Source-oxidized and (super-)wet magmas explain porphyry copper fertility vectors: A thermodynamic modeling approach

We use phase-equilibrium and trace-element modeling to address the effects of magmatic H2O, oxygen fugacity (fO2), and differentiation depth of mantle-derived magmas on the generation of porphyry copper deposit (PCD)−forming magmas. We explore how these factors control first-order compositional trends indicative of high PCD-forming potential [Sr/Y and (Eu/Eu*)/Yb versus SiO2 and fO2] and test different models for the ideal amount of H2O and fO2 of PCD-forming magmas. Our findings suggest that initial oxidized conditions (i.e., magmas derived from an oxidized source) are necessary for the generation of PCD-forming magmas. We show that wet to super-wet magmatism (H2O >4−8 wt%) combined with an initially oxidized state better matches the required conditions for PCD generation. Importantly, we find that garnet is not necessary for PCD formation; amphibole-bearing but garnet-free assemblages can also produce PCD-forming magmas.