Fluid inclusions of the Ilkwang Cu−W-bearing breccia-pipe deposit, Kyongsang Basin

Geoscience Journal Vol. 2, No. 1, p. 15--25, March 1998

Fluid inclusions of the Ilkwang Cu-W-bearing breccia-pipe deposit, Kyongsang Basin

Kyounghee Yang

Jaeyeong Lee

Department of Geology, Pusan National University, Pusan 609-735, Korea (e-mail: yangkyhe@ hyowon, cc.pusan.ac.kr) Department of Geology, Kyungpook National University, Taegu 702-701, Korea

ABSTRACI': The copper-tungsten mineralization of llkwang mine, Kyongsang Basin was associated with crystallization of a small stock of granodiorite and quartz monzonite, explosive brecciation, and exsolution of magmatic hydrothermal fluids. Fluid inclusions in quartz phenocrysts of granitic rocks and hydrothermal quartz in breccia matrix trapped aqueous fluids that had caused brecciation and precipitation of copper- tungsten ore mineais. Four groups of fluid inclusions are identified: Type I (vapor-rich and low salinity), Type II (liquid- rich and low to moderate salinity), Types IIla and l l lb (liquid- rich and high salinity) and Type IV (CO2-bearing, vapor-rich, and low salinity). The earliest high-saline fluid (Type Ilia) and the following CO2-bearing fluid (Type IV) were exsolved from crystallizing melt and trapped at relatively high pressure (> 1 kbar). As water was partitioned from melt to aqueous fluid, the volume expansion caused overpressuring of the magmatic hydrothermal system and led to an explosive hydrothermal brecciation. The pressure quenching related to the brecciation was followed by crystallization of the matrix producing early- hydrothermal quartz and tourmaline. Additionally, a subse- quent decrease in pressure produced boiling of aqueous fluid, generating Types I and l l lb inclusions. They were associated with sulfide-mineral precipitation at 493 3430C and 450 120 bars. The separation of vapor phase including acid volatile species CO2 from the liquid phase might have raised pH of the hydrothermal fluid, which was favorable for the formation of sulfides. The late-magmatic, high-salinity fluids (Type IIIb) were mixed with circulating meteoric fluids and then were cooled and diluted to form lower-temperature and lower- salinity hydrothermal fluids that were trapped as inclusions during the f'mal stage of hydrothermal activity. The fluid- inclusion data are comparable with those from breccia-pipe deposits of porphyry-copper systems. It is suggested that hydrothermal activities and favorable structure factors in the Ilkwang mining district played a role for the generation of favorable P-T conditions and helped provide sufficient energy to form a breccia zone.

Key words: brecciation, exsolution, boiling, overpressuring, pressure quenching

1. INTRODUCTION

The Cu-W-bearing breccia-pipe deposit of Ilkwang mine located in the southeastern part of Kyongsang Basin (Fig. 1) was one of the largest copper producer in Korea during the 1930- 1940s. Previous studies (Jin, 1975;

Kang et al., 1976; Fletcher, 1977; Hwang, 1979; So and Shelton, 1983) have shown that emplacement of a granodiorite-quartz monzonite stock was responsible for the formation of copper-tungsten ores and the associated alteration during the late stage of crystallization. Hwang (1979) and So and Shelton (1983) examined characteristics of fluid inclusions that had been remnants after the ore formation; yet, full range of homogenization temperature was failed to be determined but sainities were estimated at 5--12 wt% NaCI equiv. In comparison with other breccia-pipe deposits, So and Shelton (1983) conjec- tured that the salinity of the fluid inclusions measured in the Ilkwang deposit was low to moderate, different from most other breccia-pipe deposits, but similar to those of tungsten deposits associated with Cretaceous granite in Korea. However, our new fluid-inclusion data compare favorably in fluid composition and temperature with those from many other breccia-pipe deposits (e.g., Sillitoe and Sawkins, 1971; Carson and Sawkins, 1980).

The Ilkwang stock system contains distinctive fluid systems from hypothermal to epithermal environments and offers a good opportunity to study fluid evolution associated with emplacements of granitic rocks. This paper presents a new interpretation of the magmatic hydrothermal fluids that were related to the ore formation and brecciation by delineating pressure-temperature (P-T) paths during the fluid evolution. Along with the previous works (Jin, 1975; Kang et al, 1976; Fletcher, 1977, Hwang, 1979; So and Shelton, 1983), our data are considered to reflect the physicochemical conditions under which the breccia-pipe mineralization occurred.

2. GENERAL GEOLOGY AND PETROGRAPHY

In the study area (Fig. 1), Cu-W bearing breccia-pipe deposits occur in a granodiorite to quartz-monzonite stock which intruded a gently dipping Uppcr Cretaceous sequence of siliciclastic sediments and pyroclastics. The pyroclastic unit is located to the west of the stock, and the underlying sedimentary rocks consist of shale and sandstone. The Ilkwang quartz-monzonite stock and the

16 Kyounghee Yang and Jaeyeong Lee

129"12' E 35019 ' N.

35"18' N.

