The orebodies consist of several thin tabular orebodies of hydrothermal replacement type where they are hosted by carbonate rocks. The Pb-Zn(-Ag) ...
Mineral. Deposita 27, 58-65 (1992)
i ep'one,
ralium
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9 SpringeroVerlag 1992
Application of arsenopyrite geothermometry and sphalerite geobarometry to the Taebaek Pb-Zn(-Ag) deposit at Yeonhwa I mine, Republic of Korea Yong-Kwon Koh 1, Seon-Gyu Choi 1, Chil-Sup So 2, Sang-Hoon Choi 1 and Etsuo Uchida 2 i Department of Geology, Korea University, Seoul 136-701, Republic of Korea 2 Department of Mineral Resources Engineering, Schooi of Science and Engineering, Waseda University, Tokyo 169, Japan Received: July 10, 1990/Accepted: November 27, 1991
Han (1969a, 1969b, 1972) briefly summarized the geology of the Yeonhwa I mine and Yun (1978a, 1978b, 1979), Yun and Silberman (1979) and Yun and Einaudi (1982) did regional studies on sedimentary, structural, and igneous geology, K-Ar geochronology and skarn formation at the Bonsan deposit. However, no detailed mineralogical and geochemical studies have been made previously on the Taebaek deposit, although Je and Lee (1987) recently described the exploration and development of this deposit. In this paper we describe the sulphide mineralization and present the chemical compositions of the sphalerites and arsenopyrites with their mode of occurrence and paragenetic sequence. Finally an attempt is made to decipher the physico-chemical environments under which the Taebaek Pb-Zn(-Ag) deposits have been formed - in particular their formation temperature and pressure - utilizing arsenopyrite geothermometry and sphalerite geobarometry respectively.
Abstract. The Taebaek Pb-Zn(-Ag) deposit of the Yeonhwa I mine, Republic of Korea, occurs in a broadly folded and reverse-faulted terrain of Paleozoic sedimentary rocks: the Taebaeksan basin. The orebodies consist of several thin tabular orebodies of hydrothermal replacement type where they are hosted by carbonate rocks. The Pb-Zn(-Ag) mineralization can be divided into four distinct stages based upon the mode of occurrence of ore minerals, ore textural relationships and their composition. Based on temperatures inferred from arsenopyrite compositions by means of electron microprobe and fluid inclusions, the estimated temperatures for the stages I, II, III and IV reach 330 to 350 ~ 270 to 340 ~ 230 to 250~ and < 220 ~ respectively. The sulphur activity (atm) of ore formation at the Taebaek deposit was estimated for each stage as 10 -11 to 10 -11'5, 10 -9"5 to 10 - 13, 10 - 13.5 to 10 - 15, and < 10-15, respectively. Even though application of sphalerite geobarometry is problematic because of the absence of good mineral assemblages, sphalerite coexisting with pyrite but not with pyrrhotite was used to estimate the minimum mineralization pressure (about 1 kbar).
The Yeonhwa I mine, which is the largest producer of zinc and lead in the Republic of Korea, is located near the northern margin of the Taebaeksan basin, approximately at latitude 37~ and longitude 129~ (Fig. 1). The mine is largely divided into three deposits (Bonsan, Dongjeom and Taebaek). Since 1960, the Bonsan and Dongjeom deposits have produced more than 10 million metric tons crude ore averaging 6% zinc, and 2% lead with silver and cadmium as byproducts. The Taebaek deposit, discovered in 1982 by diamond drilling, has produced 2.5 million metric tons of lead and zinc ore. The average ore grades of the deposit are 4.52% Pb, 4.54% Zn, and 100 g/ton Ag with traces of copper and gold. The total ore reserves at present are estimated at more than 10 million metric tons.
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.
