厄立特里亚阿斯马拉VMS矿床S、Pb同位素对成矿物质来源的约束

摘要

阿斯马拉(Asmara)地区位于非洲东北部的厄立特里亚,是重要的VMS型矿床成矿带.对其矿石结构特征和成矿物质来源的分析研究有利于区域成矿规律的总结.在分析矿床成矿地质条件的基础上,研究Emba Derho和DebarwaVMS型矿床的矿石结构特征、矿石硫、铅同位素组成及闪锌矿中的流体包裹体特征.结果表明:Debarwa和Emba Derho矿床成矿作用在经过VMS过程之后,其后期的变形变质过程对成矿的改造和叠加作用有限.闪锌矿中的流体包裹体研究显示其成矿流体具有低盐度、中-高温的特征.Debarwa和Emba Derho矿床中黄铁矿、闪锌矿和黄铜矿等原生硫化物矿物的δ34ScDT值变化范围较窄,数据集中在0.01％～0.48％之间,重晶石S同位素为高正值(δ34S=1.66％～1.73％),具有地幔硫和海水硫的混合来源特征.Debarwa和Emba Derho矿床矿石铅同位素组成稳定,显示正常铅的特征.对矿石铅同位素进行特征参数示踪、铅构造模式示踪和△β-△y图解示踪的结果表明:Debarwa和Emba Derho矿床中矿石铅主要为地壳与地幔混合的俯冲铅.%Volcanogenic massive sulfide (VMS) deposit is an important type of nonferrous deposit.In Eritrea,Neoproterozoic volcanogenic massive sulphide deposits and occurrences,to a large extent,distribute in Asmara-Nakfa Belt (ANB).The most important mineral deposits and occurrences known in Asmara-Nakfa Belt are those of the Emba Derho and Debarwa VMS deposits.Based on the research related to the Debarwa and Emba Derho deposit's oreforming geological conditions,the compositions of S,Pb isotopes as well as the characteristics about fluid inclusions in sphalerite and ore textures were studied,and the origin of oreforming materials was discussed.The results show that the transformation and superposition of regional metamorphism for deposits are limited.Ore-bearing fluids are characterized by medium-high temperature,low salinity.The δ34ScDT values of primary sulfide ore minerals distribute narrowly (0.01％-0.48％) in Debarwa and Emba Derho deposits,δ34ScDT value of barite in Debarwa deposit is 1.66％-1.73％.The characteristics of sulfur isotopic composition show that the sulfur in these two deposits is derived from the mantle mixed seawater.Meanwhile,the Pb isotopic composition of ore sulfides is stable and the ore lead is ordinary common lead with little U and Th radiogenic lead.According to the tracer analysis regarding the characteristic parameters,lead composition model and Aβ-Aγ diagram,the conclusion is drawn that the ore lead is typically crustmantle mixed subduction lead.