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GEOCHEMICAL CRITERIA OF THE UPPER SINIAN HYDROTHERMAL CHERT ON THE SOUTHEAST CONTINENTAL MARGIN OF THE YANGTZE PLATE

PENG Jun 1,2 , YI Hai sheng 2, XIA Wen jie 2 (1.Southwest China Petroleum Institute; 2.Chengdu University of Technology)  
The Upper Sinian bedded chert is widely distributed on the southeast continental margin of the Yangtze plate. Its thickness ranges from 20 m to 150 m. The chemical compositions of the chert are very pure, and the contents of siliceous minerals exceed 90 percent with those of any other minerals very low. The total content of the trace elements change to much degree. The contents of a large part of the trace elements are low compared with crust clarke values of these elements. But the chert is very rich in the trace elements Ba, As, Sb, Ag and U coupled with relatively high contents of Fe, Mn and low contents of Al, Ti, Mg. It can be identified as hydrothermal chert by the values of Fe/Ti, (Fe+Mn)/Ti, Al/(Al+Fe+Mn) and U/Th. In the Al Fe Mn, Fe Mn (Cu+Ni+Co)×10 triangle diagrams and lg[U] lg[Th], Cr Zr diagrams, all samples in the study area fall in the hydrothermal sediment field. All these characteristics show the cherts are mainly the prodocts of hydrothermal processes. That δ 30 Si values of the chert range from 0.0‰ to 0.7‰ with δ 18 O from 20.1‰ to 23.6‰ clearly manifests that the submarine hydrothermal water is the main source of silica.
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