西藏乌鲁穷含铜磁铁矿床的发现及地质特征

The discovery of the Wuluqiong magnetite deposit in Tibet and its geological characteristics

继尼雄、弗野、材玛等铁矿床的发现之后,新一轮1∶5万区域地质调查又在班公湖—怒江成矿带西段乌鲁穷地区发现了较大规模的含铜磁铁矿矿床。野外路线调查、剖面实测和大比例尺填图表明,乌鲁穷磁铁矿形成于南羌塘南缘中—晚侏罗世中酸性岩体与上三叠统日干配错群二组(T3R2)碳酸盐岩外接触带中,矿体受后期近南北向高角度正断层改造,成矿作用表现出多期次、多阶段特征。通过对矿体邻近的石英闪长岩、二长花岗岩和控矿断层中同构造碳酸盐脉进行锆石LA-ICP-MS U-Pb同位素测年,获得石英闪长岩、二长花岗岩成岩年龄分别为:(161.3±0.72)Ma、(157.5±1.7)Ma,并获得同构造碳酸盐脉热液锆石年龄为(84.2±1.1)Ma。结合前人研究和1∶5万地质调查成果,认为乌鲁穷含铜磁铁矿在中—晚侏罗世(157.5～161.3 Ma)班公湖—怒江洋北向俯冲过程中初步富集成矿,形成矽卡岩型含铜磁铁矿矿床,矿体在晚白垩世(84.2 Ma)陆内伸展过程中再次富集;提出班公湖地区在晚白垩世中期存在一期横跨缝合带的陆内伸展成矿事件,成矿作用与班公湖—怒江洋闭合后地幔软流圈底辟及其相关的地壳线性热隆伸展是一个机理关联的系统。

A relatively large magnetite(including copper)deposit was newly discovered during the 1∶50,000 regional geological1215 survey in Wuluqiong area,Tibet,following the discovery of the Nixiong iron mine,Fuye iron deposit,Caima iron deposit,and some other deposits.Located in the western part of Bangong Co-Nujiang River mineralization belt,this ore deposit has not been studied in such aspects as geological features,genesis,age and formation structural environment.According to the field survey,the authors found that the Wuluqiong magnetite ore deposit was formed in the exo-contact zone between the intermediate-acid intrusive rocks in Middle-Late Jurassic and carbonate rock in 2 nd member of Riganpeicuo Group in Upper Triassic(T3R^2).It was reconstructed by the later NS-trending high-angle normal fault and showed characteristics of multiple phases and stages of mineralization.Based on the LA-ICPMS zircon U-Pb geochronological study of the quartz diorite and the monzogranite adjacent to the orebody and the isostructural carbonate veins in the fault controlling the orebody,the authors obtained two weighted mean 206 Pb/238 U ages of(161.3±0.72)Ma,(157.5±1.7)Ma for the crystallized zircons in the quartz diorite and the monzogranite,and obtained a weighted mean 206 Pb/U 238 age of(84.2±1.1)Ma for the hydrothermal zircons in the isostructural carbonate veins.Combined with the previous research and 1:50,000 geological survey,the authors hold that the Wuluqiong magnetite ore deposit was initially concentrated in skarn-type magnetite during the process of northward subduction of Bangong Co-Nujiang River in Middle-Late Jurassic(157.5-161.3 Ma),and then underwent another enrichment in the process of intracontinental stretch in Late Cretaceous(84.2 Ma).It is considered that mineralization events related to the intracontinental stretch occurred in the middle of Late Cretaceous in Bangong Co area and mineralization might have existed on both north and south sides of the Bangong Co ophiolitic mélange belt.The mineralization,together with the diapirism of asthenosphere after the closure of Bangong Co-Nujiang River and related linearly thermal uplift and stretch of the crust,constituted a system of correlative mechanism.