Gold-Silver Deposits Which Are a Principal Mineral Resource in Korea

Home , Stephanite

MININGGEOLOGY, 36(6), 555-572, 1986

Gold and Silver Ores from the Geumwang Mine in South Korea and Their Mineralization

Asahiko SUGAKI*, Ok Joon KIM** and Won Jo Kim***

Abstract: Gold and silver bearing quartz veins of the Geumwang mine in South Korea occur in Cretaceous granite

altered hydrothermally. Ores from the mine are exceptionally silver rich as the Korean type gold deposit. The ore

minerals occur in two or three sulfide bands formed at early to middle stages and a sulfosalt band at the late stage of

mineralization. Electrum and silver minerals of polybasite, pyrargyrite and argentian tetrahedrite appears in sulfide

band or stringer consisting of pyrite, arsenopyrite, galena, sphalerite, chalcopyrite and quartz in the vein. Also

silver-antimony sulfosalt minerals such as pyrargyrite, polybasite, miargyrite, diaphorite and argentian tetrahedrite

with native silver occur as a band in central portion of the quartz vein in association with some amounts of pyrite,

sphalerite, arsenopyrite and quartz. Composition of electrum is 48.4 to 52.7 wt% Ag (63.0 to 67.4 at% Ag). Mean-

while sphalerite has 2.2 to 4.2 mole% Fes. Homogenization temperatures of fluid inclusion (two phases) in quartz of

the sulfide band are from 183•‹to 310•Ž(240•Ž in average) with a peak at 240•Ž, commonly 220•‹to 270•Ž. The ore

mineralogy suggests that temperature (T) and sulfur fugacity (fs2) of the formation of the sulfide and sulfosalt bands

are estimated as T: 190•‹to 240•Ž, fs2: 10-15.5 to 10-13.5 atm, and T: 130•‹to 170•Ž, fs2: 10-19.5 to 10-110 atm, respec-

tively. Such low temperature and low sulfur fugacity mineralization crystallized silver-antimony sulfosalts are con-

sidered.to be singular as the Korean type gold deposit.

amounts of gold. Commonly amounts of Introduction silver in such gold ores are from less than Gold-silver deposits which are a principal those of gold to its several times. Occurrence mineral resource in Korea are thought to be of high grade silver bearing gold ore is rather mainly formed by mineralizations related to rare in Korea. However, as an exceptional ex- activities of granitic magmas during periods ample the gold-silver ore from the Geumwang from Jurassic to Cretaceous or early Tertiary mine is given. There are found many silver (KIM, 1971a; PARK, 1981; LEE, 1981; minerals such as pyrargyrite, polybasite, SHIMAZAKIet al., 1984; KIM, 1985). Many of diaphorite, miargyrite, stephanite, argentian them belong to quartz veins of meso- or tetrahedrite and native silver besides electrum hypothermal type, and are named as the in the ore. Consequently its silver content Korean type gold deposits (TSUCHIDA,1944; gives distinctly such high values as 1,000 to LEE, 1981; Kim, 1985). They are in general 3,500 g/t for gold content of 7 to 25 g/t, poor in silver contents in comparison with sometimes 50 to 200 g/t. It is characteristic of the Geumwang's ore, and there is recognized a Received on August 11, 1986, accepted on November singularity as the gold ore from Korea. 25, 1986 Therefore, the present authors would like to *Institute of Mineralogy , Petrology and Economic describe about such silver rich gold ore, Geology, Tohoku University, Sendai 980, Japan (Pre- especially occurrence, paragenesis and mineral sent Address: Kadan 4-30-503, Sendai 980, Japan) **Department of Geology assemblage of silver minerals and electrum, , College of Sciences, Yonsei and condition of gold-silver mineralization in University, Seoul, Korea. this paper. ***Korea Institute of Energy and Resources , Seoul, The Geumwang mine being at about 100 km Korea. southwest of Seoul is situated at Bonggog-ri of Keywords: Geumwang mine, Gold-silver deposit, Silver Geumwang-myeon; Eumseong-gun, Chung- sulfosalts, Fluid inclusion, Gold-silver mineralization, Chemical composition, Electrum, Diaphorite, cheongbug-do as shown in Figure 1. It is X-ray powder data. located at 2 km southwest of the Mugeug mine

555 556 A. SUGAKI, O. J. KIM and W. J. Kim MINING GEOLOGY:

(Kim, 1971b). Ore deposits of the Geumwang mine occur in biotite and leucocratic granites. The granites near ore veins suffer hydrothermal alteration by silicification and sericitization. In altered granite, plagioclase changes distinctly to fine aggregate of sericite and kaoline although alkali-feldspar remains relatively fresh, and biotite completely decomposes to chlorite and calcite. Also dissemination of fine grained pyrite is usually observed in altered granite. According to SFIMAZAKiet al. (1986), K-Ar age for sericite from altered granite as country rock of the Mugeug mine is 98 Ma.

Ore Deposits Fig. 1 Location map of the Geumwang mine. The gold-silver deposits of the Geumwang mine are quartz veins filling up fissures which is one of representatives of the Korean developed in granitic rock altered hydrother- type gold deposit (GALLAGHER, 1963; Kim, mally (KANEDA et al., 1984; PARK et al., 1985). 1982). This area with both the mines is known There are found eight veins in the mine, but as principal gold producing district in South among them, principal ones are Nos. 1, 2, 4

Korea. The ore deposits of both the mines are and 8 veins which are being worked, as shown thought to belong the same type gold-silver in Figure 2. No. 1 vein runs to N-S direction vein. Annual productions of gold and silver in (N10•‹E to N10•‹W) dipping 75•‹to 85•‹E or W, metal from the Geumwang mine were 47 kg and has scale of about 600 m in length, 200 m and 1,215 kg in 1983, 78 kg and 1,610 kg in in depth and 30 to 100 cm in width. No. 2 vein

