The Tantalite Valley Shear Zone is a major Precambrian, southeast-trending tectonic lineament extending for some 500 km (possibly as much as 800 km) along strike in southern Namibia (South West Africa) and the northern Cape Province of South Africa. A minimum right-lateral displacement of 85 km has been estimated for this shear zone, which is one of a number with similar orientations found in southwestern Africa. The shear zones may represent slip-lines produced during continental collision about 1000-1300 m.y. B.P. The shear zones have acted as the locus for the intrusion of high-alumina tholeiitic magmas which have led to the development of a number of mafic to ultramafic complexes situated in or near the zones, and particularly the Tantalite Valley Shear Zone. Igneous activity and tectonism took place over an extended period of time and some bodies have been partly or completely metamorphosed to metagabbro or amphibolite. Three complexes have been studied in detail and they are geochemically distinct from each other, such that they cannot be related to one another by simple processes of fractionation although the rock types within any one complex may be so related. All show broad similarities in that they are depleted in lithophile elements (Ba, Rb, Sr, Nb, Zr) and enriched in nickel relative to similar basalts found elsewhere.