Be–Nb–Ta mineralization in granites: An example from the Nyalam area in the Himalayan belt, southern Tibet

In the Himalayan orogenic belt, numerous Cenozoic granites are enriched in rare metals, and three giant rare metal deposits have been discovered. However, the mechanisms responsible for rare metal enrichment and mineralization remain poorly understood. In the Nyalam area, a suite of granites intruding the Southern Tibet Detachment System have high rare metal contents (100–284 ppm Be, 24–117 ppm Nb, 15–102 ppm Ta). Zircon UPb analyses revealed that these granites formed ca. 17.3 Ma. Compared with barren granites in nearby areas, the rare metal-rich granites are characterized by (1) higher Na₂O and MnO contents and lower TiO₂ and MgO contents; (2) higher Rb and Rb/Sr values but lower Sr and Ba contents; (3) lower REE contents, nearly flat REE patterns and pronounced negative Eu anomalies; (4) higher Be, Nb, Ta, Rb, Cs, Ga, and Tl contents but lower B contents; and (5) lower ⁸⁷Sr/⁸⁶Sr(t) and ε_Nd(t) values. Significantly, when Nb/Ta < 5 and Zr/Hf < 20, melt structures undergo changes characterized by increasing M/F ratios (network formers/modifiers). These geochemical compositional variations between the two types of granites imply the combined effects of fractional crystallization and hydrothermal alteration but ultimately derived from melt structural changes. Depolymerization reduces melt viscosity and increases NbTa solubility, thereby enhancing the efficiency of fractional crystallization and facilitating rare metal enrichment.