In situ observations of tungsten speciation and partitioning behavior during fluid exsolution from granitic melt

钨在熔体-流体间分配行为的在线观测

Most economically important tungsten(W)deposits are of magmatic-hydrothermal origin.The species and partitioning of W during fluid exsolution,considered to be the controlling factors for the formation of ore deposits,are thus of great significance to investigate.However,this issue has not been well addressed mainly due to the significant difference in reported partition coefficients(e.g.,from strongly incompatible to strongly compatible)between fluid and melt(D_(W)^(fluid/melt)).Here,we used an in situ Raman spectroscopic approach to describe the W speciation,and to quantitatively determine the Dfluid/melt of individual and total W species in granite melts and coexisting Na2WO4 solutions at elevated temperatures(T;700–800C)and pressures(P;0.35–1.08 GPa).Results show that WO_(4)^(2-)and HWO4are predominant W species,and the fractions of these two species are similar in melt and coexisting fluid.The partitioning behaviors of WO_(4)^(2-)and HWO4are comparable,exhibiting strong enrichment in the fluid.The total DW fluid/melt ranges from 8.6 to 37.1.Specifically,DW fluid/melt decreases with rising T–P,indicating that shallow exsolution favors enrichment of W in evolved fluids.Furthermore,Rayleigh fractionation modeling based on the obtained D_(W)^(fluid/melt)data was used to describe the fluid exsolution processes.Our results strongly support that fluid exsolution can serve as an important mechanism to generate W-rich oreforming fluids.This study also indicates that in situ approach can be used to further investigate the geochemical behavior of ore-forming elements during the magmatic-hydrothermal transition,especially for rare metals associated with granite and pegmatite.

流体出溶是控制钨成矿的重要机制,研究钨在熔体-流体间的分配行为具有重要科学意义.针对这一问题,前人开展了大量高温高压实验和地质样品分析.然而,已有的钨在熔体间-流体间的分配系数差异巨大,甚至高达4个数量级,严重影响了对成矿过程的定量刻画.究其原因,主要是实验设计存在问题和典型地质样品可获得性差所致.本文应用热液金刚石压腔模拟流体出溶的高温(700~800℃、高压(0.35~1.08 GPa)环境,收集了花岗质熔体和共存流体的原位拉曼光谱,揭示了岩浆-热液过渡阶段钨的主要赋存方式,建立了应用拉曼光谱测定钨的不同赋存方式在流体-熔体间分配系数的方法,并获取了上述温度、压力条件下钨在流体和熔体间的分配系数(8.6~37.1).基于本文获取的分配系数和瑞利分馏模型,计算了流体出溶过程中钨在流体中的瞬时浓度和累积浓度,结果得到了地质实例的良好印证.