Altered pyroclastic rocks in South African iron-formation

Hundreds of layers composed dominantly of stilpnomelane occur in thick extensive iron-formation in the Transvaal system in South Africa. Examination of thin sections reveals that many of these layers contain axiolitic shard-like structures, suggesting that they are altered pyroclastic material, presumably water-laid tuffs. A comparison is made with similar stilpnomelane-rich layers in iron-formation from Western Australia. The presence of these altered pyroclastic layers indicates that there was volcanic activity during much of the time the iron-formation was being deposited.