Barite and witherite deposits in Lower Cambrian shales of South China; stratigraphic distribution and geochemical characterization

Abstract

In south China, black shales of the Lower Cambrian series cover approximately one million square kilometers. Barite and witherite deposits in this black shale series are closely associated with black cherts and belong to a class of chert-hosted bedded barium deposits. Witherite deposits are confined to the Daba Mountains in the Qinling area, whereas barite deposits are found in both the Qinling area and the Jiangnan area to the south. The barium ore beds are usually underlain by beds with prominent phosphatic concretions and overlain by black shales.The barite ores can be divided into four textural types: massive, laminated, banded, and nodular; witherite ores are generally compact, massive, and nodular. There are carbonaceous mudstone interlayers in the barium ore beds, and organic carbon content may reach as high as 14 wt percent. Radiolarians have been found in the black cherts associated with witherite and barytocalcite ores.From the geosyncline to the platform, delta 34 S values of the barites increase, reaching values as high as 50 per mil. The lowest delta 34 S value of barite (8ppm) is within witherite ore beds, suggesting that some sulfur was derived from volcanic sources. However, most delta 34 S values either are close to the sulfur isotope composition of Cambrian seawater or are significantly higher. The highly positive values suggest that bacterial sulfate reduction in closed basins led to strong fractionation of sulfur isotopes and concentration of delta 34 S in residual sulfates. Carbon and oxygen isotopes of the witherite and barytocalcite have mean values of delta 13 C PDB and delta 18 O PDB of-18 and -11 per mil in the Ziyang witherite deposit, and -14 and -13 per mil in the Chengkou deposit. The light carbon values indicate that the carbon in the barium carbonates was derived from organic matter decomposition, and thus, that these minerals formed during early diagenesis.Based on regional geology and geochemical analyses, a genetic model of mineralization is suggested that includes submarine eruption and hot spring activity during late-stage volcanism that provided barium to seawater in the basin of deposition. Deep, euxinic conditions prevailed in the late Sinian, which changed in the Early Cambrian to even greater restriction with a failure of resupply of sulfate by the open ocean. Decomposition of plankton settling through the water column may also have released barium either to the basin waters or to the pore water of the sediment, leading to deposition of barite and witherite in different sedimentary and geochemical environments.