한국지구시스템공학회지 Vol. 43, No. 2 (2006) pp. 106-117

연구논문

Fluid Inclusion Study of a Magmatic Cobalt Mineralization at the Boguk Mine, Korea

Chul-Ho Heo1), Jae-Ho Lee2)*, Seong-Taek Yun3) and Chil-Sup So3)

보국 마그마성 열수 코발트 광화작용의 유체포유물연구

허철호1)․이재호2)*․윤성택3)․소칠섭3)

요 약 : 보국 코발트 광산의 열수성 석영 ± 탄산염 ± 녹섬석 맥들은 백악기 후기의 화강암내 열극을 충진했다. 맥의 광물조성은 코발트, 몰리브덴, 구리, 납, 아연, 비스무스 및 금의 광석광물을 함유하고 있는 다금속 성향을 보이며, 광화작용은 5개의 광화시기로 구분된다. 맥상광물은 광화시기에 따라 체계적으로 변화하며 다음과 같은 광물공생군을 보인다: 녹섬석과 석영을 수반한 함코발트 비화물, 유비화물 및 휘수연석 → 천금속 황화물, 금, 철산화물 → 탄산염. 광석광물공생에 대해 평형 열역학을 적용하면 다음과 같다: 광화 1, 2기 코발트 광화작용은 T = 560-360℃, log fs2 = -6.2~-12.0 atm의 광화유체에서 일어났으며, 광화 3기의 천금속 황화물 및 금은 T = 380-275℃, log fs2 = -7.5~-10.6 atm 유체에서 침전되었다. 코발트 광화작용에서 천금속 황화물 침전으로 광화작용 이 진행되면서 온도감소와 산소분압의 증가가 수반되었을 것으로 사료된다. 코발트의 침전은 마그마성 염수의 냉각 및 환원에 의해 야기되었을 것으로 사료된다. 이 냉각 및 희석은 초기 마그마계가 쇠퇴하면서 다량의 천수성 지하수의 혼입에 의해서 발생했으며, 계속해서 천금속 황화물, 금, 비스무스가 침전되었다. 광물학 및 유체포유물 연구에 의하면, 코발트, 비소, 몰리브덴은 마그마 정출작용중 직접 용리된 고온(<~585℃), 고염농도(<67 wt. NaCl) 의 마그마 염수로부터 용리되어 분별된 것으로 사료된다. 마그마 염수가 냉각되면서, 이 금속들은 석영 ± 녹섬석 맥내 비화물과 유비화물로 침전되었다. 약 350℃의 온도에서 마그마성 열수계가 쇠퇴하면서, 천수성 지하수의 거대순환이 마그마성 열수계를 붕괴시키고 점진적으로 열수유체의 냉각, 희석, 산화가 촉진된다. 첨금속, 금, 칼슘 은 천수순환중 주변의 퇴적암에서 용탈되며 광화 3기에서 5기의 광화작용과 관련된 유체를 형성하게 된다. 주요어 : 물질흐름분석, 자원관리, 지속가능한, 자원이용지수 Abstract : Hydrothermal quartz carbonates actinolite veins of the Boguk cobalt mine filled the fractures in a granite stock of Late Cretaceous age. They show the polymetallic nature consisting of Co-, Mo-, Cu-, Pb-, Zn-, Bi-, and Au-bearing ore minerals, and is divided into five stages. The vein mineralogy changes systematically with time: cobalt-bearing, arsenides and sulfarsenides and molybdenite with actinolite and quartz → base-metal sulfides, gold,and Fe oxides → barren carbonates. Equilibrium thermodynamic considerations of ore mineral assemblages are as follows: cobalt mineralization in stages I and II, T = 560-360℃, log fs2 = -6.2 to -12.0 atm deposition of base-metal sulfides and gold in stage III, T = 380-275℃, log fs2 = -7.5 to -10.6 atm. With the transition from cobalt mineralization toward base-metal sulfide deposition occurred the temperature decrease and concomitant increase in fo2. The deposition of cobalt probably occurred as a result of cooling and reduction of the magmatic brines. This cooling and dilution occurred by mixing with progressively larger volumes of meteoric groundwater as an early magmatic system waned, and resulted in successive deposition of base-metal sulfides, gold and bismuth, Fe oxides, and carbonates. By combining the mineralogic, fluid inclusion and petrochemical data, the following model is proposed for ore genesis at Boguk: during the Late Cretaceous, a micrographic granite stock intruded

2005년 5월 27일 접수, 2006년 3월 20일 채택 1) 국립공원관리공단 국립공원연구원 2) 한국지질자원연구원 지질기반정보연구부 3) 고려대학교 지구환경과학과 *Corresponding Author(이재호) E-mail; [email protected] Address; Geology & Geoinformation Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea

106 Fluid Inclusion Study of a Magmatic Cobalt Mineralization at the Boguk Mine, Korea 107

volcanosedimentary rocks at near surface. Cobalt, arsenic, and molybdenum were partitioned into high-temperature (up to ~585℃), high-salinity (up to 67 wt. % NaCl) magmatic brines exsolved directly from the crystallizing magma. As the magmatic brine cooled, these metals precipitated as arsenides and sulfarsenides in quartz actinolite veins. Following the waning of the magmatic hydrothermal system at temperatures around 350℃, a huge circulation of meteoric groundwater formed to collapse the system, resulting in progressively larger degrees of cooling, dilution and oxidation of hydrothermal fluids. Base metals, gold, and possibly calcium were leached from surrounding sedimentary rocks during the meteoric water circulation, and formed the fluids related tostage III to V mineralization. Key words : Cobalt deposit, Mineralogy, Fluid inclusion

