辽宁省小佟家堡子金矿床流体包裹体及同位素地球化学特征

Fluid Inclusions and Isotopic Geochemistry Characteristics of the Xiaotongjiapuzi Gold Deposit,Liaoning Province,China

辽宁小佟家堡子金矿床位于华北克拉通北缘。矿区出露地层为元古宇辽河群大石桥组大理岩和盖县组片岩,断裂构造控制着矿体的产出。矿石类型包括石英脉型和蚀变岩型。围岩蚀变类型有硅化、绢云母化和碳酸盐化。成矿过程划分为早、中、晚3个阶段,依次为石英±黄铁矿阶段、石英-多金属硫化物阶段和石英-碳酸盐阶段,金主要沉淀于石英-多金属硫化物阶段。流体包裹体研究表明,小佟家堡子矿床发育富液两相包裹体、富气两相包裹体、含CO_2包裹体和纯CO_2包裹体。成矿早阶段石英中仅见富液两相包裹体,包裹体均一温度介于311~408℃之间,盐度介于5.9%~14.3%NaCl eqv之间;成矿中阶段石英中发育富液两相包裹体、富气两相包裹体、含CO_2包裹体和纯CO_2包裹体,包裹体均一温度介于268~376℃之间,盐度介于4.1%~13.0%NaCl eqv之间;成矿晚阶段石英中仅见富液两相包裹体,均一温度介于201~254℃之间,盐度介于1.6%~7.6%NaCl eqv之间。成矿流体具中温、低盐度、富CO_2的特征,属于H_2O-NaCl±CO_2体系。流体不混溶作用是金沉淀的主要机制。成矿流体的δ^(18)OW值为0.3‰~2.3‰,δD_W值为-99.8‰^-96.2‰,表明成矿流体以岩浆水为主,混合部分变质水和大气降水。金属硫化物的δ^(34)S值介于+4.6‰^+12.9‰。金属硫化物的铅同位素比值变化较小,^(206)Pb/^(204)Pb=17.671~18.361,^(207)Pb/^(204)Pb=15.569~15.659,^(208)Pb/^(204)Pb=37.695~37.937。S-Pb同位素组成表明成矿物质主要来自辽河群变质岩和晚三叠世岩浆岩。黄铁矿中流体包裹体3He/4He值为0.27~0.53 Ra,地幔流体参与成矿作用的比例为2.9%~5.8%,地壳流体占主导地位。

The Xiaotongjiapuzi gold deposit is located in the northern North China Craton,eastern Liaoning Province,NE China. The strata in the Xiaotongjiapuzi gold deposit are composed of the marble and schist of the Dashiqiao and Gaixian formations of the Proterozoic Liaohe Group. The faults control the distribution of the orebodies. The ores contain quartz vein and altered rock types. The wall-rock alterations in the deposit can be divided into silicification,sericitization and carbonatization. Three stages of mineralization were identified,with the early stage represented by quartz ± pyrite vein,the middle stage by quartz-polymetallic sulfide vein,and the late stage by quartz-carbonate vein. Four types of fluid inclusions were distinguished： liquid-rich two-phase,gas-rich two-phase,CO_2-bearing and pure CO_2 fluid inclusions. The early stage quartz only contains liquid-rich two-phase fluid inclusions with salinities of 5. 9 to 14. 3 wt. % NaCl equiv. and homogenization temperature of311 to 408 ℃. The middle stage quartz contains all four types of fluid inclusions,of which liquid-rich twophase,gas-rich two-phase,CO_2-bearing fluid inclusions yield homogenization temperatures of 268 to 376 ℃,salinities of 4. 1 to 13. 0 wt. % NaCl equiv. In the late stage quartz only the liquid-rich two-phase fluid inclusions were observed,which have relatively low salinities of 1. 6 to 7. 6 wt. % NaCl equiv. and relatively low homogenization temperatures of 201 to 254 ℃. The ore-forming fluid is characterized by medium temperature,low salinity and CO_2-rich,roughly belonging to H_2O-NaCl ± CO_2 system. The fluid phase separation or immiscibility in the middle stage caused rapid precipitation of ore-forming materials. The δ^（18） OWvalues of the ore-forming fluid vary from 0. 3‰ to 2. 3‰,and δDWvalues vary from-99. 8‰ to-96. 2‰,indicating that the ore-forming fluid derived mainly from magmatic fluid mixed with minor metamorphic water and meteoric water. δ^（34） S of metallic sulfides ranges from ＋ 4. 6‰ to ＋ 12. 9‰. Pb isotopic ratios of metallic sulfides show small variations：^（206） Pb/^（204） Pb = 17. 671 to 18. 361,^（207） Pb/^（204） Pb = 15. 569 to 15. 659 and ^（208） Pb/^（204） Pb = 37. 695 to 37. 937. Sulfur and lead isotopes suggest that the ore-forming materials were derived from the metamorphic rocks of the Liaohe Group and Late Triassic magmatic rocks.3 He/4 He ratio of fluid inclusions in pyrite ranges from 0. 27 to 0. 53 Ra. The mantle helium involved in the ore-forming fluid is 2. 9% to 5. 8%,which implies mantle-and crust-derived fluids were involved in the gold mineralization together.