冈底斯白容-岗讲富电气石斑岩成矿系统的岩浆演化和矿质沉淀研究

Porphyry Cu deposits related to the India-Asia continental collision mainly cluster in the Gangdese Metallogenic belt, Tibet. Yet the magma chamber formation processes and ore precipitation mechanism, two main controlling factors for the collision-related porphyry Cu deposits, are still unclear. In this proposal, we take the Bairong-Gangjiang porphyry Cu system as the research objective to study these two factors. Based on the systematic geological mapping, we conduct detailed microstructure observations on various magmatic minerals from different igneous rocks, and hydrothermal tourmaline and coexisted minerals at different stages. We analyze in-situ major, trace elements on these minerals, and also apatite Sr-Nd isotope and tourmaline B isotope by EPMA, LA-(MC)-ICPMS and Nano-SIMS. Combined with previous studies, we discuss the magmatic mineral crystallization environment and formation processes, and evolving paths of S-Cl content in the magma, to reveal the formation processes of magma chamber under the tourmaline-rich porphyry Cu system in the collisional environment. Also, we constrain the boron-rich fluid properties, provenance and evolution history, and refine the ore precipitation mechanism from the perspective of tourmaline studies. This study will provide a better understanding on magma chamber formation processes for collision-related Cu porphyry system, and a new view to demonstrate the controlling factors of porphyry ore precipitation, which has important significance to improve the metallogenesis of Tibetan collision-related porphyry deposit.

西藏冈底斯斑岩铜矿的形成主要与印亚大陆碰撞过程有关。然而对碰撞环境斑岩矿床“中间岩浆房过程”和“浅部矿质沉淀机制”这两个关键控制因素，尚未得到很好揭示。本申请拟以冈底斯中段白容-岗讲斑岩铜矿为研究对象，在详实的野外工作基础上，对各岩相岩浆矿物和热液阶段的电气石进行精细显微结构观察，运用电子探针、激光剥蚀多接收等离子体质谱和纳米离子探针原位微区分析技术等手段，着重对中新世岩体内钾长石巨晶、磷灰石、黑云母、角闪石和石英斑晶等岩浆矿物，以及不同热液阶段电气石和共生矿物，开展原位微区主-微量元素、磷灰石Sr-Nd同位素和电气石B同位素等研究，探讨岩浆矿物结晶环境、形成过程和成矿元素S-挥发分Cl演化规律，揭示碰撞环境富电气石斑岩系统的中间岩浆房过程，精细限定斑岩系统中富B流体的来源、演化和矿质沉淀机制。以上研究有助于进一步理解碰撞环境斑岩铜矿的中间岩浆房过程和浅部成矿过程，深化斑岩型矿床成矿理论。

西藏白容-岗讲矿区为冈底斯斑岩成矿带典型的斑岩型铜钼矿，且发育典型电气石蚀变，前人对于该类斑岩成矿体系的蚀变-热液蚀变特征、岩浆演化过程和热液矿化中心研究较为薄弱。本研究工作围绕“冈底斯白容-岗讲富电气石斑岩成矿系统的岩浆演化和矿质沉淀中心”这一关键研究问题，取得重要研究进展，包括：1）利用岩性条件、蚀变与脉系类型及其水平-垂直分布特征揭示白容-白容西富硼斑岩体系岩浆-热液成矿特征，综合判定岩浆-热液成矿中心，判定白容-白容西矿区仅仅剥露了大部分火山岩盖层，并推测主矿体隐伏在海拔4500m（甚至4000m）之下，埋深甚大；2）利用钾长石化与矿化关系特征推测西藏岗讲斑岩体系岩浆-热液成矿中心，揭示出该矿床是一个典型的剥蚀较深的斑岩铜钼矿床的底部；3）结合蚀变-脉系特征综合分析和锆石同位素定年-微量元素研究，示踪白容-岗讲铜钼矿田岩浆过程，即白容-白容西斑岩铜钼矿的岩浆侵位时间集中在14-11Ma，辉钼矿年龄13.2-14.0Ma，其中二长花岗斑岩、花岗闪长斑岩与成矿有关，英安斑岩和煌斑岩成矿后侵位，起到了破坏矿体的作用，构建白容-白容西矿区成矿的三阶段模型；4）通过白容-岗讲斑岩成矿系统研究揭示青藏高原演化对冈底斯带中新世斑岩成矿的控制作用，揭示白容-岗讲富硼斑岩系统成矿潜力，即尼木矿田的构造背景从适合斑岩成矿的挤压到伸展过渡的阶段（13.8Ma）转换到超钾质煌斑岩（11.1Ma）代表的伸展阶段所代表的不到3Ma的快速伸展过程引发的持续部分熔融过程，可能是造成尼木矿田各斑岩矿床岩枝众多、岩性复杂的背后的动力学原因之一；5）对比西藏尼木矿集区与世界典型斑岩矿床成矿系统和冈底斯带主要斑岩矿床，阐明冈底斯白容-岗讲矿床的成矿特色，揭示出斑岩-矽卡岩-脉状多金属成矿系统是冈底斯斑岩成矿体系下一步找矿的重点。