Physical and Chemical Parameters of Processes Producing Rare-Metal Deposits in Granitoid Systems with Fluorine: Experimental Data

The origin of rare-metal deposits in granites is considered with regard for experimental data, which place constraints onto interpretations of geological materials and the 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 and vary from ~2–5 to ~0.1 wt %. These concentrations depend much less significantly on temperature and pressure. In granite melt in equilibrium with fluorine-bearing fluid, Ta and Nb are strongly partitioned into the melt. The paper demonstrates principal difference in the partitioning of W and Ta, Nb, Sn in melt granite–salt systems. The fluoride water–salt phase is a very effective extractant of W, while Ta, Nb, and Sn are completely retained in the aluminosilicate melt. The model magmatic fluid in equilibrium with Li–F granite melt is multiphase and contains significant amounts of SiO₂ and Na, Al, Li and K fluorides. The solubility of ore minerals in this fluid is insignificant, with the concentration of Nb much higher than that of Ta. The HF concentrations in high-temperature magmatic fluids were estimated at ~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 with sufficiently concentrated HF and, possibly, KF solutions (fluids). In sodium alkaline solutions, hydrothermal transport is quite probable for Nb but difficult for Ta, and the pyrochlore solubility is thereby higher than that of columbite.