内蒙古西沙德盖钼矿床辉钼矿Re-Os同位素年龄及其地质意义

Re-Os isotopic dating of molybdenite from Xishadegai molybdenum deposit in Urad Front Banner of Inner Mongolia and its geological significance

西沙德盖钼矿床是近年来在内蒙古中西部地区找到的一处中型钼矿床。钼矿化在西沙德盖斑状钾长花岗岩株及其与太古界乌拉山群变质岩接触带内呈浸染状和脉状产出,并且构成透镜状矿体。本次研究过程中,对9件代表性辉钼矿样品进行了Re-Os同位素年龄测定,Re-Os同位素模式年龄值变化范围为222.4～226.3Ma,等时线年龄为(226.4±3.3)Ma,与赋矿围岩花岗斑岩的成岩年龄一致,由此认为西沙德盖钼矿床与斑状钾长花岗岩的形成时间均为晚三叠世,属印支晚期构造-岩浆活动的产物。结合矿区外围其他钼矿床同位素年龄数据,可以推测,内蒙古中西部大规模钼矿化发生的时间为印支晚期。古大陆内部张裂构造作用所诱发的岩浆活动是导致钼矿床形成的主导因素。

Located in the western section of the northern margin of the North China Plate,the Xishadegai ore deposit is a newly discovered Mo deposit.Molybdenum mineralization occurs within the Xishade porphyritic K-feldspar granite stock or along its contact zone with Archean metamorphic rocks of the Ulashan Group.Ore bodies consist of veins,veinlets and disseminated blocks.Re-Os dating of nine molybdenite samples collected from the main ore body shows that the isochron age is（226.4±3.3） Ma and the model ages range from 222.4 Ma to 226.3 Ma with an average value of（225±1） Ma,consistent with the age of the ore-bearing wall rock K-feldspar porphyritic granite.It can thus be considered that the K-feldspar porphyritic granite and the molybdenum deposit were both formed in Late Triassic,and that magmatism and mineralization took place in late Indosinian period in the northern margin of the North China Block.In consideration of other isotopic age data from the molybdenum deposits in adjacent areas,it can be inferred that the large-scale molybdenum mineralization in the central and western regions of Inner Mongolia occurring during the late Indosinian.magmatic activity induced by internal tectonic extension of paleo-continent plates seems to have been the dominant factor responsible for the formation of the molybdenum deposit.