From low-H2O parental magmas to ore-forming porphyry systems: magmatic water enrichment and high oxygen fugacity as key controls in the Sanjiang metallogenic belt

• Ore and barren porphyries are cogenetic but differently evolved, forming a sequence. • Ore-forming porphyries are wet to super-wet and gain H 2 O during evolution. • Porphyry Cu favors wet, oxidized magmas nd hydrous but reduced ones are barren. Wet (>4 wt% H 2 O) or super-wet (>6 wt% H 2 O) magmas are widely considered to be necessary for the formation of porphyry copper deposits (PCDs). However, whether their high H 2 O contents are mainly inherited from the source region or acquired during magmatic evolution remains debated. To address this question, we focus on one of the world’s largest post-collisional porphyry copper belts, the Sanjiang metallogenic belt in southwestern China. A comparative study of zircon geochemistry and whole-rock compositions from five ore-forming and six barren porphyries shows that all porphyries have similar ages (ca. 39–32 Ma), and that their zircon compositions define a regional evolutionary array. With increasing degrees of magmatic evolution, melt H 2 O contents calculated from zircon, together with zircon Yb/Dy, (Ce/Nd)/Y and (Eu/Eu*)/(Dy N /Yb N ), increase systematically, indicating progressive enrichment of water during differentiation. Our calculations show that some barren porphyries (e.g., Yangshizi and Bailiancun) have low H 2 O contents (2.38–4.19 wt%), interpreted as relatively less evolved end-members of the magmatic system. Other barren porphyries (e.g., Shigu and Weishan) are H 2 O-rich (7.06–7.25 wt%) but have low oxygen fugacity (ΔFMQ = –0.10 to +0.72). In contrast, the ore-forming porphyries are characterized by both high H 2 O contents (6.17–10.68 wt%) and high oxygen fugacity (ΔFMQ = +1.45 to +1.64), implying that high oxygen fugacity is required for mineralization in wet magmas. Furthermore, the ore-forming porphyries exhibit generally lower zircon crystallization temperatures (667–708 °C), notably high zircon Hf contents (∼9867–11634 ppm), and markedly lower Eu/Eu* ratios (0.55–0.75), compared with the broader temperature range (675–819 °C) and Eu/Eu* values (0.62–0.90) of the barren porphyries. These features indicate that the ore-forming magmas experienced more complex and advanced histories of magmatic evolution and crystal–melt segregation. Taken together, our results suggest that the initial H 2 O contents of ore-forming magmas may not have been particularly high (<4 wt%), but that they became progressively enriched in H 2 O through protracted evolution to form wet to super-wet magmas. In such systems, the combination of elevated magmatic H 2 O contents and high oxygen fugacity appears to be especially favorable for the development of porphyry copper mineralization.