Quartz monzonite

Granodiorite

Pyroclastic rock

I. I Sedimentary rock

129"14' E

V~ ++ +~; ++;;;;;; ~ " ~ " loo __1 + - + + - I t + + + + + + + + + + ~ + - + + + + + + j r + + + + + q Jr ~ + i i i i i ~ i Ha-r,

+ Jr+++ ++ ++ ++;; ++; ++ + + ; ; : ++ ++ +lilY, an ++Jr++Jr++$~+~§ , Jr Jr ++ ~ ++ ~ ++~ ++ Jr Jr~++++Jr

~~~++++++++++++::a++++++++++~~i L § §247247247 §247247 : : ++~++++++++++++++++++++++++++jrjrjr

- + + + j r + + + + + + + + + + + +

1< ++ ++ + ++ +~++ ++; 2 ++ ++ ++ ++ ~ ++

-370N ~"-" t

Kyongsang] ^ . 1 Basin .(. ~]uoy

o , / o ~ " area -35 N ~

1280E 130,~

Fig. 1. Geologic map of the Ilkwang mine area (after Fletcher, 1977). Contours are in meters.

sericite from the brecciated fragments within the pipe have been dated at 81 • and 69_+3 Ma, respectively (Fletcher, 1977).

The breccia pipe consists of strongly altered, brecciated fragments and various cementing minerals forming a network. The paragenetic sequence and petrographic fea- tures of the hydrothermal alterations are summarized by Jin (1975) and Fletcher (1977). The alteration phase consists of propylitic and sericitic assemblages composed of epidote, chlorite, sericite, quartz, tourmaline, and garnet. Hornblende and biotite were altered to epidote and chlorite. K-feldspar and plagioclase were converted to sericite. The presence of recrystallized granular quartz suggests that the fluids responsible for the alteration were saturated with silica.

According to Fletcher (1977), open-space fillings between brecciated fragments were formed in three overlapping stages. Quartz was deposited through all stages. In the initial stage, milky fine-grained anhedral quartz was precipitated on the walls of cavities followed by the deposition of prismatic, euhedral milky or clear quartz. Clear quartz and calcite were then deposited in the remaining space. Tourmaline forms fine-grained mass sur-

rounding the fragments or is locally disseminated as rosettes within the fragments. Blue--green and yellow-- brown pleochroisms are very common in the tourmaline. Some tourmalines are present as fine acicular crystals intergrown with quartz and sulfides. The second stage consists mainly of deposition of milky quartz, scheelite, arsenopyrite, pyrrhotite, and chalcopyrite, representing the main phase of sulfide precipitation. Chalco- pyrite occurs as disseminated or veinlet-filling along the fractures of tourmaline, arsenopyrite, or quartz grains. Scheelite occurs as light green-colored, fractured crystals surrounded by quartz. Chalcopyrite, arsenopyrite, and pyrrhotite are the dominant sulfides in the ore. The final stage of mineralization consists largely of deposition of clear euhedral quartz, chalcopyrite, sphalerite, galena, bis- muthinite, and calcite. Sphalerite is commonly associated with chalcopyfite. Calcite represents the final precipitate and is often associated with clear euhedral quartz. Comb structures are observed.

Replacement textures are common in the sulfide minerals of Ilkwang mine. They include the oxidation of chalcopyrite into covellte along the fractures, sphalerite grains partially "replaced by the blebs and rods of chalcopyrite as

Fluid inclusions of the Ilkwang Cu-W-bearing breccia pipe 17

copper-rich fluids reacted with the sphalerite (Vaughan and Craig, 1997), replacement of pyrrhotite by marcasite, and atoll structures due to the selective replacement of arsenopyrite by chalcopyrite. Tear-shaped droplets of bis- muthinite occur within arsenopyrite.

3. FLUID INCLUSIONS

3.1. Petrography

4+

+ r

...... rr~+l ffiW . . . . . . ii)ii+,+!!l! ~+++~+~+,++ '

At present, it is not accessible to the adits of the mine; samples for this study were obtained from outcrops of host rocks and three separate dumps in front of the adits of the pipe deposit. Rock-forming magmatic quartz, hydrothermal milky and clear quartz, and calcite from the breccia matrix were examined. Doubly polished sections varying in thickness from 0.07 to 0.1 mm were used for the analyses of fluid inclusions. Four types of fluid inclusions are identified based on the phase behavior at room temperature (Fig. 2). Type I inclusions are vapor-rich, with vapor bubble(s) occupying greater than 80 vol% of the inclusion (Fig. 2a). Type II inclusions are liquid-rich, with vapor bubble(s) occupying less than 30 vol% of the inclusion (Fig. 2b, c). Type III inclusions are liquid-rich and contain more than three daughter minerals including halite (Fig. 2d, e). Type IV inclusions contain liquid and gaseous CO2 (Fig. 2f). Based on the criteria of Roedder (1984), the four fluids are regarded to have been trapped as primary, pseudosecondary, and secondary inclusions.

3.1.1. Rock-forming (magmatic) quartz Quartz phenocrysts in quartz monzonite are cloudy and

highly embayed. They often contain crystallized melt inclusions and abundant, small, secondary inclusions, either Type II (< 2 ~tm in diameter) or Type IV (< 20 l.tm). Type III inclusions contain large halite, a vapor bubble, birefringent crystals, and sometimes opaque crystals, indicating dense brines (Fig. 2e). One of the birefringent crystals is recognized as anhydrite on the basis of strong birefringence and acicular form. Type III inclusions are relatively small in size ( ~ 10 ~tm) and are randomly distributed or occur along the microfractures on the boundaries of quartz grains. They may be primary or pseudosecondary in origin. Type IV inclusions are randomly distributed or occur along the microfractures.

