Sedimentary iron ores

Sedimentary iron ores worth mining comprise about 80% of the world s Fe-ore production and ca 90% of the world s reserves. To be of value, ores should contain at least 0.6 g g Fe, preferably in the form of Fe oxides. Because hematite and magnetite contain more Fe per unit weight than goethite, ores in which the two former oxides predominate, are preferred. Table 15.3 summarizes the main characteristics of sedimentary iron ores. 

Genetic type Fe source Texture Main minerals Geological environment Type name 

Hydrothermal- sedimentary hypogene banded Mt, Hm Greenstone belts banded iron formations Algona 

Marine- sedimentary banded Mt, Hm Miogeosyndine Lower Proterozoicum Superior 

Terrestrial- sedimentary massive, earthy Gt limnic Amberg 

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Bodies of Water and the Chemical Sediments ,— The chemistry of the deposition of salts from sea-water has already been made the subject of special research, and van t Hoff s results in this field are already familiar. The deposition of calcium carbonate awaits a similar thorough study. Allied questions are the formation of dolomite, the deposition of various salts from inclosed bodies of water, the deposition of phosphate rocks, the precipitation of colloidal suspensions of clay and other substances, and the origin of the great deposits of sedimentary iron ore. 

Our knowledge about the forms and genesis of neoformed Fe " oxides in sediments and sedimentary rocks is still rather limited because the low concentrations and poor crystallinity of these oxides hinder their identification and description. Two groups of Fe-containing sediments, whose Fe oxides have, however, attracted more interest than usual, are red beds and sedimentary iron ores (Fiichtbauer, 1988). 

Tab. 15.3 Types of sedimentary iron ores (modified from Fuchtbauer, 1988 with permission)...

Mvumorphic Ores. These ores include sedimentary iron-ore deposits, which have been metamorphosed, as well as ores associated with mctamorphic rocks, m which the origin of the ore is obscured by recrystallizaiion. Essentially all of the Pre-Cambrian sedimentary iron formations are of this type. 

Butuzova, G.Yu., 1969. Recent volcanogenic-sedimentary iron ore process in the caldera of Santorin island (Aegean Sea) and its effect on the geochemistry of the sediments. Izd. Nauka, Moscow, 112 pp. (in Russian). 

Formozova, L.N., 1963. Exhalative-sedimentary iron ore deposits of Lahn-Dill type and the geological environment of their formation. In Vulkanogenno-osadochniye formatsii i terrigennyye formatsii (Volcanogenic-sedimentary and clastic formations), Moscow, pp. 161-208 (in Russian). 

Formozova, L.N., 1968. Regularities of formation of volcanogenic-sedimentary iron ores. Trudy Cleol. Inst. Akad. Nauk S.S.S.R. (Trans. Geol. Inst. Acad. Sci. U.S.S.R.), Moscow, 196 7-153 (in Russian). 

Iron ore is a chemical sedimentary rock that forms when iron and oxygen (and sometimes other substances) combine in solntion and deposit as a sediment. Hematite is the most common sedimentary iron ore mineral. Rock salt (halite) is also a chemical sedimentary rock that forms from the evaporation of ocean or saline lake. It is rarely fonnd at Earth s snrface, except in areas of very arid climate and is often mined for use in the chemical indnstry or for nse as a winter highway treatment. Limestone is a rock that is composed primarily of calcinm carbonate. It can form organically from the accumulation of shell, coral, algal, and fecal debris. It can also form chemically from the precipitation of calcinm carbonate from lake or ocean water. 

Siderite. Chalybite FeCOj Gray, yellow, brown Vitreous, pearly, silky 48,2 Important mineral in sedimentary iron ores all over the world. In Europe for instance in Erzberg, Austria and Bilbao, Spain... 

Sources. Iron ore deposits were formed by many different processes, eg, weathering, sedimentation, hydrothermal, and chemical. Iron ores occur in igneous, metamorphic, and sedimentary deposits. Normally, as-mined iron ore contains 25 to 68% iron. 

By convention, rocks are divided into three groups magmatic (volcanic or extrusive and plutonic or intrusive), metamorphic and sedimentary rocks. Iron ores being the source of iron as a metal, are also rocks and are common in all three groups. Most rocks contain iron oxide minerals of varying nature and abundance. This chapter collects information about their occurrence (Tab. 15.1), properties and formation. 

