黔中瓮安早震旦世磷块岩的形成环境及成因机制  

Formation environment and metallogenic mechanism of Weng'an phosphorite in the Early Sinian,Central Guizhou Province

作　　者：杨海英 肖加飞[1] 胡瑞忠[1] 夏勇[1] 何洪茜 Yang Hai-Ying;Xiao Jia-Fei;Hu Rui-Zhong;Xia Yong;He Hong-Xi(State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China;University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院地球化学研究所矿床地球化学国家重点实验室,贵州贵阳550081 [2]中国科学院大学,北京100049

出　　处：《古地理学报》2020年第5期929-946,共18页Journal of Palaeogeography：Chinese Edition

基　　金：国家自然科学基金联合基金项目(编号:U1812402);国家自然科学基金项目(编号:41972095,41830432);贵州省科技计划项目(黔科合基础[2017]1194)联合资助。

摘　　要：瓮安磷矿为鄂—湘—黔成矿带的重要组成部分,位于黔中古陆的东侧,其中,大塘矿段位于发育完整的下震旦统陡山沱组中,该组地层自下而上为下白云岩段、A矿层、上白云岩段和B矿层。元素地球化学特征表明:A矿层P2O5高于B矿层,而Sr含量、Mg/Ca值却相反,表明从A矿层到B矿层,气候干旱炎热程度升高。碳酸盐C、O同位素表明:δ13C、δ18O和Z值在A矿层分别为-4.30‰^-2.03‰、-11.30‰^-7.53‰、112~119,B矿层分别为-1.16‰~1.49‰、-8.41‰^-2.26‰、123~129,表明A矿层的盐度较低,B矿层的盐度较高。Ceanom在A矿层均大于-0.1,在B矿层均小于-0.1,另外,Mo、U、V元素在A矿层高于B矿层,表明氧化还原条件从A矿层的还原转变为B矿层的氧化条件。A矿层稀土元素呈“左倾型”配分曲线和热水元素的富集,指示海相热水输入;Si、Al、Ti含量占一定比例,指示了陆源风化物质输入;δ13C阶段性变化指示了上升洋流带来的深海海水,因此,陡山沱组磷块岩成矿物质可能有多个来源。A矿层“左倾型”稀土配分曲线以及磷酸盐球状、团块状结构和较高磨圆度,表明其成矿作用机制主要为海水机械动力;B矿层稀土元素呈“帽型”配分模式和大量微生物碎屑发育,表明其成矿机制主要为微生物生长成矿作用,伴随海水机械动力。Weng'an phosphorites are located at the east of Central Guizhou oldland and are important constituent of Hubei-Hunan-Guizhou phosphorite belt.Among them,the Datang ore block developed intact Early Ediacaran stratigraphic sequence,and the phosphorite occurred in the Doushantuo Formation which are divided into the lower layer of dolomite,layer A,the upper layer of dolomite,and layer B.Geochemistry showed that the P2O5 concentration of phosphorites in the layer A was higher than that of layer B,while the Sr and Mg/Ca were opposite.These characteristics indicate the degree of aridity and torridity increased from layer A to layer B.δ13C,δ18O,and Z value in layer A are-4.30‰-2.03‰,-11.30‰-7.53‰,and 112119,respectively,and those of layer B are-1.16‰1.49‰,-8.41‰-2.26‰,and 123129,respectively,indicating that the salinity increased from layer A to layer B.Ceanom of the layer A are greater than-0.1,whereas those of the layer B are less than-0.1,suggesting the redox conditions changing from anoxic to oxic.The NASC-normalized REE distribution patterns of the layer A show“left-inclining”,indicating the input of hydrothermal fluids.Furthermore,the Si,Al,and Ti content in the phosphorite and the periodical change ofδ13C indicate the input of terrigenous weathering and deep seawater.The“left-inclining”REE plots,high psephicity,and intraclast and blocky textures of phosphate from the layer A collectively suggest the phosphorite formed via mechanical power of seawater.The“hat-shaped”REE plots and microbial debris of phosphate from the layer B indicate the phosphorite formed via microbially mediated accretionary growth.

关 键 词：地球化学 形成环境 成矿物质 成因机制 黔中 

分 类 号：P588.24[天文地球—岩石学]