3.1.2. Hydrothermal milky quartz Milky granular or prismatic quartz cemented the

brecciated fragments of granodiorite. The quartz is typically cloudy and contains all types of inclusions. Types I, II, and IV inclusions are by far the most abundant. Negative- crystal-shaped Type IV inclusions are randomly distributed in three dimensions and are considered as primary in origin (Fig. 2f). Liquid and gaseous C02 are greater

Fig. 2. Photomicrographs of inclusion types observed in quartz, but c is from calcite. (a) Type I vapor-rich inclusion (30 p_m). (b)Type II liquid-rich inclusion (40 ~tm). (e) Type II liquid-rich inclusion (70 ~tm) in hydrothermal calcite. (d) Type III halite-beating inclusion (30 ~tm) with four daughter minerals in hydrothermal quartz. (e) Type III halite-bearing inclusion (10 p+m) with four daughter minerals in magmatic quartz. Note the relatively large halite crystal and a small vapor bubble. (f) Type IV liquid-COz-bearing inclusion (20 p.m). (g) Growth zones defined by aqueous immiscibility pair of numerous primary Types I and III fluid inclusions in hydrothermal quartz. Note that vapor-rich inclusions (Type I) are generally:larger than the saline inclusions (Type III). Scale bar =80 ~m. (h) Type III inclusion (20 ~tm) of aqueous immiscibility pair from hydrothermal quartz before heating. (i) The same inclusion as shown in h, but after heating. Note that erythrosiderite (KzFeC15 . H~O) did not return to the original crystal after homogenization. L=liquid, V--vapor bubble, O=o- paque crystal, H=halite, E=erythrosiderite, A=anhydrite, C=calcite, B--unidentified birefringent crystal, VC=COz-bearing vapor bubble, LC=liquid CO2.

than 70--80 vol% of the inclusion. They are characterized by visible amounts of both liquid CO2 and H 2 0

and generally regular in shape. The narrow range of volume ratios (70--80%) of CO2 suggests that these inclusions have been trapped from an originally homo- geneous mixture of CO2 and H20, i.e., in the one-phase field.

Type I inclusions coexisting with Type III along the growth zones may indicate fluid boiling. If these inclusions were trapped in the two-phase field (L+V), they are valuable P-T indicators because the homogenization temperature equals the formation temperature. Unfor- tunately, this approach cannot be used to determine the P- T conditions during the formation because vapor-rich inclusions always trap only small amounts of liquid along with the vapor phase (Bodnar et al., 1985a). Therefore, the salinities and homogenization temperatures of vapor-


rich inclusions do not represent compositions of the vapor phase nor temperatures at trapping. Their homogenization temperatures are always higher than the trapping temperatures. In addition, homogenization temperatures of the vapor-rich inclusions cannot be determined so as to confirm that they were trapped at the same P-T conditions as the coexisting halite-bearing inclusions, owing to the difficulty in visual estimation when the vapor phase fills the inclusion.

The formation conditions of fluid inclusions trapped in the two-phase field (L+V) can be studied by petrographic and pressure, volume, temperature, and composition techniques (PVTX). Saline- and vapor-rich inclusions must coexist spatially, indicating coeval trapping. Figure 2g shows the growth zone decorated with Types I and III inclusions, indicating boiling conditions. Although petrographic evidence is suggestive of fluid boiling, the halite- bearing and vapor-rich inclusions can not represent an immiscible pair in the case that the halite-bearing inclusions may homogenize by halite dissolution (at temperature higher than that of vapor-bubble disappearance temperature). Phase equilibrium constraints do not permit inclusions which homogenize by halite dissolution to be trapped in equilibrium with a vapor phase. Type III inclusions assigned as the boiling pair in this study homogenized by vapor-bubble disappearance as described in the later section.

Type I inclusion sometimes contains a round birefringent grain and an opaque solid phase (Fig. 2a). Type III inclusions (Fig. 2d) contain halite, two birefringent daughter crystals, one or two opaque grains, and a vapor bubble. The birefringent grains of somewhat rounded rec- tangular and rhombic forms are thought to be erythrosiderite (KzFeC15- H2 O) and calcite, respectively. Ery- throsiderite is characterized by transparent to pale green in color, high refractive index, and strong birefringence (e.g., Stefanini and Williams-Jones, 1996). Chalcopyrite- like solid phases with typical shape are also observed. Vapor-rich Type I inclusions coexisting with numerous one-phase, liquid-filled inclusions probably formed as a result of continued healing and necking after the vapor phase had nucleated within the inclusions. Type II inclusions with a relatively large vapor bubble are common in milky quartz, implying low density of the fluid.

3.1.3. Hydrothermal clear quartz and calcite Inclusions in clear quartz and calcite contrast markedly

with those in milky quartz. The inclusions are rare and relatively large in size (up to 150 [tm in diameter). Type II inclusions are dominant. Calcite contains only Type II inclusions, representing the latest fluids trapped (Fig. 2c). They record either the latest fluids from which the quartz and calcite grew, or fluids that circulated at some later period after most quartz and calcite crystals formed.

3.2. Microthermometry

