Iron and graphite associated with fossil bacteria in chert

Abstract

The mineralization associated with fossilized bacteria in 2.0-Ga stromatolitic grey chert from the Gunflint Iron Formation has been examined using electron microprobe techniques and transmission electron microscopy (TEM). X-ray diffraction showed that the chert consisted mainly of quartz with small amounts of calcite, lepidocrocite, hematite and graphite. In petrographic thin sections, fossilized remains of both filamentous and coccoid microorganisms were detected by optical microscopy. Electron microprobe analyses of these fossilized bacteria revealed high concentration of Si and O, as expected for the quartz matrix of the chert. Trace amounts of Al, S, C, Fe and Cl were also present. Fragments of microfossils were observed by TEM examinations of finely ground chert. Selected areas of this cellular debris generated graphitic electron diffraction patterns with spacings at d = 3.38, 2.04 and 1.23 å. Individual cell fragments were also found in successive stages of mineralization from amorphous to microcrystalline iron oxides.