Magali Perez

A black shale protolith for gold-tellurium mineralisation in the Dalradian Supergroup (Neoproterozoic) of Britain and Ireland

ABSTRACT The Dalradian Supergroup of Britain and Ireland is mineralised by gold-tellurium vein deposits. The host succession includes carbonaceous, pyritic shales (pelites) which were a source of trace elements, including gold and tellurium. LA-ICP-MS mapping of pyrite crystals shows that late stages are enriched in gold, tellurium and lead, representing concentration of these elements during metamorphism and related hydrothermal activity. The sulphur isotope composition of the pyrite varies with stratigraphic position, reflecting an origin for the pyrite in the depositional environment through microbial sulphate reduction. Where pyrite was converted to pyrrhotite, trace element contents are much lower, indicating element liberation during metamorphism. These observations are consistent with a model of black shale protoliths for orogenic gold deposits.

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

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 266u2005ppm) compared to the earlier-formed material (Se up to 181u2005ppm). 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

Liberation of selenium from alteration of the bowland shale formation: evidence from the Mam Tor landslide

The Bowland Shale Formation is anomalously rich in selenium (Se) at levels an order of magnitude greater than other black shales. The Mam Tor landslide, Derbyshire, England, offers an opportunity to measure whether the Se anomaly is conferred to the alteration products formed by oxidative water flow through the shale. Selenium in the shale is concentrated in diagenetic pyrite. Alteration of the shale causes decomposition of the pyrite to iron oxyhydroxide, which is carried in colloidal form (ochre) by springs draining the landslide. The iron oxyhydroxide contains anomalously high Se, and anomalously high Se was measured in water ponded where the ochre precipitated, although not in flowing groundwater. Other trace elements including cadmium and thallium also occur at concentrations higher than in other ochres. Given the widespread nature of the Se anomaly in the Bowland Shale Formation and equivalents across Britain and Ireland, any alteration products derived from workings through the shale should be disposed of with care.