Variability in distribution of major and trace elements in Lower Eocene siliceous sections of the Transuralian Region, Russia

This paper presents lithologic and geochemical data from the sequence of the Eocene Irbit formation siliceous rocks (Transuralian Region) outcropping in a quarry in the Irbit deposit (thickness of 15 m) and in a large natural outcrop, Belaya Gorka (thickness of 13 m). The data show that both outcrops are composed of diatomites and clayey diatomites, both characterized by a certain degree of lithologic heterogeneity around their chemical, granulometric, and mineralogical compositions; microstructural features; and degree of diatom preservation. The values of indices important for the classification of siliceous rocks and determination of prospects for their industrial application—SiO2, Al2O3, Fe2O3, and clay fraction content—ranged from 66% to 77%, 7% to 14%, 3.00% to 5.60%, and 23% to 50%, respectively. In all studied lithologic varieties, element abundances of V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Ge, and Sb were two to three times higher than their respective abundances in the Earth’s crust. This is probably related to these elements’ involvement in the biological cycle and favorable conditions for transport. Rb, Cs, Ba, and Sr, as well as rare earth elements, are considered the most reliable indicators of lithologic and geochemical subdivision of a sequence of siliceous rocks, as they are associated mainly with clayey minerals. Variations in these indicators have recorded, with great probability, even short-term cycles and semi-cycles of silica sedimentation in the Transuralian Region, as well as tectonic regime and involvement of terrigenous influx.