Rare metal pegmatites of the Cape Cross-Uis pegmatite belt, Namibia: geology, mineralisation, rubidium-strontium characteristics and petrogenesis

Rare metal pegmatites of the Cape Cross-Uis pegmatite belt (Namibia) containing sub-economic to economic concentrations of Sn, Nb, Ta, Li, occur in three main pegmatite swarms. Unzoned rare metal pegmatites are characterised by disseminated cassiterite mineralisation and varying Li, Ta and Nb concentrations, whereas zoned and poorly zoned pegmatites of the LI-rich rare metal type, beside low grade cassiterite mineralisation, contain higher Nb, Ta and Li concentrations (500 ppm Nb; 800 ppm Ta average). Occurrences of both types are confined to areas of extensive shearing in Pan-African greenschist facies metasediments within the tensional environment of a half-graben structure.

Rb/Sr isotope studies of various mineral phases in zoned and unzoned pegmatites in the study area indicate a late Pan-African emplacement age of 490 Ma for the pegmatitic melts. Petrogenetically, high ⁸⁷Sr/⁸⁶Sr initial ratios reflect partial melting of crustal material (basement rocks). Rare metal pegmatites in the study area are believed to have originated at the peak of regional metamorphism. This is indicated by higher metamorphic grades towards the Atlantic Ocean, which is combined with an increase in Li, Nb and Ta concentrations.