1984, and 104 kg and 1,716 kg in 1985, respec- occurs parallel (N5•‹-10•‹E) to No. 1 vein at tively. east side of it, dipping 75•‹to 85•‹E. Its scale is 350 m long, 200 m deep and 30 to 70 cm wide. Geology Meanwhile Nos. 4 and 8 veins (Figure 2) strike Topography of the mine and its neigh- to N59•‹to 70•‹W dipping 30•‹SW, and have borhood shows a hilly feature of 100 to 120 m scale of 100 to 200 m in length and 10 to 80 cm elevation. Geology of the hill around the mine in width. They consist mostly of quartz consists of Cretaceous biotite granite and associated with small amounts of pyrite, leucocratic granite (Kim, 1982; PARK et al ., arsenopyrite, chalcopyrite, sphalerite, galena, 1985) although Jurassic granite widely distrib- silver-antimony sulfosalt minerals, native utes in this area according to geological map silver, electrum, calcite, sericite, chlorite and of Daejeon (1:250,000) issued by Geological kaoline. Ore minerals of sulfides, silver-an- and Mineral Institute of KOREA(1973). Quartz timony sulfosalts, native silver and electrum porphyry partly intrudes into biotite granite. etc. often appear in thin band or stringer, 0.5 Biotite granite is grayish white colored and to 3.0 cm, sometimes 5.0 cm wide, at most coarse grained hollocrystalline rock having an outer side of the quartz vein along boundaries equigranular texture. It is composed of with altered granite walls and in middle or cen- quartz, microcline, perthite, plagioclase and tral parts of the vein. biotite as essential constituent minerals. K-Ar There are gold-silver veins of the Mugeug age for alkali feldspar from biotite granite as mine adjoining to this mine. They are the country rock of the Mugeug mine neighboring same type deposits as those of the Geumwang with the Geumwang mine (Fig. 1) is 112 Ma mine, occurred in Cretaceous granitic rock 36(6), 1986 Gold and Silver Ores from the Geumwang Mine, Korea 557

Au and 500 to 3,500 g/t Ag, generally 10 to 30 g/t Au and 1,000 to 2,000 g/t Ag, in the ore shoot of the vein. For instance, contents of gold and silver in shipping ore from the mine after hand picking are as follows: Au (g/t) Ag (g/t) 6.5 2,955 8.5 3,585 9.0 2,768 15.1 3,131 24.0 2,708 31.3 2,917 35.6 3.294 The, silver contents of the ore are approximately 100 to 300 times of gold contents as given above. However the amounts of silver are not in proportion to those of gold. Such silver rich gold ore is uncommon as the Korean type gold-silver ore.

Ore and Minerals

The gold-silver ore from the Geumwang mine is composed mostly of massive quartz associated with some amounts of ore and other gangue minerals. As ore minerals pyrite, arsenopyrite, chalcopyrite, sphalerite, argentian tetrahedrite, pyrargyrite, polybasite, miargyrite, diaphorite, stephanite, electrum and native silver are accompanied by quartz. Also small amounts of calcite, sericite, chlorite and kaoline as gangue minerals besides quartz are found. Among them massive quartz is granular aggregate of grayish white and milky white in color and oc- cupies main portion or both ores and barrens of the vein. Meanwhile sulfide and sulfosalt minerals mentioned above appear as band and stringer forms in the quartz vein. The sulfide band and stringer, 0.2 to 5.0 cm wide, are com- pact and dark gray in color consisting mainly Fig. 2 The ore veins at the - No. 2 level in the of pyrite, arsenopyrite, sphalerite, galena and Geumwang mine. quartz associated with small amounts of chalcopyrite, argentian tetrahedrite, polyba- altered hydrothermally, and have. been site, pyrargyrite, stephanite and electrum. developed greatly in the past (GALLAGHER, They in general occur as two or three bands or 1963; Kim, 1982). stringers at most outer or inner portions of the Although gold and silver grades of the vein quartz vein (Figure 3-A). However the bands in the Geumwang mine changes distinctly-in are discontinuous and occasionally teared to place by place, they are roughly 7 to 200 g/t pieces or fragments by deformation after ore 558 A. SUGAKI, O. J. KIM and W. J. KIM MINING GEOLOGY:

Fig. 3 Photographs of gold-silver ores from No. I vein of the Geumwang mine. A: Pyrite (py) and sphalerite (sp) with quartz (qz) in sulfide band. gr: Rock fragment. (Specimen No. 81112108a); B: Silver-antimony sulfosalt (sst) band in quartz (qz) vein. (No. 81112108b); C: Silver-antimony sulfosalt (sst) band with native silver (Ag) in quartz vein. (No. 81112108a); D: Ring ore consisting of quartz, sphalerite (sp), pyrite (py) and rock fragment (gr). (No. 81112107). formation. They partly consist mostly of ag- miargyrite and diaphorite with some amounts gregate of euhedral pyrite, 0.1 to 0.6 mm in of polybasite, argentian tetrahedrite, native size, associating with quartz, sphalerite, silver, sphalerite and quartz occurs occasional- galena, arsenopyrite, electrum, polybasite and ly in the central portion of the quartz vein pyrargyrite. Once in a while sulfide ring ore, (Figure 3-B and C). In the sulfosalt band 3.0 to 10 cm in size, with a core of fragment of small amounts of pyrite, arsenopyrite and altered granitic rock is found instead of the chalcopyrite are also found in association with sulfide band in the quartz vein (Figure 3-D). It native silver, argentian tetrahedrite and is composed concentric band or rim, 0.2 to pyrargyrite etc. 0.5 cm wide, of pyrite, arsenopyrite and As mentioned above, both the sulfide band quartz with sphalerite, galena, argentian (including ring ore) and sulfosalt band in the tetrahedrite, pyrargyrite and polybasite. quartz vein contain many kinds of silver-an- While silver-antimony sulfosalt band, 0.5 to timony sulfosalt minerals, electrum and native 1.0 cm in width, consisting of pyrargyrite, silver. Therefore they often form high grade 36(6), 1986 Gold and Silver Ores from the Geumwang Mine, Korea 559 gold-silver ore. Constituent minerals of the sulfide and sulfosalt bands and comparative amounts of them are shown in Figure 4. Among them, base-,metal sulfide minerals such as pyrite, arsenopyrite, sphalerite and galena are essential constituents of the sulfide band (and ring ore) together with quartz. They assemble with each another intimately in association with electrum, pyrargyrite, polybasite and argentian tetrahedrite. In the meanwhile silver-antimony sulfosalts of pyrargyrite, miargyrite and diaphorite become principal minerals of the sulfosalt band. Ore minerals assembled with electrum and silver minerals in both the sulfide and sulfosalt bands are shown collectively in Figure 5. Elec- turm assembles with commonly pyrite and Fig. 4 Constituent minerals consisting of the sulfide galena, and occasionally sphalerite, chalco- and sulfosalt bands, and comparative amounts pyrite, arsenopyrite, pyrargyrite and polyba- among them. site in the sulfide band (Figure 6-A and B). Pyrargyrite, polybasite and argentian tetrahedrite associate closely with each another ac- companying by base-metal sulfides in the sulfide band. While miargyrite and diaphorite assemble with each other, and accompany by pyrargyrite, polybasite, argentian tetrahedrite, sphalerite and quartz in the sulfosalt band. Native silver appears only in the sulfosalt band. It assembles with miargyrite, diaphorite, pyrargyrite, quartz and occasionally pyrite, arsenopyrite, sphalerite, galena and chalcopyrite. Occurrence, mineral assemblage and paragenesis, optical properties, chemical compositions, and crystal data of ore and gangue minerals are described below. Ore minerrls Electrum Electrum usually occurs as granular or polygonal forms, 0.01 to 0.2 mm in size, associating with base-metal sulfides and silver minerals, commonly pyrite and galena in the sulfide band of the quartz vein as Fig. 5 Mineral assemblages with gold and silver shown in Figure 6-A and B. It also is frequent- minerals in the sulfide and sulfosalt bands, and ly found as an inclusion, 5 to 50 um in size, their frequency. in pyrite crystal (Figure 6-C). Electrum sometimes appears as an individual grain, 1 to (63.0 to 67.4 at.% Ag) as given in Table 1, but 100 um in size, filled up interstice of quartz ag- it is limited in a narrow range without composi- gregate. Its chemical composition obtained by tional zoning. PARKet al. (1985) also obtained electron microprobe X-ray analyzer is com- 35.7 to 51.9 wt% Ag (50.1 to 66.6 at.% Ag) as paratively silver rich as 48.4 to 52.7 wt% Ag composition of electrum. 560 A. SUCAKI. 0. J. Kim and W. J. KIM MINING Gi ot(xw:

