A Fluid Inclusion Study of Au-Bearing Quartz Vein Systems in the Central and North Deborah Deposits of the Bendigo Gold Field, Central Victoria, Australia
A Fluid Inclusion Study of Au-Bearing Quartz Vein Systems in the Central and North Deborah Deposits of the Bendigo Gold Field, Central Victoria, Australia
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DOI：
10.2113/gsecongeo.95.3.467
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发表时间：
2000-05
期刊：
影响因子：
5.8
通讯作者：
Yiefei Jia;X. Li;R. Kerrich
中科院分区：
文献类型：
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作者：
Yiefei Jia;X. Li;R. Kerrich
中部和北部Deborah金矿床产Au约7吨，位于澳大利亚维多利亚州中部本迪戈金矿田的15个穹顶之一，是重要的中热液脉金省。含金石英脉系统主要赋存于经历了下绿片岩相变质作用的下奥陶统浊积岩中的逆断层中。在德博拉中部和北部矿脉中，石英脉的发育顺序为：(1)层状脉，具有Au-As-Sb-Pb-Zn-Ni的金属组合；(2)刺(Au)；(3)块状贫脉(主要扩张阶段)；(4)角砾脉，具Au-As；(5)石英-铁角闪石细脉；(6)晚期纯石英或方解石细脉。在3个矿化脉期(1、2、4)中，只有1个矿化脉期早于主变形事件或与主变形事件同时代，2、4期矿化脉期晚于主变形事件。脉体结构特征表明，脉体发育受控于流体压力的反复波动导致的水力压裂。脉状石英中的流体包裹体含有不同成分的C-O-H流体。室温下流体包裹体主要有三种类型：I型两相原生(CH4)V+(H2O)L流体包裹体，II型两相或三相原生拟次生(CO2)V或L+(CH4)V或L+(H2O)L流体包裹体，包括IIa，CH4-CO2-H2O和IIb，CO2-H2O流体包裹体，以及III型，(H2O)V+(H2O)L流体包裹体。III型流体包裹体进一步分为三类：IIia，原生-拟次生，富气相；IIib，原生-拟次生，富液；IIIc，次生，富液。流体包裹体分布、显微测温和拉曼光谱数据表明，与金矿化石英脉(1期、2期和4期)有关的流体具有中等盐度，范围为0-12wt%氯化钠当量(模拟盐度约为7-8wt%氯化钠当量)。这些矿脉的形成温度为325~375℃，压力为200~300兆帕，侵位深度约为8~12千米。与贫瘠的石英脉有关的流体(阶段3、阶段5和阶段6)的盐度较低，约为0-3wt%的氯化钠当量(模拟的盐度约为2-3wt%的氯化钠当量)，温度较低。有证据表明，流体在所有脉期都是不混溶的，同一来源的流体之间也存在混合，但不同的fO2也可能影响金的沉积(Cox等人，1995年)。热液成矿流体为CH2-CO2-H2O-NaC l体系，块状贫脉(3期)和细脉(5期和6期)分别为CO2-H2O-NaC l和H2O-NaC l体系。I型和II型流体包裹体的碳相范围从纯CH4到混合CH4和CO2，再到纯CO2。金矿成矿流体为弱酸性，fO2为10~(-25)~10~(-37)bar，一般高于CO2/CH4，低于SO2/H_2S缓冲界面。相对减少的成矿流体类似于其他浊积岩容矿的脉状金矿床和火山-深成地体中的相应成矿流体。
The Central and North Deborah gold deposits, with a production of about 7 tons Au, are located in one of 15 domes in the Bendigo gold field, central Victoria, Australia, an important mesothermal lode gold province. The gold-bearing quartz vein systems are hosted predominantly by reverse faults in Lower Ordovician turbidites, which underwent lower greenschist facies metamorphism. In the Central and North Deborah mines, there are six stages of quartz veins that developed in the following sequence: (1) laminated veins, with a metal association of Au-As-Sb-Pb-Zn-Ni; (2) spurs (Au); (3) massive barren veins (main dilation stage); (4) brecciated veins, with Au-As; (5) quartz-ankerite veinlets; and (6) late, pure quartz or calcite veinlets. Of the three mineralized vein stages (1, 2, and 4), only the first one predated or was coeval with the main deformation event, whereas stages 2 and 4 postdated the main deformation. The vein structures indicate that vein development was controlled by repeated fluctuations of fluid pressure inducing hydraulic fracturing. Fluid inclusions in vein quartz contain C-O-H fluids of variable compositions. Three main types of fluid inclusions are recognized at room temperature: type I, two-phase, primary (CH4)V + (H2O)L fluid inclusions; type II, two- or three-phase, primary-pseudosecondary (CO2)V or L + (CH4)V or L + (H2O)L fluid inclusions, including IIa, CH4-CO2-H2O, and IIb, CO2-H2O fluid inclusions; and type III, (H2O)V + (H2O)L fluid inclusions. Type III fluid inclusions are further divided into three groups: IIIa, primary-pseudosecondary, vapor-rich; IIIb, primary-pseudosecondary, liquid-rich; and IIIc, secondary, liquid-rich. Data from fluid inclusion distribution, microthermometry, and Raman spectroscopy indicate that fluids associated with Au mineralized quartz veins (stages 1, 2, and 4) have moderate salinity ranging from 0 to 12 wt percent NaCl equiv (modeled salinities around 7–8 wt % NaCl equiv). These veins formatted at temperatures from 325° to 375°C, and pressures of 200 to 300 MPa, with emplacement depths of about 8 to 12 km. Fluids associated with barren quartz veins (stages 3, 5, and 6) have a low salinity of about 0 to 3 wt percent NaCl equiv (modeled salinities about 2–3 wt % NaCl equiv) and lower temperatures. There is evidence of fluid immiscibility in all vein stages and mixing between fluids of the same souce, but differing fO2 may also have influenced gold deposition (Cox et al., 1995). Hydrothermal ore-forming fluids responsible for gold mineralization are in the CH2-CO2-H2O-NaCl system, whereas those of massive barren veins (stage 3) and veinlets (stages 5 and 6) are in the CO2-H2O-NaCl and H2O-NaCl systems, respectively. The carbonic phases of type I and II fluid inclusions range from pure CH4, through mixed CH4 and CO2, to pure CO2. Gold ore-forming fluids were weakly acidic, with f O2 estimated to have been 10–25 to 10–37 bars, and generally above the CO2/CH4 and below the SO2/H2S buffer boundaries throughout gold mineralization. The relatively reduced mineralizing fluids are similar to those of other turbidite-hosted lode gold deposits and counterparts in volcanic-plutonic terranes.