-TAEBAEK Pb-Zn DEPOSIT
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paleogene granites Mid cretaceous
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Precsmbrian rocks
Fig. 1. General geologic map of the Republic of Korea showing location of the Taebaek Pb-Zn(-Ag) deposit at Yeonhwa I mine
59
Geologic setting
Table 1. K-Ar data of specimen from the Taebaek Pb-Zn(-Ag) de-
posit
The Yeonhwa I mine area lies within a EW-NWW trending syncline of a broadly folded and thrust-faulted terrain of Cambro-Ordovician carbonates (Chosun Supergroup) and Carboniferous to Triassic elastic sedimentary rocks (Pyeongan Group) overlying Precambrian basement. The Precambrian basement of granite gneiss, exposed in the southern part of the mine area, has been dated by KoAr methods at approximately 1750 Ma (Yun and Silberman 1979). The stratigraphic sequence in this area is, in ascending order, the Jangsan, Myobong, Pungchon, Hwajeol, Dongjeom, Dumudong and Magdong Formations (Chosun Supergroup) overlain unconformably by a sequence comprising the Hongjeom, Sadong, Kobangsan and Nogam Formations (Pyeongan Group). The total thickness of the Chosun Supergroup has been estimated to be greater than 1500 m. The Myobong and Pungchon Formations, which are host rocks of the mineralization, consist mainly of cleaved mudstone containing limestone interbeds, and limestone and dolomitic limestone with mudstone respectively. The dominant faults in the mine area include (1) reverse fault trending EW and with a dip of about 60~ (2) reverse faults trending N25-40~ and dipping steeply southwestwards; and (3) normal faults trending N to N25~ with dips of 45 to 85~ A large number of quartz porphyry dikes, ore veins, and ore pipes were emplaced along the NNW- and NNE-trending fault systems. Their intersections may have acted as a suitable channelway for ascending ore-forming fluids. The stocks or dikes of Triassic lamprophyre (K-Ar age: 213__+4Ma on biotite; Yun and Silberman 1979) have intruded both the Precambrian basement and Paleozoic sedimentary cover. Dikes of quartz porphyry also crop out the northern part of the mine area, yielding a K-Ar age of 78 Ma (Park et al. 1987).
Sample No.
Description
%K
Radiogenic 4~ (moles/g) STPxl0-9
% Radiogenie 4OAr
Date (Ma+ lcr)
TB-1
alteration sericite
7.14
0.7107
56.6
56.6 + 1.4
one, has a strike length of more than 300 m, and a vertical extent of more than 600 m. The Taebaek deposit has been exploited at five levels from the highest (-480 m) to the lowest (-720 m) level below the surface, of which the -600 m level is the most extensively mined. The known downward extension of Pb-Zn mineralization parallel to the dip of the enclosing beds exceeds about 1 km. In this study, specimens below the -720 m level were only obtained from drill cores. The distribution of sulphide minerals shows a marked variation with depth within the orebodies; an abundance of galena together with rhodochrosite in the uppermost portion; the predominance of pyrite and monoclinic pyrrhotite in the upper levels; and the predominance of hexagonal pyrrhotite in the lower levels. The Pb/(Pb + Zn) values for the ores of the Taebaek deposit are higher than those for the lower orebodies of the Bonsan deposit, but are similar to those for the upper orebodies of the Bonsan deposit (Je and Lee 1987). Also, silver grades for the Taebaek ores increase as lead grades increase. The Pb/(Pb + Zn) values for the crude ore of the Taebaek No. 1 orebody on the -660 m level range from 18 to 53 with an average of 43, in marked contrast with ore from the -300 m level in the Weolam No. 1 orebody of the Bonsan deposit which has Pb/(Pb + Zn) values of less than 27.
