Metal scavenging by sulfide oxidation in porphyry copper deposit root zones

Abstract

Porphyry copper (-gold) deposits in both arc and non-arc settings are hosted by oxidized magmatic rocks, but the exact mechanism by which the source magmas of these non-arc deposits become oxidized remains obscure. Through whole-rock geochemistry, mineral chemistry, and zircon isotopic analyses, here we show that the Neoproterozoic root of the Cretaceous Shaxi porphyry copper-gold deposit in the Jiangnan orogen is reduced and copper-sulfide rich, whereas the Cretaceous ore-forming magmas are oxidized. An oxidized mafic melt flushed the Neoproterozoic root zone, extracted copper and gold, and transported them to the upper crust where the Cretaceous Shaxi deposit formed. Evidence is recorded in growth zones of clinopyroxene, zircon, spinel, and plagioclase. Our results indicate that the relatively reduced, sulfide-enriched sub-continental lithospheric mantle or lower crust is essential but not sufficient for the formation of fertile non-arc porphyry magmas. Addition of a newer oxidized magma is crucial for the formation of this mineralization style.