Geochemical mechanism of Nb-, Ta-mineralization during the late stage of granite crystallization

The mechanism of Nb-, Ta-mineralizatio is discussed in the light of the properties of Nb-, Ta-complexes in different phases. Experiments show that Nb and Ta are essentially enriched in the melts when Nb-, Ta-bearing albite granites are completely melted (800–850°C) and in equilibrium with a HF-bearing vapor phase. It is also demonstrated from the experiments that the hydrolysis of Nb-, Ta-fluorine complexes in aqueous solutions takes place with increasing temperature reaching a maximum value in the vicinity of critical temperature and becoming stable under super-critical conditions. Under this circumstance, Nb-, Ta-complexes can be transported in the vapor phase. Ta exhibits a great ability of transport in the vapor phase as compared with Nb, while Nb is more soluble than Ta under hydrothermal conditions.