东秦岭秋树湾铜钼矿流体包裹体和稳定同位素特征及其地质意义

秋树湾铜钼矿是东秦岭钼矿带上典型的受斑岩体控制的矽卡岩-斑岩角砾岩筒复合型矿床,矿体赋存于成矿母岩花岗岩及矽卡岩和角砾岩筒中。根据矿物共生组合、矿石组构、围岩蚀变及脉体的穿插关系,可划分为早(Ⅰ)、中(Ⅱ)、晚(Ⅲ)3个矿化期,再将Ⅰ期细分为干矽卡岩-钾长石化-石英阶段(Ⅰ₁)、爆破角砾岩阶段(Ⅰ₂)、湿矽卡岩阶段(Ⅰ₃)、磁铁矿阶段(Ⅰ₄);Ⅱ期分为斑岩型铜(钼)矿阶段(Ⅱ_b)和石英硫化物阶段(Ⅱ_s);Ⅲ期为方解石、重晶石、石英阶段(Ⅲ)。流体包裹体可划分为S型含子矿物多相包裹体、L型纯液相包裹体、C型含CO₂三相包裹体、W型气液两相包裹体、G型纯气相包裹体5种类型。按时间先后顺序,成矿流体的温度、盐度、氧化还原环境具有规律性的演化特征。均一温度范围: Ⅰ期为222~406℃,Ⅱ期为152~315℃,Ⅲ期为119~189℃;盐度w(NaCl_eq): Ⅰ期介于4.2%~36.5%,Ⅱ期为3.3%~34.8%,Ⅲ期为4.2%~11.9%。激光拉曼光谱及群体包裹体成分分析结果表明,第Ⅰ期流体以H₂O、CO₂、CH₄、H₂S为主,表现为还原环境;第Ⅱ期流体以H₂O、CO₂、N₂、O₂、SO₄^2-、Cl^-、F^-为主,为氧化环境,暗示流体源于岩浆。流体包裹体岩相学及包裹体测温表明,流体由早期的高温、高盐度、含CO₂的H₂O-NaCl-CO₂体系的岩浆流体在成矿Ⅰ期发生沸腾作用和相分离,伴随着流体沸腾、CO₂逸失、温度下降、大气水的加入、盐度下降等过程,导致大量金属硫化物沉淀。在成矿Ⅱ、Ⅲ期成矿体系趋于开放,流体存在大气降水混入,逐渐演化为晚期的低盐度、中低温度、贫CO₂的流体体系。H、O、S同位素结果表明有地幔流体参与成矿作用。

Fluid inclusions and stable isotopes of Qiushuwan copper-molybdenum deposit in East Qinling orogenic belt and their geological implications

The Qiushuwan copper-molybdenum deposit in the East Qinling molybdenum belt is atypical skarn-porphyry breccia pipe controlled by the porphyry.Ore bodies occur in the biotite granite porphyry,skarn and breccia pipes.According to mineral paragenesis,ore fabrics,wall-rock alteration and intersecting relationship of the veins,the formation of the ore deposit can be divided into three periods,i.e.,high-temperature alteration-skarn period(Ⅰ),sulfide precipitation period(Ⅱ) and low-temperature mineral-free period(Ⅲ),composed of six ore-forming stages: dry skarn-K-feldspar-quartz stage(Ⅰ1),explosion breccia stage(Ⅰ2),wet skarn stage(Ⅰ3),magnetite stage(Ⅰ4),porphyry copper(molybdenum) ore stage(Ⅱb) and quartz sulfide mineralization(Ⅱs),and calcite,barite,quartz stage(Ⅲ).Mineral-forming fluids in quartz,garnet and calcite can be divided into five types,i.e.,S-type multi-phase inclusions containing daughter minerals,L-type pure liquid inclusions,C-type three-phase CO2-bearing inclusions,W-type gas-liquid two-phase inclusions,and G type pure gas inclusions.Ore-forming fluid temperature,salinity and redox environment are characterized by regular evolution: homogenization temperature values of Ⅰ period,Ⅱ period and Ⅲ period are 222~406℃,152~315℃,and 119~189℃ respectively,whereas salinities w(NaCleq) of Ⅰ period,Ⅱ period and Ⅲ period are 4.2%~36.5%,3.3%~34.8% and 4.2%~11.9%,respectively;Raman spectroscopy and analysis of group inclusions show that the fluid compositions are mainly H2O,CO2,CH4,H2S in Ⅰ period,implying a reduction environment,H2O,CO2,N2,O2,SO2-4,Cl-,F-in Ⅱ period,suggesting an oxidation environment and the derivation of fluid from the magma.Inclusions petrography and thermometry shows that the fluid was originally high temperature,high salinity,CO2-bearing H2O-NaCl-CO2 system magmatic fluid,and experienced boiling and phase separation in Ⅰ period,accompanied by fluid boiling,CO2 escaping,temperature drop,addition of atmospheric water,and decrease of salinity,which led to the precipitation of large quantities of metal sulfides.In Ⅱ and Ⅲ period,the ore-forming system tended to become open,and meteoric water infiltrated into the fluid,leading to the evolution into the late low-salinity,low temperature,poor CO2 fluid system.H,O,S isotopes show that mantle materials participated in the ore-forming process.