Abstract
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.
Highlights
Abundant karst-like features occur within Fe-rich substrate of the iron ore mining regions of Brazil
Fe(III)oxides and chert or other non-Fe phases [3]; (2) ore-grade Fe deposits that form upon depletion of gangue minerals in banded iron formation (BIF), giving rise to solids that are relatively enriched in Fe [4,5,6]; and (3) canga, which is a surficial breccia of weathered BIF and iron ore clasts that are cemented by secondary Fe(III)oxides, and quite resistant to weathering processes [7,8]
In an effort to better constrain our model for biospeleogenesis in iron ore caves” (IOCs), we evaluated the susceptibility of these different Fe(III) phases to reductive dissolution by the Fe(III) reducing bacteria (FeRB) Shewanella oneidensis MR-1
Summary
Abundant karst-like features occur within Fe-rich substrate of the iron ore mining regions of Brazil (primarily the Carajás region in Paráand the “Iron Quadrangle” region in Minas Gerais). This includes over 3000 caves, referred to as “iron ore caves” (IOCs; Figure 1) [1,2]. Fe(III) (hydr)oxides and chert or other non-Fe phases [3]; (2) ore-grade Fe deposits that form upon depletion of gangue minerals in BIFs (e.g., silica layers), giving rise to solids that are relatively enriched in Fe [4,5,6]; and (3) canga, which is a surficial breccia of weathered BIF and iron ore clasts that are cemented by secondary Fe(III) (hydr)oxides, and quite resistant to weathering processes [7,8]. IOCs are rare in other major iron ore-producing regions of the world, such as the Lake Superior Iron Range and Sokoman Iron Formation of North America, the iron ore ranges of western Australia and South Africa, and the “Kursk Magnetic Anomaly” of Russia
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