Hematitic iron ores of hydrothermal-sedimentary origin and Palaeozoic in age, are those of the Lahn-Dill-type in West and Central Europe (Harder, 1964). Hydro-thermal solutions associated with submarine volcanic activities have transported Fe (as FeCl3) into a marine environment, where after hydrolysis, hematite was formed (via ferrihydrite) at the margin of the basin, whereas siderite (after reduction) was formed in its centre. These ores are - in contrast to true sedimentary ores - low in Al,Ti and trace elements, which betrays their volcanic origin. 

Iron ores occur in igneous, mctamorphic or sedimentary rocks, or as weathering products of various primary iron-bearing materials- For convenience of analysis, iron ores are grouped into (I) igneous tires. 121 contact ores. i3i hydrothermal ores. (4) sedimentary ores, with several suhclassihcaiions of the latter ores. Brief definitions follow ... 

Minerals of economic importance within sedimentary formations include, hut are not limited to fluorite, barite, phosphorite, and oolitic hematite. Fluorite is utili/ed us a flux in steelniakiitg and when of high quality as lenses and prisms in the optical industry. Barite is an essential mineral used m gas- and oil-well drilling. Phosphorite, a product of chemical precipitation from seawater, when ircaled with sulfuric acid, produces superphosphate fertilizer, (.killtic hematite deposits of extensive size are important sources of iron ore. 

There seems little doubt that coordination geochemistry does play a role in ore formation in a number of environments. Much more work in this field remains to be done however. It is possible that sedimentary iron formations also derive from similar cycles to those mentioned above for manganese.330 The uptake of iron by bacteria which are capable of synthesizing magnetite, Fe304,... 

Iron ore (sedimentary) Kanker district Central-east India 100 Pandey et al. (2006)... 

The sedimentary jaspilite formation, the stratotype of which is the Krivoy Rog iron-ore sequence, is characterized by an association with pelitic and psammitic sedimentary rocks whose sources were products of subaerial weathering. The sediments of this formation were deposited in the Krivoy Rog-Kremenchug marginal depression and also in individual parageosyncli-nal zones the Orekhov-Pavlograd, Western Pri-Azov, Mangush, etc. 

In some works (Parak, 1975) even the apatite-iron ores of Kiruna in Sweden, which are believed to be intrusive-magmatic, are attributed to exhalative-sedimentary deposits. 

The participation of volcanic material in iron-ore accumulation is confirmed by the distribution of trace elements, in particular germanium (Grigor yev, 1971). The explanation of the formation of banding in the case of a volcanic source of the iron and silica and sedimentary mode of deposition presents considerable difficulties. Usually, pulsating volcanic activity, and as a result of this, periodic entry of iron and silica into solution. 

Malakhovskiy, V.F., 1956. Geochemistry and main problems of the origin of the Kerch supergene iron ores. In Voprosy mineralogii osadochnykh obrazovaniy (Problems of the Mineralogy of Sedimentary Formations). Izd. Lvov Univ., 3-4 190-202 (in Russian). 

Mel nik, Yu.P., 1975. Sedimentary minerogenesis in the history of Krivoy Rog iron ore deposits. In Osadochnoye mineraloobrazovaniye (Sedimentary Minerogenesis). Izd. Naukova Dumka, Kiev, Vol. 2, pp. 3-9 (in Russian). 

In general, the minerals now identified as chamosite are found in iron ore bodies of sedimentary origin (e.g., Maynard, 1986 Fernandez and Moro, 1998 Wiewora et al, 1998 Kim and Lee, 2000). Chamosite associated with iron oxides appears to follow a compositional trend from iron oxides plus kaolinite to chlorite, as indicated in Figure 8, using the data of Velde (1989). The recombination of iron oxide in the presence of kaolinite gives an aluminous, ferrous mineral, chamosite. This mineral is formed under burial conditions where ferric iron oxide is reduced to feiTous iron which is rapidly incorporated into a 7 A chlorite mineral. Both chamosite and berthierine result from the reduction of ferric iron to ferrous iron. 

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