Selenium and tellurium concentrations of Carboniferous British coals

Standard mode and collision/reaction cell mass spectroscopy methods have been utilized in order to overcome spectral interferences and provide ultra‐low quantification of selenium (Se) and tellurium (Te) in British Carboniferous coals for the first time. The accurate detection of Se and Te in coals is becoming increasingly important, as coals and pyrite have been identified as potentially significant trace element sources. The mean Se concentration of British coals bear comparison to that of world coals, with anomalous Se content (concentrations above 4 mg/kg) across westerly exposures, often coinciding with high sulphur (S) content and visible pyrite. New Te data for British coals gives a mean concentration of 0.02 mg/kg, with anomalous Te in Ayrshire. There is a positive correlation in the Te/Se ratio across the sample set. The close relationship between Se and Te, as well as Se–Te with both early syngenetic and later cleat‐filling pyrite, confirms an important role for sulphides in Se and Te sequestration in British coals. The high Se‐Bowland Shale and/or Ordovician volcanics may have provided the trace element source for British coals of similar or younger age. Regional intrusive activity (shallow tabular intrusions or more extensive plutons) and episodes of intense deformation can alter the thermal maturity of coals and may have driven the movement of trace element‐rich fluids through strata, locally enriching coals in Se and Te.

, and pyrite has been noted as a potential source of economic interest for Se (Keith, Smith, Jenkin, Holwell, & Dye, 2017). Both Se and Te occur in low abundances in the Earth's crust (Se = 0.05-0.09 mg/kg; Te = 0.002 mg/kg ;Rudnick & Gao, 2003) but are known to be concentrated in specific geological and environmental reservoirs and may be associated with mining activities and reserves (UKERC, 2012). These elements may also form in high concentrations in organic-rich sediments, such as coal (in raw form and as coal ash following combustion). An evaluation of concentrations and controls on Se and Te occurrence in coal is important for understanding resource potential and well as environmental threats relating to their occurrence and processing.
The enrichment of Se and Te in coals compared to crustal abundances reflects the affinity of these trace elements for both organic matter and sulphur (Spears, 2015;Spears, Borrego, Cox, & Martinez-Tarazona, 2007), and possibly for the addition of Se and Te during diagenesis (Diehl, Goldhaber, & Hatch, 2004). Limited data also suggest that trace element contents, including Se, increase with the thermal maturity of coal (Liu, Wang, & Oakey, 2006;Raask, 1985). High Se in British and Irish Carboniferous sediments has been previously identified in organic-rich black shales (Bowland Shale of mid-Visean age; Parnell, Brolly, Spinks, & Bowden, 2016), and it was suggested that Ordovician volcanics may have provided an anomalous source in the watershed. The high Se-Bowland Shale and/or Ordovician volcanics may therefore provide a source of Se (and other trace elements such as Te) to coal strata of similar or younger age. Despite the extensive studies into trace elements in coal, there is still a limited understanding of the processes of preferential accumulation of Se and Te in coal, how they relate to coal properties, and their spatial and stratigraphic distribution in British coalfields. Developments in analytical methods for measuring Se and Te using cold plasma and collision/reaction cell (CRC) interface allow the detection of Se and Te down to ultra-low levels of detection in British coals. This study reports new data for Carboniferous coals from across Great Britain (Figure 1) and, in particular, (a) establishes a database for Se and Te concentrations in a set of coals of similar stratigraphic age; (b) assesses how variable or constant the concentrations are over a wide area; (c) assesses any variability in the ratio of Te/Se; and (d) assesses any relationships between concentrations of Se and Te with percentage organic carbon, percentage sulphur (S), and thermal maturity.
Samples were analysed using both standard mode mass spectrometry methods and CRC microwave-assisted wet digestion methods (Henn et al., 2018) for better detection of low concentrations.

| Sampling and coal properties
Sample localities include exposures and collieries from the coalfields of North West Scotland, across the Midland Valley of Scotland, Northern FIGURE 1 Map of Great Britain, with Carboniferous exposed coalfields shown and sample localities (see Table 1) England, the Midlands, Wales, and Kent (see Table 1 and Figure 1). In total, 44 seam sub-samples were analysed for Se and Te content. Coal rank and thermal maturity indicator information has been compiled based on vitrinite reflectance values. Vitrinite reflectance values for British coals have been collected from a range of sources (Armstroff, 2004;BCURA (The British Coal Utilisation Research Association), 2002;Burnett, 1987;DECC, 2013;Durucan, Ahsan, & Shi, 2009;Parnell, 1992;Vincent & Rowley, 2004).

