Cold arc magma differentiation linked to porphyry copper deposit formation?

The key processes responsible for the genesis of world-class porphyry copper deposits remain ambiguous. A high water content of parental magmas evolved in the deep crust is posited to be one key control on their formation, yet remains controversial. Here, we report trace element compositions of whole-rocks and zircons that pre-, syn- and post-date mineralisation from several major porphyry copper districts. We find clear temporal trends of decreasing zircon Ti and whole-rock Zr. Across all studied deposits and literature data, we find low zircon Ti and whole-rock Zr concentrations are inherent to intrusive rocks associated with porphyry Cu deposits. According to the Ti-in-zircon thermometer and zircon solubility models, these concentrations are consistent with porphyry Cu-related magmas being cold relative to other intermediate-felsic magmas (<800 degrees C). We perform thermodynamic modelling to demonstrate that such low temperature magma differentiation can be readily explained by elevated water concentrations (>6 wt.%) of porphyry Cu magmas at depth which displaces the liquidus to lower temperatures. By integrating this with the latest zircon solubility models, we show that wet magma differentiation leads to zircon appearing as a relatively early crystallising phase at low temperature. Our study therefore provides new evidence that magmas related to porphyry Cu deposits contain characteristically high water contents (>6 wt.%).