Major element distribution in Archean banded iron formation (BIF): influence of metamorphic differentiation

The Archean (2.8 Ga) Banded Iron Formation (BIF) of the Bell Lake region of Yellowknife greenstone belt, Canada is recrystallized to metamorphic assemblages of the amphibolite facies. This BIF is characterized by centimetre-scale Fe-rich and Si-rich mesobands. In the Si-rich mesobands, thin layers of magnetite microbands are developed in a quartz matrix. The Fe-rich mesobands are composed mainly of Ca-amphibole (hornblende), Fe–Mg amphibole (grunerite), and magnetite. The metamorphic foliation locally cuts across the mesoband boundaries, indicating the mesobanding was formed prior to peak metamorphism. Variations in mineral modal proportions between Fe-rich mesobands and microbands are diagnostic of depositional compositional differences between beds. Micro-X-ray fluorescence imaging reveals metamorphic differentiation within Fe-rich mesobands, with segregation of Fe–Mg amphibole, and the incompatible element Mn is concentrated at the margins of the Fe-rich mesobands during the amphibole-forming reactions. Ti was relatively immobile during metamorphic segregation and its distribution provides a record of the original structures in the Fe-rich mesobands.