Mafic magma injection as a driver for the formation of Au-rich porphyry–hydrothermal systems in thickened crustal setting

• The Yao’an deposit represents Au mineralization from oxidized magmas in thick crust. • Felsic magmas experienced early sulfide saturation and strong oxidation, losing chalcophiles. • Gold analyses reveal the limited metallogenic potential of felsic magmas. • Late mafic potassic dikes induced sulfide recycling, enhancing Au enrichment. Gold-rich mineralization in porphyry systems, including porphyry-type and related hydrothermal deposits, generally requires Au-enriched magmas, followed by efficient hydrothermal extraction and precipitation. While moderately oxidized magmas with delayed sulfide saturation are typically favorable in thin-crust settings, the Yao’an Au–polymetallic deposit, associated with Cenozoic alkaline magmatism at the western margin of the Yangtze Craton, represents a distinctive exception. The orebody morphology and alteration zoning suggest mineralization of this deposit is characteristic of a hydrothermal vein-type rather than a typical porphyry system. Geochemical evidence from this study shows that the felsic porphyries in the deposit underwent early sulfide saturation, while Fe isotope and zircon oxybarometry results indicate that they were characterized by high oxidation states (δ 56 Fe = 0.14–0.59 ‰; ΔFMQ + 2.89 to +3.41). Such conditions were unfavorable for Au enrichment, consistent with the low and stable Au contents (3.11–4.61 ppb) in the peripheral intrusions. Exsolved fluids from these magmas produced specularite-dominated orebodies but likely hindered effective chalcophile metal retention. Thus, this magmatic–hydrothermal stage was unlikely to solely form an economic deposit. Instead, the subsequent emplacement of mafic ultrapotassic dikes likely played an important role in facilitating further Au enrichment. Mineralogical and geochemical evidence indicates they became sulfide-saturated and host elevated Au (up to 64.1 ppb). They also display light Fe isotope signatures (δ 56 Fe = 0.05–0.07 ‰), implying a relatively low oxidation state. We propose that these mafic magmas intruded into oxidized felsic porphyries, triggering sulfide breakdown, metal remobilization, and promoting further Au concentration in the orebodies