Biota and Organic Matter in the Late Sinian and Early Cambrian Black Rock Series of West Hunan and Its Significance for Metallization

A great amount of black rock series has been found in the Upper Sinian Members 2 and 4 of the Jinjiadong Formation, middle-upper Liuchapo Formation and the Lower Cambrian Xiaoyanxi Formation in West Hunan, which is associated with periodic sea-level changes. By the studies of relationships between the distribution and development of the biota and the abundance of Au, Ag, U, V, Ni, Mo and Cu in the Upper Sinian and Early Cambrian black rock series in Cili, Dayong, Yuanling, Xupu and Qianyang of West Hunan, central China, it has been revealed that the enrichment of Ag, V, and Mo is related with the development of multi-cell plants and vendotaenides, and that of Cu and Ni is related to flourishing of bacteria and shelly fossils. The black rock series in the study area contains abundant organic matter, among which the siliceous shale contains the highest TOC, amounting to 4.51-13.4%. All the values of the equivalent vitrinite reflectance (medium values) determinated with the IR-spectroscopic method in the area are over 2.65%, indicating that the organic evolution was at an overmature stage. Due to the effect of enhancement of organic maturation by hydrothermal fluids, the Upper Sinian and Lower Cambrian had a reverse organic maturity profile in most areas of West Hunan. The enrichment of Ag, V, Ni, Mo and U is a result of organic absorption, and that of Au might be attributed to the migration by hydrothermal fluids.