Highly fractionated Eocene granites in the southern Lhasa subterrane: Implications for tungsten metallogeny and exploration

Abstract

Highly differentiated granites are worldwide the primary source of W, Sn, and rare metals. Recently, a tourmaline granite was discovered in the Dagejia area in the western section of the southern Lhasa subterrane, southern Tibetan Plateau, where W mineralization is represented by quartz-tourmaline-wolframite veins in the granite. Two samples from the Dagejia tourmaline granite yielded weighted ²⁰⁶Pb/²³⁸U mean ages of 41.0 ± 0.5 Ma and 42.4 ± 0.5 Ma (Eocene). The Dagejia tourmaline granite is a typical S-type granite, characterized by high silica (SiO₂ = 72.78–78.29 wt%) and high total alkali (Na₂O + K₂O = 5.01–9.93 wt%) contents, has a high aluminum saturation index (A/CNK = 1.06‒2.72), and high normative corundum contents (average: 4.0 wt%; range: 0.7‒10.1 wt%). The zircon grains have negative ε_Hf(t) values (−3.96 to − 1.96) and Mesoproterozoic two-stage Hf model ages (T_DM2 = 1243–1372 Ma). These characteristics, along with low whole-rock CaO/Na₂O and low Nb/U ratios, and high Rb/Sr and high Rb/Ba ratios, point out that the Dagejia tourmaline granite derived from melting of a pelitic, mid- to upper crustal source. The Dagejia tourmaline granite displays high differentiation indexes (DI = 88–96), has low Zr/Hf and Nb/Ta ratios, and is depleted in Ba, Nb, Sr, Eu, and Ti, indicating that the granite is highly fractionated and has undergone a significant crystallization differentiation, involving amphibole, K-feldspar, plagioclase, and titanite. The W contents of the relatively primitive Dagejia tourmaline granite samples (DI = 88–94) are higher than those of the upper continental crust, showing that the magma source was pre-enriched in W. In addition, the W contents increased significantly when the DI exceeded 95, revealing an exceptionally high degree of differentiation triggering the further enrichment of the Dagejia tourmaline granite in W. Based on these geochemical characteristics and the presence of quartz-tourmaline-wolframite veins, the Dagejia tourmaline granite has the potential to have generated a fertile W mineralization. Combined with the W occurrences at Hahaigang, Nuri, Jiagang, and Jiaoxi, the Lhasa Terrane has a significant metallogenic potential for further W discoveries, particularly linked with highly fractionated granites.