Structural and geochemical features of coal-bearing sediments and sources of rare element impurities in coals of the Rakovka depression, Primorsky Krai, Russia

Abstract

There are known brown coal deposits in Primorsky Krai (Russian Federation), where coals contain rare elements (Ge, U, REE, etc.) up to industrial concentrations. One of the known metalliferous coals is the Rakovka coal-bearing depression, located on the southern edge of the Khanka (Prikhankaiskaya) Lowland, with an area of about 70 km2. Rare-metal granites with a developed weathering crust are spread in its basement and flanks. The structural conditions forming the depression were studied using measurements of orientations of structural elements (layering, contacts, veins, and dikes) with regard to kinematic conditions of faulting. The coal and host rocks were sampled in sections in the cross strike of the coal seam. Granites and dikes cutting them were sampled by separate rock chip samples. The chemical composition and element content were determined for all samples. It was found that the adjacent rocks played a major role in the formation of the Rakovka rare-metal-coal deposit. The rare-elements enrichment is not associated with active tectonics, faults, and hydrothermal sources, as was previously assumed. Rather, it was caused by the hydrogenic and clastogenic removal of these metals from the weathering crust of granites of the depression's flanks and argillated basite dikes cutting the granites. The hydrogenic nature of the anomalous accumulation of U in sorbed form on organic matter of coals is confirmed by the predominant enrichment of low-ash beds. Rare earth elements entered coal seams both in mineral and dissolved forms. A model of REE and uranium input into the coal-bearing sediments of the Rakovka depression was proposed.