Introduction Konchonri Formation which consists mainly of shale with minor intercalations of sandstone and limestone. There are few cobalt-bearing deposits in Korea, and The bedding strikes 275° to 320° and dips 5° to can be grouped into two genetic types(Nakamura, 15°SW. The Yucheon group rocks extrude or intrude 1942): (1) deposits associated with hydrothermal Cu, the Konchonri Formation and consist mainly of Zn, Au and Ag mineralization in a genetic tie with andesite and andesite porphyry(Yun and Youm, 1997). felsic igneous rocks; (2) deposits associated with A granite stock with an outcrop size of about 2×4 nickel in basic igneous rocks, where cobalt-bearing km intrudes the Hayang and Yucheon group rocks, and minerals occur only as a by-product. The Boguk cobalt hosts the hydrothermal veins of the Boguk cobalt mine deposits in this study share many features with the (Fig. 2). The granite is composed mineralogically of granite- related hydrothermal deposits. Until the quartz, plagioclase, orthoclase and biotite with minor mining activity was stopped at 1970, the Boguk cobalt amounts of hornblende, apatite, zircon, chlorite and mine has produced an average 0.5 to 1.0 wt. % Co per hematite. Along the intrusive contacts with sedime- metric ton of ores, with trace amounts of gold. ntary rocks occur the intrusion-related prophyllitic Only a few studies of cobalt mineralization in South alteration assemblages. The calc-alkaline granite stock Korea were carried out. Park(1990) and Yun and is occasionally uneven in grain size, ranging from fine- Youm(1997) have described a xenothermal feature of to medium-grained. Inward from the margin, the grain the Boguk cobalt deposits, based on ore mineralogy. size tends to be increased. The granite also shows However, the source and physicochemical conditions miarolitic cavities and micrographic texture, suggesting of the cobalt ore mineralization have not been their epicrustal emplacement and the presence of understood. The purposes of this study are to describe abundant volatile components in magma. A Rb-Sr age the complex ore mineralogy, to elucidate the fluid dating of the granite suggested a Late Cretaceous age evolution and to propose genetic model for the (around 86 Ma) of the intrusion and associated ore Co-bearing hydrothermal system. mineralization (Yun and Youm, 1997).

Geologic Setting Ore Veins and Mineralogy

The Boguk cobalt mine, located at latitude of 35° The hydrothermal mineralization of the Boguk mine 47’N and longitude of 128°45’E, is situated within the consists of narrow(each 0.1-0.5 m thick), fracture- middle western part of the Gyeongsang Basinin which filling quartz, carbonate and actinolite veins. These occur the non-marine, sedimentary and volcanic- veins occur within a calc-alkaline granite stock. The plutonic rocks of Cretaceous age (Fig. 1). The geology ore mineralogy is relatively complex and consists of of the mine area is composed mainly of volcano- cobalt-bearing arsenides or sulfarsenides(in the decre- sedimentary rocks of the Hayang and Yucheon groups asing order of amounts, loellingite, cobaltite and glau- that are intruded by a small granite stock (Fig. 2). The codot), arsenopyrite, molybdenite, base-metal sulfides Hayang group rocks inthe mine area belong to the (chalcopyrite, sphalerite, pyrite, pyrrhotite, etc.), and

제43권 제2호 108 Chul-Ho Heo․Jae-Ho Lee․Seong-Taek Yun․Chil-Sup So

Fig. 1. Simplified geologic map of the Republic of Korea, showing the location of the Boguk cobalt mine within the Cretaceous Gyeongsang Sedimentary Basin.

Fig. 3. Generalized paragentic sequence of minerals in veins of the Boguk cobalt mine. Temperature scale (*) is based on fluid inclusion temperatures and on thermo- dynamic considerations of ore mineral assemblages. See the “Fluid Inclusions” section for fluid inclusion types.

During stages I and II, cobalt was deposited as loellingite, cobaltite and glaucodot. These cobalt-bearing minerals are associated intimately with arsenopyrite, molybdenite and pyrrhotite. Stage I veins are charac- Fig. 2. Geologic map of the Boguk cobalt mine area terized by the occurrence of green-colored amphibole (modified after Yun and Youm, 1997). Hydrothermal veins are developed restrictedly within the granite stock. (actinolite) in association with minor amounts of quartz. Actinolite occurs as massive aggregates which contain cobalt-bearing ore minerals, and is commonly rare amounts of oxides(magnetite and hematite) and replaced by stage IV brown carbonates(siderite and electrum. Gangue minerals are quartz, carbonates and dolomite). Ore minerals consist mainly of Co-rich actinolite. loellingite(2.3-11.5 wt. % Co, average 6.9 %) and Based on investigation of the mineral assemblages arsenopyrite(up to 8.9 wt. % Co) with rare amounts of and textural relationships(e.g., cutting, banding) of cobaltite, glaucodot, molybdenite and pyrrhotite. Stage veins, the vein miner alization at Boguk is divided into II mineralization is characterized by less amounts of five mineralization stages(I to V; Fig. 3). Co-bearing loellingite in clear quartz veins without

