Advanced Argillic Alteration and Geochemistry of Alunite. in an Evolving Hydrothermal System at Baguio, Northern Luzon, Philippines

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1 RESOURCE GEOLOGY, 43(3), 155 `164, 1993 Advanced Argillic Alteration and Geochemistry of Alunite in an Evolving Hydrothermal System at Baguio, Northern Luzon, Philippines Masahiro AOKI, Elveta C. COMSTI, Florinio B. LAZO Yukihiro MATSUHISA Abstract: Acid alteration related to epithermal Au mineralization in the Baguio district, northern Luzon, Philippines, reflects the nature of the past hydrothermal activity. The acid alteration is characterized by an assemblage of pyrophyllite, diaspore and dickite, sometimes with strongly zoned alunite solution. This style of alteration is associated with a large zone of leached rock, mainly residual silica now recrystallized to quartz. The alunite has cores rich in Ba, Sr, and PO4; together with 34S values of +15 to +24 permil, these data suggest formation a high temperature fluid which contained reactive magmatic components such as H2SO4 and HCl. The extensive ridge-forming residual silica and surrounding advanced argillic alteration are underlain by district-wide propylitic alteration and Au-bearing quartz veins. Some of the veins have halos of acid alteration containing quartz, sericite and pyrophyllite, with or without alunite, diaspore and dickite. The pervasive acid alteration formed about Ma, 1 m.y. after the intrusion of microdiorite in the district. At about 0.6 Ma, quartz veins formed a subsequent hydrothermal system. The neutral ph waters of this system may have had a local input of magmatic acid components, particularly in the vicinity of the. Kelly vein system. This younger system was responsible for epithermal Au mineralization in the district. Introduction Epithermal precious metal mineralization is well developed in the Baguio district of northern Luzon, Philippines, with over 800 tonnes of Au produced by 1990 (MITCHELL and LEACH, 1991). Acid alteration is common throughout the district (COMSTI et al., 1990), and has a close spatial relationship to the ore deposits (Fig. 1). We examined the nature of the acid alteration several locations throughout the district in order to better understand its origins and relationship to mineralization. Acid fluids form in two distinct environments in the earth's crust. In a volcanic setting, condensation of high-temperature magmatic Received on March 1, 1993, accepted on April 15, 1993 * Mineral Resources Department, Geological Survey of Japan, Higashi, Tsukuba 305, Japan ** Mi nes and Geosciences Bureau, North Avenue, Diliman, Quezon City, Philippines Keywords : Baguio, Magmatic hydrothermal system, Advanced argillic alteration, Epithermal gold mineralization, Alunite, K-Ar age, Sulfur isotopic composition. vapors containing SO2 and HCl form acid chloride-sulfate waters with a ph<2 (e.g., GIGGENBACH and SHEPPARD, 1989; SHINOHARA et al., 1993). These highly acid waters may have a high temperature, in excess of 300 Ž (REYES and GIGGENBACH, 1992), and leach the rock of most components (STEVEN and RATTE, 1960). This leaching leaves a vuggy silica residue characteristic of high sulfidation alteration (STOFFREGEN, 1987), similar to that in the Nansatsu district (URASHIMA et al., 1981; HEDENQUIST et al., 1993). The only other nonweathering environment of acid fluid formation occurs in the shallow depths of geothermal systems, in zones of steam-heating (SCHOEN et al., 1974). Vapor boiled deep, neutral ph water will contain H2S and other gases. As the vapor condenses into groundwater within the vadose zone, H2S oxidizes to sulfuric acid. These steamheated acid sulfate waters attain a minimum ph of about 2 and form a relatively thin cap of acid alteration, with maximum temperature not much above 150 Ž (REYES, 1990; REYES and GIGGENBACH, 1992). 155

