International Geologiical Congress - Oslo 2008


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MRD-13 Ore deposits associated with black shales: from their origin to their environmental impacts


Discussion on origin of the witherite deposits in large barium ore belt, southern Qinling Mountains, China


Jiajun Liu, China University of Geosciences (Beijing) (China)
Zhenjiang Liu, China University of Geosciences (Beijing) (China)
Wenchao Su, Institute of Geochemistry, Chinese Academy of Sciences (China)
Yan Yang, China University of Geosciences (Beijing) (China)
Jianping Wang, China University of Geosciences (Beijing) (China)


Barium in the form of barite deposits is wide distribution in the world, but in the form of witherite deposits is extremely rare. A large number of stratiform witherite and barite deposits are occurred in early Paleozoic silicalite formation in southern Qinling Mountains. The two types of deposits show both paragenesis and separation in space distribution, which constitute the unique large barium ore belt in the world. The systematic study of fluid inclusions in witherite mineral show that the homogenization temperatures are 110-250°, the components are characterized by CH4, N2, CO2, H2S and other components.
Based on these evidences the authors suggested that the thermochemical sulfate reduction was main metallogenic mechanism of the witherite deposits When late Sinian to early Cambrian, witherite deposits of the southern Qinling Mountains were located in the long and narrow faulted depression basin, which distribute along the NWW-SEE direction. There are multiple sets of cross-crust contemporaneous fault systems, which provided channels for hydrothermal activity. The outspread of contemporaneous fault conduced the development of a series of uplift and low-lying land. In large witherite mineralized zone, there are no lack of typical hydrothermal sedimentation features. Although some researchers raised the hydrothermal sedimentary genesis, it do not come down to the mechanism of association and separation of witherite and barite. However, why witherite and barite sometimes isolated and forming their independent deposit type such as witherite deposit, barite deposits, sometimes intimate paragenetic and forming witherite-barite deposit? Especially, the obvious zoning character of spatial distribution of witherite deposits and barite deposits, and in some deposits, the existence of witherite and barite occurred interactively forming typical characteristics of banded and laminated structure, which may indicate that the genesis of the witherite deposits in the study area might be related with the extreme environment of submarine.
In early Cambrian period, the abroad developed activity of oceanic currents, volcanoes and hydrotherm in Dabashan depression basin of the South Qinling area, not only provide abundant nutrients which survive and impel extreme growth of creatures for seawater, also provide abundant Ba2+. By means of the biogenicagency, Ba2+ in the seawater concentrated and precipitated on the sea floor in the form of biogenic barite, and formed the present barite deposits. But the effusional hydrothermal fluids rich of CH4 and other hydrocarbon mixed with seawater in the active state of hydrotherm. Since seawater rich of SO42-, hydrocarbons transit from reduction state carbon to the oxidation state of carbon inevitably, and thus, provide necessary CO32- for the forming of witherite. The special mechanism of forming the witherite deposits can be summarized in the following general form: Sulfate+Hydrocarbos-BaCO3+H2S+H2O+Other Productions


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