西藏仁布县新夏岩金矿床成矿时代探讨

Discussion on the Ore Forming Age of Xinxia Primary Gold Deposit, Tibet, China

西藏仁布县新夏岩金矿的矿体赋存于切割辉石闪长岩的近南北向断裂中,矿区中还发育有呈小角度斜切赋矿断裂的辉绿岩脉。通过对矿区内辉石闪长岩和辉绿岩脉中锆石进行LA-ICP-MS分析,获得两种岩浆岩中锆石的不一致U-Pb线的上交点和下交点年龄分别为543±23 Ma和14.2±2.4 Ma(n=14,MSWD=0.19)以及579±21 Ma和5.3±1.9 Ma(n=13, MSWD=0.074)。锆石的阴极发光显示的内部结构以及微区原位微量和稀土元素等地球化学特征显示,构成不一致U-Pb年龄的锆石测点记录了由岩浆捕获的围岩锆石的变质作用,下交点年龄应是代表岩浆侵位引发的热事件。这些年轻的年龄数据显示,新夏岩金矿的矿体应是形成于印度-亚洲陆陆碰撞的后碰撞伸展阶段,明显不同于马攸木和邦布岩金矿,代表了青藏高原南部在陆陆碰撞背景下一次尚待进一步揭示的金成矿事件。

The orebodies of Xinxia primary gold deposit are located in the north-south fault system in the pyroxene diorite, and some of these faults are cutted by diabase vein. LA-ICP-MS are taken on zircons from these two kinds of magmatite in this paper, and the analysis spots yield discordant U-Pb ages. The upper and lowwer intercepted ages are 543±23 Ma and 14.2±2.4 Ma（n=14, MSWD=0.19） for pyroxene diorite, and 579±21 Ma and 5.3±1.9 Ma（n=13, MSWD=0.074） for diabase vein respectively. The internal structure of these zircons revealed by cathodoluminescence and geochemical characteristics of trace and rare earth elements in these zircons show they are inherited zircons captured and metamorphosed by igneous rock that host them. These discordant U-Pb ages are the records of metamorphism induced by magmatism, and the lowwer intercepted ages represent the emplacement of the pyroxene diorite and diabase vein respectively. These ages may imply the ore-forming age of Xinxia primary gold deposit, Tibet, China, is much younger than Mayoumu in the west and Bangbu in the east of IYS, and the orebodies are formed in post-collisional extension setting of India-Asia continental collision. This gold metallogenic event still needs further discovery along IYS in southern Tibetan Plateau.