2 156M. Aoxt, E. C. COMSTI, F. B. LAZo and Y. MATSUHISARESOURCE GEOLOGY: systems of the Baguio district. Fig. 1 Simplified geologic map of the Baguio district showing geographical distribution of the advanced argillic cap and Au-bearing quartz veins. Solid circles indicate studied sample locations. We have characterized the mineralogy associated with acid alteration a number of localities in the Baguio district. This information and K-Ar dating of some of these minerals, in conjunction with chemical and isotopic data on the composition of alunite -solution, help to constrain the evolution of hydrothermal activity in the district. Direct measurement of the fluid composition and temperature in geothermal systems of the Philippines, coupled with study of alteration minerals associated with neutral ph and acid waters (REYES, 1990; REYES and GIGGENBACH, 1992), assists in the interpretation of altera-tion related to the extinct hydrothermal Geology and Alteration of the District We have used previous studies of the geology, alteration and mineralization of the Baguio district as a basis for our sampling and data interpretation. The geological mapping of the district has recently been updated (FERNANDEZ and DAMASCO, 1979; BALCE et al., 1980; United Nations, 1987a). The Upper Miocene Klondyke Formation consists of a variety of clastic rocks and is widespread throughout the district (Fig. 1). This formation is intruded by a series of microdiorites (the Kelly Diorite) in the vicinity of the Kelly and Baguio Gold deposits. The Kelly Diorite, the Acupan dacite plug and associated breccias (e.g., the GW Breccia at Acupan) are closely related spatially to gold mineralization (SAWKINS et al., 1979; COOKE and BLOOM, 1990). The steeply dipping vein systems of the Acupan and Baguio Gold deposits trend northeast, whereas the Antamok veins trend northwest (Fig. 1). FERNANDEZ and DAMASCO (1979) suggested that these northeast- and northwest-trending vein systems are a conjugate set of fractures resulting an east-west compression in the northern Luzon after the Pliocene. The Kelly and Baco Aubearing quartz veins are also steeply dipping and trend east-west, possibly reflecting a different stress field that responsible for the other conjugate veins systems in the district. Tensional fractures parallel to a horizontal principal stress of compression can be formed where fluid pressure exerts a strong upward stress, as indicated by the linear arrangement of flank volcanoes in Japan

3 43(3), 1993 Advanced argillic alteration and geochemistry of alunite 157 and the Chilean Andes (NAKAMURA, 1977). Similarly, it is possible that the east-west-trending veins of the Kelly deposit reflect an upward force caused by the high fluid pressure due to a proximal intrusive heat source, in addition to the regional east-west compressive stress. There are two types of alteration mineralogy in the district, one indicating alteration by neutral ph fluids, the other by acid fluids. There are also two styles of each type of alteration, pervasive to massive, and vein-controlled. The rocks in the district commonly show a widespread style of propylitic alteration; this is cut by east-west-, northeast- and northwest-trending quartz veins (Fig. 1). The veins consist of quartz, sericite and calcite with minor adularia, and are mineralized with Au. The wallrock alteration related to the veins Table 1 Structural control, mineral assemblage in the veins and wallrock alteration at the major gold deposits in the Baguio area. abbreviations: qz=quartz, adul=adularia, cal=calcite, rc=rhodochrosite, m=rhodonite, an=anhydrite, py=pyrite, sp=sphalerite, gl=galena, cp=chalcopyrite, th=tetrahedrite, tn=tennantite, bo=bomite comprises chlorite, sericite and less frequent smectite (SAWKINS et al. 1979; COMSTI et al. 1990; COOKE and BLOOM, 1990); good examples are seen at vein locations in the Acupan, Kelly and Antamok deposits (Table 1). In contrast to this neutral ph type of alteration, alteration associated with acid fluids tends to be more spatially restricted. There is a large, horse shoe-shaped area of pervasive acid alteration (Fig. 1) characterized by a leached cap, more restricted in extent than the district-wide propylitic alteration. This leached cap is typical of that formed by acid fluids of volcanic origin (STEVEN and RATTE, 1960). Its massive nature forms highrelief ridges, extending the Loakan airport north towards Baguio City, then turning northeast to Mines View Park and southeast to Bua Ridge. At Mines View Park, quartz, alunite solution, pyrite and native sulfur occur in vertical zones of brecciation. The alteration underlying this leached cap consists of pyrophyllite, dickite and diaspore, minerals which indicate a relatively high temperature of formation, 150 to 250 Ž, based on experimental (IMLEY et al.,1980) and geothermal observations (REYES, 1990). Pyrophyllite can form at temperatures as low as about 150 Ž in equilibrium with amorphous