Petrological and geochemical characteristics of Palaeogene low-rank coal on the Faroe Islands: Restricted effects of alteration by basaltic lava flows
2016
The first combined petrographic and geochemical investigation
of coal from the Faroe Islands was performed as a case study to
understand thermal effects from basaltic lavaflows on immature
coal. The samples were divided into two distinct
groups:“normal”coal (xylite and detroxylite) and“altered
organic matter”(charcoal and organic particles dispersed in
samples rich in altered clastic mineral components or enriched
via hydrothermal fluids). The“normal”coal consists primarily of
huminite-group material dominated by ulminite. The proportions
of material from inertinite and liptinite groups vary from
sample to sample. The studied macerals are anisotropic with no
observed reaction rims or vacuoles. According to the mean
ulminite reflectance in combination with ultimate and proximate
analyses, the coal reached the lignite and subbituminous
stages. The maceral compositions together with coal palynology
indicate a predominance of gelified wood-derived tissues and
demonstrate that the coal evolved in wet forest swamps under
limno-telmatic to telmatic conditions. Alteration effects on
immature coals from overlying basalt flows were relatively
limited. Due to relatively rapid heat loss from the basaltic
lava, as verified by the presence of volcanic glass
(tachylyte), its imposed thermal effects resulted only in
development of a thin“anthracite-like”crust on samples with no
elevated coal rank. Associated hydrothermal fluids induced coal
hydrofracturing with subsequent mineral precipitation and
decomposition of the ambient feldspar-rich volcaniclastic
sediments. Altered organic matter is enriched in SiO2, Al2O3
and FeOtot, as well as in trace elements such as Ni and Cr. In
contrast, these samples are depleted in Hg (<10 ppb).
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