The Upper Sinian bedded cherts are widely distributed on the southeast continental margin of the Yangtze plate,with a total thickness ranging from 20 to 150m,The cherts are very simple in chemical composition,with the contents of siliceous minerals exceeding 90% and those of other mineral species being very low.The total content of trace elements is considerably varialbe,The contents of most trace elements are low as compared with their crust clarke values.But these cherts are very rich in the trace elements Ba,As,Ab,Ag and U,coupled with relatively high contents of Fe,Mn and low contentsof Al,Ti,and Mg,It can be identified as the cherts of hydrothermal origin in terms of the values of Fe/Ti,(Fe+Mn)/Ti,Al/(Al+Fe+Mn)and U/Th,In the Al-Fe-Mn,and Fe-Mn-(Cu+Ni+Co)×10 diagrams and lg[U]-lg[Th],Cr-Zr diagrams.all the samples from the area studied fall within the hydrothermal sediment field .All these characteristics show that the cherts are mainly the product of hydrothermal processes.The fact that the δ30Si values of the chert range from 0.0‰ to 0.7‰ with δ18O from 20.1‰to 23.6‰ cleraly manifests that the submarine hydrothermal water is the main source of silica.
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