Manganese deposits of the Cape Peninsula, South Africa
The outcrops, petrography and geochemistry of manganese deposits were studied at several sites on the Cape Peninsula (Skeleton Gorge, Kasteelpoort Path, Kommetjie and Hout Bay) as well as at Rooi Els on the opposite side of False Bay. The purpose of this study was to understand the origin and depositional history of these manganese deposits. Manganese ore samples contain between 16 and 86 wt% MnO, 0.3 to 29 wt% Fe2O3 , 1.2 and 5.6 wt% K2O and a P2O5 content of 0.3 to 1.8 wt%. The primary manganese oxide mineral present is cryptomelane (KMn8O16). The iron and quartz content of bulk rock samples have a positive correlation, and a negative correlation to manganese. From the field data, petrography and chemistry by electron microprobe analysis, it was established that the method of deposition was lateral secretion and the deposits fit into Harrison’s (1998) manganese classification as a Type 1 deposit. Lateral secretion involves the reduction of manganese and iron oxides within the soil profile by acidified rain water, further leaching of oxides as reducing groundwaters flow through regolith and bedrock, and the eventual precipitation as oxides at groundwater seeps and springs. Field observations and the chemistry of water and rock samples support the lateral secret ion model in which the manganese and iron are chemically leached from the Peninsula Format ion quartzarenite sandstone bedrock and transported along fractures and faults to be deposited as oxides upon emergence at the surface. Minor surface coatings of iron and manganese oxides are ubiquitous in the study area but can become concentrated in locally historic economic deposits where the groundwater f low is focused by topography, aquitards and faults. Trace element composition of the manganese-rich deposits indicates that they are low-temperature, hydrogenous freshwater deposits. The proposed Eh - pH evolution of surface and ground water flow paths is consistent with the reduction of manganese and iron within the soil profile, and the precipitation of first iron and then manganese at springs and seeps. The Rare Earth Element (REE) pat terns suggest variable sources and transport of the manganese and iron. The manganese deposits of the Cape Peninsula are no longer economic but do provide insights into the remobilisation of metals by low temperature groundwaters during the process of lateral secretion.
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