内蒙古霍各乞矿床中高温、低盐度、富碳质流体中Cu的迁移机制

The Huogeqi deposit in western Inner Mongolia is characterized by its low-salinity, mesothermal, and carbonic Cu-forming fluid, which shares many similarities with the ore-fluids of orogenic gold deposits. Traditional thermodynamic calculations suggested that Cl- is the predominant ligand for Cu complexation in medium to high temperature fluids, and excluded the low-salinity orogenic-type fluid as an ideal solution for Cu transport. The fact that a low-salinity fluid is responsible for Cu mineralization of the Huogeqi deposit indicates that Cu complexation in mesothermal fluids needs to be revisited. In the traditional thermodynamic calculations, the role of reduced sulfur as a ligand for metal complexation might be greatly underestimated at high temperatures, for the effect of prograde dissolution of Fe-S minerals were neglected. In our recent thermodynamic modeling, the interaction between Fe-S mineral dissolution and Cu comlexation was taken into consideration, and Cu-hydrosulfide is proved to be the predominant Cu specie in mesothermal fluids. The primary goal of this study is to reveal the predominant Cu specie in the low-salinity, mesothermal, and carbonic ore-fluid of the Huogeqi deposit. The composition of Cu-forming fluid is to be investigated using in-situ LA-ICP-MS and PIXE analysis on single fluid inclusions. Combined with thermodynamic modeling that based on petrographic observations and fluid inclusion compositions, the Cu complexation in the ore-forming fluid will be envisaged. In addition, the major factors that may influence Cu solubility in the ore-fluid and the possible mechanism that may lead to Cu precipitation will also be discussed based on the results of the results of thermodynamic modeling.

内蒙古西部霍各乞Cu矿床成矿流体具有中高温、低盐度、富碳质特征，与造山型金矿流体类似。该矿床以及其他造山型Cu矿床的典例研究表明部分造山型成矿流体也具有一定的Cu矿化潜力。然而，现有的成矿理论难以解释为低盐度的造山型成矿流体可以造成Cu的矿化，因为传统观点认为Cl是中高温条件下Cu搬运迁移的主要配体。前人的热力学计算忽略了Fe-S矿物的递进溶解与Cu络合作用间的耦合，进而可能低估中高温下S作为Cu配体的重要性。我们前期的热力学模拟工作考虑了上述因素，初步揭示中高温条件下Cu的硫氢络合物为具有较好的稳定性，暗示S等配体可能对造山型Cu矿化有重大贡献。本项目拟通过对霍各乞矿床的岩矿相学观察、流体包裹体显微测温、PIXE及LA-ICP-MS单个流体包裹体成分分析，结合热力学模拟，揭示Cu在中高温、低盐度、富碳质的造山型成矿流体中的主要迁移方式、Cu溶解度的主要控制因素、以及可能的Cu沉淀机制。

在项目支持下，完成了霍各乞成矿期单个包裹体PIXE微量元素分析。结果表明成矿流体具有较高的铜含量，普遍达到n×102至n×103 ppm（中位数1338 ppm），具有较大的成矿潜力。PIXE测试分析了成矿流体中Cl、S含量，揭示流体中的Cl与Cu含量不存在正相关；流体中S含量高达11000 ppm，表明Cu元素在霍各乞中温、低盐度、富碳质流体中主要以硫氢配合物形式迁移，而非氯配合物。作为研究工作的拓展，采用热力学模拟方法，从理论上定量论证了Au、Cu、Pb、Zn等元素在变质脱水过程中的成矿潜力。模拟结果表明，泥质岩在跨域绿片岩相—角闪岩相时，会释放大量流体，并萃取岩石中多数的Au及少部分的Cu、Pb、Zn等贱金属元素。伴随变质地体中形成超大型造山型Au矿床，有可能形成中等规模的Cu、Pb、Zn矿床。研究结果表明，在变质过程中，Cu等贱金属成矿潜力不如造山型金矿，但仍有可能形成部分贱金属矿床。此类矿床具有与造山型金矿类似的地质、地球化学特征，具有中温、低盐度、富碳质的变质成矿流体，霍各乞即为一典型实例。相关研究成果发表于Geochimica et Cosmochimica Acta。此外，在项目支持下，对与霍各乞矿床临近的东升庙Zn-Pb矿床的成因进行了研究。该矿床与霍各乞同属于狼山—渣尔泰山成矿带，研究表明东升庙矿床具有与霍各乞类似的成因，经历了元古代同生沉积期成矿物质预富集，并在燕山期变质、剪切变形过程中发生了成矿物质的再活化、加富，形成了受剪切带控制的Zn-Pb矿床。相关研究成果发表于Geoscience Frontiers.