Heating and cooling experiments were carried out using Linkam Th 600 heating and freezing stage. Repli- cate measurements of homogenization temperature (Th) showed a reproducibility within _+2.0~ at temperatures near 350~ Repeated measurements of melting temperatures of H20- and CO2-rich standard fluid inclusions showed a reproducibility within • 0.3~

Aqueous inclusion measurements were made to obtain Th and compositions (Figs. 3 and 4). Salinities of aqueous inclusions were calculated using the program SALTY of Bodnar et al. (1989) and Bodnar and Vityk (1994); salinities of CO2-bearing inclusions were calculated from clathrate melting temperatures using the equation of Darling (1991). Although fluid inclusions seemed to be one group of generation, those with inconsistent homogenization temperatures were excluded from the study. Obvious secondary inclusions were also excluded from the microthermometry.

5 0 ~ a Type V inclusions

in Mqtz 30

10 0

b Type III inclusions I--'t--I t-" 30 [ ] Th(H)in Mqtz I~ I I I

~,-'~ ~ (/)O lO [ ] T h ( V ) ~

O 0 , . c[-] Type II inclusions t'~ 50 I I [ ] Inclusions in Hqtz E ~ ~ n calcite

Z 30

1( 0

lo d Type l inclusions

100 200 300 400 500 600 Homogenization Temperature (~

Fig. 3. Histogram of homogenization temperatures of fluid inclusions. Th(H)=homogenization by halite dissolution, Th(V) =homogenization by vapor disappearance, Mqtz=magmatic quartz, Hqtz=hydrothermal quartz.


50

": ' . 4O > ~ "m

(b

O 30

Z

v 20 .__>, c-

O9 10

I �9 Type l in Hqtz.

OType II in Hqtz O Type II in Calcite

nType Ilia in Mqtz

==Type IIIb in Hqtz

I>Type IV in Hqtz

o O

O0 200 300 400 500 600

Homogenization Temperature (~

Fig. 4. Homogenization temperatures of fluid inclusions plotted against salinities. For abbreviations, see Figure 3.

The different melting behaviors of halite-bearing indus- ions indicate that they were trapped in different regions of pressure-temperature conditions (Fig. 5). If inclusions homogenize by halite disappearance followed by vapor- bubble disappearance, these inclusions would follow the path L + V + H - - ~ L + V ~ L . It indicates that they were trapped in the Field A. The L + V + H curve is apparently not shown in Figure 5 because of the pressure scale, but is located below the L + V curve. If inclusions homogenize by simultaneous disappearance of halite and vapor, these inclusions would follow the path L + V + H - - * L . It indicates that they were trapped along the dashed isochore B. If inclusions homogenize by vapor-bubble disappearance followed by halite disappearance, these inclusions would follow the path L + V + H - - * L-t-H--* L. It indicates that they were trapped in Field C (Fig. 5). Therefore, Field A is the P-T space where any inclusions display vapor bubble disappearance at a temperature above the halite disappearance, whereas Field C is the P-T space where any inclusions display halite disappearance at a temperature above the vapor-bubble disappearance. These inclusions in Field C have never been in Field L-FV, indicating that they were not generated from fluid boiling. The arrow directions here mean the heating on the heating stage and would show the opposite behaviors in nature. A similar melting behavior is observed in inclusions from magmatic quartz of Ilkwang mine and the same principle can be applied to calculate trapping conditions (Fig. 6).

3.2.1. Rock-forming (magmatic) quartz Based on the homogenization behavior, Type III inclu-

sions are divided into two subtypes: Type IIIa inclusions in magmatic quartz homogenized by halite dissolution (Fig. 2e)and Type IIIb inclusions in hydrothermal quartz by vapor'bubble disappearance (Fig. 2d). Type IIIa inclusions, either primary or pseudosecondary, are evaluated by microthermometry. Type IIIa inclusions homogenized at temperatures between 251 and 411~ salinities vary from 35 to 49 wt% NaCl equiv. (Figs. 3 and 4). Trans- parent crystals or birefringent daughter crystals such as anhydrite in Type IIIa inclusions are always present after the halite dissolution.

Although Type IV inclusions in magmatic quartz are secondary, they are evaluated by microthermometry because of their consistent phase ratios. They can be temporally compared with Type IV inclusions in hydrothermal quartz. Temperatures of homogenization to CO2 gas phase range from 26 to 28~ indicating that density of the CO2 is 0.30--0.25 g/cm 3 (Bodnar et al., 1985b). Total homogenization occurred between 300 and 330~ Melting temperatures of CO2-clathrate were about 8--9~ corresponding to salinities of about 4.0--2.3 wt% NaC1 equiv. (Darling, 1991). The values of Th CO2 L-V, melting temperature of clathrate and Th H20-CO 2 indicate that the content of CO2 is about 15--20 mol% in the system H20-CO2-NaCI (Hendee and Hollister, 1981; Brown and Lamb, 1989). The low CO2 content compared


|

5

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n 1

0 , 0

I ' a I I i f ' I ' i ss- ,

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~" 0~'~ L+H ," L+V+H

/ ,

L j / o, .Y

.

400 600 800

Temperature (~

d Q),~"

J .'i

200

Fig. 5. Partially schematic diagram showing the relationship between the P-T formation conditions and the mode of homogenization of H20-NaC1 fluid inclusions with 40 wt% (after Bodnar and Vityk, 1994). The field is largely divided into three spaces. The left side, defined by L(40)+H and L(40)+ V, is the space where liquid and halite are stable. The right, defined by L(40)+H and L(40)+V, is the space where only liquid is stable. The right lowermost comer is the immiscibility field that liquid and vapor coexist. The inclusions trapped a- long isochore A follow the path L + V + H - - ~ L + V - - , L , whereas inclusions trapped along isochore C follow the path L + V + H ~ L + H --, L. Inclusions trapped along the dashed isochore B separate the P-T space into two regions and homogenize by simultaneous disappearance of halite and vapor. These inclusions follow the path L + V + H - - , L.

