西藏念青唐古拉矽卡岩型铅锌矿床成因机制:来自石榴子石U-Pb年代学约束

Lacking of suitable dating minerals leads to slow progress for the geochronology of Pb-Zn skarn deposit. The success of U-Pb dating for skarn garnet in recent years has provided a new method for determining the age of this type of deposit. A series of Pb-Zn polymetallic deposits developed in the Nyainqêntanglha belt which belongs to a significant part of the giant Gangdese metallogenic belt, Tibet. The current research has the controversy between the exhalative-sedimentary and magmatic-hydrothermal Pb-Zn mineralization and is also difficult to determine whether a multistage magmatic hydrothermal mineralization has developed. One of crucial factor is that no effective dating minerals can be used to accurately limit the Pb-Zn mineralization age. Therefore, this project intends to choose the most controversial Lawu and Yaguila Pb-Zn skarn deposits, for the first time to carry out the in suit analysis of garnet elements and U-Pb geochronology to constrain the mineralization age of Pb-Zn skarn, to reveal the incorporation mechanism and genetic significance of the trace elements in the garnet, particularly the U, to identify the coupling relationship between the multi-stage magmatism and Pb-Zn skarn mineralization in the area, and finally to and discuss the regional mineralization geodynamics to enrich and improve regional metallogenetic theory.

由于缺乏合适的定年矿物，矽卡岩型铅锌矿床的年代学研究进展缓慢。近年来矽卡岩石榴子石U-Pb定年的成功为确定该类矿床的形成时代提供了新的方向。西藏念青唐古拉地区发育一系列矽卡岩型铅锌多金属矿床，是冈底斯巨型成矿带的重要组成部分。当前的研究存在海底喷流沉积成矿作用与岩浆热液成因之争，亦存在是否发育多期岩浆热液成矿作用的判定之难。归根结底在于缺乏行之有效的定年矿物来精确约束铅锌矿化的年龄。本项目拟以最具成因争议的拉屋与亚贵拉矽卡岩型铅锌矿床为研究对象，首次开展石榴子石微区元素与U-Pb年代学分析，约束矽卡岩铅锌矿化的形成时代，揭示石榴子石微量元素，尤其是U的配分机制与成因意义，查明该地区多期岩浆活动与矽卡岩铅锌（钼）成矿作用的耦合关系，探讨区域成矿动力学背景，丰富和完善区域成矿理论。

项目以西藏冈底斯成矿带的拉屋与亚贵拉矽卡岩型铅锌矿床为主要研究对象，针对矽卡岩矿物，尤其是石榴子石开展了LA-ICP-MS微量元素和U-Pb定年研究。结果显示，石榴子石为钙铁榴石-钙铝榴石系列，结构较为复杂，振荡环带发育，成分变化较大（And100Gr0~And20Gr80）。石榴子石是主要的稀土元素载体，而辉石基本不含稀土元素。钙铁榴石和钙铝榴石的球粒陨石稀土元素配分曲线显示一定的差异性，其中钙铁榴石显示富集轻稀土元素、亏损重稀土元素、Eu多为正异常特征；而钙铝榴石显示亏损轻稀土、富集且相对平坦的重稀土配分模式；稀土元素进入石榴子石的机制为钇铝（YAG）榴石型。石榴子石还含有一定量（高达数百ppm）的矿化微量元素，如Sn、W、As、In等。U在石榴子石的含量相对较低（<1~10 ppm），且在同一颗石榴子石的分布也不均匀。结合透射电镜研究，提出U可能是以U6+的形式置换钙铁榴石中的Fe3+。U-Pb年代学结果显示拉屋矿床形成于54.6±2.9 Ma，亚贵拉矿床形成于65.0±4.7 Ma。结合区域其他矽卡岩矿床的U-Pb年龄，查明了该地区多期岩浆活动与矽卡岩铅锌（钼）成矿作用的耦合关系，同时指出冈底斯西段有关的Cu、Au矿床形成于晚白垩世（85 Ma），可能代表了新特提斯洋向北俯冲时期的成矿事件；而冈底斯大量分布的与Fe-Pb-Zn-Ag-W-Mo相关的矽卡岩矿床集中形成于65~50 Ma之间，代表了在古新世到始新世早期，新特提斯洋板片开始回卷诱发了软流圈物质大规模的上涌，从而诱导地壳熔融而引发的区域成矿事件。