斑岩型矿床母岩浆中水的来源及其成矿机理

母岩浆富水(H₂O质量分数高于4%)被认为是斑岩体中Cu、Au等成矿元素富集成矿的关键因素,是形成岩浆-热液成矿系统的必要条件之一,主要表征为成矿斑岩中角闪石斑晶发育,轻稀土元素富集而中稀土元素亏损,w(Sr)/w(Y)和w(La)_N/w(Yb)_N值高(w(Sr)/w(Y)>40,w(La)_N/w(Yb)_N>20)等特征。弧环境下形成的斑岩成矿系统中的水主要来源于俯冲洋壳或交代岩石圈的脱水作用;而后碰撞环境下形成的斑岩成矿系统中的水可能主要源自矿区内同时代、共空间产出的镁铁质岩浆的混合注入,或含矿岩浆自身的分异结晶。尽管不同地球动力学背景下形成的斑岩成矿系统母岩浆中水的来源和富集机制不同,然而它们仍然被认为具有统一的成矿机理:母岩浆在地幔源区或地壳岩石部分熔融过程中,水的加入可以降低其熔点,促进源区含Cu、Au硫化物重熔,或萃取岩石中Cu、Au成矿元素,或聚集岩浆中分散分布的金属元素,形成富金属的岩浆;含矿岩浆就位后,富水岩浆很快达到水饱和,大量流体出溶,岩浆水会与各类卤化物一起以独立流体相形式从熔体中分离出来,这种富含挥发分和高盐度的流体具有很强的金属携带能力,为随后发生的斑岩矿化提供热液和金属来源,提高母岩浆的成矿潜力。

Origin and Mineralization Mechanism of the Water in Parent Magma of Porphyry Deposit

The parent magma rich in water (mass fraction of water is more than 4%) is the critical factor for the enrichment and mineralization of ore-forming elements (Cu, Au, etc.), and one of the key to the formation of magma-hydrothermal systems. The main characteristics include that the occurrence of amphibole phenocrysts, enriched light rare earth elements, depleted middle rare earth elements, and w(Sr)/w(Y)>40 and w(La)_N/w(Yb)_N>40. The water of porphyry metallogenic system in arc settings is derived from the dehydration melting of oceanic plate or metasomatized lithospheric mantle; however, the water of porphyry metallogenic system in post-collisional settings, which is lack of direct subduction source, is likely to be derived from mixing of water-rich mafic magma with melt at lower and/or upper-crustal depths, or from crystallization differentiation of ore-related magmas themselves. Although the geodynamic background of porphyry metallogenic system and the enrichment mechanism of magmatic water in ore-related parent magma are different, they are still considered as the uniform mineralization mechanism. The higher magmatic water is the necessary conditions of forming magmatic hydrothermal mineralization system. During the process of partial melting, the addition of H₂O can reduce the melting point of magma, promote the re-melting of the source containing Cu, Au sulfide, or the extraction of Cu, Au ore-forming elements from rock, or the aggregation of the scattered distribution of metal elements in magma, forming metal-rich magma; after the emplacement of ore-related magma, the water-rich magma will quickly reach saturation, making abundant fluids exsolve from magma and magmatic water with all kinds of halide separate from the melt in the form of independent fluid phase. The high-salinity fluid, which is rich in volatile possessing strong capacity of carrying metals, can prompt copper and gold clathrates, and other metal fractions enriched in the fluid, and provide main ore-forming fluids and metal source for the porphyry mineralization, and generate (or improve) the metallogenic potential of parent magma.