Abstract
The majority of the world’s porphyry deposits and all the giant ones formed above active subduction zones either at oceanic or continental margins, but they form at discrete moments in the evolution of an arc and not all magmatic arcs are mineralized, consequently understanding the tectonic triggers is critical for the discovery of new deposits. Recent work has shown that giant porphyry deposits are commonly associated with periods of tectonic complexity including ridge subduction and arc reversal. Ridge subduction can lead to slab flattening, crustal thickening, cessation of magmatism and possibly increased sediment subduction, all of which may play a role in the formation of giant ore deposits. These events can be recognised through changes in the geochemistry of the pre-, syn- and post-mineralization rocks often resulting in spikes in La/Yb ratios and changes in the isotopic composition. However, not all porphyry systems show these geochemical signatures suggesting that a variety of processes can lead to their formation.
Recent advances in analytical methods mean that mineral chemistry can now provide powerful insights into the evolution of fertile, ore forming magmas. Detailed studies of magmatic minerals like plagioclase and amphibole have shown that emplacement of mafic magmas into felsic/intermediate magma chambers can trigger mineralization in porphyry systems. Similarly, zircon chemistry is now widely used to investigate the oxidation state and water content of arc-related intrusions to assess their potential fertility, with negative Eu and Ce anomalies being characteristic of reduced magmas whereas positive anomalies suggest oxidized and/or hydrous magmas which are more conducive to mineralization.