后碰撞环境斑岩铜矿成矿金属及硫的来源：以西藏驱龙铜矿为例

Most porphyry Cu deposits (PCDs) formed in association with subduction-related calc-alkaline magmas, and widely occur in magmatic arcs worldwide. A widely accepted model is that such deposits were formed from hydrothermal fluids exsolved from hydrous, high-fO2, sulfur-rich arc magmas, derived from a mantle wedge metasomatized by the subduction-slab fluids. Recent studies have documented that such deposits also occur in post-collisional settings, e.g., the Gangdese porphyry Cu belts in Tibet. Formation of such PCDs is very difficult to be explained by using the classic PCDs model, which results in a supplementary model to be proposed to interpret genesis of the PCDs in such setting. In the supplementary model, metals and sulfur of the post-collisional PCDs were thought to be derived from the subduction-modified lower crust, not the metasomatized mantle wedge. On base of detailed analysis, however, we found the model still cannot explain well on sources of metals and sulfur for such PCDs. Recently, abundant magmatic sulfides were found to be developed in mafic enclaves hosted by mineralization-related intrusion in the Qulong deposit by our pilot study. It was well known that mafic enclaves are products of magma mixing, thus input of mafic magma can supply metals and sulfur to mineralization-related felsic magma to form giant deposit such as Qulong, which provides us a new idea on sources of metals and sulfur for PCDs in post-collisional setting. To check the hypothesis, we plan to investigate: (1) mixing process between the mafic and felsic magmas, (2) S isotope of sulfides/sulfates in the mafic enclaves and hydrothermal veins, (3) composition and variation of initial ore-forming fluid, and (4) origin of the mafic enclaves, aiming to clarify sources of metals and sulfur for the PCDs in post-collisional setting.

立足于弧环境斑岩铜矿床研究而建立的经典斑岩铜矿成矿模式，很难解释后碰撞环境产出的西藏冈底斯斑岩铜矿带的成因，这促使前人基于该铜矿带的研究，提出了后碰撞环境斑岩铜矿成矿模式，该模式强调后碰撞环境斑岩铜矿成矿金属及硫来自俯冲改造的下地壳，而非楔形地幔。细致的分析发现，该模式目前在解释后碰撞环境斑岩铜矿成矿金属及硫来源时仍存在明显不足，这是因为仅下地壳的熔融很难提供形成超大型斑岩铜矿所需的金属和硫。为此，项目在前期重要发现基础之上（在与成矿斑岩近同期的暗色包体中发现大量岩浆硫化物），拟选择西藏驱龙这一典型后碰撞环境斑岩铜矿开展研究，尝试以暗色包体成因及其内硫化物的硫同位素研究为突破口，结合矿床早期成矿流体组分及硫同位素变化情况，查明矿床成矿金属及硫的来源。并通过综合对比，揭示后碰撞环境斑岩铜矿成矿物质来源，完善后碰撞斑岩铜矿成矿理论。

大陆碰撞造山带是斑岩铜矿产出的一种新环境，由于该环境缺乏活动的大洋俯冲，形成斑岩铜矿所需的金属、S及H2O无法像弧环境下的斑岩铜矿一样，通过大洋板片俯冲脱水过程得以实现。因此，大陆碰撞环境斑岩铜矿含矿斑岩起源、成矿金属、S及H2O的来源是理解该环境斑岩铜矿床形成机制的核心，也是近年来斑岩铜矿研究中的热点。为解决上述问题，特别是后碰撞斑岩铜矿成矿金属及S的来源问题，项目选择冈底斯带最大的斑岩铜矿床——驱龙矿床开展研究，重点调查了该矿床中新世高镁闪长岩、埃达克质斑岩及其内闪长质包体矿物学及地球化学，以及闪长质包体全岩及其内岩浆硫化物的S同位素特征，并与整个冈底斯中新世斑岩铜矿系统进行了对比。.研究表明，西藏下地壳脱水熔融无法提供形成冈底斯含矿斑岩的高H2O含量，下地壳部分熔融时，需要额外H2O的加入。额外的H2O主要来自印度大陆的俯冲脱水：热俯冲条件下（~15 oC/km），印度大陆上地壳中主要的含水矿物会在俯冲至100km深之前发生脱水反应，失去矿物中的大部分水，释放的水将不断上升水化上覆岩石圈地幔或下地壳，并诱发地幔和下地壳部分熔融，形成镁质碱性岩浆和埃达克质岩浆。另外，镁质碱性岩浆上侵到下地壳下部，也可提供水和/或热，进一步促进下地壳熔融，最终形成富水、成矿的埃达克质岩浆。俯冲大陆的热结构及俯冲量是控制富水成矿斑岩形成的两个最关键因素。驱龙矿床富硫化物闪长质包体地球化学及其内硫化物的S同位素结果显示，该矿床成矿金属和S起源于俯冲改造的下地壳中弧岩浆阶段沉淀硫化物的再熔融。系统对比西藏冈底斯带早侏罗纪世及中新世岩浆及成矿事件，也发现俯冲阶段沉淀于下地壳的硫化物后碰撞阶段发生再熔融，是冈底斯其他中新世斑岩铜矿成矿金属及S的主要来源。.项目在Geology、Journal of Petrology等国际主流期刊发表论文9篇，项目成果受到国内外关注和认可，产生了重要影响。