In Person SEES Seminar --- Dr. Frances Cooper, University College London Title - The role of exhumation in generating high-grade copper deposits: Insights from the Central Andes

Abstract: Copper (Cu) is a vital part of the global effort to transition away from fossil fuels towards a greener, low-carbon economy. It is an essential component of most clean energy technologies, from wind turbines and solar panels to electric vehicles and energy storage. Today, 75% of the world's Cu is mined from porphyry Cu deposits (PCDs), which are metal-rich magmatic-hydrothermal systems typically associated with subduction zones. However, there are two problems: (1) finding these deposits is becoming increasingly difficult because most of the “easy” ones on the surface have already been discovered; and (2) they are notoriously low-grade (≲1% Cu), meaning that huge amounts of rock must be removed to retrieve a tiny amount of Cu. This. Means that, as the global demand for Cu continues to grow, mining companies not only need to look for more elusive, buried deposits but also those that will have the least environmental impact. To achieve this requires finding regions that are likely to host the most Cu-rich (highest-grade) deposits, which minimise the amount of extraction required.

I suggest that the exhumation history of a PCD plays a critical role in determining its Cu grade and can be used to help narrow down search space. During the initial formation of a PCD, rapid exhumation is needed to advect magmatic heat and metal-carrying fluids into the shallow crust. Exhumation must then slow down so the deposit has time to linger close to the surface where it can be enriched via leaching and reprecipitation of the Cu by meteoric water. During this time, it is not only essential that the climate is wet enough to drive enrichment but also that the deposit is not buried beneath cover rocks that might inhibit the process. I present an example from the Middle Eocene-Early Oligocene copper belt in northern Chile, which hosts some of the world's premier PCDs, but seems to disappear towards the border with Peru. Could there be (potentially lucrative!) buried deposits there or were conditions never conducive for them to form in the first place? By combining U-Pb geochronology, Al-in-hornblende geobarometry, and low-temperature thermochronology to track exhumation histories, we show that this apparent disappearance is due to excessively high exhumation rates followed by burial beneath a large ignimbrite, which left little or no time for enrichment to occur.