ИСТИНА

Most types of rare-metal granites and related pegmatites are characterized by increased concentrations of fluorine and lithium compared to other types of granites. Fluorine is one of the most important (after water) components of the gas phase of natural granite melts, and lithium, along with sodium and potassium, is one of the main representatives of the alkaline components of these melts. Both elements (F and Li) form their own minerals or are isomorphically included in a number of minerals in granites and pegmatites. The presented work is based on the results of experiments obtained in the study of a granite silicate-salt system of the Si-Al-Na-K-Li-F-H-O system saturated with water, with limiting fluorine contents and lithium concentrations of 1-1.5 wt.% at temperatures 1250-500⁰C and pressure 1 kbar. Some of the experiments were carried out at pressures of 2, 3, and 5 kbar. The system is a model for the most differentiated granite melts, from which alkaline granites, ongonites, elvans, cryolite-bearing and Li-F-granites, their pegmatites and rare-metal deposits associated with all these rocks are formed in nature. Fluorine and lithium are incompatible elements that accumulate during the differentiation of magmatic melts, in which the liquation separation of melts rich in fluorine and lithium is also possible. The authors carried out systematic experimental studies of a model granite system saturated with fluorine in water in a wide range of compositions. They made it possible to outline the composition area aluminosilicate melt in equilibrium with cryolite, villiomite, topaz and salt aluminofluoride melt formed due to liquid immiscibility. It was shown that the introduction of lithium into the system leads to a significant expansion of the field of the salt aluminofluoride melt. The distribution of lithium between aluminosilicate and alkali-alumina-fluoride salt melts in a granite system with the limiting contents of water and fluorine at temperatures of 700 - 800⁰C and pressures of 1-5 kbar has been experimentally studied. It is shown that lithium is distributed in favor of the salt melt under all conditions. The experiments carried out made it possible to study the influence of temperature, pressure and water concentration in the Si-Al-Na-K-Li-F-O-H system on the phase relations and distribution of Li between the phases - aluminosilicate and salt melts and the quench fluid. Conclusions. 1. It has been experimentally shown that in granite melts with a limiting content of fluorine of more than 3 wt.% and lithium of more than 1 wt.%, liquid immiscibility phenomena arise, as a result of which the separation of the salt melt from the aluminosilicate melt occurs. 2.The distribution of lithium between aluminosilicate and alkali-alumina-fluoride salt melts in a granite system with the limiting contents of water and fluorine at temperatures of 700 - 800⁰C and pressures of 1-5 kbar has been experimentally studied. It is shown that lithium is distributed in favor of the salt melt under all conditions. An increase in the water content in the system by more than 20wt.% and a pressure from 1 to 5 kbar leads to a decrease in the partition coefficients of lithium between the salt and aluminosilicate melts by 2-4 times. 3. During the differentiation of granite melts in nature, the accumulation of F and Li is possible, which will lead to the formation and separation of salt melts from aluminosilicate ones. Crystallization of cryolite-bearing granites and pegmatites is possible from such magmas. Magmas can have both agpaitic and plumasite compositions. 4. The ability of fluoride lithium-containing salt melts to concentrate rare elements such as W, Nb, Hf, Sc, U, Th and, especially, REE has been experimentally proven, which suggests an important role of these melts in the formation of rare-metal-rare-earth deposits.