Original Articlerge_104 374..387

Mesothermal Gold Vein Mineralization of the SeolhwaMine, Asan Mining District, Korea

Chul-Ho Heo,1 Se-Jung Chi1 and Chil-Sup So

2

1Mineral Resources Group, Geology and Geoinformation Division, Korea Institute of Geoscience and Mineral Resources(KIGAM), Yuseong-gu, Daejeon, and 2Emeritus Professor, Department of Earth and Environmental Sciences, Korea University,Seongbuk-gu, Seoul, Korea

Abstract

Gold mineralization of the Seolhwa mine occurs in a single stage of massive quartz veins which filled thenorth-east-trending fault shear zones in the Jurassic granitoid of 161 Ma within the Gyeonggi Massif. The veinquartz contains three main types of fluid inclusions at 25°C: (i) aqueous type I inclusions (0–15 wt.% NaCl)containing small amounts of CO2; (ii) gas-rich (more than 70 vol. %), vapor-homogenizing, aqueous type IIinclusions; and (iii) low-salinity (less than 5 wt.% NaCl), liquid CO2-bearing, type III inclusions. The H2O-CO2-CH4-N2-NaCl inclusions represent immiscible fluids trapped earlier along the solvus curve in the tem-perature range 250–430°C at pressures of ~1 kb. Detailed fluid inclusion chronologies suggest a progressivedecrease in pressure during the mineralization. Aqueous inclusion fluids represent either later fluids evolvedthrough extensive fluid unmixing from a homogeneous H2O-CO2-CH4-N2-NaCl fluid due to decreases intemperature and pressure, or the influence of deep circulated meteoric waters. Initial fluids were homogeneousH2O-CO2-CH4-N2-NaCl fluids as follows: 250° to 430°C, 16–62 mol% CO2, 5–14 mol% CH4, 0.06–0.31 mol% N2

and salinities of 0.4–4.9 wt.% NaCl. The T-X data for the Seolhwa mine suggest that the hydrothermal systemhas been probably located nearer to the granitic melt, which facilitated the CH4 formation and resulted in areduced fluid state indicated by the predominance of pyrrhotite. Measured and calculated isotopic composi-tions of the hydrothermal fluids [d18O = 5.3–6.5‰; dD = -69 to -84‰] provide evidence of the CH4-H2Oequilibria and further indicate that the auriferous fluids were magmatically derived. Both the dominance of d34Svalues of sulfides close to the meteoric reference (-0.6–1.4‰; d34SSS values of 0.3–1.1‰) and the available d13Cdata (-4‰) are consistent with their deep igneous source. The Seolhwa mine was probably formed by extensivefracturing and veining due to the thermal expansion of water derived from the Jurassic granitoid melt.

Keywords: fluid inclusion, gold, Korea, mesothermal, stable isotope.

1. Introduction

In South Korea, most gold-silver vein deposits areintimately associated with Jurassic and Cretaceousgranitoids (Shelton et al., 1990; So et al., 1995; So andYun, 1997). The Cretaceous granites have been shownto be lower level intrusions than the Jurassic granites

(Cho & Kwon, 1994). The gold-silver deposits inSouth Korea display a consistent relationship amongmineralization age, depth, water to rock ratio, and Agto Au ratio (So & Shelton, 1987; Shelton et al., 1988;Shelton et al., 1990; So et al., 1995; Heo et al., 2003;Pak et al., 2006): Jurassic, gold-rich mesothermal-type deposits; Early Cretaceous, Korean-type gold-

Received 17 January 2007. Accepted for publication 13 June 2008.Corresponding author: C.-H. Heo, Mineral Resources Group, Geology and Geoinformation Division, Korea Institute of Geoscienceand Mineral Resources (KIGAM), 30 Gajeong-dong, Yuseong-gu, Daejeon 305-350, Korea. Email: chheo@kigam.re.kr

doi: 10.1111/j.1751-3928.2009.00104.x Resource Geology Vol. 59, No. 4: 374–387

374© 2009 The Authors

Journal compilation © 2009 The Society of Resource Geology

silver deposits; Late Cretaceous, silver-rich epithermaldeposits.

Mesothermal-type deposits seem to be geneticallyassociated with Jurassic granitoids (Shelton et al., 1988;So et al., 1995; Heo et al., 2003; Pak et al., 2006) and arecharacterized by low Ag/Au (0.13–3.8) ratio. They arecomposed of quartz veins of milky or gray color withsmall amounts of sulfide minerals. Fluid inclusionstudies indicate that gold deposition occurred at tem-peratures of 300–370°C in response to unmixing ofCO2-rich hydrothermal fluids at depths more than4.5 km. Korean-type gold-silver deposits are geneti-cally associated with late Jurassic to early Cretaceousgranites, and are characterized by low Ag/Au (0.5–3.0)ratios and a paucity of sulfide minerals. Gold deposi-tion occurred at temperatures near 270°C in responseto boiling and cooling at depths near 1.25 km (So &Shelton, 1987). Silver-rich, epithermal-type deposits aregenetically associated with late Cretaceous to Tertiarygranites and are characterized by relatively highAg/Au ratios (10–200) and complex sulfide and/orsulfosalt mineralization. Geochemical studies indicatethat gold-silver deposition occurred at depths less than0.75 km (Shelton et al., 1990).

The Seolhwa mine (N 36°43′, E 127°02′) is locatedabout 100 km south of Seoul (Fig. 1). The mine wasexploited until 1987, but is now closed. The aims of thepresent study are to document the nature of gold min-eralization and understand the origin and physico-chemical conditions of the auriferous hydrothermalfluids at Seolhwa.

