中国早古生代若干高硒黑色岩系中层状硅质岩的地球化学特征及其成因意义

在中国早古生代若干高硒黑色岩系中发育有数层的厚度不等的层状硅质岩,SiO₂含量一般在 90%以上,岩石富Fe,相对贫Mn、Al、Ti。亲石元素大量亏损,但富典型的热水沉积指示元素Ba、As、Sb,也有一定的铂族元素(PGE)的异常,稀土元素的配分模式呈左倾,有明显的Ce的负异常和轻微的Eu的正异常。δ3 0 Si值变化范围为- 0 .7‰～ 1.2‰,δ18O值变化范围为 17.6‰～ 2 9.0‰。硅质岩的地球化学表明其为热水沉积作用的产物。文章最后讨论了热水成因硅质岩的形成与硒富集的关系,认为形成硅质岩的构造环境可能是地壳深部Se得以上升到浅部聚集的前提,而含硅热液则可能是硒的良好的”溶剂”。

Geochemistry and Genesis of Bedded Cherts in Some Typical Eopaleozoic High Selenium Black Shales, China

Bedded cherts, with varying thickness, are widely distributed in some typical eopaleozoic high Se black shales in China.Bedded cherts are mainly composed of SiO₂in chemical composition, generally more than 90%with relatively high content of Fe and low content of Mn, Al, Ti.Lithophile elements deplete in cherts, however some elements like As, Ba, Sb, etc., which are typically indicative elements for hydrothermal sedimentation, are rich in cherts.REE distribution pattern show obvious left slope with clearly negative Ce anomaly and slightly positive Eu anomaly.δ30Si values of cherts range from -0.7 ‰ to 1.2 ‰, and δ18O values of cherts range from 17.6 ‰ to 29.0 ‰.Above much evidence suggests that origin of bedded cherts mghit be ascribed to hydrothermal sedimentation.At last of the paper, the relationship between hydrothermal cherts and enrichment of Se is discussed.Based on some indirect evidence, it can be concluded that the tectonic environment that hosts cherts should be a prerequisite for the ascension of Se from the deep depth in the crust and for its enrichment at the shallow depth.In the meantime, Si-bearing hydrothermal solutions might be a desirable ”solvent ” for Se.