Geochemical Evolution of the Banded Iron Formations of the Voronezh Crystalline Massif in the Early Precambrian: Sources of Matter and Geochronological Constraints

摘要

The banded iron formations (BIFs) of the Voronezh Crystalline Massif occur at three stratigraphic levels: Mesoarchean, Neoarchean, and Paleoproterozoic. In comparison with Paleoproterozoic BIFs, the Archean BIFs are enriched in TiO2, Al2O3, Cr, Ni, and REEs. All the BIFs are characterized by positive Eu anomalies, absence of Ce anomalies, and predominance of HREEs over LREEs. The Paleoproterozoic BIFs show no evidence of clastic or hydrothermal contamination. The low Ni/Fe ratios indicate that the BIFs are younger than 2.7 Ga and their formation was followed by a sharp drop of the level of the mantle Ni supply. On the other hand, very low (<1 ppm) U contents indicate the upper age of iron accumulation-no later than the Great Oxidation Event of similar to 2.47 Ga.