2. Geologic setting and ore veins

The mine area is mainly underlain by biotite graniteintruded Precambrian basement of banded biotitegneiss (Fig. 2). The biotite granite mainly containsquartz, feldspar and biotite with small amounts ofamphibole and chlorite as accessory minerals. It ischaracterized by fine- to medium-grained equigranu-lar texture and yields Rb-Sr two-point isochron age of161 � 4 Ma (Table 1). Gold-bearing quartz vein strikesN10–20°E and dips 55–60°NW. The vein width rangesfrom 0.05 to 0.1 m, but ore grades are relatively high(5.6–102.7 g t-1 Au; 3–17 g t-1 Ag). Electrum-base metalsulfide mineralization at the Seolhwa mine occurs inmassive quartz veins, which are mineralogicallysimple. The veins contain minor amounts (usually <5vol.%) of ore minerals including molybdenite,scheelite, pyrite, marcasite, pyrrhotite, sphalerite, chal-copyrite, galena, tellurobismuthite, bismuthinite,

native gold, electrum. Wall-rock alteration occurs asnarrow (<0.4 m wide) linear bleached zones adjacent tothe vein margins.

3. Mineral paragenesis

Veins of the Seolhwa mine were formed during twomineralization stages separated by a major tectonicfracturing event (Fig. 3).

3.1 Stage I

Economic quantities of gold, together with quartz andsulfides, were introduced during this stage (Fig. 3).Stage I mineralization at Seolhwa mine is characterizedby dominant and massive milky quartz with sulfides.Massive quartz is frequently cut by randomly orientedfractures (mostly less than 1 mm in width). Molybden-

SEOUL

38o

36o

127o 129o

100km0

N

MID CRETACEOUS TO PALEOGENE GRANITES

MID CRETACEOUS SED. & VOLCANIC ROCKS

MID JURASSIC TO LOWER CRETACEOUS GRANITES

CAMBRIAN TO UPPER JURASSIC ROCKS

PRECAMBRIAN ROCKS

Fig. 1 General geologic map of Korea, showing the loca-tion of the Seolhwa mine. Modified from Geology ofKorea Geological Society of Korea (1999).

Mesothermal Gold Seolhwa

© 2009 The AuthorsJournal compilation © 2009 The Society of Resource Geology 375

ite is the first formation of sulfide phase and sheeliteoccurs as dissemination in veins and wall rocksgreisen. Scheelite occurs as anhedral masses intimatelyintergrown with chalcopyrite, pyrrhotite and sphaler-ite. Pyrite occurs as both wall-rock alteration productsand vein mineral. Fine-grained pyrite aggregatescontain rare anhedral pyrrhotite grains. Pyrite is fre-quently replaced by marcasite along margins and/orfractures. Pyrrhotite occurs as subhedral masses inti-mately intergrown with chalcopyrite, electrum andsphalerite. It also occurs as an intimate intergrowthwith bismuthinite and chalcopyrite. Sphalerite (10.6–14.8 mol% FeS, Fig. 3) usually occurs as anhedral

masses throughout the veins and is closely intergrownwith pyrrhotite and chalcopyrite. Later sphaleriterarely occurs within microfractures cutting quartz andsulfides, and is associated with late quartz, chalcopy-rite, and electrum. Late stage I mineralization is char-acterized by deposition of tellurobismuthite andbismuthinite. These minerals are associated with elec-trum (20.2–30.3 atom.% Ag, Fig. 3) at central portionsof each vein where galena is disseminated. Tellurobis-muthite also occurs along small, galena-rich fracturescutting earlier vein materials. Bismuthinite occur withchalcopyrite and pyrrhotite and intergrows with elec-trum. These are also disseminated within anhedral

SeolhwaMine

55

N

0 500 1000 m

65

60

65

6260

LEGEND

Quaternary

Cretaceous

Jurassic

Precambrian

Alluvium

Unconformity

Acidic dyke

Intrusion

Biotite granite

Intrusion

Banded biotite gneiss

Quartz vein with dip

Fault

127 02'

36 43'

Fig. 2 Geologic map of the Seolhwa mine.

Table 1 Rb-Sr two-point isochron data of granitoid hosting the Seolhwa mine

Sample Description 86Sr(ppm)

87Rb(ppm)

87Sr/86Sr 87Rb/86Sr Isochron parameters Age(Ma � 1s)

Slope(¥10-3)

Intercept(87Sr/86Sr)0

SH-2 Whole-rock 50.0 34.5 0.7240 0.68 2.287 0.7224 161 � 4Biotite 4.08 144.5 0.8025 35.0

ppm, parts per million.

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galena as tiny (mostly less than 10 mm) inclusions.Native gold (9.1–20.0 atom. % Ag, Fig. 3) occurs asdiscrete grains of gold within the fracture of quartz andis intergrown with chalcopyrite. Electrum (20.2–30.3atom. % Ag, Fig. 3) is frequently associated with bis-muthinite, chalcopyrite, galena, sphalerite, and telluro-bismuthite infilling the fractures of quartz and sulfides.

3.2 Stage II

Stage II of the Seolhwa mine is represented by massivewhite calcite, which filled open-spaces created by post-ore fault and brecciation (Fig. 3). Calcite occasionallyoccurs as clear rhombohedral crystals up to 1 cm inlength.

4. Fluid inclusion study

Vein samples collected from the Seolhwa mine wereinvestigated by microthermometry in order to docu-ment fluid compositions and mineralization tempera-tures, and to examine the thermal history of auriferoushydrothermal fluids. Investigated samples from theSeolhwa mine were restricted to vein quartz and somecalcite. Sphalerites were not suitable for the study, dueto their opacity and massive occurrence.

