西准噶尔包古图还原性斑岩铜矿富CH4流体来源和CH4形成机制研究

Most of porphyry Cu deposits (PCD) show oxidized features, and reduced porphyry Cu deposits (RPCDs) are scarce in the world, but the Baogutu large Cu deposit from western Junggar is recently recognized as a typical RPCD, characterized by widely development of CH4-bearing ore-forming fluids at several hydrothermal stages. Previous investigations on Baogutu RPCD were mainly focused on magmatic and mineralization chronology, petrology and geochemistry. However, the formation mechanism of CH4 developed at several stages of ore-forming fluids and the origin of CH4-bearing fluids are still unknown, which significantly restricts the understanding of mineralization processes. In order to solve these questions, the newly recognized Baogutu RPCD is chosen as the research object in this study. Laser-Raman spectrum analysis will be used to detect the typical fluid/melt inclusions developed at different stages (from magmatic to hydrothermal) for carbon species in ore-forming fluids, combined with the analyses of microthermometric and fluid compositions of gas/liquid phase. Several independent isotopes will be analyzed by gas mass spectrometer, including H-O isotopes, C isotope of CH4 and CO2 in fluid inclusions enclosed in different stages of quartz, C isotopes of calcite, and He-Ar isotopes in fluid inclusions enclosed in hydrothermal sulfides. In addition, electron microprobe and laser ablation (multi-collector) inductively coupled plasma mass spectrometry will be conducted on hydrothermal minerals including apatite, titanite and epidote for major-trace elements and in situ Sr-Nd isotopes determination. All these results will be combined to investigate the formation mechanism of CH4, and to decipher the origin of CH4-bearing fluids. This research has important implications for the process of mineralization on RPCDs.

全球斑岩型铜矿绝大多数为氧化性，还原性铜矿实例很少，国内新近在西准噶尔确认包古图大型铜矿为典型还原性斑岩矿床，其热液各阶段流体均富含甲烷 (CH4)。前人对该矿床的研究主要集中在成岩-成矿年代学和岩石地球化学方面，缺乏对其中各阶段包裹体普遍发育的CH4成因及成矿流体来源探究，制约了对其成矿过程的理解。本申请拟以包古图还原性斑岩铜矿为研究对象，通过激光拉曼光谱对岩浆至热液各阶段典型熔体/流体包裹体进行系统的C赋存形式分析，辅以包裹体显微测温和气-液相成分分析，并运用气体质谱仪分析不同阶段石英流体包裹体H-O和CH4-CO2 C同位素、碳酸盐C同位素和硫化物He-Ar同位素组成，结合电子探针和LA-MC-ICP-MS获取蚀变矿物（磷灰石、榍石和绿帘石）主微量元素和Sr-Nd同位素组成，从而揭示成矿流体中CH4的形成过程，限定富CH4成矿流体来源。该研究对还原性斑岩铜矿成矿过程具有重要指示意义。

全球斑岩型铜矿绝大多数为氧化性，还原性铜矿实例很少。由于全球缺乏还原性斑岩铜矿实例，导致其成岩-成矿机制缺乏深入了解。我国西准噶尔包古图为一典型还原性斑岩铜矿，以发育大量富CH4流体为典型特征，为研究还原性斑岩铜矿富CH4成矿流体中CH4的形成机制，CH4流体来源及岩浆性质和成因提供了典型实例。为此，我们系统研究了岩浆至热液各阶段流体包裹体C赋存形式，流体H-O-C-He-Ar同位素、碳酸盐C同位素、硫化物S同位素、磷灰石Nd同位素、热液绿帘石Sr同位素、全岩磁化率-Fe2+/Fe3+-主微量元素-Sr-Nd-Pb同位素、锆石U-Pb-Hf-O同位素及微量元素等组成，取得了如下主要结论：1）包古图斑岩铜矿成矿流体中的CH4是由岩浆阶段的CO2经费托反应转变而来，形成于早期高温热液蚀变阶段，而非来自含碳地层的热解作用或地幔成因；2）包古图富CH4流体流体来自深部大型长英质岩浆房，该岩浆房形成过程中经历了显著的镁铁质岩浆混合作用和大规模去气作用；3）包古图成矿岩石为相对还原的钛铁矿系列、I型花岗质岩石，形成于新生下地壳来源的长英质岩浆与地幔来源的镁铁质岩浆混合作用，可能与扩张中脊俯冲形成板片窗构造背景有关。同时，我们对西邻哈萨克斯坦Aktogai巨型、高氧化性斑岩铜矿进行系统的对比研究发现，Aktogai在成岩-成矿作用方面存在显著差异：如成矿岩石为磁铁矿系列、I型花岗质岩石，形成于加厚下地壳部分熔融作用；成矿流体不含CH4，发育大量的热液硬石膏，表明成矿流体为氧化性特征；高氧化性、高岩浆结晶温度和埃达克质特征可能是Aktogai矿床形成的内在有利因素。上述研究结果极大地丰富了还原性斑岩铜矿成矿理论，证实还原性和氧化性斑岩铜矿在成岩-成矿作用方面的差异，同时为揭示我国境内斑岩铜矿规模较小提供了重要依据。

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