SIDERITE IS AN IDEAL PRECURSOR FOR LARGE ACCUMULATIONS OF IRON OXIDE CEMENT

Iron oxide mineralization in sandstones produces outcrops and rocks that are spectacular visually. Many of these accumulations are characterized by thick bands of Fe-oxide cement that occur as rinded spherical or box-like concretions, or continuous, subparallel bands. We have concluded that these accumulations are the products of evolving fluid-rock-microbe systems. The presence of concentrations of Fe-oxide cement in sandstone requires that Fe was transported in aqueous solution. Meter-scale Fe transport apparently occurred under reducing conditions during the formation of a reduced Fe precursor (typically siderite). The precursor later dissolved and the ferrous Fe was oxidized by Fe-oxidizing microbes. The geometry of many of the accumulations is evidence that Fe transport occurred at cm to dm-scales during production of Fe-oxide mineralization.

Why is siderite (rather than pyrite or another Fe carbonate) the preferred precursor for the formation of thick bands of Fe-oxide cement? Conditions at the Earth’s surface have been hostile to both siderite and microaerophilic, neutrophilic Fe-oxidizing microbes since the development of an oxygenated atmosphere. Invasion of siderite-mineralized aquifers by oxygenated groundwaters allows microbes to begin oxidation of Fe²⁺ under conditions of low dissolved O₂. Microaerophilic Fe-oxidizing bacteria utilize the ferrous Fe as an electron donor at a redox gradient located mm to cm from the siderite-solution interface. Carbonates that contain less Fe than ankerite will oxidize abiotically upon exposure to oxygenated waters but are apparently not suitable substrates for Fe-oxidizing microbes. The subsequent precipitation of Fe-oxide during hydrolysis 1)generates the bands of Fe-oxide cement that characterize many of these accumulations, 2)generates acid that promotes siderite dissolution and 3)helps to maintain a strong gradient in Fe²⁺ concentration from siderite mineral surface to the redox boundary. Oxidation of pyrite occurs when electrons are transferred from pyrite directly to aqueous Fe³+ in contact with the mineral surface. Precipitation of ferric oxide depresses the activity of Fe³⁺, the species needed to remove electrons from the pyrite and to effect oxidation.