한국지구시스템공학회지 Fluid Inclusion Study of a Magmatic Cobalt Mineralization at the Boguk Mine, Korea 109 actinolite. Toward the stage II mineralization, cobalt- alization are carried out to trace the changes in bearing minerals abruptly decrease in amounts, whereas physicochemical conditions of hydrothermal fluids at arsenopyrite and other sulfides increase. Stage II ore Boguk. Representative ore mineral assemblages accor- mineralogy consists dominantly of Co-rich arsenopyrite ding to mineralization stages in veins are summarized (up to 8.8 wt. % Co) and loellingite(4.7-10.2 wt. % Co, in Table 1. average 7.1 %) with rare amounts of molybdenite, Within stage I and II veins, arsenopyrite is coexisting pyrrhotite and pyrite. with loellingite and pyrrhotite. Chemical compositions Stage IIImineralization is represented by deposition and elemental substitutional relationships of arsenopyr- of relatively abundant base-metal sulfides within white ites have been studied by Yun and Youm(1997). Most quartz veins and consists of quartz, carbonates, arseno- of stage I and II arsenopyrites are typically cobalt-rich pyrite(mostly <0.5 wt. % Co), chalcopyrite, sphalerite, (up to 8.9 wt. % Co; average = 3.9 wt. % and 2.9 wt. pyrite, pyrrhotite, tetrahedrite, bismuthinite, native bis- % for stage I and stage II , respectively), resulting in muth, electrum and Fe oxides(magnetite and hematite). quite restricted applicability of arsenopyrite as a Electrum(39.8-45.6 atom. % Ag) rarely occurs as tiny geothermometer(Kretschmar and Scott, 1976). How- grains associated with bismuthinite, native bismuth and ever, small numbers of arsenopyrites with minor amo- chalcopyrite along fractures of earlier ore minerals. unts of Co and Ni(totally <1 wt. %) may indicate their Toward the end of stage III mineralization occurs depositional conditions as follows(Table 1): about 440˚ typically the assemblage of pyrite + magnetite + to 560℃ and log fs2 values of 9.3 to 6.2 atm for stage hematite in carbonates, which indicates the oxidation of I mineralization; and about 360˚ to 460℃ and log fs2 hydrothermal fluids. Stage IV and Vmineralizations are values of 12.0 to 8.8 atm for stage II mineralization. represented by deposition of barren carbonatesand Thus, cobalt mineralization(arsenides and sulfarseni- chalcedony. Stage IV veins characteristically show des) of stage I and II veins occurred at high temper- repeated rhythmic banding which consists of black to atures between 360°and 560℃ from the ore fluids. pale brown carbonates. Stage III arsenopyrites occur along vein margins and Weak hydrothermal alteration(<0.1 m thick) of host form an mineral assemblage with pyrrhotite, bismu- rocks shows potassic, sericitic and chloritic assem- thinite and native bismuth. Compositional data of stage blages. Adjacent to stage I veins occurs the weak III arsenopyrites(31.3-32.2 atom. % As) indicate the potassic alteration(a few millimeters thick) which can formation temperatures of 335°to 380℃ and log fs2 be recognized easily by the occurrence of pink-colored values of 10.6 to 9.2 atm(Kretschmar and Scott, 1976; K-feldspar, indicating that the chemistry of Co-deposi- Barton and Skinner, 1979). In the middle to late ting hydrothermal fluids was largely controlled and mineralization of stage III veins, stannite(10.9-11.2 wt. buffered by the K-feldspar-rich granite. On the other % Zn) and sphalerite(4.9-6.0 mole % FeS) form an ore hand, stage III and IV veins are associated with the mineral assemblage with pyrite. Based on the parti- wider, pale green-colored sericitic alteration zone which tioning of Fe and Zn between stannite and sphalerite consists of sericite, chlorite, carbonates and pyrite. (Nakamura and Shima, 1982; Shimizu and Shikazono, In summary, the vein mineralogy of the Boguk cobalt 1985), stannite+sphalerite+pyrite assemblage indicates mine changed systematically with time as follows(Fig. the depositional temperature and sulfur fugacity(log fs2) 3): actinolite, cobalt-bearing minerals and molybdenite values are 345 to 360℃ and 7.8 to 7.6 atm, respe- (stages I and II) → base-metal sulfides, Fe oxides and ctively. The assemblage of electrum(39.8-45.6 atom. % gold(stage III) → barren carbonates(stages IV and V). Ag) + sphalerite(2.3-4.8 mole % FeS) + chalcopyrite bismuthinite in central parts of stage III veins also Physicochemical Conditions of indicates the depositional temperatures between 275°to Mineralization 350℃, corresponding to the log fs2 values of 9.8 to 7.5 atm(Barton and Toulmin, 1964; Scott and Barnes, Equilibrium thermodynamic considerations of miner- 1971). Therefore, the deposition of base-metal sulfides

제43권 제2호 110 Chul-Ho Heo․Jae-Ho Lee․Seong-Taek Yun․Chil-Sup So

Table 1. Representative equilibrium mineral assemblages and their calculated depositional conditions, Boguk cobalt mine

