东秦岭鱼池岭斑岩型钼矿床成矿作用过程——来自成矿流体的约束

摘要

鱼池岭斑岩型钼矿床钼成矿期可分为石英-钾长石、辉钼矿-石英、绢英岩化和石英-碳酸盐化4个阶段.流体包裹体显微测温及流体成分研究显示,成矿流体由早阶段高温、高盐度、高氧化性、富CO2、富金属元素的CO2-NaCl-H2O体系岩浆热液,向晚阶段低温、低盐度、还原性、贫CO2、贫金属元素的NaCl-H2O体系大气降水热液演化,成矿深度也由石英-钾长石阶段的5.85km,变为辉钼矿-石英阶段的4.11km.流体H、O、S同位素组成表明,鱼池岭钼矿早期以岩浆水为主,晚期有较多的大气水混合,而S则可能来自于深部,具幔源S的特征.氧逸度、pH值、Eh值从早阶段到晚阶段均具有依次降低的趋势,Eh值显示钼矿床的形成总体属于弱还原环境.鱼池岭钼矿主成矿阶段前,钼成矿物质可能以钼酸、离子对为主,少量以钼-氧-氯络合物形式存在:主成矿阶段钼成矿物质主要以钼-氧-氯络合物形式进行迁移.%The hydrothermal period related to molybdenum mineralization of the Yuchiling Mo deposit can be divided into 4 s tages,i.e.,quartz-potash feldspar stage,molybdenite-quartz stage,sericite-quartz alteration stage and quartz-carbonation stage.Fluid inclusion studies and isotope geochemical analysis of the 4 stages show that the ore-forming fluids of the 4 stages changed physicochemically from magmatic fluid characterized by high temperature,salinity,oxygen fugacity,CO2 and high content of metal elements of the earlier stages to meteoric fluid characterized by low temperature,salinity,oxygen fugacity,CO2 and low content of metal elements of the later stages.The ore-forming depth of quartz-potash feldspar stage was about 5.85km,that of the molybdenite-quartz stage was 4.11km,and that of the last stage was near the surface.δ18OH2o shows that the ore-forming fluid was dominated by magmatic water at the early stage,whereas at the late stage,it could be produced by mixed magrnatic and meteoric water.The compositions of sulfur isotope show that sulfur might have been derived from the mande.The oxygen fugacity,pH values and Eh values decreased from early stage to the late stage,and the Eh values suggest a weak reducing environment during the precipitation of the Mo element.Before the main ore-forming stages,Mo element probably existed as H2MoO4 and ion pair,with a small amount of Mo-O-Cl complex.Mo-O-Cl complex was probably predominant when it entered the main ore-forming stages.The boiling and mixing action of the fluid as well as the water-rock interaction and evolution of the fluid from the depth to the surface could change the physicochemical properties of the ore-forming fluid and resulted in the precipitation of Mo and other metal elements.