The origin of propylitic alteration halos in porphyry systems

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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.