Microthermometric data were obtained on a gas-flow heating/freezing stage (USGS/Fluid Inc.) cali-brated with synthetic H2O and CO2 inclusions andvarious organic solvents (Hollister, 1981; Shepherdet al., 1985). Heating rates were varied but were main-tained at 1°C min–1 for the determination of meltingtemperatures and carbonaceous-phase homogeniza-tion temperatures, and near 10°C min–1 for the mea-surement of total homogenization temperatures.During freezing experiments, the sequential repeatedfreezing technique described by Haynes (1985) wasused over the expected temperatures for optimum pre-cision. The temperatures of the total homogenizationand carbonaceous phase homogenization, and themelting temperatures of carbonaceous phase, ice andclathrate, are within standard errors of �1.0° and�0.2°C, respectively. The measured number of fluidinclusions is 618.

4.1 Occurrence and types of fluid inclusions

Massive vein quartz from the Seolhwa mine is rich influid inclusions, probably due to repeated fracturingand healing both during and after quartz deposition.The 618 fluid inclusions were measured and the sizevaried from 3 to 27 mm with an average of 11mm. Threemain types of fluid inclusions were identified at 25°C(Nash, 1972), combined with their behavior duringcooling (down to about -100°C) and slight heating (upto about 30°C). They are, by order of decreasing abun-dance: type I (liquid CO2 is not present at room tem-perature), type II (gas-rich), and type III (liquid CO2-bearing at room temperature). Two subtypes of type Ifluid inclusions have been distinguished on the basis oftheir occurrence and compositional characteristics:type Ia (primary and/or pseudosecondary aqueousfluid inclusions indicating the presence of a nucleatedclathrate upon heating after cooling) and type Ib(aqueous inclusions having the inability to nucleate aclathrate). Type III inclusions are subdivided into twotypes by their homogenization behavior: type IIIa(homogenized into aqueous phase) and type IIIb

Chlorite

Sericite

Epidote

Quartz

Molybdenite

Scheelite

Pyrite

Marcasite

Pyrrhotite

Sphalerite

Chalcopyrite

Galena

Tellurobismuthite

Bismuthinite

Electrum

Calcite

gnirutcarf

MINERALS

14.8–10.6 mol% FeS

20.2–30.3 atom. % Ag

9.1–20.0 atom. % Ag

STAGE I STAGE II

Native Gold

Fig. 3 Generalized paragenetic sequence of mineralsfrom veins and alteration zones of the Seolhwa mine.The FeS contents of sphalerite range from 10.6–14.8mol. The silver contents of native gold range from 9.1to 20.0 atomic %. The silver contents of electrum rangefrom 20.2 to 30.3 atomic %.

Mesothermal Gold Seolhwa

© 2009 The AuthorsJournal compilation © 2009 The Society of Resource Geology 377

(homogenized into carbonaceous phase). All types offluid inclusions have been observed in stage I veinquartz. Stage II calcite samples contain type Ib inclu-sions only.

4.2 Heating and freezing data

Six-hundred and eighteen fluid inclusions include 565primary, pseudosecondary and 13 secondary inclu-sions in stage I vein quartz and 29 primary, pseudosec-ondary and 11 secondary inclusions in stage II calcite.Salinity data were reported based on freezing-pointdepression in the system H2O-NaCl (Potter et al., 1978)for H2O-rich, type Ib and II, and on clathrate meltingtemperatures (Bozzo et al., 1975; Collins, 1979) for CO2-bearing type Ia and III inclusions.

Primary and pseudosecondary inclusions of stage Imilky quartz include CO2-bearing, type Ia and III inclu-sions. A wide range of fluid inclusion homogenizationtemperatures in vein quartz reflects several hydrother-mal episodes rather than a single specific event, asindicated by textural evidence of multiple opening andfilling of the veins.

4.2.1 Type I inclusions

Melting temperatures of the solid CO2 in type Ia inclu-sions range from -61.6° to -56.8°C. Clathrate meltingtemperatures of type Ia inclusions range from 0.1° to10.0°C. The measured clathrate melting temperaturescorrespond to salinities ranging from 0.0–15.4 wt.%equivalent NaCl (Fig. 4). The total homogenization tem-peratures range from 226–432°C for type Ia inclusions(Fig. 5). Type Ib inclusions are unable to nucleate aclathrate on cooling, suggesting a maximum concentra-tion of less than 2.7 wt.% CO2 (Collins, 1979; Hedenquist& Henley, 1985). Type Ib inclusions homogenize at tem-peratures ranging from 138°C to 388°C, and have salini-ties of 0.2–15.2 wt.% equivalent NaCl (Figs 4, 5).Homogenization temperatures of primary and pseudo-secondary inclusions (all type Ib) in stage II calcite rangefrom 171°C to 284°C (Fig. 5), which are lower than thoseof primary and pseudosecondary inclusions in stage Iquartz. This indicates that stage II mineralizationoccurred from much cooler fluids possibly as a result ofmuch influx of meteoric waters in the hydrothermalsystem. The estimated salinities of stage II fluids rangefrom 2.4 to 4.8 wt.% equivalent NaCl (Fig. 4).

4.2.2 Type II inclusions

Most type II inclusions were not suitable for freezingexperiments due to their small size. Two melting tem-

peratures of the iced phase in the experiments were-3.0° and -9.1°C, indicating salinities of 4.96 and 12.96wt.% equivalent NaCl (Potter et al., 1978). Type II inclu-sions were homogenized to vapor phase at 224°C to406°C (Fig. 5).

4.2.3 Type III inclusions

Melting of the solid CO2 occurred at temperaturesranging from -59.0°C to -56.6°C: type IIIa, -59.0° to-56.7°C and type IIIb, -58.9° to -56.6°C. Homogeniza-tion of the carbonaceous phase occurred at the follow-ing temperatures: type IIIa, 15.1°C to 26.6°C and type

Wt. % NaCl equiv.

