西藏恰功铁矿岩浆演化序列及斑岩出溶流体特征

冈底斯成矿带是碰撞造山过程形成的最重要的成矿带,恰功矽卡岩铁(铜)矿床即位于冈底斯成矿带中部.本文在详细的野外地质填图和室内研究基础上确定了恰功铁矿与成矿有关的斑岩体为二长花岗斑岩,锆石的U-Pb定年结果为68.8±2.2 Ma,明显早于冈底斯东部与后碰撞伸展有关的斑岩型矿床和主碰撞期形成的矽卡岩型矿床,其应代表了冈底斯一次尚未被充分认识到的成矿事件.通过对成矿斑岩的岩石学、岩石化学、岩浆出溶流体的包裹体岩相学、显微测温分析及包裹体成分的LRM、LA-ICP-MS和PIXE分析,本文探讨了与该矽卡岩矿床有关的斑岩的岩浆起源、斑岩侵位机制及出溶流体特征.结果表明,成矿斑岩为矿区最早的侵入岩单元,具有壳幔源混源特征,其常量元素组成与冈底斯东部斑岩铜矿带的埃达克质岩相近,但稀土和微量元素组成与东部斑岩明显不同,为地幔物质上涌并诱发角闪岩相下地壳熔融的产物,岩体侵位深度大(>7 km)、剥蚀程度高.早期岩浆出溶的流体为高温、高压、高盐度流体,其中富含Fe、Pb、Zn、Cu等成矿金属,与世界其他斑岩铜矿床相比,相对富铁、铅而贫铜.流体的沸腾作用发生于钾硅化阶段,形成了一套富气、高盐、高固相和气液包裹体组合,不同于早期出溶流体的包裹体组合.从岩浆起源和出溶流体性质可以看出,该斑岩具有形成与斑岩有关的铁铅多金属矿床的成矿潜力,但从该矿区地表出露的蚀变和包裹体测压结果可以看出,该斑岩体目前剥蚀深度较大,对斑岩型矿床保存不利,对该斑岩体及与成矿关系的认识对理解该区内的矿床成因和指导区域找矿具有重要的指示意义.

Magmatic Evolution and Characteristics of Magmatic Fluid in the Qiagong Porphyry System

Collision between the Indian and Asian plates has produced several metallogenic belts in the Tibet, Yunnan, Sichuan, and Qinghai provinces of China since ～65 Ma. Based on their contained resources, the Gangdese belt is the most important economically, and it is the host to the Qiagong iron skarn in the central part of the belt. The mineralising intrusions at Qiagong are monzonite granite porphyries, which were emplaced at 68.8±2.2 Ma based on U-Pb (zircon) age determinations. Qiagong therefore formed at the start of the main collision stage, and is much older than porphyry copper and Cu-Au-Mo skarns in the eastern part of the Gangdese belt. New geological mapping, petrological and geochemical studies of chemistry of Qiagong have identified features typical of a porphyry copper system associated with the Fe skarn. Isotopic data are consisted with mixing of mantle and lower crustal material. Microthermometric, PIXE and LA-ICP-MS analyses of fluid inclusions related to the Qiagong porphyry system have identified high temperature, high pressure and high salinity magmatic-hydrothermal fluids that have high Fe (up to 23%), Pb (up to 10% ) and Zn (up to 1.4%) and low Cu (up to 464×10~(-6)) concentrations compared to other porphyry copper systems. The porphyry system is inferred to have formed at depths of 6～7 km based on pressure estimation results, and has therefore been deeply eroded.