Mineral composition1) Physicochemical environments 2) Mineral apy sp st el log fS2 Stage Temp. (℃) References assemblage(s) (atom. % As) (mole % FeS) (wt. % Zn) (atom. % Ag) (atm) Kretschmar and I apy + loe + po 35.3－36.7 (4) 440－560 -9.3－-6.2 Scott (1976) II apy + loe + po 34.3－35.6 (7) 360－460 -12.0－-8.8 ditto Early-Middle: ditto; Barton and III apy + po 31.3－32.2 (5) 335－380 -10.6－-9.2 Skinner (1979) + bi + bm Nakamura and Middle-Late: Shima (1982); a) sp + st 4.9－6.0 (7) 10.9－11.2 (4) 345－360 -7.8－-7.6 Shimizu and + py Shikazono (1985) Barton and Toulmin b) el + sp (1964); 2.3－4.8 (16) 39.8－45.6 (12) 275－350 -9.8－-7.5 + cp ± bm Scott and Barnes (1971) IV sp + gn ± mt 0.9 2.4 (7) 1) Number of analysis is indicated in parenthesis. 2) Arsenopyrites with high cobalt and nickel contents (>1 wt. % Co + Ni) are not included in the range. Abbreviations: apy = arsenopyrite, bi = native bismuth, bm = bismuthinite, cp = chalcopyrite, el = electurm, gn = galena, loe = loellingite, mt = magnetite, py = pyrite, po = pyrrhotite, sp = sphalerite, st = stannite. and gold in stage III veins occurred at lower tem- 1993)for aqueous fluid inclusions, and on the dissol- peratures(between 275°and 380℃) than that of the ution temperature of halite for halite-bearing inclusions cobalt mineralization in stage I and II veins. (Chou, 1987; Sterner et al., 1988). The mineralogical change of dominant Fe-bearing Three main types of fluid inclusions(<4 to 40µm in minerals from pyrrhotite(in stages I and II) to mag- size, average about 10µm) were distinguished based on netite + hematite(in stage III) suggests the progressive the microthermometric behavior and phase relations at oxidation of the Boguk hydrothermal fluids with room temperature(Table 2). paragenetic time. Type I inclusions are aqueous two-phase and liquid-rich inclusions with a vapor bubble comprising Fluid Inclusion Study 5 to 30 volume percent(usually 10 to 20 vol. %) of the total inclusion volume. The bubble is determined to be In order to determine the variations of temperature essentially water vapor by crushing. No gas hydrates and composition of ore fluids, fluid inclusions in about formed recognizably during freezing experiments. They 80 samples of quartz and calcite from veins were homogenize readily into a liquid phase upon heating. examined by microthermometry. Sphalerite and car- Type I inclusions are the most abundant in samples bonates(except calcite) were not suitable for the study examined(except stage I quartz which contains typi- because of their opacity and tiny size(if present) of cally type III inclusions), and occur as both primary fluid inclusions. Microthermometric data were obtained and secondary inclusions(Roedder, 1984). using a USGS Fluid Inc. gas flow-type heating/freezing Type II inclusions are aqueous two-phase and system. Salinity data are reported based on the freezing vapor-rich inclusions containing more than 70 percent point depression in the system H2O-NaCl(Bodnar, of vapor bubble, and homogenize to a vapor phase

한국지구시스템공학회지 Fluid Inclusion Study of a Magmatic Cobalt Mineralization at the Boguk Mine, Korea 111

Table 2. Summary of fluid inclusion characteristics, Boguk cobalt mine

1) Inclusion Occurrence and Microthermometer data (℃) Phases present type paragenetic association Tm-ice Tm-H Th-L/V predominant in stage II and III quartz and in stage IV -17.1 to 0.0 98－495 (L) I L + V and V carbonates (as P and S); abundant in stage I quartz - (as S)

II V + L common in stage II quartz (as P) -1.1 to -5.2 - 298－402 (V)

L + V + H ± Sy predominant in stage I quartz (as P and S); abundant in - 313－584 287－573 III ± other solids stage II quartz 1) Data range for primary fluid inclusions. Abbreviations: H = halite, L = liquid, P = primary, S = secondary, Sy = sylvite, Th = homogenization temperature, Tm = melting temperature, V = vapor upon heating(if not decrepitated). They occur charact- 10-20℃ lower than the halite dissolution temperatures). eristically in stage II quartz as regular-shaped inclusi- ons with no fracture controls, indicating their primary Microthermometric data origin. Type II inclusions in stage II quartz occur Final homogenization temperature and salinity data of locally in the same areas as clusters of type I and type primary fluid inclusions are shown in Figures 4 and 5. III inclusions, suggesting the existence of hetero- Halite dissolution is the final phase transition geneous fluids during trapping. Within the stage I observed in type III fluid inclusions in clear quartz of quartz rare type II inclusions occur along healed stage I, and occurs at temperatures of 407° to 584℃, fractures, indicating their secondary origin. with a mode at 480° to 540℃. Assuming an H2O-NaCl Type III inclusions are high-salinity, multiphase system, these dissolution temperatures indicate salinites (liquid + vapor + halite other solids) inclusions. The between 46 to 67 wt. % NaCl equiv. Final homo- vapor bubble comprises 5 to 25 volume percent of the genization temperatures of primary type I, type II, and inclusion volume. They always contain halite crystals type III inclusions in stage II quartz are 297°to 495℃ which occupies 10 to 85 percent of the inclusion (to liquid), 290°to 403℃(to vapor) and 310° to 487℃ volume. Other solid phases are observed in less than (to a liquid by halite dissolution, corresponding to 10% of type III inclusions and include sylvite and salinities between 39 and 56 wt. % NaCl equiv.), unidentified minerals. Halite and sylvite can be easily respectively. Estimated salinities of type I and type II distinguished by the shape and optical isotropy inclusions in stage II quartz range from 9.9 to 20.3 wt. (Roedder, 1984). The rare unidentified solids are opti- % and 1.9 to 8.1 wt. % NaCl equiv., respectively. cally birefringent with round or prismatic forms, and Quartz and calcite from stage III to V veins contain do not dissolve upon heating. Type III inclusions are type I inclusions only. Ranges of homogenization observed only within stage I quartz and stage II quartz temperature and salinity of primary inclusions are: as both primary and microfracture-controlled secondary 219°to 352℃ and 4.8 to 14.8 wt. % NaCl equiv for inclusions. These inclusions homogenize finally by stage III; 124°to 222℃ and 0.0 to 12.9 wt. % NaCl halite dissolution upon heating. During heating, the equiv for stage IV; and 98° to 203℃ and 1.1 to 7.3 dissolution of sylvite(if present) is the first phase wt. % NaCl equiv for stage V. transition and occurs at temperatures between 135°and Figures 4 and 5 show that both temperature and 227℃(mostly between 170°and 190℃) for primary salinity of hydrothermal fluids decreased progressively fluid inclusions. With continued heating, liquid-vapor with increasing paragenetic time, likely due to the homogenization by vapor bubble disappearance occurs increasing amounts of influx of cooler(~100℃) and at temperatures of 287°to 573℃(which are usually dilute(~0 wt. % NaCl) meteoric groundwater into the