| Se-Te determination
The Se and Te whole-rock content of coals was determined by two techniques: First, selected whole rock samples were analysed as part of a suite of 51 elements using standard mode aqua regia inductively coupled plasma-mass spectrometry (ICP-MS) techniques at ALS Minerals (Loughrea, Ireland). Samples were milled and homogenized, and 0.25 g were digested with aqua regia in a graphite heating block.
The residue was diluted with deionized water, mixed and analysed using a Varian 725 instrument (ICP-MS method code ME-MS41). The Hydrofluoric acid (40%) was used without previous purification for digestion by MAWD. Water was purified using a Milli-Q system and used to prepare all standard solutions and reagents. Concentrated HNO 3 (65%) was distillated in a subboiling system for MAWD and preparation of reference solutions. Reference solutions of Se and Te (0.01 to 10 μg/L) were prepared by sequential dilution of a stock solution (1,000 mg/L) in HNO 3 . Argon (99.998%) was used for plasma generation, nebulization and as auxiliary gas and hydrogen (99.999%) was used in the CRC.
In order to evaluate the MAWD method, select samples were digested by MAWD using three digestion solutions: (a) 4 ml HNO 3 + 1 ml

| Total organic carbon and sulphur content
Total organic carbon (TOC) and sulphur (S) contents were measured using a LECO CS225 elemental analyser, after decarbonatization with hydrochloric acid, to a precision of ±0.05%. Analyses were run concurrently with CRMs 501-024 (Leco Instruments, 3.23 ± 0.03% C, 0.047 ± 0.003% S, instrument uncertainty ±0.05% C, ± 0.002% S) and BCS-CRM 362 (Bureau of Analysed Samples Ltd., 1.48% S). The instrument was calibrated using these CRMs.  (Spears, 2015). The main coal-bearing intervals comprise the Lower and Middle Coal Measures, formed during the Westphalian A and Westphalian B to Lower C in a fluvial coastal plain environment (Gayer, Peŝek, Sykorova, & Valterova, 1996). This was dominated by overbank mudrocks, representing deposition in extensive interdistributary lakes (Gayer et al., 1996). The start and end of the Carboniferous (into the Stephanian) was marked by a climate that was (at least seasonally) arid (Waters et al., 2007). An increase in sand content and a climatic shift to drier conditions led to a decrease in the number of coal seams during the Westphalian C, Westphalian D and Stephanian (Besly, Burley, & Turner, 1993;Kombrink, 2008). The Coal Measures were strongly affected by post-depositional Variscan fold and thrust deformation, and such deformation resulted in a set of superimposed tectonic fractures on the coals, partially obliterating a cleat fracture system (Frodsham, Gayer, James, & Pryce, 1993;Gayer et al., 1996;Gayer & Nemcok, 1994;Owen, 1974;Woodland & Evans, 1964 Group sample (Namurian), and Clackmannan Group (Visean; Figure 2).

| Se-Te content of British coals
Results are shown in Table 1 and Figures 3-9. The world mean Se content of coals has been previously estimated in several studies, generally giving a range of 1-3 mg/kg (Nalbandian, 2012;PECH, 1980;Valkovic, 1983;Yudovich & Ketris, 2006 The arithmetic mean British coal Se content for samples analysed in this study is 2.1 mg/kg, in line with the world mean content and slightly higher than the British mean of Spears and Zheng (1999). This value is slightly higher than the world mean Se content for bituminous coals of 1.6 mg/kg (Yudovich & Ketris, 2006). Overall, samples with detectable Te show an increase with increasing Se, with a strong positive correlation (linear trendline R 2 value = 0.85, calculated using Microsoft Excel; Figure 3). The standard mode method shows a mean Se content of 2.5 mg/kg, similar to that of the CRC method of 2.3 mg/ kg. Ayrshire (3.7 mg/kg) and North Wales (3.7 mg/kg) show the highest mean regional Se content from the sample set ( Figure 4).

