The article presents the results of optical, electron microscopic and electron microprobe studies of columbite group minerals, collected during heavy mineral concentrate sampling of alluvial deposits in the Mayoko region (Republic of the Congo). The aim of the study is revealing tantalum niobates ore body in this region. We found that these minerals in loose deposits are represented by two grain-size groups: less than 1.6 mm (fine fraction) and 1.6-15 mm (coarse fraction). The grains of both fractions belong mainly to columbite-(Fe), less often to columbite-(Mn), tantalite-(Mn) and tantalite-(Fe), contain impurities of Sc, Ti, and W. The crystals have micro-scaled zoning (zones varies slightly in the Ta/Nb ratio values) and contains a lot of mineral inclusions and veins represented by zircon, pyrochlore supergroup minerals and others. Columbite-(Fe) and columbite-(Mn) are characterized by an increased content of Ta 2 O 5 up to the transition to tantalite-(Fe) and tantalite-(Mn). This allows us to exclude the formation of subalkaline rare-metal granites, their metasomatites (albitites and greisenes) and carbonatites, from the list of possible columbite ore rocks source in the Mayoko district. Thus, beryl type and complex spodumene subtype rare-element pegmatites of the mixed petrogenetic family LCT-NYF (according to P.Černý) should be considered as a probable root source. The results of the research should be taken into account when developing the methodology for prospecting in this area.
The unique size of monolithic blocks of granite in the «Vozrozhdenie» deposit («Kawantsaari») is due to the stability of these rocks in the steeply dipping and probably crosscutting part of the massif, where the rocks have retained their massive texture and have been least modified at the stage of formation of rare-metal pegmatites. The shape and size of monolithic blocks are determined by the pattern of disturbances, in the formation of which the leading role belongs to three steeply dipping zones of tectonic fractures of the North-West strike with the haloes of changes in the conditions of low-temperature acidic metasomatism. Later tectonic disruptions in these zones led to the destruction of rock blocks between them within a series of transverse fractures and to subhorizontal discontinuities of rocks caused probably by gravitational unloading of the massif and glacier action. The distribution of fractures in all directions is characterized by equal spacing («step»).
Geological and mineralogical research were conducted by the scanning probe microscopy and showed that in plagioclase pegmatites of North Baikal muscovite province the average thickness of the peristerites increases from early graphic zones to late trochlear zones and pegmatoid zones of pegmatite veins. The thickness reaches a maximum in the pegmatites of pegmatoid structure which contain large-crystalline muscovite. This regularity can be used for development mineralogical criteria for evaluation of pegmatite veins productivity at the large size muscovite source.
Physico-mathematical modeling has shown that color of iridescent depends on spacing of peristerite lattice. The more spacing lattice, the color of iridescent is higher. The phenomenon of iridescent in plagioclases has an interferential nature. We can select two genetic types of peristerites: peristerites of decomposition and segregation peristerites. Segregation peristerites are formed from postcrystallizational consolidation of peristerites of decomposition. The form of segregation pertisrerites is one of the reasons of polychromatic iridescent genesis in plagioclases.