Probing amagmatic fluids in porphyry deposits via a mono- intrusion system at Tongchang (Dexing, SE China)

Abstract

Abstract Porphyry deposits contain large amounts of base metals, and bear important fingerprints of magmatic and hydrothermal processes in convergent and collided plate margins. For decades, source of amagmatic fluid and its role in sulfide deposition in porphyry systems has been equivocal. The Tongchang mono-intrusion porphyry deposit is studied for fluid sources and depositional mechanisms. In-situ quartz O and anhydrite Sr isotope analyses revealed a coexistence of magmatic and amagmatic (groundwater and residue metamorphic) fluid reservoirs, spatially separated by an impermeable zone formed by quartz deposition near brittle-ductile transition in phyllite. The impermeable zone sealed inside amagmatic components. The early and early-intermediate fluid stages were dominated by magmatic fluids forming high-T disseminated Cu sulfides (>400 °C). The late-intermediate stage was marked by retreating of the brittle-ductile interface and creation of an intermediate zone below the impermeable quartz seal. Rupturing of the intermediate zone at 350 °C to 300 °C caused incursion of the sealed amagmatic fluids leading to deposition of vein-type Cu sulfides. The late fluid stage was characterized by massive invasion of evolved groundwater and ore remobilization in response to volumetric contraction of the fluid system. Four universal models are envisaged to account for variabilities in amagmatic availability and thermal histories in porphyry deposits.