PORPHYRY cu±mo±au DEPOSITS IN a CONVERGENT OROGENIC CONTEXT

Porphyry Cu±Mo±Au deposits form in convergent margin environments ranging from ocean–ocean and ocean–continent subduction, through arc–continent and continent–continent collision, to post-collisional or post-subduction settings. Porphyry Sn-W and Mo deposits also form in related tectonic settings, commonly associated with more felsic magmas towards continental interiors.

The common theme in all of these deposit types is the exsolution of a water-rich fluid phase from upper crustally emplaced magmas, which partitions metals from the magma and transports them upwards into cooler cover rocks where metals are precipitated. This primary requirement requires relatively high magmatic water contents (>4 wt.%), which are most commonly found in subduction-related environments. Porphyry Cu±Mo±Au deposits additionally require significant amounts of Cl and S in the hydrothermal fluids, to transport (as chloride and sulfide species) and precipitate metals (mostly as sulfide minerals). A subduction source (principally from seawater) is indicated for all of these volatile components, whereas most of the metals are mantle-derived.

An increasing number of porphyry Cu±Mo±Au deposits are being recognized to have formed after subduction ceased beneath the host crustal section, either due to collisional events or migration of the subduction zone (e.g., due to subduction reversal, arc rifting, etc.). The key to post-subduction porphyry metallogeny appears to be the water locked up in hydrous cumulate minerals in the deep upper-plate lithosphere, left behind by fractionation of hydrous mafic magmas during the main arc stage. Partial melting of these amphibolitic residues during later tectonothermal events, especially during rifting, generates relatively small volumes of magma that may remobilize metals from residual cumulate sulfide phases, and differ only from main stage calc-alkaline magmas in having slightly to distinctly elevated alkali contents.