Abstract
Mid-Jurassic pyritic coals exposed at the village of Brora, northern Scotland, UK, contain a marked enrichment of tellurium (Te) relative to crustal mean, average world coal compositions and British Isles Carboniferous coals. The Te content of Brora coal pyrite is more than one order of magnitude higher than in sampled pyrite of Carboniferous coals. The Te enrichment coincides with selenium (Se) and mercury (Hg) enrichment in the rims of pyrite, and Se/Te is much lower than in pyrites of Carboniferous coals. Initial pyrite formation is attributed to early burial (syn-diagenesis), with incorporation of Te, Se, Hg and lead (Pb) during later pyrite formation. The source of Te may have been a local hydrothermal system which was responsible for alluvial gold (Au) in the region, with some Au in Brora headwaters occurring as tellurides. Anomalous Te is not ubiquitous in coal, but may occur locally, and is detectable by laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS).
Highlights
Due to its low abundance in the Earth’s crust (2 μg·kg−1 ) [1] and analytical limitations for low detection, tellurium (Te) is seldom recognised in many geological settings
Brora coals show a dark brown to dull black lustre, are soft and break up typical of subbituminous ranking coals
Jurassic coals exposed at the town of Brora in northern Scotland contain pyritic bands with anomalous Te content, recognised by LA-ICP-MS methods
Summary
Due to its low abundance in the Earth’s crust (2 μg·kg−1 ) [1] and analytical limitations for low detection, tellurium (Te) is seldom recognised in many geological settings. Te has been previously suggested to concentrate in low-temperature sedimentary environments, controlled by redox variations and forming inclusions in sulphide minerals and red bed successions [5,6]. This suggests that Te may locally concentrate across sediments. Te in exposed coal seams, historic coal mining sites and associated tailings may pose a source of contamination, resulting in potentially toxic levels and a risk to soils, surface and groundwater systems. This study reports the rare occurrence of elevated Te in pyritic banded Jurassic coals of Brora (northern Scotland), recognised by Minerals 2017, 7, 231; doi:10.3390/min7120231 www.mdpi.com/journal/minerals laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) methods. Results highlight (northern Scotland), recognised by laser ablation inductively coupled plasma-mass spectrometry the(LA-ICP-MS)
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