Precambrian iron formations contain both banded and oolitic ores. Associated rocks include argillaceous and arenaceous beds, and volcanics. Detailed microscopic study suggests that ores were deposited as gelatinous iron hydroxides, iron-aluminum silicates, iron carbonate, and clastic quartz. Both shallow-and deeper-water environments are recognized. Diagenesis and metamorphism produced goethite, lepidocrocite, hematite, magnetite, chamosite, kaolinite, prochlorite, and ankerite. On the assumption that the atmosphere was oxidizing, solutions from terrestrial weathering are considered an inadequate source of dissolved iron, and a theory of submarine volcanic exhalations is favored.