西藏多不杂斑岩矿床铜金沉淀机制研究

Effective precipitation of metals is a critical step for the formation of porphyry copper deposit. However, the mechanism of effective precipitation still remains controversial in the scientific community. China has made significant progress in terms of exploration of porphyry copper deposits, and the discovery of world-class Duolong ore district along the Bangong-nujiang suture zone in Tibet is the latest proof to that effort. The Duobuza gold-rich porphyry copper deposit is the most representative deposit in the Duolong ore district. This project is designed to study the Duobuza deposit. We plan to conduct detailed field investigations to identify the relationships between mineralization and alteration and develop the overall mineral paragenesis, which will be assisted by the results of computer modeling of copper-gold mineralization. Based on the previous field study, we plan to conduct cathodoluminescence analysis of quartz veins, as well as microthermometry and LA-ICP-MS analysis of fluid inclusions to constrain the relationships among temperature, pressure, salinity, and metal contents of ore-forming fluids. In addition, we plan to carry out the oxygen and hydrogen isotope analysis of quartz veins from different stages, and discuss the effects of fluid-mixing on metal precipitation. Consequently, the mechanism of metal precipitation could be identified based on our analytical results. This work will provide an example for studying metal precipitation of porphyry copper deposits, which will not only enrich the theory of ore genesis, but also help exploration for porphyry copper deposits in the future.

金属元素的有效沉淀是形成斑岩铜矿的关键环节，但目前学术界对金属元素沉淀机制的认识仍存在着争议。近年来我国斑岩铜矿找矿勘查工作取得了重大突破，在西藏班公湖-怒江缝合带发现了具有世界级规模的多龙矿集区，多不杂是该矿集区内具显著代表性的大型富金斑岩铜矿。本申请拟以多不杂矿床为研究对象，对该矿开展详细的野外地质调查，并结合矿体建模软件分析结果，厘清铜金矿化与蚀变作用关系并划分成矿阶段。在此基础上，对不同阶段石英脉开展阴极发光实验、流体包裹体测温、单个包裹体成分分析，以此查明流体温度、压力、盐度特征与金属元素含量变化的相互作用关系，同时辅以H、O同位素测试分析，进一步探讨流体混合作用对金属元素沉淀的影响，并综合上述实验结果明确多不杂矿床铜金沉淀机制。本次工作不仅可为斑岩铜矿金属元素沉淀机制的研究提供范例，而且可以丰富斑岩铜矿成矿理论，并为后续找矿勘查工作发挥重要的指导作用。

本项目以西藏多龙矿集区多不杂斑岩铜金矿床为研究对象，综合地质编录与实验测试，探讨金属元素的沉淀机制，同时探索建立找矿标志。项目取得了三项重要进展。第一，厘清了该矿床蚀变与矿化对应关系。根据地质编录与矿物学研究结果，明确了区内蚀变矿物组合类型及分布特征，并通过对比铜金矿体建模数据，确定成矿作用主要与钾硅酸盐化蚀变有关，而绿泥石-绢云母化蚀变也有部分矿化发育。第二，查明了多不杂矿床流体演化过程与矿质沉淀机制，并建立了找矿标志。流体包裹体和氧同位素数据指示，岩浆热液在成矿早、中阶段占据主导地位，而成矿晚阶段则有大气降水的加入。硫同位素研究进一步表明，不同蚀变类型、成矿阶段的硫化物δ34S值具有显著差异，具体表现为：钾硅酸盐化（-4.8‰~-0.4‰）相比青磐岩化（+1.2~+4.8‰）具有更低的δ34S值，绿泥石-绢云母化（-2.6‰~+0.6‰）相比钾硅酸盐化具有更高δ34S值，晚成矿阶段三类热液脉体δ34S值逐渐升高（-3.5‰~+0.5‰，-2‰~+0.6‰，-0.7‰~+2.3‰）。综合流体包裹体、岩相学等证据分析，上述变化分别与沸腾作用、水岩反应、去气作用相关，并且矿质沉淀机制主要为沸腾作用和水岩反应。硫同位素分带与矿体品位变化对比显示，该矿床矿体富集部位硫化物具有更低的δ34S值（矿体Cu品位＞0.2%，黄铜矿δ34S（‰）＜-1，黄铁矿δ34S（‰）＜0），表明硫同位素对圈定矿化中心具有重要指示意义。第三，初步揭示了多龙矿集区内典型矿床热液演化过程。通过对比区内硫同位素数据与模拟计算结果，本项研究提出区内典型矿床流体演化过程可分为2类：流体演化主要受控于温度变化（拿顿）；流体演化受温度和氧逸度变化共同控制（波龙、拿若、拿厅和铁格隆南）。整体而言，多龙矿集区典型矿床矿化阶段硫化物通常具有较低的δ34S，指示成矿流体为高氧化性流体，该特征也可为斑岩铜矿系统找矿勘查工作提供指导和帮助。