The origin of propylitic alteration halos in porphyry systems

斑岩矿系统是世界上铜和钼的主要来源，也是金银的主要储藏库（Cooke et al., 2014a）。这些矿床起源于大量含金属的热液，这些热液是从结晶的地壳岩浆储层中溶解出来的。最近的研究表明，丙基蚀变晕——与斑岩中心相关的最广泛的热液蚀变带——可以从矿床本身延伸5公里以上，即使在如此遥远的距离上，岩浆流体也可能有助于其发育（Pacey et al., 2020）。我们现在也知道，在这些晕中发育的一些蚀变矿物，如绿绿石和绿泥石，可以结晶出典型的斑岩环境特征成分，并且可以随着距离系统中心的距离而系统地变化（例如Cooke等人，2014b； Wilkinson等人，2015,2017,2020）。然而，尽管有了这一新的认识（见Hollings和Orovan， 2020），我们仍然不知道如此大量的蚀变是如何发展的，涉及流体的起源和性质及其流动路径。因此，该项目的目的是了解斑岩矿系统中全区和局部丙质蚀变的控制作用，并限制热液流体的性质。这项研究将把野外测绘与大规模取样、岩石学、矿物化学、地质年代学和流体包裹体研究结合起来。这将允许开发一个模型来约束流体流动事件的相对时间，流体流动的构造和岩石地层控制，以及所涉及的流体的压力-温度-成分演化。数值模拟可用于测试可解释观测结果的备选方案。该研究将为与斑岩中心相关的区域尺度蚀变的起源及其与斑岩成矿事件之前通常存在的长期岩浆活动的潜在联系提供新的见解。这将对斑岩勘探产生重大影响，包括更好地解释目前工业上广泛应用的矿物化学分带模式、改进蚀变事件的地质年代学和更好地预测斑岩生育信号。这种加深的理解将有助于我们的总体研究目标，即降低寻求发现低碳能源转型所需金属的矿物勘探的风险和环境足迹。

Porphyry ore systems represent the world's principal source of copper and molybdenum and are major repositories of gold and silver (Cooke et al., 2014a). These deposits originate from huge volumes of metal-bearing hydrothermal fluid that exsolves from crystallising crustal magma reservoirs. Recent studies have shown that the propylitic alteration halo - the most extensive zone of hydrothermal alteration associated with porphyry centres - can extend for more than 5 km from the ore deposit itself, and that magmatic fluids are likely to contribute to its development even over such large distances (Pacey et al., 2020). We also now know that some of the alteration minerals that develop within these halos, such as epidote and chlorite, can crystallise with characteristic compositions that are typical of the porphyry environment and which can vary systematically with distance from the centre of the system (e.g. Cooke et al., 2014b; Wilkinson et al., 2015, 2017, 2020). However, despite this new understanding (see Hollings and Orovan, 2020), we still do not know how such huge volumes of alteration develop, in terms of the origin and nature of the fluids involved and their flowpaths. Thus, the aim of the project is to understand the controls of district-wide and localised propylitic alteration in porphyry ore systems and constrain the properties of the hydrothermal fluids. The study will integrate field mapping with large-scale sampling, petrography, mineral chemistry, geochronology and fluid inclusion studies. This will allow a model to be developed that constrains the relative timing of fluid flow events, the structural and lithostratigraphic controls of fluid flow, and the pressure-temperature-compositional evolution of the fluids involved. Numerical modelling may be utilised to test alternative scenarios that can account for the observations. The research will provide new insights into the origin of district-scale alteration associated with porphyry centres and its potential connections to the long-lived magmatism that typically precedes porphyry ore-forming events. There will be significant implications for porphyry exploration in terms of better models for interpreting mineral chemistry zonation patterns that are now widely applied by industry, improved geochronology of alteration events and better prediction of porphyry fertility signals. This enhanced understanding will feed into our overarching research goal to decrease the risk and environmental footprint of mineral exploration that seeks to discover the metals needed for the low carbon energy transition.