Formation of Early-Cambrian Stratiform Barite Deposits in South China

Stratiform barite deposits located on the Yangtze Platform, South China, provide a large portion of the world's barite ore. However, useful descriptions of the deposits are remarkably limited, so their origin has remained fairly speculative. The host rocks are marine black shales of early-Cambrian age, and the deposits seem to lack high concentrations of Pb and Zn. This has led to suggestions that the barite deposits of South China have an exhalative, cold-seep origin. We visited seven active barite mines in the Gongxi, Fuluo, Dahebian and Bilin districts of Hu'nan and Guizhou Province, measured stratigraphic sections, and collected numerous samples of barite ore and surrounding black shale. Powdered samples were analyzed using an X-ray diffractometer, and rock sections were slabbed to examine mineralogy and geochemistry using Laser-Ablation ICP-MS. All mines tap into a single stratigraphic horizon of high purity barite that averages 2m in thickness, and extends over about 210 km2. The horizon is generally 80-100% barite (BaSO4). Minor minerals, including quartz (SiO2), sal-ammoniac (NH4Cl), goethite (FeO(OH)), wulfenite (Pb(MoO4)), and matlocite (PbClF) occur sporadically. Chert beneath and shale above the barite horizon contain pyrite (FeS2). Given the purity of the barite, the areal extent of the overall deposit, and the underlying and overlying pyrite, the overall horizon appears to be a primary depositional feature, where barite precipitated from Ba-rich water when a change in sulfur chemistry occurred. Specially, sulfur existed as sulfide before the barite, but as sulfate when the barite precipitated. The origin of barium is uncertain. Positive correlations between the amounts of barium, copper, nickel, cobalt, yttrium and rare earth elements suggest that hydrothermal fluids venting into seawater may have been a source. This input would have been distal and into sulfide-rich water so that most metals precipitated as sulfide minerals but the barium accumulated in the water.