The importance of H2O in arc magmas for the formation of porphyry Cu deposits

Abstract

Porphyry copper deposits (PCDs) hosted in subvolcanic intrusions at convergent margins are the primary world's copper resources. However, the set of magmatic processes that lead to the generation of ore-bearing magmatic provinces remains unclear. In this paper we review the systematic of Cu evolution during arc magma differentiation using new and existing global compilations of whole rock geochemistry data. We trace the Cu evolution from primitive arc magmas through lower crustal plutonic to volcanic rocks. We focus on the well-known tholeiitic and calc-alkaline fractionation sequences, where arc tholeiitic series represents damp primitive melts (<2 wt% H₂O) evolving with iron enrichment, and calc-alkaline series are wet primitive melts (>2 wt% H₂O) that differentiate with iron depletion. Our study shows that the Cu concentration in primitive arc basalts (~80 ppm) is indistinguishable from that of primitive melts formed at mid-ocean ridges (MORBs) implying that Cu is mainly sourced from the mantle wedge in arcs with a limited contribution from the subducted oceanic lithosphere. A global compilation of plutonic rocks whole rock geochemistry (lower crustal cumulates and derivative melts) indicate no systematic difference in Cu concentrations between cumulates associated with tholeiitic or calc-alkaline series. Yet a complementary global compilation of arc volcanic whole rock geochemistry highlights the contrasting behavior of Cu in tholeiitic and calc-alkaline series during magmatic differentiation. In tholeiitic arc series, Cu shows an incompatible or compatible behavior during magma differentiation influenced by the crustal thickness. In calk-alkaline arc series, Cu is compatible during magma differentiation independently to the crustal thickness. This relates to the timing of sulfide saturation, which is controlled by the liquid lines of descent (LLD) and/or crustal thickness at redox conditions relevant for arc magmas. We demonstrate that the initial melt H₂O content in primitive arc melts controls the LLD and the volume of remaining melt mass at fluid saturation. We show that the remaining H₂O-saturated melt mass positively correlates with the total mass of Cu transferred into degassing fluids. The mass of extractable Cu ranges from ~3 to ~10 Mt (i.e., large PCD) for calc-alkaline series, and ranges from ~0.3 to ~2.5 Mt for tholeiitic series. The ore-forming potential of calc-alkaline arc magmas is at least ~4 to ~10 times higher relative to tholeiitic arc magmas. Despite the compatible behavior of Cu during magmatic differentiation, we propose that a single stage model for the formation of large economic PCDs (as opposed to multi-stage model for Cu-sulfides storage and remobilization) is most applicable for the calc-alkaline melts. The importance of the initial melt H₂O content ultimately reflects the key role of flux melting associated with wet calc-alkaline series and high ore-forming potential, in opposition to decompression melting associated with damp tholeiitic series in arc.