(Reif, 2004; Excursion Guide…, 1995; Reif, 2008)

The area of the Ermakovka F-Be deposit (Fig.10, page 20) being the largest one in Russia is a part of Siberian Platform southern fold rim, and it localizes in West Transbaikalia, 140 km east of Ulan-Ude. The deposit lies near the Kizhinga Mesozoic riftogenic depression and occurs in carbonate-terrigenous strata preserved as comparatively big roof pendant in distribution area of Paleozoic granitoids. The host strata represents sublatitudinal asymmetrical syncline (Fig.11, page 21). Metasomatic sandstones compose the core of syncline, whereas the strata of interbedded dolomites, limestones and amphibole-pyroxene-biotite schists forms limbs of the syncline.

Of economical value are fenacite-microcline fluorite deposits with high content of BeO to 4%, 1.19% averagely) that formed by replacement of limestones subjected to grinding. The poorer veinlet-impregnated mineralization is not widely spread, mostly in scarns, alumosilicate schists and igneous rocks that formed in the following succession: 1) sill of gabbrodiorites (318 ± 2 Ma); 2) the by layered and cutting bodies of biotite, sometimes gneiss-like granites (261 ± 5 Ma); 3) pre-ore suite of basic and acid composition dykes – trachydolerites, trachyandesites, trachyrhyolites (225 ± 5 Ma) (Lykhin et al., 2001). The later leucocratic aegerine-containing granites (224 ± 1 Ma) form the weakly eroded (0.01 km² at the surface), widening with depth protrusion of Ermakovka intrusion. Ore bodies are grouped around the protrusion being 100-400 m away, but the F-Be mineralization was not found in granites themselves, though their local fluoritization is followed by 2-4 fold increase in Be content. In addition, alkaline granites are considered as syn-ore ones, felsite dyke without signs of hydrothermal alteration is post-ore in places of ore deposit crossing.

The hydrothermally altered rocks are represented by the following varieties: (1) albite and quartz-albite-microcline metasomatites on the Pre-Mesozoic granites and pegmatites; (2) garnet-pyroxene and pyroxene-vesuvianite scarns that develop on carbonate and silicate rocks, including some magmatic formations; (3) microclinites that replace both metasedimentary and igneous rocks; (4) vein-like zones of fluoritization controlled by tectonic dislocations, being characteristic of all rock types within the ore field.

The Ermakovka granite stock represents discordant intrusive body with numerous apophyses. The Ermakovka stock is composed by rocks of three intrusive phases in the depth interval being accessible for observation. Early porphyry-like granites (Gr1) occur among metamorphic rocks of isolated dykes and form rounded and sharply angular xenoliths of various size (to several m³) in the younger granite-porphyries (Gr2) that make up more than 90% of the intrusive volume (Fig.10, inset B). Both Gr1 and Gr2 are crossed by numerous thin dykes (<20 cm) of fine- and middle-grained granites (Gr3, Fig.10, inset C). Veins and lenses of pegmatites (Pgm) 0.1-0.4 m thick and 10-40 m long occur in apical part of the stock, among granites of Phase II (Gr2). Cm-sized schlierens also occur in some types of Phase III. These pegmatites were formed due to segregation of residual melt as e result in situ crystallization of Gr2 and G 3. Contents of SiO₂, Fe₂O₃, Rb, Zr, Nb, Th increase, and those of Sr and Y decrease in granites in succession from Phase I to Phase III (Gr1 Gr3) (Table 2, page 22). It shows that partial separation of feldspar occurred during the period of Gr 1, Gr 2 and Gr 3 introductions. Compared to alkaline granitoids of Transbaikalia (Zanvilevich et al., 1985), granites of Phase II that compose bulk volume of the Ermakovka stock are more than twice enriched in F, Rb, Zr, Nb, Ni, Cu, Pb, Mn and depleted in Sr and Ba. Variations in Be, Y and Mo are not significant.

Granites are composed by quartz and perthitic alkaline feldspar that form rounded idiomorphic phenocrysts 5-7 mkm in size and fine-grained matrix in the pegmatite inner zones. Quartz and feldspar form coarse-grained aggregates or isolated megacrysts (to 6 cm long) within fine-grained aplite-like granite that composes marginal zones. Needle-shaped microcrystals of aegerine occur in outer zones of some quartz phenocrysts. However, they are also usual in quartz matrix. Aegerine is also present in granite matrix as in other alkaline granites of Transbaikalia. But in this case, it is usually replaced by aggregate oh hematite, albite and quartz (±siderite). Aegerine amount increases from 1-2 to 5% from G 1 to Gr3. It is also particular for zircon, ilmenite and other concentrators of trace elements like monazite, ilmenoruthile and florencite that occur in pegmatites. Minerals of beryllium lack in granites and pegmatites.

