吉尔吉斯斯坦赋存于角闪岩相变质岩中的Ishtamberdy金矿床成因机制研究：硫化物显微结构、化学组成与年代学制约

Orogenic gold deposits are generally associated with low- to medium-grade metamorphism, whether significant gold deposits can be formed through medium- to high-grade metamorphic event has been hotly disputed. Ore-forming mechanism of this kind of gold deposits remains unclear. The Ishtamberdy gold deposit in Kyrgyzstan Tian Shan is hosted by upper amphibolitic facies metamorphic rocks and thus provides an ideal target for the investigation of the ore-forming mechanism of the deposit as well as the relationship between gold mineralization and medium- to high-metamorphic event. The proposed project will focus on the ore texture and sulfide microstructure of the Ishtamberdy gold deposit based on detailed field survey and microscopic petrography observation. SEM、EPMA and EBSD will be first utilized to observe the microstructure of the sulfides and identify their origin and evolvement information as well as deformation and metamorphic overprint. The occurrence of gold and its relationship with sulfides will also be investigated. Subsequently, the distribution and variation of trace elements and sulfur isotopes will be analyzed with LA-ICP-MS or SIMS. In addition, Re-Os dating with pyrite and arsenopyrite, or U-Th-Pb dating with U-Th bearing accessory minerals is used to provide constraints on the geochronology of the gold mineralization. These results will be combined together to reveal the gold source and enrichment mechanism as well as inquire into the ore genesis and its relationship with amphibolitic facies metamorphism in the Ishtamberdy gold deposit, which would provide a better understanding on the gold mineralization associated with medium- to high-metamorphic rocks and on the knowledge of orogenic gold deposit.

造山型金矿床多与中低级变质作用有关，对于中高级变质作用能否形成重要金矿床目前还存在较大争议，成矿机制尚不清楚。吉尔吉斯斯坦天山Ishtamberdy大型金矿床赋矿围岩为高角闪岩相变质岩，金矿床成因机制及其与角闪岩相变质作用的关系是亟待解决的关键问题。本申请拟以Ishtamberdy金矿床为研究对象，在详细的野外地质调查和岩/矿相学工作基础上，利用SEM、EPMA、EBSD等方法系统研究矿石结构构造及硫化物显微结构，识别硫化物成生演化和变形变质特征，查明金的赋存状态及其与硫化物结构变化的关系。在此基础上，利用LA-ICP-MS或SIMS开展硫化物微量元素和硫同位素组成的微区原位分析，并结合黄铁矿和毒砂Re-Os同位素定年或热液成因含铀-钍副矿物U-Th-Pb定年，查明成矿物质来源及富集机理，揭示矿床成因机制及其与变质作用的关系，深化中高级变质岩中金矿床的成因认识，丰富造山型金矿床的研究。

造山型金矿床金成矿与变质作用的关系、成矿物质来源和成矿机制是亟待解决的关键问题，吉尔吉斯斯坦天山赋存在中高级变质岩中的Ishtamberdy金矿床提供了良好的研究对象。项目针对该矿床开展了详实的野外调研，并系统识别了矿石结构构造和硫化物显微结构特征，分析了硫化物微量元素和原位硫同位素组成，并开展成岩成矿年代学等研究，取得以下主要认识：1）石英-黄铁矿-毒砂阶段（阶段II）是主要金成矿阶段，不仅形成较多的可见金（Au/Ag=3.36~4.41），在该阶段黄铁矿Py3和毒砂Apy2中也分布大量固溶体金或纳米颗粒金。该阶段黄铁矿核部Py3a具有较低的Au、As和Cu含量（平均4.73ppm Au），边部Py3b具有高的Au、As和Cu含量（平均97.65ppm Au，16700 ppm As，215 ppm Cu）；毒砂多孔核部Apy2a平均Au含量为172.92ppm，具有震荡环带的边部Apy2b平均Au含量为1159.47ppm。不同世代硫化物微量元素分析及典型环带状黄铁矿扫面结果显示，成矿期黄铁矿Au、As、Cu、Ag、Sb、Tl等元素相对富集，毒砂Au和Cu等具正相关关系。2）SIMS和LA-ICP-MS原位硫同位素分析表明热液成矿阶段硫同位素组成总体为δ34S =4.8~11.0‰，但不同阶段硫同位素组成存在一定差异，指示在成矿过程中存在物理化学条件的变化。值得注意的是，主阶段高Au-As含量黄铁矿Py3b（δ34S =4.7~6.7‰，平均5.6‰）与矿区外围矽卡岩中黄铁矿（平均4.8‰）和同区域Unkurtash与侵入岩有关的金矿床（3.8~7.1‰，平均5.7‰）具有相似的硫同位素组成，表明岩浆热液可能参与了成矿过程。3）锆石、榍石U-Pb和辉钼矿Re-Os年代学研究将金成矿时代限定在305~280Ma之间，明显晚于围岩石榴石角闪岩变质作用时代（318Ma）。并且，热液硫化物硫同位素组成（平均7.2‰）远小于变质成因的硫化物（平均14.7‰），且变质成因的硫化物具有非常低的Au、As等元素含量（<0.04ppm Au, 43 ppm As）；因此，围岩变质脱挥发分无法为金成矿提供直接的金属和硫来源。综上，Ishtamberdy金矿床成矿过程中可能有岩浆流体的加入，物理化学条件的变化形成了具有不同显微结构、元素组成和硫同位素特征的黄铁矿、毒砂等硫化物。

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