The iron mining regions of Brazil contain thousands of “iron ore caves” (IOCs) that form within Fe(III)-rich deposits. The mechanisms by which these IOCs form remain unclear, but the reductive dissolution of Fe(III) (hydr)oxides by Fe(III) reducing bacteria (FeRB) could provide a microbiological mechanism for their formation. We evaluated the susceptibility of Fe(III) deposits associated with these caves to reduction by the FeRB Shewanella oneidensis MR-1 to test this hypothesis. Canga, an Fe(III)-rich duricrust, contained poorly crystalline Fe(III) phases that were more susceptible to reduction than the Fe(III) (predominantly hematite) associated with banded iron formation (BIF), iron ore, and mine spoil. In all cases, the addition of a humic acid analogue enhanced Fe(III) reduction, presumably by shuttling electrons from S. oneidensis to Fe(III) phases. The particle size and quartz-Si content of the solids appeared to exert control on the rate and extent of Fe(III) reduction by S. oneidensis, with more bioreduction of Fe(III) associated with solid phases containing more quartz. Our results provide evidence that IOCs may be formed by the activities of Fe(III) reducing bacteria (FeRB), and the rate of this formation is dependent on the physicochemical and mineralogical characteristics of the Fe(III) phases of the surrounding rock.
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