The genesis of coal measure blackband ironstones

Abstract

Blackband ironstones are essentially thin (c.75 cm thick) siderite rich (total iron up to 40%), carbonaceous, layered mudrocks which commonly occur in grey coal measure sequences at coal seam horizons. In the United Kingdom they are restricted in commercial development to the Midland Valley of Scotland (Namurian E) and the North Staffordshire Coalfield (Westphalian C) although they are now no longer exploited. During the course of this study, in addition to material from these two areas, examples of blackband ironstone developments were also obtained from the Nottinghamshire Coalfield although the majority of the work was concentrated on an opencast coal site a few miles to the north of Newcastle-Under-Lyme and nearby collieries (National Coal Board) and boreholes in the North Staffordshire Coalfield. Similar blackband 'developments are also recorded occurring in the Westphalian of the Ruhr, Germany and the Pennsylvanian of Ohio, USA. In addition to their conspicuous layered macro-texture the blackbands exhibit many other primary textures including root disturbed layers, sideritised unflattened spores and preservation of plant cell detail which all point to a very early, in some cases pre-compaction siderite formation. A study of their enclosing sediments clearly shows that they were deposited in an environment intermediate between a complex delta top alluvial flood plain and coastal plain swamps; an environment likely to have been one in which small areas were subjected to periods of lacustrine deposition. The presence of varved mudstones and oil shales in the measures directly above many of the blackband ironstones and the distinctive fine layering of the blackbands themselves are evidence of such a depositional environment. The blackbands are considered to have been formed in a similar way to Recent bog iron ores. They are therefore fossil bog iron ores formed contemporaneously in the proto-coal peat bogs; the iron being concentrated by precipitation due to oxidation in the surface waters of the bog and subsequently reduced to siderite in the top few metres of the carbon-rich sediment of the substrate. In support of these conclusions detailed mineralogical, petrological, sedimentological and stable isotope data are presented herein.