0 105 150 105 150 105 150 105 150 105 150 105 150 105 150 105 150 105 150 105 150 105 150 105 150 105 150 105 150 105 150 105 15

v

0 105 15

Num

ber

of m

easu

rem

ents

10

20

30

40

50

0

10

0

Stage I Quartz

Stage II Calcite

Type IIType Ia

Type Ib

P+PS S

Type Ib

P+PS S

v

Wt. % NaCl equiv.

Type IIIa

Type IIIb

P+PS

Fig. 4 Frequency diagrams of salinities of fluid inclu-sions in stage I quartz and stage II calcite from theSeolhwa gold mine. P, primary fluid inclusion; PS,psuedosecondary fluid inclusion; S, secondary fluidinclusion.

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IIIb, 16.1° to 30.4°C. Clathrate melting of type III inclu-sions ranged from 7.5° to 9.8°C. Measured clathratemelting temperatures correspond to salinities rangingfrom 0.4–4.9 wt.% equivalent NaCl (Fig. 4). The homog-enization temperatures of type III inclusions were 269°to 429°C for type IIIa and 253° to 414°C for type IIIb,respectively (Fig. 5).

4.3 Fluid composition

4.3.1 Bulk gas analyses

Gas analyses of bulk inclusion fluids for some repre-sentative quartz specimens were carried out by meansof the gas chromatograph technique of the Departmentof Mineral Development Engineering, University ofTokyo in Japan (Zaw et al., 1991). Fine-grained quartzspecimens sieved between 0.1 and 0.25 mm in diam-eter have been provided for gas analyses of inclusionfluids. The fine-grained quartz of 5 g for one experi-mental run was decrepitated by heating at 500°C in a

vacuum-line system to extract inclusion fluid. Theinclusion fluid was classified into two groups of H2Otrapped by icing effect and condensed gases pushedinto a gas holder. The volume of H2O was calculated bydetermination of H2 gas transferred from ice. While, thegas species and each volume of condensed gases weredetermined by a gas chromatograph analyzer. In thepresent study, four gases such as H2O, CO2, CH4, andN2 have been extracted. CH4 and N2 exist in smallamounts (less than 1 mol%).

The amounts of H2O, CO2, N2, and CH4 released fromfluid inclusions of the given samples are shown inTable 2. The results show that auriferous hydrothermalfluids consist dominantly of H2O and CO2 with smallamounts of CH4 and N2 (Table 2).

4.3.2 Composition of initial fluid inclusion

As described earlier, the type III inclusions are generallyhigh in temperature, low in salinity, and rich in liquidCO2 as well. It thus appears likely that type III fluidinclusions are well representative of initial inclusionfluids related closely to mineralization.

Bulk compositions and densities of type III inclu-sions were estimated from data on visual volumes ofcarbonaceous and aqueous phases, combined with thecompositional data and densities of the carbonaceousand aqueous phases (Table 3). The quantitative V-XCH4

projection of the CO2-CH4 system (Heyen et al., 1982),combined with the CO2 melting and homogenizationtemperatures, was used to estimate the CH4 contents intype III inclusions. Visually estimated mol% of CH4 inthe non-aqueous part of type III inclusions are 5–14mol% CH4: 5–8 mol% in type IIIa and 8–14 mol% intype IIIb (Fig. 6). CH4 is supposed to be the sole agentresponsible for the observed CO2 melting and homog-enization temperature depressions. This presumptionis likely because the P-V-T-X properties of coexistingcarbonaceous liquid and vapor for CO2-CH4-N2 mix-tures containing relatively small amounts of N2 are

100 200 300 400 500

100 200 300 400

Th ( )Co

stne

merusaem f

o reb

muN

0

10

0

20

10

0

10P+PS

P+PS

P+PS

S

S

Type Ib

Type Ib

Type Ia

Type II

Type IIIaType IIIb

P+PS

Stage II Calcite

Stage I Quartz

Fig. 5 Frequency diagrams of total homogenization tem-perature of fluid inclusions in stage I quartz and stageII calcite from the Seolhwa gold mine. P, primary fluidinclusion; PS, psuedosecondary fluid inclusion; S, sec-ondary fluid inclusion.

Table 2 Bulk chemistry of the vein quartz from theSeolhwa mine

Sample no. H2O(mol%)

CO2

(mol%)CH4

(mol%)N2

(mol%)

SH-1 96.97 2.43 0.42 0.18SH-3 97.18 2.62 0.06 0.14SH-5 97.00 2.43 0.25 0.31SH-7 92.41 7.30 0.35 0.06SH-90 95.77 3.28 0.71 0.24

Mesothermal Gold Seolhwa

© 2009 The AuthorsJournal compilation © 2009 The Society of Resource Geology 379

Table 3 Microthermometric data of selected carbonaceous inclusions in stage I vein quartz from the Seolhwa mine

Type TmCO2 Tmcl. Thco2 Th Vol.aq. dco2 daq. dtotal NaCl Xco2 XH2O XNaCl

(°C) (%) (g cc-1) (wt.%) (mol fraction)

IIIa -59.0 9.5 26.6 269 30 0.71 0.76 0.73 1.0 0.47 0.53 0.00-58.8 9.8 23.5 282 20 0.74 0.74 0.74 0.4 0.62 0.38 0.00-56.7 9.6 23.6 307 30 0.74 0.69 0.73 0.8 0.51 0.49 0.00-57.8 9.0 24.1 320 60 0.73 0.66 0.69 2.0 0.23 0.76 0.00-56.8 9.7 25.6 307 70 0.72 0.69 0.70 0.6 0.16 0.84 0.00-58.1 9.2 25.5 298 55 0.72 0.71 0.71 1.6 0.26 0.74 0.00-57.7 9.3 20.2 308 40 0.78 0.69 0.74 1.4 0.41 0.59 0.00-57.8 9.2 15.1 307 35 0.82 0.69 0.77 1.6 0.48 0.52 0.00-57.7 8.8 15.2 326 45 0.81 0.65 0.74 2.4 0.39 0.61 0.00-57.9 8.6 19.8 318 40 0.79 0.67 0.74 2.8 0.42 0.57 0.01-57.8 8.9 19.7 325 45 0.79 0.65 0.73 2.2 0.38 0.61 0.00-57.4 8.2 25.5 291 30 0.72 0.72 0.72 3.6 0.49 0.50 0.01

