Abstract

Formations rich in organic matter occurred throughout most of the geologic history of the Bohemian Massif. Accumulation of organic matter was connected with basic submarine volcanic activity, with transgressive sedimentary regimes, and with limnic and paralic environments.The largest carbon-rich marine sediment accumulation occurred in late Proterozoic and early Paleozoic time. In these periods, substantial concentrations of trace elements and sulfur accumulated in black claystones, where the metal associations were determined mostly by synchronous volcanicity. In spite of the substantial amount of accumulated trace elements, only subeconomic concentrations of pyrite-pyrrhotite and U-V-Sb-Au mineralizations were formed in association with Proterozoic stromatolites. Subsequently, Cadomian and Variscan metamorphic processes caused substantial remobilization of metals and sulfur in black claystones and produced disseminated gold and uranium deposits. During the emplacement of postkinematic Variscan granitoids, organic matter and some metals were leached from the surrounding low metamorphic, carbon-rich sediments by hydrothermal solutions of deep crustal origin. A typical feature of these deposits is the presence of isotopically light bitumens (delta 13 C PDB = -40.8 to -56.5ppm) and the biological tracers phytane and pristane, indicating a biogenic origin. As a reducing agent, organic carbon (graphite) controlled the localization of Variscan hydrothermal uranium mineralization which occurs in graphitized shear zones in the highly metamorphosed basement of the Bohemian Massif. The crystallinity of the graphitized organic matter decreases sharply in these zones, probably due to hydrothermal decomposition of the graphite lattice. The low crystallinity of graphite and anomalously light isotopic composition of the hydrothermal carbonates in this type of deposit (delta 13 C PDB = -6 to -ppm) indicate that graphitized organic matter was involved in the reduction of uranium-bearing hydrothermal solutions.During the formation of sandstone-type uranium deposits in the carbon-rich platform sediments of the Bohemian Massif, the organic matter of host rocks became oxidized by the action of oxygen dissolved in ore-bearing solutions. This process gave rise to organic polymers similar to humic acids, which reacted with UO (super +2) 2 to form insoluble uranium organic compounds.

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