Crustal magma oxidation state and endowments in porphyry copper deposits

Porphyry copper deposits (PCDs) supply the majority of the world’s copper (Cu) as well as significant amounts of molybdenum (Mo) and gold (Au). The formation of these deposits is controlled by a variety of geological factors, among which the magmatic oxidation is one of the focuses of current research. This study investigates the redox characteristics of arc magmas by analyzing the ratio of vanadium (V) to scandium (Sc) and its correlation with the oxygen fugacity (fO) of magmas from arcs of various crustal thicknesses globally. We compiled an extensive dataset of published V/Sc values and studied the partition coefficients of V and Sc between minerals and silicate melts across various magmatic environments. The results demonstrate that fractional crystallization of magnetite markedly reduces V/Sc values of the residual melt. Furthermore, magnesium number (Mg#) at the onset of magnetite fractionation is strongly positively correlated with fO of the equilibrium magma. Based on this, the oxidation of arc magmas from different crustal thicknesses can be evaluated. The result suggests that magmas generated in crusts thicker than 20 km tend to be more oxidized with increasing crust thickness. This trend is attributed to enhanced intracrustal differentiation processes involving garnet and amphibole, particularly in mature island and continental arcs. In contrast, magmas in thinner arc crusts (<20 km), which are significantly influenced by subducted slab materials, display higher oxidation states than most of their thicker crust counterparts. These findings suggest that both subduction dynamics and crustal thickness play a vital role in determining the oxidation state of arc magmas and, consequently, their capacity to generate PCDs. Highly oxidized, primitive arc magmas are less favorable for PCD formation due to that they tend to experience magnetite fractionation and thus sulfide saturation in the early stage of magma evolution, a process leading to copper depletion. On the contrary, thick arcs where magnetite crystallization is widely suppressed and therefore more conducive to retaining Cu in evolved magmas. This study highlights the coupled roles of subduction input and intra-crustal differentiation in the genesis of arc magmatic oxidation, and provides geochemical indications for the metal endowment of porphyry copper deposits based on the V/Sc systematics.