流体在岩浆型铬铁矿和铂族元素成矿过程中的作用及意义

镁铁- 超镁铁岩是揭示地幔物质组成和壳幔相互作用的重要窗口，也是Ni- Cu- PGE- Cr等金属矿产资源的重要载体。不同的镁铁- 超镁铁岩体赋矿特征明显不同：蛇绿岩以产出铬铁矿床为特征，阿拉斯加型岩体主要赋含铂族元素(PGE)矿床，大型层状岩体则可同时产出铬铁矿床、PGE矿床和Cu- Ni硫化物矿床。这种成矿差异显然与赋矿岩体形成的构造背景、母岩浆经历的岩浆演化过程有关，但缺少关键控制因素的研究。前人对上述不同种类矿床的研究工作主要集中于地幔源区的部分熔融、上升过程中或岩浆房内的围岩混染和结晶分异等岩浆过程，而极少关注流体作用。近年来，实验岩石学和岩石地球化学的研究均表明幔源岩浆演化过程中的流体活动可能对成矿元素的富集迁移起到至关重要的作用，同时这些成矿元素的赋存状态和分配系数也在不断更新。厘清Cr和PGE在熔体演化——尤其是流体出溶过程中的地球化学行为，刻画并揭示其迁移富集、分离和再富集的成矿过程及控制因素，已成为当前岩浆矿床研究的热点。本文围绕富水流体与铬铁矿和PGE成矿关系的科学问题，总结了不同镁铁- 超镁铁岩体的成矿差异以及铬铁矿和PGE矿床成矿过程中的流体活动记录，提出流体性质和组分对铬铁矿和PGE迁移富集的控制作用，强调有必要开展蛇绿岩、大型层状镁铁- 超镁铁岩体和阿拉斯加型岩体的对比研究。

Role and significance of fluids in magmatic- type chromite and platinum- group element mineralization

Mafic- ultramafic rocks are key windows to probe mantle composition and mantle- crust interaction process, and also important hosts of Ni, Cu, PGE and Cr metal resources. Their various occurrences are related to different genetic types of mineralization. Ophiolites mainly host chromite deposits, while Alaskan- type complexes could contain PGE deposits. Chromite, PGE as well as Ni- Cu sulfide deposits may occur together in an individual large layered complex. Such mineralization difference is basically interpreted as dependence of tectonic setting, and composition and evolution of parental magmas, but critical controls have not been well constrained. Previous studies are focused on partial melting of mantle sources, wall- rock assimilation, and fractional crystallization during magma upward ascent. Role of fluids in mineralization receives little attention. Recent experimental and petrogeochemical results have revealed that fluid activity during magma evolution may have played a significant role in metal transportation and enrichment, particularly with growing knowledge of metal occurrence and partitioning behavior. A hot topic in magmatic deposits is to constrain geochemical behavior, transportation and deposition of Cr and PGE during fluid exsolution from magmas. This paper presents a summary of mineralization differences in various mafic- ultramafic complexes and records of fluid activities during chromite and/or PGE mineralization, and addresses dependence of fluid property and composition for deposit formation. We thus suggest that systematic comparisons between ophiolite, layered complex and Alaskan- type complex will well document the topic.