Critical Differences between Typical Arc Magmas and Giant Porphyry Cu ± Au Systems: Implications for Exploration.

This article focuses on the magmatic processes controlling the formation of giant porphyry copper (Cu) and copper–gold (Cu–Au) deposits associated with arc magmatism in thick and thin crustal settings. Using rare earth element (REE) modeling, the study finds that ore-forming magmas in both thick (>50 km) and thin (<40 km) crusts fractionate significantly more amphibole and less plagioclase than barren reference arc magmas, indicating higher water contents in ore-associated magmas. Thick-crust magmas are inferred to be wetter than thin-crust magmas, with high water content being critical for giant porphyry Cu deposit formation in thick crusts, while both elevated water and chalcophile element concentrations (e.g., Cu and Au) are necessary for giant Cu–Au deposits in thin crusts. The study also demonstrates that yttrium (Y) versus MgO plots provide a more reliable geochemical tool than Sr/Y ratios for identifying economically prospective porphyry systems, due to Y’s monotonic behavior during fractionation and lower susceptibility to alteration.