湖北宜昌白竹坪金矿床的成因:来自流体包裹体及H-O-S-Pb同位素地球化学的证据

Genesis of Baizhuping Gold Deposit in Yichang, Hubei: Evidence from Fluid Inclusion and H-O-S-Pb Isotope Geochemistry

宜昌白竹坪石英脉型金矿床位于湘西-鄂西成矿带西部黄陵背斜核部东北缘,是该地区代表性矿床之一。成矿过程可划分为无矿化石英脉阶段(Ⅰ)、石英-黄铁矿-方铅矿阶段(Ⅱ)和石英-碳酸盐阶段(Ⅲ)。流体包裹体研究表明:不同阶段石英中均含有大量的流体包裹体,以气液两相和富液相为主,总体为中低温度(151~341℃)、中低盐度(3.53%~14.10%NaCleqv)和低密度(0.752~1.019 g/cm^(3))的流体;估算的流体捕获压力为12.2~33.7 Mpa;推测成矿深度为1.22~3.37 km。流体包裹体群体成分及单个流体包裹体激光拉曼分析表明:成矿流体总体属于NaCl-H_(2)O-CO_(2)体系。氢、氧同位素分析显示,成矿流体的δ^(18)OH_(2)O变化于+1.0‰~+5.3‰,δD_(H_(2)O)变化于-56.1‰~-35.0‰,表明成矿流体主要由变质水和大气降水组成,成矿主阶段以变质水为主,晚期有较多大气降水的加入;金属硫化物的δ^(34)S值为0.93‰~6.69‰,^(206)Pb/^(204)Pb为15.634~15.677,^(207)Pb/^(204)Pb为15.238~15.318,^(208)Pb/^(204)Pb为36.634~36.796,表明成矿物质主要来源于下地壳及上地幔。总体上,白竹坪金矿床成因类型应为造山型金矿,受区域深大断裂以及次级的韧-脆性剪切带控制。

Yichang Baizhuping gold deposit is located in the northeast edge of Huangling anticline core in the west of western Hunan-western Hubei metallogenic belt,one of the representative deposits in this area.The ore type is mainly quartz vein gold deposit.The mineralization process can be divided into Non-mineralized quartz vein stage(Ⅰ),quartz-pyrite-galena stage (Ⅱ) and quartz-carbonate stage (Ⅲ).The study of fluid inclusions shows that there are large number of fluid inclusions in quartz at different stages,mainly gas-liquid two-phase and liquid-rich types with general medium-low temperature (151~341℃),medium-low salinity(3.53~14.10%NaCleqv) and low density (0.752~1.019 g/cm~3).The estimated fluid capture pressures range from 12.2 MPa to 33.7 MPa,with the formation depth of 1.22~3.37 km.Laser Raman analysis of group fluid inclusions and single fluid inclusions shows that ore-forming fluids generally belong to the NaCl-H_(2)O-CO_(2)system.Oxygen and hydrogen isotope data from quartz show that the δ^(18)O_(V-SMOW)values range from 13.0‰to15.3‰,δ^(18)O_(H_(2)O)values from+1.0‰to+5.3‰,and ^(δ)D_(H_(2)O)values between-56.1‰and-35.0‰,which revealed that the ore-forming fluids of Baizhuping gold deposit were composed of metamorphic water and meteoric water,while the ore-forming fluid of main metallogenic stage mainly came from the metamorphic water,with the addition of more meteoric water at the late stage.The δ^(34)S values of sulfides range from 0.93‰to 6.69‰.Among Pb isotopes,the^(206)Pb/^(204)Pb,^(207)Pb/^(204)Pb and^(208)Pb/^(204)Pb ratios of sulfides are in the range of 15.634 to15.677,15.238 to 15.318 and 36.634 to 36.796 respectively,which revealed that the ore-forming materials mainly came from the lower crust and upper mantle.The genetic type of Baizhuping gold deposit should be orogenic gold deposit with obvious ore-controlling effects from regional deep faults and ductile-brittle shear zones.