Origin of iron ore by diagenetic replacement of calcareous oolite

Abstract

MANY non-uniformitarian¹ or quantitatively improbable² hypotheses have been offered to explain the origins of extensive sedimentary rocks rich in iron. Ironstones and iron formations³, here termed non-cherty and cherty iron ores, have, however, two consistent characteristics which suggest that their origins were fundamentally similar and involved processes which are still operative at present⁴. The first is the occurrence of mudrocks, or their metamorphosed equivalents, overlying individual ore beds within a few metres, with the ore beds themselves covered by non chemical sedimentary rock and not eroded. The second is the occurrence of oolite texture in unmetamorphosed deposits which are not closely associated with volcanic rock or shales bearing pyroclastic material. The internal structures of well preserved ferriferous ooids closely resemble those of Recent calcareous ooids. Some Recent ferriferous spherules are concentrically layered⁵ but, unlike the ooids in iron ores, they contain clay minerals other than ferrous septechlorite, and occur collectively only as moderately iron-rich sediments, no thicker than a few tens of centimetres. Extensive beds of ferriferous oolite thicker than 1 m have formed repeatedly throughout the history of the Earth, even within the last 5 Myr (ref. 6).