Optimal magmatic oxidation conditions for giant porphyry copper deposits

It is widely accepted that moderately to highly oxidized magmas are needed to form porphyry copper deposits (PCDs). However, only a few studies have attempted to investigate the effects of variable magmatic fO₂ values on the formation of PCDs. Based on previously published studies, the magma oxygen fugacity of giant PCDs is mainly concentrated between ΔFMQ -0.5 and ΔFMQ +2.5, with the first quartile, median, and third quartile being +0.8, +1.3, and +1.7, respectively. In this work, we have carried out numerical modeling, which shows that the S^2- used to precipitate Cu accounts for approximately 90% of total sulfur at ΔFMQ +1 but drops to 33%-38% and 15%-25% at ΔFMQ +2 and ΔFMQ +3, respectively. This leads to a decrease of 88%-96% in Cu precipitation efficiencies from ΔFMQ +1 to ΔFMQ +3. Our petrological model further shows that the maximum Cu precipitation efficiency (and, therefore, Cu endowments) occurs at around ΔFMQ +1. We, therefore, highlight that moderately oxidized magma (ΔFMQ = ∼1) rather than highly oxidized ones (ΔFMQ > ∼2) is optimal for the generation of giant PCDs.