Mobilisation of arsenic, selenium and uranium from Carboniferous black shales in west Ireland

The fixation and accumulation of critical elements in the near surface environment is an important factor in understanding elemental cycling through the crust, both for exploration of new resources and environmental management strategies. Carbonaceous black shales are commonly rich in trace elements relative to global crustal averages, many of which have potential environmental impacts depending on their speciation and mobility at surface. This trace element mobility can be investigated by studying the secondary mineralisation (regolith) associated with black shales at surface. In this study, Carboniferous shales on the west coast of Ireland are found to have higher than average shale concentrations of As, Cd, Cu, Co, Mo, Ni, Se, Te and U, similar to the laterally equivalent Bowland Shales, UK. Groundwater penetration and oxidative weathering of these pyritic black shales produces oxide deposits, dominated by goethite and jarosite, which are significantly enriched in As (44-468 ppm), Se (12-184 ppm), U (6-158 ppm) and other trace elements, compared to concentrations in the parent shales. Major elemental abundances vary in composition from 3.5-29.4 % sulphate, 0.6-9.1 % phosphate and 36.6-47.2 % iron-oxide. Phosphate substitution within jarosite is observed in these samples, formed under ambient pressure and temperature conditions.
The major and trace elements forming these secondary deposits are predominantly sourced from the underlying black shales through mobilisation by groundwater. This discovery is critical for the environmental assessment of black shale lithologies during shale gas exploration and production, where the demonstrated mobility of in situ elemental enrichments may indicate a significant source of produced- and groundwater contamination during and after hydraulic fracturing processes. The proportions of the major oxide phases exhibit a clear control on the trace elemental enrichments within the secondary deposits, where increasing Se and As concentrations correlate with increasing phosphate content of the jarosite mineralisation. This has implications for the remediation of acid mine drainage seeps, where phosphate-rich jarosite phases could be utilised for more efficient trace element removal.