Geologic and Stable Isotope Evidence for Submarine Diatreme Formation, and Emplacement of the Boyongan and Bayugo Porphyry Copper-Gold Deposits, Philippines

摘要

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 (?~(18)O: +8.3‰ to +9.1‰; ?D: -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 +- 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. These are interpreted as mixtures of evolved magmatic fluids (?~(18)O: +7.6 ?o +10.6 ; ?D: -42‰ to -20‰) with evolved Pliocene sea water (?~(18)O: + 4.0‰; ?D: +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.