1.8 Ga iron-mineralized microbiota from the Gunflint Iron Formation, Ontario, Canada: implications for Mars Original Research Article

A ∼1.8 Ga banded-iron stromatolite from the Mink Mountain locality (PPRG 336) of the Gunflint Iron Formation, Ontario, Canada was investigated as an analogue to Martian hematite deposits which have the potential to contain fossilized Martian life. The stromatolitic sample was primarily composed of quartz (SiO2) with fractional amounts of hematite (Fe2O3), greenalite ((Fe,Mg)3Si2O5(OH)2), and minor amounts of stilpnomelane (K(Fe2+,Mg,Fe3+, Al)8(Si,Al)12(O,OH)27 · 2H2O). Iron-bearing minerals were present within thin, discontinuous bands aligned roughly parallel. Octahedral pseudomorphs of hematite after magnetite occurred as localized elongate clusters, as well as on the outer rims of detrital oncolitic structures. Microcrystalline greenalite was present as a clay occurring in layers subparallel to the stromatolitic layering. Greenalite was observed in both the silica- and iron-rich layers, as well as within oncolitic structures. Irregular aggregates of radiating stilpnomelane needles extended into the silica matrix. Optical microscopy (OM) and scanning electron microscopy (SEM) revealed 5–10 μm diameter iron-mineralized or siliceous, concentrically laminated, ooid-like structures in localized patches. OM, SEM, and energy dispersive X-ray analyses (EDX) were used to characterize microfossils mineralized by iron-oxides in the stromatolite. The microfossils consisted of coccoidal and filamentous forms. The coccoids (∼0.5 to 2 μm diameter) were present both in chains and individually throughout the silica- and iron-rich bands, while the filamentous remnants (∼1–3 μm diameter, and up to 200 μm length) were localized and patchy in the sample. While the Gunflint Iron Formation does not provide us with a perfect analogue to the Martian hematite site, it does establish the potential for finding iron-mineralized microorganisms within Martian sediments.