Redox states and genesis of magmas associated with intra-continental porphyry Cu–Au mineralization within the Jinshajiang–Red River alkaline igneous belt, SW China

Abstract

The Jinshajiang–Red River alkaline igneous belt and the associated Cenozoic Cu–Au mineralization are located in an intra-continental strike-slip fault zone in SW China. The Ce4+/Ce3+ ratios of zircon from representative Cu-mineralized, Au-mineralized and barren porphyry intrusions from the belt indicate that the Cu–Au ore-bearing porphyry intrusions had much higher fO2 of magma than the barren porphyry intrusions. Elemental and Sr–Nd isotope ratio data indicate that both the Cu–Au ore-bearing and barren porphyry intrusions were derived from partial melts of the ancient enriched metasomatized mantle sources (EMII type). The mantle source was possibly modified by subduction of the Paleo-Tethyan oceanic slab beneath the Changdu–Simao block between the early Permian and the late Triassic. The oxygen fugacity of the magma was likely related to the redox state of the source, and the different fO2 calculated for the magmas that gave rise to Cu–Au ore-bearing and barren porphyry intrusions are a product of magmas from the different sources. The sources of the barren porphyry intrusions were influenced mainly by the slab-derived fluids, whereas the sources of the Cu–Au ore-bearing porphyry intrusions were modified by both the slab-derived fluids and slab-derived melts. Cenozoic strike-slip faulting in this region caused lithospheric-scale extension and upwelling of the asthenosphere; the heat produced by this process produced partial melts of the ancient enriched metasomatized mantle sources, resulting in the emplacement of alkaline porphyry intrusions and associated Cu–Au mineralization at ~40–30Ma.