Within the Welwitschia Inlier of basement rocks occurs the approximately 10x5 km Oas hornblende quartz syenite intrusion. About 1 km north of Oas, according to a map in Hoffman et al. (1996), are two small bodies (approximately 1.5x1.5 km) that comprise the Lofdal nepheline syenite, of which no details are given. Also north of Ofdal and covering an area of some 13x8 km are numerous east-west-trending felsic and carbonatite dykes. The carbonatites carry abundant fluorite in places and up to 180 ppm Th and 260 ppm W; up to 460 ppm Nb has been recorded in the nepheline syenite (Verwoerd, 1993).