Hydrothermal fluid modelling for pH in alkaline hosts (NERC grant NE/M010848/1)

This study explored the links between host rock composition, hydrothermal fluid composition (particularly pH), and the resulting ore minerals and deposits. The progressive water–rock reaction between 1 kg of initially acidic, condensed magmatic vapour and a series of different rock compositions was modelled with CHILLER (Reed, 1982, Reed, 1998), and follows the design of the water-rock reactions of Reed (1997). The thermodynamic data used in the numerical experiments are from the database SOLTHERM.H08 (Reed and Palandri, 2013). Data and calculations within SOLTHERM include: equilibrium constants calculated with SUPCRT92 (Johnson et al., 1992); mineral thermodynamic data for silicates, oxides, hydroxides, carbonates, gases (Holland and Powell, 1998) and sulphides (Shock, 2007). Mineral solid solutions are represented by end-member compositions that are mixed using an ideal multisite mixing scheme. Rock compositions used in the modelling represent a sub-alkaline andesitic control, and a number of alkaline compositions associated with world-class Au deposits. All starting rock compositions are derived from whole rock geochemical data, and have been recalculated to a 100% basis without TiO2 or P2O5 (excluded as minor phases with little to no effect on hydrothermal mineral assemblages). Original total Fe (as Fe2O3) has been recalculated to FeO and Fe2O3 using the method of Müller et al. (2001). The andesite is representative of calc-alkaline, silica saturated compositions, and is derived from and discussed in detail in Reed (1997). The Luise “Phonolite” (a trachyandesite using the Le Maitre et al., 1989 TAS plot; Fig. 1) and Trachyandesite are from the vicinity of the Ladolam epithermal Au deposit, Lihir Island, Papua New Guinea (Müller et al., 2001). The Porgera Mugearite and Feldspar Porphyry represent unaltered host rock compositions (Richards, 1990) from the Porgera Au deposit (Papua New Guinea). The Cripple Creek Phonolite is part of the host suite to the Cripple Creek epithermal Au deposit, Colorado (Kelley et al., 1998). The Savo trachyte (Smith et al., 2009) represents a typical host rock of the active hydrothermal system (Smith et al., 2010), on Savo island, Solomon Islands. With the exception of the Andesite, all compositions are alkaline using the total alkali versus silica definition of Irvine and Baragar (1971). The Savo sample is not associated with known epithermal Au mineralisation; this composition was selected on the grounds that it represents an evolved (SiO2-rich) silica-saturated, alkaline composition. The initial fluid composition is based on a condensate from Augustine volcano (Symonds et al., 1990) mixed 1:10 with pure water (Reed, 1997; Table 2). A single starting fluid for all models was chosen so as to demonstrate the effect of host rock alone.本研究探讨了宿主岩石成分、热液流体成分（尤以pH值为核心）与所形成的矿石矿物及矿床之间的内在关联。 本研究以1千克初始呈酸性的冷凝岩浆蒸气与一系列不同岩石成分之间的渐进水-岩反应为建模对象，采用CHILLER软件（Reed, 1982; Reed, 1998）开展数值模拟，且严格沿用了Reed（1997）提出的水-岩反应实验设计方案。本次数值实验所使用的热力学数据源自SOLTHERM.H08数据库（Reed与Palandri, 2013），该数据库包含的内容与计算结果如下：通过SUPCRT92软件计算得到的平衡常数（Johnson et al., 1992）；硅酸盐、氧化物、氢氧化物、碳酸盐、气体（Holland与Powell, 1998）以及硫化物（Shock, 2007）的矿物热力学数据。矿物固溶体采用端元成分进行表征，并通过理想多位点混合方案实现端元间的混合。 本次建模所采用的岩石成分包含一套亚碱性安山岩对照样品，以及一系列与世界级金矿床相关的碱性岩石组分。所有初始岩石成分均取自全岩地球化学测试数据，并被重新计算至不含TiO₂和P₂O₅的100%干基——这两种微量组分对热液矿物组合的影响极微甚至可忽略，故予以剔除。原始总铁（以Fe₂O₃计）已按照Müller等人（2001）提出的方法被重新换算为FeO与Fe₂O₃。该安山岩代表钙碱性、硅饱和的岩石成分，其相关细节已在Reed（1997）中进行了详细阐述与讨论。 “Luise响岩（Phonolite）”（依据Le Maitre等人1989年的总碱-二氧化硅（Total Alkali-Silica，TAS）图解，其岩石学分类应为粗安岩；图1）与粗安岩样品取自巴布亚新几内亚利希尔岛Ladolam浅成低温热液金矿床周边区域（Müller et al., 2001）。Porgera穆杰安山岩（Mugearite）与长石斑岩代表了巴布亚新几内亚Porgera金矿床的未蚀变宿主岩石成分（Richards, 1990）。Cripple Creek响岩是美国科罗拉多州Cripple Creek浅成低温热液金矿床的宿主岩石组合之一（Kelley et al., 1998）。Savo粗面岩（Smith et al., 2009）代表了所罗门群岛萨沃岛活跃热液系统的典型宿主岩石（Smith et al., 2010）。除安山岩之外，所有岩石成分均符合Irvine与Baragar（1971）提出的总碱-二氧化硅分类标准中的碱性岩石范畴。Savo样品并未与已知的浅成低温热液金矿床矿化作用相关联，其入选的依据在于其代表了一种演化程度较高（富SiO₂）、硅饱和的碱性岩石成分。 初始流体成分取自奥古斯丁火山的冷凝流体（Symonds et al., 1990），并以1:10的比例与纯水混合（Reed, 1997；表2）。本研究为所有数值模型选用了统一的初始流体，以单独凸显宿主岩石成分对反应过程的影响。