甘肃北山地区小西弓金矿床成矿流体性质及矿床成因

小西弓金矿床位于甘肃北山造山带南部,矿体受NWW向次级断裂控制,赋矿围岩主要为中元古界西尖山群变质岩。成矿过程可分为3个阶段:石英-黄铁矿阶段(早阶段)、石英-多金属硫化物阶段(主成矿阶段)和石英-碳酸盐阶段(晚阶段)。对主成矿阶段石英中的流体包裹体、微量元素和氢、氧同位素开展研究,以期查明成矿流体性质,探讨矿床成因。主成矿阶段石英中主要发育气液两相水溶液包裹体、CO2-H2O三相包裹体和纯液相CO2包裹体,均一温度介于194~397℃,盐度为2.2%~8.9%NaCleqv,密度为0.63~0.98 g/cm^3。利用CO2-H2O三相包裹体计算出主成矿阶段流体包裹体捕获压力为257~395 MPa,成矿深度为9.5~14.6 km。流体包裹体测温、激光拉曼光谱分析与石英微量元素特征表明,成矿流体为中高温、低盐度、中低密度的CO2-H2O-NaCl±CH4体系,且具有相对还原性的特点。主成矿阶段石英的δDV-SMOW值为-100.2‰^-75.6‰,δ18OH2O值为+4.15‰^+8.31‰,主成矿阶段流体以变质水为主,流体不混溶作用是金富集成矿的重要机制。综合分析认为,小西弓金矿床为中深成造山型金矿床。

Properties of Ore-forming Fluids and Genesis of the Xiaoxigong Gold Deposit in the Beishan Region,Gansu Province

The Xiaoxigong gold deposit is located in the south part of the Beishan orogenic belt. The orebodies are hosted in metamorphic rocks of the Middle Proterozoic Xijianshan Group, and are controlled by the NWW- trending secondary faults. Mineralization process can be divided into three stages, including quartz-pyrite stage (early stage), quartz-polymetallic sulfide stage (main ore stage) and quartz-carbonate stage (late stage). In this paper, fluid inclusions, trace elements and H-O isotopes of the main ore stage are studied in order to determine the properties of the ore-forming fluids and the genesis of the deposit. The quartz of the main ore stage mainly contains H₂O-NaCl, CO₂-H₂O and pure CO₂ inclusions. The homogenization temperatures, salinities and densities of H₂O-NaCl and CO₂-H₂O inclusions vary from 194 ℃ to 397 ℃, 2.2% to 8.9% and 0.63 g/cm³ to 0.98 g/cm³, respectively. According to the calculation from CO₂-H₂O three phase inclusions, the trapping pressures in the ore stage are 257 MPa to 395 MPa and the ore-forming depths are 9.5 km to 14.6 km. The results of fluid inclusion microthermometry, laser Raman spectroscopy and quartz trace elements show that the ore-forming fluid is a CO₂-H₂O-NaCl±CH₄ system and somewhat reduced, with medium-high temperature, low salinity and medium-low density. The values of δD_V-SMOW and δ¹⁸ {\mathrm{O}}_{{\mathrm{H}}_2\mathrm{O}} in the ore stage are -100.2‰ to -75.6‰ and +4.15‰ to +8.31‰.The fluid inclusion and H-O isotope characteristics indicate that the ore-forming fluids in the main ore stage were mainly metamorphic water, and the fluid immiscibility is an important mechanism for gold precipitation. Comprehensive analyses show that the Xiaoxigong deposit belongs to a mesozonal to hypozonal orogenic gold deposit.