花岗岩体系矿物/熔体成矿元素分配行为的实验研究

W, Sn, Mo, Cu deposits are typical magmatic-hydrothermal deposits and most of them are related to highly differentiated granites. For this type of ore deposits, one prerequisite is the magmatic crystallization differentiation which leads to the enrichment of metallogenic elements in the residual melts. And the partition coefficients of W Sn Mo and Cu between mafic minerals, Fe-Ti oxides, sulfides and melts are the key to control whether or not metal elements can be enriched during magmatic processes. Because trace element partition coefficients vary with physical and chemical conditions, therefore, it is necessary to investigate the influence of physical and chemical conditions on W, Sn, Mo, Cu partition coefficients between minerals and melts. Based on the fact that the mineral/melt metal element partition coefficients are still lack in the granite system and the study of the effects of physical and chemical conditions are nearly blank, this project will conduct high temperature and high pressure experiments to systematically investigate the effects of physical and chemical factors such as temperature, pressure, mineral and melt compositions, oxygen fugacity and volatile components on W, Sn, Mo, Cu partition coefficients between mafic minerals, Fe-Ti oxides, sulfides and melts. These partition coefficients can be applied to model the geochemical behaviors of W, Sn, Mo, Cu during magmatic processes and to discuss the key factors which control their enrichment in the residual melts.

花岗岩有关的W,Sn,Mo,Cu矿床是典型的岩浆热液矿床，绝大部分与高分异花岗岩有关；成矿的先决条件是岩浆结晶分异导致成矿元素在残余熔体中预先富集；而铁镁矿物、铁钛氧化物、硫化物与熔体之间成矿元素分配系数的大小是决定它们在岩浆过程中能否富集的关键；由于成矿元素分配系数是随岩浆的物理化学条件变化而变化的，因此，研究物理化学因素对W,Sn,Mo,Cu在矿物与熔体之间分配系数的影响是必要的。基于目前花岗岩体系矿物/熔体成矿元素分配系数仍然十分缺乏，物理化学因素的影响研究更是几近空白，本项目拟通过高温高压实验，系统调查温度、压力、矿物和熔体组成、氧逸度和挥发份等物理化学因素对铁镁矿物、铁钛氧化物、硫化物与花岗质熔体之间W,Sn,Mo,Cu分配系数的影响，应用系统配套的分配系数定量研究花岗岩结晶过程中W,Sn,Mo,Cu等金属元素的地球化学行为、残余熔体成矿元素富集的过程和机理以及各元素高度富集的关键物理化学控制因素。

本项目通过高温高压实验，调查温度、压力、矿物和熔体组成、氧逸度等物理化学因素对铁镁矿物、铁钛氧化物、锆石、硫化物与花岗质熔体之间W, Sn, Mo, Cu, Nb, Ta分配系数的影响，研究花岗岩体系W, Sn, Mo, Cu, Nb, Ta等金属元素的地球化学行为、富集的过程和机理以及各元素高度富集的关键物理化学控制因素。在以下两方面取得重要进展：.（1）使用新的实验方法，即Pt95Cu05为样品管和双管技术控制氧逸度，系统准确地测定了上地幔-地壳条件下Cu在铁镁矿物，铁钛氧化物与中酸性熔体之间的分配系数，结果表明，Cu分配系数范围：石榴石/熔体（0.01-0.06）≤橄榄石/熔体（0.04-0.20）≈斜方辉石/熔体（0.04-0.24）≈角闪石/熔体（0.04-0.20）≤单斜辉石/熔体（0.04-0.45）≤铁钛氧化物/熔体（0.18-1.83）。应用这些新的分配系数，探讨了Cu在岩浆结晶过程中的行为及成矿。 .(2) 使用中钾及高钾两种玄武岩作为初始物进行高温高压实验部分熔融实验产生中酸性性熔体，获得不同温度、压力、水含量、氧逸度和钛含量不同条件下角闪石/熔体间的铌钽分配系数(DNb和DTa)：DNb范围从0.16到0.90， DTa从0.13到0.68，而DNb/DTa从0.76到2.81；DNb和DNb/DTa与温度、角闪石Mg#和熔体H2O含量呈负相关，而与熔体聚合度呈正相关。使用角闪石/熔体Nb Ta分配系数探讨了对弧岩浆演化、大陆壳形成及成矿的应用。