Abstract

Petrographic studies of a bore core section through the Dales Gorge Member of the Brockman Iron Formation, supported by experimental gel runs, indicate a complex paragenesis of associated ferroan silicate minerals in the Wittenoom Gorge area. The primitive silicate phases appear to have been a mixture of montmorillonite and greenalite, while depending on the nature of subsequent diagenesis, these gave way to stilpnomelane and minnesotaite, respectively, with occasionally a little chlorite. Three varieties of stilpnomelane are recognized and can be correlated with different stages of diagenesis. Riebeckite and crocidolite together represent the end products of this sequence, but whereas the former largely represents a metasomatic replacement product after stilpnomelane, crocidolite is essentially a direct crystallization product, which is confined to marked intraformational breccia horizons apparently formed through slumping of the partially consolidated sedimentary mass. Experiments on the formation conditions of crocidolite, show that soda availability and concentration are essential factors, but almost equally critical are an abundance of Fe (super ++) ions and low Eh conditions. Associated magnetite, despite its apparent seeding role, is a serious contender for Fe (super ++) , so to minimize magnetite formation, relatively low pH values are also probably more favorable towards crocidolite formation.

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