Fluid Inclusion Study of Gold Mineralization at the

한국지구시스템공학회지

Vol. 44, No. 1 (2007) pp. 20-27 연구논문

Fluid Inclusion Study of Gold Mineralization at the Yuryang Mine, Korea 1)

Chul-Ho Heo *

유량 금광화작용의 유체포유물 연구 허철호* 요 약 : 유량 금광상은 충남 천안지역의 선캠브리아기 편마암내 배태되어 있고 페그마타이트와 밀접하게 관련되 어 있다. 석영맥의 조직은 괴상이며 맥주변에는 약한 열수변질작용을 보인다. 광석광물의 종류는 단순하며, 은을 함유하고 있는 광물상의 부재로 낮은 은/금 비율을 나타낸다. 광화작용은 2회에 걸쳐 진행되었으며, 초기 광화작 용의 철-황화광물은 전형적으로 맥주변에서 산출하며, 후기의 금-텔루륨-비스무스 광화작용에는 방연석, 섬아연 석, 자류철석, 텔루륨함유 광물, 에렉트럼등의 광물이 열극을 충진하는 특징을 보인다. 유체포유물은 특징적으로 이산화탄소를 함유하며 상변화에 따라 4개 유형(Ia, Ib, IVa 및 IVb)으로 구분된다. 유체포유물 자료에 의하면 본 광상이 저 내지 중염농도(< 12 wt. % eq. NaCl)와 메탄(1～22 mole %)을 함유한 광화유체로부터 고온에서 생성되었음을 지시한다. 광화유체로부터 금의 침전 메카니즘은 이산화탄소 불혼화작용에 기인된다. 유량 금광상 에서 관찰된 물리 · 화학적 조건들을 고려하면 본 광상이 상당한 심도에서 텔루륨-비스무스 광화작용을 수반하는 조산성 금광상과 유사한 것으로 사료된다. 주요어 : 조산성 금광상, 유체포유물

Abstract : The Yuryang gold deposit is located in Cheonan area, Chungcheongnamdo province. It is hosted in Precambrian gneiss and is spatially related to pegmatite. The quartz veins display massive textures with weak potassic alteration. The ore mineralization is simple, with a low Ag/Au ratio of 1.5 : 1, due to the rarity of Ag-bearing minerals. Ore mineralization took place in two stages. The early Fe-sulfide mineralization occurred along the vein margins, and the late Au-Te-Bi mineralization is characterized by fracture fillings of galena, sphalerite, pyrrhotite, Te-Bi-bearing minerals. Fluid inclusions in vein quartz characteristically contain CO2 and can be classified into four types (Ia, Ib, IVa and IVb) according to the phase behavior. The results of fluid inclusion data indicate that the ore deposit was formed at a distinctively high temperature from fluids with moderate to low salinity (< 12 wt. % eq. NaCl) and CH4 (1～22 mole %). The dominant ore-deposition mechanisms were CO2 effervescence which triggered gold mineralization. The physicochemical conditions of the Yuryang gold deposit indicate that the Yuryang gold deposit may be an orogenic gold deposits with Te-Bi mineralization at a considerable depth. Key words : Orogenic gold deposits, Fluid inclusion

The Cheonan mineralized area is one of the major Au-Ag productive zones including more than 50 goldsilver mines within an area of about 400 km2. The time of peak production in this area was between 1938 and 1942 when a total of 3.9 tons of gold were as produced from ore with an average of approximately 16g per metric ton. The Au-Ag deposits in the Cheonan area show common features, such as association with pegmatite dykes, massive vein morphology, low Ag/Au ratios, simple mineralogy, weak hydrothermal alteration and Jurassic emplacement ages. However, research on gold-silver mineralization in the area has been limited mostly to studies of a few individual ore deposits

