华南下寒武统黑色岩系铂多金属矿中黄铁矿流体包裹体的He-Ar同位素体系

He-Ar Isotopic Systematics of Fluid Inclusions in Pyrites from PGE-Polymetallic Deposits in Lower Cambrian Black Rock Series, Southern China

利用惰性气体同位素质谱仪测定了华南下寒武统具代表性的黑色岩系铂多金属矿中黄铁矿流体包裹体的He-Ar同位素组成,发现其~3He/~4He均较低,为0.43×10^(-8)～26.39×10^(-8),R/R_α为0.003～0.189,而^(40)Ar/^(36)Ar为258～287,接近大气饱和水。同位素地球化学示踪研究显示,该矿成矿流体主要由建造水(盆地热卤水)和大气饱和水(海水)组成,基本不含地幔流体或深源岩浆水。黑色岩系铂多金属矿的成矿可能与华南克拉通南缘加里东冒地槽的演化有关。在早寒武世,接受了巨厚沉积的加里东盆地中的建造水由于上覆沉积物的压力,顺层侧向迁移,并沿断裂上升与海水混合而成矿。

He-Ar isotopic compositions of fluid inclusions trapped in pyrites from some representa- tive PGE-polymetallic deposits in Lower Cambrian black rock series in Southern China were anal- ysed by using an inert gas isotopic mass spectrometer. The results show that the ore-forming fluid possesses a low ~3He/~4He ratio, varying between 0.43×10^(-8) and 26.39×10^(-8), with an R/R_a value of 0.003～0.189. The ^(40)Ar/^(36)Ar are 258～287, closing to that of air-saturated water (ASW). He-Ar isotopic indicator studies show that the ore-forming fluid was mainly composed of formation water or basinal hot brine and sea water, while the content of mantle fluid or deep-de- rived magmatic water might be neglected. The PGE-polymetallic mineralization might be related to evolution of Caledonian miogeosynclines distributed along southern margin of the South China Craton. In the Early Cambrian Period, the formation water (basinal hot brine) trapped in the Caledonian basins where accumulated extremely thick sediments was expelled and migrated later- ally along strata because of the pressure generated by overlying sediments. The basinal hot brine ascended along faults, mixed with sea water and finally deposited ore minerals.