Ironstones, Iron Formations, and Iron Ore

Abstract

Ironstones and iron formations are iron-rich chemical precipitates from the ocean that were deposited episodically throughout Earth's history, displaying different sedimentological and geochemical characteristics through time. The variety of iron formations found throughout Earth history is a result of changes in iron sources, plate tectonics, depositional setting, and oceanic and atmospheric redox state. The iron source for the Precambrian iron formations was submarine hydrothermal activity in anoxic basins and oceans, whereas that for Phanerozoic ironstones is poorly constrained being either submarine volcanism during short periods of ocean anoxia or intense chemical weathering on the continents; either source could have been associated with and triggered by the emplacement of Large Igneous Provinces. Precambrian iron formations deposited below the storm wave-base display a characteristic banding and are called banded iron formations, while those deposited above the fair-weather wave-base consist of iron-oxide granules cemented by chert and are called granular iron formations. Phanerozoic ironstones were deposited above the fair-weather wave-base and consist of iron-oxide ooids cemented by iron silicates or carbonates. Even after a century of research on iron formations, we still do not know the identity of their original mineral precursor. Several have been suggested: Fe-(oxyhydr)oxides, Fe-silicates, green rust, and Fe-carbonates. The identity of the mineral precursor has a strong bearing on models proposed to explain iron formation precipitation: biological and non-biological iron oxidation for oxides and non-redox controlled precipitation of iron for Fe-carbonates, Fe-silicates, and green rust. This issue is critical, because the use of iron formations and ironstones to interpret seawater chemistry and redox state of ancient oceans depends on the inferred mineral precursor and mechanism for iron precipitation. Many post-depositional factors have influenced and changed iron formation mineralogy and geochemistry since they were deposited. Tectonic (e.g., orogenic) events are capable of introducing hydrothermal fluids along faults and shears from deep to shallow crustal levels, and thus upgrading iron formations by further concentrating the iron minerals. Ultimately, Iron formations and ironstone upgraded by hydrothermal leaching of silica and carbonates to iron ore are primary exploration targets for iron resources.