Introduction The Cheonan mineralized area including Yuryang mine is situated along the margin between Jurassic Daebo granitoids and the Precambrian Gyeonggi massif. 2005년 5월 3일 접수, 2006년 12월 21일 채택 1) 한국지질자원연구원 지질기반정보연구부 광물자원연구실 *Corresponding Author(허철호) E mail; [email protected] Address; Mineral resources group, geology and geoinformation division, Korea institute of geoscience and mineral resources, 30 Gajeong-dong, Yuseonggu, Daejeon 305-350, Korea

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Fluid Inclusion Study of Gold Mineralization at the Yuryang Mine, Korea

(Choi et al., 2001; Heo et al., 2001a and b, 2002; Choi, 2002). The geology and mineralization of the Yuryang gold deposit have not previously been studied. From the Yuryang gold deposit, petzite is firstly reported in Korea by this study. Ag tellurides in gold-silver vein deposits has only rarely been reported in Korea (Shelton et al., 1990; Choi and Wee, 1992; Choi et al., 1996). Only a few investigators have studied the geochemistry and genesis of Te-bearing precious gold-silver vein mineralization in Korea, and little is known about the mineralogical association and geochemical controls of telluride mineralization in the gold- silver deposits associated with Jurassic magmatism. The aims of the present study are to document the nature of auriferous hydrothermal mineralization, with special reference to Yuryang, to understand the origin and physicochemical conditions of the auriferous hydrothermal fluids based on mineralogical and fluid inclusion data, and to suggest a genetic model.

Ore Veins and Mineral Paragenesis The Yuryang deposit is located close to the boundary between Precambrian gneisses, comprising granitic and banded biotite gneiss, and Jurassic granitoids (Fig. 1). Two hydrothermal gold-quartz veins occur as narrow (usually 10～50 cm thickness), fault-related fracture-fillings in Precambrian gneiss. They generally strike N20°～ 40°W, with dips of 35～45°NE and can be traced intermittently for approximately 55 to 110m along the strike. Au and Ag ore grades of the veins are 230 g/t

Fig. 1. Geological map of the Yuryang gold deposit.

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Au and 173 g/t Ag, respectively. The ore textures, including cross-cutting and brecciation textures, indicate that the Au-Ag-Te bearing veins formed during repeated episodes of fracturing and healing. Careful examination of the temporal variation of ore mineral assemblages indicates that the vein mineralization can be divided into two stages(Fig. 2); early Fe-sulfide mineralization and late Au-Ag-Te mineralization with late Fe-sulfide mineralization. Early Fe-sulfide Mineralization Stage The early minerals typically occuring along the vein margins consist of white quartz and pyrite with amounts of chlorite and rutile. Pyrite occurs as euhedral to subhedral aggregates, forming poorly developed mineral bands near the vein margins. The pyrite is commonly fractured and cemented by late base metal sulfides and Au-Ag-Te minerals. Dark brown Fe-rich sphalerite (17.3～11.6 mole % FeS) displays stellar texture within pyrite and/or occurs as subhedral to anhedral massive aggregates at intermediate vein portions. Late Au-Ag-Te Stage Late stage mineralization is characterized by deposition

Fig. 2. Generalized paragenetic sequence of minerals from the Yuryang gold deposit.

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Chul-Ho Heo

of sphalerite, hexagonal pyrrhotite, electrum, and tellurides such as petzite, altaite, hessite and Bi-Te mineral. It can be further subdivided into two mineral assemblage I and II. Electrum (79.4∼66.6 atomic % Au) coexists with galena and tellurides (I) and/or occurs as fracture fillings within pyrite (II). Telluride mineralization also occurs along small, galena-rich fractures cutting earlier deposited vein materials. Telluride minerals occur closely with assemblage I electrum in central vein portions where galena is disseminated. Sphalerite occurring in assemblage I is characterized by low FeS content. Electrum of assemblage II, infilling fractures in pyrite, coincides with pyrrhotite and sphalerite. The Au content of assemblage II electrum is slightly lower than that of assemblage A. The occurrence of tellurides is characteristic of the late periods of vein mineralization, which suggests a distinct change in the geochemical conditions of fluids with paragenetic time. Compared to common epithermal deposits (Shelton et al., 1990), the ore bodies of the Yuryang deposit show a more complex tellurium mineralogy, including Pb-, Bi-, and Ag-bearing minerals.

