高板河黄铁矿Re-Os定年研究-同步二级蒸馏技术探索及应用

Re-Os isotope system is the most direct and effective approach for the study on metal deposit metallogenic epoch and source of ore-forming materials. Nevertheless,it is difficult to achieve a satisfied accuracy and precision for Re-Os dating of sulphide samples with pg/g level content of osmium. Two aspects are included in this study. ⑴ A technology of synchronization of the secondary Os distillation is put forward in the study. Various factors that influence the moore acidity of a trapping solution and Os recovery during the process of distillation will systematically be studied. The optimum conditions for achieveing a high Os yield will be investigated. An effective way to decrease the moore acidity of trpping solution will be explored when 1ml volume of the trpping solution is utilized. The repeatability and stability of Os separation will be investigated using standard materials and unknown pyrite samples with low Re and Os concentration.⑵ The high precision instrument ELEMENT XR has some outstanding advantages,such as ultralow dark current and superhigh sensitivity(two million cps/ppb)which are conducive to gaining much higher precision for Os isotope ratios measurement after careful optimizing related parameters of the instrument.The satisfied accuracy and precision of Re-Os dating for sulphide samples with low content(pg/g level) of osmium will be achieved at last.On the basis of the new developed method, Re-Os isotopes composition of laminated pyrite in the structure of syngenetic sedimentary in Gaobanhe (east district of Hebei province) SEDEX deposit will be determined. The accuracy sedimentary isochron age of Re-Os for this deposite will be dated clearly.It is an effective Os isotopes composition restrict for the study of sulfide deposit on material sourcse and mineralization mechanism in Gaobanhe district. The newly developed Re-Os isotope dating method is to important for Re-Os dating study on hydrothermal deposit and sedimentary ore deposits for samples with ultra-trace Re and Os concentration.

Re-Os体系是研究金属矿床成矿时代和成矿物质来源最直接有效的方法，但是Os含量在pg/g级的硫化物样品Re-Os定年很难获得满意的精度。因此，本研究：⑴针对同步二级蒸馏技术，系统研究蒸馏过程中影响捕集液酸度及Os回收率的各种因素，以期提高Os回收率、降低捕集液酸度，实现小体积捕集液高Os信号测量；⑵利用ELEMENT XR仪器的超低噪声（＜0.2cps）、超高灵敏度(2Mcps/ppb)优势，进行仪器参数和分析条件优化，以显著提高低含量Os同位素测量精度。从而实现低含量硫化物样品精准Re-Os定年。 在此基础上，选择高板河喷流沉积硫化物矿床同生沉积组构中纹层状黄铁矿进行Re-Os同位素定年研究，厘定该矿床的沉积成矿年龄，为探讨该矿床成矿物质来源和机制提供有效的Re-Os同位素制约。 本研究建立的Re-Os定年方法对热液矿床及沉积岩容矿矿床等低含量硫化物Re-Os定年研究具有重要意义。

本研究建立了高氯酸-逆王水溶样、同步二级Os蒸馏分离的Re-Os定年新方法。方法优势如下：（1）溶解量提高: 常规方法溶样量小于1.2g，新方法黄铁矿可以达到3g，其它类型硫化物可以达到4g，溶样量是原来的2-3倍。（2）成本降低：传统方法溶解1g金属硫化物需要20 ml以上溶剂，新方法17 ml氧化剂即可使溶样量达到传统方法的3倍；（3）降低能耗：传统方法Carius管要在220℃以上的条件下加热24h-48h，新方法在205℃下加热24h即可使管内达到同位素交换平衡。（4）提高测量准确度：以同步二级蒸馏方式有效克服了Os吸收液酸度过高所导致的测量不准确的问题。方法应用于模拟低Os含量样品(基体内加入GBW HLP)分析，测定的模式年龄平均值为219.4±2.1 Ma (n=9)，所得结果与标准值221.4±5.6 Ma在不确定度范围内吻合一致。.在上述方法实验基础上，本项目对华北克拉通北缘中段的高板河Sedex型黄铁矿矿床中代表原生特征的层状及发生重结晶作用的胶状黄铁矿进行了Re-Os同位素体系定年研究。喷流沉积矿床虽为同生矿床，但由于成矿时代久远常常受到后期地质作用改造。华北克拉通北缘发育了很多喷流沉积矿床，成矿后的叠加改造作用对研究矿床的成矿过程带来较大困难。研究结果表明，高板河矿床成矿过程分为两个期次，分别为1505±55Ma的原生年龄和149±5.8Ma的改造年龄。早期为喷流沉积成矿，晚期为热液叠加改造成矿。原生年龄的获得不但确定成矿年龄，同时也为约束高于庄组地层的沉积上限时间提供了依据。改造年龄的获得说明矿区矿体受到岩浆热液的叠加改造，并指出岩脉形成的时间应为晚侏罗世而非前人认为的中侏罗世。Re-Os同位素体系的初始值显示，在早期的成矿过程中，Os初始值为0.18，显示成矿流体具有壳幔混合特征；到了晚期成矿阶段，Os初始值为1.87，成矿流体以壳源为主。