One of the biggest Nb-Ta pegmatites in the Tantalite Valley area.
Ref.:
Baldwin, J. R. (1989): Replacement phenomena in tantalum minerals from rare-metal pegmatites in South Africa and Namibia. Mineral. Mag. 53, 571-581. [http://www.minersoc.org/pages/Archive-MM/Volume_53/53-373-571.pdf]