Abstract

Altered pyroclastic rocks, now composed almost entirely of ferrostilpnomelane, occur in numerous layers in the Proterozoic Brockman iron-formation in the Hamersley range of Western Australia. A number of well-preserved volcanic structures, combined with a unique mineralogy, indicate that the layers are derived almost entirely from volcanic materials, and show further that volcanism was active during much of Brockman time. The stilpnomelane-rich layers appear to have formed by alteration of the volcanic material. Included layers of normal shale suggest that the stilpnomelane has not formed from detrital materials.

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