Geochemistry and rare-metal potential of coals of the Sakhalin coal basin, Sakhalin island, Russia

The study presents new data on the geochemistry coal deposits of the Cretaceous, Paleogene, and Neogene age in the Sakhalin Basin. A wide range of trace elements were studied in 240 coal samples and 138 coal-bearing rock samples from the main fields of the Basin that are under development and available for study. This study demonstrated that the coals in the Sakhalin Basin are generally characterized by concentrations of most trace elements close to the average estimates for the world coals along with, in addition, a mixed chalcophile-siderophile and lithophile geochemical assemblage typical for the transition areas of the continent-ocean zone. On average for the Basin, the coals are enriched in Sc, V, Co, Br, Sr, and Ba compared to the world average. Ash of coals from the Sakhalin Basin is characterized by the above-Clarke content of rare alkali metals (Li, Rb, Cs), Cr, Ni, Cu, Y, and Zr, in addition to the above elements. HREE, Br, Cu, Ag, Hg, Nb, Ge, Mo, W, As, Ni, and Pb specialization was identified for some coalfields. The levels of accumulation of Sc, Y, Sr and heavy lanthanides in coal ash in some coalfields exceed the minimum possible commercial concentrations. The study demonstrated that the geochemical trace element characteristics of coal-bearing sediments in the Sakhalin Basin reflects the peculiarities of the geological evolution of this crustal block. Despite the high tectonic activity and volcanism in the Cenozoic, the specific composition of the rocks in the flanks and the basement of the coal-bearing depressions defined the low rare-metal potential of the basin. Scandium, V, Co, Ni, Ba, and Sr anomalies are associated with the predominantly basic composition of the flanks of the depressions, while local Hg, Sb, W, Cu, and Ge anomalies are connected to the hydrothermal activity. •The coals in the Sakhalin Basin have a geochemical specialization typical for transition areas of the continent-ocean zone.•The coals are enriched with Sc, V, Co, Br, Sr, Ba, Br, Cu, Ag, and Hg relative to the average estimates for the world coals.•Some coals have high concentrations of Sc, Y, Sr, Hg, and medium and heavy lanthanides reaching commercial values.•Distribution of Sc, Y, Sr, Hg, M and HREE with high concentrations suggest a hydrogenous mechanism of their accumulation.