IIIb -57.4 9.1 30.4 280 20 0.63 0.74 0.65 1.8 0.59 0.41 0.00-57.2 9.2 29.9 287 20 0.60 0.73 0.63 1.6 0.58 0.42 0.00-56.6 9.6 29.7 305 30 0.61 0.69 0.63 0.8 0.46 0.54 0.00-58.9 7.5 18.0 311 40 0.78 0.68 0.74 4.9 0.42 0.57 0.01-58.4 8.3 17.4 288 30 0.81 0.73 0.79 3.4 0.52 0.47 0.01-57.1 9.0 24.8 253 20 0.73 0.79 0.74 2.0 0.61 0.39 0.00-58.5 9.1 19.6 325 40 0.78 0.65 0.73 1.8 0.43 0.57 0.00-57.5 8.7 27.5 282 30 0.68 0.74 0.70 2.6 0.47 0.52 0.00-58.0 8.9 16.1 314 45 0.82 0.67 0.75 2.2 0.38 0.61 0.00-58.1 9.0 18.2 322 40 0.82 0.66 0.76 2.0 0.44 0.56 0.00-57.9 8.7 27.6 314 50 0.68 0.67 0.68 2.6 0.30 0.70 0.01

daq., density of aqueous; dCO2, density of CO2; dtotal, total density; Th, total homogenization temperature; ThCO2, homogenization temperatureof CO2; TmCO2, melting temperature of solid CO2; Tmcl., melting temperature of clathrate; Volaq, volume percent of aqueous phase at 25°C;XCO2, mol fraction of CO2; XH2O, mol fraction of H2O; XNaCl, mol fraction of NaCl.

–57 –58 –59 –60

90

70

50

30

5

0–5–10–15–20

0 4 8 12 16 20 24 28

Type IIIa

Type IIIb

Mole % CH4

Mol

ar v

olum

e cm

. m

ol3

-1

Final melting of solid CO ( C)2 o

Hom

ogenization of CO

( C) (L+

V L)

2o

TM

CO

2

critical curve

20 C

15 C

10 Coo

o

Fig. 6 Graph for calculating themol composition of CO2-CH4

mixtures using the final meltingtemperature of solid CO2 (TmCO2)and the temperature of liquid-vapor homogenization (ThCO2).Solid curves are for ThCO2, lightcurves are for TmCO2. The mol%CH4 is given by the intersectionof the appropriate ThCO2 andTmCO2 curves. This diagram refersto inclusions that homogenizeinto the liquid state. This diagramwas modified from Shepherdet al. (1985).

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very similar with those of the CO2-CH4 systembetween -20° and +15°C (Arai et al., 1971; Sarashinaet al., 1971).

4.4 Variations in temperature and compositionof auriferous hydrothermal fluids

A wide range of homogenization temperature of fluidinclusions probably reflects several hydrothermal epi-sodes rather than one specific event (Fig. 7). The upperend (ª450°C) of homogenization temperature for stageI mineralization indicates that the vein mineralizationat the Seolhwa mine belongs to a mesothermal-typedeposit.

As described previously, hydrothermal fluids princi-pally observed in stage I minerals are composed of twotypes: (i) dominantly aqueous fluids of moderate salin-ity (type Ib), containing minor amounts of CO2 (typeIa); and (ii) mixed CO2-H2O fluids of low salinity (typeIII). CO2 contents of the type III fluid inclusions arehighly variable within individual samples. CO2-richtype IIIb inclusions homogenized at nearly the sametemperatures as H2O-rich type IIIa inclusions. Theseobservations indicate that the type III fluid inclusionsrepresent trapping of immiscible H2O-CO2 fluids,which evolved through CO2 effervescence. The rela-tionship between homogenization temperature andsalinity for stage I fluids is shown in Figure 7. LiquidCO2-bearing type III fluid inclusions of stage I showdecreases in salinity from 5–0 wt.% NaCl as the tem-perature decreases from 450°C to 250°C. This isexplained by boiling of CO2-rich fluids. CO2-poorfluids after extensive degassing of CO2 were probably

trapped at the temperature between 370°C and 250°Cas the CO2 clathrate-forming type Ia fluid inclusions ofstage I. Type Ia inclusions tend to show the increase ofsalinity from 2–15 wt.% equivalent NaCl with decreas-ing temperature, likely indicating the extensive contin-ued boiling coupled with unmixing of CO2-poor fluids.Such continued boiling of hydrothermal fluids isthought to be the result of pressure decrease during theascent of hydrothermal fluids. Following the completedepletion of CO2 from stage I fluids, the residual fluidswere trapped as CO2-absent type Ib fluid inclusions inquartz (Th°C = 200–300°C; Fig. 7).

Fluid boiling accompanying CO2 effervescence inhydrothermal systems may result in abrupt chemicalchanges in the residual liquid (e.g. fO2, fS2, pH, SH2S, etc.).These changes favor the deposition of precious metalsthrough destabilization of metal complexes (Seward,1973; Drummond & Ohomoto, 1985). Given the fre-quent association of galena, sphalerite and chalcopy-rite with gold in the Seolhwa mine, the role of sulfideprecipitation accompanying boiling is critical. Decreaseof sulfur activity with boiling, through sulfide deposi-tion and/or H2S loss, is likely the most importantmechanism for gold deposition at the Seolhwa mine.

