西藏山南努日矽卡岩型铜钼钨矿床黑云母花岗岩锆石U-Pb定年及其成矿意义

Zircon U-Pb Age of the Biotite Granite of Nuri Skarn Type Cu-Mo-W Deposit in Shannan,Tibet and Its Metallogenic Significance

西藏努日矽卡岩型铜钼钨矿床是冈底斯成矿带东段南缘的大型矿床,也是近年来靠近雅鲁藏布江缝合带发现的首例大型白钨矿矿床。笔者对采自矿区北矿段与白钨矿化矽卡岩有密切关系的黑云母花岗岩进行了锆石LA-ICP-MS的U-Pb定年研究。分析锆石具有明显的振荡环带、Th/U比值大于0.1等典型岩浆成因锆石的特点。22个测点数据206Pb/238 U年龄可分为3组,799Ma、55.6 Ma、45.6~52.7 Ma,分别对应为拉萨地块形成年龄、捕获锆石的年龄、黑云母花岗岩形成年龄。在锆石原岩判别图解U-Y、Nb/Ta-Y中,所有点都很好的落入了花岗岩类的范围内,暗示黑云母花岗岩可能来源于新元古代的拉萨地块,测年数据表明,岩体成岩时代为始新世,可能系印度板块向欧亚板块北向俯冲碰撞阶段中晚期岩浆活动事件,该年龄早于矿区辉钼矿形成年龄。结合矿区第一期黄铜矿Re-Os年龄其与岩体形成年龄基本一致,指示矿区内存在两期成矿事件即早期白钨矿化、黄铜矿化和晚期黄铜矿、辉钼矿矿化。这也是首次在冈底斯成矿带南缘发现的与主碰撞期岩浆活动有关的成矿事件,同时也丰富了冈底斯带岩浆岩侵入序列与成矿事件的耦合关系,对于研究区域找矿具有十分重要的理论指导意义。

The Nuri Cu-W-Mo deposit,located in the east-south gandise belt,is a large skarn deposit associated with the biotite granite,in the Northern yarlung zangbo suture zone.Zircon U-Pb dating and trace element of the biotite granite were studied to understand relationship with scheelite in north ore,based on the LAICP-MS U-Pb geochronological investigation of 18 zircon grains in biotite granite.The results shows that the growth zonal apparently,and Th/U ratio range between 0.1~1.23（except 18point）more than 0.1,is the typical magmatic zircon.The U-Pb ages of zircon can be divided into three groups：799Ma,55.6 Ma and 45.6~52.7 Ma,representing the formation time of Lhasa block at neoproterozoic,the captured zircons age,the formation age of biotite granite.The U-Y、Nb/Ta-Y discrimination diagram of zircon trace element indicates that all of the points fall in granitoid rock range,indicating that the biotite granite derived from the Lhasa block.The dating data show that they formed during the Eocence epoch.This age is exactly the middle-late time at the main collision of the India-Eurasia,corresponding to the India plate subduction to Eurasia plate collision middle-late stage magma event,and the age is earlier than molybdenite formation age but same as the first stage of Re-Os age of chalcopyrite.The two phases in the metallogenic events are early stage such as scheelite and chalcopyrite mineralization and late stage such as molybdenite and chalcopyrite.This is the first study in the southern margin of the metallogenic belt,its enriched the magmatic intrusion sequence and metallogenic events and have a great theoretical guidance significance for regional prospecting.