西藏列廷冈-勒青拉铅锌铁铜钼矿床硫化物Re-Os和Rb-Sr年龄及其地质意义

列廷冈-勒青拉矿床位于西藏冈底斯北缘多金属成矿带东侧,是该成矿带内一个独特的同时发育Pb、Zn、Fe、Cu、Mo五种元素矿化的典型矽卡岩型矿床。矿区的成矿作用均与东南侧的居布扎日复式岩体密切相关,目前已发现了位于岩体与碳酸盐岩地层接触带部位的Fe-Cu-(Mo)矿化和远离岩体外接触带部位的Pb-Zn-(Cu)矿化。因此,铅锌与铁铜钼矿体之间成矿关系的精确厘定,是明确这种具有不同来源属性的多金属共生矿床的成矿物质来源和成矿机制等科学问题的前提。文章通过辉钼矿Re-Os同位素和闪锌矿Rb-Sr同位素测年工作,从时间上对Fe-Cu-(Mo)矿体和Pb-Zn-(Cu)矿体的成矿时代进行限定。结果表明,与黄铜矿共生的辉钼矿Re-Os同位素等时线年龄值为(59.4±4.5)Ma,闪锌矿Rb-Sr同位素等时线年龄为(58±2)Ma。空间上,近岩体部位Fe-Cu-(Mo)矿化和远端矽卡岩部位Pb-Zn-(Cu)矿化发育相似的钙质系列矽卡岩矿物。因此,从时间和空间上均表明Pb-Zn-(Cu)矿化与Fe-Cu-(Mo)矿化为同一成矿系统。5件辉钼矿样品中w(Re)为1.5121×10~(-5)～7.4442×10~(-5),7件闪锌矿样品中初始~(87)Sr/~(86)Sr比值为0.71446～0.71471,指示Fe-Cu矿化成矿物质具有壳幔混合的特征,而Pb-Zn-(Cu)矿化的成矿物质则偏重于地壳来源的特征。对矿区内黄铁矿和闪锌矿成分分析结果显示:黄铁矿Co/Ni比值均大于1,闪锌矿Zn/Cd比值平均为292.11。结合前人资料分析可知,近岩体Fe-Cu-(Mo)矿化是在高温、高氧逸度的碱性环境下形成,远端外接触带Pb-Zn-(Cu)矿化是在中高温、还原碱性的环境下形成。列廷冈-勒青拉矿床形成于印-亚大陆碰撞的主碰撞阶段,该时期新特提斯洋板片的回卷可能是矿床形成的深部动力学机制。

Sulfide Re-Os and Rb-Sr ages of Lietinggang-Leqingla Pb-Zn-Fe-Cu-Mo deposit in Tibet and its geological significance

The Lietinggang-Leqingla deposit has five kinds of metals, i.e., Pb, Zn, Fe, Cu and Mo, in the same ore district. It is the most typical skarn-type deposit in the north Gangdise polymetallic belt. The mineralization of this deposit was closely related to the Jubuzhari composite intrusion in the southeast of the ore district, the Fe-Cu- (Mo) and Pb-Zn-(Cu) mineralization was explored in the contact zone between intrusion and carbonate rocks and distal carbonate strata respectively. Therefore, constraining the metallogenic relationship between Pb-Zn-(Cu) and Fe-Cu-(Mo) orebodies is the premise to understanding the source of metallogenic material and ore-forming mechanism of this deposit, which is considered to be polymetallic symbiosis deposit with different source attributes. In this paper, the authors used the molybdenite Re-Os dating and sphalerite Rb-Sr dating to constrain the ore-forming ages of Fe-Cu-(Mo) and Pb-Zn-(Cu) orebody. The result shows that the Re-Os isochron age of molybdenite co-existent with chalcopyrite is (59.4±4.5) Ma, and the Rb-Sr isochron age of sphalerite is (58±2) Ma. In space, the Fe-Cu-Mo mineralization near the intrusion and Pb-Zn-(Cu) mineralization far from the intrusion have developed similar calcium series skarn minerals, indicating that the Fe-Cu-Mo mineralization and Pb-Zn-(Cu) mineralization were formed in the same metallogenic system in time and space. The rhenium content of five molybdenites ranges from 1.5121×10^-5 to 7.4442×10^-5, and the initial Sr isotope ratios (⁸⁷Sr/⁸⁶Sr) of seven sphalerites range from 0.71446 to 0.71471, which indicates that the metallogenic material of Fe-Cu mineralization was derived from the mixture of the crust and the mantle, and the metallogenic materials of Pb-Zn-(Cu) exhibit the cha-racteristics of crustal origin. The chemical compositions of pyrite and sphalerite show that the Co/Ni ratios of all pyrites are higher than 1, and the average Zn/Cd ratio of sphalerites is 292.11. Based on the previous analysis, the authors hold that the Fe-Cu-(Mo) mineralization near the intrusion was formed in an environment of high temperature, high oxidation and alkaline, whereas the Pb-Zn-(Cu) mineralization far from the intrusion took place in an environment of middle-high temperature, reduction and alkaline. The Lietinggang-Leqingla deposit was formed at the initial stage of the India-Asian continental collision, and the deep geodynamic process which controlled the formation of this deposit was the rollback of the Neo-Tethys slab in this period.