Fluorides and Fluorcarbonates in Rocks of the Katugin Complex,Eastern Siberia: Indicators of Geochemical Mineral Formation Conditions

The paper discusses the chemical composition and parageneses of fluorides and fluorcarbonates in rocks of the Katugin Complex, with which a unique deposit of REE–Nb–Ta ore with cryolite is associated. In mineralogy and chemical composition, the rocks correspond to biotite, biotite–amphibole, arfvedsonite, and aegirine–arfvedsonite granites, which were regarded in earlier publications as granite-like metasomatic rocks. Aegirine–arfvedsonite granite contains a cryolite–gagarinite assemblage, which reflects depletion of Ca in the mineral-forming medium and enrichment in Na and F. Arfvedsonite granite is characterized by intergrowth of yttrofluorite with fluocerite and gagarinite, which indicates a relative enrichment in Ca and low CO₂ content. Biotite granite is characterized by an assemblage of fluorite with titanite, apatite, and monazite as evidence for an elevated Ca concentration along with moderate F and P contents in the system. Neighborite, coulsellite, gagarinite, fluocerite, and tveitite-(Y) appear in biotite–amphibole granite along with replacement of annite with riebeckite and development of albite after microcline. All this indicates that a moderately alkaline Na-fluoride solution with a low Ca concentration affects biotite granite.