湖北竹山县银洞沟矿床成矿流体特征及矿床成因

Fluid inclusion and H-O isotope geochemistry and ore genesis of the Yindonggou deposit, Zhushan County, Hubei, China

银洞沟大型银多金属矿床位于南秦岭造山带,矿体主要受东西向的银洞岩背斜控制,呈脉状产于武当山群变火山岩。热液成矿作用包含4个阶段:(Ⅰ)细粒石英-闪锌矿-方铅矿阶段;(Ⅱ)细粒石英-银金矿化阶段;(Ⅲ)粗粒石英阶段,含少量方铅矿-闪锌矿-黄铜矿;(Ⅳ)块状铁白云石-石英阶段。银洞沟矿床流体包裹体可分为3类:NaCl-H2O型(W型)、CO2-H2O-NaCl型(C型)和CO2-CH4型(PC型)。Ⅰ阶段石英中的流体包裹体以W和C型为主,含少量PC型,相比Ⅰ阶段,Ⅱ阶段的C型包裹体更多,而Ⅲ阶段仅发育W型包裹体。显微测温表明,Ⅰ阶段流体包裹体均一温度为308～436℃,盐度为5.1%～10.2%NaCleqv;Ⅱ阶段均一温度为220～375℃,盐度为2.0%～10.7%NaCleqv;Ⅲ阶段均一温度为122～272℃,盐度为0.4%～7.2%NaCleqv。根据C型包裹体估算前两个阶段压力分别为330～463MPa和180～363MPa,相应成矿深度分别为12.5～17.5km和6.8～13.8km。从Ⅰ到Ⅲ阶段,δ18OW、δD平均值分别由8.4‰和-72‰,变化至0.9‰和-67‰,指示初始成矿流体来自变质热液,晚阶段混入了大气降水。流体包裹体与氢氧同位素数据表明,银洞沟矿床成矿流体为中温、低盐度、富CO2的变质热液,属于造山型矿床,流体的混合可能是金属沉淀的主要机制。

The Yindonggou Ag-Au-Pb-Zn deposit is a quartz-vein system hosted in metamorphosed volcanic rocks of the Wudangshan Group in the southern Qinling Orogen. The Ag-Au mineralization is controlled by the Yindongyan anticline. The hydrothermal ore-forming process can be divided into four stages according to mineral assemblages and crosscutting relationships of the veinlets, from early to late ： （ 1 ） fine grained quartz-sphalerite-galena vein ; （2） fine grained quartz-silver-gold vein, containing minor pyrite; （3） coarse grained quartz vein, with minor galena, sphalerite and chalcopyrite; and （4） ankerite-quartz vein. Most of the Ag-Au ores are formed during stage 2, and Pb-Zn ores are formed during stage 1. Three types of fluid inclusions （FIs） are distinguished in this study based on petrographic and microthermometric criteria, NaC1-H2O, CO2-H2O-NaC1 and CO2-CH4 fluid inclusions, respectively. All of the three types of FIs can be observed in the hydrothermal quartz of stages 1 and 2 ; while only the NaC1-H2O inclusions occur in the stage 3. FIs of stage 1 are mainly homogenized between 308℃ and 436℃, with salinities ranging from 5. 1% to 10. 2% NaCleqv. The stage 2 FIs yield homogeneous temperatures of 220 - 375℃ and salinities of 2. 0% - 10. 7% NaCleqv. FIs of stage 3 are homogenized between 122℃ and 272℃, with salinities of O. 4% N 7. 2% NaCleqv. The estimated pressures range from 330MPa to 463MPa in stage 1, 180MPa to 366MPa in stage 2, corresponding to the depths of 12. 5 - 17.5km and 6. 8 N 13.8km, respectively, suggesting regional compression might cause somewhat tectonic overpressure. The 8TM Ow decrease from 6. 6%0 in the stage 1, to - 1.2%0 ～ 0.4%0 in the stage 3, with the 8D changing from - 74%0 in stage 1 to - 48%0 ～ - 73%0 in stage 3. The δD vs δ^18TM OH2o plots show a shift from metamorphic water towards meteoric water line. Thus, we suggest that the original ore-forming fluids are metamorphic in origin which is characterized by mesothermal, low-salinity and CO2-rieh. The fluid mixing have took place during ore-froming process and resulted to metal deposition. Therefore, the Yindonggou deposit can be genetically assigned to be orogenic-type mineral system.