Geochemical Characteristics Of Soils Derived From The Lower-cambrian Black Shales Distributed In Western Hunan, China

This study carries out a geochemical approach to the soils derived from the Lower-Cambrian black shales (the black-shale soils) in Ganziping areas of western Hunan province (China). During field working, one weathering profile numbered as Z and 10 topsoils numbered as GS were selected for surveying and sampling. The concentrations of major elements, mineral composition,heavy metals and the other trace elements as well as rare earth elements of soils, and the parent rocks (the black-shale) were determined using ICP-MS, XRF and XRD techniques.Results show that the black-shale soils were strongly weathered, with CIA values of higher than 74. Due to the higher intensity of chemical weathering, the chemical compositions of the soils are characterized by distinct depletion of CaO and Na2O and by significant enrichment of Al2O3 and Fe2O3.The XRD results indicate that quartz, muscovite, albite, hedenbergite and microcline comprise main mineral species in the soils,and quartz content is high in most samples.Clay minerals in the samples are composed mainly of Kaolinite, goethite, clinochlore and montmorillonite,together with minor other minerals. The kinds and contents of minerals in profile might be increased by aggravation of weathering of the soils.Compared to the China average soils, the soils are enriched with heavy metals including Ba,Sc,V,Cr,Co,Ni,Cu,Zn,Pb,Th,U among which U,V and Ni are in highest degrees. The complex enrichment indexes (EI) values of the weathering profile and topsoils are mostly higher than 2. The soils might be heavy-metal contaminated by reflecting the extreme chemical composit7ions of their parent black shales. Assessment using the method of enrichment factor (F) shows that the soils are contaminated by heavy metals U,Ba,V,Cu and Ni, among which heavy metals U is the highest degrees of contamination, with F values ranging from 5 to 20.On these bases, the authors assess V, Zn, Cu, Pb and Ni pollution in soils with the mineralogical method. The result show that the contents of heavy elements in the soils are inconsistent with their assessment values of ecologic effect. The contents of V and Zn in the weathering profile fail to reach the critical values of pollution. There exists pollution of Cu, Pb and Ni in the studied profile, which appears to pose a threat to the local inhabitants, and appropriate environmental countermeasures, should be taken into account.The results of correlation study and step regression analysis both indicate that heavy metals of the samples might be bounded mostly by clay minerals and Fe-oxides (goethite), but different metals might be bounded by different mineral, among which Sc,Co and Th by clay minerals, and Cr and Zn by Fe-oxides (e.g., goethite). However, heavy metals V,Pb,U,Ba,Mn,Ni and Cu might be free from these two phrases.Elemental ratios Zr/Hf, Ta/Nb and Nd/Sm of the samples from the profile are markedly consistent with corresponding parent rock. Additionally, the samples show the similar REE distribution patterns which are consistent with that of corresponding parent rocks, indicating that rare earth elements are not differentiated during pedogenesis. Therefore, it is suggested that the heavy-metal contamination of the samples was caused originally by metals from the parent rocks (black shales) in the areas, which is a natural source for soil heavy-metal contamination.Plots of heavy metal mobility (Change%) to chemical weathering index and Pearson correlation between heavy metal mobility and some relative factors show that the metal Ni might be a long lasting heavy-metal contamination for the soils, and the contamination extents of metals V and Pb might be reduced by releasing of the metals during weathering of the soils. However, the secondary enrichment of metals Zn,Mn,Th,Ba,and Co during the soil weathering might also cause the development of the heavy-metal contamination in future for the soils by these metals.