钨的气态迁移与岩浆-热液成矿作用:实验研究及其成矿学意义

低密度的水热蒸气和超临界似气流体广泛存在于中地壳至地球表面的各种地质环境中, 是成矿金属搬运和富集的重要介质。火山喷气凝结水、火山结壳和升华物、矿床的流体包裹体气相中均存在具有地质意义的W含量或含钨矿物, 表明W同样可在含水气相中溶解和迁移。本文在350 ℃~400 ℃和压力为60~200 bar的实验条件下, 测定了WO₃-H₂O体系中W在水蒸气和似气流体中的溶解度, 考察了水蒸气压力对W溶解度的影响。结果显示, W在水蒸气中的逸度(或含量)远高于依据无水体系中固体WO₃挥发性数据计算的蒸气压力, 证明气态溶质W与溶剂水蒸气之间存在促进W溶解的水合作用。经热力学方法分析, 认为可能形成了WO₃·nH₂O(g)形式的水合气体物种, 其水合数n在350 ℃、370 ℃和400 ℃时分别为1.4、1.6和2.9。因此WO₃·3H₂O(g)或H₂WO₄·2H₂O(g)及H₆WO₆(g)在温压较高的岩浆-热液或气成-热液成矿环境中(如斑岩系统)对W的气态迁移和浓集可能具有重要作用, 而在温压较低的水热蒸气条件下, W的迁移形式可能以水合数较小的WO₃·H₂O(g)(或H₂WO₄)和WO₃·2H₂O(g)(或H₂WO₄·H₂O)物种为主, 其含量或比例随水蒸气的压力而改变。某些斑岩型和脉型钨(钼)矿床常存在富气体包裹体, 伴随酸性岩浆结晶出溶的以低盐度含水蒸气占优势的岩浆流体对斑岩系统中W、Mo在高温阶段的气态迁移和矿质在花岗岩体顶部和上覆岩层的聚集具有重要意义, 之后蒸气冷凝可产生高盐度的含矿卤水或与渗流地下水混合形成低-中等盐度的成矿流体, 流体的减压沸腾(相分离)和对围岩的交代蚀变导致W、Mo等金属在不同阶段和构造-岩性部位沉淀富集。

Gaseous transport and magmatic-hydrothermal mineralization of tungsten: Experimental study and its metallogenic implications

Low-density hydrothermal vapor and vapor-like fluid occur widely in various geological environments from middle crust to terrestrial surface, and they are important agents for the transport and enrichment of ore-forming metals. Geologically significant tungsten contents or tungsten-bearing minerals are found in fumarolic condensates, sublimates and incrustations in volcanic areas and in vapor phase of fluid inclusion in ore deposits, showing tungsten can also be dissolved and transported in aqueous vapor. The solubility of tungsten in water vapor and vapor-like fluid in the WO₃-H₂O system was experimentally determined at temperatures of 350 ℃~400 ℃ and pressures of 60~200 bar, and whereby the influence of water vapor pressure on the solubility was investigated. The results indicate that the fugacity or contents of tungsten in water vapor are much higher than the vapor pressure of solid WO₃ calculated with the volatile data in water-free system, demonstrating hydration takes place between the gaseous solute of tungsten and the solvent of water vapor, which promotes the dissolution of tungsten in the vapor. Based on thermodynamic analysis, the solubility is attributed to the formation of hydrated gas species WO₃ · nH₂O(g), and the hydration numbers are 1.4 at 350 ℃, 1.6 at 370 ℃, and 2.9 at 400 ℃, respectively. Thus, WO₃ · 3H₂O(g)or H₂WO₄ · 2H₂O(g) and H₆WO₆(g) is likely to play an important role in the gaseous transport and concentration of tungsten in the magmatic-hydrothermal or pneumatolytic-hydrothermal circumstances such as porphyry system under high temperature and pressure conditions, whereas the complexes with less hydration numbers, WO₃ · H₂O(g)(or H₂WO₄)and WO₃ · 2H₂O(g)(or H₂WO₄ · H₂O), whose proportion varies with the water vapor pressure, will probably predominate in the hydrothermal vapor at lower temperatures and pressures. Vapor-rich inclusions occur commonly in some porphyry and vein-type W(-Mo) deposits, the magmatic fluid dominated by low-salinity aqueous vapor exsolved from acid magma during crystallization is mostly responsible for the gaseous transport and gathering of tungsten and molybdenum in the apical space of the granitic pluton and overlying wall rocks. Subsequently vapor can evolve into metal-bearing high-salinity liquid or brine through condensation or further produce low to moderate salinity mineralizing fluid by mixing with infiltrating groundwater. Fluid boiling or phase separation caused by pressure drop and replacement and alteration in wall rocks will result in the deposition and enrichment of W, Mo and other metals in different stages and structural-lithologic positions.