to the volume of C O 2 phase (70--80 vol%) is owing to the presence of C02 vapor-rich inclusions with low density. Solid C02 melted at -57.0 to -58.0~ denoting nearly pure C02.

3.2.2. Hydrothermal quartz Initial melting of ice between - 7 5 and -70~ was

observed in some vapor-rich Type I inclusions along the growth zones in milky and clear quartz (Fig. 2g). The next phase change (melting) occurred at -56.6 to -57.2~ denoting nearly pure CO2 gas phase although liquid CO2 at room temperatures and CO2-clathrate during cooling experiment were not observed. Final melting of ice occurred at temperatures between -9 .2 and -1.0~ indicating salinities of 1.7 to 13.1 wt% NaCI equiv. Th varies from 432 to 519~ Vapor-rich inclusions, upon heating, remained unchanged until the temperature appro- ached 50~ below the Th of the inclusions and then gradually homogenized by the vapor bubble expansion, which made it difficult to observe the bubble disappearance and resulted in an approximate measurement.

Type II inclusions trapped in clear and milky quartz show first and final ice-melting temperatures at about -50~ or below, and between -2 .8 and -21.4~ (corresponding to salinities of 4.7-24.0 wt% NaCI equiv.),

respectively. They have the Th between 128 and 337~ These eutectic temperatures require the presence of additional cations besides sodium (Roedder, 1984). Low to moderately saline inclusions (4.7--10.2 wt% NaC1 equiv.) show the highest Th (304--337~ Fig. 4) in Type II inclusions. A great number of secondary inclusions in clear quartz show first melting at around -50~ and final ice-melting between -9.1 and -16.9~ (corresponding to salinities of 12.9--20.2 wt% NaC1 equiv.). They have the Th between 128 and 190~ (Fig. 4).

Type IIIb inclusions are present in hydrothermal quartz, and have the Th of 343--493~ with salinities of 40--48 wt% NaC1 equiv. These inclusions are randomly distributed or occur along the growth zone, indicating fluid immiscibilities (Fig. 2g). Upon heating of these Type IIIb inclusions, erythrosiderite (K2FeC15 �9 H20) dissolution was the first phase at 195--222~ with a mode at 200--205~ As temperature decreased again following homogenization, erythrosiderite renucleated not as a single crystal but as aggregates of small granular grains (Fig. 2h, i). The next phase change during heating is halite dissolution at temperatures of 324--405~ With continued heating, vapor bubble disappeared at 343--493~ as the final phase changes. Calcite and opaque daughter minerals or a transparent crystal persisted after the halite dissolution.

The microthermometric data of Type IV inclusions in hydrothermal quartz are similar to those in magmatic quartz described above. They may have been trapped as secondary inclusions in magmatic quartz and as primary ones in hydrothermal quartz from the same fluid.

3.2.3. Calcite Primary inclusions trapped in hydrothermal calcite

show final ice-melting temperatures between -3.1 and -19.3~ (corresponding to salinities of 5.1--21.9 wt% NaCl equiv.) and Th between 172 and 270~ Although the whole range of the salinity tends to decrease, the values of secondary inclusions in quartz and primary inclusions in calcite indicate that the salinities remained rather constant during the final stage of mineralization (Fig. 4).

4. TRAPPING CONDITIONS OF FLUIDS

Two points should be concerned in order to discuss trapping conditions of the fluids exsolved from the crystallizing melt: solidus temperatures of the granitic rocks and temporal relationships of fluids. In this paper, solidus temperature of the Ilkwang granitic rocks is temporarily defined as 600~ under the water-saturated conditions. Based on the microthermometry and the calculation of mineral equilibria, solidus temperatures of the Cretaceous- Tertiary granitic rocks in Kyongsang Basin are reported to fall in the range of 580--670~ (Yang and Bodnar,


~ 4 ..Q v

�9 2 13_

~ 3

~ 2

~C" 10I C H20-NaCl

C.P.

if- o

5

"s 4 t~

v

a~ 3 "3

co 2

13. 1

d B

0 I ~ I I I

200 400 600 Temperature (* C)

Fig. 6. Pressure-temperature diagram illustrating trapping conditions. (a) Inclusions in which liquid-vapor homogenization occurs between 238 (Point A1) and 359~ (Point B1). The inclusions homogenize by halite dissolution between 251 (Point A2) and 411~ (Point B2). (b) Inclusions in which total homogenization occurs between 300 and 330~ (Points C1 and D1)

1994; Lee et al., 1995; Yang, 1996). Recent studies have demonstrated that solidus temperature can extend as low as 450 to 500~ in a peraluminous, fluorine-enriched melt (e.g., Webster et al., 1987). The Ilkwang stock contains significant amount of fluorine (285--103 ppm; Jin, 1975) and shows a petrochemical feature of peraluminous granitic rocks (Yang and Kim, 1998). Therefore, the assumed solidus temperature of 600~ is not inappro- priate for the discussion of the trapping conditions in the followings. With this assumption, the upper temperature limit of water saturation is defined as 600~ (Fig. 6).