Anomalous Se is recognized in individual coal sample localities from
Fife-Stirling (6 mg/kg), Northumberland (5.9 mg/kg), Nottinghamshire (5.3 mg/kg), Argyllshire (5.7 mg/kg), Ayrshire (5.3 mg/kg), North Wales (5.6 mg/kg), and Staffordshire (5.6 mg/kg). Figure 5 shows that high Se tends to occur in westerly coal exposures (e.g., Western Scotland, North and South Wales, Cumberland, and Staffordshire) as well as in Northumberland and Fife-Stirling.   concentrations are values considered above 1.3%, the cut-off value for S content of coals acceptable for local power generation in Northumberland (Turner & Richardson, 2004). Figure 9 shows that some higher Se and Te content coals coincide with higher vitrinite reflectance values (e.g., Kent, South Wales, Fife-Stirling, and Northumberland).

| Comparison to world coals
Results show that British coals contain typical Se concentrations comparative to world Se coal content. Previous databases for Australia, the United States, UK, and Iran record mean Se values of 1.02, 1.7, 1.8, and 1.2 mg/kg, respectively (Coleman, Bragg, & Finkelman, 1993;Pazand, 2015;Riley et al., 2007;Spears & Zheng, 1999). Elevated Se levels in coal have led to a suggestion that coal might be a global resource for the element, of up to 500,000 tonnes, based on a content of 0.5 mg/kg (Speirs, Houari, Contestabile, Gross, & Gross, 2013). Two regions where coal waste is regarded as an environmental problem, British Columbia (Wellen, Shatilla, & Carey, 2015) and West Virginia (Lindberg et al., 2011), have mean Se contents of about 2.6 and 3.6 mg/kg, respectively (Kennedy, Day, Mackie, & Pesonen, 2015;Vesper et al., 2008). All of these mean values are lower than the 4 mg/kg level considered anomalous, above which 6 of the 15 British regions measured. The regions of high Se content in British coals do not match to regions of anomalously high stream water Se content (Salminen, 2005).
Previously, Te was seldom recognized in coals worldwide due to its low abundance. The mean Te content of world coals has been taken at 1/100 of world mean Se content, giving a mean world Te content of 0.015 mg/kg (Davidson & Lakin, 1973). As a result (and with no known previous analysis of Te in British coals for comparison), any Te above this world mean value is considered anomalous, though concentrations are too low to be considered significant in any economic or environmental sense. Limited previous data for Te in coals from China, Japan, Australia, and UK are all in the range 0.02-0.08 ppm (Hashimoto et al., 1989;Spears & Tewalt, 2009;Woo, Watanabe, Hashimoto, & Lee, 1987), similar to the values determined in British coals.

| Source and enrichment of Se and Te in British coals
High Se in the Bowland Shale and underlying Ordovician volcanics (Parnell et al., 2016) may have provided the initial source for Se and Te for British coals of similar or younger age. This is particularly true of upper Visean-Namurian Clackmannan Group (Waters et al., 2007(Waters et al., , 2011 samples, which were deposited at a similar time to the Bowland Shale. The limited overall Se and Te range of Westphalian A-C samples suggests there was little variation during the Westphalian period, which suggests that local variations in thermal maturity and mineralization may play a role in the regional and stratigraphic trace element patterns of enrichment throughout Great Britain. There are some correlations of Te and Se with high vitrinite reflectance (Figure 8), which may indicate that more thermally mature coals contain higher trace element content, previously attributed to uptake from groundwaters during burial (Raask, 1985).
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., 2018) and Jurassic coals in Brora (Bullock et al., 2017). 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., 1996;Turner & Richardson, 2004). The Clackmannan Group coals in Fife-Stirling shows higher mean Se and Te, which may be attributed to the 18-m thick Midland Valley Sill complex causing local heating of strata (Vincent, Rowley, & Monaghan, 2010), which may be responsible for the higher thermal maturity, and may have provided a potential source and/or fluid transport mechanism for chalcophile elements through the strata. Enrichment of Se (and possibly) Te regions such as Northumberland and the northern Pennines may be additionally influenced by seawater distribution, with periodic inundation by the sea into low-lying deltaic planes (Cann and Banks, 2001;McKay and Longstaffe, 2003;Turner and Richardson, 2004;Bouch et al., 2006;Spears, 2015;2017).
There is also a positive correlation in the Te/Se ratio (Figure 2), suggesting a close relationship between the two elements, both substituting for S in pyrite. 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). In coals, Te also has an indirect association with organic matter, and microbial activity can concentrate both Se and Te. High Se and Te coals recognized in this study typically contain visible pyrite (e.g., samples from Ayrshire, Northumberland, South Wales, and Staffordshire). The role of pyrite is therefore pivotal in sequestration of Se and Te in coals. However, the cross-plot of Se versus S indicates only a slight a positive correlation, with several outliers (Figure 8a). This may therefore suggest that pyritic S is important in Se and Te sequestration, but sulphate and organic sulphur content in coals do not play and later (epigenetic) cleat-filling pyrite (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).  Enrichment in some areas may be additionally influenced by seawater distribution and inundation into low-lying deltaic planes.