Determining the impact of magma water contents on porphyry Cu fertility: Constraints from hydrous and nominally anhydrous mineral analyses

Most large-scale porphyry-style mineralization is genetically associated with oxidized, sulfur-rich, and hydrous magmas, the sources of which were enriched in mantle-derived or juvenile crust−derived materials. Geochemical indices for these features can distinguish fertile magmas and barren magmas. However, fertile magmas that generate different-sized porphyry deposits are usually geochemically indistinguishable, and the factors controlling the size of porphyry mineralization are poorly understood. Here, we used zircon H2O contents and the compositions of apatite hosted in zircon, phenocrysts, and groundmass to compare the water contents and volatile evolution of fertile magmas in the Yulong ore belt, Tibet, which generated giant-, large-, and medium-sized porphyry deposits. We found that these porphyries have comparable major- and trace-element and zircon Hf-O isotope compositions, with high oxygen fugacity and sulfur contents. A negative correlation between [see PDF for equation] versus [see PDF for equation] of apatites suggests that the magmas consistently achieved H2O saturation before zircon crystallization, and zircon hydroxyl contents reveal that the water contents of the melt positively correlate with the size of the deposits, consistent with larger deposits achieving H2O saturation at deeper crustal levels. The deeper H2O saturation also caused larger amounts of Cl extraction, as indicated by the trends of increasing [see PDF for equation] and decreasing [see PDF for equation] with larger deposit sizes. These data sets can be reconciled by magmas forming larger deposits having higher H2O contents and suggest that the H2O content is a key control on their fertility. Our study highlights the utility of integrated hydrous and nominally anhydrous mineral analyses for constraining enigmatic magmatic volatile processes in magmatic ore-forming systems.