295 D22. From Processes to Stratigraphy

Wednesday, 25 October 2017
8:00 AM - 12:00 PM
Ballroom 6C

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

Paper number: 295-6
Program: Technical Sessions
Time: 9:15 AM – 9:30 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-308449

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