Origin of Proterozoic metal-rich black shales from the Bohemian Massif, Czech Republic

Abstract

Proterozoic metal-rich black shales (metasiltstones) in the Bohemian Massif are always closely associated with submarine mafic volcanic rocks and occur in (semi)isolated basins that formed most likely in connection with the evolution of an Upper Proterozoic intracontinental rift. The black shales are characterized by an average of 3.5 wt percent C organic , and 5.2 wt percent S and carry up to 0.42 wt percent Zn, 0.21 wt percent V, 0.12 wt percent Ni, 0.12 wt percent Cr, 0.12 wt percent Cu, 0.06 wt percent As, 0.03 wt percent Mo, 132 ppb Au, 102 ppb Pd, 25 ppb Pt, 1.7 ppb Rh, and 1.3 ppb Ru. Their thickness varies from about 1 to 45 m. Sulfide minerals often include abundant framboidal pyrite, which occurs widely in C organic -rich layers. Grainy pyrite, Zn, Ni, Cu, and Mo sulfides appear to be restricted to late quartz and carbonate veinlets. Mo selenide, Mo telluride, galena, berthierite, and gold are very rare. Sulfur isotope results suggest the dominant source of sulfur to have been seawater sulfate, reduced by bacteria under different conditions. The delta 13 C values of the organic matter (-24.2 to -37.5ppm) reflect the combination of the carbon isotope composition of dissolved HCO 3 , its high concentration, and the type of organisms. The results of Rock-Eval pyrolysis as well as laser Raman spectroscopy show a high maturity, close to that of well-crystallized graphite. Fluid inclusion and stable isotope data from quartz and carbonate veinlets of very low grade regionally metamorphosed metalrich black shales at Kamenec and highly metamorphosed facies at Hromnice indicate that they resulted from the interaction between 18 O-rich fluids, volcanics, and metasediments. The interaction could have occurred during subsea-floor or later regional metamorphism.