4.5 Pressure consideration

The densities of carbonaceous phase (dcarb.) were deter-mined from carbonaceous-phase homogenization tem-peratures using phase diagram for CO2 (Angus et al.,1973; Hollister, 1981) and the isothermal d-P diagramfor CO2 at 40°C (Burruss, 1981). The results are shownin Table 3. The densities of CO2 for type III fluids are asfollows: type IIIa, 0.71–0.82 g cc-1 and type IIIb, 0.60–0.82 g cc-1. Isochores for type III fluids (d = 0.60–0.82 g cc-1) intersect the temperature planes of 253°C to326°C at pressures of 70–150 MPa (Roedder & Bodnar,1980) (Fig. 8). The density of the aqueous phase of typeIII inclusions was estimated using an equation byBodnar (1983). The bulk densities (dtotal) of the selectedcarbonaceous inclusions were calculated from the esti-mated densities and volumes of the carbonaceous andaqueous phases. The estimated bulk densities of inclu-sions range from 0.63 to 0.79 g cc-1 (Table 3). Bulk com-positions were calculated from the data of densities,volumes and bulk compositions of the carbonaceousand aqueous phases. The calculated mol fraction ofcarbonaceous species (XCO2) range from 0.16 to 0.62 fortype IIIa and from 0.30 to 0.61 for type IIIb (Table 3).

Based on the calculated fluid compositions and esti-mated fluid trapping temperatures, several immiscibil-

0

2

4

6

8

10

12

14

16

150 200 250 300 350 400 450

Th (°C)

Wt.

% N

aCl e

quiv

.

Type Ia

Type Ib

Type II

Type IIIa

Type IIIb

Fig. 7 Homogenization temperature versus salinitydiagram for fluid inclusions from the stage I quartzfrom the Seolhwa mine.

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ity P-T-X curves for the system H2O-CO2-NaCl (Bowers& Helgeson, 1983a, b) have been applied to estimatetrapping pressures of fluids. The maximum pressure isestimated to have not exceeded about 300 MPa basedon the depth of emplacement of the Jurassic Daebogranites that are interpreted to relate to the vein-forming events (Cho & Kwon, 1994). CO2-H2O inclu-sion fluids from quartz samples of stage I are plotted inthe two-phase region at 1 kb in the H2O-CO2 system(Hendel & Hollister, 1981; Fig. 9).

5. Stable isotope studies

Sulfur isotope compositions of nine sulfides (threechalcopyrite, one galena, three pyrrhotite and twosphalerite), oxygen isotope compositions of 12 veinquartz and six calcite, carbon isotope compositions ofsix calcite, and hydrogen isotope compositions of 13inclusion fluids were measured in order to elucidatethe origin and evolution of auriferous hydrothermalfluids in the Seolhwa mine. Conventional techniques ofextraction and analysis were used, as described byMcCrea (1950), Grinenko (1962), Hall and Friedman(1963). Data are reported in conventional d-notationrelative to the Canyon Diablo Troilite (CDT) standardfor S, the Pee Dee Belemnite (PDB) standard for C, and

Vienna SMOW for O and H. The standard error of eachanalysis is approximately �0.1‰ for S, C and O, and�2‰ for H.

5.1 Sulfur isotope

The d34S values of stage I sulfide minerals from theSeolhwa mine are as follows (Table 4): pyrrhotite, -0.4–0.9‰; chalcopyrite, 0.2–0.8‰; sphalerite, 1.1–1.4‰;galena, -0.6‰. Assuming depositional temperaturebased on fluid inclusions and paragenetic constraints,the following d34S values of H2S of fluids are calculated(Field & Fifarek, 1985): chalcopyrite, 0.3–0.9‰;sphalerite, 0.8–1.1‰; galena, 1.1‰ (Table 4). The pre-dominance of pyrrhotite and the alteration assemblagesericite � kaolinite indicates that sulfur in the Seolhwahydrothermal fluids was dominantly H2S. Therefore,the d34SH2S value of 0.3–1.1‰ (avg. = 0.8) is a goodapproximation of d34SSS values of the fluids, indicatingderivation of sulfur mainly from an igneous source.

5.2 Carbon, hydrogen and oxygen isotopes

d13C, d18O and dD values of calcite, quartz, and inclu-sion waters were measured (Table 5). The d18O values ofwaters were calculated using the isotope fractionationequations of Friedman and O’Neil (1977) and Matsu-

o

2

C.P.

Temperature ( C)

Pre

ssur

e (b

ars)

1200

800

400

100 200 300 4000

0

0.60

CO1600

2000

0.82

Isochores

Fig. 8 P-T diagram for the CO2 system. Data fromKennedy (1954). Isochores for type III fluids (d = 0.60–0.82 g cc-1) intersect the temperature planes of 253–326°C (taken from the homogenization temperaturesof type III inclusions, Fig. 5) at pressures of 0.7–1.5 kb.C.P., critical point.

o 0

Two phase region

Single fluid

1 kb400

300

200

0.0 0.2 0.4 0.6 0.8 1.0

Th(

C)

XCO2

2.6

Type IIIa

Type IIIb

Fig. 9 Total homogenization temperature versus calcu-lated mol fraction of CO2 for the type III fluid inclu-sion. The curves are the upper limit of the two-phaseregion at 1 kb in the H2O-CO2 systems for 2.6 and 0wt.% NaCl, respectively. Numerals 2.6 and 0 in thefigure correspond to NaCl wt.% of the fluids. Dia-grams after data from Hendel and Hollister (1981);Bowers and Helgeson (1983a, b).

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hisa et al. (1979), respectively, for quartz and calcite,coupled with fluid inclusion temperatures.

