华南下寒武统黑色页岩中锌的富集机制

The mechanism of zinc enrichment in the lower Cambrian black shale in South China

华南大面积下寒武统黑色页岩具有多元素超常富集的显著特征,常被认为与古海洋的氧化还原条件、海洋生产力和热液活动等作用有关。已有成果显示,该区黑色页岩中存在锌(Zn)的富集现象,w(Zn)从几十到上万个10-6。本文通过不同沉积相剖面的元素分布规律、岩相学观测和硫同位素分析研究了锌的来源及其沉淀机制。结果显示:1)华南不同沉积相,甚至同一剖面上存在显著Zn的差异富集(高达15000×10^(-6)),Zn的富集期短暂且峰值出现于520Ma左右;2)锌主要以细粒或者粗粒分散状的闪锌矿形式产出,与钡冰长石和黄铁矿紧密共生,指示锌的来源与热液有关;3)硫同位素组成变化范围较宽(δ^(34)S=-26.5‰~+44.5‰),暗示着闪锌矿与共生黄铁矿沉淀时有微生物硫酸盐还原作用产生的硫化氢参与。结合埃迪卡拉纪-寒武纪时期海洋的氧化还原分层结构(即表层水体氧化,深部水体缺氧和共存的硫化水体),提出华南下寒武统黑色页岩中Zn的富集是520 Ma左右的热液活动与普遍存在硫化水体相互作用的结果。

The lower Cambrian black shale of South China is characterized with the high concentrations of multi-elements which were proposed to be related to the palaeoceanic redox conditions,marine productivity and hydrothermal activity.Previous research results showed that there is obvious zinc(Zn)enrichment in the black shale in South China with Zn contents widely ranging from hundreds to tens of thousands of ppm.In this paper,the origin and precipitation mechanism of zinc have been studied through the analysis of distribution patterns of Zn in different sedimentary facies sections,petrographic observations,and sulfur isotope analyses.The results are shown below.1)There are significant differences of enriched Zn(10-15000 ppm)in different lower Cambrian sedimentary facies in South China,or even in different facies of the same section.Especially,the Zn enrichment occurred in a short period with the peak at about 520 Ma;2)Zn mainly occurred in forms of fine and coarse disseminated sphalerite grains which are closely intergrown with hyalophane and coarse-grained pyrite,indicating that the zinc was sourced from the hydrothermal fluid;3)The wide range of sulfur isotopic compositions(δ^(34)s values varying from-26.5‰ to+44.5‰)suggested that hydrogen sulfide produced by the microbial sulfate reduction was involved in the precipitation of sphalerite and intergrown pyrite.Combined with the knowledge of stratified structure of the redox for the palaeocean in the Ediacaran-Cambrian transition period,namely,the overlying oxic surface water,the anoxic deep water,and the accompanied mid-depth sulfidic water(e.g.,euxinia),it is proposed that the Zn enrichment in the lower Cambrian black shale in South China was resulted from the interaction of hydrothermal fluid and commonly existed sulfidic water at about 520 Ma.