Outline of ore deposits Ore mineralogy and petrology The Taebaek Pb-Zn(-Ag) deposit is emplaced mainly within the limestones of the Pungchon Formation and within minor limestones in the underlying cleaved mudstones of the Myobong Formation, both belonging to the Chosun Supergroup (Je and Lee 1987). A quartz porphyry dyke near the orebodies suffered silicification, sericitization, pyritization and argiUization. Radiometric dating of alteration sericite has yielded a K-Ar age of 56.6+ 1.4 Ma, indicating a Tertiary age for the mineralization (Table 1). The Taebaek deposit is considered to be of hydrothermal-metasomatic origin. It consists of six significant orebodies: Taebaek No. 1, No. 2, No. 3 and No. 5 orebodies and Jeolgol No. 1 and No. 2 orebodies. These orebodies are controlled by faults which are commonly approximately parallel to the principal NNW-trending systems and which dip 75-85~ These orebodies commonly display pinches and swells both vertically and horizontally. The Taebaek No. 1 orebody, the most productive
Sphalerite, galena, pyrite, arsenopyrite, and monoclinic pyrrhotite are the principal sulphide minerals in the Taeback lead-zinc ores, although the proportions of the minerals in each orebody are variable from place to place. In addition, minor or trace amounts of tin-bearing and silver-bearing sulphides, sulphosalts (starmite, argentite, tetrahedrite-freibergite and pyrargyrite) and metal alloys (electrum, antimony-bearing silver and native silver) have been observed. The mineralogy of the gangue is largely dominated by carbonates: calcite and rhodochrosite. Four distinct stages can be distinguished on the basis of the textural relations of the various minerals and the occurrence of iron sulphides. The paragenetic sequence of sulphide minerals changes from early lead-zinc to late gold-silver mineralization (Fig. 2). Stage I is characterized by the presence of hexagonal pyrrhotite as an early stage. Hexagonal pyrrhotite occurs
60 as grains of irregular shape, and is closely associated with sphalerite (Fig. 3 a). By using the magnetic colloid coating procedure described by Scott (1974), it is seen that this mineral is rarely replaced by monoclinic pyrrhotite along cleavage, traces or cracks, and along the boundaries with sphalerite as well as with other minerals. Native bismuth occurs as inclusions in hexagonal pyrrhotite. Most of the native bismuth is intimately intergrown with galena. Arsenopyrite assumes an euhedral or subhedral shape and occurs in the form of fine-grained masses or individual grains up to 3 mm in diameter. Stage II, which corresponds to the most productive stage of lead-zinc mineralization, is characterized by the presence of pyrite and the absence of hexagonal pyrrhotite. The sulphide minerals formed during this stage consist mainly of sphalerite, pyrite, galena and arsenopyite with minor or trace amounts of chalcopyrite MINERALS
STAGE 11 STAGE ]I
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H F:"fRRI-IOTITE ARSENOPYRITE
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GALENA STANNITE TETRAHEDRITE FRE~BERGiTE MARCASITE
-
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ELECTRUM Sb-SILVER ARGENTITE PYRARGYRITE NATIVE SILVER QUARTZ CALCITE RHOOOCHROSITE
Fig. 2. Generalized paragenetic sequence of minerals from the Taebaek Pb-Zn(-Ag) deposit. Width of lines corresponds to relative abundance. Abbreviations; H. pyrrhotite, hexagonal pyrrhotite; M. pyrrhotite, monoclinic pyrrhotite
Fig. 3. a Photomicrographs showing hexagonal pyrrhotitte (hpo) intergrown with sphalerite (sp) and galena (gn) of stage I. b Photomicrograph showing monoclinic pyrrhotite (mpo), sphalerite (sp)