The presence of microscopic (20-80 mkm) fluorite inclusions in quartz phenocrysts is a specific feature of granites. Fluorite amount increases in quartz of Phases I and II granite matrix, and separate grains to 0.5 mm in size originate in granites of Phase III in many cases. Quartz grains along with fluorite contain microcline inclusions that indicates their magmatic origin for both of them.

In some cases, apophyses and relatively thin (0.6-3 m) endocontact zones of the Ermakovka stock composed by regular-, fine- and middle-grained granites contain dendrite crystals of quartz and aegerine being perpendicular to contact. Such one-oriented textures form in those cases when the melt is overcooled, and there is a sufficient thermal gradient. Respectively, zones with dendritic crystals can be considered as quenching zones

Quenching zones in granite-porphyries on contact with granite-pyroxene and vesuvianite scarns show that the latter were formed prior to introduction of the Ermakovka stock. In contrast, more ancient granite bodies do not have quenching zones, and processes of scarn formation are associated just with them.

The dyke suite is represented by numerous not large intrusions from 0.5 to 15 m thick and from 100 to 1000 m long that are oriented in submeridional direction along the border with the adjacent tectonic depression. Dykes of porphyry-like diorites and quartz monzonites that represent the earliest formations of the dyke series occur both within the deposit and outside it, whereas the younger dykes of porphyry-like quartz syenites mostly localize near the deposit and only sometimes are 1-2 km outside it. The youngest felsite dyke is within the ore body and differed by north-western orientation from the previous ones.

Concepts about relative temporary succession of dyke, ore and granite stock formation are based on the following data. The observations in the open pit show that the stock of aegerine granites undoubtedly hosts only the felsite dyke, while the dykes of other composition cross numerous not large granite bodies that were previously considered as apophyses of stock, but differed from it by lack of quenching endocontact zones or regular-grained, sometimes gneiss-like structure. Therefore, these granites are rather apophyses of Pre-Mesozoic granites, but not the Ermakovka intrusive. Moreover, granites contain large (tens m³) schist xenoliths that host veins of quartz syenite-porphyries being similar to typical dykes of quartz syenites by composition and texture. Thus, all dykes, except, felsic one are pre-ore and intruded prior to formation of the Ermakovka stock.

The ore-forming Be mineralization of the deposit is represented by some loads being rich in fluorite. The largest ones of them occur in limestones grinded by numerous tectonic dislocations. The present stratiform massive ore bodies (>1 mass.% Be) have complex form in detail and demonstrate transitions to stockwork ores (<1 mass.% Be). Ores within limestones are largely composed by fluorite (to 60 vol.%) and subordinate bertrandite and/or fenacite, microcline, quartz, calcite, pyrite and sporadically occurring galena and sphalerite. The loads being rich in fluorite within alumosilicate rocks (brecciated scarns, schists, gabbroids) rarely occur. They are enriched in quartz and highly depleted in Be minerals. The Be mineralization was not revealed in granites, though Be content in granites that contained traces of postmagmatic fluorite was twice higher than in unaltered rocks.

The molybdenum mineralization in the Ermakovka deposit is represented by several types. Dispersed molybdenite, pyrite and postmagmatic fluorite that are confined to albitization zones in Gr2, represent the first type of Mo mineralization (Mo1). Compared to the unaltered granites (Gr2), albitized rocks are highly enriched in Mo, depleted in Zr and Nb and have various contents of Be. The second type of Mo mineralization (Mo2) is confined to western apophyses of the Ermakovka stock and represented by a thin net of quartz veinlets that contain minor fluorite, and enriched in molybdenite, monazite, ilmenoruthile, and lack Be minerals. The similar net of veinlets being rich in molybdenite crosscuts biotite schists 300 m to the north-west from stock. It is the third type of Mo mineralization (Mo3). In this case, the veins consist of oligoclase, andradite-grossular garnet, calcite, subordinate vesuvianite, pyrite and a great amount of late molybdenite. Fluorite is not specific of this type of mineralization. Be minerals were not revealed.