silica, though dickite and diaspore coexistence indicate higher temperatures, at least 230 Ž (REYES, 1990) to 280 Ž (HEMLEY et al., 1980). Acid alteration is also associated with veins present in valleys south of Tuding Silica Pit. Here the alteration mineralogy consists of pyrophyllite, dickite, diaspore, kaolinite, sericite and pyrite. The lack of alunite and native sulfur distinguishes these vein halos the massive style of acid alteration. At the Kelly deposit, pyrophyllite, dickite, diaspore and sericite occur in the wallrock adjacent to veins. The occurrence of these hightemperature acid alteration along the E-W trending quartz veins at Kelly also suggests an intrusive body at a proximal depth that provided magmatic acid components. Analytical Procedure For the determination of K-Ar age and sulfur isotope composition, purified alunites, sericitic clays and a bulk rock sample were used. Alunites were hand-picked after crushing and sericitic clays were hydraulically elutriated. A small amount of quartz and pyrophyllite were the only accessory phases in the purified materials. The K-

4 158 M. AOKI, E. C. COMSTI, F. B. LAZO and Y. MATSUHISA RESOURCE GEOLOGY: Ar ages were determined by Teledyne Isotopes Co. Ltd. The constants are: 40K/K=1.167 ~10-2ƒÃatom% used for the age calculation (STEIGER and JAGER, 1977). The sulfur isotope composition of alunites was measured by a Finigan MAT 250 mass spectrometer. Sulfur was extracted the alunite bearing sample by the method of SASAKI et al. (1979). Conversion of Ag2S to SO2 was carried out by the method of FRITZ et al. (1974). and Table 2 K-Ar age data for alteration minerals and igneous rocks the Baguio area. Relation between Alteration and Mineralization The gold concentration in the vuggy silica zone related to the pervasive acid alteration is usually <0.1 ppm (United Nations, 1987b). There are no clear cross-cutting relations between the pervasive acid alteration and the Au-bearing quartz veins. COMSTI et al. (1990) describe two distinct stages of vein style Au mineralization (hereafter are referred as stage-i and -II) at the Kelly deposit. Acid alteration associated with late veins (stage-ii) cuts an earlier quartz vein (stage- I) related to neutral ph type of alteration in an adit of the mine. The pyrophyllite-sericite of the acid alteration formed saline fluids ( `45 eq. wt % NaCl) at temperatures >330 Ž, based on fluid inclusion results for vein quartz (stage-ii), though there is also an indication for some acid fluids having a lower temperature, averaging about 250 Ž. In contrast, the tem-peratures and salinity of fluids associated with the earlier neutral ph quartz-sericite-calcite vein alteration (stage-i) were lower, averaging about 215 Ž and < 1 eq. wt % NaCI (COMSTI et al., 1990). However, the Kelly deposit consists of numerous vein structures, so it is difficult to make a general conclusion about the temporal relation-ship between the two styles of gold mineralization based on the limited observations of cross cutting relations. Nevertheless, both the quartz veins and the acid alteration appear to be controlled by the same overall structure trending east-west (COMSTI et al., 1990). K-Ar dating of alteration minerals was conducted to help determination of the temporal relation between the massive acid alteration caps and the Au-bearing quartz veins. Distinction between the Kelly vein-style acid alteration (stage-ii) and the earlier neutral ph-related quartz veins (stage-i) by radiometric dating was not possible due to the low K contents of minerals and the difficulty in discriminating the two types of alteration in the field. Five samples were dated (Table 2), including alunite related to massive acid alteration the Tuding Silica Pit and the Bua Ridge, clays associated with Au-quartz veins the Kelly and Acupan deposits, and nearly fresh microdiorite (the Kelly Diorite) in the valley north of the Tuding Silica Pit (Fig. 1). The results of our age determination are compiled together with previous age data obtained for igneous rocks in this area (Table 2). The K-Ar ages of alunite in the pervasive acid alteration are 0.9 }0.1 and 1.4 }0.2 Ma, while