to observable dimensions for optimum precision. The temperatures of the total homogenization and carbonaceousphase homogenization and the melting temperatures of carbonaceous phase, ice and clathrate, had standard errors of ±1.0° and ±0.2℃, respectively. Seventy fluid inclusions were measured in this study. Occurrence and Types of Fluid Inclusions The size of the fluid inclusions ranged from 6 to 24 ㎛. Two main types of fluid inclusions were identified based on their appearance at room temperature, combined with their behavior during cooling down to about -100 ℃ and slight heating up to about 30 ℃ (Nash, 1972): types I(H2O-rich) and type IV(CO2-rich) inclusions (Fig. 3). Two subtypes of type I fluid inclusion have been distinguished on the basis of their occurrence and compositional characteristics: Type Ia (primary and/or pseudosecondary aqueous fluid inclusions showing the presence of a nucleated clathrate upon heating after cooling) and Type Ib (aqueous inclusions). Type IV inclusions are subdivided into two types: type IVa (homogenized into aqueous phase) and type IVb

Fluid Inclusion Study Vein mineral samples collected from the Yuryang deposit were investigated by microthermometry in order to document the ranges of fluid compositions and temperatures of ore forming fluids and to investigate the thermal histories of auriferous hydrothermal fluids. Twenty vein quartz samples were collected for fluid inclusion study. Sphalerite was not suitable for the study due to its opacity and massive occurrence. Microthermometric data were obtained by gas-flow heating/freezing stage and calibrated with synthetic H2O and CO2 inclusions and various organic solvents (Hollister, 1981; Shepherd et al. 1985). The heating rates were varied, but were maintained 1 ℃/min for determinations of the melting temperatures and carbonaceous-phase homogenization temperatures, and maintained about 10 ℃/min for measurement of the total homogenization temperatures. During the freezing experiments, the sequential repeated freezing technique, described by Haynes (1985), was employed over the expected temperatures in order to make phases coarsen 한국지구시스템공학회지

Fig. 3. Photomicrographs showing the occurrence of primary and pseudosecondary fluid inclusions in quartz. A. Type IV inclusion showing the CO2 double rings B. H2O-rich Type I inclusion. Scale bars = 20 ㎛.


(homogenized into carbonaceous phase) upon slight heating (up to about 30 ℃). Type I inclusions: These fluid inclusions consist of two phases (liquid and vapor) at room temperatures. Type Ia inclusions are generally tabular or negativeshaped, and their gas bubbles comprises 10 to 45 % of the total inclusion volume. Type Ia inclusions contain minor amounts of CO2. The occurrence modes of type Ia inclusions are summarized as follows: (1) isolated inclusions, which are distributed randomly and contained within the regular-shaped vacuoles; (2) trails of inclusions, which commonly represent healed fractures relevant to gold-containing quartz veinlets. According to Roedder (1981, 1984), mode (1) indicates a primary origin, whereas mode (2) reflects a pseudosecondary origin. The occurrences of type Ib inclusions (less than 15 vol. %, mostly < 5 % of gas) are as follows: 1) inclusions are parallel to the growth zone or crystal faces, indicating a primary origin; and 2) irregular in shape, occasionally showing necking phenomena, and they occur along the planes crosscutting the quartz grains, suggesting their secondary origin. Type IV inclusions: These inclusions consist of three (liquid water, CO2-rich liquid and vapor) phases at room temperature. Their volumetric proportions of carbonaceous phases (liquid+vapor) at 25℃ range from 30 to 80 percent (average = 62 %). The whole range of carbonaceous-phase volumetric proportions has been observed within individual samples, which is thought to be a result of fluid unmixing. However, some inclusions having nearly constant phase ratios and often form inclusion clusters, suggesting an entrapment of a homogeneous fluid. Type IV inclusions occur as follows: (1) small groups usually consisting of negative crystals that are relatively isolated and exhibit no planar orientations, (2) isolated, randomly distributed inclusions that are contained within the negative-shaped, subrounded, or irregular vacuoles, and (3) trails of small-sized (< 6 m) inclusions, which represent healed fractures, and these trails occasionally continue to quartz veinlets containing sphalerite and gold. Based on the classification scheme of Roedder (1981, 1984), modes (1) and (2) are primary and mode (3) is secondary or pseudosecondary.

