Chapter 10 proterozoic iron-formations

Although many Proterozoic iron-formations have undergone various degrees of metamorphism, most of the interpretations in this overview are based on data derived from iron-formations that have undergone only low-grade metamorphic (lower greenschist) facies conditions. These relatively unmetamorphosed iron-formations consist mainly of chert, magnetite, various carbonates (siderite, members of the dolomite-ankerite series, and calcite), hematite, and silicates, such as greenalite, stilpnomelane, minnesotaite, and riebeckite. The mineralogy of iron-formations as a function of diagenesis and metamorphism is reviewed in the chapter. Several well-known Proterozoic iron-formations have undergone only lowgrade (greenschist) metamorphism, whereas others show a considerable range in metamorphic grade. Very-low-grade metamorphic mineral assemblages make up most of the iron-formation lithologies of the Hamersley Basin. Medium-grade metamorphic assemblages of iron-formation are characterized by the common development of amphiboles, mainly members of the cummingtonite–grunerite series. High-grade metamorphic iron-formations consist of essentially anhydrous assemblages in which variable amounts of ortho- and clinopyroxene predominate. Fayalite may be present, as well as carbonates and garnet, and lesser amounts of amphiboles; quartz, magnetite, and/or hematite are the major constituents of oxide-rich iron-formations.