제43권 제2호 112 Chul-Ho Heo․Jae-Ho Lee․Seong-Taek Yun․Chil-Sup So

Fig. 6. Homogenization temperature versus salinity diagram for primary fluid inclusions in vein minerals from the Boguk cobalt mine. See text for fluid inclusion type.

high temperature(up to ~600℃) and high salinity(up to ~67 wt. % NaCl) fluid. With time, the hydrothermal fluids changed in composition from high-saline, type III(during stages I and II) toward very dilute, type I fluid. Fig. 4. Homogenization temperatures of primary fluid inclusions in vein minerals from the Boguk cobalt mine. Fluid evolution A systematic temperature decrease with increasing paragenetic time is remarkable. See text for fluid incl- The relationships between homogenization tempera- usion type. ture(Th) and salinity of primary fluid inclusions are shown in Figure 6. Halite-bearing type III inclusions in stage I quartz have the highesthomogenization tempera- tures(up to ~580℃) and show a positive correlation between salinity and Th because all of the inclusions examined are homogenized by halite disappearance. These high-temperature and high-salinity brines pro- bably had exsolved from a crystallizing granitic melt, as have suggested for many porphyry copper and/or molybdenite systems(Eastoe, 1978; Henley and McNa- bb, 1978; Kamilli, 1978; Ahmad and Rose, 1980; Bloom, 1981; Reynolds and Beane, 1985; Samson, 1990; So et al., 1991; Cline and Bodnar, 1994). The high-temperature and high-salinity brines(46-67 wt. % NaCl equiv.) that was trapped in type III inclusions during the stage I quartz do not coexist with any liquid- and vapor-rich fluid inclusions. This fact cannot Fig. 5. Estimated salinities of primary fluid inclusions in be explained adequately by the typical aqueous fluid vein minerals from the Boguk cobalt mine. The salinity immiscibility model and may likely indicate that the decreases systematically with increasing paragenetic high-salinity fluids were generated by direct exsolution time. See text for fluid inclusion type. from the crystallizing silicate melt(Cline and Bodnar,

한국지구시스템공학회지 Fluid Inclusion Study of a Magmatic Cobalt Mineralization at the Boguk Mine, Korea 113

1994). Here, we cannot rule out a specific fluid immi- low salinity(<15 wt. % NaCl equiv.) and show a scibility model as follows: as a granitic magma general decrease of salinity with decreasing temper- crystallized down to ~600℃, the residual melt under- ature(Fig. 6). The relationship of Th vs. salinity gone subcritical aqueous fluid separation and subse- indicates that cooling and dilution of hydrothermal quent fluid immiscibility(due to the release of highly fluids by mixing with meteoric water occurred from built vapor pressure and the associated adiabatic stage III to V mineralizations(accompanying deposi- decompression; Henley and McNabb, 1978; Burnham tion of base metal sulfides, gold and carbonates). It and Ohmoto, 1980) to forma low-density vapor phase may suggest that as the early magmatic hydrothermal and a high-density brine; the low-density vapor first system(responsible for the stage I and II minerali- rose buoyantly and subsequently condensed to form a zation) waned, progressively larger volumes of me- high-salinity brine(or released out to the surface before teoric water inundated the system. the deposition of stage I vein minerals). However, we prefer the direct exsolution model for the genesis of Pressure-depth conditions stage I orefluid. Trace amounts of cobalt(and arsenic) As described above, the intimate association of type accompanied this magmatic brine as it separates II inclusions with type I and III inclusions in the stage directly from the granitic melt, and precipitated as II quartz indicate that the fluids were trapped along a arsenides and sulfarsenides with quartz and actinolite. two-phase boundary in the system H2O-NaCl. The All types of inclusions trapped in stage II vein quartz P-T-X data for the system at temperatures of ~300°to homogenized in a similar temperature range(~300°to 500℃(Sourirajan and Kennedy, 1962; Bodnar et al., 500℃). The absence of salinity values with ~20 to 40 1985; Chou, 1987) indicate pressures below ~600 bars, wt. % NaCl equiv. in stage II ore fluids indicates that corresponding to depths of <2.3 km and <6.5 km under stage II cobalt mineralization occurred with typical lithostatic and hydrostatic pressure conditions, res- aqueous fluid immiscibility. Boiling effect of ore fluids pectively. with moderate-salinity(~10 to 20 wt. % NaCl equiv.) also may explain the origin of high-salinity(~40 to 55 Discussions and Conclusions wt. % NaCl equiv.) fluids in stage I and stage II mineralizations. However, this boiling model would Hydrothermal cobalt deposits generally have been require very extensive volatilization of water in the viewed as the products of deposition directly from hy- system in order to produce the necessary salinity drothermal solution of magmatic origin. Recently, increase of at least 200%. Therefore, we suggest that however, many cobalt-bearing deposits of non-magmatic immiscibility of subcritical aqueous fluid occurred hydrothermal origin have been discovered (Kerrich et throughout the stage II mineralization and formed a al., 1986; Kissin, 1988). In fact, Co(and Ni) is rare in dilute vapor(trapped as type II fluids) and a high- minerals from ore deposits of magmatic hydrothermal salinity brine(trapped as type III fluids) at the same origin owing to its strong partitioning to the magmatic time. Type I inclusions with moderate-salinity values phase(typically, mafic minerals of crystallizing silicate may represent either original fluids before the phase melts) and depletion in residual hydrothermal fluids, as separation or condensates of immiscible vapors. Previ- well as due to very low solubility under general ous studies of hydrothermal W-Mo and Cu deposits in hydrothermal conditions(Crerar et al., 1985; Susak and the Gyeongsang Basin of Korea also show that aqueous Crerar, 1985). According to Halls and Stumpfl(1972), fluid immiscibility was main mechanism for the ore the cobalt deposits can be classified into four genetic deposition during hydrothermal fluid evolution in groups: magmatic/hydrothermal(Badham, 1975, 1976; southeastern Korea(e.g., Gyeongchang W-Mo mine, So Horrall et al., 1993); metamorphic/ hydrothermal(Goodz et al., 1991; Andong area Cu mines, So et al., 1997). et al., 1986; Kerrich et al., 1986); sedimentary syngenetic Fluid inclusions in stage III to V minerals are (Schneider, 1972); non-magmatic(Kissin, 1988). The exclusively of type I with low temperature(<350℃) and Boguk cobalt deposit shares many features with the

