东秦岭上宫金矿流体成矿作用:矿床地质和包裹体研究

HYDROTHERMAL METALLOGENY OF THE SHANGGONG GOLD DEPOSIT,EAST QINLING:STUDIES ON ORE GEOLOGY AND FLUID INCLUSION GEOCHEMISTRY

上宫金矿产于东秦岭熊耳地体的熊耳群火山岩中,属于断控脉状造山型金矿床。上宫金矿赋矿构造由压剪向张剪演化,成矿过程经历了3个阶段:早阶段为黄铁矿-铁白云石-石英脉,脉体和矿物变形、破碎,形成于挤压或压剪环境;中阶段细粒多金属硫化物网脉贯入共轭节理或裂隙,没有遭受变形,形成于张剪环境;晚阶段具梳状构造的石英-碳酸盐细脉形成于伸展引张环境。流体包裹体组合从早阶段富CO2型,经中阶段富CO2型、贫CO2型和纯CO2型组合,向晚阶段贫CO2型演化。3个阶段的流体包裹体均一温度分别为380℃～320℃,300℃～220℃和200℃～120℃;早、中阶段的压力分别为285MPa～200MPa和160MPa～100MPa。中阶段流体离子(如SO42-,Cl-,K+)质量分数高,(K+Na)/(Mg+Ca)比值高,pH值高,Eh值低,属于碱性还原性质的过饱和流体,有利于多金属硫化物、碲化物和Au,Ag,Te等自然元素的快速沉淀。中阶段包裹体可见异相均一现象,具有较低的CO2/H2O比值,表明发生了以CO2逸失为特征的沸腾现象。东秦岭(含熊耳地体)三叠纪-早侏罗世地壳缩短、增厚、隆升,中侏罗世-早白垩世造山带挤压向伸展转折和隆升,白垩纪造山带伸展、断陷。这种区域构造演化的3阶段特征与上宫金矿含矿构造、矿石组构、成矿过程和包裹体组合的3阶段演化特征完全对应。

The Shanggong Au deposit,with reserves of about 30 t Au,in the Xiong′er Group volcanic rocks of the Xiong′er Terrane, East Qinling,is one of the fault-controlled orogenic-type lode gold deposits in China. Ore-hosting structures are recognized to have evolved from compressional shear to tensional shear.The hydrothermal ore-forming process is divided into early,middle and late stages,characterised by pyrite-ankerite-quartz, polymetallic sulfides and carbonate-quartz respectively. The early stage veins and minerals are structurally deformed and broken, suggesting they have formed in a compression or compressional shear setting. The middle stage poly-metallic sulfide stockworks are intruded into the early stage veins and conjugate joints or cracks, and show no clear deformation, which suggests a tensional shear setting. The late stage quartz-carbonate veinlets, with comb structure,are likely formed in an extensional tectonic environment. The composition of fluid inclusions changed from CO_2-rich in the early stage,to CO_2-rich, pure CO_2 and CO_2-poor in the middle stage,and to CO_2-poor in the late stage. Homogenization temperatures of fluid inclusions are between 380℃~320℃ for the early stage, 300℃~220℃ for the middle stage and 120℃~200℃ for the late stage. The trapping pressures for the early and middle stage fulid inclusions are estimated at 285 MPa^200 MPa and 160 MPa^100 MPa respectively. The middle stage fluid has the highest contents of cations and anions (e.g. SO_4^(2-),Cl^-,K^+), the highest (K+Na)/(Mg+Ca) ratios,the highest pH and lowest Eh values, which probably resulted from fluid-boiling charaterised by CO_2 phase separation indicated by the lowest CO_2/H_2O ratios and divergent homogenization to gas and liquid respectively. The alkaline,supersaturated fluid in the middle stage is most conducive to the rapid deposition of polymetallic sulfides, tellerides and native elements such as Au, Ag and Te. In the east Qinling area, the collision between Yangtze and North China continents progressed from crustal thickening, shortening and uplift during Triassic-Early Jurassic,to compression-extension transition and uplift during Middle Jurassic-Early Cretaceous,to orogenic extension and fault-controlled depression during Cretaceous. These three evolutionary stages correspond to the three-stages of ore-hosting structures, ore textures, ore-forming processes and fluid inclusion populations of the Shanggong Au deposit. This shows that the Shanggong gold deposit formed during the Mesozoic continental collision and can be well interpreted by the tectonic model of collisional orogeny, metallogeny and fluid flow (CMF). During the collision, ore-froming fluids were generated from metamorphic dehydration of the underthrust slab which downwent beneath the Xiong′er Terrane along the Machaoying fault, a north-dipping A-type subduction zone. The metamorphic fluids migrated upward along the ore-hosting faults of the Shanggong gold deposit, boiled at the brittle-ductile transiton level, and then mixed with downgoing meteoric water, resulting in rapid deposition of ore-metalls.