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Heating and Freezing Data A total of 70 primary+pseudosecondary fluid inclusions in vein quartz were examined from the Yuryang gold deposit. Salinity data were calculated based by freezingpoint depression in the system H2O-NaCl system (Potter et al., 1978) for H2O-rich type Ib and by clathrate melting temperatures (Bozzo et al. 1975; Collins, 1979) for CO2-bearing type Ia and IV inclusions. The wide range of homogenization temperatures in fluid inclusion reflects several hydrothermal episodes, rather than one specific event, which is indicated by the textural evidence of multiple opening and filling of the veins. The results of the microthermometric analyses are as follows(Figs. 4 and 5). Type I inclusions: The total homogenization temperatures ranged from 221 ° to 344 ℃ for type Ia inclusions (Fig. 4). Clathrate melting temperatures of type Ia inclusions ranged from 7.1 ° to 9.6 ℃ (Fig. 5). The measured clathrate melting temperatures corresponded to salinities ranging from 0.8 to 7.9 wt. % eq. NaCl. Type Ib inclusions had an inability to nucleate a clathrate on cooling, suggesting the maximum presence of < 2.7 wt. % CO2 (Hedenquist and Henley, 1985). They homogenized at clearly lower temperatures ranging from 195 ° to 269 ℃, and had salinities of 0.0 to 10.1 wt. % eq. NaCl. Type IV inclusions: Melting of the solid CO2 (TmCO2) occurred at temperatures ranging from -56.7 ° to -62.2 ℃(type IVa, -56.7 ° to -62.2 ℃ type IVb, -56.7 ° to -60.1 ℃). Homogenization of the carbonaceous

Fig. 4. Histogram of homogenization temperatures of fluid inclusions of the Yuryang gold deposit.

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and 228 ° to 331 ℃ for type IVb inclusions (to the carbonaceous phase) (Fig. 4).

Fig. 5. Histogram of the final ice-melting temperature and clathrate-melting temperatures of the fluid inclusions from the Yuryang gold deposit.

phase (Th-CO2) occurred at the following temperatures type IVa, 4.4 ° to 29.1 ℃ and type IVb, 19.8 ° to 28.3 ℃. Clathrate melting temperature of type IV inclusions could be determined by the sudden appearance of a CO2-rich liquid, which was distorted and blocked from view by the clathrate (Collins, 1979). They range from 5.3 ° to 9.6 ℃, which were lower than the melting of pure CO2 clathrate at 10.0 ℃ (Fig. 5). The measured clathrate melting temperatures corresponded to salinities ranging from 0.82 to 8.6 wt. % eq. NaCl. Although type IV inclusions were easily decrepitated prior to total homogenization, 33 total homogenization temperatures were recorded. In cases that decrepitating occurred before the anticipated homogenization, the decrepitation temperatures were used as minimum homogenization temperatures. They include 248 ° to 332 ℃ for type IVa inclusions (to the aqueous phase) 한국지구시스템공학회지

