碰撞造山带逆冲推覆背景碳酸盐岩容矿铅锌矿床富氟流体成因——以青海莫海拉亨矿床为例

The fluorite-rich carbonate-hosted Pb-Zn deposits are mainly located in three kinds of tectonic environments in the world. For this kind of deposits, the controversial origins of the fluorine-rich hydrothermal fluids are the most interesting scientific questions to geologists. Due to recent research, the mineralizing fluids of fluorite-rich carbonate-hosted Pb-Zn deposits were usually added fluorine by alkaline magmatic fluids in extension rift environment, while they have two origins in orogenic foreland basin environment, including single basin brine and basin brine mixing with magmatic fluid. However, in the third important tectonic environment, the thrust belt in orogen, the existence meaning of the fluorite-rich carbonate hosted Pb-Zn deposits has not caused geologists’ attention and the origins of the fluorine-rich fluid are also never clearly answered by anyone. For this reason, a scientific question is proposed which is what the origins of fluorine-rich fluid in carbonate-hosted deposits produced in thrust belt in collisional orogen are. To answer the scientific question, the project plans to choose Mohailaheng deposit, a typical fluorite-rich carbonate-hosted Pb-Zn deposit located in ‘Sanjiang’ thrust belt in Tibetan orogen, as a case to investigate its fluorine-rich fluid origins. The analysis of noble gas isotope and the ratio testing of halogen for the fluid will be the most critical contents for this research. Other works, such as the geological mapping of alteration and mineralization, the investigation of the fluid’ chemic-physic characteristics and the study of mineral isotopes and elements also will be done. By synthetical contrast, this research will answer the scientific question above. It also will provide important information for understanding the origins of fluorite-rich carbonate-hosted Pb-Zn deposits occurring in thrust belt in collisional orogen and be valuable to the genetic classification of this kind of sediment-hosted Pb-Zn deposits in the world.

富氟型碳酸盐岩容矿铅锌矿床主要产于三类大地构造环境，多年来，其富氟流体因成因不清而广受关注。近年研究提出，在伸展裂谷背景，富氟流体多有碱性岩浆流体加入；在汇聚造山前陆盆地背景，有纯盆地卤水来源和岩浆作用参与两种可能。然而，在第三种重要环境，汇聚造山逆冲推覆背景，富氟型碳酸盐岩容矿铅锌矿床的研究却刚刚起步，富氟流体成因尚未被明确回答。为此，项目选择莫海拉亨——产于青藏高原碰撞造山带逆冲推覆系统的典型富氟碳酸盐岩容矿铅锌矿床为对象，以流体成分惰性气体同位素和卤素比值分析为突破口，结合典型矿带大比例尺蚀变矿化填图、流体物化性质分析和淀积矿物元素、同位素示踪，查明矿区富氟流体特征和来源。通过综合对比，回答碰撞造山带逆冲推覆背景碳酸盐岩容矿铅锌矿床富氟流体成因这一科学问题。研究对理解汇聚造山逆冲推覆背景富氟型碳酸盐岩容矿铅锌矿床成因具重要科学意义，对富氟型碳酸盐岩容矿铅锌矿床成因归属具重要科学价值。

项目在2015.1-2017.12期间执行，主要开展青藏高原三江逆冲推覆系莫海拉亨富氟型碳酸盐岩容矿铅锌矿床成矿作用研究。执行期间，项目开展了地表填图和钻孔编录、流体包裹体测温和惰性气体同位素-卤素含量分析，以及H-O-S-Pb同位素分析，并与区域成矿作用进行了对比。.研究表明，莫海拉亨铅锌矿体赋存在下石炭统杂多群碳酸盐岩组，矿体受逆冲断层上盘灰岩褶皱核部破碎带控制，呈切层板状产出，矿石以角砾状、细脉状构造为主。矿物组合主要为闪锌矿+方铅矿+黄铁矿+白云石+方解石+萤石+重晶石，萤石发育在铅锌硫化物沉淀阶段。.莫海拉亨铅锌矿床成矿阶段的流体包裹体均一温度峰值介于130~180 °C，盐度峰值介于1~24 wt. % eq. NaCl；成矿流体O和D同位素范围分别为-140‰~-94‰和-2.1‰~11.2‰；包裹体群体成分Na/Br、Na/K和Cl/Br比分别介于46~232、1~16、25~296之间；3He/4He比值为0.69 ± 0.04~1.3 ± 0.6 Ra。这些特征表明莫海拉亨矿床成矿流体具两种来源，其一为一套低温高盐流体，为来自矿区附近碳酸盐岩地层封存的蒸发浓缩海水形成的盆地卤水；其二为一套中低温、中高盐流体，为来自区域碳酸盐岩地层中封存的蒸发浓缩海水+少量岩浆水形成的区域流体。重晶石和硫化物S同位素介于-30.0‰~24.2‰，反映了细菌还原硫和有机质对硫酸盐热还原为主要硫源，两种流体混合为矿质主要沉淀机制。方铅矿206Pb/204Pb=18.298~18.492、207Pb/204Pb=15.298~15.539、208Pb/204Pb=38.169~38.504，反映金属来自区域各套地层，F来自区域富钾岩浆活动的幔源挥发份，金属及F通过区域流体在区域上运移时的萃取和搬运到达矿区。.综合分析三江带沉积岩容矿铅锌矿床和区域逆冲推覆构造的时空关系，提出区域铅锌成矿作用发育在逆冲推覆构造后的伸展阶段，逆冲阶段的构造挤压导致热液释放并沿逆深部拆离带运移，富集成矿物质（金属+F），逆冲后伸展作用导致热液上升到达矿区还原硫储库成矿。结合惰性气体同位素和卤素含量分析，首次建立了碰撞造山带逆冲推覆系富氟碳酸盐岩容矿铅锌矿床区域流体成因模型。.项目发表论文16篇（国外SCI 4篇，国内核心12篇），获2016年度国土资源科学技术奖一等奖1项。