EPITHERMAL AND SUBJACENT PORPHYRY MINERALIZATION, ACUPAN, BAGUIO DISTRICT, PHILIPPINES - A FLUID-INCLUSION AND PARAGENETIC STUDY

The Acupan gold-silver deposit is a major precious-metal producer in the Baguio gold district, Northern Luzon, Philippines. Paragenetic and fluid-inclusion studies have confirmed the presence of two contrasting hydrothermal fluid types, each characterized by distinctive fluid-inclusion populations, ore and gangue associations and alteration assemblages. Mineralization related to both porphyry and epithermal ore-forming events are recognized. The spatial proximity of magmatic-hydrothermal to epithermal mineralization has not previously been described from an adularia-sericite deposit such as Acupan. Two vein stages of porphyry origin (stages I and II) are related to emplacement of a subvolcanic porphyry dacite. Stage I (quartz-magnetite-chalcopyrite) veinlets are associated with K-silicate alteration. Biotite, magnetite and anhydrite are the dominant alteration minerals. 'White sericite' and propylitic alteration halos envelop stage II (anhydrite-quartz-pyrite-chlorite) veinlets, which cross-cut the earlier stage I mineralization. The association K-mica-quartz-pyrite-anhydrite-chlorite typifies the white sericite assemblage. Propylitic alteration is dominated by chlorite-pyrite-anhydrite; epidote is notably absent. Paragenetic stages III to V of epithermal character occur together as veins up to one meter wide and are superimposed on porphyry-style mineralization. The highest precious metal concentrations are present in fine-grained 'grey quartz' (stage III), recognized predominantly along vein margins in the upper levels of the deposit and as clasts enclosed by later paragenetic stages. Fragments of altered wall rock are hosted by stage III gangue. Stage IVa mineralization is coarse-grained and typified by 'white quartz' together with K-mica, adularia, rhodochrosite and mangano-calcite. Opaque minerals include pyrite, sphalerite, chalcopyrite, galena, free gold and gold-silver tellurides. Vuggy, colloform and brecciation textures are all recognized in stage IVa gangue. Later-stage IVb gangue infills central portions of the veins and is dominated by coarse-grained calcite and quartz. Stage V (anhydrite-pyrite-quartz) veins reopen and crosscut stages III and IV. Stage IVb and V veinlets develop locally intense propylitic envelops where they are isolated from the larger veins. Sericitic alteration (K-mica-quartz-pyrite-illite) is in general symmetrically disposed about the epithermal veins in envelopes up to a meter wide, and is surrounded by a large, weakly altered propylitic zone characterized by the mineral association epidote-chlorite-pyrite-carbonate-magnetite. Fluid-inclusion studies reveal that stage I veinlets were deposited from hypersaline (34 to over 74 eq. wt. % NaCl) magmatic fluids. Homogenization temperatures (TH) ranged from 350°C to in excess of 600°C. Stage II was deposited from saline fluids (31-63 eq. wt. % NaCl), and TH measurements from 300°C to in excess of 600°C were observed. Entrapment temperatures for stage III fluids were between 240°C and 320°C, with salinities from 0 to 2.6 eq. wt. % NaCl. Stage IVa formed from low-salinity fluids (0-5.1 eq. wt. % NaCl) at temperatures between 220°C and 310°C. Deposition of stage IVb occurred between 180°C and 280°C from low-salinity fluids (0-1.7 eq. wt. % NaCl). Temperatures declined approximately 60°C from the beginning of stage III deposition to the end of stage IVb. A late resurgence in temperatures is present during anhydrite deposition (stage V), with TH between 214°C and 299°C and salinities from 0.2 to 3.4 eq. wt. % NaCl. Magmatic-hydrothermal fluids exsolved during crystallization of the subvolcanic porphyry dacite produced stage I and II mineralization. A co-existing magmatic vapor generated by phase separation was ubiquitous throughout these stages. Epithermal mineralization commenced at least 1 Ma after stage II, with near-surface rupturing of a hydrothermal seal producing hydrothermal breccias, widespread boiling and gold mineralization (stage III). Later self-seal/rupture cycles and minor hydrothermal brecciation produced apparent paragenetic reversals of stage IVa gangue sealed by stage III, and stage IVb gangue sealed by IVa. Boiling, recognized at greater depths in stage IVa gangue, was more restricted in extent than for stage III. Gold deposition below the boiling zone in stage IVa was localized at structural intersections and controlled by pressure fluctuations. Stage IVb may have been deposited in response to boiling and/or fluid mixing. Temperature increase and/or fluid mixing are the likely mechanisms for stage V deposition. © 1990.