Research on fluid inclusions of the Baiyun gold deposit, Liaoning Province: implications for fluid evolution and metallogenic mechanism
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摘要:
辽宁白云金矿位于华北克拉通东北缘, 是胶东-辽东-吉南成矿带内重要的大型金矿床之一。在矿床地质特征研究的基础上, 通过矿床的流体包裹体特征的系统研究, 探讨成矿流体的性质、演化及金的迁移沉淀机制。依据脉体间的穿插、矿物共生组合及矿物交代关系, 将白云金矿的成矿作用过程划分为石英-黄铁矿(早阶段)、石英-多金属硫化物(主阶段)和石英-碳酸盐(晚阶段)3个阶段。岩相学观察显示, 白云金矿主要发育的流体包裹体类型为H2O两相包裹体(W型)、CO2-H2O三相包裹体(C型)和纯CO2包裹体(PC型)。显微测温结果显示, 主成矿阶段流体包裹体的均一温度范围集中在240~280℃, 流体包裹体盐度范围集中在6%~8%NaCleq., 计算出相应的流体密度为0.74~0.91g/cm3, 具有中温、中低盐度、中等密度成矿流体的特征。氢-氧同位素结果显示, 成矿流体早期为岩浆热液, 成矿过程中逐渐有大气降水的混入。W型、C型和PC型包裹体存在同视域共存的现象, 并且这些包裹体的均一温度相近, 而均一方式不同, 表明流体发生不混溶作用。白云金矿围岩蚀变中发育与矿化密切相关的硅化、绢云母化等蚀变, 以及黄铁矿-绢云母-石英的矿物组合特征, 暗示水岩反应的发生。因此, 流体不混溶和水岩反应是造成白云金矿床中金矿物沉淀富集的主要机制。在此基础上, 估算成矿压力为43.4~87.5MPa, 对应的成矿深度为1.6~3.3m, 并结合前人对区域剥蚀深度的估算, 推测白云金矿深部仍具有较大的资源潜力。
Abstract:The Baiyun gold deposit in Liaoning province is situated at the northeastern edge of the North China Craton and is one of the significant large-scale gold deposits in the Jiaodong-Liaodong-Jinan metallogenic belt.Based on its geological features, this study thoroughly examined the fluid inclusion characteristics of the deposit, analyzed the fluid properties, evolution, and gold movement and precipitation mechanisms.According to the interlacing of veins, mineral assemblage, and metasomatism, the metallogenic process of the Baiyun gold deposit can be divided into three stages: quartz-pyrite(early stage), quartz-polymetallic sulfide(main stage)and quartz-carbonate(late stage).Petrographic observation reveals that the fluid inclusions developed in the Baiyun gold deposit are primarily H2O two-phase inclusions(W type), CO2-H2O three-phase inclusions(C type)and pure CO2 inclusions(PC type).Microthermometry results show that the homogenization temperature of fluid inclusions during the main ore-forming stage is between 240℃ and 280℃, and their salinity varies between 6 and 8% NaCl eq.The corresponding fluid density is calculated to be 0.74~0.91 g/cm3.These exhibit medium-temperature, medium-low salinity and medium-density ore-forming fluid the characteristics.H-O isotopic results suggest that the ore-forming fluid was originally magmatic hydrothermal, and atmospheric water was gradually mixed in during the ore-forming process.W type, C type and PC type inclusions coexist in the same field of view, and while the homogenization temperatures of these inclusions are comparable, their modes of homogenization are distinct, indicating the presence of fluid immiscibility.The existence of water-rock interaction is suggested by silicification and sericite alteration, which are closely connected with mineralization in the surrounding rock alteration, as well as the pyrite-sericite-quartz mineral assemblage features.Therefore, the main mechanisms of gold mineral precipitation in the Baiyun gold deposit are fluid immiscibility and water-rock interaction.On this basis, it is estimated that the metallogenic pressure is 43.4~87.5 MPa, and the corresponding metallogenic depth is 1.6~3.3 km. Combined with the previous estimation of regional denudation depth, it is inferred that the Baiyun gold deposit still has great resource potential in the deep.
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表 1 白云金矿流体包裹体显微测温结果
Table 1. Microthermometry results of fluid inclusions in the Baiyun gold deposit
阶段 寄主矿物 包裹体类型 Tm-CO2/℃ Tm-cla/℃ Tm-ice/℃ Th-CO2/℃ Th/℃ 盐度/% NaCl eq. 早阶段 石英 WL -3.3~-9.0 256~388 5.4~12.9 C -57.7~-56.6 3.3~8.9 23.2~30.5 281~323 2.2~11.5 主阶段 石英 WL -1.1~-8.1 179~332.6 1.9~11.8 WV 275.5~348 C -57.9~-56.6 4.0~9.2 21.3~30.6 230~325 1.6~10.5 PC -57.8~-56.6 21.3~26.8 晚阶段 方解石 WL -0.1~-4.0 125.6~249 0.2~6.5 石英 WL -0.1~-3.8 150.8~228.2 0.2~6.2 注: Tm-CO2—CO2三相点温度；Tm-cla—CO2相笼合物融化温度；Th-CO2—CO2相部分均一温度；Tm-ice—冰点温度；Th—完全均一温度 
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表 2 白云金矿石英氢氧同位素组成
Table 2. Hydrogen and oxygen isotopic compositions of quartz in the Baiyun gold deposit
样品号 矿化阶段 温度/℃ δ18O/‰ δ18OH2O/‰ δ18DH2O/‰ BY022 早阶段 326 12.0 5.9 -86.8 BY024 326 12.6 6.5 -80.3 BY035 主阶段 260 12.2 3.6 -97.1 BY042 260 14.5 5.9 -97.7 BY043 260 14.2 5.6 -93.5 BY216 260 12.3 3.7 -93.4 BY090 晚阶段 179 11.9 -1.3 -90.2
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