新疆包古图斑岩铜矿成矿岩浆氧化-还原特征及其控制因素研究

Classic porphyry copper deposits are relatively oxidized in magmatic stage, with logfO2 varying between the hematite-magnetite (HM) and nickel-nickel oxide (NNO) buffers, mostly exceeding QFM+2. The porphyries are magnetite-series I-type granitoids, bearing primary magmatic anhydrite. However, a small amount of reduced porphyry copper deposits have been recognized recent years. They show relatively low oxidation state, with logfO2 less than or equal to the quartz-fayalite-magnetite (QFM) buffer. These deposits contain abundant hypogene magmatic sulfides (such as pyrrhotite) and are associated with ilmenite-series I-type granitoids. Magmatic redox state plays a crucial role in the formation of large porphyry copper deposits. Magmatic sources with high oxidation state will breakdown the magmatic sulfide phases (S2-), and ensure that sulfate (SO42-) is the exclusive sulfur species in the melt. This process likely releases large amounts of chalcophile (e.g., Cu, Ni) and siderophile (e.g., Au) elements to melts, which can transport these ore-forming metals to the shallow crust. Therefore, most ore-forming magmas of large porphyry copper deposit show relatively high oxygen fugacity. However, it is not well understood why magmatic stages in some porphyry deposits have such low oxidation state. One possible explanation will be their magmatic sources are reduced. Additionally, magmatic differentiation such as fractionation and contamination upon emplacement can also lower magmatic oxidation state. Both oxidized-magnetite and reduced-ilmenite porphyries are recognized in Baogutu area, West Junggar, Xinjiang. In this study, we will focus on the oxidation state of the mineralized intrusions in Baogutu region based on mineralogy and geochemistry. Moreover, the controlling factors on oxidation state will be discussed based on petrology, mineralogy and geochemistry. This study will likely lead to a significant advance in our understanding of porphyry ore genesis and improve exploration criteria for them.

一般认为，斑岩铜矿成矿岩浆氧逸度较高（logfO2＞QFM+2），其中含岩浆成因硬石膏，成矿岩体类似磁铁矿系列I型花岗岩。而部分还原型斑岩铜矿成矿岩浆氧逸度较低（logfO2＜QFM），含有岩浆成因硫化物，成矿岩体类似钛铁矿系列I型花岗岩。氧逸度是斑岩成矿作用的关键制约因素，高的氧化状态确保岩浆源区中的硫以硫酸盐（SO42-）的形式存在，将亲铜（例如Cu）和亲铁元素（例如Au）释放进入岩浆熔体。而关于还原型成矿岩浆的制约因素（受控于岩浆源区还是岩浆演化过程中的结晶分异或地壳物质混染）和成矿意义，目前仍然存在争议。新疆西准噶尔包古图斑岩铜矿区同时发育氧化型（磁铁矿型）和还原型（钛铁矿型）矿化岩体，本项目拟通过岩石-矿物学和锆石微量元素地球化学研究，探讨包古图斑岩铜矿成矿岩浆的氧化-还原特征。同时，岩石学与地球化学相结合，探讨成矿岩浆氧逸度的控制因素，并分析其成矿意义，为成矿理论研究提供资料。

成矿岩浆的氧逸度是控制斑岩矿床成矿作用的关键因素之一，本项目以新疆西准噶尔包古图斑岩铜矿为研究对象，通过多个矿化岩体的对比研究，探讨成矿岩浆的氧化-还原状态及其控制因素，在丰富斑岩矿床成矿作用理论的同时，为西准噶尔地区的找矿勘探和区域成矿对比提供依据。在项目执行的3年时间里，按计划完成各年度研究任务，达到了预期研究目标。已发表学术论文8篇，其中SCI 3篇，EI 1篇。总经费25万元，已支出23.8819万元，经费使用合理。在开展了两次野外地质调查和样品采集的基础上，对不同类型的母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）通过平衡共生的磁铁矿-钛铁矿组合、角闪石成分特征、新鲜样品Fe3+/Fe2+比值、锆石Ce4+/Ce3+和Eu/Eu*比值等手段，获得包古图斑岩铜矿成矿岩浆的氧逸度值为ΔFMQ 2.5 ~ 2.72。（2）获得矿化岩体以花岗闪长岩和石英闪长岩为主，侵位于310Ma ~ 319Ma。富集Cs、Rb、Ba、Th、U和LREE，亏损Nb、Ta和Ti；(87Sr/86Sr)i为0.70360 ~ 0.70382，εNd(t)为6.6 ~ 8.3，具有年轻的Nd模式年龄TDM1 = 435 ~ 588Ma。成矿岩浆是受交代的亏损地幔部分熔融生成的玄武质岩浆经历高度结晶分异之后的产物（可能发生岩浆混合），古老地壳物质的混染很弱。（3）矿化特征发育较弱的中酸性侵入体处于岩浆-热液演化的早期阶段，成矿流体高温、富S；随着流体向低温的方向演化，体系的硫逸度逐渐降低，As含量相对上升，矿化作用广泛发育。（4）岩浆-热液作用过程中氟有助于铜迁移进入流体相。（5）在III号岩体与包古图组地层接触带发育锑矿化，包括早期自然锑阶段（自然锑+磁黄铁矿），中期硫锑铁矿阶段（硫锑铁矿+磁黄铁矿+黄铁矿+毒砂）和晚期辉锑矿阶（辉锑矿+硫锑铁矿+黄铁矿+毒砂），锑矿物的沉淀主要受氧逸度和温度控制。