Precipitated Ores.—Many deposits of iron ore owe their origin to the precipitation in seas, lakes and swamps of iron de rived from the decay of rocks in the zone of weathering. Iron goes into solution in this zone either by the oxidation of the sulphides of that metal, or by the breaking up of ferro-magnesian silicates. Soluble iron compounds are also formed by the action of certain bacterial organisms present in the soil. Reduced by these organ isms to the ferrous state, the iron combines either with the organic acids of the humus, or with the carbonic acid of the at mosphere, to form a soluble bicarbonate.
On exposure to air, or in the presence of plant life, carbon di oxide is given off and iron carbonate is precipitated, as shown by the following reaction : The ferrous carbonate rapidly oxidizes to ferric hydrate, and is deposited as or A continuous layer of this pan consequently tends to form below insufficiently drained clayey and peaty soils. The formation of ferric hydrate in stagnant swamp water, where ferrous carbonate is undergoing oxidation, may actu ally be seen in operation, the iridescent films that form on such water being an indication that the reaction is taking place.
When the waters deposit iron in the presence of much carbonic acid, or of decaying organic matter, siderite (FeCO3) is deposited. In muddy waters the silt goes down with the iron carbonate lead ing to the formation of These clay-ironstones or spathic ores as they are also termed, occur in the coal measures of the British Isles and in those of Westphalia and North America, and are an important source of iron. Though the iron content does not on the average amount to much more than 30%, the compara tive leanness of the ore is compensated by the low percentage of phosphoric acid, which seldom rises above one per cent. The black band ores of the coal measures are a similar deposit, but differ from the bulk of the clay-ironstones by the high percentage of vegetable or peaty matter they contain; sufficient to furnish the fuel required to calcine them. Such ores represent what was once a carbonaceous mud in which ferrous carbonate was precipitated. The Scandinavian are an interesting example of pre cipitated iron ores.
The Jurassic iron ores of England and the similar minette ores of Lorraine and Luxembourg on the Continent, which constitute the greatest reserves of iron ore in Europe, form well-defined beds in a sedimentary series of sandstones, shales and limestones.
Although low in iron (25 to 40% metallic iron) in many cases their large lime content makes them largely self-fluxing, and hav ing a high percentage of phosphorus they are admirably suited for the production of pig iron for the basic process of steel-making. Their oolitic structure (so named from the resemblance to fish roe) is produced by the deposition of carbonate of lime and carbonate of iron around sand grains or fragments of fossils. In the zone of weathering, the spathic ores are oxidized to limonite, a change in colour from green to red being characteristic. Some secondary enrichment also takes place under favourable condi tions. These iron ores are worked in England on three horizons, namely the Inferior Oolite (Dogger or Northampton stone) the Middle Lias (Marlstone) and the Lower Lias (Frodingham stone).
The metal content of the copper-bearing sandstones so widely distributed throughout the world, owes its origin, partly to precipi tation and metasomatic replacement, partly to sedimentation. The prevalence of these deposits in the red beds of the Upper Carbon iferous, Permian and Trias formations indicates sedimentation in the shallow seas of desert regions where evaporation was The copper was no doubt leached out, from earlier copper deposits in continental areas, by meteoric waters rich in sodium chloride and calcium sulphate. Transported as sulphate or chloride it was precipitated in inland seas or lakes and reduced to chalcocite by decaying organic remains. The replacement of tree-trunks and other plant remains as well as of the cementing material of the sandstones by chalcocite is a feature of these deposits.
The accumulation of cupriferous detritus while sedimentation was in progress, was no doubt a factor of the concentration in some cases, but such material of the original deposit would, of course, be subsequently modified by the circulation of the ground waters. Associated minerals are calcite, gypsum and barytes. In the zone of weathering the copper ores are oxidized to the green and blue carbonates—malachite and azurite.