应用辉钼矿Re－Os定年技术研究青海什多龙矽卡岩型钼铅锌矿床的地球动力学背景

摘要

青海什多龙矽卡岩型Mo－Pb－Zn矿床位于鄂拉山多金属成矿带，该矿床成矿时代制约了成矿过程、动力学背景的探讨。前人通过对矿区及外围岩体的K－Ar法间接取得了188～208 Ma和368 Ma的数据，由于分析测试手段限制，时间跨度较大，对于成岩成矿时代的厘定尚缺乏精确的同位素年龄数据。本文采用辉钼矿Re－Os同位素定年方法测定该矿床的成矿时代，为矿床模型的建立和反演成矿地球动力学背景研究提供可靠的基础资料。分析结果显示，6件辉钼矿样品Re－Os同位素模式年龄的变化范围小，集中在234．2～240．1 Ma之间，加权平均年龄值为（236．2±2．1）Ma，与等时线回归计算得到的年龄值（233．4±9．6）Ma在误差范围内相吻合，表明成矿时代为（236．2±2．1）Ma。辉钼矿的铼含量为0．3939×10－6～0．6621 × 10－6，反映了成矿物质以壳源为主，且变质流体参与了成矿作用。本研究Re－Os定年结果显示什多龙矽卡岩型Mo－Pb－Zn矿床与鄂拉山造山带成岩年龄（约220～230 Ma）接近，表明成岩成矿受控于相同的动力学背景，是在鄂拉山造洋裂谷体制向碰撞－伸展造山体制转变的过程中，导致加厚下地壳的部分熔融而引起的印支晚期岩浆－热液事件。%The Shiduolong skarn Mo-Pb-Zn deposit is located in the multi-metal metallogenic belt of E’lashan.The precise age of the ore deposit limits our understanding of the ore-forming processes and geodynamical setting.In spite of the K-Ar methods of analysis and testing in the Shiduolong mining area and the periphery of intrusives (1 88-208 Ma and 368 Ma),whether or not thest ages can represent real mineralization times requires to be tested further.In this study,we carried out a Re-Os study of molybdenite from the Shuangqing deposit,providing fundamental data to build the mineral deposit model and reflect the geodynamical setting in this area.6 molybdenite samples of the deposit yield Re-Os isotopic model ages ranging from 234.2 Ma to 240.1 Ma,with a weighted mean age of (236.2 ±2.1 )Ma.A similar isochron age of (233.4 ±9.6 )Ma is also obtained by the isochron regression.This result suggests that the Mo mineralization occurred in last Triassic.The Re contents of molybdenite indicate that ore-forming materials are derived from crust and aqueous fluids involved in the ore-forming stage. Based on the same of rock-forming in E ’lashan and mineralization in Shiduolong Mo-Pb-Zn deposit,rock-forming and ore-forming were regarded as the same geodynamic background of India-Eurasia collision.The tectonic regime changing from developing oceanic rift to collision-extension make the thickened lower crust melt partially in the Indosinian magmatic-tectonic-metallogenic event.