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Using Ge/Si ratios to decouple iron and silica fluxes in Precambrian banded iron formations

¹ School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
² Department of Earth Sciences, Cardiff University, P.O. Box 914, Cardiff CF10 3YE, UK
³ Division of Exploration Geoscience, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Wembley, WA 6014, Australia

Banded iron formations are prominent sedimentary deposits of the Precambrian, yet the source of their silica remains unresolved. Here we show that Ge/Si ratios preserved in banded iron formation chert layers are indicative of weathering of continental landmass. This conflicts with the accumulation of evidence suggesting that chemical components were sourced at mid-ocean-ridge hydrothermal systems. Instead, it implies that the sources of silica and iron were decoupled during banded iron formation deposition, silica being dominantly derived from weathering of continental landmass and iron having a hydrothermal origin. Thus, the chemistry within banded iron formation depositional basins underwent clear switching that varied on a periodic basis and is recorded in the alternation of iron- to silica-rich layers.

Key Words: Hamersley banded iron formation • Ge/Si ratios • Archean-Proterozoic ocean chemistry

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