Vol 67, No 3 (2025)

The article considers the geological position and mineral composition of rare-metal (TR) metasomatites with a pegmatoid structure and forming lenticular bodies. TR metasomatites are spatially and genetically related to carbonatites and form single bodies. The latter are spatially related and have the same geological position with industrial bodies of vein, granular quartz. Unlike cabonatites, metasomatites are crushed and boudinated. The uniqueness of metasomatites is due to unusually large xenotime crystals. Most minerals involved in the structure of metasomatites contain abnormally high isomorphic impurities of Y. In the composition of yttrium-bearing epidote and yttrium-bearing titanite, its amount is slightly more than 8 wt. % Y₂O₃. Fergusonite-(Y) and a mineral whose composition approaches the idealized formula YAlSiO₅ are present as inclusions in yttrium titanite. In apatite-II, formed in certain areas during recrystallization (granulation) of apatite-I, the total content of yttrium and heavy rare earth elements significantly exceeds 10 wt. %. Fergusonite-(Y) and a microinclusion, the composition of which corresponds to a complex oxide with the formula Ca₂Y₂O₅, are present as inclusions at the grain boundary. The unusual composition of metasomatites and the specific structure of TR-metasomatites indicate a long process of their formation and metasomatic nature. At present, metasomatite lenses occur in rocks that have undergone metamorphism at the level of amphibolite facies.

Economical mineralization of the Bystrinsky Сu-Аu-Fе-porphyry-skarn and Shakhtama Мо-роrрhуrу deposits is confined to multiphase granitoid plutons of the Middle-Late Jurassic Shakhtama complex. The composition of biotite from magmatic rocks of ore-bearing and barren intrusions has been studied to identify the specifics of Cu-Au porphyry and Mo porphyry mineralization. The magmatic origin of the studied biotite and the absence of secondary processes have been demonstrated. Biotite from ore-bearing intrusions of the Bystrinsky and Shakhtama deposits is characterized by high MgO content (>15 wt.%). Low values of IV(F) and IV(F/Cl) calculated from biotites of ore-bearing intrusions indicate enrichment of F and Cl in the fluid phase. It has been determined that the rocks of the ore stocks were formed from oxidized magmas. New data showed that published diagrams for biotite composition are incorrect for separating ore-bearing and barren rocks. Linear discriminant analysis was performed on the obtained biotite compositions of the Bystrinsky and Shakhtama deposits and the author's new discriminatory diagram was proposed. Unlike published diagrams, new discriminant diagram allows to be distinguished potentially ore-bearing intrusive complexes (and their type of mineralization) from barren intrusive complexes by biotite composition and can be used with other indicator minerals in the search for porphyry mineralization.

The throughout lightening up to the whitening of basement granites, ore-bearing alluvial sediments, volcanogenic-sedimentary strata and fracture zones of plateau basalts are shown on an example of Vershinnoye uranium ore deposit of Khiagda ore field of Vitimskiy uranium ore region. The contribution of deep carbonated waters with high reducing capacity to this process is shown. The ore-localizing and ore-preserving role of the ore-bearing alluvial sediments lightening process is proofed. The key similarities and differences of young (Vitim type) and ancient (Dalmatov type) paleovalley deposits in contrast to the classical bedded-infiltration deposits of the Tien Shan uranium megaprovince are shown.