德兴金山金矿床成矿流体来源:小尺度构造和同位素地球化学证据

Source of ore-forming fluids in Jinshan gold deposit of Dexing County:Constraints from microstructures and stable isotopes

德兴金山金矿床位于扬子板块和华南克拉通之间的江南造山带东段赣东北深大断裂带的次级剪切带中,是一个与韧-脆性剪切带有关的超大型金矿床。金山金矿床的矿石类型包括蚀变岩型和含金石英脉型,均为与断裂(或者裂隙)充填有关的不同尺度石英脉系统。纹层状和透镜状含金石英脉结构表明金山金矿床的成矿作用是同构造的,金的矿化与递进变形作用密切相关,变形过程中产生的变质流体参与了成矿作用。石英-钠长石-铁白云石-黄铁矿蚀变带中与金共生的黄铁矿流体包裹体的3He/4He比值为0.13～0.24Ra,40Ar/36Ar比值变化范围为575～1090,说明成矿流体主要以地壳端员的流体为主,有很少量的地幔流体参与。铁白云石的碳、氧同位素值分别变化于-5.0‰～-4.2‰和4.4‰～8.0‰之间,与世界上大多数脉状金矿床的碳、氧同位素值基本一致。含金石英脉中石英的氧同位素变化于12.4‰～15.3‰之间,其中的流体包裹体氢同位素值变化于-62‰～-73‰。根据这些同位素地球化学数据,结合金山金矿床小尺度地质构造特征,笔者认为金山金矿的成矿流体主要为变质流体,并有少量地幔流体和大气降水的参与。金山金矿形成于地体碰撞过程中的转换压缩汇聚构造背景中。

The Jinshan gold deposit, a superlarge deposit with 200 t Au, is hosted in the Jinshan ductile shear zone and located in the northeast J iangxi deep crustal fault. The zone is a magnificent Neo-Proterozoic melange-suture zone. The mineralized fault-filling quartz veins that were elongated and boudinaged along with mylonitic folia tions serve as main ore bodies. Textural and structural data indicate that mineralization was syn-tectonic, and the opening of the foliation planes played a fundamental role in facilitating gold mineralization. Auriferous quartz veins were laminated due to repeated foliation opening and fluid filling and precipitation. Banded and laminated textures suggest a crack and seal mechanism for gold precipitation and the continuation of the shear deformation during the emplacement of the veins. This means that progressive deformation is closely associated with gold mineralization. Fluid inclusions in pyrite associated with quartz have ^3He/^4He ratios of 0.13-0.24 Ra, whereas their ^40Ar/36Ar ratios are from 575 to 1 090. δ^18O values calculated from quartz are between 5.5‰ and 8.4‰, and 3D values of the fluids in the fluid inclusions of quartz are between - 61‰ and - 73‰. The δ^13C values of ankerite range from 5.0‰ to - 4.2‰, whereas the δ^18O values range from 4.4‰ to 8.0‰. These ranges demonstrate a mixing of mantle-derived fluids and crustal-derived fluids. The crustal end-member is composed of metamorphic water and meteoric water. The mierostructures along with noble gas isotope （He and Ar） data as well as stable isotope （C, O and D） data strongly suggest that diverse sources were involved in the hydrothermal activity in the Jinshan gold deposit. Ore-forming fluids were dominantly derived from the crustal source with the participation of less than 5 % mantle components. The deposit was formed in a transpressive tectonic setting.