and tetrahedrite. Gangue minerals are gray quartz and calcite. Sphalerite occurs as large anhedral grains or aggregates commonly associated with pyrite. Arsenopyrite disseminated throughout the ores is commonly associated with pyrite. Stage III is characterized by the presence of both monoclinic pyrrhotite and pyrite. Stage III can be subdivided into three successive substages: III a, III b and III c. During this stage, the paragenetic evolution of sulphide and sulphosalt minerals follows the trend from silver-free to silver-rich assemblages: (1) stannite-tetrahedrite; (2) electrum-antimony-bearing silver; and (3) freibergite-argentite-pyrargyrite-native silver. Stage III a, mainly base-metal mineralization, is characterized by abundant sphalerite, monoclinic pyrrhotite, pyrite, arsenopyrite and galena with minor or trace amounts of marcasite, chalcopyrite, stannite, and tetrahedrite. The gangue minerals are rhodochrosite, calcite, and quartz. Arsenopyrite and pyrite in this stage are similar to those in stage II, but monoclinic pyrrhotite occurs as a matrix to the earlier minerals (Fig. 3 b) and as veinlets cutting them. Stage III b, which is characterized by gold-silver mineralization, is represented by arsenopyrite, sphalerite and monoclinic pyrrhotite with trace amounts of electrum and antimony-beating silver. Gold occurs as small grains of electrum (less than 10 I.tm) containing 81.4 to 87.2 atomic percent silver. Electrum forms fine irregular grains intimately associated with sphalerite and galena, filling the boundaries of arsenopyrite and pyrite. Stage III c, which is characterized by silver mineralization, is represented by galena, sphalerite, monoclinic pyrrhotite and/or pyrite with minor amounts of silverbearing sulphide and sulphosalt minerals. Galena includes many minute exsolution grains (1 - 10 larn) of silver-bearing sulphides and sulphosalts; freibergite (29.9 to 33.0 atomic % Ag), argentite, pyrargyrite and native silver. Argentite and pyrargyrite occur as anhedral grains along the boundaries of late sphalerite and galena. Native
and galena (gn) replacing and/or fiUing the interstices of arsenopyrite (asp) and pyrite (py) of stage III. All scale bars are 0.1 mm
61 silver is intimately intergrown with late sphalerite and galena. Stage IV represents the formation of rhodochrosite veinlets, up to 10 cm wide, clearly cutting all the earlier Pb-Zn ores. Minor sphalerite and galena occur only as fine-grained aggregates concentrated within veinlets, and are rarely associated with pyrite, chalcopyrite and/or monoclinic pyrrhotite.
12 ~ 10
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4
Methods of investigation Polished sections and doubly polished thin sections of the ore samples were examined using reflected light microscopy. The chemical compositions of pyrrhotite, sphalerite and arsenopyrite were determined using JEOL JXA733 electron microprobes at Yonsei and Waseda Universities. The analyses for pyrrhotite were carried out using synthetic troilite as a standard and a low accelerating voltage of 15 kV. During all analyses for sphalerite and arsenopyrite, the accelerating voltage was 20 kV, the PCD current was 0.2 ~tA, and a fixed time method was used for each element analyzed (30 s). The standards used were synthetic ZnS for Zn and S, natural chalcopyrite and cobaltite for Fe, As, Co, and S and pure metal for Ni, Cd and Mn. After corrections for dead time and background, matrix effects corrections were made with reference to atomic number, absorption and fluorescence (ZAF corrections) and calculation was performed by MZAF originally written by the JEOL Company. The analysis of sphalerites was restricted to Zn, Cu, S, Fe, Mn and Cd. Three elements - iron, manganese and cadmium - in sphalerites from some specimens were measured by a partial analysis method. Zinc and sulphur contents of sphalerite were calculated assuming that the atomic proportion of metals including zinc, iron, manganese and cadmium to sulphur is 1 : 1. In order to detect any zoning, line scanning over the grains was carried out.
