钼同位素在岩浆热液系统的分馏机制及其对斑岩型矿床的成因指示

It is still unclear whether significant fractionations of Mo isotopes occur in a magmatic and hydrothermal system, as well as fractionation mechanism. We have recognized that chalcopyrites and pyrites in the porphyry deposits can show significant fractionation of rich- heavy Mo isotope. This proposal plans to investigate Mo isotopic composition and variation of igneous rocks and sulfides in the representative porphyry Cu-Mo deposits, to confirm that significant fractionation of Mo isotope can be taken place in a hydrothermal system. The mechanism and regularity of Mo isotope fractionation in a hydrothermal system are summarized. Meanwhile, based on the analysis of the Mo isotopic compositions of the porphyry and related igneous rocks as well as the rock-forming minerals, it is found whether the magmatic differentiation can result in Mo isotope fractionation and possible mechanism. Afterwards, using Mo isotopes as main means, we will compare Mo isotope compositions and fractionation features between subduction and non-subduction type porphyry deposits in Sanjiang area, to find out the similarities and differences of Mo isotopic composition of the both, the Mo isotope identification marks of two types of porphyry deposits will be suggested. Moreover, according to the change trend of Mo isotopes in sulfides of different stages, the Cu, Mo precipitation stage and the mineralizing process will be traced. This study not only can better understand the change rule of Mo isotopes in the high temperature condition of nature, but also has important reference value for revealing the genesis of porphyry deposits.

钼同位素能否在岩浆热液系统显著分馏及其机制仍是不清楚的，我们已在斑岩型矿床的黄铜矿、黄铁矿中识别出富集重钼同位素的显著分馏。本项目拟查明代表性的斑岩Cu-Mo矿床中火成岩和硫化物的钼同位素组成和变化范围，证明岩浆热液系统演化导致钼同位素极端分馏是普遍现象，阐明岩浆热液系统钼同位素的分馏机制和规律；同时通过对成矿斑岩和相关火成岩以及造岩矿物钼同位素成分的高精度分析，揭示岩浆演化过程能否导致钼同位素分馏及其可能的分馏机制。然后利用钼同位素作为主要研究手段，对比研究三江地区俯冲型和非俯冲型斑岩型矿床硫化物和成矿斑岩的钼同位素组成和分馏特征，查明两者钼同位素组成的异同，总结区分两类斑岩型矿床钼同位素鉴别标志，同时依据不同阶段硫化物钼同位素的变化趋势揭示成矿热液中Cu、Mo沉淀期并示踪成矿过程。本研究不仅能更好地认识钼同位素在自然界高温条件下的分馏变化规律，而且对揭示斑岩型矿床的成因有重要参考价值

钼（Mo）元素及同位素因对氧化还原环境变化的敏感特性使其成为研究岩浆热液过程的新型地球化学工具。然而，Mo同位素能否在岩浆热液系中分馏及机制仍是不清楚的，本项目查明了代表性斑岩铜矿中金属硫化物和相关火成岩的Mo同位素组成和变化特征，证明岩浆热液系统演化是导致Mo同位素极端分馏是普遍现象，阐明岩浆热液系统Mo同位素的分馏机制和规律。同时通过对成矿斑岩和相关火成岩的Mo同位素组成的分析，揭示岩浆演化过程是否发生Mo同位素分馏。本项目对以三江地区普朗为代表的斑岩铜矿床开展了系统的矿床解剖研究，划分了岩浆热液成矿的四个阶段，对不同阶段的金属硫化物，以及成矿斑岩和围岩进行了高精度的Mo同位素分析测试，主要取得以下新认识和进展：（1）证明了在同一斑岩型岩浆热液成矿系统的低Mo含量硫化物存在显著的Mo同位素分馏，并且以富集重Mo同位素组成为特征；（2）揭示出与成矿相关的斑岩岩浆系统无明显的Mo同位素分馏，成矿斑岩和矿床围岩具有相对轻的Mo同位素组成；（3）热液系统的辉钼矿显示轻钼同位素组成，而低钼含量的黄铜矿、黄铁矿和磁黄铁矿具有显著的重钼同位素分馏，反映了在岩浆热液演化过程中重/轻钼同位素的非均匀分布，Mo同位素遵循瑞利分馏演化；（4）Cu-Mo主沉淀期的青磐岩化阶段金属硫化物对应着Mo同位素最大分馏程度并有富集重Mo同位素特征，暗示金属硫化物Mo同位素显著变化和重Mo同位素富集的识别可能指示斑岩铜矿的主成矿阶段和成矿地段。