云南中甸地区燕山晚期花岗岩浆分异演化与铜钼钨成矿的矿物化学约束

Theoretical and experimental geochemistry researches show that the mechanism of Cu-Mo-W differentiation during the evolution process of granitic magmas mainly controlled by magmatic oxidation state, the relative timing of crystallization and volatile phase exsolution. Investigating the chemical content of minerals from the ore-forming intrusions will constrain on the mechanism of Cu-Mo-W differentiation and establish the indexes for exploration. The Xiuwacu, Relin, and Hongshan are three Late Yanshanian media to large scale Cu-Mo-W deposits in the Zhongdian area. They were formed genetically related to the same period granitic intrusions and with remarkable Cu-Mo-W differentiation during the fractional crystallization process. Therefore, these three deposits are ideal cases for this research. In this study, on the basis of detailed field investigation and previous studies, the chemical content of the sphene, zircon, hornblende and apatite from each phase of the intrusions will be in-situ analyzed (LA-ICP-MS, etc.). Then, based on the chemical content data of those minerals, geochemical indexes will be calculated to constrain on the magmatic oxidation state, crystallization degree, emplacement level, and variation of the metal and volatile of each phase of the intrusions. Finally, those geochemical content indexes of the minerals will be used to constrain on the mechanism of Cu-Mo-W differentiation during the evolution process of granitic magmas.

理论及实验地球化学研究显示，铜、钼、钨自花岗岩浆分异至热液而成矿，受控于岩浆源区以及岩浆结晶分异程度、氧逸度和挥发分逸出的时间等因素。针对这些因素，开展花岗岩矿物化学研究并建立相应指标体系，对正确理解与花岗岩有关的铜、钼、钨差异化成矿机制和指导找矿预测具有重要意义。云南中甸地区休瓦促、热林和红山三个形成于燕山晚期并分别以钼钨、钼铜和铜矿化为主的矿床，均与起源相似的同期花岗岩浆活动有关，相关花岗岩是开展该项研究的理想对象。本项目拟选取与这三个矿床相关花岗岩体为对象，深入研究花岗岩（各岩相）中普遍存在的岩浆成因榍石、锆石、磷灰石及角闪石的（LA-ICP-MS等）原位矿物化学组成特征，建立不同岩体的矿物化学指标体系，系统揭示各花岗岩体结晶分异程度、氧逸度、侵位深度以及成矿元素和挥发分含量的变化规律，对铜、钼、钨在花岗岩浆演化过程中的差异性成矿机制及过程进行有效限定。

花岗岩浆相关的Cu、Mo、W矿化通常受到岩浆结晶分异程度、氧逸度和挥发分含量的影响，研究岩体的这些物化性质对探讨Cu、Mo、W成矿作用具有重要意义。本项目对云南中甸地区休瓦促、热林、红山岩体进行了详细的岩相划分，并对各岩相开展了锆石Hf-O同位素及锆石、榍石和磷灰石微量元素分析，结合全岩地球化学特征，提出了弧岩浆演化与Cu-Mo成矿作用之间的模型：中甸地区早阶段弧岩浆作用起源于交代地幔楔的部分熔融具有相对较低的氧逸度和温度，而晚阶段弧岩浆主要受到板片断离作用的影响，而具有较高的温度和氧逸度，且由于高温软流圈地幔熔体的底侵而引发早阶段新生下地壳的部分熔融并导致大规模斑岩型Cu-Mo成矿作用。借助实验地球化学研究，证实成矿流体中W并不以NaHWO4为主要迁移形式，而以钨酸形式迁移，在得到钨酸热力学数据的基础上，模拟计算显示W易溶解于高温、高盐度、高pH或低pH和低Ca、Fe的热液中，降温、水岩反应及流体混合作用均易导致钨矿物的沉淀。此外，本项目执行中对中甸邻区邦达A型花岗岩和如多正长岩的发现，指示着藏东地区广泛发育有白垩纪的成岩成矿作用，主要延龙目-双湖缝合带及德格-乡城断裂分布，且以陆内W-Mo-Pb-Zn矿化为主。