Geochemical Characteristics Of Weathered Lower-Cambrian Black Shales In Central Hunan, China

This study systematically analyzed the weathering characteristics of lower Cambrian black shale in central Hunan, China. By analyzing chemical composition(i.e., major and trace elements) of fresh and weathered black shales, we established a new chemical weahter index for evaluating the weathering intensity of black shale, and explored the mobility of elements and the relationship between the enrichment of heavy metals and characterisitc of rare earth elements(REEs). The main findings were summarized below:(1) Previsous chemical weathering indices such as CIA(MIA), CIW,CIX, PIA, STI, R, WIP, V, and W, were established on the basis of geochemical studies on igneous rocks that are chemically homogenous.These chemical weathering indices are not suitable to assess the weathering degrees of black shale, because their values are less sensitive,and do not reflect the actual weathering degrees of weathered black shales. Based on the difference in chemical composition between fresh and weathered black shale, a new chemical index(Weathering degree of Black shale, WB) was established using typical discriminant function of major elements: WB = 28.8 × ln(SiO2) + 16.67 × ln(TiO2) + 10.52× ln(Al2O3) + 5.62 × ln(Fe2O3) ﹣ 2.01 × ln(MgO) + 4.10 ×ln(CaO)﹣4.24 × ln(K2O)﹣5.06 × ln(Na2O) + 5.07 × ln(LOI)﹣158.13. This new chemical index can overcome the weakness of previous chemical indices, and can be used to precisely assess the degree of black shale weathering.(2) During weathering of balck shale, Mn, Sr, Cs, Ba, Pb, U, V, Cr,Co, Ni, Cu and Zn were highly leached, as a result of the decomposition of primary minerals. In contrast, Th, Rb and Sc were less mobile and marginally leached. No significant differentiation of REEs was observed,and the mobility of REEs is somehow related to pH, and weathering intensity. The behavior of trace elements and REEs during mobilization and redistribution differs, due to the influences of pH, weathering intensity, and secondary minerals, especially Fe/Al/Mn oxides or hydroxides.(3) The values of enrichment factor shows that the weatheirng of black shales could result in the contamination of Mo, Cd, Sn, Sb, U, Ba,V and Cu. Principal component analysis, variance analysis and correlation analysis collectively suggest that i) δCe can be used to indicate the pollution levels of Mo, Sb, U, Sn, Ba, Cu, and V; ii) δEu can be used to indicate the pollution levels of Mo, Ba and Cu; and iii)(La/Yb)n and(La/Sm)n can be used to indicate the pollution level of Cu.