Banded iron formations (BIFs) are prominent sedimentary deposits of the Precambrian, but despite a century of endeavor, the mechanisms of their deposition are still unresolved. Interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation, in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy. On the basis of chemical analyses from BIF units of the 2.5 Ga Hamersley Group, Western Australia, we show here that even during periods of maximum iron precipitation, most, if not all, of the iron in BIFs could be precipitated by iron-oxidizing bacteria in cell densities considerably less than those found in modern Fe-rich aqueous environments. Those ancient microorganisms would also have been easily supported by the concentrations of nutrients (P) and trace metals (V, Mn, Co, Zn, and Mo) found within the same iron-rich bands. These calculations highlight the potential importance of early microbial activity on ancient metal cycling.

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A drill core consisting of unaltered banded iron-formation from Paraburdoo, Western Australia, representing the entire Dales Gorge Member of the Brockman Iron Formation, has been analyzed in short units of about 300 mm or in groups of units of up to 1 m. These analyses have been combined to give average compositions for each of the 33 macrobands of the member. The average compositions for the 17 oxide "facies" BIF macrobands and the 16 intercalated chert-carbonate and silicate "facies" S macrobands are, respectively, 43.15 and 45.78 percent SiO2, 0.09 and 2.04 percent A12Oa, 46.37 and 27.77 percent Fe•Oa (total iron), 2.69 and 5.21 percent MgO, 1.65 and 2.85 percent CaO, 0.04 and 0.02 percent Na•O, 0.01 and 0.09 percent TiO2, 0.21 and 0.16 percent P2Os, 0.05 and 0.02 percent MnO, and 0.03 and 0.32 percent S. The analyses are similar to those reported by Trendall and Pepper (1977) for a part of the Dales Gorge Member at Wittenoom, 130 km away. These data, together with documented evidence of lateral continuity of both major and minor banding across the Hamersley Iron Province, support an assumption that the primary deposits were chemically similar throughout the area. Mineralogical variations between sites indicate low but variable metamorphism in the area, and magnetite-hematite ratios and textural relations suggest increasing grade from Paraburdoo to Wittenoom to Tom Price.A short length of drill core from Tom Price, representing two complete macrobands and parts of two others, was also analyzed, and the results were found to vary significantly from the Paraburdoo and Wittenoom data. These differences are attributed to metasomatism of the rocks at Tom Price. Primary precipitation of iron in BIF macrobands was caused by oxidation and was dominated by gels containing iron(Ill) hydroxyoxides and silica in varying proportions, depending both on seasonal conditions and on supply. These variations were responsible for the wide range in composition and in thickness of banding, from tens of micrometers (submicrobanding), to millimeters (microbanding), to tens of millimeters (mesobanding). Preservation of micro-and submicrobanding within specific mesobands is due to very early consolidation and accentuation of primary depositional layering during diagenesis. In other mesobands, crystallization was evidently delayed and, under deeper burial conditions, tended to destroy primary lamination. In S macrobands, iron was precipitated as siderite and silicates. Mineralogical and chemical data show that the apparently sharp lithological boundaries between BIF and S macrobands are in fact gradational. Such contacts are attributed to a more or less continuous precipitation of silica and iron, interrupted periodically and changed in character by increase in pH and/or fco•. These changes were initiated by additions of volcanic ash. S macrobands are characterized by increases in the overall content of CO•, Ca, Mg, Mn, A1, Ti, K, and Zr.Podding, and other localized distortion, of mesobands is attributed mainly to lateral flow of silica...

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