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A tentative discussion on the variation of selenium isotopes in black shale of Niutitang Formation,Zunyi,Guizhou Province,and its implications for depositional environment

ZHU Jian-ming1,2,JOHNSON Thomas M2,LUO Tai-yi3 and ZHU Xiang-kun4(1.State Key Lab of Environmental Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550002,China;2.Department of Geology,245 NHB,University of Illinois at Urbana-Champaign,Urbana,IL 61801,USA;3.State Key Lab of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550002,China;4.Key Lab of Isotopic Geology,Ministry of Land and Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China)  
Black shale of the Lower Cambrian Niutitang Formation is widely distributed in the Yangtze Platform of South China,and there is regionally developed a typical conformable polymetallic sulfide horizon.A lower profile of the black shale enriched in Se,Mo and other metal elements in Xiaozhu of Zunyi area was selected for Se isotope determination so as to investigate the probable source of Se and its depositional environment.The results show that remarkable variation of δ82/76SeSRM 3149 exists in the studied black shale sequence.The middle carbonaceous shale,Ni-Mo layer and carbonaceous carbonate have a narrow range of δ82/76SeSRM 3149,from +0.65‰ to +1.15‰ with a mean of 0.91‰±0.23 ‰(n=4);the upper carbonaceous shale has a value of-1.24‰;the bottom K-bentonite and phosphorite have a wide range of δ82/76SeSRM 3149 between-4.35‰ and +4.11‰.Combined with the published δ98/95MoJMC data of Ni-Mo layer in the Huangjiawan section,it is held that the middle black shale was deposited in an anoxic/euxinic environment,but an uncertain mixing process between basin seawater and oxic ocean water or hydrothermal fluid must have happened at that time.The bottom K-bentonite experienced a strong weathering and alteration,and the oxidization-reducing process probably occurred near the boundary between oxidizing water and anoxic water.Se in seawater might have been leached from the Se-rich bottom K-bentonite or tuff or from submarine hydrothermal fluid.Se isotopes indicate that the depositional environment of black shale in the Niutitang Formation of Zunyi area was possibly situated on the continental margin of a restricted basin and experienced different evolution stages from oxidizing-suboxic-anoxic/euxinic to suboxic.
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