攀西层状岩体成矿氧逸度研究

This project is to investigate the oxidation states during magma evolution and ore formation in the Fe-Ti-V oxide ore-bearing layered intrusions of the Panxi region, the Permian Emeishan Large Ignious Province, and to perform anhydrous one atmosphere melting experiments on the mixtures of ores and associated gabbros from the Panzhihua intrusion. The oxidation states of the magmatic systems will be determined using silicate and oxide mineral assemblages in the Permian layered intrusions in the Panxi region including Panzhihua, Hongge and Xinjie. The pre-eruption oxidation states of the coeval picrites and basalts, which are complientary to the studies of the intrusive rocks, will be determined using suitable phenocrysts in the lavas. Selected oxybarometers will include those that are based on titanomagnetite-ilmenite equilibria, ferrous-ferric ratios in minerals, multivalent element distributions in mineral pairs, and intrinsic oxygen fugacity measurements by electrochemical experiments. The relationship between liquid composition, temperature and oxygen fugacity in the magmatic systems saturated with Fe-Ti oxides will be determined by the melting experiments. The data from this study for the Panzhihua intrusion will be used to constrain the nature of the parental magma for this intrusion and the controls on ore formation in the intrusion. Ultimately, all of the data from this study will be used to address three outstanding important questions: (1) whether or not Fe-Ti oxide saturation in the magma was due to a sudden change in oxidation state (i.e., dramatic increase in fO2); (2) what is the most impotant factor to controll the oxidation state of ore-forming parental magma; (3) wether or not liquid immiscibility played a critical role in the formation of the Panzhihua Fe-Ti oxide deposit. More broadly, this study will improve our understanding of metallogeny associated with mantle plume activity in a continental setting.

本项目以峨眉山大火成岩省攀西层状岩体岩浆演化及成矿过程氧逸度测算为重点，辅助开展氧逸度约束下的熔融实验研究。氧逸度测算包括层状岩体主要岩相带及矿层的各类岩矿石矿物组合，同时开展有关苦橄岩、玄武岩岩浆氧逸度测算，界定原始岩浆、成矿母岩浆及主要成矿阶段的岩浆氧逸度，分辨攀西地区氧逸度可能存在的区域性特色及原因。熔融实验以获得不同温度和氧逸度下钛铁氧化物饱和的液相线岩浆成分为目的，建立钛铁氧化物饱和状态下的液相线岩浆成分与温度、氧逸度的关系；利用温度和氧逸度的测算结果，对攀枝花岩体成矿母岩浆可能的组成进行评估、并讨论成矿岩浆的形成和演化。这些研究致力于探讨攀西层状岩体成矿机理研究中的3个突出问题：①攀西层状岩体是否为高氧逸度成矿；②攀西层状岩体成矿岩浆氧逸度是否具有宏观背景并对构成对区域性成矿作用的约束；③岩浆不混熔与成矿。其研究有助于攀西层状岩体成岩成矿理论研究的深入。

攀西层状岩体钒钛磁铁矿成因是一个长期争议的问题，其中，突出的问题包括如：攀西层状岩体是否为高氧逸度成矿、岩浆不混熔成矿的可能性。本项目以攀枝花、红格岩体为主要研究对象，系统开展了攀枝花、红格岩体初始岩浆及成矿氧逸度测算，在对攀枝花岩体针对性的地质地球化学研究基础上，开展了攀枝花天然岩矿石样品的熔融实验。氧逸度测算方法选择性采用包括钛铁氧化物平衡氧逸度计算、基于矿物中铁氧化状态的氧逸度计算和利用变价元素钒在矿物-熔体间分配的氧逸度计算。通过对各项氧逸度的测算，基本界定了攀枝花、红格岩体初始岩浆和成矿岩浆的氧逸度，为攀西层状岩体成矿机理研究提供了更全面可信的氧逸度约束条件。研究成果攀西层状岩体并非高氧逸度成矿，其成矿氧逸度基本上在正常幔源岩浆的氧逸度范围内；对攀枝花天然岩矿石样品的熔融实验表明在岩浆房温度条件下可以出现富铁熔体相，综合分析结果支持攀枝花铁矿岩浆不混熔成因。