Geochemical and geological controls on the genesis of gem-quality "Paraiba Tourmaline" in granitic pegmatites from northeastern Brazil

Abstract

The electric blue Paraiba Tourmaline (PT) occurs in northeastern Brazil only in four deposits of zoned and highly fractionated granitic pegmatites of the Borborema Pegmatitic Province (BPP). These PT-bearing pegmatites are rich in spodumene or lepidolite, and are emplaced in iron-poor quartzites or metaconglomerates of the Neoproterozoic Equador Formation. Black comb-textured dravitic tourmaline in the border zone indicates crystallization from a boron-bearing granitic magma. The PT occurs at the transition between the albite-rich inner intermediate zone (IIZ) and the quartz core of the pegmatites, in primary wedge-shaped crystals of near-end-member color-zoned elbaite, arranged in fan-like, radial or comb-textured groups of crystals. The PT most commonly forms the intermediate growth-zone of color-zoned elbaite crystals having a pink to red core and a green to dark green rind. The PT-bearing pegmatites are distinguished by intensive late replacement of primary K-feldspar, spodumene, elbaite and Li-phosphates by lepidolite. The lepidolite replacement of zoned elbaite is highly selective; it begins in the core, then affects the light blue intermediate zones, and preferentially leaves intact the green rim. PT crystals or crystal zones preserved from lepidolite replacement are rare, and occur enveloped in quartz of the pegmatite core or in vug fillings. According to electron-microprobe data on color-zoned elbaite crystals, PT crystal-zones are distinguished not only by high Cu contents, but also by higher values of Li and F (mostly fluor-elbaite), Fe concentrations near zero, and lower concentrations of Mn than the contiguous zones with other colors. Increasing Fe + Mg concentrations are responsible for the green color in the rims enveloping PT, even if Cu values remain elevated. Values of Al, X □ and Mn are slightly elevated in the pink zones in comparison with PT in the same crystal. Textural relations indicate that PT formed by direct crystallization from a pegmatite-forming melt rich in B, Li and F, at the early stage of formation of the quartz core, and prior to the appearance of secondary lepidolite and other late hydrothermal minerals. The difference between PT-bearing and other elbaite-rich pegmatites may not necessarily be the failure to produce primary PT, but also the failure to preserve it from late conversion to lepidolite. The formation of PT is linked with low content of mafic minerals in the wallrock and in the pegmatite-forming melt itself.