镁铁—超镁铁质岩浆中铂族元素的富集机理:综述与实例

Enrichment mechanism of platinum-group elements in mafic-ultramafic magmas:Review and case studies

目前陆地上可利用的铂族元素(PGE)资源主要来自与镁铁—超镁铁质岩浆密切相关的岩浆硫化物矿床。岩浆硫化物矿床成矿理论关注的一个重要问题就是镁铁—超镁铁质岩浆中PGE的富集机理。经典成矿理论认为,由于PGE在平衡的硫化物熔体与硅酸盐熔体之间具有极高的分配系数(10^(5)~10^(6)),PGE富集成矿主要与成矿体系中硅酸盐熔体与硫化物熔体的质量比有关(R-factor)。但是近些年来,许多新的实验岩石学结果和天然矿石样品纳米尺度PGE赋存状态的观测结果对这一经典理论提出了挑战。本文列举了一些相关的研究实例,显示硅酸盐熔体中的PGE纳米颗粒可以被硫化物或铬铁矿机械捕获、并通过定向附着生长、聚集、粗化和融合,最终形成纳米颗粒集合体和纳米合金。另外,岩浆中半金属元素(TABS,即Te、As、Bi、Sb、Sn)和Se可以与PGE优先形成各种互化物,从而富集于砷化物、铋化物、碲化物或硒化物中,而非硫化物中。因此,镁铁—超镁铁质岩浆体系中PGE的富集可能不仅受其在硫化物熔体中极高的分配系数控制,一些物理过程导致的PGE分配以及半金属元素对PGE的富集作用也不容忽视。由于矿石中的铂族矿物一般为纳—微米级,采用聚焦离子束(FIB)和高分辨透射电镜(HRTEM)、纳米二次离子质谱技术(Nano-SIMS)和原子探针(APT)等先进的微区分析技术对矿物相中PGE的赋存状态进行深入研究,有望获得PGE富集机理的新认识,并不断完善和发展经典的岩浆硫化物矿床成矿理论。

Platinum-group elements(PGE)mineralization is mainly hosted in magmatic sulfide deposits associated with mafic-ultramafic magmas.The mechanism of PGE enrichment in magmatic systems is an important issue of the genesis of magmatic sulfide deposits.In classical theory,the high PGE tenors in the sulfides are attributed to the extremely high sulfide-silicate partition coefficient(10^(5)~10^(6))and the mass of silicate melt with which a given mass of sulfide melt has been equilibrated(R-factor).However,this conventional model has been challenged by recent experimental and empirical results of PGE occurrences in micro-to nano-scale.Here,we briefly introduce some case studies showing that PGE nanoparticles can be physically trapped in sulfide and/or chromite and then form PGE nanoalloy by oriented attachment and flocculation of many coexisting nanoparticles.On the other hand,PGE(e.g.,Pt and Pd)incline to combine with semimetal elements(TABS:Te,As,Bi,Sb and Sn)and Se,forming PGE-rich arsenides,bismuthides,tellurides or stannides.Therefore,besides thermodynamically partitioning of PGE between sulfide and silicate melts,the assemblage and coalescence of PGE nanoparticles and the existence of TABS may play important roles in the collection of PGE in sulfides.As platinum-group minerals are commonly micrometer/nanometer-sized,the detailed investigation of PGE occurrences in variable mineral phases using advanced micro-scale analytical techniques such as FIB,HRTEM,nano-SIMS and APT will help for a better understanding of the PGE enrichment mechanism,and may provide some new insights for the metallogenic theory of magmatic sulfide deposits.