Aspects of iron incorporation into sediments with special reference to the Yorkshire Ironstones

Abstract

Summary Ironstones of former economic value occur in both Coal Measures and Jurassic sequences in Yorkshire. The distribution of pyrite and siderite in the Coal Measures sediments is explained by the stability relationships between the two minerals and the controls on the rate of pyrite formation, particularly the sulphate availability, but also involving the amount and quality of metabolizable organic matter and the iron availability. In marine black shales early diagenetic pyrite formation may have immobilized much of the available iron, whereas in non-marine sediments iron was available over much longer diagenetic periods to form siderite in the mudrocks and pyrite in the coals. The qualitatively more important siderite forms clay ironstones by a process of diagenetic redistribution and concentration of original iron. In the marine Jurassic sediments pyrite and siderite again show a cyclic distribution. The same explanation holds for pyrite in the marine black shales, but in the grey shales loss of metabolizable organic matter under oxic conditions mainly allows post-oxic reactions, which includes reduction of iron but little sulphate reduction. In the oolitic ironstones efficient loss of organic matter in an important oxic zone is seen as a key factor. The oolitic ironstones do represent an original enrichment of iron in contrast to the other ironstones considered. Post-oxic reactions are thought to be important and produced berthierine and siderite. The possibility of some diagenetic enrichment of iron in the oolitic ironstones should be recognized.