Abstract: This paper reports on the results of studies of the carbonate–fluoride isolations with extremely high Y and REE concentrations from biotite–arfvedsonite granite of the Katugin rare metal deposit. We obtained the chemical data on that minerals that compose these isolations—gagarinite-(Y), tveitite-(Y), fluocerite-(Ce), bastnaesite, and fluornatropyrochlore. A carbonate–fluoride globule with a well-preserved crystallization order was revealed in quartz of the host granite. This finding might be evidence of silicate–fluorine immiscibility suggested before for the Y–REE isolations in the Katugin granites. We assumed that the fluoride melt occurred at the magmatic stage of the formation of biotite–arfvedsonite granite, which resulted in depletion of the host granite with fluorine and the redistribution of the REE and Y in the fluoride salt melt.