成矿流体的相态变化可对热液矿床的形成产生重要影响(Heinrich,2007)。除了流体出溶及沸腾作用外,实验研究发现部分典型成矿流体(含REE-SO
4、Zn-SO
4、Li-SO
4、Na-SO
4等流体)在一定的温-压范围内可发生较为特殊的液–液不混溶作用,即均一流体分离出高密度液相与低密度液相(Valyashko,2008;Wang等,2013,2016,2017;Wan等,2021;Cui等,2021)。原位拉曼光谱定量分析显示相关成矿金属在两种液相中的分配行为存在明显差异,因此推测液–液相分离可对成矿元素的迁移、富集产生重要影响(Wang等,2016,2017;Wan等,2021a)。然而,由于缺少对不混溶液相物理性质(密度、粘度)的限定,研究人员难以精确刻画由物理性质差异而导致的两种液相分离过程,进而限制了对液–液不混溶作用效果的评估(Zhang等,2020)。本研究结合原位拉曼光谱定量分析、质量守恒计算及硅管中流体体积分析等方法,测定了不混溶液相的密度。结果显示,贫溶质流体相与纯水密度相近,而富溶质相密度远高于贫溶质相,可达2 g/cm
3。不混溶液相密度存在巨大差异,这将导致不混溶液相迁移活性明显不同,并促进两种液相高效分离及高品位矿床的形成。此外,建立合适的地球化学指标,是判识地质条件下液–液不混溶作用并探讨其成矿效应的前提。本研究通过拉曼光谱分析两种典型组分(SO
42-:液–液不混溶关联组分;ClO
4-:液–液不混溶非关联组分)在ZnSO
4–Zn(ClO
4)
2–H
2O体系液-液不混溶及沸腾过程中的分配行为,开展了定量分析。结果显示,沸腾过程中SO
42-与ClO
4-被同等程度地富集于残余液相中。而在液–液相分离过程中,二者的分配行为存在明显差异,SO
42-在高密度液相中明显富集而在低密度相中浓度很低,而ClO
4-在两不混溶液相中的浓度相近(Wan等,2021b)。综合已有研究结果,我们提出了一套地质流体液–液不混溶的判识标志:1、流体中含一定的液–液不混溶关联组分并具备发生液–液不混溶的潜力,且流体的
T–P–x演化路径与该体系的液–液不混溶稳定域存在重叠;2、由单期流体所形成的流体包裹体群中液–液不混溶关联组分浓度存在明显差异,而低挥发性的液–液不混溶非关联组分不呈现明显的浓度差异;3、结合其他分析手段,评估外来流体混入、水岩反应等过程的影响,综合判别液–液相分离的发生。
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