Fig. 6 Photomicrographs of gold and silver minerals found in the ore from No . 1 vein of the Geumwang mine. A: Electrum (el) and galena (gn) with polybasite (p1) assembled with pyrite (py). (Specimen No. 81112108a); B: Elec- trum (el) and galena (gn) with polybasite (pl) filled up interstice of aggregate of pyrite (py) and quartz (qz) . (No. 81112108a); C: Electrum (el) of irregular form enclosed in pyrite (py) crystal. qz: Quartz. (No. 81112108a); D: Miargyrite (mg) and diaphorite (dp) with native silver (Ag) found in the sulfosalt band . (No. 81112108b).

Native silver Native silver is occasionally Gyu, personal communication). Also no found as irregular grain and polygonal platy native silver occurs in the sulfide band. Its form, 0.01 to 1.0 mm in size, assembling with gold content is very slight less than 0.1 wt% silver-antimony sulfosalt minerals in the Au corresponding to almost pure silver as sulfosalt band at central portion of the quartz given in Table 2. vein (Figure 3-C). Native silver occurs Pyrargyrite Pyrargyrite is one of common sometimes as an inclusion, 10 to 30ƒÊm in size, silver minerals from this mine, and occurs as in diaphorite and pyrargyrite (Figure 6-D). granular and polygonal forms, 0.1 to 0.7 mm Such native silver associated with silver-an- in size, or their aggregate in close association timony sulfosalts has mainly appeared from with base-metal sulfides, electrum, other silver the sublevel of the -No. 2 level (82 m below minerals and quartz in the sulfide and sulfosalt the surface) of the No. 1 vein, but there is bands as seen in Figure 5. It is also found as found no native silver in the No. 1 vein at the granular and island-like forms, 0.3 to 1.0 mm deeper level than the -No. 4 level (Choi Seon- in size, in aggregate of polybasite with 36(6), 1986 Gold and Silver Ores from the Geumwang Mine, Korea 561 sphalerite in the sulfosalt . band, and Under ore microscope, pyrargyrite presents sometimes appears as irregular veinlet (Figure a weak pleochroism from bluish gray to gray 7-A) assembling with galena, tetrahedrite, with light bluish tint in color, and a moderate sphalerite, polybasite and stephanite along anisotropism changing its 'interference color cracks in pyrite of the sulfide band or rim of from dark gray to gray. It shows internal reflec- the ring ore. Pyrargyrite in the sulfosalt band tion of bright deep red. Reflectivity of shows partly micrographic texture (Figure 7- pyrargyrite is given in Table 3. B) formed by invasion of miargyrite. Its chemical composition is given in Table 4. It has the composition near pure pyrargyrite, but it contains 1.0 to 4.0% as proustite Table 1 Chemical compositons of electrum found in the sulfide band of the No. 1 vein in the Geum- (Ag3AsS3) molecule. X-ray powder diffraction wang mine, obtained by electron microprobe. data for pyrargyrite from the mine are given in Table 5 as compared with that of pyrargyrite from the Sanru mine, Japan. The cell parameters calculated from the powder data as hexagonal cell are a=11.040A and c = 8.720 A. Polybasite Polybasite occurs in granular, polygonal and irregular forms, 0.03 to 0.5 mm, sometimes 1.0 mm in size, as a common silver mineral same as pyrargyrite in the sulfide and sulfosalt bands. It assembles with usually pyrargyrite, base-metal sulfides and quartz, and occasionally electrum and argentian tetrahedrite in the sulfide band. Aggregate of polybasite with pyrargyrite and galena fills up a gap of crystal aggregate of pyrite or Table 2 Analytical data of native silver in the sphalerite (Figure 5-C). It sometimes appears sulfosalt band of the No. 1 vein in the Geumwang as irregular form as rimmed partly along mine. margin of galena or chalcopyrite grains. Polybasite is also found as an individual grain in quartz aggregate. Polybasite shows distinct pleochroism from grayish white with pale yellowish green to grayish white with light purple, and has strong anisotropism changing its interference color from dark brown to gray or yellowish gray.

