Development of magnetite in iron formations of the Lake Superior region

Petrographic study indicates that siderite, some chert, greenalite, and fine-grained hematite--minerals commonly regarded as sedimentary--had original grain sizes that were very small (less than 30 microns). The grains define or are closely related to primary features such as bedding and granules. Magnetite has a wide range in grain size (1 to 100 microns), commonly bears no relation to granule boundaries, and exhibits replacement textures and cross-cutting relationships. Development of magnetite and associated minnesotaite and stilpromelane tends to destroy fine laminations and obliterate granule texture. Most magnetite is, therefore, believed to have formed by oxidation of primary siderite and greenalite during low-grade metamorphism.