The low-grade carbonate banded iron formation of the Aguas Claras mine, from the transition of the Caûe to the Gandarela Formation (2.4 Ga, Itabira Group, Brazil), is composed of porous and microsparitic dolomite, quartz, iron oxides and (oxy)hydroxides. Iron oxides occur as bands, veins and as inclusions in the dolomite crystals, whereas iron (oxy)hydroxides only occur as inclusions. Combined mineralogical analyses (X-ray diffraction, Raman spectroscopy, focused-ion-beam thinning and transmission electron microscopy) identified the inclusions in porous dolomite as hematite and minor goethite and/or ferrihydrite, whereas microsparitic dolomite only hosts hematite. Curie balance analysis on whole rock reveals that, at temperatures between ∼680 °C and ∼900 °C, hematite and iron (oxy)hydroxide inclusions react with the surrounding dolomite resulting in the assemblage: magnesioferrite (MgFe2O4), srebrodolskite (Ca2Fe2O5), lime (CaO), portlandite (Ca(OH)2) and periclase (MgO), whereas hematite and pure dolomite do so only at ∼900 °C. This difference is an indirect argument for the presence of iron hydroxide inclusions within the dolomite. The inclusions are either single crystals or form clusters in pores. The fast-Fourier-transform diffraction pattern of one single crystal can be indexed as goethite and ferrihydrite. This finding suggests an incipient solid-state transformation of ferrihydrite to goethite rather than a dissolution-precipitation process. It is suggested that the clustered hematite and the goethite/ferrihydrite precipitated from Fe- and Si-bearing fluid inclusions, which were trapped during early diagenesis at about 2.4 Ga under oxygenated conditions. Microstructural evidence (e.g. dislocations in dolomite from hematite inclusions) point to maximum T < 420 °C. The preservation of goethite/ferrihydrite clustered inclusions during this low-temperature event may be due to their silica contents increasing their stability.
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Research Article|
September 22, 2017
Iron (oxy)hydroxide and hematite micro- to nano-inclusions in diagenetic dolomite from a 2.4 Ga banded iron formation (Minas Gerais, Brazil)
Beate Orberger;
Beate Orberger
*
1
GEOPS, UMR 8148, Univ. Paris-Sud, CNRS, Université Paris-Saclay
, rue du Belvédère, Bât. 504-509, 91405Orsay, France
*
Corresponding author, e-mail: [email protected]
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Christiane Wagner;
Christiane Wagner
2
Sorbonne Universités, UPMC, Univ Paris 06, CNRS, Institut des Sciences de la Terre de Paris (ISTeP)
, 4 place Jussieu, 75005Paris, France
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Alina Tudryn;
Alina Tudryn
1
GEOPS, UMR 8148, Univ. Paris-Sud, CNRS, Université Paris-Saclay
, rue du Belvédère, Bât. 504-509, 91405Orsay, France
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Benoît Baptiste;
Benoît Baptiste
3
Sorbonne Universités, UPMC, Univ Paris 06, CNRS, IMPMC
, 4 place Jussieu, 75005Paris, France
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Richard Wirth;
Richard Wirth
4
Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences
, Section 3.3, Telegrafenberg, 14473Potsdam, Germany
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Rachael Morgan;
Rachael Morgan
1
GEOPS, UMR 8148, Univ. Paris-Sud, CNRS, Université Paris-Saclay
, rue du Belvédère, Bât. 504-509, 91405Orsay, France
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Serge Miska
Serge Miska
1
GEOPS, UMR 8148, Univ. Paris-Sud, CNRS, Université Paris-Saclay
, rue du Belvédère, Bât. 504-509, 91405Orsay, France
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Beate Orberger
*
1
GEOPS, UMR 8148, Univ. Paris-Sud, CNRS, Université Paris-Saclay
, rue du Belvédère, Bât. 504-509, 91405Orsay, France
Christiane Wagner
2
Sorbonne Universités, UPMC, Univ Paris 06, CNRS, Institut des Sciences de la Terre de Paris (ISTeP)
, 4 place Jussieu, 75005Paris, France
Alina Tudryn
1
GEOPS, UMR 8148, Univ. Paris-Sud, CNRS, Université Paris-Saclay
, rue du Belvédère, Bât. 504-509, 91405Orsay, France
Benoît Baptiste
3
Sorbonne Universités, UPMC, Univ Paris 06, CNRS, IMPMC
, 4 place Jussieu, 75005Paris, France
Richard Wirth
4
Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences
, Section 3.3, Telegrafenberg, 14473Potsdam, Germany
Rachael Morgan
1
GEOPS, UMR 8148, Univ. Paris-Sud, CNRS, Université Paris-Saclay
, rue du Belvédère, Bât. 504-509, 91405Orsay, France
Serge Miska
1
GEOPS, UMR 8148, Univ. Paris-Sud, CNRS, Université Paris-Saclay
, rue du Belvédère, Bât. 504-509, 91405Orsay, France
*
Corresponding author, e-mail: [email protected]
Publisher: Deutsche Mineralogische Gesellschaft, Sociedad Española de Mineralogia, Societá Italiana di Mineralogia e Petrologia, Société Francaise de Minéralogie
Received:
27 May 2016
Accepted:
16 Jun 2017
First Online:
22 Mar 2018
Online ISSN: 1617-4011
Print ISSN: 0935-1221
© 2017 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart
European Journal of Mineralogy (2017) 29 (6): 971–983.
Article history
Received:
27 May 2016
Accepted:
16 Jun 2017
First Online:
22 Mar 2018
Citation
Beate Orberger, Christiane Wagner, Alina Tudryn, Benoît Baptiste, Richard Wirth, Rachael Morgan, Serge Miska; Iron (oxy)hydroxide and hematite micro- to nano-inclusions in diagenetic dolomite from a 2.4 Ga banded iron formation (Minas Gerais, Brazil). European Journal of Mineralogy 2017;; 29 (6): 971–983. doi: https://doi.org/10.1127/ejm/2017/0029-2679
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Index Terms/Descriptors
- banded iron formations
- Brazil
- carbonate rocks
- carbonates
- chemically precipitated rocks
- diagenesis
- dolomite
- dolostone
- electron microscopy data
- ferrihydrite
- goethite
- hematite
- hydroxides
- inclusions
- iron formations
- iron hydroxides
- iron ores
- lime
- magnesioferrite
- magnetic properties
- metal ores
- microstructure
- Minas Gerais Brazil
- oxides
- oxyhydroxides
- periclase
- portlandite
- Raman spectra
- sedimentary rocks
- South America
- spectra
- TEM data
- X-ray diffraction data
- srebrodolskite
- Aguas Claras Mine
- focused ion beam
Latitude & Longitude
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