| 其他摘要 | Within a few million years from Neoproterozoic to Early Cambrian (~544Ma), the types and quantities of biological materials had explosively increased to a huge amount. This event is called as "Cambrian Exploration". Simultaneously, there also existed a global anoxic event, called as “Oceanic Anoxic Event (OAE)”. The geological events resulted in the worldwide distribution of black shales in the lower Cambrian. In Southern China, the distribution areas of lower Cambrian black shales include Anhui, Zhejiang, Jiangxi, Hunan, Guizhou and Yunan provinces with the approximate range of 2000 Km. The formation of these black shales can partly represent the geologic evolution under the global background.
South China continent is marked by the Early Cambrian black shales which are characterized by abundant organic matter and various metals, such as Mo, Ni, Se, V, Ag, Au and PGE etc. Because its important scientific significance and potential value economically, these black shales are causing widespread interest by global geologists. Therefore, in this paper, the Early Cambrian black shales-hosted Ni-Mo poly-metallic layer in Zunyi area is selected to study in terms of mineralogy, major and trace element, C, Se, S stable isotopes etc. Some new conclusions are listed as following:
⑴ The Ni-Mo layer is characterized by lower SiO2 compared to host rock-black shales. Their contents of MgO, Na2O, K2O, MnO and TiO2 are similar to these of carbonaceous shalet. In the Ni-Mo layer, Fe, Ca and P are significantly higher than the carbonaceous shale, which is related to the increase of calcite and phosphorus in the Ni-Mo layer.
⑵ The trace elements, such as Se, Mo, Cd, Ag, Tl, U, Ni, Bi, Zn, Cu, V are enriched in the Ni-Mo layer with the larger enrichment coefficient ranging from n to n×103. The enrichment coefficient of Se and Mo is beyond 30,000 with largest enrichment coefficient of 50,014 for Mo.
⑶ Some geochemical parameters, such as selenium variations in black shales, U/Th ratio, negative Ce anomaly, slightly positive Eu etc, indicated the mixing of submarine hydrothermal and biological effects. In addition, the ratio of V/ (V+Ni) in black shales also indicated the anoxic sedimentary environment.
⑷ Mineral assemblage in Ni-Mo layer mainly included pyrite, vaesite, gersdorffite, Fe-bearing gersdorffite, violarite, heazlewoodite and C-Mo-S mixed-layer phase (MoSC). Most minerals experienced two mineralization stages, namely sedimentary - hydrothermal early stage and hydrothermal-biological stage.
⑸By the EMPA analysis, Ni is mostly hosted in nickel minerals . Mo is occurred in MoSC. The variation of As and Se in different minerals is complicated. In pyrite Se is less while As got a higher content, however, in MoSC the situation is the opposite. It was believed that due to different geochemical behavior of Se and As in the hydrothermal system, so the two different mineral phases formed.
⑹ Se stable isotopic measurement showed that the Ni-Mo layer has a narrow isotopic fractionation as δ82/76Se ranging from -1.56‰ to +1.85‰ and with average of +0.25‰. This result does not support the mechanism that metals remove and deposit from water by redox reactions. Based on the Se isotopic variations in Ni-Mo layer, a model is built: when Se was released from hydrothermal fluid, Se was preferentially incorporated into sulfide phase. This process did not cause the larger isotope fractionation. The small residual portion of Se in fluid was mixed into seawater, and was adsorbed by plankton. This process will result in about 1-2% shift of Se isotopic ratio. The model is favor to explain Se isotopic variations in the Ni-Mo layer and Se occurrence in different phase.
⑺The organic carbon contents of Ni-Mo layer and host rock range from 1.75% to13.9%. Organic carbon isotopic composition (δ13Corg) varied from -30.6‰ to -32.2 ‰. The negative shift may be due to the early Cambrian transgression in these area. |
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