The d18O values of 12-vein quartz range from 9.4 to14.37‰ and those of six-vein calcite ranged from 9.97to 10.61‰. Using the equation of Matsuhisa et al.(1979), calculated d18Owater values of vein quartz are5.29–6.51‰. Using the equation of Friedman andO’Neil (1977), calculated d18Owater values of vein calciteare 6.20–7.27‰. Inclusion fluids extracted from 10quartz and three calcite specimens by crushing(Shelton, 1983) have dD values of -69 to -84‰. Therelatively narrow dD range indicates that the bulkchemical compositions of inclusion fluids from exam-ined samples are within a limited variation and theextracted fluids are representative of the mineralizingfluids.

d13C values of stage II calcite are -3.14 to -5.52‰.Using calcite-CO2 isotope fractionation equation ofFriedman and O’Neil (1977), coupled with fluid inclu-sion temperatures, calculated d13CCO2 are -1.87 to-4.0‰. The carbon isotope values may be compatiblewith a deep igneous carbon source (Burrows et al.,1986).

5.3 Interpretation of hydrogen and oxygenisotope results

Figure 10 shows the calculated isotopic compositionsfor the Seolhwa hydrothermal fluids on a hydrogenversus oxygen isotope diagram. Calculated oxygen iso-topic compositions for the mineralizing fluids overlapthe field of primary magmatic water, but their hydro-

gen isotope compositions have values up to 10‰ lowerthan the values of magmatic water. Their d18O-dD com-positions are significantly deviated from the field oftypical metamorphic waters.

Relatively uniform d18O values for the Seolhwa minemay be interpreted to represent deeply circulatedmeteoric waters, which have been highly evolvedthrough substantial fluid-rock interaction at elevatedtemperatures and low water-to-rock ratios (Nesbittet al., 1986; Shelton et al., 1988). Such a meteoric modelfor the Seolhwa mine seems adequate to explain thedepleted but variable dD values of the ore fluids.However, the meteoric model can not be successfullyapplied for the Seolhwa mine because: (i) post-ore cal-cites from the Seolhwa mine yielded a d13C value of-4‰, which is likely compatible with a deep igneouscarbon source (Burrows et al., 1986), and the d18Owater

values of around 6.0‰ (which are also within the mag-matic water value); (ii) the mixing between magmaticand highly-evolved meteoric waters (Burrows et al.,1987) also can not best explain the isotope systematicsbecause the input of a primary magmatic fluid from theadjacent granitic magma source could be essential inthe present case.

For the application of a magmatic model for theSeolhwa mine, the light dD values of the fluids must beexplained. Such depleted dD values can be bestexplained by the fractionation effect between water andCH4. Sheppard (1986) suggested that such a dD deple-tion might reflect either the fractionation effectsbetween water and other gas species, and noted thatwaters containing a significant quantity of D/H woulddeplete in dD values through the exchange reactionsinvolving reduced gas species such as CH4. This meansthat if a magmatic fluid encounters a strongly reducingenvironment containing graphite, some of the H2O willreact to form CH4 and the latter will have a relativelylow dD value, whereas the remaining H2O will bericher in deuterium by material balance. Therefore, it issuggested that the d18O-dD data for the Seolhwa mineare compatible with the deposition of quartz veinsfrom deeply sourced homogeneous fluids generatedduring the Jurassic granite intrusion, which had beenassociated with the regional tectonism (Cho & Kwon,1994). Other evidence for magmatic sources of orefluids is the common occurrence of liquid CO2-bearingfluid inclusions in Korean granites exclusively of Juras-sic age (Watanabe, 1981), possibly indicating a geneticassociation between the magmatic reservoirs and theore fluids (Shelton et al., 1988). Repeated fracturing andrehealing of the veins might have been associated with

Table 4 Sulfur isotope data of vein sulfides from theSeolhwa mine

Sample no. Mineral d34S(‰) T(oC)† d34SH2S(‰)‡

SH-1 Po 0.1SH-2 Po -0.4SH-3 Cp 0.2 300 0.3SH-5 Cp 0.8 340 0.9SH-7A Sp 1.1 340 0.8SH-7B Cp 0.5 340 0.6SH-9A Sp 1.4 340§ 1.1SH-9B Gn -0.6 340§ 1.1SH-11 Po 0.9

†Based on fluid inclusion and paragenetic constraints.‡Calculated sulfur isotope compositions of H2S in ore fluids,

using the isotope fractionation equation in Ohmoto and Rye(1979).

§Sulfur isotope temperature calculated using the equation forthe Sp-Gn pair in Ohmoto and Rye (1979) is 330 � 50°C. Cp,chalcopyrite; Gn, galena; Po, pyrrhotite; Sp, sphalerite.

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late-stage processes in orogenic belts. If uplift andunloading were accompanied during the mineraliza-tion in the Seolhwa mine, meteoric waters could reachthe mineralizing sites during the late stages of veinmineralization. In fact, the variations in fluid inclusionstypes for the veins indicate a pressure decrease duringmineralization, and furthermore the occurrence ofaqueous inclusions in Seolhwa mines is thought to rep-resent the late stage of involvement of deep-circulatedmeteoric waters.

6. Summary

1 Mesothermal gold mineralization of the Seolhwamine were deposited in a single stage of massivequartz veins, which filled the mainly north-east-trending fault shear zones exclusively in the grani-toid within the Gyeonggi Massif.

