Formation of Cryolite in the Granites of the Katugin Deposit: Constraints from Experimental Data on the Fluorine–Lithium-Bearing Granite System

The assemblages of cryolite-bearing granites of the Katugin rare-metal deposit are studied and compared with experimental results in the fluorine–lithium-bearing granite system. The experiments demonstrate that cryolite crystallization begins from the salt melt, which is in equilibrium with the aluminosilicate melt, at 700°C and 1 kbar. In the temperature range of 500−600°C and a pressure of 1 kbar, cryolite crystallizes together with quartz from the aluminosilicate melt. In the fluorine–lithium-bearing granite system, rare-earth elements are partitioned mainly in the salt alkali–aluminofluoride melt. It is shown that cryolite and associated rare-earth minerals of the Katugin deposit are likely formed at the magmatic stage through the silicate–salt immiscibility in granite melts at the late stages of differentiation.