제43권 제2호 114 Chul-Ho Heo․Jae-Ho Lee․Seong-Taek Yun․Chil-Sup So magmatic-hydrothermal group which commonly occurs result of cooling and reduction(as suggested by the as Co-sulfarsenides-bearing, fracture- filling veins in association of pyrrhotite without Fe oxides) of the Mesozoic terrigeneous or volcanosedimentary rocks magmatic brines(Kissin, 1993). (which are intruded by granitoids) within young fold Primary type I, II, and III inclusions coexist in stage regions(Krutov, 1977). II quartz, and homogenize at similar temperature range Hydrothermal quartz carbonates actinolite veins of (between ~300° and 500℃, in good agreement with the the Boguk cobalt mine filled the fractures in a granite temperature estimates based on the thermodynamic stock of Late Cretaceous age. The granite intruding the consideration). Type III inclusions have salinities bet- Konchonri Formation consisted of mainly shale shows ween 39 and 55wt. % NaCl equiv. These observations the petrographic features implying its epicrustal indicate that the subcritical aqueous immiscibility emplacement. Hydrothermal vein mineralization of the occurred during the formation of relatively cobalt-poor, Boguk cobalt mine shows the polymetallic nature stage II quartz veins. Fluid inclusions in stage III to V represented by Co-, Mo-, Cu-, Pb-, Zn-, Bi-, and minerals record the progressive cooling (from ~350° Au-bearing ore minerals according to five minerali- down to ~100℃) and dilution(from ~15 down to 0 wt. zation stages. The vein mineralogy changes systemati- % NaCl equiv.) of hydrothermal fluids. This cooling cally with paragenetic time: cobalt-bearing, arsenides and dilution occurred by mixing with progressively and sulfarsenides(Co-rich loellingite, cobaltite and larger volumes of meteoric groundwater as an early glaucodot) and molybdenite with actinolite and quartz magmatic system waned, and resulted in successive (in stages I and II) → base-metal sulfides, gold, and Fe deposition of base-metal sulfides(chalcopyrite, sphale- oxides(in stage III) → barren carbonates(in stages IV rite, galena, etc.), gold and bismuth, Fe oxides, and and V). Equilibrium thermodynamic considerations of carbonates. ore mineral assemblages yield the following physi- Petrochemical analyses of various rocks in the Boguk cochemical conditions: (1) cobalt mineralization in mine area(Yun and Youm, 1997) show that the granite stages I and II: T = 560°-360℃, log fs2 = 6.2 to 12.0 stock is enriched in cobalt(in average, twelve times atm (2) deposition of base-metal sulfides and gold in higher than worldwide average granitoids) but is stage III: T = 380°-275℃, log fs2 = 7.5 to 10.6 atm. relatively depleted in base metals; whereas the Kon- With the transition from cobalt mineralization toward chonri Formation shale is typically enriched in copper base-metal sulfide deposition occurred the temperature (avg. ~90 ppm) and zinc(avg. ~135 ppm) but is decrease and concomitant increase in fo2. depleted in cobalt. These data may support the diverse Three types of fluid inclusion were identified in vein metal source model: cobalt was derived directly from quartz and carbonates. These are liquid-rich, low- a granitic magma; base metals(and probably gold) were salinity type I; vapor-rich, low-salinity type II; and derived from surrounding sedimentary rocks through halite-bearing type III. Within stage I quartzoccur only remobilization by circulating meteoric water. the type III inclusions which homogenize by halite By combining the preceding discussions(based on dissolution at high temperatures(407° -584℃, correspo- mineralogic, fluid inclusion and petrochemical data), nding to salinities between 46 and 67 wt. % NaCl the following model is proposed for ore genesis at equiv.). These high-salinity brines were probably gen- Boguk: during the Late Cretaceous micrographic gra- erated by direct exsolution from the crystallizing nite intruded volcano-sedimentary rocks at near surface. granitic magma, and were related to the transport Cobalt, arsenic, and molybdenum were partitioned into - (probably as chloro complexing such as CoCl3 and high-temperature(up to ~585℃), high- salinity(up to 67 2- CoCl4 , Susak and Crerar, 1985; Uchida et al., 1996) wt. % NaCl) magmatic brines which were exsolved and deposition of cobalt(as arsenides and sulfarsenides directly from the crystallizing magma. As the magmatic within actinolite + quartz veins). The deposition of brine cooled, these metals precipitated as arsenides and cobalt(and associated arsenic) probably occurred as a sulfarsenides in quartz actinolite veins. Following the

