Sodic metasomatism in the Palaeoproterozoic Hotazel iron-formation, Transvaal Supergroup, South Africa: implications for fluid–rock interaction in the Kalahari manganese field.

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]