Abstracts

The Carajs mineral province, located in northern Brazil, hosts the largest high-grade iron ore resource in the world (18 Gt @ 66 wt.% Fe). The iron ore deposits formed from banded iron formation (BIF) units of the Neoarchean Carajs Formation, and are situated on the northern (Serra Norte) and southern (Serra Sul) limbs of the WNW-striking Carajs fold.Unweathered BIF samples from two Serra Norte ore bodies (N7, N8) and an ore body from the Serra Sul (S11D), have a simple mineralogy, consisting of alternating magnetite and quartz-rich bands. This mineral assemblage is characteristic for oxide facies BIF. Sedimentation of the BIF units occurred in distal basin parts, as shown by low abundances of Al2O3, TiO2 and high field strength elements (HFSE). Seawater-like rare-earth element (REE) distribution patterns indicate a formation by marine chemical sedimentation. The absence of true Ce anomalies shows that suboxic to anoxic conditions were prevalent in the Carajs basin, at the time of BIF deposition. Unusually strong positive Eu anomalies were detected in all BIF samples. These anomalies record a high hydrothermal flux to the Carajs basin. The strictly positive Fe isotope composition of magnetite in BIF samples indicates derivation from Archean seawater by partial oxidation of aqueous Fe(II). The low variability between the Fe isotope composition of iron-rich bands at the 10-cm scale demonstrates that BIF formation took place with remarkable consistency over several thousand years. Rayleigh modeling of Fe isotope fractionation during Fe(II) oxidation suggests that BIF sedimentation occurred at low ocean temperatures.The iron ore deposits consist mainly of friable hematitegoethite ore (so-called "soft ore"). Martite, i.e. hematite pseudomorphic after magnetite, is the main constituent of soft ore. The homogenous Fe isotope composition of martite in individual samples strongly resembles the invariable Fe isotope composition of magnetite, which suggests that iron remained immobile during geological history. This argues against proposed models of hypogene iron ore formation, since hydrothermal overprint would have introduced a large scatter in the Fe isotope composition of magnetite and martite, combined with a considerable shift towards a more positive Fe isotope composition. Microplaty hematite is a minor constituent of soft ore and often occurs as crystals nucleating on martite. This observation and the homogenous Fe isotope composition of microplaty hematite and martite, detected in samples of BIF and soft ore, indicate that microplaty hematite formed by recrystallization of pre-existing martite. Pressure and temperature conditions during weathering should not allow this degree of recrystallization. Thus, martitization and microplaty hematite formation must have occurred earlier, possibly during retrograde metamorphism, when the prevalent P-T conditions shifted the stability of magnetite towards hematite. By contrast, goethite, formed in an open system during subrecent/recent weathering, has a more variable FeAdvisors/Committee Members: Lehmann, Bernd.