Carlos Alberto Rosière

Aerobic bacterial pyrite oxidation and acid rock drainage during the Great Oxidation Event

The enrichment of redox-sensitive trace metals in ancient marine sedimentary rocks has been used to determine the timing of the oxidation of the Earth’s land surface. Chromium (Cr) is among the emerging proxies for tracking the effects of atmospheric oxygenation on continental weathering; this is because its supply to the oceans is dominated by terrestrial processes that can be recorded in the Cr isotope composition of Precambrian iron formations. However, the factors controlling past and present seawater Cr isotope composition are poorly understood. Here we provide an independent and complementary record of marine Cr supply, in the form of Cr concentrations and authigenic enrichment in iron-rich sedimentary rocks. Our data suggest that Cr was largely immobile on land until around 2.48 Gyr ago, but within the 160 Myr that followed—and synchronous with independent evidence for oxygenation associated with the Great Oxidation Event (see, for example, refs 4–6)—marked excursions in Cr content and Cr/Ti ratios indicate that Cr was solubilized at a scale unrivalled in history. As Cr isotope fractionations at that time were muted, Cr must have been mobilized predominantly in reduced, Cr(iii), form. We demonstrate that only the oxidation of an abundant and previously stable crustal pyrite reservoir by aerobic-respiring, chemolithoautotrophic bacteria could have generated the degree of acidity required to solubilize Cr(iii) from ultramafic source rocks and residual soils. This profound shift in weathering regimes beginning at 2.48 Gyr ago constitutes the earliest known geochemical evidence for acidophilic aerobes and the resulting acid rock drainage, and accounts for independent evidence of an increased supply of dissolved sulphate and sulphide-hosted trace elements to the oceans around that time. Our model adds to amassing evidence that the Archaean-Palaeoproterozoic boundary was marked by a substantial shift in terrestrial geochemistry and biology.

The tectonic evolution of the Quadrilátero Ferrífero, Minas Gerais, Brazil

Abstract The structural framework of the Quadrilatero Ferrifero is the result of two main deformational events. The first is an extensional event of Palaeoproterozoic age (between 2 100 and 1700 Ma, the Transamazonian Event), which resulted in the formation of Archaean granite-gneiss domes and the nucleation of regional synclines in the overlying Rio das Velhas and Minas supergroups strata. Such regional synclines are continuous with each other and are bordered by structural granite-gneiss highs. At the contact between these units, ductile-brittle to ductile extensional shear zones are developed, representing the dislocation surfaces of Archaean blocks. This tectonism is probably related to the evolution of a metamorphic core complex, formed in the hinterland to the west of a Transamazonian collision zone, which structured the Paramirim Craton. The second event is compressive and associated with the closure of the Pan-African/Brasiliano proto-ocean (650–500 Ma), situated to the east of Quadrilatero Ferrffero (QF). During this event, a west-verging fold-and-thrust belt (FTB) developed, causing inversion, amplification, translation and rotation of the basinal synclines. The FTB affected mainly the eastern portion of QF and obliterated many of the tectonic features of the extensional event.

Microstructures, textures and deformation mechanisms in hematite

Abstract Deformation and recrystallization of hematite in iron formations and high-grade ore bodies of the Caue Formation from the Quadrilatero Ferrifero District, Minas Gerais, Brazil, resulted in several types of fabrics, depending on the tectonic and metamorphic imprint. The fabrics vary from randomly oriented granoblastic to strongly oriented lepidoblastic/mylonitic with local development of cataclastic features. Complete pole figures of the crystallographic planes (003)-basal plane; {110}-prism and {104}-rhombohedron were measured on several samples from different areas by means of neutron diffraction revealing a typical crystallographic preferred orientation for the different fabric types. The configuration and intensity of the maxima are directly related to the magnitude of the deformation and the type of response of the ore. The analysis of the textures and microstructures of the iron ores and comparison with experimental deformation data lead us to the conclusion that three main mechanisms are active: basal slip, diffusion processes and anisotropic grain growth. Post-tectonic recrystallization and secondary grain growth did not affect the textures developed during deformation.

