Banded iron formation

Classification：Sedimentary Rock

Chemical sedimentary rocks, mainly formed by interbedding of siliceous ribbons and rich iron ribbons, are the main source of iron.

The fine banded siliceous hematite deposits discovered in the Precambrian rocks are stratified, with a thickness of hundreds of meters, lasting up to 150 kilometers. It is mainly composed of chemical (or biochemical) sedimentary chert and one or several iron rich minerals (oxide, carbonate, silicate or sulfide) thin layer.

The thickness of the monolayer can reach several centimeters, and the inner layer is millimeter level. Between the thin layer and layer and its interior, often abrupt change and gradual change from material composition, display characteristics of graded layer. It may be due to the organic chemical action during the deposition of small debris in the stable shallow basin, so it is a syngenetic deposit. In many granulites and gneiss areas in the Archean, there are some lenticular and layered metamorphic banded iron bearing formations, which are small in scale and often symbiotic with mica schist, marble, diagonal amphibolite and parechite, and their original rocks are shallow water quartzite, argillite carbonaceous rock, metamorphic amphibolite and granulite. Lithofacies, probably formed in the early continental shelf environment, called the original superior type (proto-Superior type). The oldest strip iron formation is found in West Greenland. The surrounding rock is calcareous quartzite and amphibolite metamorphosed from greenschist facies to amphibolite facies, and the Pb-Pb age has been obtained for 3 billion 770 million years. Archean greenstone belts often produce banded iron formations, which are symbiotic with various metamorphic volcanic rocks, hard sandstone and schist and form lenticular bodies in volcanic rocks. As far as the world is concerned, Palaeoproterozoic is the peak period of the production of banded iron bearing formation, which develops in Paleoproterozoic basins or geosyncline adjacent to the boundary of Archean craton. The main development period is between 2 billion 600 million and 1 billion 800 million years, of which a considerable portion of the hematite changed into magnetite. The Proterozoic banded iron - bearing formation belongs to the Soviet prolle type, with few clastic materials, most of which are symbiotic with flint, dolomite, quartzite, charcoal black shale and argillaceous slate, less symbiotic with volcanic rocks. It extends to hundreds of kilometers and can be used for long distance comparison. The situation in China is different. From the middle of the Archean (such as the eastern Hebei region) to the new Archaean (such as the saddle and Eastern Hebei region), a huge strip of iron - bearing ore deposits is produced, showing remarkable characteristics different from the Archean geology around the world. Banded iron formation is mainly divided into two types: Algoma type and superior type. These two types of ore deposits are the most important iron ore resources in the world, such as the Hymes Leigh of Western Australia, Lake Superior in the United States, the Labrador in Canada, Russia, Ukraine and Brazil.