PRECAMBRIAN BANDED IRON-FORMATIONS (BIFS) WORLD-WIDE: THEIR GEOLOGIC SETTING, MINERALOGY, METAMORPHISM, AND ORIGIN

BIFs are the diagenetic product of a chemical precipitate in a very iron-rich-system: SiO₂-FeO-Fe₂O₃-CaO-MgO-CO₂-H₂O (with minor Al₂O₃-Na₂O and variable MnO). They occur in the geologic record from 3.8 Ga (Isua, West Greenland) to about 1.8 Ga with a maximal abundance at about 2.5 Ga, and a reoccurrence in Neoproterozoic time (from about 0.8 and 0.6 Ga). Many of the 3.8 to 1.6 Ga BIFs have very similar average chemistries and the late diagenetic assemblages consist of chert, magnetite, hematite, greenalite, stilpnomelane, minnesotaite and carbonates (siderite, dolomite to ankerite, calcite). Regional metamorphism results in assemblages rich in various amphiboles, and at higher grades, various pyroxenes. Several major BIFs in Brazil with an age of about 2.4 Ga are much richer in Fe³⁺ (almost all hematite-rich) than normal, possibly as a result of deep weathering and secondary oxidation. The Neoproterozoic BIFs are chemically distinctly different from most others because about 95% of their total iron is Fe³⁺.

All BIFs between 3.8 and 1.8 Ga show REE patterns with pronounced positive Eu anomalies, negative Ce anomalies and depletion in the light REE. These patterns are the result of chemical precipitation from solutions that represent mixtures of seawater and hydrothermal input (of Fe and Si) from spreading centers in oceanic crust. The Neoproterozoic BIFs (e.g., Rapitan iron-formation, Yukon) display a lack of the Eu anomaly, and their overall REE pattern is very similar to that of modern ocean water at 100 m. This suggests that the hydrothermal input was highly diluted by ocean water at this late Precambrian time. The Neoproterozoic iron-formations commonly show a close association with glaciogenic deposits. The Rapitan BIF is interpreted as having been deposited during a major transgressive event with a rapid sea-level rise during an interglacial period, after earlier buildup of ferrous iron in solution in deeper water during a glacial period.