Table 3 Reflectivities of silver-antimony sulfosalt minerals from the No. 1 vein of the Geumwang mine. 562 A. SCGAKI, O. J. KIM and W. J. KIM MINING GEOLOGY:

Fig. 7 Photomicrographs of silver-antimony sulfosalts found in the ore from No. I vein of the Geumwang mine. A: Pyrargyrite (pr) and galena filled up interstice or crack of pyrite (py) in the sulfide band of quartz vein. (Specimen No. 81112107); B: Micrographic texture of pyrargyrite (pr) and miargyrite (mg) with argentian tetrahedrite (tt) and sphalerite (sp) in the silver-antimony sulfosalt band. qz: Quartz. (No. 8112108b); C: Polybasite (p1) and galena (gn) filled up interstice of pyrite (py) crystal in the sulfide band of the vein. (No. 81112108a); D: Argentian tetrahedrite (tt) invaded and cut sphalerite (sp) in aggregate of miargyrite (mg) and diaphorite (dp) in the sulfosalt band. qz: Quartz. (No. 81112108b).

Light etching is conspicuously recognized. silver mineral in the sulfosalt band occurs as Also reflectivity of polybasite is given in Table granular aggregate, 0.07 to 0.5 mm in size. It 3. The chemical composition of polybasite is associates with other silver-antimony given in Table 6. It contains 20 to 33% in sulfosalts, native silver, sphalerite and quartz. arsenpolybasite (Cu, Ag)16As2S11mole. The In this case, diaphorite and pyrargyrite are data of X-ray powder diffraction for often found as island-like form in host or polybasite from the mine are given in Table 7 matrix of miargyrite aggregate. Also in comparison with those of polybasite from miargyrite occasionally invades into pyrargy- the Keley mine, Canada. Lattice constants rite as fine veinlet and lamella, and forms a calculated from the powder data as pseudohex- micrographic texture with pyrargyrite (Figure agonal cell are a=15.076 A and c=23.886 A. 7-B). It rarely encloses tiny crystals of Miargyrite Miargyrite which is a principal arsenopyrite. 36(6), 1986 Gold and Silver Ores from the Geumwang Mine, Korea 563

Table 4 Analytical data of pyrargyrite from the Table 6 Chemical composition of polybasite from sulfide band of the No. 1 vein, obtained by electron the sulfide band of the No. 1 vein microprobe.

1-6: Specimen No. 81112100a. 1-6: Specimen No. 81112108a. anisotropism changing from dark brown to

Table 5 X-ray powder diffraction data of yellowish gray. It sometimes presents deep red internal reflection, but less than that of pyrargyrite from the sulfide band in the No. 1 vein. pyrargyrite. The reflectivity of miargyrite is given in Table 3 together with that of other silver-antimony sulfosalts. The analytical data for miargyrite are given in Table 8. It has

almost stoichiometric composition without arsenic. The data of X-ray powder diffraction for miargyrite in comparison with those from the Braubsdorf mine, Germany are also given

in Table 9. Both the data are in good accor- dance with each other. The unit cell parameters of miargyrite calculated from the powder data as monoclinic cell are a=13.224•ð, b = 4.382•ð, c=12.861•ð and 6=98.28•‹. Diaphorite Diaphorite is found as granular and island-like forms, 0.07 to 0.2 mm in size, in miargyrite, or as a thin band, 0.3 to 1.0 mm in width, consisting of its aggregate in the sulfosalt band. Diaphorite assembles with pyrargyrite, argentian tetrahedrite and sphalerite besides miargyrite.it is occasionally replaced by miargyrite. It also encloses fine crystals of native silver and 'quartz. Diaphorite has distinct pleochroism from light grayish white with pale yellowish green to grayish white with bluish tint, and presents a moderate anisotropism changing its in- *After Sugaki et al. (1982) terference color from bluish gray to yellowish brown. It shows rarely internal reflection of Under ore microscope, miargyrite has visi- deep red in color, and. also presents distinctly ble pleochroism from white to grayish white light corrosion. Reflectivity of diaphorite is with pale bluish tint, and shows strong given in Table 3. The chemical composition of 564 A. SUGAKI, O. J. KIM and W. J. KIM MINING GEOLOGY:

Table 7 The data of X-ray powder diffraction for Table 8 Analytical data of miargyrite found in the polybasite from the sulfide band of the No. 1 vein silver-antimony sulfosalt band of the No. 1 vein, obtained by electron microprobe.

1-5: Specimen No. 81112108b.

Table 9 X-ray powder diffraction data of miargyrite from the silver-antimony sulfosalt band (No. 1 vein)

* After JCPDS 8 -123 * After JCPDS 19-1136 ** Specimen No . 81112108a. * Specimen No . 81112108b. diaphorite is given in Table 10 . It has 33 to 35 the powder data as monoclinic cell are mole% PbS less than that of its stoichiometric a=17.913•ð, b=5.880•ð, c=15.841•ð and composition Ag3Pb2Sb3S8 (40 mole% PbS) . ,8=116.68•‹. Its composition corresponds to Ag3.17-3.31 Stephanite Stephanite is uncommon as Pb 1.57-1.63Sb3.14-3.1757.89-8.03.X-ray powder silver minerals from this mine, but rarely ap- diffraction data for diaphorite are given in pears as irregular grain, 0.05 to 0.2 mm in Table II as compared with that of diaphorite size, in pyrargyrite with argentian tetrahedrite from the Pribram mine, Czechoslovakia . Both filled up crystal interstice of pyrite aggregate data for diaphorite accord well with one in the sulfide band. another. Its lattice parameters calculated from Stephanite has a visible pleochroism from 36(6), 1986 Gold and Silver Ores from the Geumwang Mine, Korea 565

Table 10 Chemical composition of diaphorite Table 12 Analytical data for stephanite found in found in the silver-antimony sulfosalt band (No. 1 the sulfosalt band of the No. 1 vein vein).

1-3: Specimen No. 31112103b.

