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 fO 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 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.