成矿热液分类兼论岩浆热液的成矿效率

The classification of ore-forming fluid and the efficiency of ore formation of magmatic hydrothermal solution

根据流体中H2O、CO2、NaCl三组分的相对含量,结合H2O-NaCl体系临界压力和石盐融化曲线温度,可以将地壳中的成矿流体分为5类:大气降水、海水、盆地卤水、变质流体和岩浆流体。这种基于地壳环境中普遍存在的三元系结合流体临界性和石盐融化温度的分类,对于理解地壳中沿地温地压梯度流动流体的多样性和流体混合与不混溶具有重要意义。相比其他热液,岩浆热液金属富集效率即成矿效率最高,其原因为:岩浆热液具有岩浆所提供的成矿所需的作用能量;岩浆出溶热液含有高浓度的挥发组分,它们不仅极大影响热液酸碱度和氧逸度,从而增强金属萃取效率,而且释放的巨大能量造成围岩渗透率扩大和产生流体对流;高温高盐度特征也导致岩浆热液具有远高于其他流体的金属萃取和富集效率。与岩浆熔体相比,岩浆热液系统成矿效率更高,可以归结于多数成矿元素高的溶液/熔体分配系数。最新的流体相和熔体相电负性和化学硬度的计算表明,相比熔体相,流体相具有更大的电负性和硬度。根据硬软酸碱原理和最大硬度原理,由于金属表现出更高的电负性和硬度,因此相对于硅酸盐熔体相,热液流体对金属有更强的吸引力和萃取能力。这也与绝大多数金属在溶液/熔体之间更为偏向于进入热液的实验结果相一致。

Ore-forming fluids are divided into meteoric,seawater,basinal settings,metamorphic and magmatic fluids,according to the contents of H2O,CO2 and NaCl phase components,as well as the critical pressure and halite melt-curve-temperature in the H2O-NaCl system.This kinds of classification has a great value in understanding the variety of flowing fluids along geothermal and geopressure gradient,and fluid mixing and unmixing in the earth＇s crust.Compared with other hydrothermal fluids,the magmatic hydrothermal fluid has a highest metal-enrichment efficiency by the following reasons： 1） the magmatic hydrothermal fluid has the energy required to carry out the mineralization from the magma;2） it has a high concentration of volatile components,which not only greatly affects the hydrothermal pH and oxygen fugacity,but also enhances the extraction efficiency of metals,and the huge released energy causes the expansion of wall rock permeability and produces fluid convection;and 3） the characteristics of high temperature and high salinity also lead to much higher metal extraction and enrichment efficiency of magmatic-hydrothermal fluids than other fluids.Compared with the magmatic melts,the hydrothermal system has higher mineralization efficiency,which can be attributed to the high partition coefficient of liquid-melts for most ore-forming elements.The latest estimates of electronegativity and chemical hardness of the fluid phase and the melt phase indicate that,compared with the melt phase,the fluid phase has a greater electronegativity and chemical hardness.According to the hard and soft acid-base theory and the principle of maximum hardness,as the metals show the higher electronegativity and chemical hardness,the fluids are very attractive and of higher extraction ability for the metals compared to the silicate melt phase,which is consistent with the experimental results showing that the vast majority of the metals favor the hydrothermal fluid than the melt phase.