Magmatic Evolution of the Supergiant Rio Blanco-Los Bronces Porphyry Cu Deposit, Central Chile: Implications for Ore-Forming Processes

This study investigates the magmatic evolution of the world’s largest porphyry copper deposit, Rio Blanco-Los Bronces in central Chile, with special emphasis on the role that the timing of sulphide saturation plays in magma fertility. We report new whole-rock geochemical data (including the platinum group elements), along with zircon oxygen isotopes, trace elements and U–Pb ages for the intrusions from the Rio Blanco side of the deposit. Our results confirm that the intrusive magmatism that produced this deposit was continuous for at least 4 Myr and show that these magmas are mantle-derived and experienced negligible crustal contamination. The most primitive sampled intrusions have <3 wt % MgO, showing that the magmas have undergone extensive fractional crystallization in a deep chamber before emplacement into the upper crust. This fractionation was controlled by amphibole, with garnet playing a minor role (≤2.5%), suggesting that most of the fractionation took place in the upper part of the lower crust. Calculations of ∆FMQ values suggest that the magma became more oxidized during the later stages of its evolution. The PGE geochemistry shows that the Rio Blanco-Los Bronces magmatic system reached sulphide saturation in the deep magma chamber early in its evolution before its MgO content fell to ~3 wt %. However, the early sulphide saturation did not prevent the formation of this supergiant Cu-dominant deposit, as has been previously proposed. We attribute this to the small amount of sulphide formed, with a high monosulphide solid solution/sulphide liquid ratio and the high fO2 of the magma. We propose that other factors, such as the enormous size of the magmatic system (>10 000 km3) and the long duration of the magmatic activity (≥4 Myr), were key in the formation of this supergiant Cu deposit.