1-5: Specimen No. 81112108b. Table 11 The data of X-ray powder diffraction of diaphorite from the sulfosalt band (No. 1 vein) tion almost corresponds to the ideal formula Ag5SbS4 of stephanite. Argentian tetrahedrite It is a principal silver mineral from the mine, and occurs as granular crystal or polygonal form, 0.1 to 0.3 mm in size. Argentian tetrahedrite assembles with pyrargyrite, miargyrite, diaphorite and base-metal sulfide minerals. It often appears as rim-like form along margin of sphalerite or quartz grains in the sulfosalt band. In this case, argentian tetrahedrite invades into sphalerite without chalcopyrite dots, and cuts it as veinlet (Figure 7-D). Also veinlets of argentian tetrahedrite with pyrargyrite, galena, polybasite and sphalerite develop along cracks in crystal aggregate of pyrite in the sulfide band. The chemical composition of argentian tetrahedrite is given in Table 13. It has 0.0 to 23.0% as tennantite molecule, and also contains 22.0 to 30.0 wt% (13.1 to 18.2 at%) Ag, 1.7 to 3.3 wt% (1.9 to 3.9 at%) Fe and 3.0 to 5.2 wt% (3.0 to 5.1 at%) Zn, approximately same as analytical values by PARKet al. (1985). * After JCPDS 9-126 ** Specimen No . 81112108b. Pyrite Pyrite which is a most common ore mineral in the sulfide band occurs as euhedral brownish gray to gray with yellowish tint, and or subhedral cubic crystals, 0.1 to 1.0 mm in presents distinct anisotropism changing its in- size, or their aggregate. It sometimes forms a terference color from dark brown to yellowish band, 2 to 30 mm wide, in massive quartz, and gray. Its light etching is conspicuous. Reflec- assembles with most of ore minerals from the tivity of stephanite is given in Table 3 together mine except miargyrite and diaphorite as with that of other silver-antimony sulfosalts. shown in Figure 5. Pyrite occasionally appears The data analyzed for stephanite are given in as outer side rim, 2 to 5 mm in width, of the Table 12. According to the data, its composi- ring ore in the quartz vein (Figure3-D). It is 566 A. SUGAKI,O. J. KIM and W. J. KIM MINING GEOLOGY:

Table 13 Chemical compositions of argentian table 14 Analytical data of sphalerite in the sulfide tetrahedrite from the sulfide and sulfosalt bands in and sulfosalt bands (No. 1 vein) the No. 1 vein

1-5: Sulfide band (Speciemen No. 81112108a), 6-7: Sulfosalt band (Specimen No. 81112108b).

1-7:with chalcopyrite dots, sulfide band sometimes penetrated by veinlet, 0.05 to 0.2 (Specimen No. 81112108a), mm wide, consisting of pyrargyrite, poly- 8-14: without chalcopyrite dots, sulfosalt band (Specimen No. 81112108b) basite, argentian tetrahedrite, galena and sphalerite along its crack. It often has an inclu- tions are given in Table 14. As seen in the sion of electrum and galena. Dissemination of table, the composition of sphalerite*1 with fine grained pyrite, 0.01 to 0.4 mm in size, is chalcopyrite dots from sulfide band is 1.1 to usually seen in hydrothermally altered granite 2.5 wt% Fe (2.2 to 4.2 mole% FeS). Sphalerite adiacent to the vein. without chalcopyrite dots from sulfosalt band Arsenopyrite Arsenopyrite also is a com- includes 1.1 to 2.4 wt% Fe. There is no mon ore mineral similar to pyrite, and usually compositional difference between both appears as individual crystal of rhombic or sphalerite. prismatic forms, 0.04 to 0.1 mm, sometimes Galena Galena occurs as granular or 0.3 to 0.5 mm in size, or their aggregate in the polygonal forms, 0.03 to 0.3 mm in size, in sulfide band. Arsenopyrite assembles with association with base-metal sulfides, polyba- most of ore minerals excepting some silver-an- site, pyrargyrite, argentian tetrahedrite, elec- timony sulfosalts same as pyrite. It occasional- trum and quartz in the sulfide band. It often ly encloses fine grains of galena, chalcopyrite fills up interstice of crystal aggregate of pyrite, and polybasite. Arsenopyrite composition ob- arsenopyrite and quartz. Also the veinlets of tained from the d131value (1.6292 to 1.6300 A) galena assembling with polybasite, pyrargyrite is 30.9 to 31.5 at% As (KRETSCHMARand and argentian tetrahedrite develop along SCOTT,1976). cracks in pyrite aggregate of the sulfide band. Sphalerite Sphalerite is often found in Galena is occasionally replaced by polybasite some amounts as granular form, 0.05 to 0.5 along margin of its crystal. mm in size, in the sulfide and sulfosalt bands Chalcopyrite Chalcopyrite is found in of the quartz vein, but its quantities are small quantities as irregular form, 0.03 to 0.3 distinctly less than those of pyrite and mm in size, in association with arsenopyrite, arsenopyrite. Sphalerite in the sulfide band pyrite, sphalerite, galena, polybasite, quartz, contains generally tiny dots of chalcopyrite, and sometimes electrum and argentian but it in the sulfosalt band has no chalcopyrite tetrahedrite in the sulfide band. dots. Sphalerite in the sulfosalt band . is rimmed by argentian tetrahedrite, and cut by *1Analysis for sphalerite with chalcopyrite dots has been its veinlet (Figure 5-D). Sphalerite composi- tried apart from them. 36(6), 1986 Gold and Silver Ores from the Geumwang Mine, Korea 567

Gangue minerals

Quartz The ore veins almost consist of quartz, and it occurs as massive aggregate of granular crystal, 0.2 to 2.0 mm, sometimes 3.0 to 5.0 mm in size. It shows occasionally

prismatic form, 0.4 to 0.7mm•~2.0 to 4.0 mm, grown perpendicular to wall of country rock. Quartz generally is turbid by very fine

dusty inclusions. It associates intimately with slight quantities of calcite, sericite, chlorite and sulfide minerals such as pyrite and

arsenopyrite dotted in barren zone of the vein. Some quartz, especially prismatic one presents flamboyant or feather-like extinctions which are usually observed about chalcedonic quartz

crystallized under condition of shallow depth Fig. 8 Histogram of homogenization temperature at low temperature. It also shows occasionally of fluid inclusion in quartz from the vein of the undulatory extinction affected by stress of Geumwang mine. deformation or shear. There is often found ag-

gregate of fine grained quartz, 0.01 to 0.1 mm Kaoline It appears in slight amounts as in size, with sericite and chlorite embedded in- very fine grained aggregate together with terstice of aggregate of coarse grained quartz sericite in white and light gray colored clay described above. Such fine grained quartz is found as film, stringer or veinlet in the quartz thought to be a relict of silicified rock vein or along its boundaries with wall rock. It fragments in the vein. also occurs as an alteration product with Calcite Calcite appears as a irregular sericite from feldspar of granitic rock frag- form, 0.1 to 0.5 mm in size, filling up in- ment in the vein. terstice of quartz aggregate, but its amounts are very slight. Also calcite is sometimes Fluid Inclusion

