西南天山萨瓦亚尔顿金矿床构造-流体控矿作用研究

Structural deformation and fluid evolution associated with the formation of the Sawayardun gold deposit in Southwestern Tianshan Orogen

【研究目的】构造-流体与成矿的耦合关系属于目前矿床学研究的前沿问题,造山型金矿作为典型受构造变形控制的热液矿床,是窥探构造-流体与成矿作用内在联系的理想研究对象。【研究方法】萨瓦亚尔顿金矿床是西南天山地区规模最大的造山型金矿,通过野外构造解析,流体包裹体和C-H-O-S同位素地球化学数据,研究矿区的构造变形特征,分析成矿流体性质及其成矿物质来源。【研究结果】矿区变形可分3期:早期为韧性变形、中期为脆-韧性变形、晚期为脆性变形。根据脉体穿切关系和变形特征,识别出3期与构造相对应的石英脉(Qz1,Qz2,Qz3),其中Qz2为含金石英脉体,金矿主要形成于第二期的脆-韧性变形期。室内观测发现,早、中期石英中发育CO_(2)-H_(2)O型、纯CO_(2)型和H_(2)O溶液型3种类型流体包裹体,晚期仅发育水溶液型包裹体。早期石英中包裹体均一温度为237~386°C,盐度为1.4%~9.2%NaClequiv.;中期石英获得CO_(2)-H_(2)O和水溶液包裹体均一温度为204~310°C,盐度为0.5%~16.6%NaClequiv.;晚期水溶液包裹体具有较低的均一温度(125~235°C)和盐度(0.2%~10.6%NaCl equiv.)。根据CO_(2)-H_(2)O型包裹体计算早、中期的流体压力分别为267 MPa和208~253 MPa,对应形成的深度分别为10 km,8~9 km。同位素分析结果揭示,成矿的流体具有变质流体以及大气降水的特征,成矿物质主要来源于赋矿地层-古生代碎屑岩-碳酸盐岩建造。【结论】萨瓦亚尔顿金矿床晚古生代经历了早期挤压向晚期走滑伸展的转变;早期NW-SE向的挤压作用促使地层变质脱水产生了大量富CO_(2)、低盐度的变质流体,形成无矿石英脉;在构造变形转向走滑伸展时,造山带抬升剥蚀,流体压力降低并发生不混溶或沸腾作用,CO_(2)等气体逃逸,诱发浅源大气降水加入并与变质热液混合,导致大量成矿物质快速沉淀成矿。

[Objective]The relationship between structure-fluid and mineralization is a frontier problem in modern study of mineral deposit.The orogenic-type gold deposits are hosted in fault zone,and are representative target to study structural deformation and fluid evolution responsible for mineralization process.[Methods]The Sawayardun orogenic-type gold lode system is the largest gold deposit in the Southwestern Tianshan Orogen,northwestern China.Basing on results of structural analysis,fluid inclusions and C-H-O-S isotope,natures of ore-controlling structure and ore-fluid,and metal source were studied.[Results]The structural deformation at the Sawayardun mine can be divided into the early ductile,middle ductile-brittle,and late brittle stages,according to the crosscutting relationships of veins,and macro and micro structures.The three-stage deformation process associated with three kinds of quartz veins(Qz1,Qz2,Qz3)from early to late,with gold mineralization being mainly introduced in middle stage.Quartz formed in the early and middle stages(Qz1 and Qz2)contains three compositional types of fluid inclusions,i.e.pure CO_(2),CO_(2)-H_(2)O and NaCl-H_(2)O,but the late-stage minerals only contain the NaCl-H_(2)O inclusions.The inclusions trapped in the early,middle and late stages yield total homogenization temperatures of 237°C-386°C,204°C-310°C,and 125°C-235°C,respectively,with corresponding salinities of 1.4-9.2,0.5-16.6,and 0.2%-10.6%NaCl equiv.,respectively.The minimum trapping pressures estimated from CO_(2)-H_(2)O inclusions are 267 MPa and 208-253 MPa in the early and middle stages,corresponding to lithostatic depths of 10 km and 8~9 km,respectively.The isotope systematics shows the nature of metamorphic and meteoric fluids and the hostrocks(Palaeozoic clastic and carbonate sediments)to be a significant source of ore metals.[Conclusions]The structural deformation features at Sawayardun indicate that the mineralization was associated with the tectonic transition from the early NW-SE-trending compression to the late strike-slip extension.The early compression caused the metamorphism and metamorphic dehydration of the Palaeozoic clastic and carbonate sedimentary rocks,resulting in the formation of the low salinity,CO_(2)-rich fluids and the early-stage barren quartz veins.Coupling with the tectonic transition from compression to strike-slip extension,the crust was rapidly uplifted and eroded,the fluid system depressurized and boiled,and mixed with and input by and mixed with the meteoric water,causing rapid escape of gases such as CO_(2) and deposition of ore-metals such as Au.