5 43(3), 1993 Advanced argillic alteration and geochemistry of alunite 159 pyrophyllite-sericite a quartz vein (stage II) in the Kelly deposit indicates a younger age of <0.6 Ma. Four km southeast of Kelly, Au-bearing quartz veins of the Acupan deposit cut a dacite diatreme (Balatoc plug), the latter dated at 1 Ma (JICA, 1983). Sericite one of these neutral ph-type veins has an age of 0.65 }0.07 Ma, supporting the less reliable date the Kelly vein system. These data indicate nearly a million year evolution of hydrothermal activity related to the formation of diatremes, andesite dikes and breccia pipes, though it is likely the hydrothermal activity was sporadic rather than continuous for this long period. The massive acid alteration appears early, at Ma, temporally related to diatreme formation and the volcanic activity that likely accompanied it. Vein-related Au mineralization followed at about 0.6 Ma, possibly associated with the formation of a district-wide fracture system. The microdiorite, spatially related to much of the mineralization of the district (United Nations, 1987a; COOKE and BLOOM, 1990), is appreciably older, by approximately 1 m.y., than any of the dated alteration (Table 2). Therefore, this intrusion appears genetically unrelated to the hydrothermal activity responsible for the veinstyle Au miner-alization. Chemical and Isotopic Composition of Alunite Alunite is a common mineral associated with acid alteration. It has a wide range of chemical (AOKI, 1983, 1991; STOFFREGEN and ALPERS, 1987, 1992; VELINOV et al., 1991) and isotopic composition (RYE et al., 1992) that provides constraints on its conditions of formation. Hypogene alunite, that formed by ascending acid fluids related to volcanic condensates (RYE et al., 1992), commonly has a large variation in chemical composition one locality, with a variety of growth textures (AOKI, 1991). The sulfur isotopic composition is usually relatively heavy compared with coexisting sulfide minerals (RYE et al., 1992), due to fractionation between sulfide and sulfate in solution at formation temperatures of Ž or higher. The core of hypogene alunite is commonly enriched in PO4 and multi-valent cations such as Ca (crandallite and woodhouseite), Sr (svanbergite), Ba (gor- Fig. 2 X-ray powder diffraction profiles of the (006) and (0012) spacings of alunite solution and minamiite. The presence of asymmetric and multiple peaks indicates wide compositional variations between the three endmembers; alunite, natroalunite, and minamiite. For a better understanding of these complex X-ray diffraction profiles, see the internal growth textures shown in Fig. 3. Specimens of advanced argillically altered rock east of Baguio City. ceixite), Pb (hinsdalite) and REE (florencite) (STOFFREGEN and ALPERS, 1987; AOKI, 1991; VELINOV et al., 1991). These mineralogicallycomplex cores are usually rimmed by minamiite (a Ca-analogue of alunite with doubled c-axis length) and rhythmic bands of alunite solution, alunite and natroalunite (AOKI, 1991). In contrast, alunite formed in a steam-heated acid sulfate environment, overlying boiling, neutral ph geothermal waters, lacks the complex core described above, and has a range of composition limited to the alunite-natroalunite solution series (AOKI, 1983, 1991). The sulfur isotopic composition of alunite formed by steam-heated waters is similar to that of coexisting sulfide minerals due to the relatively low temperature of formation and lack of isotopic equilibration (RYE et al., 1992). Four samples containing alunite were collected the horse shoe-shaped massive alteration zone (Table 2; Fig. 1). The length of the c-axis of alunite solution is a function of its chemical composition, and this may be estimated its basal spacing, determined by X-ray diffraction analysis (XRD). XRD traces for the (006) plane of alunite -solution and (00 12) of minamiite are shown in Figure 2. End member alunite and

6 160 Fig. RESOURCE GEOLOGY: M. AOKI. E. C. COMSTI. F. B. LAzo and Y. MATSUIIISA 3 Scanning image Ba. of alunite Polished View tion thin Park. b. exhibits regularly alunite Note electron (BEI) grey Bua solution, pictures cores Ridge. Wh=Srrepresents solution (medium Numerous and etch 30 Đm on the of a broken habit alunite grey). Ba-bearing (SEI) ratios. within with d. BEI crystals, narrow on the crystal woodhouseite, specimen. The iron surface sharp edges massive the higher Baguio reflectivity which surface is indicate Qz=Quartz, rock as white dissolution Gt=Goethite Mines at Bua Variation solution during Note some of a broken a younger or other iron episode. oxides. silica Sr and at Mines Alunite quartz. c. in brightness of early