新疆西准噶尔包古图地区Au-Cu成矿作用研究

Porphyry is one of the most important Cu, Au and Mo-bearing ore system. The ore-forming process of porphyry system can be divided into magmatic, magmatic-hydrothermal and hydrothermal stage. Although abundant studies had been done to the evolution of porphyry system, most of the publications focused on the magmatic and magmatic-hydrothermal stages. Experimental researches on magmatic evolution indicated that fractional crystallization of pyrrhotite and magnetite in early magmatic stage would lead to obvious decrease of Au and Cu content in the magma. Magma rich-in volatile and sulfur, with high fugacity is favorable to porphyry mineralization. The ore-forming material such as Au, Cu transport from magma to hydrothermal fluid via the exsolution of volatile. The magmatic and magmatic-hydrothermal stages are critical to porphyry mineralization. Certainly, the evolution of hydrothermal fluid is also an important stage of the porphyry ore-forming system, which determines the types and distribution of the mineralization. Zoning of mineralization is always found in porphyry ore-forming system: porphyry Cu-Au/Mo mineralization hosted in intrusion; skarn or porphyry-epithermal transition mineralization in contact zone; epithermal or quartz vein type ore in distal part. Porphyry mineralization happened in early hydrothermal stage. Proximal and distal deposits formed in middle to late hydrothermal stage. Studies about the hydrothermal evolution concentrated on porphyry mineralization, however, it is still controversial about the relationship between porphyry and proximal, distal deposits, whether it contributes ore-forming material, ore-forming fluid, force for the transportation of fluid or all of those to proximal and distal deposits..Porphyry ore-forming system was recognized in Baogutu region, Western Junggar, NW China. The style of the ore system is as follow: (1) intrusion-hosted porphyry Cu-Au mineralization; (2) Au-Bi-bearing sulfide veins hosted in contact zone; (3) distal (1-3km) quartz vein type Au-As-Sb mineralization. In this study, we will take the Au-Bi and Au-As-Sb deposits in Baogutu porphyry system as examples to discuss the contribution of porphyry to proximal and distal mineralization. The genesis of gold mineralization as well as their genetic relation with porphyry will be systematically studied. We will also summarize the Au-Cu mineralization regularity of Baogutu region and establish the metallogenic model.

斑岩成矿系统的演化由岩浆、岩浆-热液、热液阶段组成，岩浆、岩浆-热液阶段决定着斑岩系统的成矿能力。研究表明，富含挥发分、高氧逸度且富硫的岩浆通过挥发分出溶可以形成具有成矿潜力的热液；热液演化阶段决定系统中形成的矿床类型及其分布，从岩体向外围常形成明显的矿化分带性，热液演化早期主要在斑岩体内形成斑岩型矿化，而关于热液中晚期演化过程以及斑岩体系对外围矿床的贡献（提供成矿物质、成矿流体或热驱动力），目前依然是斑岩成矿系统研究的薄弱环节。新疆西准噶尔包古图地区发育斑岩成矿系统，从岩体内向外围依次发育：斑岩型Cu-Au矿化（岩体内）、硫化物脉型Au-Bi矿化（与围岩接触带）、石英脉型Au-As-Sb矿化（外围1-3km），本项目拟通过研究包古图地区Au-Bi矿化、AuAs-Sb矿化的成因及其与斑岩体系的关系，建立成矿模型，探讨热液中晚期演化过程及斑岩体系对外围矿床的贡献，为成矿理论研究提供资料。

西准噶尔包古图地区出露多个金矿床，产于斑岩体与围岩接触带或岩体外围1~3km范围内，对包古图地区Au-Cu成矿作用的研究，将有利于理解和揭示斑岩岩浆体系热液晚期阶段的演化过程，以及斑岩体系与外围金矿化之间的成因联系。为此，本项目选择包古图地区的金矿床和斑岩型矿床为研究对象，对斑岩体系和金矿化之间的关系进行了讨论。经过3年的工作，本项目达到了预期目标。已发表的学术论文12篇（SCI 6篇， 其中一篇为SCI收录会议论文）。总经费25万元，已支出25万元，经费使用合理。在开展了两次系统野外地质调查和采集样品的基础上，对典型围岩和矿石样品进行了多种地球化学分析，获得了大量高精度地球化学数据。主要成果包括：（1）包古图地区存在两类金矿化：硫化物脉型Au-Bi矿化和石英脉型Au-As-Sb矿化，分别产于岩体与围岩接触带和岩体外1-3km范围内，金矿化与Bi矿化或As-Sb矿化具有成因联系；（2）在包古图金矿发现了自然锑、自然砷、方锑金矿、硫锑金银矿和Paakkonenite (Sb2AsS2)等罕见矿物，与金矿化关系密切，Au-As-Sb矿化主要受热液体系氧逸度、硫逸度等因素控制；（3）包古图金矿区中酸性脉岩主要为闪长玢岩和石英闪长斑岩，属于钙碱性系列，具有LREE、LILE富集和Nb、Ta亏损的微量元素地球化学特征，侵位年龄为310-320Ma，与区内斑岩体为同一期岩浆活动的产物；（4）不同阶段矿物组合、流体包裹体以及黄铁矿微量元素数据指示，成矿流体主要为中低温（150-300℃）、低盐度（1-10 wt.%）流体，从成矿早期到晚期，成矿流体的温度、硫逸度和氧逸度逐渐降低，pH升高，这些因素的变化导致了金矿化的发生；（5）成矿流体H-O同位素、S同位素以及Osi显示，包古图金矿成矿流体主要为经历了水-岩反应的岩浆水，成矿流体和成矿物质主要来自于岩浆；包古图金矿化年龄为318Ma（黄铁矿Re-Os），与包古图地区中酸性岩浆活动同时代，指示金矿化与中酸性岩浆活动具有成因联系；（6）包古图地区出露的斑岩氧化还原状态存在差异，其中I、II和V号岩体属于磁铁矿系列，具有较高氧逸度，III号岩体属于钛铁矿系列，氧逸度较低；（7）通过对比，发现包古图金矿为与侵入岩有关金矿成矿系统中的浅成金矿床，指示包古图III、IV号岩体深部可能存在角砾岩型、席状Au-As-Bi矿化。