한국지구시스템공학회지 Fluid Inclusion Study of a Magmatic Cobalt Mineralization at the Boguk Mine, Korea 115 waning of the magmatic hydrothermal system at Issue (Japan), V. 8, p. 1-11. temperatures around 350℃, a huge circulation of Chou, I.M., 1987, Phase relations in the system NaCl- meteoric groundwater formed to collapse the system, KCl-H2O. III: Solubilities of halite in vapor-saturated resulting in progressively larger degrees of cooling, liquids above 445℃ and redetermination of phase equili- ℃ dilution and oxidation of hydrothermal fluids. Base brium properties in the system NaCl-H2O to 1000 and 1500 bars, Geochimica et Cosmochimica Acta, v. 51, p. metals(Cu, Zn, etc.), gold, and possibly calcium were 1965-1975. leached from surrounding sedimentary rocks(largely the Cline, J.S., and Bodnar, R.J., 1994, Direct evolution of Konchonri Formation shale) during the meteoric water brine from a crystallizing silicic melt at the Questa, New circulation, and formed the fluids related to stage III to Mexico, molybdenum deposit, Economic Geology, V. V mineralization. 89, p. 1780-1802. Crerar, D.A., Wood, S., Brantly, S., and Bocarsly, A., 1985 References Chemical controls on solubility of ore-forming minerals in hydrothermal solutions, Canadian Mineralogist, V. 23, Ahmad, S.N., and Rose, A.W., 1980, Fluid inclusions in p. 333-352. porphyry and skarn ore at Santa Rita, New Mexico, Eastoe, C.J., 1978, A fluid inclusion study of the Panguna Economic Geology, V. 75, p. 229-250. porphyry copper deposit, Bougainville, Papua New Badham, J.P.N., 1975, Mineralogy, paragenesis and origin Guinea, Economic Geology, V. 73, p. 721-748. of the Ag-Ni, Co arsenide mineralization, Camsell River, Goodz, M.D., Watkinson, D.H., Smejkal, V., and Pertold N. W. T., Canada, Mineralium Deposita, v. 10, p. 153-175. Z., 1986,Sulfur-isotope geochemistry of the silver- sul- Badham, J.P.N., 1976, Orogenesis and metallogenesis with farsenide vein mineralization, Cobalt, Ontario, Canadian reference to the silver-nickel-cobalt-arsenide ore associ- Journal of Earth Sciences, v. 23, p. 1551-1567. ation, in Strong, D.F., ed., Metallogeny and Plate Tec- Halls, C., and Stumpfl, E.F., 1972,The five-element (Ag- tonics, Geological Association of Canada, Special Paper, Bi-Co-Ni-As) vein deposits - A critical appraisal of the v. 14, p. 541-548. geological environments in which it occurs and of the Barton, P.B., Jr.,and Skinner, B.J., 1979, Sulfide mineral theories affecting its origin [abs.]: 24th International stabilities, in Barnes, H.L., ed., Geochemistry of Hydro- Geological Congress, Montreal, Section 4, p. 540. thermal Ore Deposits (2nd ed.)., New York, Wiley Henley, R.W., and McNabb, A., 1978, Magmatic vapor Intersci., p. 278-403. plumes and groundwater interaction in porphyry copper Barton, P.B., Jr., and Toulmin, P., III, 1964, The electrum emplacement, Economic Geology, V. 73, p. 1-20. tarnish method for determination of the fugacity of sulfur Horrall, K.B., Hagni, R.D., and Kisvarsanyi, G., 1993, in laboratory sulfide systems: Geochimica et Cosmochi- Mafic and ultramafic plutons associated with the New mica Acta, v. 33, p. 841-857. Madrid Rift Complex - A possible major source of the Bloom, M.S., 1981, Chemistry of inclusion fluids; stock- copper-cobalt-nickel mineralization of southeast Misso- work molybdenum deposits from Questa, New Mexico, uri, Economic Geology, V. 88, p. 328-343. Hudson Bay Mountain, and Endako, British Columbia, Kamilli, R.J., 1978, The genesis of stockwork molybdenite Economic Geology, V. 76, p. 1906-1920. deposits: Implications from fluid inclusion studies at the Bodnar, R.J., 1993, Revised equation and table for deter- Henderson mine [abs.], Geological Society of America

mining the freezing point depression of H2O-NaCl Abstracts with Programs, V. 10, p. 431. solutions, Geochimica et Cosmochimica Acta, v. 57, p. Kerrich, R., Strong, D.F., Andrews, A.J., and Owsiacki, L., 683-684. 1986,The silver deposits at Cobalt and Gowganda, Bodnar, R.J., Burnham, C.W., and Sterner, S.M., 1985, Ontario: III. Hydrothermal regimes and source reservoirs Synthetic fluid inclusions in natural guartz. III. Deter- -evidence from H, O, D, and Sr isotopes and fluid

mination of equilibrium properties in the system H2O- inclusions, Canadian Journal of Earth Sciences, V. 23, NaCl to 1000℃ and 1500 bars, Geochimica et Cosmo- p. 1519-1550. chimica Acta, V. 49, p. 1861-1873. Kissin, S.A., 1988, Nickel-cobalt-native silver (five- Burnham, C.W., and Ohmoto, H., 1980, Late-stage pro- element) veins: A rift-related ore type, in Kisvarsanyi, cesses of felsic magmatism, Mining Geology Special G., and Grant, S.K., eds., Proceedings of North American

