Biologically induced iron ore at Gunma iron mine, Japan

The mineralogy of sedimentary iron ores from the Gunma iron mine are descri bed to evaluate the role of microorganism and plants in ore formation. The iron ore is composed of nanocrystalline goethite, well-crystallized jarosit e and very small amounts of strengite. The ore characteristically occurs as thick-bands of alternating goethite and jarosite bands, thin-bands of diff erent goethite grain sizes, and fossil-aggregate ore rich in moss and/or le aves. Algal fossils are clearly preserved in the goethite bands in the thic k-banded ore. Lattice imaging showed characteristic crystallographic orient ations of the goethite nanocrystals. The thin-banded iron ores consist of m icrometer-sized chestnut-burr-like goethite aggregates, probably formed by bacterial iron biomineralization. The bands may be attributed to biological or seasonal rhythms. Various products of biomineralization are found in th e present-day pH 2-3, Fe2+-, and SO42--rich streams. Bacterial precipitatio n had variations from amorphous Fe-P-(S) precipitates near the outlet of mi neral spring to Fe-P-S precipitates and to Fe-S-(P) (schwertmannite-like) p recipitates in the midstream. Mosses and green algae are also collecting Fe precipitates in and around the living and dead cells. Comparison of the pr ocesses occurring in the present-day streams and the iron-ore specimens sup ports the interpretation of these ores as the product of biomineralization.