EPITHERMAL COLD MINERALIZATION, ACUPAN, BAGUIO DISTRICT, PHILIPPINES - GEOLOGY, MINERALIZATION, ALTERATION AND THE THERMOCHEMICAL ENVIRONMENT OF ORE DEPOSITION

A large gold-rich adularia-sericite-style epithermal vein system forme d at 0.65 Ma at Acupan, Baguio district, Philippines, during a period of high K magmatism Low-salinity (approximate to 0.5 nit % NaCl equiv) , gassy (up to 0.41 m CO2) meteoric fluids with temperatures of up to 300 degrees C or more were focused into ct series of steeply dipping i nterconnected faults and joints ill the Virac granodiorite, Balatoc di atreme, Lucbuban gabbro, and Zig-Zag Formation. Prolonged and varied g eothermal activity resulted in the formation of a series of composite banded epithermal veins with an average width of 1 m that extend to de pths of at least 1 km (relative to tile present-day surface). The epit hermal event was characteried by periods of hydrothermal brecciation i nterspersed with more quiescent periods when delicate to coarsely band ed vein material was precipitated. Five distinct mineral assemblages h ave been defined in the epithermal veins: type A (characterized by fin e-grained chalcedony), type B (fine-grained gray quartz), type C (coar se-grained white quartz), type D (coarse-grained calcite), and type E (coarse-grained anhydrite). Electrum and Au-Ag tellurides have been re cognized in type B, C, and D bands. Interaction of ascending chloride brines with die wall rocks over the lifetime of the system produced in tense, symmetrical, meterwide halos of altered rock characterized by s ericitic, silicic, and sericite-chlorite mineral assemblages that grad e 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((O2)) between -2 8 and -33, log f((H2)) between -1.5 and -3.7, and log f((H2S)) between -0.6 and -1.8 at 300 degrees C. Abundant quartz-K mica-pyrite alterat ion and vein mineralization are consistent with silica-saturated, redu ced, weakly acidic to weakly alkaline fluid compositions. Boiling caus ed the fluids to evolve to more alkaline conditions, leading to the sp oradic precipitation of adularia in the veins and altered wall rocks. Dissolved CO2 concentrations were sufficiently high (up to 0.41 m) to stabilize calcite at tile expense of calc-silicate minerals. Mg2+ conc entrations were low in the mineralizing fluids but increased during di ssolution of primary hornblende and biotite in the country rocks, resu lting in the development of a sericite-chlorite alteration assemblage in the deep mine levels. H2S and H2CO3 were the predominant sulfur- an d carbon-bearing species, and the low-salinity, gassy nature of the fl uids favored gold and silver transport as bisulfide complexes. H2Te an d HTe- 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, a nd log f((Te2)) ranged between -7.8 and -8.6 at 300 degrees C during p eriods of hessite deposition. Magmatic contributions also account for the CO2- and H2S-rich nature of the fluids and for magmatic helium iso tope signatures.