BANDED IRON FORMATIONS AS GUIDES FOR THE HISTORY OF THE LITHOSPHERE, ATMOSPHERE, AND HYDROSPHERE

Algoma-type BIFs have similar mineralogy, paragenesis, and geochemistry to Lake Superior-type BIFs, and they occur in geologic formations of all ages. Therefore, BIFs must have formed in similar geochemical environments and by the same geochemical processes through geologic history. BIFs, like volcanogenic massive sulfide deposits (VMS), accumulated in seafloor depressions by two concurrent processes: (i) the precipitation of fine-grained minerals by mixing locally discharged hydrothermal fluids with local seawater, and (ii) the transformation of earlier minerals (e.g., goethite/hematite to magnetite) by reactions with later hydrothermal fluids. Submarine hydrothermal fluids were products of the seawater-rock interaction.

The tectonic settings and thermal structure of hydrothermal systems for Lake Superior-type BIFs were probably similar to those for sediment-hosted massive sulfide and Mississippi Valley-type deposits; they accrued in intra-continental basins and their heat sources were probably deep-seated mantle plumes. In contrast, Algoma-type BIFs and VMS mostly deposited in deep (>2 km) basins that developed in back-arc or intra-continental rift systems; their heat sources were probably shallow-seated magmas and intrusives. The higher abundance and general larger size of Archean BIFs to younger BIFs probably reflects higher heat flow in the Archean crust-mantle system rather than a difference in atmospheric chemistry.

Differences in the morphology (e.g., dimensions) and mineralogy (e.g., oxides, carbonates, sulfides) of BIFs and VMS were most likely generated by varying relative concentrations of: (a) Fe2+ and, (b) H2S in hydrothermal fluid, and (c) O2 (aq), (d) CO2 (+ HCO3-), and (e) H2S (+ HS-) in local seawater. While (a) and (b) were largely controlled by the thermal structure of a hydrothermal plumbing system, (c), (d), and (e) were greatly influenced by atmospheric pO2 and pCO2 values and by the hydrological regimes of depositional basins (e.g., restricted vs. open; stratified vs. non-stratified). For example, lower O2 levels in local seawater would tend to spread oxide BIFs thinly and widely. Siderite BIFs are favored in evaporitic basins under a high pCO2 atmosphere. The common occurrence of ferric (hydr)oxides as primary minerals in Algoma-type BIFs suggests the atmosphere-ocean system has been basically oxic since ~3.8 Ga ago.