Abstract
Organic matter is preserved in unmetamorphosed and low-grade metamorphic pyrite and/or Mn-bearing black shales, pyrite-Mn stratiform ores, cherts and siliceous stromatolites of Proterozoic age in the Barrandian area of the Bohemian Massif. This organic matter consists mostly of kerogen with an isotopic composition and relative concentration of free radicals which depend on the metamorphic grade of the host rock ( δ 13 C PDB =-30.7‰ to -36.9‰ ; N rel , = 0.161 to 0.132). Enrichment of the kerogen in the light isotope of carbon, a marked isotopic shift towards more positive δ 13 C values with increasing metamorphic grade, and the very low crystallinity of the incipient graphite lattice reflect the original aliphatic (alicyclic) character and algal origin of the kerogen; its composition is similar to that of shungite. Chiefly aliphatic and alicyclic hydrocarbons were identified in the solvent extracts of these Proterozoic rocks. Alkane patterns differ according to host rock type, indicating their possible syngenetic origin. Other identifiable organic compounds (amino acids, carboxylic acids, phenols and aromatic aldehydes) are probably a result of later contamination. The concentrations of U, Th, V, Cr, Sb, Ag, Au, Zn and Cu in pyrite and pyrite-Mn ores are relatively low. In contrast, stratiform concentrations of U, V and chiefly Au, Ag, Sb and Zn are confined to siliceous stromatolites rich in kerogen, or to adjacent jaspilites. From a palaeogeographic viewpoint, the highest concentrations of metals occur at the boundary between the oxidic facies (stromatolites, jaspilites and intercalations of other rocks) and the reduced facies (black shales) where a syndiagenetic geochemical barrier developed. Mineralization was significantly influenced by subalkaline volcanism. Stromatolites of an intertidal type apparently formed where volcanoes rose to sea level. Stromatolitic breccias are intruded by chromium-rich green quartz veins and locally covered by tuffs.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
