Porphyry ore deposits are the source of much of the copper, molybdenum, gold and silver used by humans. Porphyry ore typically forms in magmatic arcs above subduction zones. However, generation of the largest deposits is often restricted to specific arc segments and limited periods of time. Here, I outline a hierarchy of four key triggers that may be involved in the formation of large porphyry deposits. The first process is characterized by a cyclical enrichment of magmas with metals and water in the deep crust. Second, saturation of the magma with sulphide facilitates the concentration of metals into smaller volumes of material from which they can later be released. The third process is an efficient transfer of metals into hydrothermal fluids that are exsolved from the magmas. Finally, localized processes trigger the precipitation of ore minerals in the crust. Although some or all of these processes must act in concert to generate large ore deposits, I argue that sulphide saturation of the magma is the most important step and that this can explain the temporal and spatial distribution of ores. Consequently, the fingerprint of sulphide saturation in igneous rocks could be used to identify those parts of magmatic arcs that are particularly predisposed to ore formation.