Precious Metals in Black Shales of the Baltic Basin (Leningrad Region)

This article presents the results of studies on the noble metal contents of the Paleozoic dictyonema shales of the Baltic sedimentary basin. On the territory of the studied Kaibolovo–Gostilitsy prospecting area, high, up to potentially industrial, concentrations of noble platinum group metals (PGMs), comparable with the concentrations in sedimentary ore types taken into account by the State Balance of Mineral Reserves of the Russian Federation, have been confirmed with a large factual material (more than 600 shale samples). Increased gold and silver contents have also been noted. Palladium predominated in the composition of platinum group metals. The article presents the identified features of the PGM distribution over the area and in the section of the shale-bearing sedimentary strata. Precious metals in the dictyonema shales of the Baltic basin are distributed in the shales in the following forms: in terrigenous scattered finely dispersed native minerals (forms), in impurities in sulfide minerals (mainly in pyrite), and in species localized in the organic matter (Ag and partly Au). The conditions of formation of precious metal mineralization in the dictyonema shales have been reconstructed. The source of precious metals were rocks of the provenance area – the Baltic Shield – and seawater of the ancient sedimentation area, a supplier of a number of rare metals deposited on the geochemical reducing barrier of the dictyonema shales. Increased and stable concentrations of precious metals in combination with large-scale (billions of tons) geological resources of the dictyonema shales indicated the possible presence of a large unconventional mineral resource base of precious metals in the Baltic region of Russia.