Abstract

Several supergene ferromanganese wad deposits, mined on small scale for industrial applications, are developed on dolomite of the Neoarchaean Malmani Subgroup of the Transvaal Supergroup in South Africa. At the West Wits Gold Mine on the plateau of the Witwatersrand escarpment near Johannesburg, the ferromanganese wad is developed in manganiferous dolomite of the Oaktree Formation at the base of the Malmani Subgroup. The wad represents an ancient saprolite developed in the dolomite below a major unconformity with incised valleys filled by ferruginous silty mudstone and gravels containing abundant reworked ferromanganese soil nodules. The saprolite is up to 80 m thick. The incised valley deposits are cut by a second erosion surface below which another palaeosol is developed with characteristics of a ferric podzol. Large root marks extend from the top of this palaeosol through the ferruginous silty mudstone channel-fill deposits, into the underlying ferromanganese wad, up to depths of several tens of metres. The root marks are filled with yellow Kalahari sand. The second erosion surface is flat and draped by a poorly sorted pediment stone lag. Stones on the pediment were derived from the underlying channel-fill succession. The pediment is in turn overlain by a dark yellowish brown sandy transported soil (the Hutton soil). Finally the Hutton soil is incised by modern stream erosion and overlain by a thin modern soil covered by grass. The ferromanganese wad in the saprolite at the base of the incised valley succession is mainly composed of amorphous manganese- and iron-oxyhydroxides containing some crystalline birnessite, lithiophorite, and haematite, as well as subordinate quartz, muscovite, and kaolinite. Original sedimentary bedding in the dolomite is partly preserved in the saprolite, although mass balance calculations suggest that the saprolite has undergone 60 to 70% compaction in the transformation of dolomite to wad. Carbonaceous chloritic shale beds interbedded with the Oaktree dolomite are altered to kaolinitic clays in the saprolite. Ferromanganese soil nodules present in the incised valley-fill are composed of a nucleus of older abraded or broken nodules concentrically overgrown by several coatings of cryptomelane+ or -goethite. The saprolite and overlying erosion surfaces, sediments, and soil profiles provide new information on the Post-Gondwana tectonic and climatic history of the Witwatersrand plateau. This plateau forms the watershed between rivers draining into the Indian and Atlantic Oceans from the Highveld of South Africa. The ferromanganese wad apparently formed after the breakup of Gondwanaland, perhaps during the late Cretaceous to early Tertiary African cycle of erosion. Conditions must have been humid and warm to have allowed deep chemical weathering with wad formation, kaolinitization, and lateritization. The incised valley succession is correlated with the post-African I cycle of erosion, which took place between 30 and 2.5 Ma. During this period the climate became more arid with development of savannah-type vegetation and trees with very deep tap roots. The arid conditions climaxed with the formation of the pediment stone lag on top of the incised valley-fill succession and influx of Kalahari sand in the middle Pliocene. It is thought that the pediment developed at the start of the post-African II cycle of erosion in the Pliocene. After that the climate apparently became more humid, resulting in reworking of Kalahari sediment into the Hutton soil. This soil is at present being incised by stream erosion and covered by grassland below which a thin modern humic soil is developed.

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