湘西下寒武统牛蹄塘组黑色岩系的微量元素地球化学特征

Trace Element Geochemistry Characteristics of Niutitang Formation from Lower Cambrian Black Rock Series in Western Hunan

为探讨下寒武统黑色岩系在深水区域的沉积环境及其伴生的Ni-Mo多金属富集层的金属元素来源,对湘西寒武纪初期盆地相区龙鼻嘴剖面进行系统采样,分析了样品中干酪根的微量元素含量和全岩V、U、Mo与TOC含量并就微量元素比值与TOC关系进行了探讨。结果显示,干酪根中的Th/U、V/Sc和V/Cr值在剖面上均具有相似的变化规律,V、U和Mo与TOC的相关性可知剖面沉积过程中存在硫化环境,Ni-Co-Zn三角图解中多数样品分布于热水沉积区域。初步推断剖面下部的NiMo多金属富集层,其具有微量元素明显富集和碳同位素较大负值的特征,可作为地层对比的标志;寒武纪初始阶段深水区域可能处于短暂的硫化环境,伴随着沉积环境的充氧之后变为广泛的厌氧环境;黑色岩系沉积过程中有热液活动参与,Ni-Mo多金属富集层中部分元素的极度富集可能是还原的海水和海底热液共同作用的结果。

The Early Cambrian is a period with great change in Earth＇s history. In order to determine palaeoenvironment of black rock series in the deep sea area and the source of Ni-Mo-PGE layer, samples were systematically collected from the Longbizui section in the Lower Cambrian basin facies area, western Hunan. We analyzed trace element ratios in kerogen, correlations of TOC-（V, U and Mo） and Ni-Co-Zn diagram in bulk rocks. The results indicated that, Th/U, V/Sc and V/Cr values showed a same trend in the section. TOC-（V, U and Mo） correlation indicated euxinic conditions, and most samples were distributed in the hydrothermal area of the Ni-Co-Zn diagram. It suggested that a Ni-Mo-PGE polymetallic layer, which enriches trace elements and has relative negative δ^13Corg values, might exist in the lower part of the section and this layer can serve as a marker for stratigraphic succession. The results indicated that the deep sea areas of early Cam- brian had been through a short euxinic environment period and then turned into a widespread anoxic environment after oxygenated events in the bottom water. Hydrothermal activities existed during the deposition of black rock series. The extreme enrichment of some metals in the Ni-Mo-PGE layer might be resulted from reducing environment of bottom water and hydrothermal activities.