The 2.4-km diameter Agate Mountain Complex intrudes basalts and red quartz latites of the early Cretaceous Etendeka Formation on the inland extension of the Walvis Ridge. It appears to be a satellite vent located on the cauldron subsidence ring fracture of a slightly older and much larger alkaline volcano centred offshore. Reworking of pyroclastic deposits of this early volcano produced epiclastic conglomerates containing pebbles of quartz latite and fenitised basement rocks within the caldera structure. The quartz latites and conglomerates were intensely ferruginised adjacent to the ring fracture. All movement on the ring fracture ceased shortly thereafter. The Agate Mountain Carbonatite Complex was then emplaced. Fenitisation of the red quartz latites to a pervasive green colour preceded local ring fracturing, brecciation of the fenite and emplacement of early breccias, radial carbonatite dykes and the main sövitic carbonatite. Progressive alteration of the sövite to a bastnaesite-bearing beforsite was followed by explosive emplacement of a core breccia. This was followed by intrusion of three small basic plugs. Thirty-one satellite plugs and diatremes and a microgranite dyke were emplaced during or after formation of the central ring complex. Green phonolite dykes and plugs are late-stage intrusions. Tertiary erosion produced a silcrete duricrust, aragonite-filled solution cavities and oxidation of REE minerals.