Fluid exsolution and degassing pathways control post-subduction porphyry ore system

Abstract

Porphyry Cu ore-forming arc segments have been shown to have distinct magma compositions in contrast to segments that do not host porphyry Cu deposits. However, within some fertile magmatic belts, such as the Miocene Gangdese porphyry copper belt (Tibet), contemporaneous intrusions can either be fertile or barren despite similar post-subduction settings and ages. This study systematically compiled zircon and apatite composition data from the Gangdese belt, with a detailed focus on the Miocene barren quartz monzonite porphyry (~14.7 Ma) and the fertile rhyodacite porphyry (~13.0 Ma) from the Gangjiang deposit. By analyzing resistant minerals like zircon, titanite, and their included apatite, we investigated the differences and similarities in key parameters such as temperature, oxygen fugacity, and water content between the fertile and barren systems. The results show that both systems have similarly high oxygen fugacity (ΔFMQ ≈ +1 to +3), high water content (>4 wt.%), and similar initial magma temperatures (average Ti-in-zircon temperature: fertile intrusions ~705°C; barren intrusions ~707°C). Crucially, the estimated initial melt chlorine content, based on apatite inclusions, is also similar (barren melt initial Cl: ~500–600 ppm; mineralizing magmas initial Cl: ~400–500 ppm). However, the key difference lies in the efficiency of fluid exsolution and degassing. In the barren quartz monzonite porphyry, the melt Cl content remained stable (~500 ppm) during cooling, suggesting formation in a predominantly volatile unsaturated system. In contrast, in the fertile rhyodacite porphyry, the melt Cl content decreased significantly from ~400 ppm to ~50 ppm as temperature declines indicating early fluid saturation. Numerical simulations further support that the formation of fluid channel networks during second boiling in the magma system accelerated fluid exsolution and degassing. The efficient fluid degassing allowed large volumes of Cu-charged hydrothermal fluids to be released, thereby forming a large Cu mineralizing hydrothermal system. Our study indicates that the emplacement of oxidized, wet magmas is not the only pre-requisite to porphyry Cu deposit formation, but efficient exsolution and degassing of fluids from mature magma reservoir is additionally required.