Based on the fluid-inclusion petrography and microthermometry described in the previous sections, the following is a probable temporal sequence of fluids trapping: Type Ilia inclusions in magmatic quartz ~ Type IV in hydrothermal milky quartz ~ Types IIIb and I in hydrothermal milky and clear quartz ~ Type II in hydrothermal clear quartz and calcite. In order to understand P-T evolution of fluids, P-T phase diagrams are constructed but show relative relations of isochores in the P-T space (Fig. 6). Since all fluid components are not identified and the phase equilibria for systems containing these components are not known, the effects of these components cannot be precisely quantified.

4.1. Type IIIa Inclusions

Type IIIa inclusions in magmatic quartz were trapped in the liquid-stable, vapor-absent field. Figure 6a is a P-T diagram :showing the trapping conditions for Type IIIa inclusions. We selected data points that encompass the liquid-vapor homogenization between 238 (Point A1) and 359~ (Point B1) and the halite dissolution between 251 (Point A2) and 411~ (Point B2). These dissolution temperatures indicate salinities between 35 and 49 wt% NaC1 equiv. Liquidi and isochores of 35 and 49 wt% NaCI equiv, were calculated from the data summarized by Bodnar and Vityk (1994). Isochores A3-A6 and B3-B6 are analogous to Isochore B in Figure 5 separating the P- T space into two regions. Inclusions of 35 wt% NaC1 equiv, homogenized by the halite dissolution were trapped along Isochore A1-A5 similar to the Isochore C in Figure 5. The same interpretation can be applied to Isochore B1-B5. Thus, inclusions trapped within the region A2-B2-B5-A5 would homogenize to the liquid by vapor disappearance between 238 (Point A1) and 359~ (Point B1) and exhibit final homogenization by the halite dissolution between 251 and 411~ (Points A2 and B2). Vertical Lines A5-B5 and A4-B4 are defined by the

containing about 3 wt% NaC1 equiv, and 20 mole% CO 2. (e) See Figure 7 for details. (d) The path of the fluids inferred from the data of aqueous inclusions shown in a--e. Region D is the P-T space where Type II inclusions were trapped during the final stages of hydrothermal activity.


solidus temperature (600~ of granitic rocks and the highest Th of Type IIIb inclusions (493~ respectively. Isochores in Figure 6a illustrate that trapping of fluid inclusions might have occurred under almost any pressure because of small difference between the liquid-vapor homogenization temperatures and the halite dissolution temperatures. Type Ilia fluids in magmatic quartz were, however, trapped earlier than Type IIIb fluids in hydrothermal quartz, probably above 493~ and in the P-T space of A4-B4-B5-B4.

4.2. Type IV Inclusions

Data points that encompass inclusions showing total homogenization between 300 and 330~ (Points C1 and D1; Fig. 5b) and salinities of 2.3--4.0 wt% NaCI equiv. were selected for the evaluation of trapping condition. In order to find the pressures at the homogenization temperatures (Points C1 and D1) and the slopes of isochores (C1-C3 and D1-D3) for 20 tool% CO2 and 3 wt% NaC1 equiv, in the P-T space, the diagram of CO2-H20 system containing 20 mol% CO2 and that of CO:-H:O-NaC1 system containing 20 mol% CO2 and 6 wt% NaCI equiv. (Hendee and Hollister, 1981; Bodnar et al. 1985b; Brown and Lamb, 1989; Sterner and Bodnar, 1991) were combined for the extrapolation. Vertical Lines C3-D3 and C2- D2 are defined by the solidus temperature (600~ of granitic rocks and the highest Th (493~ of Type IIIb inclusions. The region C2-D2-D3-C3 is the possible P-T conditions under which Type 1V inclusions were trapped.

4.3. Type IIIb Inclusions

Data points that encompass inclusions with the Th and salinity of 343--493~ and 40--48 wt% NaCI equiv., respectively, were selected for the evaluation of trapping condition. We assume that Type IIIb inclusions in hydrothermal quartz homogenized by vapor-bubble disappearance were trapped in the fluid immiscibility field, that is, in a field that liquid and vapor can coexist. Thus, these Th and salinity data are referred to vapor-pressure curves for high salinity H20-NaC1 solutions (Fig. 7). The inter- section of the isopleth corresponding to the inclusion composition with the measured homogenization temperature in P-T space defines the pressure at the time of trapping. Thus, halite-bearing inclusions with halite dissolution temperature of 323--405~ (corresponding to a salinity of 40--48 wt% NaC1 equiv.) and with a homogenization temperature of 343--493~ would have been trapped at about 450 to 120 bars (shaded regions in Figures 6c and 7).

Figure 6d illustrates the path of the magmatic-hydrothermal fluid inferred from the data from aqueous inclusions shown in Figure 6a--c. Region D is probably the P-T

O o O v - -

, . , . , . , , , / /

&

m m m

C.P.

C I I , I I I I I . i .

100 200 300 400 500 600 700

Temperature (~

Fig. 7. An enlargement of Figure 6c. Vapor-pressure curves for HgO-NaC1 solutions having salinities of 30--70 wt% NaC1 (after Bodnar and Vityk, 1994). Also shown is the liquid-vapor curve for H20 (0), the HzO critical point (C.P.), and the three-phase (L+V+H) curve in the H20-NaC1 system. In- clusions with a salinity of 40--48 wt% NaC1 equiv, and temperature of 343--493~ would have been trapped in the shaded region. Point A indicates Th=343~ 40 wt%, P=120 bars; Point B indicates Th=493~ 40 wt%, P=450 bars; Point C indicates Th=493~ 48 wt%, P=370 bars.

space where Type II inclusions in calcite or hydrothermal clear quartz were trapped.

5. EVOLUTION OF THE MAGMATIC HYDROTHERMAL SYSTEM

