Abundant bedded chert deposits occurred at the Ediacaran-Cambrian transition interval at the Yangtze Platform, South China. However, there is not a non-controversial and integrated model for the origin of these chert deposits from trace element patterns and oxygen isotopic evidence. To understand the origin and the oceanic environment of these chert deposits, we analyzed the Si isotopic composition, major and trace elements of chert samples from two stratigraphically correlated sections with a depositional age around 542 Ma. The relationship between Al2O3 content and Si isotopic values indicated three end-member sources of Si derivation. The most negative delta Si-30 values (-0.3% to -0.5%.) reflect the silicon derived from hydrothermal fluids due to intense tectonic activity. The most positive delta Si-30 values (up to +1.2%.) could reflect the increase of Si-30 in the hydrothermal fluid by precipitation, or the signature of seawater. The medium delta Si-30 value (+0.2 parts per thousand to +0.7 parts per thousand) could be interpreted as the mixing of the hydrothermal fluid and volcanic materials with negative delta Si-30 and ambient seawater with positive delta Si-30. It is obvious that the injection of Si-rich hydrothermal fluid would affect the delta Si-30 and trace element distribution of seawater. In addition, we estimated that the delta Si-30 values of Ediacaran-Cambrian transitional seawater range from +2.2 parts per thousand to +3.5 parts per thousand based on assumed fractionation factors -2.3%.< epsilon < -1.0 parts per thousand between precipitated Si in chest deposits and initial dissolved Si in seawater. According to Si isotopic composition and other geochemical evidence, we maintain that hydrothermal activity played an important role not only in the formation of bedded chest and polymetallic ore deposits, but also in the oceanic environment and concurrent evolution of life during Ediacaran-Cambrian transition interval. (c) 2012 Elsevier B.V. All rights reserved.

Publication name	PRECAMBRIAN RESEARCH　Volume: 224 　Pages: 23-35　 Published: JAN 2013
Author(s)	Fan, Haifeng; Wen, Hanjie; Zhu, Xiangkun; Hu, Ruizhong; Tian, Shihong
Corresponding author	FAN Haifeng tonyfanhaifeng@gmail.com 1.Chinese Acad Sci, Inst Geochem, State Key Lab Ore Deposit Geochem, Guiyang 550002, Peoples R China 2.Chinese Acad Geol Sci, Inst Geol, Beijing 100037, Peoples R China .

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