雅鲁藏布江缝合带念扎造山型金矿床白云母Ar-Ar年代学及其地质意义

念扎金矿床是近年来新发现的位于雅鲁藏布江缝合带南侧仁布构造混杂岩带与蚀变闪长岩接触带的大型造山型金矿床,目前探明资源储量已达25 t以上,平均品位3. 08×10-6。对成矿时代的精确厘定,是理解矿床成因机制和雅鲁藏布江缝合带金矿床成矿地质背景的关键。系统的野外踏勘、岩相学观察和电子探针工作表明,念扎矿床中发育与金成矿同期的含白云母石英脉。对白云母进行Ar-Ar同位素测年工作显示,白云母Ar-Ar坪年龄为(43. 66±0. 50) Ma,等时线年龄为(44. 21±0. 46) Ma,说明念扎金矿床形成于印-亚陆陆碰撞的主碰撞汇聚成矿阶段的晚期。目前在雅鲁藏布江缝合带的南缘已相继发现了邦布、马攸木、折木朗等造山型金矿床,成矿时代介于59. 5~49. 5Ma之间,已俯冲的新特提斯洋壳板片的回卷和断离很可能是该类矿床形成的深部动力学机制。

Ar-Ar dating on muscovite of Nianzha orogenic gold deposit in Yurlung-Zangbo suture zone and its geological significance

The Nianzha gold deposit is a newly discovered large orogenic gold deposit in recent years, which is located in a fracture zone bordered by altered diorite and the Renbu tectonic mélange in south of the Yurlung-Zangbo suture zone. It has reserves of 25 tons with average Au grade of 3.08 g/t at present. The accurate determination of the time of mineralization of Nianzha Au deposit is crucial for understanding of the genetic mechanism of the deposit and the geological background of the deposit. The systematic field work, microscopic observation and electron microprobe analyses show that muscovite-bearing quartz veins formed simultaneously with the gold mineralization in the Nianzha deposit. Ar-Ar dating on muscovite show that the age of plateau and muscovite Ar-Ar is 43.66±0.50 Ma and the age of isochron is 44.21±0.46 Ma, suggesting that the Nianzha deposit was formed during the late period of mineralization at the collisional stage of Indo-Asian collision (65-41 Ma). Orogenic gold deposits named Bangbu, Mayum, Zhemulang have been found continually along the Yurlung-Zangbo suture zone with the metallogenic age ranging 59.5~49.5Ma, and the rollback and break off of the subducted Neo-Tethys slab may be the deep geodynamic process of these deposits.