Physical and chemical parameters of processes producing rare-metal deposits in granitoid systems with fluorine: experimental data

Abstract

The problem of the origin of rare-metal deposits in granites is considered, taking into account the results of experimental studies, which make it possible to introduce restrictions on the interpretation of geological materials and the construction of genetic models. The role of both magmatic and hydrothermal-metasomatic factors in the formation of various types of rare-metal deposits is discussed. The saturation concentrations of Ta and Nb in granite melt significantly depend on the melt composition, varying from ~ 2–5 to ~ 0.1 wt.%. They depend on temperature and pressure, to a lesser extent. The partitioning of Ta and Nb between the fluorine-containing fluid and the granite melt is sharply biased in favor of the melt. The principal difference in the partitioning of W and Ta, Nb, Sn in melt granite-salt systems is shown. Fluoride water-salt phase is a very effective extractant of tungsten, while Ta, Nb and Sn completely remain in the aluminosilicate melt. The model magmatic fluid, that is in equilibrium with Li-F granite melt, is multiphase and contains significant amounts of SiO 2 and fluorides of Na, Al, Li and K. The solubility of ore minerals in this fluid is insignificant, but the concentration of Nb substantially exceeds that of Ta. The concentrations of HF in high-temperature magmatic fluids were estimated that could reach ~ 0.5–1 M HF. The experimentally determined solidus temperatures of Li-F granites are ~ 570–630°C at a pressure of 100–200 MPa. At T = 300–550°С and P = 50–100 MPa the actual hydrothermal transfer of Ta and Nb is possible only by sufficiently highly concentrated solutions (fluids) of HF and, possibly, KF. At alkaline sodium solutions, hydrothermal transport of Nb is quite probable, and for Ta it is difficult, at that the solubility of pyrochlore is higher than the solubility of columbite.