Chemical compositions of pyrrhotite The paragenetic relations among the iron sulphide minerals from stage I to stage III at the Taebaek deposits are: hexagonal pyrrhotite~pyrite~monoclinic pyrrhotite. The identification of hexagonal and monoclinic pyrrhotite can be made by use of a magnetic colloid coating procedure (Scott 1974) and powder X-ray diffraction to detect the presence of a single (102) or a double (408, 408) peak in the same part of the X-ray pattern (Vaughan and Craig 1978). Scans over the relevant part of the X-ray pattern show that the pyrrhotite from stage I is hexagonal pyrrhotite (single peak) and the pyrrhotite from stage III is monoclinic pyrrhotite (double peak). The chemical composition of hexagonal pyrrhotite was estimated be measuring the value of dlo2 by X-ray powder diffraction and using the equation given by Yund and Hall (1969). Values ranged from 47.6 to 48.1 atomic % Fe. As shown in Fig. 4, the results of microprobe analyses indicate that hexagonal pyrrhotite at stage I tends to be higher in Fe content compared to monoclinic pyrrhotite at stage III
45
i
46
47
48
49
50
Atomic % Fe in Pyrrhotite
Fig. 4. Frequency histogram for Fe content (atomic %) of pyrrhotite from the Taebaek Pb-Zn(-Ag) deposit
(Fig. 4). The average Fe content of pyrrhotites calculated using the EPMA data are somewhat different from the value estimated by X-ray data. Chemical compositions of arsenopyrite The Taebaek arsenopyrite, which is associated with various iron sulphide minerals, may be subdivided into four varieties, corresponding to stages I, II, IIIa and IIIb. Arsenopyrites were initially checked for Co, Ni, Bi and Sb, but none of these elements were detectable by microprobe analysis. Emphasis was placed on selecting arsenopyrite grains in mutual contact with pyrite and/or pyrrhotite. In many cases, more than one grain in each sample was analyzed. Generally the variation within a single grain is less than the variation among different grains of the same polished section. This indicates that no clear compositional differences exist between the cores and rims of single crystals. From these data we conclude that the arsenopyrites are not chemically zoned. Figure 5 is a triangular diagram of Fe-As-S which illustrates the compositional variations of arsenopyrites from the various stages. Ideal arsenopyrite contains 33.3 atomic % Fe, but Klemm (1965) and Kretschmar and Scott (1976) have found that natural arsenopyrites consistently show a slight Fe deficiency (less than 1 atomic %). Arsenopyrites from the Taebaek deposit show a variation from 32.7 to 33.8 atomic % Fe. As seen in Fig. 5, the As content of the arsenopyrites from the Taebaek deposit exhibit a variation from 28.1 to 31.5 atomic % As, although all are As-deficient and S-excess species. A significant difference in arsenopyrite composition is generally found between the various stages. Arsenopyrite of stage I coexisting with hexagonal pyrrhotite shows a comparatively high content of arsenic with a mean value of 30.9 atomic %, ranging from 30.2 to 31.5 atomic %, whereas the As content ofarsenopyrite at stage II coexisting with pyrite varies between 29.4 and 30.8 atomic % As with an average of 30.3 atomic %. The As content of
62
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Sphalerite, which is the most abundant sulphide mineral at the Taebaek deposit, is associated with various iron sulphide minerals. Approximately 165 spot analyses were made for sphalerites at the various stages I, II, III a, III b, III c and IV. The analyzed sphalerites were in close contact with iron sulphides and are free of obvious exsolved lameUae or dots of chalcopyrite, pyrrhotite and stannite. The CdS contents of sphalerite are very low and nearly constant but the FeS and MnS contents of sphalerite vary from stage to stage. The FeS content of sphalerites at the Taebaek deposit vary from 9.3 to 23.0 mol % (Fig. 6). The sphalerite of stage I associated with hexagonal pyrrhotite ranges from 13.9 to 18.9 mol % FeS with an average of 16.3 mol % FeS. The variation of FeS within a single grain of some samples of sphalerite of stage I is up to 4.2 mol % FeS and poor in FeS. It seems that these sphalerites with low iron contents represent disequilibrium during deposition. The sphalerite of stage II, the most abundant sphalerite, coexisting with only pyrite, contains 9.3 to 19.4mol % FeS, with an average of 16.7 reel % FeS. Although this sphalerite exhibits larger variations in FeS content, no zoning in individual grains was found. There are no significant variations in FeS content among the sphalerites of stages III a, III b and III c. The sphalerites of these stages, less abundant and coexisting with monoclinic pyrrhotite and/or pyrite ex-
t
20
Mole % FeS
arsenopyrites at stages IIIa and III b, coexisting with monoclinic pyrrhotite and/or pyrite, vary from 28.1 to 30.1 atomic % with an average of 29.5 and 29.1 atomic %, respectively.
