The origin of propylitic alteration halos in porphyry systems

斑岩矿石系统是世界上铜和钼的主要来源，也是金和银的主要储存库(Cooke等人，2014a)。这些矿床来自大量含金属的热液，这些热液从结晶的地壳岩浆储集层中析出。最近的研究表明，早生蚀变晕--与斑岩中心有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，即使在如此长的距离内，岩浆流体也可能对其发展起到促进作用(Pacey等人，2020年)。我们现在还知道，在这些光晕中形成的一些蚀变矿物，如绿帘石和绿泥石，可以结晶出具有斑岩环境典型特征的成分，这些成分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，尽管有了这种新的认识(见Holling和Orovan，2020)，我们仍然不知道如此巨大的蚀变是如何发展起来的，就所涉及的流体的来源和性质及其流动路径而言。因此，该项目的目的是了解斑岩矿石系统中区域范围和局部的丙硫酸盐蚀变的控制，并限制热液流体的性质。这项研究将把野外测绘与大规模采样、岩石学、矿物化学、地质年代学和流体包裹体研究结合起来。这将允许开发一种模型，限制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。可以利用数值模拟来测试能够解释观测结果的替代方案。这项研究将为与斑岩中心相关的区域规模蚀变的起源及其与典型的斑岩成矿事件之前的长期岩浆活动的潜在联系提供新的见解。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，以及更好地预测斑岩丰度信号，将对斑岩勘探具有重大意义。这种加深的理解将有助于我们的总体研究目标，即降低矿产勘探的风险和环境足迹，寻求发现低碳能源过渡所需的金属。

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.