Carbonaceous Composition of Fluid Inclusion The bulk compositions and densities of type IV inclusions were estimated from data on the visual volumes combined with the compositional data and densities of the carbonaceous and aqueous phases of type IV inclusions. Estimation of the relative volumes of the carbonaceous and aqueous phases was carried out by measuring inclusions with a graduated ocular and assuming that volume was to be proportional to the area, although this method was subject to the uncertainties arising from measuring the phase volumes and ignoring the solubility of CO2 in the aqueous part (Roedder, 1984). The quantitative V-XCH4 projection of the CO2-CH4 system (Heyen et al., 1982), combined with the CO2 melting and homogenization temperatures, was used to estimate the CH4 contents in type IV inclusions. The estimated mole percent of CH4 in the nonaqueous part of type IV inclusions were 1 to 22 mole% (type IVa, 1 to 22 mole %; type IVb, 2 to 16 mole %). CH4 is supposed to be the sole agent responsible for the observed CO2 melting and homogenization temperature depressions. This presumption is likely because the P-V-T-X properties of the coexisting carbonaceous liquid and vapor for CO2-CH4-N2 mixtures containing relatively small amounts of N2 are very similar with those of the CO2-CH4 system between -20 ° and +15 ℃ (Arai et al., 1971; Sarashina et al., 1971). Thermal and Compositional Evolution of Hydrothermal Fluids As described previously, the auriferous hydrothermal fluids principally observed in vein stage minerals are composed of two types: (1) dominantly aqueous fluids of moderate salinity containing minor amounts of CO2 (type Ia), and (2) mixed CO2-H2O fluids of low salinity (type IV). The CO2 contents of type IV fluid inclusions are highly variable within individual samples. The CO2-rich type IV inclusions homogenized at nearly the same temperatures as H2O-rich type IV inclusions. These observations may indicate that type IV fluid inclusions represent the trapping of immiscible H2OCO2 fluids which evolved through CO2 effervescence


Fig. 6. Total homogenization temperature versus salinity diagram for the fluid inclusions in quartz from the Yuryang gold deposit. Note: liquid CO2-bearing type IV inclusions show an increase in salinity with decreasing temperature, indicating the effervescence of CO2-rich fluids.

(Fig. 6). The relationship between homogenization temperature and salinity for gold-depositing stage fluids is shown in Figure 6. Liquid CO2-bearing type IV inclusions in vein stage quartz show an increasing salinity (from 0.8 to 11.9 wt. % NaCl) with decreases in temperature from 331 ° to 276 ℃. This trend is explained by the extensive boiling of CO2-rich fluids. The CO2-poor fluids which had remained after extensive escape of CO2 gas were probably trapped at temperatures between 221 ° and 344 ℃ (Fig. 6) as CO2 clathrateforming type Ia inclusions in vein stage quartz. Type Ia inclusions tend to show an increase in salinity (from 0.8 to 7.6 wt. % eq. NaCl) with decreasing temperature, likely indicating the likelihood of extensive continued boiling of CO2-poor fluids. Such continued boiling of auriferous hydrothermal fluids is thought to be the result of a pressure decrease during the ascent of hydrothermal fluids. Following the complete loss of CO2 from vein stage fluids, the residual fluids were trapped as CO2-absent type Ib fluid inclusions (Th ℃ ＝ 195 °～269 ℃) in quartz. Pressure Consideration Combined with the abundance of liquid CO2-bearing fluid inclusions, such apparent homogeneous temperatures for hydrothermal fluids need to be recalculated due to