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Conference on Tectonic Control of Ore Deposits and the Schneider, H.J., 1972, Stratabound polymetallic and Fe-Ba Vertical and Horizontal Extent of Ore Systems. Univ. deposits of the Sarrabus-Gerrei region, southeastern Missouri-Rolla, Rolla, p. 268-279. Sardinia: I. Mineral deposits and geology, Neues Jahr- Kissin, S.A., 1993, The geochemistry of transport and buch fÜr Mineralogie Monatschafte, V. 12, p. 529-541 deposition in the formation of five-element (Ag-Ni-Co-- (in German). As-Bi) veins, in Maurice, Y.T., ed., Proceedings of the Shimizu, M., and Shikazono, N., 1985, Iron and zinc Eighth Quadrennial IAGOD Symposium, Stuttgart, E. partitioning between coexisting stannite and sphalerite: Schweizerbart'sche Verlagsbuchhandlung, p. 773-786. A possible indicator of temperature and sulfur fugacity, Kretschmar, U., and Scott, S.D., 1976, Phase relations Mineralium Deposita, v. 20, p. 314-320. involving arsenopyrite in the system Fe-As-S and their So, C.S., Shelton, K.L., Chi, S.J., and Yun, S.T., 1991, application, Canadian Mineralogist, v. 14, p. 364-386. Geochemical studies of the Gyeongchang W-Mo mine, Krutov, G.A., 1977, Deposits of cobalt, inSmirnov, V.I., Republic of Korea: Progressive meteoric water inund- ed., Ore Deposits of the USSR, London, Pitman Publi- ation of a magmatic hydrothermal system, Economic shing, V. 2, p. 80-105. Geology, V. 86, p. 750-767. Nakamura, K., 1942, A report of Korean cobalt mine, So, C.S., Choi, S.H., and Shelton, K.L, 1997, Geochemistry Journal of Geology (Japan), V. 49, p. 218-220 (in Ja- and genesis of hydrothermal Cu deposits in the Gyeo- panese). ngsang Basin (Andong area), Korea: A link between Nakamura, Y., and Shima, H., 1982, Fe and Zn partitioning porphyry and epithermal systems, Neues Jahrbuch fÜr between sphalerite and stannite [abs.], Joint Meeting of Mineralogie Abhandlungen, V. 171, p. 281-307. Society of Mining Geologists of Japan, Japanese Asso- Sourirajan, S., and Kennedy, G.C., 1962, The system H2O- ciation of Mineralogists, Petrologists and Economic NaCl at elevated temperatures and pressures, American Geologists, and Mineralogical Society of Japan, 1982, Journal of Science, V. 260, p. 115-141. Abstracts, p. A-8. Sterner, S.M., Hall, D.L., and Bodnar, R.J., 1988, Synthetic Park, M.E., 1990, Mineralization and paragenesis of the fluid inclusions. V. Solubility of the system NaCl- cobalt-bearing sulfide and arsenide minerals in Gyeon- KCl-H2O under vapor-saturated conditions, Geochimica gsan area, Geological Society of Korea Journal, V. 26, et Cosmochimica Acta, v. 52, p. 989-1005. p. 18-31 (in Korean). Susak, N.J., and Crerar, D.A., 1985, Spectra and coordi- Reynolds, T.J., and Beane, R.E., 1985, Evolution of hydro- nation changes of transition metals in hydrothermal thermal fluid characteristics at the Santa Rita, New solution: Implications for ore genesis, Geochimica et Mexico, porphyry copper deposit, Economic Geology, V. Cosmochimica Acta, V. 49, p. 555-564. 80, p. 1328-1347. Uchida, E., Goryozono, Y., and Naito, M., 1996, Aqueous Roedder, E., 1984, Fluid inclusions, Reviews in Miner- speciation of magnesium, strontium, nickel and cobalt alogy, V. 12, p. 644. chlorides in hydrothermal solutions at 600℃ and 1 kbar, Samson, I.M., 1990, Fluid evolution and mineralization in Geochemical Journal, V. 30, p. 99-109. a subvolcanic granitestock: The Mount Pleasant W- Yun, S.T., and Youm, S.J., 1997, Temporal variations of Mo-Sn deposits, New Brunswick, Canada, Economic ore mineralogy and sulfur isotope data from the Boguk Geology, V. 85, p. 145-163. cobalt mine: Implication for genesis and geochemistry of Scott, S.D., and Barnes, H.L., 1971, Sphalerite geothe- Co-bearing hydrothermal system, Economic and Enviro- rmometry and geobarometry, Economic Geology, v. 66, nmental Geology (Korea), v. 30, p. 289-301. p. 653-669.

한국지구시스템공학회지 Fluid Inclusion Study of a Magmatic Cobalt Mineralization at the Boguk Mine, Korea 117

허 철 호 이 재 호 현재 국립공원관리공단 국립공원연구원 책임연구원 현재 한국지질자원연구원 지질기반정보연구부 선임연구원 (本 學會誌 第42卷 第5号 參照) (本 學會誌 第42卷 第5号 參照)

윤 성 택 소 칠 섭 1985년 고려대학교 지질학과 이학사 1966년 독일 뮌헨대학교 응용지질․광 1987년 고려대학교 대학원 이학석사 물학 이학석사 1991년 고려대학교 대학원 이학박사 1968년 독일 뮌헨대학교 응용지질․광 물학 이학박사

현재 고려대학교 지구환경과학과 교수 현재 고려대학교 지구환경과학과 교수 (E-mail; [email protected]) (E-mail; [email protected])

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