Ridge. f. SEI Park. electron to enriched residual View the remnants crystals. is due idiomorphic at Loakan. of alunite to be a. Back-scattered surrounding with rock aggregate silicified appears associated are interpreted within oxides of specimen and district. of the cores zone quartz-alunite of a powdery veinlets Interstitial pits cores. alunite-pyrophyllite-dickite-pyrite Specimen K/Na zones samples PO4-rich the trigonal solution. varying (Na-rich) alunite phate-bearing image platy reflects electron alunite of alunite-bearing containing of specimen Secondary crystals of powdery section views solution a characteristic zoned dark microscope Tuding Pit. e. BEI remnants of phosof specimen Al=Alunite Bar of within Silica natroalunite. surface solubei scale on each

7 43(3), 1993 Advanced argillic alteration and geochemistry of alunite 161 natroalunite have 2Į (Cu K(x) peaks at 30.9 and 32.1, respectively. Minamiite gives (00 12) reflection at These four samples have complicated patterns due to their heterogeneous compositions. The Mines View Park sample primarily consists of minamiite or natroalunite, while the Tuding Silica Pit and Bua Ridge samples are mainly alunite. The small width in the spread of the peaks for the Loakan sample suggests a small compositional variation. Cores rich in PO4 and heavy elements are relatively minor in total amount, and thus do not show up on the XRD traces. In contrast, backscattered electron images an electron probe microanalyser (EPMA) clearly indicate the compositional heterogeneity of these alunite samples (Fig. 3). The brightness in the backscattered image is proportional to the average atomic number; the brightness increases in the order of hydronium alunite, natroalunite (= minamiite), alunite and Sr-Ba-Pb-bearing woodhouseite. Samples Mines View Park and Bua Ridge (Fig. 3a and e) both show high reflectivity cores. EPMA quantitative analysis indicate these cores are woodhouseite, containing Sr and Ba (Fig. 4). The Mines View Park alunite core is mantled with Na- and Ca-rich alunite solution. In both samples there is textural evidence (e.g., irregular shape) for partial dissolution, with the smaller cores of the Bua Ridge sample suggesting a greater amount of dissolution before the alunite overgrowth. A secondary electron image (SEI) shows a smooth surface of the alunite solution for the Mines View Park sample (Fig. 3b), indicating no dissolution after formation. In contrast, a similar SEI image for the Bua Ridge sample (Fig. 3f) shows numerous etch pits, indicating that the later alunite was also partially dissolved before hydrothermal activity ceased. The Loakan sample shows simple growth banding parallel to the crystal surface (Fig. 3c). There are no bright (heavy element) cores, and the low reflectivity indicates the sample is poor in K; the lack of banding means there is little compositional zoning. These observations are consistent with XRD interpretation based on the narrow peak width. The Tuding Silica Pit sample Fig. 4 Chemical variation core to rim in an epitaxially-grown crystal of alunite solution. Specimen Mines View Park (see Fig. 3a). : core of Srand Ba-rich woodhouseite, :rim of oscillatory zoned alunite-natroalunite-minamiite. consists largely of K-rich alunite, based on its uniformly high brightness with no regular banding (Fig. 3d), and the strong (006) peak at 31.0 (Fig. 2). The occurrence of some irregularly-shaped islands of low brightness within alunite crystals suggests that these are K-exchanged natroalunite along micro-cracks during a later stage. These four alunite samples were also analyzed for sulfur isotopic composition (Table 2). They are all relatively heavy, +15 to +24 permil ĉ34s, suggesting formation at high temperature (RYE et al., 1992). Conclusions Based on our study of the acid alteration in the Baguio district, we can add further constraints to previous conclusions (SAWKINS et al., 1979; FERNANDEZ and DAMASCO, 1979; COMSTI et al., 1990; COOKE and BLOOM, 1990; MITCHELL and BALCE, 1990) concerning the evolution of hydrothermal activity in the Baguio district (Fig. 5). Intrusion of dacitic magma as dikes and diatremes, some of which now host mineralization, took place at 2-3 Ma. Radiometric dating indicates the first hydrothermal activity occurred at Ma. Magmatic intrusion, possibly accompanied by volcanism and fumarolic activity, initiated hydrothermal circulation in the district.

