Underplating of Hydrous Mantle Magma Controls Cu–Au Mineralization at the Shaxi Deposit in the Middle-Lower Yangtze River Metallogenic Belt, China

Abstract The Middle-Lower Yangtze River Metallogenic Belt, a major copper (Cu) and gold (Au) province in eastern China, experienced extensive late Mesozoic magmatism accompanied by porphyry(-skarn) mineralization. However, the nature of the ore-forming magma (e.g. magma oxidation state and melt H2O content) and the deep magmatic processes that controlled Cu–Au mineralization remain poorly constrained. In this study, we employed an integrated approach using whole-rock and zircon geochemistry to investigate the petrogenesis and metallogenic fertility of the Shaxi ore-forming porphyries. Three porphyry lithotypes (M-QDP, F-QDP, and HDP) shared a similar parental magma, underwent varying degrees of mixing, and crystallized at ca. 130 Ma. They exhibit moderately enriched Nd-Hf isotopic signatures (εNd(t) = −5.86 to −3.07; εHf(t) = −4.60 to −1.83), and highly radiogenic 206Pb/204Pb(t) ratios (17.76–18.29), indicating derivation from the Neoproterozoic lower crust that was hybridized with coeval enriched mantle-derived melts in shallow magma chambers. Inherited zircon in the Shaxi porphyries shows low δ18O values (1.10‰–5.43‰) similar to those of Neoproterozoic felsic rocks along the northern and western Yangtze Block, indicating that the lower crust likely originated from Neoproterozoic arc magma underplated at the base of the crust. Zircon oxybarometry and hydrometry indicate that the Shaxi porphyries crystallized from highly oxidized (M-QDP, ΔFMQ = −0.41 ± 0.79; F-QDP, ΔFMQ = 1.96 ± 0.31; HDP, ΔFMQ = 0.61 ± 0.39) and water-rich magmas (melt H2O contents: M-QDP, 7.56 ± 0.35 wt %; F-QDP, 10.25 ± 0.71 wt %; HDP, 4.84 ± 0.28 wt %). The crustal source, although not sampled, is inferred to have been moderately dry (melt H2O content: 3.44 ± 0.92 wt %), Cu-fertile (average = 5.0 μg/g in zircon), and oxidized (ΔFMQ = 1.87 ± 2.04), while mantle-derived magmas from the Shuangmiao Formation were hydrous (melt H2O content: 7.39 ± 0.95 wt %) and oxidized (ΔFMQ = 1.53 ± 1.59). We suggest that mafic magmas, which are stored and undergo high-pressure fractionation at the mantle–crust boundary, have increased melt H2O contents. The resulting hot, oxidized, hydrous basaltic magmas then transferred volatiles into the Neoproterozoic lower crust, triggering water-fluxed melting of the juvenile mafic lower continental crust (LCC) and generating adakite-like melts. The elevated melt H2O contents and fO2 further facilitated Cu–Au release from the Cu-enriched LCC, producing fertile porphyry magmas. We therefore conclude that during the late Mesozoic, underplating of hot, oxidized, and hydrous mantle-derived basaltic magmas triggered melting in the juvenile lower crust through fluid-fluxed processes. This underplating-induced melting of the Neoproterozoic lower crust represents the key process controlling the formation of the Shaxi porphyry Cu–Au deposit and, more broadly, the Cu–Au mineralization in the Middle-Lower Yangtze River Metallogenic Belt following a paleo-Pacific flat-slab subduction tectonic setting.