found as one of alteration products together Fluid inclusions of two phases, vapor and with sericite and chlorite in hydrothermally liquid, are often observed in quartz of the affected rock fragments in the vein. ore vein. However, because most of them are Sericite Sericite occurs in small amounts very fine measurements on homogenization as a tiny crystal less than 5ƒÊm, sometimes 50 temperature and salinity for them are selective- ƒÊ m, filled up gap of aggregate of coarse ly tried on rather larger inclusions only than 5

grained quartz or assembled with aggregate of ,um in size. The measurements were carried fine grained quartz. Very fine grained ag- out by using a heating and cooling stage of the

gregate of sericite with small amounts of Linkam TH-600 type under heating rates of kaoline is sometimes found as hydrothermal 2•Ž and 0.2•Ž per minute for homogenization alteration product of rock fragments in the and freezing temperatures, respectively. The vein. White clay consisting essentially of experimental results on the homogenization sericite with slight quantities of kaoline ap- temperature for fluid inclusions in quartz

pears as film or thin band along boundaries (Nos. 81112101, 81112106, 81112108a and b) between the vein and wall rock. from the whole of the vein are shown as a Chlorite Small amounts of chlorite occur histogram in Figure 8. According to these as veinlet, stringer, massive or irregular forms data, the homogenization temperatures are in in quartz masses. It is also found as an altera- a wide range from 171•‹to 341•Ž (243•Ž in tion product from rock forming minerals of average value) with a peak at 240•Ž, but main- granite fragment in the vein. ly from 220•‹ to 260•Ž. In the meanwhile 568 A. SUGAKI, O. J. KIM and W. J. KIM MINING GEOLOGY:

Fig. 9 Homogenization temperature of fluid inclusion in quartz from the sulfide band of the No. 1 vein in the Geumwang mine.

salinities obtained by freezing temperature for fluid inclusions are 2.5 to 5.0 wt% NaCI equivalent. Among the data of the homogenization temperatures shown in Figure 8, Fig. 10 Mineralization sequences of minerals from those for fluid inclusions in quartz (Nos. the Geumwang mine. 81112106 and 81112108a) associated with

pyrite, arsenopyrite, sphalerite, galena, as characteristically essential minerals of the polybasite, pyrargyrite, argentian tetrahedrite sulfosalt band together with pyrargyrite, and electrum in the sulfide band are from 183•‹ polybasite and argentian tetrahedrite. From to 310•Ž (240•Ž in average) with a peak at occurrence of ore minerals as above, there are 240•Ž, commonly 220•‹ to 270•Ž as shown in two kinds of mineralizations to have sulfide Figure 9. These data have approximately same and sulfosalt bands in the vein. The sulfide values as those as shown in Figure 8, and also band is found two or three in the vein from present generally a tendency similar to those outermost side adjacent to country rock to in- (180•‹ to 300•Ž) for fluid inclusions in quartz ner (or middle) portion of the quartz vein in from the Mugeug mine near this mine place by place. Thus, sulfide band mineraliza- (KANEDA et al., 1985). tion accompanied by gold and silver minerals was repeated two or three times from early to Mineralization middle stages during formation of the vein. As mentioned above, gold-silver minerals While the sulfosalt band found at central por- such as electrum; native silver, pyrargyrite, tion of the quartz vein was producedby late polybasite, miargyrite, diaphorite, stephanite stage mineralization. A lot of silver minerals and argentian tetrahedrite from the Geum- such as miargyrite, diaphorite, pyrargyrite, wang mine are found in the sulfide and polybasite, argentian tetrahedrite and native sulfosalt bands of the quartz vein. Among silver appear in association with some them, electrum, pyrargyrite, polybasite and amounts of sphalerite, pyrite, arsenopyrite argentian tetrahedrite occur as principal gold- and chalcopyrite in the sulfosalt band. From silver minerals associating with pyrite, galena, the data on occurrence, mineral assemblage sphalerite, arsenopyrite and chalcopyrite in and paragenesis, and texture of ore and the sulfide band. On the other hand gangue minerals as described above, miargyrite, diaphorite and native silver appear mineralization sequences of minerals from the 36(6), 1986 Gold and Silver Ores from the Geumwang Mine, Korea 569