8 162 M. AOKI, E. C. COMSTI, F. B. LAZO and Y. MATSUHISA RESOURCE GEOLOGY: Stage of hypogene advanced argillic alteration Ma Stage of vein-style Au mineralization 0.6 Ma Present Fig. 5 Schematic cross-sections (not to scale) showing the Baguio magmatic hydrothermal of hypogene system a stage advanced argillic alteration to an episode of low sulfidation vein-type Au mineralization and younger supergene alteration. Sporadic acid sufate alteration may have occurred at Kelly during and after the Au-mineralization stage. The model is based on K-Ar ages, sulfur isotope geochemistry and distribution of alteration assemblages. The size and position of the intrusive body and the boundary of the alteration zones are hypothetical. This developed a district wide propylitic zone of alteration and possibly proximal phyllic alteration around the intrusive body, though they were locally cut by zones of brecciation and acid fluids. These acid fluids became highly reactive at lower temperature, and caused extensive leaching of rocks near the paleosurface. Subsequently, at about 0.6 Ma, the hydrothermal system was rejuvenated, perhaps by a new intrusion beneath the vicinity of the Kelly deposit. This hydrothermal activity accompanied the formation of district-scale fractures, which were responsible for focussing the ascent of neutral ph fluids. Magmatic acid components sporadically ascended to a shallow level to form local overprints of acid alteration. Quartz-Au veins formed in these fractures. Erosion continued to the present, exposing the highly resistent zones of residual silica which cap ridges in a horse shoeshape east of Baguio City. A very similar sequence of early acid alteration, leaching and resistant quartz ledge development, followed by later quartz veining and gold mineralization, is present at Masupa Ria, Kalimantan (THOMPSON et al., 1994). Acknowledgements : We thank Mr. F. V. DAMASCO and his colleagues of the Benguet Corporation for offering us the opportunity to conduct field work in their mining area, and for their generous assistance during field work. We also thank Mrs. H. E. A. FRANCO and Mr. R.I. VILLONES, of the Mines and Geosciences Bureau, who kindly collaborated during the field work, and provided us with their unpublished data. Mr. Y. SEKI, Geological Survey of Japan, analyzed the sulfur isotope compositions of the alunite, for which we are grateful. We thank Dr. J. W. HEDENQUIST, Geological Survey of Japan, for his useful discussions and suggestions throughout the preparation of the manuscript. References AOKI, M. (1983): Mineralogical properties and geochemistry of alunite -solution in the Osorezan geothermal system. Sci. Rep. Hirosaki Univ., 30, 132 `141 (in Japanese with English abstract). Aoxt, M. (1991): Mineralogical features and genesis of alunite solution in high temperature magmatic hydrothermal systems. Rept. Geol. Surv. Japan, no. 277, 35 `37. BALCE, G. R., ENCINA, R. Y., MOMONGAN, A. and LARA, E.

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10 164 M. AOKI, E. C. COMSTI, F. B. LAZO and Y. MATSUHISA RESOURCE GEOLOGY: