Igneous rocks related to porphyry Cu‐Au mineralization at the Dizon mine, Philippines

Abstract

The Dizon Au‐rich porphyry Cu deposit, 0.67 Mt Cu and 174 t Au, is hosted by diorite and andesite porphyry intrusions, dated at ~2.5 Ma. Amphibole and Fe‐Ti oxides in relatively unaltered rocks were used to evaluate the magma conditions of intrusions. Parental magma for diorite porphyry was ~950°C at a depth of ~15 km, whereas the parental magmas for the andesite porphyry had lower temperatures, 760–820°C at a depth of ~5 km. The deposit formed at the locus of multiple intrusions, with evidence for injections of hot mafic magmas, including destabilization texture of plagioclase phenocrysts. Parental magmas at Dizon were oxidized, above FMQ + 2.0, and water‐rich, >5 wt%, comparable to conditions of many large porphyry deposits elsewhere in the world. The occurrence of thick opacitic bands of amphibole in the diorite porphyry at Dizon reflects the release of aqueous fluids from the magma; such magmatic fluids were likely responsible for magmatic hydrothermal mineralization of the Dizon porphyry deposit. Subduction of the Scarborough Seamount caused a compressive regime in the overlying plate, which likely contributed to favourable tectonic conditions for mineralization.