Genesis of Baizhuping Gold Deposit in Yichang, Hubei: Evidence from Fluid Inclusion and H-O-S-Pb Isotope Geochemistry
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摘要: 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/cm3）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-H2O-CO2体系。氢、氧同位素分析显示，成矿流体的δ18OH2O变化于+1.0‰ ~ +5.3‰，δDH2O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δ34S 值为0.93‰ ~ 6.69‰，206Pb/204Pb 为15.634 ~ 15.677，207Pb/204Pb 为15.238 ~ 15.318，208Pb/204Pb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。Abstract: 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/cm3).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-H2O-CO2system．Oxygen and hydrogen isotope data from quartz show that the δ18O V-SMOW values range from 13.0‰ to 15.3‰，δ18O H2O values from +1.0‰ to +5.3‰，and δDH2O values between -56.1‰ and -35.0‰, which revealed that the ore-forming fluids of the 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 δ34S values of sulfides range from 0.93‰ to 6.69‰ ．Among Pb isotopes，the 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of sulfides are in the range of 15.634 to 15.677, 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.
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