EVOLUTION OF AN INTRUSION-CENTERED HYDROTHERMAL SYSTEM - FAR SOUTHEAST-LEPANTO PORPHYRY AND EPITHERMAL CU-AU DEPOSITS, PHILIPPINES

There are many examples of spatially associated porphyry and epitherma l ore deposits; a genetic connection has been suggested for some and a rgued against for others. Nowhere is this spatial association better d emonstrated than in the Mankayan district of northern Luzon, Philippin es, where the Lepanto high-sulfidation epithermal Cu-Au deposit is sup eradjacent to the Far Southeast porphyry Cu-Au orebody; together they contain >3.8 million tons (Mt) Cu and >550 t Au.Quartz diorite porphyr y dikes intruded Miocene basement rocks of metavolcanic and volcanicla stic rocks to a 300-m elevation. These intrusions postdate the Pliocen e volcanic breccia and dacite porphyry that host much of the epitherma l ore. K silicate alteration, consisting of biotite-magnetite and mino r K feldspar, is centered on the quartz diorite porphyry. K-Ar ages of the biotite are 1.41 +/- 0.05 Ma (n = 6). Vitreous, anhedral quartz v eins are associated with this early alteration and contain vapor-rich and hypersaline liquid inclusions with maximum homogenization temperat ures of 450 degrees to 550 degrees C land 50-55 wt % NaCl equiv salini ties). Lithostatic pressure estimates indicate a paleosurface at a gre ater than or equal to 1,500-m elevation. Advanced argillic alteration formed over the top of the porphyry and consists of quartz-alunite, da ted at 1.42 +/- 0.08 Ma (n = 5), synchronous with K silicate alteratio n. The lower limit of extensive quartz-alunite alteration is at a appr oximate to 600-m elevation. Similar alteration and a core of leached, silicic alteration extend northwestward >4 km along the basement dacit e contact, localized by the Lepanto fault. Chemical and S isotope zoni ng of alunite along strike indicates progressively lower temperatures away from the porphyry, from 350 degrees to 200 degrees C. K silicate alteration is overprinted by alteration consisting of chlorite plus he matite and/or sericite-illite, with a marginal zone containing pyrophy llite and an outer zone of propylitic alteration. The chlorite-sericit e alteration is cut by veins of euhedral quartz that locally fill reop ened anhedral quartz veins. The euhedral quartz veins contain anhydrit e-white mica-pyrite +/- chalcopyrite +/- bornite and have halos of ser icite; illite separated from these halos has ages of 1.30 +/- 0.07 Ma (n = 10). Fluid inclusions provide evidence for boiling on inception o f this fracturing event (T-h = 350 degrees C, 5 wt % NaCl equiv) and i ndicate a depth of 1,300 to 2,000 m below the paleowater table. This b rittle-fracture event was followed by cooling and dilution of the hydr othermal fluid. The elevation of the enargite Au epithermal ore and it s host of silicic alteration increases as the unconformity between til e basement and dacite breccia rises from a 700- to 1,200-m elevation w ith increasing distance from the porphyry. Published data on enargite- hosted fluid inclusions (T-h = 295 degrees-200 degrees C, 4-2 wt % NaC l equiv! indicate that the temperature and salinity both decrease with increasing distance from the porphyry. Epithermal ore consists of sta ge 1 euhedral pyrite-enargite-luzonite, and subsequent stage 2 Au is a ccompanied by tetrahedrite-chalcoplrite-sphalerite plus telluride and selenide minerals. Anhydrite and barite gangue minerals are followed b y late vug-fulling quartz and kandite minerals. The quartz alunite alt eration halo passes outward to ''kandite'' (kaolinite-nacrite-dickite) alteration, then to chlorite or montmorillonite, depending on the hos t rock (basement or dacite, respectively).The dated minerals were also analyzed for their delta(18)O and SD compositions, and their associat ed hydrothermal water values were calculated. Water in isotopic equili brium with biotite averaged +6.3 and -45 per mil, respectively, typica l of hypersaline liquid exsolved from felsic magma. The acidic water t hat deposited the alunite formed when magmatic vapor (+7 parts per tho usand delta(18)O and -25 parts per thousand delta D) was absorbed by l ocal meteoric water (-10 parts per thousand delta(18)O and -70 parts p er thousand delta D) in a proportion of approximate to 9:1 magmatic to meteoric. Lateral Bow to the northwest and progressive mixing with gr ound water diluted the magmatic component to 1:1 at a distance of 4 km from the porphyry. At the depth of the porphyry deposit, the later wa ter isotopically stable with sericite was dominantly magmatic (+5.7 pa rts per thousand delta(18)O and -43 parts per thousand delta D) in the core. The marginal sericitic alteration (+1.5 parts per thousand delt a(18)O and -51 parts per thousand delta D water values) indicates a ma ximum 20 to 30 percent component of local meteoric water. Pyrophyllite in both the porphyry and epithermal deposits formed from water with a n isotopic composition similar to that which formed the sericitic alte ration. The late euhedral quartz veining and sericitic alteration appe ar to have been associated with the majority of Cu and Au deposition. In addition, mineralogic, paragenetic, isotopic, and fluid inclusion e vidence suggests that this water precipitated the enargite and Au with in the epithermal deposit. Our results reinforce guidelines for explor ation of such deposits. Advanced argillic (quartz-alunite) and K silic ate alteration at Lepanto-Far Southeast are coupled in origin and resu lt from vapor and hypersaline liquid separation. Thus, exploration pro grams for buried porphyry deposits should document carefully the geolo gic, morphologic, and temporal characteristics of exposed areas of adv anced argillic alteration and its origin. Sericitic alteration at Far Southeast is associated with porphyry Cu and Au ore and appears to rep resent the roots of the main-stage Cu-Au mineralization in the epither mal deposit, hosted by silicic and quartz-alunite alteration that has a lower limit near the top of porphyry Cu-Au ore. In some cases, the s ericitic overprint of a porphyry system, particularly where it is rela ted to Cu and Au enrichment, may indicate a potential for nearby epith ermal mineralization. Similarly, sericite and/or pyrophyllite underlyi ng or overprinting a zone of hypogene advanced argillic (quartz-alunit e) alteration indicates that mineralizing fluid may have ascended to e pithermal depths. Epithermal ore at Lepanto-Far Southeast reflects a p aleohydrologic regime dominated by lateral fluid flow, with a marked c ontrol by intersection of the Lepanto fault and a lithologic unconform ity. Recognizing evidence for lateral flow is critical, as p