湖南湘东钨矿含矿石英脉辉钼矿Re-Os定年及地质意义

Re-Os Dating of Molybdenite from the Ore-bearing Quartz Veins in the Xiangdong Tungsten Deposit(Hunan Province) and its Geological Significance

湘东钨矿是一个典型的石英脉型钨锡多金属矿床,发育百余条含矿石英脉,矿脉陡立,从北到南走向呈NE、NEE和EW向,反映了矿脉形成时处于近NW-SE向区域拉伸状态。矿区内的老山坳断层分割了重要的南、北两组矿脉。北组脉具有毒砂-黄铜矿-黑钨矿等矿物组合,富含金属硫化物,多具有平行脉壁的条带状构造;南组脉则主要体现辉钼矿-黑钨矿-黄铜矿-闪锌矿-方铅矿等矿物组合,金属硫化物含量较少,具有块状构造。本次研究通过对南组脉辉钼矿Re-Os同位素测年,获得等时线年龄为150.4±1.5 Ma(MSWD=0.52),模式年龄加权平均值为150.7±2.3 Ma(MSWD=0.10),代表成矿时间。含矿石英脉的形成略晚于区内燕山期二云母花岗岩(154.4±2.2 Ma)和煌斑岩(152.1±0.9 Ma)的侵位,指示含矿石英脉在成因上与燕山早期岩浆活动密切相关。事实上,包括湘东钨矿在内的南岭钨锡多金属成矿省均形成于165~150 Ma期间,具有统一的地球动力学背景,即由太平洋板块俯冲和俯冲板片断离所引起的弧后伸展环境。

The Xiangdong(also named as Dengfuxian)tungsten deposit in the eastern Hunan province consists of hundreds of steep tungsten-bearing quartz-veins.These veins trend toward NE,NEE and EW from north to south,indicating a NW-SE regional extension during formation.They are separated by the Laoshan’ao fault into two principal ore sets,the North and the South.The North ore set has banded structure and is mainly comprised of arsenopyrite,chalcopyrite and wolframite,at high crystallization temperature.In contrast,the South ore set has blocky structure and is predominately composed of molybdenite,wolframite,sphalerite and galena,at mediate to high crystallization temperature.Re-Os dating of molybdenites from tungsten-bearing quartz-veins yielded an isochron age of150.4±1.5Ma(MSWD=0.52),which is in an excellent agreement with a weighted mean model age of150.7±2.3Ma(MSWD=0.10).This indicates that the dating results are reliable and may represent the timing of ore formation.The Re-Os isochron age is slightly younger(2?4Ma)than zircon LA-ICP-MS U-Pb ages of the two-mica granite(154.4±2.2Ma)and the lamprophyre(152.1±0.9Ma),showing that the tungsten mineralization is genetically associated with the Early Yanshanian magmatism.These data indicate that metallogenic epoch of the Xiangdong tungsten deposit is consistent with that of the large-scale W-Sn polymetallic mineralization in the Nanling metallogenic province.We argue that the tungsten-bearing quartz-veins of the Xiangdong tungsten deposit or the regional mineralization might have been formed in a back-arc extensional setting,which was probably triggered by the break-off or foundering of the flat-slab caused by the rollback of the Paleo-Pacific subducted slab.