Hydrogen and oxygen isotopic constraints on hydrothermal fluid composition from the Pliocene Boyongan and Bayugo porphyry copper-gold deposits (southern Philippines ) reveal mixing of magmatic and sea water reservoirs as the hydrothermal system evolved. Magmatic-equilibrated fluids (d18O: +8.3 to +9.1; dD: -61 to -51) are represented by hypersaline brines implicated in copper-gold transport and deposition. These dominate the fluid inclusion assemblage in early comb-quartz (unidirectional solidification textures), miarolitic quartz, and in K-silicate-stage quartz-K-feldspar_x000D_
biotite veins. Fluids responsible for post-K-silicate alteration assemblages of pyrite-clay (illite, pyrophyllite and kaolinite), alunite and smectite are represented by low-moderate salinity two-phase inclusions in co-existing quartz._x000D_
These are interpreted as mixtures of evolved magmatic fluids (d18O dD: -42 to -20) with evolved Pliocene sea water (d18O: + 4.0; dD: +4). The isotopic evidence for sea water involvement in the hydrothermal system suggests that porphyry emplacement and associated diatreme formation could have occurred in a submarine setting._x000D_
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