Porphyry copper systems supply about 75% of the world's copper1. They form above subduction zones and are preferen- tially associated with calc-alkaline magmas1,2. Such magmas result from continuous iron depletion during differentiation, in contrast to tholeiitic magmas that show initial iron enrichment during differentiation3. The formation of calc-alkaline magmas is favoured by high water content and oxygen fugacity4–7. These characteristics, as well as magmatic metal contents, are thought to be imparted in the mantle source by fluids of the subducted slab8–10. Yet this process does not explain why porphyry copper systems preferentially occur in thicker arcs1,2. Here I present a statistical assessment of more than 40,000 published geochemical analyses of magmatic rocks from 23 Quaternary-aged volcanic arcs worldwide. I find that magmas of thicker arcs are systematically more calc-alkaline and more depleted in copper than magmas of thinner arcs. This implies that the missing copper in the former accumulates as copper sulphides within or at the base of thicker arcs. Such copper accumulations are an essential step in forming porphyry systems11–13. These results suggest that the thickness of the overriding plate provides a more important control on magma differentiation than the composition of the mantle source, and can explain the preferential association of porphyry copper systems with calc-alkaline magmas and thicker arcs.