Local metamorphism, probably of the hydrothermal type, adjacent to dikes, sills, quartzadularia pegmatite veins, quartz-tourmaline veins, and quartz veins has induced secondary iron silicates such as grunerite, minnesotaite, and stilpmomelane to form at the expense of the primary minerals, chert, siderite, and greenalite. The elimination of chert through the development of iron silicates is favored by an original high iron content of the rock, by primary textures which allow the reacting minerals to be in close proximity, by the formation of secondary low iron and high silica minerals, and by the introduction of iron.
Oxidation of either the original siderite or greenalite iron formation or a facies composed of secondary iron silicates and chert produces the ordinary red or yellow cherty iron formation. The soft ores, both limonite and hematite, result from the oxidation and leaching of that facies in which the chert has been largely or completely combined with iron to form iron silicates. Chert has not been removed as such in the quantities hitherto considered necessary for the formation of ore.