A geological investigation of the Waterberg hydrothermal platinum deposit, Mookgophong, Limpopo Province, South Africa

Abstract

The geology around the unusual quartz-vein hosted Waterberg platinum deposit was mapped using aerial photographs at a scale of 1 : 1000 to establish possible controls on this unique style of mineralisation. The study area is located in the Mookgophong (Naboomspruit) District of the Limpopo Province in South Africa and is dominated by a gently to moderately, NNW-dipping irregular sequence of bimodal (rhyolite–basalt) volcanics, interbedded with many sedimentary rocks, that form part of the extensive 2˙06 Ga Rooiberg Group, that are cut by variably striking, steep to subvertical quartz veins. The regional geology of the area was mapped at various scales in the 1920s and 1940s, but much of this work remains inaccessible and mapping during this study has uncovered more detail. The major platiniferous quartz vein (known as the 'Main Lode') juxtaposes volcanic rocks of Rooiberg age against Triassic sandstones and therefore occupies a fault. There is no textural evidence that quartz veining was syntectonic and may be much younger. The well-exposed vein fabrics show many features typical of low pressure epithermal quartz veins and probably formed near the present-day surface beneath a highly eroded Karoo cover, suggesting a recent age for veining and Pt mineralisation. The Main Lode can be followed for at least 3 km along strike, but Pt mineralisation is concentrated in a short section of the vein around 500 m in length. A major regional fault (the Welgevonden Fault) occurs about 2 km to the northwest of, and parallel to, the Main Lode. The Welgevonden Fault zone is suggested to extend to the northeast and merge with the Planknek–Ysterberg Fault system, south of Mokopane (Potgietersrus). The Welgevonden Fault is presently geothermally active, but this fluid system does not carry significant concentrations of Pt and may represent a younger Pt-depleted stage of hydrothermal activity related to the event that formed the Main Lode. It is unlikely that the Rooiberg volcanics and younger rocks supplied the platinum-group elements in the deposit and the source for these metals must lie elsewhere. It is suggested that shearing along the Welgevonden–Plankneck Fault system transposed PGE-bearing mafic rocks of the northern Bushveld Complex to a position at unknown depth beneath the Waterberg deposit, or provided a network of shear zones which could be exploited by a recent hydrothermal event that transported platinum and other metals westwards along the fault system and redeposited them near the surface.