Crystallographic and magnetic preferred orientation of hematite in banded iron ores

Abstract The crystallographic preferred orientation of hematite in banded iron ores and the orientation of both the measured and the calculated principal susceptibility axes are strongly related. The maximum susceptibility is aligned with the lineation and the pole of the foliation coincides with the minimum susceptibility, although there are often distinct differences between the measured and calculated values of the susceptibilities. A wide variety of configurations of c-axis pole figures modeled by varying the parameters of the Bingham distribution and Bingham–Mardia-distribution reveal that quite different c-axis patterns of hematite ores may have the same anisotropy of the magnetic susceptibility (AMS) parameters. Large deviations between calculated and experimental AMS-data should initiate further investigations to resolve a probably unnoticed heterogeneity of the fabric. The present investigations show that the structural analysis of the preferred orientation of hematite ores by means of the rather inexpensive and fast magnetic method must be accompanied by the more expensive but unambiguous determination of preferred orientation by x-ray and neutron diffraction experiments in order to accomplish a complete and sound interpretation.

Controversy in genetic models for Proterozoic high-grade, banded iron formation (BIF)-related iron deposits - unifying or discrete model(s)?

Abstract A genetic model for the genesis of high-grade (>65 wt-%Fe) Proterozoic, banded iron formation (BIF)-related iron deposits such as the Tom Price, Mount Whaleback deposits in the Hamersley Province (Australia), the N4E, N4W and N5 deposits at Carajás (Brazil), the Aguas Claras, Conceição and Casa de Pedra deposits in the Iron Quadrangle (Brazil), the Donkerpoort West, Kwagas East deposit in the Thabazimbi iron ore district (South Africa) or the Saksagan ore field at Krivoy Rog (Ukraine), needs to take into account the: (a) timing of regional metamorphism with respect to the upgrade of BIF to high-grade iron ore; (b) significance and relationship of the protoore to the hydrothermal alteration minerals and zonation and the process of iron enrichment; (c) source and oxidation state of the hydrothermal fluids that caused the iron enrichment; (d) types of geochemical processes that caused the hydrothermal alteration and iron enrichment; and (e) significance of the widespread carbonatisation of the host rocks as a prerequisite for high-grade BIF-related iron mineralisation. We argue that a unifying model for high-grade BIF-related iron deposits cannot, presently, adequately explain the geological and geochemical characteristics observed in these deposits and therefore provide two end-member models that take into account the diversity of geological and geochemical ore deposit features. Our discrete model for the genesis of high-grade, BIF-related iron deposits is based on the interpreted tectonic setting, distinct hydrothermal fluid source of, and processes observed in, major BIF-related iron deposits. We divide them into two end-members: Proterozoic deep fault-magmatic (Carajás) type and Proterozoic rift-basin (Hamersley) type. Both end-members experienced varying degrees of deep weathering episodes that effected the hypogene enrichment and ultimately, yielded the high-grade iron ore mined in many places today.

Fabric, Texture and Anisotropy of Magnetic Susceptibility in High-Grade Iron Ores from the Quadrilátero Ferrifero, Minas Gerais, Brazil

The iron protores of the Quadrilatero Ferrifero in Minas Gerais, Brazil, are quartz- or carbonate-banded laminated rocks (itabirite) which still display primary features similar to those encountered in non-metamorphic jaspilites, such as sedimentary layering, slump structures and porous granoblastic martite fabric with relictic kenomagnetite. Synmetamorphic deformation initiated recrystallization of the platy hematite variety (specularite) and by this gave rise to a variety of secondary grain fabrics and textures portrayed by the intensity of magnetic (low-field) susceptibility and by the geometry of corresponding anisotropy ellipsoids. Folded ores usually have a foliation roughly parallel to the compositional banding, an axial plane schistosity, lepidogranoblastic fabric and textures dominantly resulting from flexural slip. The stereogram of the corresponding (11.0) texture shows a single maximum parallel to the intersection line of banding and schistosity. The AMS is generally low; its ellipsoid is triaxial. Increasing strain and shearing obliterates the primary layering and induced the formation of ore mylonites with lepidoblastic (S-tectonites) or nematoblastic (L-tectonites) grain fabrics which results in varying shapes of the AMS ellipsoids. Schistose ores, finally, have girdle arrangements of hematite-(11.0) poles, high susceptibilities and oblate shapes of the AMS ellipsoids, whereas strongly lineated ores display a single maximum parallel to the stretching direction, low to moderate susceptibilities and AMS ellipsoids with prolate shape. Secondary recrystallization and grain growth does not affect the lattice-preferred orientation.

