Mineralogy and geochemistry of metalliferous black slates in the okcheon metamorphic belt, Korea: a metamorphic analogue of black shales in the South China block

Metalliferous black slates, which were locally exploited for their low-quality coal, are distributed in the Late Proterozoic to Paleozoic Okcheon Metamorphic Belt (OMB) of the Korean Peninsula. The mineralogy of the fine matrix is dominated by either quartz-(Ba, V)-mica-graphite or Quartz-Ba-feldspar-graphite. Polycrystalline submillimeter ellipsoids and elongate lenses aligned along foliations and veinlets are scattered through the fine matrix. Both ellipsoids and veinlets include many minor minerals containing rare elements: titanite, apatite, allanite, rutile, polycrase, barite, uraninite, xenotime, armenite, zircon, molybdenite, and sphalerite. Large graphite-apatite ellipsoids (nodules) with widths of several centimeters also occur in the highly carbonaceous black slates. Goldmanite occurs locally as porphyroblasts. The maximum rare element contents are: Ba 9.7%, V 2.04%, Mo 0.13%, U 0.11%, Cr 0.33%, Cu 254 ppm, Ni 479 ppm, Zn 607 ppm, Y 255 ppm, platinum-group element (PGE) + Au 309 ppb, and carbon 57%. The occurrence of the black slates and their elemental abundances suggests that most of the rare elements were accumulated from seawater in an oxygen-poor environment. However, the high Ba content of the OMB black slates indicates some hydrothermal input into an organic-rich basin. Although metamorphism and multiple deformations prevent a direct temporal and spatial correlation, metal abundances and a close association with graphite-apatite nodules and low-quality coal suggest that the OMB black slates are metamorphosed analogues of the Early Cambrian Ba-V deposits hosted by the black shales in the South China Block.