Magma evolution and fertility of the supergiant porphyry Cu deposits from Central Chile: insights from PGE and zircon geochemistry

Abstract

Copper is used in every aspect of our society and its demand is increasing due to its critical role in the transition from fossil fuel to green energy. Porphyry copper deposits, which are the world's primary source of this metal, have been widely studied; however, it is still unclear why some porphyry systems are fertile (capable of producing ore deposits) while others are not. This work explores this question by studying igneous rocks associated with the world's largest and third-largest Cu deposits, the Rio Blanco and El Teniente deposits in Central Chile, with special emphasis on the role that sulphide saturation plays in determining magma fertility. New whole-rock, major and trace element data, including platinum group elements (PGE), as well as zircon O-Hf isotopes, trace elements and U-Pb ages from intrusions related to these deposits, are presented. The results show that the magmas that produced the regional and ore-related intrusions are co-magmatic, and that the magmatism was continuous for at least 4 Myr. The magmas that gave rise to these intrusions came from a mantle wedge modified by melts derived from greywackes at the top of the underlying subducted slab. The results show that the magmatic evolution was controlled by amphibole fractionation, with garnet playing a minor role, suggesting that the magma was water-rich and that it fractionated in a chamber located in the upper half of the lower crust. The PGE geochemistry shows that the Rio Blanco and Teniente magmatic systems reached sulphide saturation early in their evolution, in the deep, unexposed magma chamber. This depleted the magma in Au relative to Cu, leading to the formation of Cu-dominated deposits. Calculations of Ce4+/Ce3+, Eu/Eu* and delta FMQ values for the different intrusive units suggest that the fO2 and water content of the magma increased towards the later stages of the magmatic evolution. The increase in water content is attributed to extensive fractional crystallization and the increase in fO2 to the crystallization of olivine, pyroxene and, to a lesser extent, amphibole. It is concluded that the timing of sulphide saturation did not play a key role in the formation of these deposits because an early sulphide saturation did not prevent the formation of two of the world's largest copper deposits. Instead, it is argued that other factors such as the small amount of sulphide melt to precipitate from the magma, and the enormous size of the deep magma chamber, were responsible for the formation of these supergiant deposits. Show more