Geochemistry Characteristic and the Environmental Implications of Chert from Ordovician in Qinhuangdao Area

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Abstract: The Qinhuangdao area is located in the northeastern margin
of the  North China Block. The Liangjiashan and Majiagou Formation  in this area are mainly composed of finegrained dolomitic limestone, finegrained dolomite, leopardlike limestone and calcirudite, which generally contain nodular, striped and lump cherts. XRD analyses show that the major minerals of the silicalite are quartz, calcite and some dolomite. The silicalite samples are wormlike, spherical and needlelike chalcedony aggregates with metasomatic structure in some of them under the microscope. The silicalites are poor in Mn and Fe, with Si / (Si + Al + Fe) ratios of 0.98  0.99, low ∑REE, and very weak negative Ce anomalies in the NASCnormalized flat REE patterns. The characteristics of Mn / TiO2 (< 0.5), U / Th (0.422  1.225), Ce / Ce* (0.92  1.02), (La / Ce)N (0.98  1.23), and (La/Yb)N (0.8  1.78) reflects a biochemical origin rather than hydrothermal origin formed in a setting of continental margin.