Development of a recrystallized grain size piezometer for hematite based on high-temperature torsion experiments

A series of hematite samples (>98 % Fe 2 O 3, Sishen Mine, South Africa) with different initial grain sizes of −5 s −1 and 4.7 × 10 −5 s −1 , resulting in finite shear strains between 0.4 and 4.7 and finite shear strengths in the range of 32–326 MPa. The data obtained at high shear strain (>3) fit to a power law creep equation with a pre-exponential constant of lnA = −5.03 ± 0.61 MPa −2.5 s −1 , an activation energy of Q = 249 ± 48 kJ mol −1 , and a stress exponent of n = 2.5 ± 0.5, indicating dislocation creep partially assisted by grain boundary sliding as the main deformation mechanism. Irrespective of the initial grain size, the final grain size ( D ) of fully recrystallized samples decreases continuously with increasing (steady state) equivalent stress (σ), yielding a piezometric relationship of the form: D = 1055 × σ 1.03 ± 0.10 . The piezometer is applied to itabiritic hematite ores of the Iron Quadrangle, Minas Gerais, Brazil, consisting mainly of layered hematite and quartz. Compared to hematite the measured recrystallized grain size of quartz is always larger, which is in agreement with the grain size range predicted by recrystallized grain size-based piezometers for hematite and quartz if deformed at similar stress below about 100 MPa.

Iron isotope and REE+Y composition of the Cauê banded iron formation and related iron ores of the Quadrilátero Ferrífero, Brazil

The Minas Supergroup banded iron formations (BIFs) of the Brazilian Quadrilátero Ferrífero (QF) mineral province experienced multiple deformational events synchronous with hypogene mineralization, which resulted in the metamorphism of BIFs to itabirites and their upgrade to high-grade iron ore. Here, we present rare earth element and yttrium (REE+Y) compositions together with iron isotope ratios of itabirites and their host iron orebodies from 10 iron deposits to constrain environmental conditions during BIF deposition and the effects of hypogene iron enrichment. The REE+Y characteristics of itabirites (positive Eu anomaly and LREE depletion) indicate hydrothermal iron contribution to the Minas basin. Iron isotope data and Ce anomalies suggest BIFs were precipitated by a combination of anoxic biological-mediated ferrous iron oxidation and abiotic oxidation in an environment with free oxygen (such as an oxygen oasis), perhaps related to increase in oxygen concentrations before the Great Oxidation Event (GOE). The similarity of the REE+Y composition of the itabirites from the different QF deformational domains, as well as to other Superior-type BIFs, indicates that the metamorphism and synchronous hydrothermal mineralization did not significantly affect the geochemical signature of the original BIFs. However, iron isotope compositions of iron ore vary systematically between deformational domains of the QF, likely reflecting the specific mineralization features in each domain.

Avaliação técnica de minérios de ferro para sinterização nas siderúrgicas e minerações brasileiras: uma análise crítica

This paper provides a critical analysis of the methodology used in Brazil for routine technical evaluation of iron ore fines used in the sintering process based uniquely on chemical and granulometric parameters. The mineralogical characterization of adherents, intermediates and nucleantes particles of sinter feed and the main attributes that should constitute its microstrutural identity have been highlighted. It should contribute a great deal to optimize the process parameters during the various stages of the sintering process as well as promote better intrinsic sinter quality. Based on these concepts, it is expected that in future years, significant criterias will be developed for technical analysis of iron ores fines used in agglomeration industry.

A estrutura do segmento oeste da Serra do Curral, Quadrilátero Ferrífero, e o controle tectônico das acumulações compactas de alto teor em Fe

The structure of western Serra do Curral, Quadrilátero Ferrífero, represents the partial exposure of a syncline that verges to NNW – Piedade Syncline. This regional structure has parasitic folds denominated as B1A which are refolded by folds B1B that also have vergence to NNW. Both of them have ENE-WSW direction, are non-cylindrical folds and configure an interference pattern that is similar to Type 3 from Ramsay. This refolding occur during progressive deformation. The occurrence of highgrade magnetitic-martitic accumulation (> 64% Fe) of medium size (~ 100 Mt) is controlled by the combination of these folds and high angle thrust fault, both with the same direction. Above these structures, there is a development of folds with NNWSSE direction and WSW vergence (B2 folds). These folds are restricted to the B1 limbs and they configure an interference pattern named “abbuting fold”. The age obtained by U/Pb SHRIMP on monazite grains is 2034 ± 11 Ma. The monazite grains occur on textural balance with the iron oxide from high-grade accumulations. This age corresponds to the formation timing of the high-grade bodies and suggests that the mineralization process is contemporary to the regional metamorphic peak of Quadrilátero Ferrífero.