Petrography of minerals and fluid inclusions and aqueous fluid phase equilibria determined from microthermometry provide constraints for the evolution of the magmatic hydrothermal system related to the breccia-type Cu-W mineralization at Ilkwang mine. The magmatic hydrothermal system can be divided into four stages according to different types of fluids and mineral assemblages: Stage I (magmatic quartz and high saline Type IIIa fluid), Stage II (milky quartz+tourmaline and liquid COz-bearing Type IV fluid), Stage III (metal sulfides and boiling fluids of Types Ilia and I inclusions), and Stage IV (barren quartz and carbonate stage and liquid-rich Type II fluid).

The first stage is defined by the exsolution of high saline fluid (Type IIIa), termed "second" or "resurgent" boiling, from crystallizing magma somewhere in Region A (> 2 kbar) (Fig. 6d). The melt had been probably emplaced at shallow depths (< 2 kbar). As the melt cooled and crystallized inward from the walls of the magma chamber at relatively shallow depth, the system encased in a crystalline rind of quartz-monzonite became overpressurized because of fluid exsolution. The concomi- tant fluid exsolution ultimately caused the explosive brecciation of the overlying granitic rocks. The exsolved

Fluid inclusions of the llkwang Cu-W-bearing breccia pipe 23

fluid was successively trapped in magmatic quartz at somewhat higher pressure (> 2 kbar) as Type Ilia inclusions homogenized by halite disappearance at temperatures above the Th (L-V). This phase equilibria constraints and the lack of cogenetic liquid- and vapor-rich fluid inclusions indicate that high-salinity fluids were exsolved directly from the crystallizing melt and were not a product of aqueous fluid boiling (Cline and Bodnar, 1994; Yang, 1996). According to experimental evidence and numerical modeling (Shinohara et al., 1989; Cline and Bodnar, 1991), partitioning of chlorine between coexisting aqueous fluids and melt is a function of the pressure in the system and large amount of chlorine exsolve from a typical calc-alkaline melt at higher pressure. The range of fluid salinity for Type IIIa inclusions (35--49 wt% NaCI equiv.) can be explained by the high pressure in the system.

The second stage is defined by the generation of CO2- bearing aqueous fluid (Type IV inclusions) in milky quartz on the walls of the breccia matrix (Region B in Figure 6d). Abrupt pressure reduction related to brecciation favored crystallization of the matrix including quartz and tourmaline. The system became resealed to trap CO2 aqueous fluid at relatively high pressure (> 1 kbar). Sub- sequently overpressurized system due to resealing and vaporization of fluids renewed brecciation and underwent a pressure decrease, producing fluid boiling pairs (Types llIb and I). It remains to be answered whether Type IV fluid was the original fluid to generate the fluid boiling. Although this type of inclusion appears to be more common in batholithic environments, its occurrence and distribution are poorly known (Bcane and Bodnar, 1995). It is also possible that low-saline fluids exsolved from the crystallizing magma were separated simultaneously into fluid boiling pairs, generating Types lIIb and I fluids.

The third stage is defined by the formation of fluid boiling and the deposition of sulfide minerals (Region C in Figure 6d). The boiling occurred at the temperatures between 343 and 493~ and at the pressures between 120 and 450 bars, and generated two types of fluids: a fluid with salinities of 1.7--13.1 wt% NaC1 equiv, and containing rare amount of CO2-gas phase (Type I) and a fluid with salinities of 4 0 48 wt% NaCI equiv. (Type IIIb) (Fig. 4). Phase equilibria of the two-phase field in the H20-NaC1 system are not completed to allow rigorous interpretations of fluid inclusion data. However, these ranges of boiling pairs are fairly consistent with some experimental data (Bodnar et al., 1985a, b). In addition, vapor-rich inclusions do not represent the composition of the vapor phase and trapping temperatures, as previously described. The separation of vapor phase from the liquid phase might have removed acid volatile species COz from the aqueous solution, thereby raising pH, which is favorable for mineral deposition. Sulfides precipitated from the

boiling aqueous fluids and cemented the fragments of granitic rocks, ultimately forming a magmatic hydrothermal breccia ore.

Finally, the fourth stage is defined by the trapping of low to moderately saline fluids in late milky quartz, clear quartz, and calcite (Region D in Figure 6d). The deposition of some sulfides such as ehalcopyrite, sphalerite, and galena continued up to this stage. The relatively high- temperature fluids associated with quartz progressively gave way to cooler Type II fluids (170--270~ associated with calcite. The late-magmatic, high-salinity fluids mixed with circulating meteoric fluids, and were cooled and diluted to form lower-temperature, lower-salinity hydrothermal fluids that were, in turn, trapped as inclusions during the final stages of hydrothermal activity.

6. DISCUSSION

