Comparison between the Helen Iron Formation (Algoma-type) and the Sokoman Iron Formation (Superior-type)

The Sokoman Iron Formation with an age of 1.88 Ga in the Labrador Trough and the Archean Helen Iron Formation in Wawa, Ontario (2.75 Ga) are examples of shallow and deep water iron formations respectively, which are of a low metamorphic grade. The two iron formations vary greatly in their mineralogy and geochemistry. The deep water Helen Iron Formation is dominated by siderite and quartz and provides an example of a direct precipitate from seawater which had a strong but distal hydrothermal component as indicated by the lack of Cu, Pb and Zn enrichment. It has a thickness of 100m. The first 20 meters are dominated by input of detrital material of both basaltic and felsic sources before deepening of the basin led to purely chemical sedimentation. Iron in the Helen Iron Formation occurs predominantly as Fe2+, indicating that oxygen played no role in the deposition of this iron formation. Negative carbon isotope excursions close to organic rich horizons in the Helen Iron Formation are the result of dissimilatory iron reduction. In contrast, the mineralogy of the Sokoman Iron Formation is dominated by magnetite, hematite and quartz and has a thickness of 180m. The depositional environment was variable and ranged from a fairly deep, reducing basin to shallow, oxidizing water. The original mineralogy probably consisted of a silica-iron hydroxide gel which was precipitated due to oxidation of Fe2+to Fe3+. The source of the Fe2+ was probably a distal hydrothermal input. Anoxic deep water allowed the transport of Fe2+over long distances to the site of iron formation deposition. When this deep, anoxic water mass came into contact with shallow water, which was enriched in oxygen due to the photosynthetic activity of cyanobacteria, a silica-iron hydroxide gel precipitated. Dewatering of this gel during burial caused the strong diagenetic overprint preserved in the section in which the original mineralogy is obscured. The high degree of diagenesis also explains the present low organic matter content of the Sokoman Iron Formation as well as the presence of Fe2+ minerals, since organic matter may have reacted as an electron acceptor reducing Fe3+ to Fe2+.Ph