Geochemistry of tourmaline from granitic pegmatites in East Qinling and its implications for mineralization

The East Qinling region is one of the most important pegmatite provinces that host rare-element deposits in China. Tourmaline is a ubiquitous mineral in various types of pegmatites in this area and commonly exhibits black to dark blue colour in barren, Be and Li pegmatites. In this study, we investigated the compositional variation of black tourmaline from barren to Li pegmatites to demonstrate that the geochemical characteristics of the tourmaline can potentially serve as a mineralization indicator. The investigated black tourmaline belongs to schorl series which is ubiquitous in most granitic pegmatites in East Qinling. In barren pegmatites at Shuangfengcun, Nianpan and Fengyuan, the tourmaline exhibits distinct sector zoning and strong pleochroism (bluish green to brown in plane-polarized light) and contains relatively high Mg, Ti and Sc, but low Li, Mn and Zn. The tourmaline from these localities also shows variable delta(11) B values (- 19. 00 parts per thousand similar to- 12. 00 parts per thousand). In the Jiezigou Be pegmatite, the tourmaline which is blue to grass green in plane-polarized light exhibits delta(11) B varying between -16. 00 parts per thousand and - 12. 00 parts per thousand, and contains lower Ti, V and Sc than the tourmaline from the barren pegmatites. In the Fengzhuang pegmatite with slight Nb-Ta mineralization, the tourmaline shows distinct sector zoning and is blue to dark blue in plane-polarized light. The tourmaline from the Fengzhuang pegmatite contains higher Li and Zn than the tourmaline from the barren and Be pegmatites and exhibits delta B-11 values ranging from - 21. 92 parts per thousand to - 14. 75 parts per thousand. In the Caijiagou Li pegmatite, the composition of the black to dark blue tourmaline evolves to schorl-elbaite series, with a Li2O content up to 1. 0 parts per thousand. In this pegmatite, the tourmalines with delta(11) B values ranging from - 18. 96 parts per thousand to - 16. 8 parts per thousand have the highest Li, Mn and Zn concentrations, while they have extremely low Mg, Ti, Sc and V contents. The results of boron isotopic analysis show that two types of tourmalines exist in the Nianpan, Fengyuan and Fengzhuang pegmatites. The Type I tourmaline is characterized by relatively heavier B isotope that is likely crystallized from the pegmatite-forming melt at an earlier stage, whereas the Type II tourmaline displays more negative delta(11) B values, which is likely precipitated from the exsolved fluids from the pegmatites. Such fluids also resulted in formation of hydrothermal tourmaline in the country rocks. The hydrothermal tourmaline exhibits delta(11) B values comparable to those of Type II tourmaline, indicating no significant B isotopic fractionation caused by the interaction between the exsolved fluids and the country rocks. Nevertheless, the interaction between pegmatites and their country rocks resulted in addition of Mg, Ti and V into the pegmatite, and meanwhile migration of Li, B, and Al into the country rocks. In the study area, from barren to Li pegmatites, with increasing differentiation degree of pegmatites, the Li, Mn, Zn and F contents of schorl tourmaline increase and exhibit negative correlation with tourmaline delta(11) B. This suggests that Li, Mn, Zn and F contents and B isotopic compositions of schorl tourmaline can indicate various pegmatite-hosted rare-element mineralization in East Qinling.

Automated summary

In this study, the compositional variation of black tourmaline from different types of pegmatites in the East Qinling region was investigated. The results showed that the geochemical characteristics of tourmaline can serve as a mineralization indicator. Li, Mn, Zn, and F concentrations in schorl tourmaline increase with increasing differentiation degree of the pegmatites, and they exhibit a negative correlation with tourmaline delta(11) B. This provides a potential tool for indicating various pegmatite-hosted rare-element mineralization in the region.