Tourmaline as a tracer of magmatic-hydrothermal evolution and potential Nb-Ta-(W-Sn) mineralization from the Lingshan granite batholith, Jiangxi province, southeast China

The Lingshan granite batholith (Jiangxi province, southeast China) is a rather unique example of Nb-Ta-rich rare metal granite, and three different types of tourmalines have been identified in the marginal granite zone, including (i) quartz-tourmaline nodules (Tur-N type) in the fine-grained biotite granite, (ii) tourmaline segregations and/or tourmaline-filled cavities (Tur-P type) and (iii) tourmaline veins (Tur-PV type) in pegmatite dykes. The extensive fractional crystallization process from the fine-grained biotite granite to pegmatite has been revealed by the geochemical and boron isotope compositions from the Tur-N to the Tur-P and/or Tur-PV type tourmalines, such as increasing Fe contents, decreasing Nb/Ta and Zr/Hf ratios, as well as restricted boron isotopes (delta 11B,-12.0 to 9.0 parts per thousand). Particularly, the Nb and Ta contents increase from the Tur-N to the core of the Tur-P type tourmaline, and then decrease towards the rim of the Tur-P type tourmaline. It is, therefore, possible that the decreasing variations of Nb and Ta from the core to the rim within the Tur-P type tourmaline more likely record the compositional and environmental evolution from the late boron-rich magma to the exsolved hydro thermal fluids. The late-stage Tur-PV type tourmaline as the hydrothermal product contains the lowest F contents. Thus, we suggest that the Nb-Ta mineralization in the Lingshan granite batholith dominantly occurs in the highly-evolved granitic melt rather than during the post-magmatic aqueous fluid. Furthermore, the Nb and Ta contents of Lingshan tourmaline overlap with previous reported tourmaline data from W-Sn rare-metal deposits instead of Au deposit. This signature is comparable to the fact that small volumes of W-Sn mineralization occur in the Songshugang Ta-Nb-(W-Sn) deposit. Hence, the Nb and Ta contents in tourmaline can be used as potential parameter to explore the Nb-Ta-(W-Sn) mineralization not only in the Lingshan granite and pegmatite but also in similar granite batholith elsewhere in the world.