Unique Geochemistry of Sedimentary Iron Deposit Formed by Biologically Induced Mineralization

Abstract: Major, trace, and rare earth element contents were determined for sedimentary iron ores from the Gunma iron deposit. The deposit was precipitated from a spring water on the hillside of the active Kusatsu-Shirane Volcano. The ores are mainly composed of goethite and jarosite with various proportions of silicified andesitic detritus. Microbial fossils are often preserved well in goethite-rich ores. Goethite was likely precipitated by both inorganic and biogenic (biologically induced mineralization; BIM) processes, whereas jarosite was precipitated inorganically from the spring water. Si, Ti, Al, Mn, Mg, Ca, Na, Co, Rb, Y, Zr, Nb, Hf, Ta, U, and middle-heavy REEs (Sm-Lu) in the ores are dominantly derived from a detrital component (silicified andesitic rocks). On the other hand, Zn and V are likely to have been inorganically coprecipitated with goethite. Preferential uptake of P, Sc, Cu, Mo, Ba, Th, and light REEs occurs in the BIM ores. Unique enrichment of these elements is a promising indicator of biomineralization for ancient sedimentary iron deposits.