Abstract

New data are presented concerning the petrography and geochemistry of the Paleoproterozoic Hotazel iron-formation of the Kalahari manganese field, South Africa. Mineralogically and texturally, the Hotazel iron-formation is very similar to Paleoproterozoic banded iron-formations of the Superior type, comprising laminae of chert, iron oxides (magnetite, hematite), iron silicates (greenalite, minnesotaitc, stilpnomelane, riebeckite, Fe mica), carbonates (calcite, ankerite, minor siderite), and pyrite. The major chemical constituents of the Hotazel rocks are SiO 2 , Fe oxides, CaO, and MgO, whereas MnO, Al 2 O 3 , Na 2 O, and K 2 O contents are negligible (below 1 wt %). Elements of detrital derivation (Ti, Zr, Rb) and transition metals such as Zn, Cu, Ni, Co, and V are present in low concentrations, on the order of a few tens of parts per million.Stratigraphically higher, the Hotazel banded iron-formation shows a conspicuous increase in CaO contents that is coupled with a subtle decrease in bulk-rock Fe number ratios (Fe (super 2+) /(Fe (super 2+) + Fe (super 3+) )), implying a gradual transition to a more oxidizing environment. Shale-normalized rare earth element (REE) patterns are similar to those of older banded iron-formations of the Transvaal Supergroup, exhibiting light REE depletion and negative Ce anomalies comparable to modern shallow-level seawater. Occasional weak positive Eu anomalies in iron silicate-rich rocks indicate minor admixtures of a hydrothermal component below the chemocline. The similarities between the Hotazel iron-formation and most Superior-type banded iron-formations, the lack of transition metal enrichments, and the absence of strongly positive Eu anomalies in the Hotazel rocks challenge volcanogenic exhalative models proposed previously for the genesis of the interbedded Kalahari Mn deposits.

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