Magmatic-hydrothermal evolution in a fractionating granite : a microchemical study of the Sn-W-F-mineralized Mole Granite (Australia)
Magmatic-hydrothermal evolution in a fractionating granite : a microchemical study of the Sn-W-F-mineralized Mole Granite (Australia)
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DOI：
10.1016/s0016-7037(00)00428-2
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发表时间：
2000-10
影响因子：
5
通讯作者：
A. Audt́at;D. Gn̈ther;C. Heinrich
中科院分区：
文献类型：
--
作者：
A. Audt́at;D. Gn̈ther;C. Heinrich
利用与众多热液矿床相关的大型花岗质岩体（澳大利亚新南威尔士州东北部的Mole花岗岩），对岩浆晚期到热液早期过渡时期硅酸盐熔体和含水流体的化学演化进行了综合研究。通过电子探针分析（EPMA）和激光烧蚀电感耦合等离子体质谱微分析，对岩浆岩斑晶中的流体包裹体和结晶熔融包裹体以及岩浆岩孔洞中的矿物进行了主量元素和微量元素组成分析。结合矿脉的流体成分数据，这些数据可以重建硅酸盐熔体的演化过程，以及由此析出的单相（超临界）或两相（盐水+蒸汽）流体，从初始流体饱和到富f残余熔体的最终凝固。分析数据和结合实验配分数据和自然系统约束的瑞利分馏模型表明，溶出液的相态、矿化度和含矿量随结晶程度的增加而发生显著变化。即使没有外部施加的参数（如压力或温度）的变化，流体性质也会发生变化，这主要是因为熔体中F的逐渐富集。由于系统的低压（≈1 kbar），早饱和流体富含cl，并立即分离成共存的盐水和气相。熔体中F含量的增加减少了Cl向流体的分配，因此，后来的溶解流体是单相的，具有低盐度（在不变压力条件下）。由于不同矿石金属的不同络合行为，这一演化导致随着岩浆结晶的进行，微量金属分配发生了显著变化。由此产生的流体成分的变化反过来又控制了在矿床中观察到的矿-金属比率的主要变化（在这种情况下特别是锡/钨）。这一结论与近年来的斑岩型体系资料一致，更普遍地表明岩浆-热液转变可能对高温热液矿床的金属含量起主导作用。
A large granitic pluton associated with numerous hydrothermal ore deposits (the Mole Granite, northeastern New South Wales, Australia) was used for an integrated study of the chemical evolution of silicate melts and aqueous fluids during the late magmatic to early hydrothermal transition. Major and trace-element compositions were obtained by electron microprobe analysis (EPMA) and laser-ablation iductively coupled plasma mass-spectrometry microanalysis of fluid inclusions and crystallized melt inclusions in magmatic phenocrysts and minerals from miarolitic cavities. Together with fluid-compositional data from the ore veins, these data allowed reconstruction of the evolving silicate melts and therefrom exsolving single-phase (supercritical) or two-phase (brine + vapor) fluids, from the time of initial fluid saturation through to the final solidification of F-rich residual melts. The analytical data and a Rayleigh fractionation model combining experimental partitioning data with constraints from the natural system demonstrate that the phase state, the salinity, and the ore-metal contents of the exsolving fluids vary dramatically with increasing degree of crystallization. Fluid properties vary even without variation of externally imposed parameters such as pressure or temperature, mainly because of the progressive enrichment of F in the melt. Early-saturating fluids are Cl-rich and immediately separate into coexisting brine and vapor phase due to the low pressure of the system (≈1 kbar). Increasing F content of the melt reduces the partitioning of Cl to the fluid, such that later exsolving fluids are single-phase and have low salinity (at unchanged pressure conditions). Due to the contrasting complexation behavior of different ore metals, this evolution leads to a significant change in trace-metal partitioning as magma crystallization proceeds. The resulting variation in fluid compositions in turn controls the major variation in ore-metal ratios observed in the ore deposits (notably Sn/W in this case). This conclusion agrees with recent data from porphyry-style systems, and indicates more generally that the magmatic-to-hydrothermal transition probably exerts the dominant control on the metal content of high-temperature hydrothermal ore deposits.