Hydrothermal alteration and fluid pH in alkaline-hosted epithermal systems
Hydrothermal alteration and fluid pH in alkaline-hosted epithermal systems
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DOI：
10.1016/j.oregeorev.2017.06.028
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发表时间：
2017-10
影响因子：
3.3
通讯作者：
D. J. Smith;J. Naden;Gawen R. T. Jenkin;M. Keith
中科院分区：
文献类型：
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作者：
D. J. Smith;J. Naden;Gawen R. T. Jenkin;M. Keith
浅成低温热液金矿化存在于各种各样的寄主岩性中，尽管存在多样性，但矿床之间的蚀变矿物组合往往相似。值得注意的例外是那些在碱性寄主岩石中的金矿床。碱性浅成热液矿床很少见，但很重要，因为它们包括一些世界上已知的最大的浅成热液矿床（如Ladolam，Cripple Creek，Porgera）。以及特殊的金含量，碱性主机系统往往表现出不同的蚀变矿物组合，少石英和广泛的硅化比亚碱性主机的等价物，更大的富集碲，和酸蚀变（高级泥质）类型的稀缺。在这项研究中，使用地球化学建模来证明，在低水/岩比下，与碱性、硅不饱和的寄主岩石平衡的300 °C热液流体达到的pH值明显高于亚碱性岩性中的等同物。一个最大的，接近中性的pH值（5.5-6）是缓冲的反应，涉及石英在二氧化硅饱和的碱性和钙碱性岩性。在硅质不饱和的碱性寄主岩石中，石英被渐进的水-岩相互作用耗尽，pH值增加到7以上。碲和金的溶解度都有利于中性到高的流体pH值，因此在这些热液系统中存在一种明确的机制，可以导致有效的运输和浓缩，从而在碱性火成岩宿主中产生碲化金矿床。这个模型表明，碱性岩石仍然可以改变先进的泥质组合，这种蚀变类型的碱性主机的缺乏，而不是指向NaCl和HCl在岩浆挥发相在热液蚀变的开始。
Epithermal gold mineralisation is found in a wide compositional range of host lithologies, but despite the diversity the alteration mineral assemblages are often similar between deposits. Notable exceptions are those gold deposits hosted in alkaline host rocks. Alkaline-hosted epithermal deposits are rare, but important, as they include some of the world’s largest known epithermal deposits by contained metal (e.g. Ladolam, Cripple Creek, Porgera). As well as the exceptional gold contents, the alkaline-hosted systems tend to exhibit different alteration mineral assemblages, with less quartz and widespread silicification than sub-alkaline-hosted equivalents, and greater enrichments in tellurium, and a scarcity of acid alteration (advanced argillic) types. In this study, geochemical modelling is used to demonstrate that 300 °C hydrothermal fluids in equilibrium with alkali, silica-undersaturated host rocks at low water/rock ratios reach significantly higher pH than equivalents in sub-alkaline lithologies. A maximum, near-neutral pH (5.5–6) is buffered by reactions involving quartz in silica-saturated alkaline and calc-alkaline lithologies. In silica-undersaturated, alkaline host rocks, quartz is exhausted by progressive water-rock interaction, and pH increases to 7 and above. Both tellurium and gold solubility are favoured by neutral to high fluid pH, and thus there is a clear mechanism within these hydrothermal systems that can lead to effective transport and concentration to produce gold telluride ore deposits in alkaline igneous hosts. This modelling demonstrates that alkaline rocks can still be altered to advanced argillic assemblages; the paucity of this alteration type in alkaline hosts instead points to NaCl ≫ HCl in magmatic volatile phases at the initiation of hydrothermal alteration.