Chemistry, paragenesis and significance of tourmaline in pegmatites of theSouthern Tin Belt, central Namibia

Numerous pegmatite bodies occur in several distinct belts within the Damara orogen of central Namibia. Many of the pegmatites have been mined extensiv ely for Sn, Li, Nh, Ta, Cs, mica, feldspar, and gem tourmaline and gem bery l. Tourmaline is widespread as an accessory and locally abundant mineral in different geological environments within the Damara Orogen. In pegmatites from the Southern Tin Belt (STB), it exhibits diverse habits and colours, f rom black to dark green, light green, and pink, as well as variations in co mposition depending on pegmatite type and location within the pegmatite. El ectron microprobe analyses of the tourmalines show a significant chemical v ariation from Fe-rich compositions with moderate Al in the Y sites to Li-ri ch compositions with high Al-Y, and relatively high F. Fe/(Fe + Mg) ratios for black, green, and pink tourmalines vary from 0.65 to 0.95, 0.66 to 0.99 , and 0.50 to 0.87, respectively. Na/(Na + Ca) ratios increase from Li-bear ing tourmaline (0.83-0.94) to black tourmaline (0.95-0.99). Tourmaline comp ositions mostly plot along trends involving proton-loss and alkali-defect s ubstitutions from the schorl additive component. These substitutions accoun t for increases of Al in the Y site, and decreases in the X site and OH occ upancies at the expense of the schorl component. Amounts of Al (Y) range fr om 0.23-0.63 in black tourmalines to 0.82-1.44 in green and pink tourmaline s. Within individual pegmatites, the abrupt transition from black tourmalin e to green tourmaline, and later pink tourmaline, reflects decreasing Fe an d increasing Li and Al via the substitution LiAlFe-2. Enrichment of Li in t he tourmaline is accompanied by increases of Be, Sn, Rb, Cs, and Pb. Insign ificant amounts of Fe3+ in the tourmalines suggest a low oxidation state of the pegmatite-forming fluids. As the precipitation of cassiterite requires an increase in fO(2), such conditions may account for the distribution of tourmaline and cassiterite in different pegmatite zones. Field relations, d istribution, petrographic features, and tourmaline chemistry provide eviden ce of two pegmatitic domains in the STB: (1) a western one, characterized b y pegmatites with Sn, Nb > Ta, (Li, Be) mineralization and containing Fe-Mn phosphates and black tourmaline, which may be explained in terms of simult aneous crystallization of two petrologenetic units; and (2) an eastern one characterized by pegmatites with Li > Be, (Sn, Nb > Ta) mineralization and elbaite, which formed via fractionation processes and sequential crystalliz ation from the margins inward to the cores. (C) 1999 Elsevier Science B.V. All rights reserved.