Multi-stage pyrite genesis and epigenetic selenium enrichment of Greenburn coals (East Ayrshire)

Abstract

Carboniferous coals of the Ayrshire Coalfield are enriched in selenium (Se) relative to average UK and world compositions, substituting for sulphur in pyrite. Greenburn surface mine coals are characterized by syngenetic concretionary pyrite (c. 15% total area), occurring as bedding-parallel banding, and later-formed (epigenetic) cross-cutting pyrite in cleats (c. 9% total area). In these, sulphur isotope compositions for both syngenetic and epigenetic pyrite include isotopically light and heavy variants, suggesting diagenetic and hydrothermal fluid formation. Late/post-Visean cleat-filling pyrite is enriched in Se (up to 266 ppm) compared to the earlier-formed material (Se up to 181 ppm).Anomalous Se may have been sourced from near-by sulphidic Dalradian metamorphic rocks. Initial Se sequestration is associated with syngenetic pyrite mineralization, absorbed from seawater and pore waters, with additional Se introduced from fluids mobilized during epigenetic pyrite formation. Cleats from local brittle fracturing provided channels for fluid flow and a locus for precipitation of comparatively high-Se pyrite. Permian dolerite intrusions may have provided an enrichment source and/or fluid distribution mechanism. The Se concentrations of the Greenburn coals relate to multi-stage mineralization, with cleat-filling pyrite showing the highest Se content, and highlight the potential for high Se in similarly altered and fractured coal deposits worldwide.Supplementary material: LA-ICP-MS maps for Fe, Se, Ag, As, Cu, Hg, Pb and Te for Greenburn coal samples from seams 9300 Lime and 6900 Burnfoot Bridge are available at https://doi.org/10.6084/m9.figshare.c.3967860

Selenium and tellurium concentrations of Carboniferous British coals

“…Causes of thermal maturity in coal, such as igneous intrusions heating the strata, may also be accompanied by trace element‐carrying hydrothermal fluids percolating through coals, precipitating as pyrite. Dissolved metals in fluids can enhance Se and Te content of coals and have been suggested for Carboniferous coals in Ayrshire (Bullock et al, ) and Jurassic coals in Brora (Bullock et al, ). The close proximity of the South Wales, Cumberland, and Northumberland coalfields to granitic intrusions (e.g., Shap and Skiddaw granites of Cumberland, Dartmoor granite in southwest England, Weardale granite in northeast England), associated deformation which produced a series of faults through the coal strata (forming cleat systems), or later Variscan activity (in the case of South Wales), may have provided a source and/or flow pathway for hot metal‐rich fluids and subsequent increased maturity and trace element enrichment to these coals (see also Gayer et al, ; Turner & Richardson, ).…”

“…In previous studies, Se has been shown to be more abundant in early forms of pyrite (Large et al, ). However, late stage epigenetic fluids are also known to be responsible for high concentrations of trace elements in some coals (Bullock et al, ; Diehl et al, ; Spears & Tewalt, ; Yudovich & Ketris, ). The presence of pyrite infilling cleats indicates a later period of formation.…”

“…The presence of pyrite infilling cleats indicates a later period of formation. Cleats can provide channels for fluid flow, and the host coal can offer a source of reduced S for bacterial sulphate reduction (Bullock et al, ; Hatch, Gluskoter, & Lindahl, ). The development of cleats provides a locus for pyrite precipitation, and associated Se and Te.…”

“…Dissolved metals in fluids can enhance Se and Te content of coals and have been suggested for Carboniferous coals in Ayrshire (Bullock et al, 2018) and Jurassic coals in Brora (Bullock et al, 2017 Gayer et al, 1996;Turner & Richardson, 2004). The Clackmannan…”

“…Selenium can substitute readily for S in many sulphide minerals and can also be precipitated by sulphate-reducing microbes (Hockin & Gadd, 2003;Parnell, Bellis, Feldmann, & Bata, 2015 (Cavender & Spears, 1995, 1997Spears & Caswell, 1986;Spears & Tewalt, 2009 shown to be more abundant in early forms of pyrite (Large et al, 2014). However, late stage epigenetic fluids are also known to be responsible for high concentrations of trace elements in some coals (Bullock et al, 2018;Diehl et al, 2004;Spears & Tewalt, 2009;Yudovich & Ketris, 2006).…”