Bog Iron Ore

Bog iron ores have provided a major source of iron in pre-industrial Britain. Exploited examples include those of upland areas in Scotland and Wales, and of lowland areas in northern England.

Bog iron ores are typically associated with the oxidation of iron-bearing groundwaters. The upper illustration shows a hard, dark iron ore (centre right) developed between large boulders. The boulders have probably been reworked from till, but the groundwater flow may have contributed to the removal of fine-grained sediment from between them. The brightly coloured deposit is a weakly enriched zone within the till.

Manganese minerals may precipitate in rather similar circumstances to those which generate bog iron ores. In the lower example a manganese pan has developed at the base of an organic-rich soil, on top of a till. The pan is richest between pockets of stones (e.g. centre left).

Bog iron ore forming a pod 50cm thick, with darker, harder ore, overlying a lean, but brightly coloured zone (North Wales).

Manganese pan (dark) lying at the interface between humic soil and underlying till (North Wales). Section approximately 60cm high.

Claystone Ironstones

Iron reduction during early diagenesis may lead to the precipitation of various iron minerals. In marine sediments, where sulphate is an abundant porewater component, sulphate reduction may lead to the precipitation of iron sulphides. However, in freshwater, sulphate concentrations are usually low, so iron may instead by fixed as iron carbonate, siderite (the bicarbonate having been produced by microbial activity). The siderite precipitates into the porespace of the unconsolidated sediment, and a claystone ironstone produced. Individual siderite crystals are usally 10-40 µm.

Thin section of claystone ironstone. Coal Measures, Glamorgan. Optical photomicrograph, under plane ploarised light. Field of view 1mm.

Oxidised claystone ironstone showing relict form of siderite crystals pseudomorphed by oxides. Jurassic, Northants. Backscattered electron photomicrograph. Field of view 400 ¶m.

Claystone ironstone concretion. Coal Measures, Pembrokeshire.

Blackband Ironstones

Sphaerosiderite

Radially-oriented bladed siderite growth from restricted nuclei produces this characterstic growth of spheroids 500µm to 2mm in diameter. Sphaerosiderite growth is linked to high degress of saturation with respect to siderite, and is commonly linked to formation in waterlogged soils (though other environments from deep marine to lacustrine have been recorded).

Thin section of sphaerosiderite from the Scalby Fm, Jurassic, Yorkshire. Transmitted light photomicrograph, under crossed polars. Field of view 1.5mm

Ooidal Ironstones

Thin section of ooidal ironstone. Grainstone from Cleveland Ironstone Fm. Optical photomicrograph, under plane ploarised light. Field of view 4mm.

Oxide vein- and cavern-hosted mineralisation

Oxidation of upwardly-migrating deep basinal fluids in near surface settings has generated significant oxide facies ores, particularly around the margins of Permo-Trias basins in the Cumbrian and Bristol Channel orefields.