Alfonso Pesquera Pérez

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

Abstract Numerous pegmatite bodies occur in several distinct belts within the Damara orogen of central Namibia. Many of the pegmatites have been mined extensively for Sn, Li, Nb, Ta, Cs, mica, feldspar, and gem tourmaline and gem beryl. 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, from black to dark green, light green, and pink, as well as variations in composition depending on pegmatite type and location within the pegmatite. Electron microprobe analyses of the tourmalines show a significant chemical variation from Fe-rich compositions with moderate Al in the Y sites to Li-rich compositions with high Y Al , 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-bearing tourmaline (0.83–0.94) to black tourmaline (0.95–0.99). Tourmaline compositions mostly plot along trends involving proton-loss and alkali-defect substitutions from the schorl additive component. These substitutions account for increases of Al in the Y site, and decreases in the X site and OH occupancies at the expense of the schorl component. Amounts of Al (Y) range from 0.23–0.63 in black tourmalines to 0.82–1.44 in green and pink tourmalines. Within individual pegmatites, the abrupt transition from black tourmaline to green tourmaline, and later pink tourmaline, reflects decreasing Fe and increasing Li and Al via the substitution LiAlFe−2. Enrichment of Li in the tourmaline is accompanied by increases of Be, Sn, Rb, Cs, and Pb. Insignificant 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 fO2, such conditions may account for the distribution of tourmaline and cassiterite in different pegmatite zones. Field relations, distribution, petrographic features, and tourmaline chemistry provide evidence of two pegmatitic domains in the STB: (1) a western one, characterized by pegmatites with Sn, Nb>Ta, (Li, Be) mineralization and containing Fe–Mn phosphates and black tourmaline, which may be explained in terms of simultaneous 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 crystallization from the margins inward to the cores.