Although inclusions with salinity of 25--34 wt% are not observed (Fig. 5), there is probably a full spectrum of salinities from 49 to 2 wt% NaCI equiv. This may be related to the fact that hydrohalite was not recognized in the inclusion with salinity of 23--26 wt% NaCI equiv. during microthermometry. Hydrohalite and ice are somewhat difficult to be identified separately as the last melting phase because of poor optics and small size. Inclusions with salinity of less than 30--35 wt% NaCI equiv, rarely nucleate a halite crystal in practice, although inclusions containing > 26 wt% NaCI equiv, should show halite (Bodnar and Vityk, 1994). For these reasons, there might be a salinity gap between 25 and 34 wt%. Furthermore, the fluid systems were not HzO-NaC1 nor H20-NaC1-CO 2 but probably included additional components such as KC1, CaCI2, and FeC12, given the first melting temperatures of Types I and II inclusions and the presence of daughter crystals in Type III inclusions.

The measured ranges of salinities and temperatures are consistent with those from other breccia-pipe-type deposits in porphyry-copper system, most of which have salinities ranging from 30 to 50 wt% NaC1 equiv, and Th ranging from 250 to 600~ and contain one or more daughter minerals (Sillitoe and Sawkins, 1971; Carson and Sawkins, 1980; Bodnar et al., 1985a; Lattanzi, 1994). The measured ranges of salinities and temperatures are also in the comparable range of those from granitic rocks in the Kyongsang Basin (Yang and Bodnar, 1994; Yang, 1997). However, the granitic intrusions of the Kyongsang Basin did not produce economic porphyry copper-type mineralization because the magmas crystallized at high pressures (relative to typical porphyry copper magmas) and were not saturated in water until a relatively late stage (Yang and Bodnar, 1994). Failure to reach water saturation resulted in most of the copper in the original melt being sequestered as a trace component in earlier-


crystallizing silicate and sulfide phases to produce sub- economic copper (Yang and Bodnar, 1994). In the case of the Ilkwang breccia-pipe copper deposits, however, the depth of crystallization was such that the energy was available to fracture the rocks when water exsolved from the magma.

Felsic magmas in granodiorite and quartz monzonite compositions can create extensive hydrothermal activity, forming highly fractured system around the stock due to the significant volume expansion (AVr) as water is partitioned from melt to vapor (Burnham, 1979, 1997; Bum- ham and Ohmoto, 1980). Burnham (1979) calculated that the total change in volume (AVr) is about 60 and 30% at lithostatic pressure 0.5 and 1.0 kbar, respectively, showing that AVr decreases with pressure. According to his calculation, these amounts of volume expansion may produce either explosive volcanism at < 0.5 kbar, extensive fracturing at 5 .0 - -1 .9 kbar, or miarolitic cavities at > 1.9 kbar (Burnham, 1997). Granitic rocks in the Kyongsang Basin are considered to have been emplaced at relatively shallow depth (Jin et al., 1981; Min et al., 1982; Jin, 1986; Hong, 1987; Lee et al., 1987). Many miarolitic cavities and fracture systems are observed at outcrops, indicating the activity of volatile component. However, breccia pipes are comparatively rare. Although previous works (Jin, 1975; Kang et al., 1976; Fletcher, 1977) suggested that any tectonic effects did not appear to have acted as controls for the sites of breccia pipes, the I1- kwang breccia deposits are found close to the ring fracture system associated with the Daeunsan cauldron complex (Yun and Cha, 1997). Therefore, it is likely that hydrothermal activities and favorable structure factors either played roles for generating favorable P-T conditions or helped mutually provide sufficient energy to form a breccia zone.

7. CONCLUSIONS

Brecciation and copper-tungsten mineralization of I1- kwang mine was associated with magmatic hydrotlaermal fluids exsolving from a crystallizing magma and aqueous fluid immiscibility. An assemblage of quartz, sulfide minerals, and calcite precipitated from the exsolving aqueous fluids and cemented fragments of brecciated wall rocks forming a magmatic hydrothermal breccia. Fluid inclusions in quartz grains in the breccia matrix trapped fluids which transported copper. An important aspect of magmatic hydrothermal system is that the early high- saline fluids were directly exsolved by a crystallizing melt rather than by being generated by fluid boiling. These early fluids raised the internal pressure of the crystalline find of crystallizing quartz monzonite and led to an brecciation, whereas the later high-saline fluid was generated by fluid boiling and precipitated copper, pro-

ducing economic grades. Additionally, the fluid inclusion data are comparable with those from breccia-pipe deposits of porphyry-copper system, indicating the dynamic nature of the magmatic-hydrothermal systems associated with porphyry copper formation. Furthermore, the pressure of the system was low enough for fluid boiling, which is characteristic of economic systems and an evidence for a shallow level of emplacement, an important metal-concentrating or depositing mechanism. The results of this study provide some insights into the mechanisms responsible for economic sulfide precipitation in porphyry-copper systems.

ACKNOWLEDGMENTS: We would like to thank Profs. S.K. Chough, I.S. Lee, and S.T. Yun for their rigorous reviews and corrections of the manuscript. They have improved this paper tremendously. This study was supported by the post-doctoral program (1996) of the Korea Research Foundation and by the Center for Mineral Resources Research.

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Manuscript received October 21, 1997 Manuscript accepted April 8, 1998

Fluid inclusions of the Ilkwang Cu−W-bearing breccia-pipe deposit, Kyongsang Basin

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