2 The mineralogy of the mesothermal-type veins issimple and consists mainly of rare sulfides andgold. Gold grains (electrum and native gold)associated with sulfides typically occur within theveinlets cutting earlier-deposited quartz. The elec-trums from the Seolhwa mine are gold-rich,ranging from 69.7–90.9 atomic % Au. The dominantiron-bearing sulfides are pyrrhotite, chalcopyrite

Table 5 Carbon, oxygen, and hydrogen isotope data for quartz, calcite and their inclusion fluids from the Seolhwa mine

Sample no. Stage Mineral d13C(‰)

d18O(‰)

dD(‰)

T(oC)†

d13CH2CO3

(‰)‡d18Owater

(‰)§

SH-3 I Quartz 10.94 -80 330 5.87SH-5 I Quartz 14.37 -83 310 6.51SH-14 I Quartz 12.71 -75 320 6.18SH-14 I Quartz 12.6 -84 320 6.18SH-16 I Quartz 13.12 -69 315 6.35SH-18 I Quartz 12.89 -75 320 6.18SH-20 I Quartz 12.22 -69 320 6.18SH-22 I Quartz 12.35 -71 320 6.18SH-24 I Quartz 13.01 -79 315 6.35SH-90 I Quartz 11.8 -79 330 5.87SH-504 I Quartz 9.8 350 5.29SH-505 I Quartz 9.4 350 5.29SH-C1 II Calcite -4.25 10.03 280 -2.62 6.20SH-C2 II Calcite -4.24 9.98 270 -2.72 6.53SH-C3 II Calcite -5.52 9.97 270 -4.00 6.53SH-C4 II Calcite -3.30 10.61 -78 260 -1.89 6.89SH-C5 II Calcite -3.14 10.55 -76 250 -1.87 7.27SH-C6 II Calcite -3.21 10.44 -81 250 -1.94 7.27

†Based on fluid inclusion temperatures and paragenetic constraints.‡Calculated H2CO3 (ap) compositions based on isotope fractionation of Friedman and O’Neil (1977).§Calculated water compositions based on quartz-water and calcite-water oxygen isotope fractionations of Matsuhisa et al. (1979) and

Friedman and O’Neil (1977), respectively.

–10

–30

–50

–70

–90

–110

–130

–10 –5 0 5 10 15

Metamorphic water

Magmatic water

Met

eoric

Wat

er L

ine

δ18O water (% )

δDw

ater (

% )

Fig. 10 Hydrogen versus oxygen isotope diagram dis-playing stable isotope systematics of hydrothermalfluid compositions from the Seolhwa gold mine. Circle,vein quartz. The fields of magmatic and metamorphicwater are from Taylor (1979).

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and sphalerite, and characteristically containBi(-Te-S) minerals.

3 Wall-rock alteration around the mesothermal veins isvery weak (<0.3 m in width. The Seolhwa biotitegranite yields two-point isochron Rb-Sr age of 161 �

4 Ma, implying a Late Jurassic granitic intrusion.

4 Three main types of fluid inclusions are identifiedwithin vein quartz from the Seolhwa mine: (i)aqueous type I inclusions (0–15 wt.% NaCl) contain-ing small amounts of CO2; (ii) gas-rich(>70 vol. %),vapor-homogenizing, type II inclusion; and (iii) low-salinity(<5 wt.% NaCl), liquid CO2-bearing type IIIinclusions. The mesothermal fluids evolved fromearly carbonaceous (CO2-CH4-bearing) to lateaqueous fluids with time, due to extensive fluidunmixing (CO2-CH4-effervescence). Homogeniza-tion temperatures of the early carbonaceous fluidsrange from 430°C to 250°C. Microthermometric dataof the carbonaceous fluid inclusions may indicate thepresence of minor amounts of CH4 in the inclusionfluids.

5 Initial original fluids of the Seolhwa mine are inter-preted to have been homogeneous H2O-CO2-CH4-N2-NaCl fluids having the temperatures andcompositions as follows: >250° to 430°C, 16–62 mol%CO2, 5–14 mol% CH4, 0.06–0.31 mol% N2 and salini-ties of 0.4–4.9 wt.% NaCl. The auriferous mesother-mal fluids have been progressively evolved throughextensive fluid unmixing due to the decrease in tem-perature and pressure, which has been successivelyfollowed by later cooling and dilution of fluids.Available geobarometers may indicate that theSeolhwa mine was formed at pressures of ~100MPa.

6 The d34S values of sulfides in the Seolhwa mine rangefrom -0.6–1.4‰, yielding the d34SH2S values of 0.3–1.1‰ (average 0.8‰). The dominance of d34S valuesof sulfides close to the meteoritic reference maysuggest that the ore sulfurs in the mesothermalfluids have been derived mainly from the sulfur froman igneous source. The d13C values (-4.0‰) of calcitemay be consistent with the deep igneous carbonsource. The calculated d18O and dD values of themesothermal fluids were: d18Owater = 5.3–6.5‰, dDwater

= -69 to -84‰. The range of d18Owater values of theSeolhwa mine is consistent with that of primarymagmatic waters, whereas the dD values are 4‰lower than those of magmatic waters.

7 The observed d18O-dD systematics of the Seolhwamine are the result of fractionation between themagmatic water and the CH4, therefore convincingthat the mesothermal fluids were magmaticallyderived. The mesothermal gold deposits in theSeolhwa mine have formed by extensive fracturingand veining, due to thermal expansion of magmaticwaters, which has been probably accompaniedeither by the decrease of the least principal stress orby the uplift and unloading of deeply buried meta-morphic terrain during the late stage of the Daeboorogeny.

Acknowledgments

The authors thank staff and students of the Center forMineral Resources Research at the Korea Universityfor extensive discussions that have improved ourunderstanding of mesothermal gold deposits. Thisarticle has been significantly improved by the incisiveand helpful reviews by the editor, Professor HiroakiKaneda. This research is financially supported by theproject titled “construction of foundation and resourceevaluation security for overseas mineral resources”(No. 09-1121), Korea Institute of Geosciences andMineral Resources.

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