Boron and lithium isotopic compositions as provenance indicators of Cu-bearing tourmalines

摘要

The Li and B isotopic compositions of gem-quality Cu-bearing tourmalines were used (1) to distinguish among Paraiba tourmalines from Brazil and Cu-bearing tourmalines from Nigeria and Mozambique; and (2) to identify the likely source of Li and B for these gem-quality tourmalines. The 11B values of tourmaline from Paraiba, Brazil, range from -42.4 to -32.9, whereas the 11B values of Cu-bearing tourmaline from Nigeria and Mozambique range from -30.5 to -22.7 and -20.8 to -19.1 respectively. Tourmalines from each locality have relatively homogeneous 11B values and display no overlap. There is slight overlap between 7Li values of Paraiba tourmaline (+24.5 to +32.9) and Cu-bearing tourmaline from Nigeria (+32.4 to +35.4), and 7Li values of Cu-bearing tourmaline from Nigeria and Mozambique (+31.5 to +46.8). Nevertheless, Cu-bearing tourmalines from each locality can be fingerprinted using a combination of their 11B and 7Li values. The very small 11B values are consistent with a non-marine evaporite source, and are among the smallest reported for magmaticsystems, expanding the global range of B isotopic composition for tourmaline by 12. The corresponding large 7Li values are among the largest reported, although they are less diagnosticof the source of the Li. The large 7Li values in conjunction with the small 11B values suggest a non-marine evaporite or brine as a source for Li and B, either as constituent(s) of the magma source region or, by assimilation during magma ascent. The large range in 11B and 7Li values suggests that B and Li isotope fractionation occurred during magmatic degassing and late-stage magmatic-hydrothermal evolution of the granite-pegmatite system.