A large gold-rich adularia-sericite-style epithermal vein system formed at 0.65 Ma at Acupan, Baguio district, Philippines, during a period of high K magmatism. Low-salinity ([asymp] 0.5 wt % NaCl equiv), gassy (up to 0.41 m CO 2 ) meteoric fluids with temperatures of up to 300 degrees C or more were focused into a series of steeply dipping interconnected faults and joints in the Virac granodiorite, Balatoc diatreme, Lucbuban gabbro, and Zig-Zag Formation. Prolonged and varied geothermal activity resulted in the formation of a series of composite banded epithermal veins with an average width of 1 m that extend to depths of at least 1 km (relative to the present-day surface). The epithermal event was characteried by periods of hydrothermal brecciation interspersed with more quiescent periods when delicate to coarsely banded vein material was precipitated. Five distinct mineral assemblages have been defined in the epithermal veins: type A (characterized by fine-grained chalcedony), type B (fine-grained gray quartz), type C (coarse-grained white quartz), type D (coarse-grained calcite), and type E (coarse-grained anhydrite). Electrum and Au-Ag tellurides have been recognized in type B, C, and D bands. Interaction of ascending chloride brines with the wall rocks over the lifetime of the system produced intense, symmetrical, meterwide halos of altered rock characterized by sericitic, silicic, and sericite-chlorite mineral assemblages that grade out to a weak, diffuse widespread propylitically altered zone. Minor adularia alteration is restricted to the deeper mine levels.Sulfide-oxide-silicate-carbonate phase relationships in the Acupan vein system constrain the mineralizing fluids to values of log f (sub (O 2 )) between -28 and -33, log f (sub (H 2 )) between -1.5 and -3.7, and log log f (sub (H 2 S)) between -0.6 and -1.8 at 300 degrees C. Abundant quartz-K mica-pyrite alteration and vein mineralization are consistent with silica-saturated, reduced, weakly acidic to weakly alkaline fluid compositions. Boiling caused the fluids to evolve to more alkaline conditions, leading to the sporadic precipitation of adularia in the veins and altered wall rocks. Dissolved CO 2 concentrations were sufficiently high (up to 0.41 m) to stabilize calcite at the expense of calc-silicate minerals. Mg (super 2+) concentrations were low in the mineralizing fluids but increased during dissolution of primary hornblende and biotite in the country rocks, resulting in the development of a sericitechlorite alteration assemblage in the deep mine levels. H 2 S and H 2 CO 3 were the predominant sulfur- and carbon-bearing species, and the low-salinity, gassy nature of the fluids favored gold and silver transport as bisulfide complexes. H 2 Te and HTe (super -) were the predominant aqueous tellurium species. Entrainment of minor amounts of magmatic volatiles during type C and D mineralization is responsible for Te enrichment in the dominantly meteoric fluids, and log f (sub (Te 2 )) ranged between -7.8 and -8.6 at 300 degrees C during periods of hessite deposition. Magmatic contributions also account for the CO (sub 2 (super -) ) and H 2 S-rich nature of the fluids and for magmatic helium isotope signatures.