Mineralogy and geochemistry of Lower Silurian black shales from the Yangtze platform, South China

Early Silurian black shales are widely deposited over the entire Yangtze platform and their origin are highly controversial. In order to identify the controlling factors of these black shales formation, the mineralogy and geochemistry of the Early Silurian organic-rich shales from the Yangtze Platform, South China, were investigated by using an optical microscopy, X-ray diffraction analysis, X-ray fluorescence spectroscopy, inductively coupled plasma mass spectrometry and field emission scanning electron microscopy. These black shales are characterized by high total organic carbon (0.37%-10.47%, avg. 3.86%) and total sulphur (0.02%-1.68%, avg. 0.67%) content as well as enrichment of some trace elements, including V (70.66 ppm 1740.52 ppm, avg. 312.75 ppm), Cr (68.72 ppm-301.94 ppm, avg. 117.97 ppm), U (3.90 ppm-52.49 ppm, avg. 16.58 ppm), Ba (316.69 ppm-2985.82 ppm, avg. 1523.38 ppm) and rare earth elements (94.26 ppm-340.50 ppm, avg. 202.46 ppm). The minerals mainly consist of quartz and clay mineral with average content of 54.8% and 29.3%, respectively. K-feldspar, dolomite, calcite and pyrite are next in abundance, followed by trace amounts of apatite, barite and Cr-bearing minerals. The dissolution of biogenic silica and recrystallization during burial is an important source of quartz cement. In addition to biological sources, terrestrial, volcanic and authigenic sources are responsible for these mineralogical and geochemical compositions. The Al2O3/TiO2, Zr/Sc, Th/Sc, LREE/HREE and Eu anom-alies indicate that the sediment-source region is derived from a mixture of felsic and intermediate rocks of the old upper continental crust. The Th/U and V/(V + Ni) ratios, Ce anomalies and authigenic U suggest that these black shales are deposited under an anoxic bottom water environment. The elevated concentrations of U-V-Ni-Cu-Zn-Cr-Ba and REE in these shales are predominated by the marine environment during deposition and influenced by biologic production in water columns and volcanic ash fall. High marine productivity resulting from volcanic nutrients promote the anoxic/sulfidic water column conditions of the restricted Yangtze basin, further resulting in organic matter enrichment. Elemental enrichment processes usually result in a valence change under anoxic environment, which is associated either with the clay minerals or direct precipitation from sea water. Under-standing the coenrichments of organic carbon and trace elements in these black shales is important for studying the formation mechanism and resource potential of hydrocarbon and critical elements.

Automated summary

The study indicates that the formation of Early Silurian organic-rich black shales on the Yangtze Platform is influenced by various factors, including biological, terrestrial, volcanic, and authigenic sources. The geochemical characteristics of these shales show influences from marine environments and volcanic ash deposition, demonstrating high marine productivity and organic matter enrichment.