Position : 2318 (28 vues)
Type de document
Titre de la publication
Geological Society, London, Special Publications
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Oolitic ironstones occur in various sedimentary environments: shallow marine to deltaic, lacustrine, fluviatile and pedogenic. Distinction between formational and depositional environment is not always possible. Most of the marine and fluvial minette-type ironstones consist of reworked ferruginous coated grains deposited in agitated water, but there exist also indicative structural features of in situ formation in the supporting medium of lateritic and hydromorphic environments. In the zone of oscillating groundwater repeated leaching and subsequent concretionary precipitation of hydrated ferric oxides take place, according to the prevailing Eh/pH-conditions and microbial activity. The moderate Al substitution of goethite from hydromorphic environments corresponds to the observed range in oolitic ironstones. The authors therefore assume erosion, reworking and subsequent fluviomarine redeposition of soil derived ooids to be the major processes of generating minette-type ironstones. Postdepositional diagenetic changes may convert the aluminous, silica-rich ferric oxides into berthierine in reducing environments if the chemical bulk composition of the primary goethite is similar. Since any aquatic milieu with appropriate fluctuations of Eh and pH can produce ferruginous coated grains, marine iron ooids associated with hardgrounds and areas of low sediment input can also occur. But there, release of ferrous iron, transport in saline interstitial waters and fixation of ferric hydroxides — usually with very low Al-substitution — take place in a much smaller scale, unable of generating the huge iron accumulations of minette-type ore deposits.