西藏邦铺大型钼铜多金属矿床Rb-Sr等时线年龄及地质意义

Rb-Sr Isochron Age Dating of the Bangpu Large-Scale Mo-Cu Polymetallic Deposit,Tibet and Its Geological Significance

西藏邦铺钼铜多金属矿床是西藏冈底斯斑岩铜矿带东段典型的大型斑岩型矿床,以钼、铜为主,共生铅、锌。本文通过二长花岗斑岩全岩Rb-Sr法和含钼铜矿石中的黄铁矿Rb-Sr等时线法,分别获得等时线年龄为(13.88±0.38)Ma(MSWD=1.7)和(11.0±1.5)Ma(MSWD=3.5),代表二长花岗斑岩成岩、钼铜晚阶段的成矿年龄,结合锆石SHRIMPU-Pb年龄(13.9±0.3)Ma及含钼铜脉石英激光显微探针40Ar-39Ar年龄(13.9±0.9)Ma,显示邦铺钼铜多金属矿床成岩、成矿年龄具有一致性,成矿时间持续约3Ma。锶同位素初始值(0.706538～0.709190)及幔源成分(Pm)含量(4.03%～51.65%)暗示邦铺二长花岗斑岩原始岩浆可能来自地幔,在冈底斯伸展构造背景下就位(14Ma前),幔源岩浆和硅铝质地壳物质发生交换并遭受地壳物质混染,以幔源物质为主,壳源物质为辅。

The Bangpu Mo-Cu polymetallic deposit is one of the typical large-scale porphyry deposits which lies in the eastern Gangdese metallogenic belt,Tibet.Its major ore elements are molybdenum and copper,and its accompanying elements are lead and zinc.A monzonite granite-porphyry whole-rock Rb-Sr isochron age of（13.88±0.38）Ma with MSWD=1.7 was obtained which represents the crystallization age of the rock.Furthermore,a pyrite Rb-Sr isochron age of（11.0±1.5）Ma with MSWD=3.5 was obtained for Mo and Cu bearing ores which represents the age of later episode of Mo-Cu mineralization.Combining with the zircon SHRIMP U-Pb age（13.9±0.3）Ma and laser micro-probe 40 Ar- 39 Ar age（13.9±0.9）Ma,it is shown that the rock-forming and ore-forming ages are mutually consistent.The molybdenum and copper mineralization lasted about 3 Myrs.The inital 87 Sr/ 86 Sr value（0.706538~0.709190）and mantle composition percentage（Pm）（4.03%~51.65%）indicate that the primary magma might be derived from the mantle,and emplaced in the Gangdese extensional setting 14 Myrs ago.The mantle magma was interacted with crustal dioritic magma and contaminated by crustal materials during its ascending process.But generally speaking,the magma source was mainly mantle-derived.