念扎金矿床热历史:锆石U-Pb、(U-Th)/He及磷灰石裂变径迹年代学的制约

念扎金矿床是近年来最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.为约束念扎矿床的冷却及剥露历史,利用锆石的U-Pb、(U-Th)/He及磷灰石裂变径迹定年对新鲜及矿化闪长岩年龄进行测定.结果表明,新鲜闪长岩锆石U-Pb年龄为(46.32±0.53)Ma,(U-Th)/He年龄介于(7.14±0.24)Ma到(9.80±0.27)Ma,矿化闪长岩锆石(U-Th)/He年龄介于(8.38±0.24)Ma到(11.19±0.31)Ma之间,两件矿化闪长岩磷灰石裂变径迹年龄分别为(5.9±0.5)Ma和(5.3±1.0)Ma.念扎金矿床自闪长岩固结以来经历了两次快速冷却过程:第一次是从46.3 Ma开始持续到43.6 Ma,温度从750℃降至350℃,冷却速率高达约148℃/Ma;第二次为8.5~2.0 Ma,温度从约200℃降至30℃,冷却速率为26℃/Ma.念扎矿床成矿深度为9.7 km;在8.5 Ma时,矿床被抬升至4.6 km处;从8.5~5.6 Ma,矿床抬升至2.8 km;从5.6~2.0 Ma,念扎矿床被剥露至地表.

Thermal History of Nianzha Gold Deposit: Constraints from Zircon U-Pb, (U-Th)/He and Apatite Fission Track Geochronology

The Nianzha gold deposit, located in the central section of the Yurlung-Zangbo suture zone is a large orogenic gold deposit which occurred in a fracture zone bordered by altered diorite in the hanging wall to the north and the Renbu tectonic mélange in the footwall to the south. In this paper, it combines zircon U-Pb, (U-Th)/He and apatite fission track dating of the fresh and mineralized diorite in order to restrict the cooling and denudation history of the Nianzha deposit. The results show that the zircon U-Pb age of the fresh diorite is (46.32±0.53) Ma and zircon (U-Th)/He ages of fresh diorite is between (7.14 ±0.24) Ma and (9.80 ±0.27) Ma, and the zircon (U-Th)/He ages of the mineralized diorite are between (8.38±0.24) Ma and (11.19±0.31) Ma. The apatite fission track ages of two mineralization diorite are (5.9±0.5) Ma and (5.3±1.0) Ma respectively. Since the consolidation of the diorite, the Nianzha gold deposit has experienced two rapid cooling processes:the first process started from 46.3 Ma to 43.6 Ma, and the temperature dropped from 750℃ to 350℃ with the cooling rate of about 148℃/Ma, while the second process happened between 8.5~2.0 Ma with the temperature dropped from 200℃ to 30℃ with cooling rate of about 26℃/Ma. The mineralization depth of the Nianzha deposit is about 9.7 km; at 8.5 Ma, the deposit was uplifted to 4.6 km; from 8.5 to 5.6 Ma, the deposit are uplifted to 2.8 km; and from 5.6 to 2.0 Ma, the Nianzha deposit was exfoliated to the surface.