藏南邦布大型金矿成矿流体He-Ar-S同位素组成及其成矿意义

He-Ar-S isotopic compositions of ore-forming fluids in the Bangbu large-scale gold deposit in southern Tibet,China

邦布金矿床位于青藏高原雅鲁藏布江结合带东段南侧,矿体受大型脆-韧性剪切带的次级断裂控制,其一系列地质地球化学特征显示该矿属于造山型金矿。其含金石英脉中黄铁矿的流体包裹体3He/4He=0.174～1.010Ra,40Ar/36Ar=311.9～1724.9,矿物δ34S=2.8‰～4.7‰,平均3.6‰;围岩中不含金黄铁矿流体包裹体3He/4He=0.01137Ra,40Ar/36Ar=1709.7,矿物δ34S=6.5‰,显示邦布金矿成矿流体主要由地壳流体组成,但其中有地幔流体的加入,幔源He占2.7%～16.7%。由壳幔相互作用导致的幔源流体的加入是邦布金矿重要的成矿条件。在印度板块与欧亚大陆板块碰撞过程中,形成切穿地壳的纵向剪切带,深源地幔流体上升,与地壳来源的富CO2流体混合,由于温度和压力下降和流体沸腾导致含金硫化物和石英结晶,并在其次级断裂构造中形成邦布金矿体。

Bangbu gold deposit is located to the south of the east section of Yarlung Zangbo tectonic suture zone in the Southern Tibet. Ore bodies are controlled by the secondary fractures in the large-scale brittle-ductile shear zone. Its geological and geochemical characteristics show that it is an orogenic gold deposit. In this study,He-Ar-S isotopic compositions of ore-forming fluids in pyrites collected from the Bangbu auriferous quartz veins were analyzed,and the results show that 3He/4He ratio is 0.174 to 1.010Ra,40Ar/36Ar ratio ranges from 311.9 to 1724.9,and δ34S is 2.8‰~4.7‰,averaging 3.6‰;whereas pyrite collected from country rocks has 3He/4He ratio of 0.01137Ra,40Ar/36Ar of 1709.7,and δ34S of 6.5‰,suggesting that the ore-forming fluid of the Bangbu gold deposit was a mixture of crust fluid and mantle-derived fluid,and the former is predominant. The ration of mantle-derived He is 2.7%~16.7%. Crust-mantle interaction and the subsequent injection of mantle-derived fluids might have played an important role in the mineralization of the Bangbu gold deposit. During collision between India plate and Asia plate,large-scale vertical lithospheric shear zones were formed,and the subsequent mantle-derived fluid went up through the shear zone,mixing with the lower crust derived CO2-rich fluid. The mixed ore-forming fluids migrated to the secondary brittle structures in the shear zone,and finally precipitated auriferous sulfide quartz ores because of decline of temperature and pressure and subsequent boiling of the ore-forming fluids