柴北缘锡铁山铅锌矿床非层状矿体成因：成矿期碳酸盐矿物原位U-Pb年代学及C-O-Sr同位素制约

In situ LA-ICPMS U-Pb dating of ore-stage carbonate minerals could be applied to constrain the age of mineralization, thus providing significant insight into ore genesis. The dominated industry orebodies of the Xitieshan Pb-Zn deposit are non-laminated orebodies hosted in marble, which might be formed by late-stage hydrothermal superimposition after the metamorphism and deformation of the wall rocks, Tanjianshan Group. Hydrothermal carbonates including calcite, ankerite, siderite and dolomite were also formed during the mineralization of non-laminated orebodies. This proposed project will apply the LA-ICPMS U-Pb dating methods to ore-stage hydrothermal carbonates to date the mineralization of non-laminated orebodies, and further find out the possible links between the hydrothermal superimposition and regional tectonic, magmatic or hydrothermal events. C, O and in situ Sr isotopic compositions of hydrothermal carbonates and marble also will be analyzed. The analysis results in combination with the age data will be employed to study the source of ore-forming fluids, fluid-rock interaction, and hence the metallogenic mechanism of non-laminated orebodies in the Xitieshan deposit.

成矿期碳酸盐矿物LA-ICPMS原位U-Pb定年可精确限定成矿作用发生时间，为矿床成因研究提供依据。柴北缘锡铁山铅锌矿床主要工业矿体为赋存于大理岩中的非层状矿体，该类矿体可能形成于围岩滩间山群火山岩变质变形期后的热液叠加成矿作用，成矿期发育方解石、铁白云石、菱铁矿和白云石等热液碳酸盐矿物。本申请拟在矿床地质、岩相学、矿相学研究基础之上，采用LA-ICPMS原位微区U-Pb定年分析技术对非层状矿体成矿期热液碳酸盐矿物开展年代学研究，精确限定非层状矿体成矿时代，查明热液叠加成矿作用与区域地质事件耦合关系，并结合热液碳酸盐矿物和围岩C-O及原位Sr同位素示踪，分析成矿流体来源及流体-围岩相互作用，进一步阐明非层状矿体成因机制。

成矿年代学是矿床学研究的前沿与热点，精确限定成矿年代对研究矿床成因、探讨成矿事件与区域地质事件的耦合关系、厘定成矿构造背景、甚至找矿勘探等至关重要，然而由于铅锌矿床成矿矿物组合相对简单，且直接和铅锌矿化相关的、可用于放射性同位素定年的矿物较少，铅锌矿床精确定年一直是地学界的难题。本项目在详细的野外地质调查基础上，应用LA-ICPMS原位U-Pb定年测试技术，系统研究非层状矿体成矿期碳酸盐矿物形成时代，进而准确限定了非层状矿体成矿时代为217 Ma，与滩间山群火山岩形成时代（440-460 Ma）相距甚远，准确限定了其后生矿床的属性；本项目还获得了切穿硫化物矿体和大理岩围岩的方解石脉年龄（184 Ma），代表了成矿后脆性变形时代。赋矿大理岩及热液碳酸盐矿物C-O-Sr同位素分析表明岩浆相关热液流体与围岩相互作用是控制同位素交换的主要因素；而硫化物LA-ICPMS微量元素分析测试结果显示黄铁矿的变质重结晶过程不但排除了其中的包裹体，也析出了Mn、Ni、Cu、Zn、Ag、In、Sn及 Sb等微量元素，In、Sn和Ag等元素在变质变形作用发生过程中优先迁移进入变质变形黄铜矿中。综合锡铁山矿床成矿时代、硫化物交代碳酸盐岩特点、矿体和硫化物变质变形特征、矿物组成、蚀变分带、成矿物质及流体来源等特点，认为与侵入体相关的交代碳酸盐矿床模式（intrusion-related carbonate replacement deposit, CRD）更适合用来解释锡铁山矿床成因。本项目有效完成了设计目标，首次实现了对交代碳酸盐矿床广泛发育的成矿期碳酸盐矿物LA-ICPMS U-Pb年代学测试，证明了CRD矿床中成矿期方解石等低U含量矿物开展LA-ICPMS U-Pb年代学研究的可行性，将为同类矿床开展成矿年代学研究提供有力借鉴。