新疆延东斑岩铜矿床中低温流体成矿机理

Large-size Yandong porphyry copper deposit is located in the center of the Eastern Tianshan, Xinjiang. It is characterized by the hydrothermal alteration assemblages and associated mineralization formed in the low-moderate temperature condition. Using microthermometry, fluid trapping conditions is at the low-moderate temperature condition. It differs with the most porphyry copper deposits in the world which are formed in the high-moderate temperature condition. The study focuses on the issue how the ore-forming fluid in low-moderate temperature condition controlled the formation of the porphyry copper deposit. Based on systemic work in field and petrology, petrography and mineragraphy indoor, we study the hydrothermal alteration, geochemistry of the ore-forming fluids, exsolved and hydrothermal fluid inclusions using the advanced geochemistry analysis measures. It will be examined that the nature of the primary magma, the exsolved condition of the magma fluids and its contribution to the metal enrichment as well as the ore-forming condition of the hydrothermal fluids and its restriction to metal deposition. We distinguish the systemic changes of the physical-chemical conditions and compositions of the exsolved- hydrothermal fluids and their controlling to the metal enrichment and deposition. Based on these studied, the ore-forming mechanism of the low-moderate temperature fluids will be examined. This study could extend the ranges of the physical-chemical conditions of the porphyry copper deposit and add the new idea to the theory of the porphyry copper deposits in the world. In addition, it can also provide the theory evidences for searching for the porphyry copper deposits in Xinjiang.

延东大型斑岩铜矿床位于新疆东天山中部，该矿床具有特征的中低温热液蚀变组合及相应的矿化，流体包裹体测温显示流体温度为中低温，明显低于世界大多数斑岩铜矿的形成温度，具有独特的成矿条件。本项目以中低温流体如何控制斑岩型铜矿床形成为研究主线，以现代地球化学方法为研究手段，在扎实的野外地质和系统的室内岩相学和矿相学研究基础上，进行热液蚀变、成矿流体地球化学、熔融包裹体和流体包裹体的综合研究。初步查明岩浆性质、流体出溶条件及其对金属元素初始富集的贡献以及热液流体成矿条件及其对金属元素沉淀的约束，从而确定从出溶流体阶段到热液流体阶段，物理化学条件和岩浆-热液流体成分的系列性变化规律及其对成矿元素富集和沉淀的控制，最终查明延东斑岩铜矿床中低温流体成矿机理。此项研究可能拓展世界斑岩铜矿成矿的物理化学条件区间，丰富斑岩铜矿成矿机理的理论，同时也可为新疆斑岩型铜矿找矿勘探突破提供理论依据。

延东铜矿原有的热液蚀变和流体包裹体研究表明，矿区缺乏钾硅酸盐化高温蚀变，成矿温度较低（260-140°C），这与大多数斑岩型矿床流体成矿发生在中高温条件（550-350°C）明显不同。延东矿区是否不存在钾化蚀变？何种原因造成了延东铜矿广泛发育中低温流体？此外，延东铜矿缺乏详细的热液蚀变分带，成岩年龄（334Ma）明显早于成矿年龄（326-318Ma），哪种岩浆岩与成矿有关以及矿床成因问题一直有不同的认识。.我们以延东铜矿广泛发育的中低温流体为切入点，开展了岩石学、矿相学、构造地质学、流体包裹体、绿泥石成分、S-He-Ar同位素地球化学、含矿岩浆氧逸度以及成岩成矿年龄测定等方面的研究，取得了以下成果：（1）矿区除了广泛发育绿泥石化和绢云母化等中温热液蚀变外，还有高温钾化蚀变的保留，石英中存在高温（>520 - 421°C）、高盐度（49.6 - 62.5wt%NaCl）原生包裹体，表明延东矿床形成经历了高温成矿阶段；（2）新识别了构造叠加改造成矿作用，即韧-脆性剪切构造叠加在斑岩型矿体上，致使成矿物质活化和再富集，形成矿体；（3）查明了矿区广泛发育的中低温流体是构造叠加改造作用，使石英中发育大量的微裂隙及其次生包裹体（250-100°C）所致；（4）提出了延东矿区发育中高温（>520-210°C）斑岩成矿作用和中低温（<200-250°C）构造叠加改造成矿作用的新认识；（5）获得的成矿年龄（334Ma）与英云闪长斑岩成岩年龄（332Ma）一致，δ34S（-1.8~+2.3‰）和3He/4He（0.21-0.83Ra）同位素指示成矿流体源于岩浆，表明矿床形成主要与斑岩成矿作用有关；（6）建立了斑岩-构造叠加改造成矿模式。延东铜矿的成矿过程与大多数斑岩铜矿不同，具有明显的构造叠加性质。该成果对区域找矿勘探具有重要的借鉴意义，也可为丰富世界斑岩铜矿成矿理论提供资料。