, as well as, in rare cases, the Ti-phases rutile and titanite. The latter two minerals are strongly intergrown with fine grained quartz. Rutile forms euhedral prismatic crystals characterized by oscillatory zoning, which occasionally reach a maximum diameter exceeding 100 μm (Figure 5A). Locally, hematite also forms individual clusters with fine grained quartz. Fibrous hematite aggregates can be observed exceeding diameters >50 μm of irregular fan or spherulite texture. However, it can be detected as very fine grained (<5 μm) pigments in interstitial volume of mineral phases like zircon, as illustrated by blue colors at the high-resolution Fe mapping in Figure 5C.

Purple colored fluorite can be observed in the cluster assemblage as well as associated with coarse grained quartz, K-feldspar, and albite (Figure 4B, Figure 6A). It is mostly of xenomorphic shape and can locally exceed a grain size of 500 μm. Besides the abundance in interstitial volume, purple fluorite is found in veins cross-cutting the central part of the Khalzan Buregtei deposit. Further, fluorite can be observed as a replacement of K-feldspar and coarse grained quartz.

**Figure 5.** (**A**) Back scatter (BSE) image of a zircon cluster. Hematite (hem) as fibrous shape crystals in interstitial volume between zircons (zrn) and one fluorite (flu) crystal. (**B**) 5 QEMSCAN© phase map with 5 μm point spacing showing the hematite intergrowth in a close up of the cluster displayed in A. (**C**) Electron probe micro analyser (EPMA) high-resolution Fe map revealing Fe fine cementation as blue color surrounding individual zircon crystals. Other minerals: quartz (qtz), kaolinite (kao), albite (alb).

The Ti-phases rutile and titanite are strongly intergrown with fine grained quartz. Rutile forms euhedral prismatic crystals characterized by oscillatory zoning, which occasionally reach a maximum diameter exceeding 100 μm (Figure 6B). In contrast to the other ore minerals, pyrochlore is not accumulated in clusters, but is mostly associated with K-feldspar and coarse grained quartz. All grains of pyrochlore are euhedral (Figure 6C) and most of the crystals show evidence of brittle deformation. Furthermore, pyrochlore grains are observed being cross-cut by lath shaped albite. Like the other ore minerals, pyrochlore crystals are dominantly <50 μm but can exceed 140 μm in rare cases. As illustrated in Figure 6C, joints within pyrochlore are also healed by hematite precipitation.

Based in the observation of textural properties such as grain size, mineral assemblage, crystal shape, and grain boundaries indicative of equilibrium or replacement reactions, mineral phases can be differentiated into groups of primary or post-magmatic formation. Coarse grained minerals of subhedral shape sharing straight grain boundaries are considered as minerals formed under magmatic

conditions. K-feldspar and arfvedsonite are two phases with coarse grain size as well as, in part, semihedral shape. Furthermore, the coarse grains of quartz are observed to share straight grain boundaries with K-feldspar. Thus, the precursor rocks of the metasomatites of the Khalzan Buregtei deposit can be identified as alkali syenite to alkali granites (QAPF, [20]). Owing to their coarse grain size, these rocks had a porphyritic appearance, which is in accordance with the observation of [5,13–15] regarding the unaltered rock suite of the main intrusive phase.

**Figure 6.** (**A**) Bastnaesite-quartz ore cluster in sample KB S9. K-feldspar is only found as relict. (**B**) Rutile-quartz cluster in granitic sample KB G6 containing fine grained quartz, zircon, fluorite and low amounts of hematite. The box indicates pyrochlore (Pcl) grain within the major association to K-feldspar (K-Fsp), being partly replaced by albite. (**C**) Euhedral pyrochlore on boundaries between K-feldsparand quartz-zircon (Qtz-Zrn) ore cluster. Joints inside the pyrochlore are healed by hematite (hem) precipitation, which also is found around the grains and within the ore cluster (ppl, reflected light, oil).

Post-magmatic phases are identified by significant change to finer grain size and irregular grain shape. Furthermore, the formation of polymineralic aggregates pseudomorphically replacing magmatic phases (e.g., amphibole) can also best be explained in a reaction of a solid mineral with a fluid, i.e., in a post-magmatic reaction. Thus, aegirine and albite, which grew at the expense of magmatic arfvedsonite and K-feldspar, respectively, are classified as post-magmatic minerals. Fine grained quartz, which shares straight grain boundaries with albite, is considered as cogenetic and thus also as post-magmatic. Post-magmatic formation is also indicated for ore and accessory minerals (e.g., hematite and fluorite) found in clusters intergrown with post-magmatic quartz. The semihedral shape of ore mineral clusters and their straight grain boundaries shared with magmatic phases are evidence that ore clusters mainly represent aggregates pseudomorphically replacing arfvedsonite and aegirine, which itself replaces

arfvedsonite before (Figures 3D and 4A). The occurrence of clusters of post-magmatic minerals thus mirrors the texture of precursor porphyritic alkaline granite rock. Veins filled with carbonates were found cross-cutting all other mineralogical and textural features of the rocks and were interpreted to be late stage post-magmatic. It is important to note that these textural observations show that even with the enrichment of Zr, the REE or Nb were probably caused by magmatic fractionation [15]; the textural properties of the rare metal ores of Khalzan Buregtei are predominantly of post-magmatic metasomatic character, which is also in line with the characterization by [5].