Geumwang mine are inferred as shown schematically in Figure 10*2. Electrum was in- termittently crystallized at two times in close relation with formation of the sulfide band, especially pyrite and galena, at stage from early to middle periods of the mineralization. By this mineralization, some silver minerals such as pyrargyrite, polybasite and argentian tetrahedrite besides electrum were produced. Silver mineralization was more intensely performed at late stage, and a lot of silver-antimony sulfosalts such as miargyrite, Fig. 11 Temperature and sulfur fugacity of diaphorite, pyrargyrite, polybasite and argen- mineralizations formed the sulfide (I) and sulfosalt tian tetrahedrite with native silver were (II) bands in the vein of the Geumwang mine. Ag: formed. However, no electrum was crystal- Silver; arg: Argentite; pr: Pyrargyrite; mg: lized by the mineralization. Miargrute; po: Pyrrhotite; py: Pyrite; tr: Troilite; Sphalerite associated with pyrite in both the asp: Arsenopyrite; As: Arsenic, NAg: Atomic frac- sulfide and sulfosalt bands contains 2.1 to 4.2 tion of silver in electrum. I and II indicate mole% FeS as given in Table 14. In the mineralizations of the sulfide and sulfosalt bands, respectively. temperature (T)-sulfur fugacity (fs2) diagram shown in Figure 11, T-fs2 conditions of sphalerite crystallization are given by two associating with electrum and pyrargyrite in solid curves of 2.0 and 4.5 mole% FeS the sulfide band. Additionally T-fs2 curve of of sphalerite in pyrite field (TOULMINand the assemblage arsenopyrite-pyrite-arsenic is BARTON,1964; BARTONand TOULMIN,1966; shown in the figure. Because no arsenic is SCOTTand BARNES,1971). Silver contents of found in the sulfide band, fs2 of pyrite- electrum with pyrite in the sulfide band are arsenopyrite mineralization in the sulfide band 63.0 to 67.4 atomic% (Table 1). T-fs2 curves becomes lower value than that of the curve. of 0.63 and 0.67 in atomic fraction of silver From these data on the T-fs2 curves of mineral (NAg)in electrum assembled with argentite in assemblages described above, possible T-fs2 equilibrium are also shown in Figure 11. condition of formation of the sulfide band is However, because there is found no electrum- shown as a hatching area (I) surrounding with argentite (or acanthite) assemblage in the the curves in Figure 11. Temperature and sulfide band, fs2 of formation of electrum is sulfur fugacity values of the sulfide band for- thought to be lower value than that of NAg mation are estimated as follows: T: 190•‹to 0.63 or 0.67 curves (solid lines) in the figure. 240•Ž and fs.: 10-15.5 to 10-13.5 atm. Also T-fs2 curves*3 of the assemblage elect- On the other hand, T-fs2 condition of rum-miargyrite-pyrargyrite obtained by sulfosalt band formation at late stage can in- calculation from thermochemical data of fer from the data on FeS content of sphalerite CRAIGand BARTON(1973) are given as NAg0.63 and T-fs2 curves of the assemblages of native and 0.67 curves (dotted lines) in Figure 11. silver-argentite (BARTONand TOULMIN,1964) The value fs2 of formation of electrum- and native silver-miargyrite-pyrargyrite. FeS pyrargyrite assemblage in the sulfide band content of sphalerite with pyrite in the becomes higher than that given by the dotted sulfosalt band are same as those in sphalerite curves because there is no miargyrite of the sulfide band. Thus, T-fs2 curves of 2.0 and 4.5 mole% FeS shown in the figure can *2 Although there are two or three sulfide bands in the use to estimate T-fs2 condition of mineraliza- vein, the figure shows a case of two sulfide bands. tion formed the sulfosalt band. Because no *3 SUGAKIand KITAKAZE:in preparation . argentite or acanthite occur in the sulfosalt 570 A. SUGAKI, O. J. KIM and W. J. KIM MINING GEOLOGY:

band, fs2 value of native silver crystallization with 112 Ma of the K-Ar age for alkali- is lower than that of T-fs2 curve of the feldspar from biotite granite of the Mugeug assemblage of native silver-argentite. Also mine (KIM, 1971b). From these data, minerali- there are found the assemblage of native zations formed the ore deposits of both the silver-miargyrite-pyrargyrite in the sulfosalt Mugeug and Geumwang mines are considera- band. The T-fs2 curve of this assemblage is ob- ted to be carried out at middle stage of the tained by calculation using the ther- Cretaceous period. As mentioned above, the mochemical data of CRAIG and BARTON (1973) ores form the Geumwang mine conspicuously as shown in Figure 11. From these T-fs2 are silver rich in comparison with amounts of curves possible T-fs2 range of sulfosalt band gold, and a lot of silver minerals such as formation is indicated as hatching area (II) sur- pyrargyrite, polybasite, miargyrite, diaphorite, rounding with the curves as shown in Figure stephanite, argentian tetrahedrite and native 11. Temperature and sulfur fugacity ranges of silver besides electrum are found in the ore. the formation of the sulfosalt band are It is characteristic that all of silver minerals estimated as T: 130•‹ to 170•Ž and fs2: 10-19.5 except electrum and native silver are sulfosalts to 10-17.0 atm. produced by silver and antimony mineraliza- A principal range of homogenization tions and there are found no silver sulfides, temperature for fluid inclusion in quartz of the argentite or acanthite being most popular sulfide band is also shown in Figure 11 without silver mineral in Korea (KANEDAet al., 1986). correction for pressure. It indicates somewhat Occurrence of such many silver minerals, higher temperature than that estimated from especially. silver-antimony sulfosalts, is singu- the T-fs2 curves of the mineral assemblages. lar as the ore from the Korean type gold- As seen in Figure 11, temperature and sulfur silver deposits, and it is remarkable that many fugacity conditions of the mineralization of them were crystallized by late stage crystallized electrum, silver minerals and base- mineralization under conditions of low tem- metal sulfides in the sulfide band at early or perature and low sulfur fugacity which are middle stages differ obviously with those of thought to be a rare case as the mesothermal the formation of silver-antimony sulfosalt deposit. Quartz which is most principal min- minerals with native silver in the sulfosalt eral of the ore vein occurs commonly as com- band at late stage. Especially, it is noticeable pact and massive form of granular aggregate that the crystallization of silver-antimony as same as it from mesothermal gold vein. sulfosalt minerals at late stage was performed There are usually found no crustified banding under conditions of low temperature and low and druse structures of quartz in the veins as sulfur fugacity which are singular as the often seen in epithermal gold-silver vein. Korean type gold-silver deposits. However, ring ore as described before and parallel growth of. prismatic quartz similar to Conclusion comb-structure are locally observed in high Gold and silver bearing quartz veins of the grade ore. Some prismatic crystals of quartz Geumwang mine occur in Cretaceous granite show framboyant or feather extinctions which which belongs to the Bulgugsa granite series in are commonly observed about chalcedonic Korea. Accordingly, the mineralization form- quartz. Low temperature and low sulfur ed the gold-silver veins of this mine is thought fugacity mineralization at late stage, occur- to be in intimate relation with activity of rence of many silver minerals, especially Cretaceous granite. In fact, K-Ar age for sulfosalts, and appearance of ring ore and sericite from altered country rock (granite) quartz showing flamboyant and feather extinc- fragment of the gold-silver quartz vein of the tions may be suggest that the ore veins of the Mugeug mine neighboring with the Geum- Geumwang mine were partly formed under wang mine was reported as 98 Ma by condition near epithermal state, especially at SHIMAZAKIet al. (1986). Its value is in keeping late stage of the mineralization. 36(6). 1986 Gold and Silver Ores from the Geumwang Mine, Korea 571

