Petrografia, geoquímica e quimioestratigrafia isotópica de C, O, Fe e Cr da Formação Ferrífera Cauê na região de Alegria, Quadrilátero Ferrífero, Brasil

Abstract

A great volume of Banded Iron Formations (BIF) was deposited during the first Great Oxygenation Event (GOE) around 2.45 Ga, for example the Minas Supergroup BIF, Quadrilátero Ferrífero (QF). Carbonatic (dolomitic/ankeritic) BIF samples were collected from a drillhole core ca. 600m deep in the Alegria Deposit. These samples are unique as they represent the Cauê Formation least deformed and metamorphosed section studied so far, presenting no sign of hydrothermal alteration, furnishing then important information on iron deposition in the basin. The variable distribution of the main mineral phases identifies a gradual variation between four main lithofacies: (1) carbonate-magnetite BIF, (2) carbonate-magnetitequartz BIF, (3) carbonate BIF and (4) magnetite BIF. Although those facies are interleaved throughout the drillhole core, there is a clear predominance of carbonate-rich facies above the 535 meters and magnetite-rich facies below that. In general, REE-Y suggest a low clastic contamination (Y/Ho = 37.74) and geochemical signatures close to seawater (GdPAAS/YbPAAS<1, LaPAAS/YbPAAS<1), with positive La and Y anomalies. No true Ce anomalies are found. The mean Eu/Eu* value is 1.62, close to the Paleoproterozoic BIF mean. U/Th ratios are very low above the 535m (< ca. 1), but show a sharp increasing-downwards pattern below this mark, up to ca. 15. Values of δ 13C are generally between -6.99‰ and -3.40‰ while δ 18O values are generally between -19.95‰ and -15.87‰, but in the 540-560 meters interval they are distinctively lower, reaching of δ 13C = -9.95‰ and δ 18O = -24.60‰. Values of δ 56Fe are mostly positive, between 0.5 and 1.3; the higher values are attained in the same 540-560 meters interval. Authigenic δ 53Cr values are between 0.08‰ and 0.26‰, thus showing small positive fractionation with respect to the main silicate reservoirs. The δ 53Cr values show an increasing-downward pattern which mirrors the U/Th variation. Associated with the positive δ 56Fe, the δ 13C and δ 18O values suggest that authigenic processes must have played an important role in the carbon, oxygen and iron isotopic fractionation and in the dolomite and ankerite formation, excluding the hypothesis of a hydrothermal origin. A possible explanation for both the low δ 13C and the high δ 56Fe is the microorganisms actuation in the dissimilatory iron reduction (DIR) of the ferric oxides in the sedimentary pile. Thus, we put forward a model for deposition of the Cauê BIF in the Alegria Deposit which starts with the generation of Fe(II)aq through hydrothermal input in the deeper basin, which upon reaching a chemocline in the platform border, oxidizes to Fe(III) and settles as iron(oxy)hydroxides to the basin floor. When reaching the soft sediment layer, Fe(III) is then reduced by microbial activity to generate magnetite and iron-rich carbonates with negative δ 13C and positive δ 56Fe. An important issue is how U(VI) and Cr(VI) were mobilized to the basin to form the positive U/Th and δ 53Cr excursions below 535m. If those elements are admitted to be available through oxidative continental weathering, then the basin could be stratified with a shallow mildly oxygenated water layer. Oxygenation conditions were, in this case, not high enough to produce Ce cycling, although enough to produce U(VI) and Cr(VI). An alternative is that U(VI) and Cr(VI) became available through local oxygenation in continental microenvironments such as biological soil crust or microbial mats, which could provide local mobilization. In this case, there is no need for free atmospheric oxygen, neither for a stratified ocean. Our results, together with data for similar age BIF sequences around the world (e.g. Kuruman, South Africa and Hamersley, Australia), suggest that microbial-mediated DIR was a common and widespread mechanism on the genesis of BIF in the Archean/Paleoproterozoic transition.