渔塘坝富硒硅质岩成因及沉积环境探讨:硅、氧、碳和硫同位素证据

Petrogenesis and sedimentary environment of the cherts from Yutangba western Hubei Province: Evidence from silicon, oxygen, carbon and sulfur isotopic compositions

鄂西渔塘坝含碳硅质岩段(P_1~3m)为硒矿床的主要赋硒层位,其硅、氧同位素组成δ^(30)Si变化范围为0.5‰～1.8‰(平均1.25‰);δ^(18)O为22.7‰～27.1‰(平均25.3‰);硅质岩中黄铁矿δ^(34)S的值变化范围为-27.7‰～5.65‰,幅度大于20‰;硅质岩层位中方解石样品的δ^(13)C值范围为4.19‰～0.52‰。综合研究表明,渔塘坝硅质岩在成因上主要表现为热水沉积特征,成岩温度为45℃,形成于半封闭的浅海至滨浅海(滞留的盆地)缺氧沉积环境。

Yutangba selenium deposit is situated in western Hubei Province, where the carbon-bearing cherts stratum (P_1~3m) is the dominating selenium-bearing layer. The silicon and oxygen isotopes of the cherts are characterized by δ^(30)Si =0.5‰～1.8‰ (average in 1.25‰) and δ^(18) O = 22.7～27.1‰(average in 25.3‰) , respectively. The sulfur isotopes (δ^(34)S) of the pyrites from cherts vary from -27.7‰ to -5.65‰, showing a large extent more than 20‰. In addition, the carbon isotopes (δ^(13)C) of the calcites from the same layer as cherts range from 4.19‰ to 0.52‰. On the basis of these isotopic studies, it is suggested that the cherts from Yutangba selenium deposit are mainly derived from SiO_2 sedimentation from hot sea water in semi-closed epicontinental sea or littoral-neritic environment, and the diagenetic temperature is 45℃. Otherwise, the hypoxia would result in large-scale Permian biological death and extinction.