Implications of element enrichment in paralic coals from the Carboniferous in England

Abstract

The enrichment of trace elements in coals is of significant economic importance because of the growing demand for the critical elements. Several enrichment processes have been identified and in this paper the role of palaeosalinity is evaluated using published data from the East Pennine Coalfield (UK). The coal and associated mudrocks for which comprehensive ICP- AES and ICP-MS analyses are available is the Parkgate Coal. This is a high volatile bituminous coal of Upper Carboniferous (Pennsylvanian) age with a variable S content mainly due to pyritic S (Total S = 2.34 ± 1.83 %, Pyritic S = 1.17 ± 1.79 %).

Supporting information on the mineralogy of this and other major coals comes from XRD analyses. Although there is an extensive geochemical data base on coals and mudrocks from this Coalfield, most of the analyses were made using XRF with a restricted suite of trace elements (13). Comprehensive analyses of marine shales are not available for this coalfield and to evaluate the marine influence use is made of analyses of samples from the Netherlands. These samples are of comparable age and from the same extensive depositional area. Enrichment values calculated for the Parkgate Coal based on detrital trace elements in the mudrocks show that the chalcophile elements are enriched and pyrite is the major host for many of them. Other elements are enriched including Ge and Be and there is an organic association, possibly reflecting an original plant origin. The importance of a seawater control is demonstrated by close similarities in the enrichment values between the coals and marine shales. Use could be made of the enrichment values determined in this work to identify a seawater influence in coals elsewhere. Elements not enriched, and for which other controls operated, are Ga, Li, Zr and Nb. Although not confirmed for the UK coals the REE are probably in the same category. Alternatives to the control of seawater on the element enrichments in the studied coals are considered and rejected as having little or no effect.