Chemical compositions of
I
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As
Fig. 5. Enlarged isosceles quadrangle in the Fe-As-S triangle, showing the variation in As/S atomic ratios of arsenopyrite from the Taeback Pb-Zn(-Ag) deposit
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Mole % FeS Fig. 7. Relationship between FeS and MnS contents in sphalerite from the Taehaek Pb-Zn(-Ag) deposit
hibit a compositional range from 12.6 to 21.3 reel % FeS with an average of 17.8. Sphalerite of stage IV tends to display the highest FeS content among all the sphalerites. The correlation between FeS and MnS contents of the sphalerite from the Taebaek deposit is shown in Fig. 7. It is worthy of note that the FeS and MnS contents of the sphalerite at the later stages of mineralization display a strong positive correlation, although correlation between the FeS and MnS contents of sphalerites at stages I and II is not clear.
63 I O 0 0 / T , OK 2.2 2.1 -46 " ~ l --
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250 500 350 400 Temperoture,oC Fig. 8. Temperature-sulphur activity diagram showing the stability field of ore minerals suggested by mineralogical assemblages observed in stages I, II, IIIa, IIIb and IIIc. Reaction curves are from Barton and Skinner (1979). At, argentite; Ag, native silver; Ap, arsenopyrite; As, native arsenic; Bi, native bismuth; Bm, bismuthinite; Hpo, hexagonal pyrrhotite; Mpo, monoclinic pyrrhotite; Py, pyrite; and NAg,atomic fraction of silver in electrum. Number of isopleths indicates the atomic % As in arsenopyrite and mol % FeS in sphalerites
200
Physicochemical environments of ore formation
An attempt was made to estimate temperature, pressure, and activities of components during ore formation using the arsenopyrite geothermometer and sphalerite geobarometer. At the Taebaek deposit, arsenopyrite occurs at various mineralization stages, and displays a wide range of As/S ratios. The arsenic content of arsenopyrite is very sensitive to sulphur activity. When arsenopyrite is buffered with respect to sulphur activity, the As/S ratio of arsenopyrite is mainly a function of the temperature. Although Clark (1960) suggested that the As/S ratio of arsenopyrite co-existing with pyrite and pyrrhotite de-. creases as a function of the total pressure, Kretschmar and Scott (1976) proposed a T-X diagram, based on experimental work in the system Fe-As-S, which may serve as an arsenopyrite geothermometer. Since then, several authors have applied the arsenopyrite geothermometer to hydrothermal or metamorphic ore deposits (Berglund and Ekstrom 1980; Lowell and Gasparrini 1982; Scott 1983; Choi et al. 1986). The arsenopyrite of the Taebaek deposit may be employed as a geothermometer through determination of As: S ratios, provided an independent estimate can be made of sulphur activity during ore formation. As shown in Fig. 5, all chemical compositions of the Taebaek arsenopyrites display S-excess and As-deficiency. The arsenopyrite of stage I seems to have been in equilibrium with hexagonal pyrrhotite and native bismuth. For this assemblage the sulpur activity can be approximately estimated from the As content of the ar-
Fig. 9. Frequency diagram of homogenization temperatures of fluid inclusions in quartz and sphalerites of the Taebaek Pb-Zn(-Ag) deposit. P, primary; S, secondary senopyrite and the stability field of native bismuth. In Fig. 8, the bismuth-bismuthinite sulphidation curve (Barton and Skinner 1979) is plotted onto a diagram given by Kretschmar and Scott (1976), showing a relationship between log fs 2 and temperature in the Fe-As-S system. The As content ofarsenopyrite coexisting with pyrrhotite and bismuth ranges from 30.2 to 31.5 atomic % As. This corresponds to a temperature range of 330 to 355 ~ and a log fs2 range between - 1 0 and -11, based upon the assumption that arsenopyrite + pyrrhotite + bismuth is an equilibrium assemblage. The conditions of formation for the main lead-zinc mineralization (stage II) may be estimated from the chemical compositions of sphalerite and arsenopyrite coexisting with pyrite. The As content of arsenopyrite and FeS mol % of sphalerite of stage II vary from 29.4 to 30.9 and 9.3 to 19.4, respectively. The ore minerals of stage II are estimated to have been formed at temperatures of 270 to 340 ~ with sulphur activity of 10- 9.5 to 10-13 atm. At stage III a, the sphalerite, which coexists with monoclinic pyrrhotite and/or pyrite, can only have been formed at temperatures below 254 ~ For the gold mineralization, stage III b, the coexistence of electrum with 0.81 atomic fraction of Ag and sphalerite with a composition of 17.7 mol % FeS gives 250 ~ for the formation temperature and 10-13.5 atm for the sulphur activity. The conditions of the main silver mineralization of stage III c were estimated by superimposing the argentite-native silver sulphidation curve in the pyrite stability field, giving 220 ~ for the temperatures and < 10-1 s arm for sulphur activity. In order to confirm the temperature estimates based on sulphide mineral equilibria, the homogenization temperatures for fluid inclusions in quartz and sphalerite were measured (Fig. 9). The homogenization temperatures of primary fluid inclusion in quartz and sphalerite of stages II and III range from 203 to 381 ~ and from 209 to 264~ respectively. These temperatures are slightly higher than those estimated from the particular mineral assemblages. Thus, the difference in mineral assemblages might be ascribed mainly to variations in temperatures and sulphur activity, with a tendency for both to decrease with time.
64 Recent experimental investigations in the system Fe-Zn-S provide an excellent basis for interpretation of the chemistry of the sphalerites. These data allow reasonable estimates of the physicochemical conditions (pressure, temperature and sulphur activity) of formation of sphalerite. Barton and Toulmin (1966) and Scott and Barnes (1971) have shown that the amount of FeS in sphalerite is a function of pressure alone in the temperature range of 254-550~ provided that the fs 2 is buffered by coexisting pyrite and hexagonal pyrrhotite. Under these conditions, sphalerite has a fixed FeS content of 20.6 mol % at 1 bar, but the FeS content decreases progressively with increasing pressure. This compositional change has been calibrated as a geobarometer to 10 kbar by Scott (1973), Lusk and Ford (1978), and Hutchison and Scott (1981). Above 550 ~ the FeS content in sphalerite varies with both temperature and pressure, but at high pressures the temperature-independent field extends to above 600 ~ Unfortunately sphaleritepyrite-hexagonal pyrrhotite assemblages are absent at the Taebaek deposit, so that in general sulphur activity was not buffered and the sphalerite compositions show a wide range as described above. The geobarometer cannot therefore be applied indiscriminately to the Taebaek sphalerites. Because the chemical composition of sphalerites for stage I, coexisting with hexagonal pyrrhotite, are heterogeneous, the sphalerites of this stage cannot serve as a geobarometer. The sphalerite of stages III and IV, representing rare mineral assemblages at the Taebaek deposit, do not serve as a geobarometer because o f the high manganese content. In general, however, the Mn and Cd contents of sphalerite o f stage II are low and the effects of minor and trace elements on the sphalerite geobarometer are negligible (Vaughan and Craig 1978). Accordingly, minimum pressure estimates based on the FeS contents of sphalerites of stage II in equilibrium with pyrite can be made. The sphalerite of stage II, which is associated with only pyrite as iron sulphide, contains up to 19.4 tool % FeS. This can be regarded as the maximum stability field of coexisting pyrite and sphalerite, and is equivalent to a minimum confining pressure of about 1 kbar. This is approximately equal to the value 0.9_+0.1 kb given by Chung (1986) for the Bonsan deposit at Yeonhwa mine.
