HEMATITE AND GOETHITE INCLUSIONS IN LOW GRADE DOLOMITIC BANDED IRON
FORMATIONS: MAGNETISATION DURING HEATING EXPERIMENTS AND ITS
IMPLICATIONS FOR ORE BENEFICIATION
Beate Orberger1, Alina Tydryn1, Christiane Wagner2, Benoît Baptiste3, Richard Wirth4, Rachael Morgan1,
Serge Miska1
1
Université Paris-Sud, Laboratoire GEOPS, UMR 8148 (CNRS-UPS), Bât. 504, 91405 Orsay, France;
beate.orberger1@orange.fr
2
Sorbonne Universités, UPMC, Univ Paris 06, CNRS, ISTeP, 4 Place Jussieu, F-75005, Paris, France.
3
Sorbonne Universités, UPMC, Univ Paris 06, CNRS, IMPMC, 4 Place Jussieu, F-75005, Paris, France.
4
Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 3.3, Telegrafenberg,
14473-Potsdam, Germany
Banded iron formations comprise complex textures and mineralogies, which result from fluid-rock interaction
related to high and low temperature alteration. The initial ironhydroxyde mineralogy and associated phases such
as carbonates, quartz, apatite and phyllosilicates were transformed leading to an upgrading of these BIF’s into
the world’s largest source of iron ore. In low grade BIFs, a large part of the Fe is related to nanometric iron
bearing inclusions within micrometric quartz and/or carbonates (mainly dolomite). We studied laminated BIF
samples from a drill core containing 27 wt.% Fe2O3, 0.2 wt.% SiO2, 0.32 wt.% MnO, 15.46 wt.% MgO, 22.32
wt.% CaO, 0.09 wt. % P2O5, 0.15 wt. % H2O and 34.08 wt. % CO2 (Àguas Claras Mine, Quadrilátero Ferrífero,
Brazil). Bright rose coloured dolomite and quartz bands alternate with massive specular hematite bands. Ramanspectroscopy, X-ray diffraction and FIB-TEM analyses reveal that the nanoinclusions in dolomite are mainly
hematite and minor goethite, partly occurring as clusters, which are derived from fluid inclusions (Fig. 1). Curie
Balance analyses were carried out at different heating steps and temperatures on whole rock samples and a
synthetic mix of decarbonated sample and pure dolomite. X-ray diffraction (Co- and Cu-tube) show that new
phases: lime (CaO), periclase (MgO), portlandite (Ca(OH) 2), magnesioferrite (MgFe204) and srebrodoskite
(Ca2Fe2O5) were formed when samples heated until 920°C. These new sample parametres: newly formed
strongly magnetic magnesioferrite accompanied by a textural modification can be used for an optimized
metallurgical process design.
Keywords: iron ore, banded iron formations, hematite, magnesioferrite, srebrodoskite, magnetic behavior,
ironoxide inclusions, carbonates
Figure 1: TEM images of the dolomite band A) HAADF overview of the dolomite foil showing low angle grain
boundary between two dolomite crystals (seen in middle) with inclusions of euhedral hematite (he) and clusters
of nanometric goethite and ferrihydrite in the porous dolomite; B) EDX analysis on the HAADF image of the
dolomite composition. The Cu signal is from the copper grid whilst the Ga signal results from the FIB-TEM
preparation; C. Bright field image of the euhedral hematite showing its heterogeneous contrast due to high
dislocation density; D. Bright field image of idiomorphic goethite in a cluster. Circular decomposition structures
caused by the electron beam are visible in the lath (inset); E and F. Clusters of iron hydroxides as a pore filling,
most likely derived from a fluid inclusion; G. A Fast Fourier Transform (FFT) diffraction pattern of D. From the
pattern it is not clear to identify goethite or ferrihydrite, however, idiomorphic shapes indicate goethite rather
ferrihydrite (the latter is poorly crystalline); H. EDS spectra of goethite indicating the presence of Si, which
retards its transformation into hematite.