The geochemistry and mineralogy of high-S coals, with examples mainly from the East Pennines Coalfield in Yorkshire and Nottinghamshire, UK: an overview

Over a number of years, coals have been analysed in the geochemical laboratories of the University of Sheffield. Much of this work has been published and this paper presents an overview and, in some cases, a reinterpretation of the data. Most of the coal samples were obtained from seam sections in the Yorkshire and Nottinghamshire coalfields (now included in the East Pennines Coalfield), and from the Parkgate Coal in particular. Sulphur variations are essentially due to pyrite. In individual plies of the seam, lateral variation in total sulphur is thought to be due to the availability of sulphate at the time of deposition. Access of sulphate into the coal swamp environment can be directly related to river channels, many of which are known to have been long-lived features. The pyrite is of concern because of S emissions, but in addition it is the location in the coal for Hg, Tl, Pb, As and Se, all of which are of major environmental concern. Concentrations were determined by direct and indirect methods. The most common of the latter methods is statistical, and some of the potential problems are reviewed. Also contained within pyrite, but not exclusively so, are Cd, Ni and Sb. Trace element concentrations from different pyrite samples are similar, strongly suggesting a common source. Comparable trace element enrichment patterns for coals and marine shales provide evidence of a seawater origin. The chloride concentration in the coals is high and can also be linked to seawater. The two elements, S and Cl, differ in that Cl is conservative, is not involved in diagenetic reactions and is retained in the coal in residual pore fluids. Illite and kaolinite are the main clay minerals present, with subsidiary chlorite and mixed-layer illite-smectite. In most coals, the clay minerals are dominantly detrital, but in low ash coals, kaolinite of diagenetic origin can be detected. In some samples from the Barnsley Coal, the source of this kaolinite, based on the trace element geochemistry, is dispersed volcanic ash (basaltic). It is also highly likely that organic matter contributes small amounts of Si and Al in the coal, and phytoliths are one potential source. However, the detrital sediment is a major contributor to the major and trace element geochemistry of the coals. Trace elements with a dominantly detrital origin are Rb, Cs, Cr, Ga, Th, Nb and Li. Trace elements are also associated with the organic matter to a greater or lesser extent. Germanium is mainly associated with organic matter, but V is also present in the clays. These elements are mainly contained in vitrinite, although Ge concentrations are more variable, suggesting some mobility during diagenesis. This is borne out by Ge, V and Be showing enrichment at the margins of the the Parkgate Coal, following 'Zilbermints Law'. Mobility during diagenesis is also demonstrated by the mineral infill of the cleat (joints). Stages of cleat mineralization have been recognized and related to diagenetic stages in siliciclastic sequences. Although there is evidence of element mobility after deposition in the coals, this is not thought to be related to the mineralizing fluids in the Pennine Orefields.