Summary The Taebaek Pb-Zn(-Ag) deposit of the Yeonhwa I mine, of hydrothermal-metasomatic origin, is formed mainly within limestones of the Pungchon Formation and a thin limestone bed in the underlying M y o b o n g Formation, both of Cambrian age. The mineralization may be divided into four distinct stages based upon the mode of occurrence and chemical compositions o f the ore minerals. The physicochemical conditions during ore deposition at the Taebaek Pb-Zn(-Ag) deposits have been estimated from a study of the fluid inclusion study and of mineral assemblages together with application of the Kretschmar and Scott (1976) arsenopyrite geothermometer. As shown in Fig. 8, the early to middle lead-zinc
mineralizations are estimated to have occurred in the temperature range of 270 to 355 ~ with sulphur activity of 10- 9.5 to 10-13 atm, using the mineral assemblages, arsenopyrite+hexagonal p y r r h o t i t e + n a t i v e bismuth and arsenopyrite + pyrite + sphalerite, together with the arsenopyrite geothermometer. The late gold-silver mineralization may have taken place in the temperature range of 220 to 250~ with sulphur activity of 10 -135 to 10-15 atm, using the mineral assemblages, monoclinic pyrrhotite + electrum + sphalerite + pyrite and argentite + native silver + pyrite. Consequently, it may be reasonably supposed that the difference in mineral assemblages is ascribed to the decrease in temperature and sulphur activity. The homogenization temperature of primary fluid inclusions in quartz and sphalerite ranges from 203 to 381 ~ and from 209 to 264~ respectively. This is compatible with the temperature estimated by the coexisting mineral assemblages as described above. Application of sphalerite geobarometry to the Taebaek Pb-Zn(-Ag) deposit, using sphalerite associated with pyrite, yielded a minimum confining pressures of about 1 kbar. Sphalerite geobarometry, however, is not, in general, an adequate method for pressure determination at the Taebaek deposit, because o f the absence of a satisfactory coexisting mineral assemblage (i.e. pyritehexagonal pyrrhotite-sphalerite).
Acknowledgements. This research was supported financially by the Korea Science and Engineering Foundation. We thank Dr. A. J. Reedman and Prof. N. Imai for constructive suggestions for revision of our original manuscript.
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Announcemen tS Geology in Europe and beyond. Mineral deposit modelling in relation to crustal reservoirs of the ore-forming elements
"Lepidolite 200". International Symposium on the Mineralogy, Petrology, and Geochemistry of Granitic Pegmatites
22 and 23 April 1992
August 29-September 3, 1992, Czechoslovakia
and
The Masaryk University and the Moravian Museum, Brno, Czechoslovakia are organizing this Symposium to commemorate the 200th anniversary of definition of lepidolite as a new species mineral, by Klaproth (1792), from Ro~m~, western Moravia, Czechoslovakia.
BGS Minerals Industry Forum 24 April 1992 To be held at the Kingsley Dunham Centre of the British Geological Survey, Keyworth, Nottingham, England.
Information: The Conference Office, The Institution of Mining and Metallurgy, 44 Portland Place, London W I N 4BR, UK
Organizers: Dr. Milan Nov~k, Department of Mineralogy and Petrology, Moravian Museum, Zeln~ trh 6, 659 37 Brno, Czechoslovakia. Dr. Josef Stan~k, Department of Mineralogy, Petrography and Geochemistry, Masaryk University, Kotlfi~sk~i2, 611 37 Brno, Czechoslovakia
