Exsolution depth and migration pathways of mineralising fluids in porphyry deposit-forming magmatic systems

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Porphyry copper deposits provide around 75% of the world's copper and are an important source of gold and other metals. One aspect of porphyry deposit models that is poorly understood is whether the mineralising fluids from which they form are derived from: (i) high-level copper-rich porphyry magmatic stocks, (ii) feeder chambers at mid-upper crustal levels, (iii) a lower crustal magmatic reservoir, or (iv) a combination of these. The high level porphyry stock is often assumed to be the fluid source as it is invariably mineralised and can usually be temporally and texturally linked to the mineralisation. However, it is unlikely that enough fluid could be derived from such a limited magma volume. It therefore seems probable that much of the fluid comes from a deeper source, possibly 5 to 15 km, although how fluids are transported from such depths is poorly understood. The aim of the studentship is to identify textural and chemical evidence for the exsolution depth and migration pathways of mineralising fluids in materials from different levels within porphyry systems, e.g. enclaves, cumulates, minerals which crystallised at different depths and breccia clasts. Crystallisation depths will be assessed from a variety of mineral-based barometers, and the composition, including relative copper fertility and water contents, of the melt from mineral indicators, including plagioclase (1). Textural evidence for fluid exsolution will include miarolitic cavities and primary fluid inclusions in magmatic minerals. The deposits to be studied are in Nevada where, due to tectonic tilting, there is the rare opportunity to study well exposed sections through porphyry systems to palaeo-depths of >10 km (2). The student will carry out two fieldwork campaigns of around 4 weeks each. Analyses will be undertaken using state-of-the-art facilities including electron microprobe, SEM, laser ablation ICP-MS, cathodoluminescence, micro-CT scanning and fluid inclusion microthermometry at Camborne School of Mines and the Natural History Museum. New concepts from the project will inform models for porphyry deposit formation and as such aid in their discovery. References:1. Williamson B. J., Herrington R. J., Morris A., 2016. Porphyry copper enrichment linked to excess aluminium in plagioclase. Nat. Geosci. 9, 237-241.2. Seedorf E., Barton M. D., Stavast W. J. A., Maher D. J., 2008. Root zones of porphyry systems: extending the porphyry model to depth. Econ. Geol. 103, 939-956.