Acknowledgment: The authors are greatly in- 21, 234-243. debted to Dr. Arashi Kitakaze for his KIM, O. J. (1971a): Study on the intrusion epoches of assistance on X-ray and microprobe works for younger granites and their bearing to orogenies in silver minerals and Dr. Shoji Kojima for his south Korea. Jour. Inst. Mining Geol., 4, 1-9. (in Korean with English abstract) cooperation on fluid inclusion study. We also KIM, O. J. (1971b): Metallogenic epochs and provinces of wish to thank Dr. No Young Park and Dr. south Korea. Jour. Geol. Soc. Korea, 7, 37•`59. Seon Gyu Choi of :Korea Institute of Energy KIM, S. E. (1982): Gold-silver deposits in Korea. Geology and Resources for their kind information and and mineral resources of Korea. (Prof. 0. J. Kim's valuable discussion about the Geumwang ore memorial volume) 257•`273. (in Korean) deposits. KIM, W. J. (1985): Metallogeny in Korea, especially on A part of the expense for this study was sup- gold-silver, tungsten-molybdenum and placer plied by a Grant-in-Aid from the Ministry 'of deposits in south Korea. Dr. thesis of Tohoku Education, Science and Culture, Japan. University, pp 61-152. KRETSCHMAR, U. and SCOTT, S. D. (1976): Phase relations References involving arsenopyrite in the system Fe-As-S and

their application. Can. Miner., 14, 364•`386. BARTON, P. B. and TOULMIN, P. III (1964): The electrum- LEE, M. S. (1981): Geology and metallic mineralization tarnish method for the determination of the fugacity associated with Mesozoic granitic magmatism in of sulfur in laboratory sulfide system. Geochim, south Korea. Mining Geol., 31, 235•`244. Cosmochim. Acta, 28, 619•`640. PARK, N. Y. (1981): Geology and mineral deposits of BARTON, P. B. and TOULMIN, P. III (1966): Phase relations Korea. Rept. Geol. Surv. Japan., No. 261, 93•`106. involving sphalerite in the Fe-Zn-S system. Econ.

Geol., 61, 815•`849. PARK, N. Y., CHOI, S. G. and PARK, S. W. (1985): Ore

CRAIG, J. R. and BARTON, P. B. (1973): Thermochemical genesis of hydrothermal ore deposits in the Jungwon

approximation for sulfosalts. Econ. Geol., 68, 498 district KIER Research Rept., 85, No. 15, 15, 193-

•` 506. 228 (in Korean with English abstract). GALLAGHER,D. (1963): Mineral resources of Korea. Issued SCOTT, S. D. and BARNES, H. L. (1971): Sphalerite geother- by Mining Branch, Industry and Mining Div. USON/ mometry and geobarometry. Econ. Geol., 66, 653 Korea. In cooperation with Geol. Surv. Republic of •` 669. Korea, 3, 7-19. SHIMAZAKI, H., KANEDA, H. and LEE, M. S. (1984): Geological and Mineral Institute of Korea (1973): Mineralization associated with Mesozoic felsic Geological map of Daejeon (1:250,000). magmatism in Korea. Granite provinces and KANEDA,H., SHIMAZAKI,H. and LEE, M. S. (1984): associated ore deposits in South Korea (Rept. Mineralogy and geochemistry of Au-Ag ore deposits Overseas Field Research) edited by A. TsusuE, 35 in the southern Korea peninsula. Granite provinces •` 60. and associated ore deposit in south Korea (Rept SHIMAZAKI, H., LEE, M. S., TSUSUE A. and KANEDA, H. Overseas Field Research), edited by A. Tsusue, 81- (1986): Three epochs of gold mineralization in South 142. Korea. Mining Geol. 36, 265•`272. KANEDA,H., TAKENOUCHI,S., SHIMAZAKI,H. and LEE M. SUGAKI, A., ISOBE, K. and KITAKAZE, A. (1982): Silver S. (1985): Study on fluid inclusion of ore deposits, minerals from the Sanru mine, Hokkaido, Japan. especially gold-silver deposits in the southern Korean Jour. Japanese. Ass. Miner. Petrol. Econ. Geol., 77, peninsula. Abstract of Joint Meeting of Soc. Mining 65•`77. (In Japanese with English abstract) Geol. Japan, Miner. Soc. Japan and Jap. Ass. TOULMIN, P. III and BARTON, P. B. (1964): A ther- Miner. Pet. Econ. Geol., 132. (In Japanese) modynamic study of pyrite and pyrrhotite. Geochim. KANEDA,H., SHIMAZAKI,H. and LEE, M. S. (1986): Cosmochim. Acta., 28, 641•`671. Mineralogy and geochemistry of Au-Ag ore deposits TSUCHIDA, T. (1944): Ore deposits in Korea, 1•`329, in the southern Korean peninsula. Mineral. Deposita, Kasumigaseki Book Co., Tokyo. (In Japanese) 572 A.SUGAKI,0.J.KIM and W.J.KIM MINING GEOLOGY:

韓国金旺鉱山産金銀鉱石とその鉱化作用

苣木浅彦・金玉準・金元祚

要旨:韓国忠清北道陰城郡金旺面逢谷里に位する金旺鉱生関係,光学的性質,化学組成および粉末X線回折資山の金銀鉱床は白亜紀黒雲母花崗岩(112Ma)中に胚胎料,石英中の流体包有物の均質化温度および塩濃度,産する石英質鉱脈で,銀分に富み,韓国の金銀鉱床として出鉱物の晶出順序,鉱化作用の温度および硫黄フユガシは特異な型に属する.金銀鉱物としてエレクトラム,雑ティーの値などについて記述し,鉱脈生成の環境が必ず銀鉱,濃紅銀鉱,ミアジル銀鉱,ダイアホル鉱,脆銀しも従来の中ないし深熱水の条件に限られず,少なくと鉱,含銀安四面銅鉱および自然銀を産し,主として黄鉄もその一部,とくに鉱化作用の末期は浅熱水型に近い状鉱,硫砒鉄鉱,閃亜鉛鉱および方鉛鉱などの硫化物に伴態が生じていたことを示唆している. われ,石英脈中に数条の縞状を呈する.また ,このほか韓国地名の漢字による表記

上記の銀・アンチモン硫塩鉱物が帯状として石英脈の中 Bonggog-ri逢谷里,Bulgugsa仏国寺,Chung-

央部にみられ,これに自然銀を随伴する.硫化物縞は鉱 cheongbug-do忠清北道,Daejeon大田,Eumseong-gun

化作用の初期ないし中期の産物であり,一方硫塩鉱物縞陰成郡,Geumwang(-myeon)金旺(面),Jincheon鎭川,

は末期の晶出物である.本論文では金銀鉱物の産状 ,共 Mugeug(-ri)無極(里),Sinyang-ri新陽里