Depositional environments of redox-sensitive trace elements in the metalliferous black slates of the Okcheon Metamorphic Belt, South Korea

Abstract

Metalliferous black slates consisting of black slates and interbedded coaly slates in the Okcheon Metamorphic Belt of South Korea were analyzed for redox-sensitive trace elements and rare earth elements (REEs) to examine their depositional conditions. Our data show that the coaly slates have elevated concentrations of redox-sensitive trace metals (U, V, Mo, and Cr), low Mn contents, high V/(V + Ni), V/Cr, and U/Th ratios, and high total organic carbon (TOC) contents. A general tendency of positive correlation between TOC and trace metals was established. The results suggest that the coaly slates were developed under a strongly reducing environment, while the black slates were deposited under a suboxic-oxic condition. The REE patterns of the coaly slates typically show positive Eu and negative Ce anomalies compared to the North American Shale Composite reference and they are essentially inherited from submarine hydrothermal fluids under high temperature reducing condition. The enrichment of the redox-sensitive elements including V (up to 3,564 ppm) and Mo (up to 358 ppm) may have been controlled by direct hydrothermal input of metals into the rift basin from hydrothermal vents, proximal from the vent sites, as supported by textural evidences, and the metals could have been fixed from seawater by means of scavenging process via exhalative oxide particles in a hydrothermal plume. The sorption of metals from seawater under anoxic conditions into organic-rich sediments occurred as well. The metal enrichments in the black slates seem to have been achieved in somewhat similar way to black shales in South China.