COMPOSITIONAL VARIATION OF TOURMALINE-GROUP MINERALS IN THE BORBOREMA PEGMATITE PROVINCE, NORTHEASTERN BRAZIL

Abstract

Various species of the tourmaline-group are common accessory minerals in the Boqueirao, Capoeira and Quintos Li–Be–Ta–Nb-bearing, LCT-family, rare-element-class granitic pegmatites from the Borborema Pegmatite Province (BPP), northeastern Brazil. Tourmaline from the border and wall zones of the pegmatites is enriched in Mg and Fe, crystallizing as Mg-rich members of the dravite–schorl solid-solution series. Tourmaline from the intermediate zone is richer in Fe and Al+Li, crystallizing as schorl. The Fe–Mg-free and Li–Al-rich elbaite is typical of the transition between the intermediate zone and the quartz core, and of replacement pockets. The bright turquoise blue elbaite known as “Paraiba tourmaline” is produced in the Capoeira 2 and Quintos pegmatites and is distinguished by high Cu contents, up to 1.07 wt.% CuO. In other occurrences of the “Paraiba tourmaline” in the BPP, up to 2.37 wt.% CuO were reported. Differences in the geochemical evolution trend of tourmalines among the pegmatite bodies investigated (vacancy at the X site, Fe, Mg, Zn, Li and F contents), suggest that they reached variable degrees of fractionation. This observation agrees with chemical data on white mica, feldspar, garnet and gahnite, and therefore permits the use of tourmaline composition as an indicator of the degree of evolution of the hosting pegmatite. According to these data,”Paraiba tourmaline”-producing pegmatites share the following characteristics: 1) they are the most evolved pegmatites so far known in the BPP; 2) they are hosted by quartzites or metaconglomerates (iron-poor host rocks); 3) they exhibit comb-textured dravite in the border zone (early saturation in tourmaline) and 4) the elbaitic “Paraiba tourmaline” is found in the most evolved parts of the pegmatites.