Geology and mineral assemblages of the early Cambrian black shales in the South Qinling: Implications for vanadium and barium mineralization

Early Cambrian black shales in the South Qinling (SQL) of China constitute one of the world's most important vanadium- and barium-metallogenic belts. It is controversial on the enrichment mechanics of vanadium (V) and barium (Ba) mineralization. The V and Ba mineralization in the Xichuan area from the northern SQL is mainly hosted by shales and chert of the early Cambrian black shales. We conducted mineralogy and geochemistry studies to reveal the mineralization process. Electron probe X-ray microanalysis (EPMA) studies show V is enriched in jarosite, illite and chlorite in shales. The jarosite develops as concretions or flakes and shows close affinity with apatite, with biogenic structures. EPMA and X-ray maps indicate a V heterogenetic distribution within illite and chlorite. Barite occurs as spots and veins in the dark chert and siliceous shale. K-feldspar in shales occurs as columns and flakes with inhomogeneous Ba concentrations. Calcite is intergrown with quartz and Ba-enriched K-feldspar closely in the shales recording the fluctuation of pH values in the diagenetic stage. An acidity environment favors silicification, and increased pH leads to silica dissolution and calcification along with illitization and chloritization. Slightly enriched heavy rare earth elements, positive Eu anomalies, and fluorite from the shales and chert suggest the hydrothermal fluid has existed. Compared with the representative V and Ba deposits associated with black shales worldwide, these deposits occurred synchronously with extinction or anoxic events as both V and Ba mineralization are redox-dependent and associated with organisms. These events could lead to the initial enrichment of V and Ba. Our study suggests that V and Ba mineralization is mainly associated with pH fluctuation in the diagenetic and biogenic processes.