滇西北衙金多金属矿床辉钼矿Re-Os同位素年龄及其地质意义

Re-Os Isotope Age of Molybdenite in the Beiya Au-Polymetallic Deposit,Western Yunnan Province and its Geological Implications

北衙金多金属矿床产出于滇西北富碱斑岩-多金属成矿带内。矿床严格受富碱斑岩及其成生的深大断裂构造控制,并发育热液交代现象,且在围岩内以产出充填型矿脉(囊)、交代型条带矿(似层状)为主,构成斑岩-矽卡岩成矿系统,具超大型金矿床规模。本文利用Re-Os同位素定年方法首次对北衙铁金多金属矿床全矿段金属硫化物成矿年龄进行精确测定,获得5件辉钼矿样品的模式年龄为37.46~39.44 Ma,其加权平均年龄为(38.48±0.54)Ma,等时线年龄为(37.9±2.5)Ma,两者在误差范围内一致,与区域及区内富碱斑岩锆石U-Pb年龄基本保持一致;辉钼矿中Re含量主体26.84×10-6~43.15×10-6之间,指示其成矿物质来源于上地幔和下地壳,为壳幔混合源;两者均指示成矿作用与该时期具幔壳混合源性质的富碱岩浆浅成侵入关系密切。

The Beiya Au-polymetallic deposit lies in the alkali-rich porphyry polymetallic metallogenic belt of northwestern Yunnan Province. The deposit is well controlled by the alkali-rich porphyry and deep genetic faults,simultaneously developing thermal contact and contact metasomatic-hydrothermal alteration,where exist ore types of metasomatite and filling veins in the host rocks,forming a mineralization system of porphyry and skarn that is a super large-size Au-polymetallic deposit. This work firstly applied Re-Os isotopic dating technique to all ore sections. Five model ages of molybdenite obtained are from 37. 46 Ma to 39. 44 Ma,with the weighted average age 38. 48 ± 0. 54 Ma and the isochron age 37. 9 ± 2. 5 Ma,which are within the error range and consistent with the Zircon U-Pb ages of the alkali-rich porphyries in the area. The Re content of the molybdenite is mostly in the range 26. 84× 10-6to 43. 15 × 10-6,suggesting that the metallogenic material originated from the upper mantle or lower crust,and should be a mixed source of the mantle and crust. Both of the results show that the gold polymetallic metallogeny was related to alkali-rich porphyry that was derived from the mixed source of the mantle and crust.