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Twelve Group II and fourteen Group I kimberlite diatremes in central South Africa were examined for upper crustal xenoliths in order to estimate the extent of various lithological units of the Karoo Supergroup in the main Karoo basin at times of kimberlite eruption, the Cretaceous erosional history of the area, and the approximate vertical extent of the kimberlite diatremes prior to erosion. Sandstone and amygdaloidal basaltic lava xenoliths from the Karoo Supergroup were specifically selected as their modal mineralogies and geochemical compositions respectively can be attributed to specific stratigraphic positions within the Karoo Supergroup. Results indicated that, at the time of Group II kimberlite eruption (∼120 Ma), basaltic lavas of the Drakensberg Group covered the entire area, but by the time of Group I kimberlite eruption (∼85 Ma), they were restricted to the south-eastern half of the study area. At the latter time, an escarpment is proposed to have existed at the basalt outcrop limit, some 180 km west of its current position. Sandstones of the Stormberg Group had a restricted original distribution in the north and east of the study area, whereas sandstones from all other Karoo groups occurred throughout the entire area. In the Kimberley area, approximately 500 m of erosion is estimated to have occurred from 120 to 85 Ma and 850 meters from 85 Ma to the present day at average rates of approximately 15 m/Ma and 10 m/Ma respectively. Both Group I and II kimberlite diatremes had vertical extents of approximately 1350 m at eruption. An inland scarp-retreat model is proposed for the Cretaceous erosion cycle in central South Africa. [ABSTRACT FROM AUTHOR]

Petrological and geochemical evidence is presented on the occurrence of aegirine in the Palaeoproterozoic Hotazel iron-formation, which hosts the giant manganese deposits of the Kalahari manganese field, South Africa. The mineral has an essentially pure Na end-member composition and occurs sporadically in iron-formation immediately bordering the manganese ore beds. The development of aegirine appears to have taken place due to the action of late-infiltrating, saline hydrothermal fluids at the expense of a pre-existing, binary quartz–hematite assemblage. It is proposed that such a process would have overprinted (and therefore post-dated) a spatially more extensive, low-temperature alteration event which brought about thorough carbonate leaching, oxidation and residual enrichment of metals in the Hotazel iron–manganese rocks. [ABSTRACT FROM AUTHOR]