Differential deposition of gold in mafic-hosted and ultramafic-hosted hydrothermal systems on the mid-ocean ridge
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摘要:
洋脊是地球上规模最大的山脉体系，主要由超镁铁质和镁铁质岩所组成，因构造和岩浆作用，这两类岩层分别孕育了超镁铁质和镁铁质含金热液系统。金首先通过水岩反应从洋脊源区岩层内迁移出来，再经历运移堆积作用汇聚到硫化物堆积体内，最后遭受后期蚀变活化迁移改造。以上过程构成了金在这两类热液系统中的完整演化历程。超镁铁质热液系统内的金在汇源端员间的比值远高于镁铁质热液系统。这种差异性聚集暗示了这两类热液系统分别演化出了独具特色的载金属性特征及富集迁移机制。相比于镁铁质热液系统，超镁铁质热液系统内围岩普遍具有的高金含量和高孔高渗特征、热液流体中溶解态非生物有机质和气态物质含量高、硫化物堆积体所处区域裂隙发育及构造稳定等因素，都是造成两者之间存在显著差异性聚集过程的主要原因。持续性地对洋脊热液系统各深部结构体进行更多更精细有关金的丰度、赋存状态及演化变迁的测试分析及模拟研究工作，是未来量化揭示金在不同类型热液系统内的物源贡献及各演化阶段中富集亏损的关键，也将为未来人工海底干预富集成矿工程累积信息。
Abstract:The global mid-ocean ridge, where the gold-bearing hydrothermal circulation occurs by tectonic and magmatic processes, is the largest mountain chain on the Earth. It consists predominantly of ultramafic and maficd rocks. A significant amount of gold is initially leached out from the two types of source rocks, and then move by similar physical-chemical processes of transportation and precipitation, and finally incorporate into the massive sulfide deposits. After formation, the sulfide deposits will suffer from extensive dissolution, remobilization and reprecipitation. It is observed that the gold ratios between the sulfide deposits and the source rocks in the ultramafic-hosted hydrothermal systems are much greater than those in the mafic-hosted hydrothermal systems. Such differences in the two types of hydrothermal systems suggest that they have significant distinctions in the occurrence, evolution and enrichment of gold in the hydrothermal circulation on the mid-ocean ridge. Four factors in favor of such distinctions in the ultramafic-hosted hydrothermal systems have been identified: 1) the nature of the surrounding rocks; 2) abiotic organic compounds and gaseous species in the vent fluids; 3) permeability of the hydrothermal fluids upflowing zones; 4) incidence of the volcanic event. For the gold deposits on the mid-ocean ridge, in-situ and micro-area analyses of high-precision for the composition, species and properties in different components of the hydrothermal systems together with the experimental and thermodynamic modelling are critical to revealing quantitatively the extent of the source contributions and enrichment or depletion of gold in different evolutionary stages of gold, and provide significant insight into the enrichment process of gold on the mid-ocean ridge under the intervention of mankind.
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表 1 超镁铁质和镁铁质热液系统在水岩反应阶段对金在海底富集的不同影响
Table 1. Different effects for the gold enrichment during the seafloor fluid-rock interaction in the mafic-hosted and ultramafic-hosted hydrothermal systems
热液系统类型 围岩的金含量/10−9 相容性 附着上浮 围岩物理属性 额外热源 镁铁质 1.0～1.2（−） − + 致密（−） − 超镁铁质 1.7（+） + − 多孔（+） 蛇纹石化放热（+） 注：+表示相对有利于富集，−表示相对不利于富集，数据来源见正文。 
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表 2 超镁铁质和镁铁质热液系统在运移堆积阶段对金在海底富集的不同影响
Table 2. Different effects for the gold enrichment during the seafloor migration and accumulation of the metallogenic material in the mafic-hosted and ultramafic-hosted hydrothermal systems
热液系统类型 流体中非生物有机化合物含量 还原性环境 丘体形态 构造活动 镁铁质 低（−） 低（−） 高耸（−） 强（−） 超镁铁质 高（+） 高（+） 平坦（+） 弱（+） 注：+表示相对有利于富集，−表示相对不利于富集，数据来源见正文。
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表 3 超镁铁质和镁铁质热液系统在后期活化阶段对金在海底富集的不同影响
Table 3. Different effects for the gold enrichment during the seafloor weathering and migration of the sulfide deposits in the mafic-hosted and ultramafic-hosted hydrothermal systems
热液系统类型 趋外活化迁移 丘体形态 水力压裂 镁铁质 + 高耸（+） 弱（+） 超镁铁质 − 平坦（−） 强（−） 注：+表示相对有利于富集，−表示相对不利于富集，数据来源见正文。
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