Ge/Si in Hamersley BIF as tracer of hydrothermal Si and Ge inputs to the Paleoproterozoic ocean

Abstract

Ge/Si and Ge/Fe ratios have been measured by laser ablation ICP-MS in selected sections of the Hamersley banded iron-formation (BIF), including ‘chert-siderite’ bands (i.e. carbonate- and silicate-facies BIF). The major Ge carriers are magnetite and hematite in the oxide-facies, and the Fe-rich silicates greenalite, minnesotaite and stilpnomelane in chert-siderite bands, while quartz has the lowest Ge/Si ratios (0.15–0.7μmol/mol), in accord with discrimination against Ge in quartz. The homogeneous Si/Fe ratio of 1.25 for Dales Gorge oxide-facies BIF is consistent with Si-ferrihydrite as precursor phase. This Si concentration is sufficiently high for formation of iron-free silica clusters at the surface of ferrihydrite, so that only a minor fractionation of the Ge/Si ratio should result from preferential Ge adsorption relative to Si. Hematite-chert bands that have been minimally altered by metamorphic reactions and/or fluid metasomatism, are the preferred material to determine the sources of silica in Hamersley BIF. Variations of Fe/Si and Ge/Si ratios in chert mesobands are interpreted as the result of diagenetic mineral segregation. A consistent Ge/Si ≈3μmol/mol is then derived by extrapolation of measured Ge/Si ratios to the average Fe/Si composition of Dales Gorge BIF. Simple mass balance calculations indicate hydrothermal inputs to the ocean of the order of 25% for Si and 90% for Ge, assuming Ge/Si=10μmol/mol for high-temperature hydrothermal fluids. The high Si concentration in siderite-chert bands (Si/Fe≈4) demands only a small amount of Si-ferrihydrite precursor and some excess silica likely precipitated in pore-water oversaturated with regard to amorphous silica. The average Ge/Si=0.80±0.1μmol/mol for bulk chert-siderite samples, including some stilpnomelane-rich cherts, is within the compositional range of modern riverine and aeolian inputs, pointing to a continental Si source during their deposition.