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pressure correction. The pressure effect on the composition of sphalerite coexisting with pyrite and hexagonal pyrrhotite has been determined in various ranges by experiments and thermochemical calculation(Lusk and Ford, 1978). The results agree with each other at relatively higher temperature (above about 400℃). Hutchison and Scott (1981) proposed equation with correlation coefficient 0.993 and standard deviation ± 0.3 kb, and has been regarded as the most reliable one. Pyrrhotite from the Yuryang gold deposits occurred as hexagonal pyrrhotite which is identified by the methods of X-ray powder diffraction and staining by Bitter’s colloid. Most of hexagonal pyrrhotite is associated with sphalerite that has relatively high Fe contents. The sphalerite coexist with pyrite and pyrrhotite from the Yuryang mine contains 16.2 ～ 17.9 mole % FeS, and 17.0 ± 0.75 in average. The compositions of sphalerite correspond to about 3.5 ～ 2.1 kb. Gold Transport and Deposition Gold and silver are mostly transported in hydrothermal solutions as bisulfide or chloride complexes (Seward, 1984; Drummond and Ohmoto, 1985). However, low Ag/Au ratios in ore due to the rare amount of base-metal sulfides and Ag-bearing phases in the deposit imply that chloride complexes did not play an important role in the precious-metal transportation. Combined with the close association of gold with sulfide as well as the sericitization in wall rock, gold was transported as a reduced sulfur complex at the Yuryang deposit. Fluid boiling accompanying CO2 effervescence in auriferous hydrothermal systems may result in abrupt chemical changes in the residual liquid. These changes favor deposition of precious metals through destabilization of metal complexes (Drummond and Ohmoto, 1985). Gold precipitation mechanisms involving the gold bisulfide complex include a decrease in temperature at constant pH, oxidation of the complex, pH decrease, and a decrease of sulfur activity by sulfide precipitation and/or H2S loss accompanying boiling. Given the frequent association of galena, sphalerite and chalcopyrite with gold in the Yuryang deposit, the role of sulfide precipitation accompanying boiling is critical. The decrease of the sulfur activity that accompanies boiling, through sulfide deposition 제44권 제1호

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and/or H2S loss, is likely the most likely important mechanism for gold deposition in the Yuryang deposit. Together with the evidence of fluid boiling and associated H2S loss during the Fe-sulfides mineralization, it was assumed that the boiling of fluids probably could result in a drop of the fugacity of sulfur in fluids following Au-Te-Bi mineralization.

Discussion and Conclusion The pressure, estimated using the sphalerite geobarometry at Yuryang, was from about 3.5 to 2.1 kb, corresponding to a depth of about 6.9 ～ 11.5 km. At these depths, the high confining pressures would limit the vein textures. The veins associated with pegmatite and displaying interlocking networks of quartz suggest that veining occurred at a deeper level, equilibrating with the host rocks. The physicochemical conditions observed in the Yuryang gold deposit with Te-Bi mineralization are consistent with those of deposits that formed in an intrusion-related gold system at a deep depth (Lang and Baker, 2001). The orogenic gold deposits are typically associated with regionally metamorphosed terranes, and formed during igneous activity related to compressional to transpressional deformation at convergent plate margins such as accretionary and collisional orogenies (Kerrich and Cassidy, 1994; Groves et al., 1998; Goldfarb et al., 2001). When the Yuryang gold deposit is compared with the orogenic gold system, a close resemblance is noted, especially with regard to the type of deposits, mineralogy, host rocks, alteration patterns, features of the ore fluids and the inferred tectonic environment. The sulfide-poor, massive quartz veining associated with muscovitebearing pegmatite at Yuryang are consistent with the typical features of Jurassic gold-rich deposits. I interpret that these Jurassic gold deposits were interpreted as being formed during changing of subduction direction of the Izanagi plate through the space made by extension as a result of a change in subduction direction. The characteristics of ore-forming fluid and Au-Te-Bi mineralization at the Yuryang indicate the ore fluids were derived from high temperature, moderate salinity and deeply sourced water. Therefore, it is concluded that the Jurassic gold lode deposits in the the Cheonan area, 한국지구시스템공학회지

including the Yuryang gold deposit, are compatible with deposition of the hypothermal-type quartz veins from deeply sourced fluids generated by the late Jurassic Daebo orogeny.

Acknowledgements This research was financially supported by the fund from a planning study for information-oriented reevaluation on the development potential of domestic metallic ore deposits by Korea Institute of Geosciences and Mineral Resources (KIGAM).

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허 철 호 현재 한국지질자원연구원 지질기반정보연구부 광물자원연구실 선임연구원 (本學會誌第42卷第5号參照)

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