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JAMSTEC Report of Research and Development, Volume 1, March 2005, 1–12

1

Ore and gangue minerals of seafloor hydrothermal deposits

in the Mariana Trough

Chihomi Iwaida* and Hirotomo Ueno**

Abstract Chimney and mound samples collected by “Shinkai 6500” from two hydrothermal vent fields in the Mariana Troughwere studied mineralogically and geochemically. One of hydrothermal vent fields is so called Forecast Vent Field at the depthof 1,450 m in the South Mariana Trough, and another deeper field is so called Alice Spring Field at the depth of 3,600 m in theMiddle (or Central) Mariana Trough. Ore and gangue minerals from the both hydrothermal fields are similar, but the quantityof them is different in each field. Main ore minerals in the South Mariana Trough are sphalerite, marcasite, chalcopyrite andtetrahedrite-tennantite, and those in the Middle Mariana Trough are wurtzite, chalcopyrite and tetrahedrite-tennantite. Asgangue minerals, gypsum is abundant and barite is poor in the South Mariana Trough, but, in contrast, anhydrite and barite arerich in the Middle Mariana Trough. Chemical compositions of ore minerals are analyzed by EPMA. Sphalerie contains FeS upto 2.5 mol %.

The trapping temperature of fluid, i.e. formation temperature is obtained after pressure correction for the measured homoge-nization temperature of fluid inclusion. Pressure-corrected trapping temperature of fluid inclusion in wurtzite and anhydrite inthe South Mariana Trough averages 240°C, and that in wurtzite in the Middle Mariana Trough averages 278°C. The measuredmaximum temperatures during this survey are 210°C and 267°C in the South Mariana Trough and Middle Mariana Trough,respectively. The salinity of fluid is determined from melting temperature depression of fluid inclusion.

Sulfur isotopes of sulfide and sulfate minerals are examined, and bulk chemical compositions of chimneys are analyzed. It isnoteworthy that some chimneys contain gold of the order of 10 to 30 ppm.

Keywords: Mariana Trough, chimney, ore mineral, fluid inclusion

1. Introduction The two hydrothermal vent fields have been known in

the Mariana Trough: the Forecast Vent Field(13°23.7'N, 143°55.2'E) of 1,450 m deep in the SouthMariana Trough, and the Alice Spring Field (18°12.9'N,144°42.4'E) of 3,600 m deep in the Middle (or Central)Mariana Trough.

Biological and earth scientific survey was performedby “Shinkai 6500” in the Mariana Trough in 1996(Fig. 1). Chimney and mound samples of Dives 353(Observer; Katsunori FUJIKURA) and 354 (Observer;Takashi OKUTANI) in the South Mariana Trough, andDive 356 (Observer; Kazunori HASEGAWA) in theMiddle Mariana Trough are mainly examined in thisstudy. Tracks of these dives are shown in Figure 2.

Many geochemical aspects on the Alice Spring Fieldof the Middle Mariana Trough have reported (Campbellet al., 1987: Craig et al., 1987: Kaster et al., 1987:Kusakabe et al., 1990: Gamo et al., 1994: Kase et al.,1995). The new hydrothermal vent field in the SouthMariana Trough has discovered in 1992 (Gamo et al.,1993), and hydrothermal minerals have been reportedbriefly (Gamo et al., 1994: Fryer et al., 1993). In this

paper, ore and gangue minerals from both hydrothermalvent fields in the South and Middle Mariana Troughsare examined together in detail. Then, ore and gangueminerals in the South Mariana Trough are firstlydescribed here. Results of fluid inclusion investigationare the first data in the Mariana Trough.

Geology of the Mariana Trough is described in detailand intelligibly by Fryer (1997). The outline and briefresult of the survey in 1996 are reported by Fujikura etal. (1997).

2. Chimney and Their Constituent Minerals2.1 Occurrence and appearance of chimney and

mound samplesThe chimneys used in this study are named such as

“dive number” – “chimney number”. Some samplesfrom the mound around the chimney are also used.Following chimneys were examined in detail; Chimneys353-1, 354-2 and 354-3 in the South Mariana Troughand Chimney 356-5 in the Middle Mariana Trough.Sampling sites of these chimneys are shown in Figure 2and listed in Figure 5.

Chimney 353-1 was collected from an active

* Department of Earth and Environmental Sciences, Faculty of Science, Kagoshima University ** Department of Environmental Security System, Faculty of Risk and Crisis Management, Chiba Institute of Science

Ore and gangue minerals in the Mariana Trough

JAMSTEC Rep. Res. Dev., Volume 1, March 2005, 1–122

hydrothermal vent of the northern slope at the depth of1,468 m (13°23.754'N, 143°55.175'E). The sphaleriteand chalcopyrite rich fragile body is covered by whitegypsum. The collected chimney itself is not so large asshown in Figure 3A. The maximum temperature of avent around sampling site was 210°C.

Chimney 354-2 from the depth of about 1,450 m(13°23.682'N, 143°55.182'E) is rich in marcasite andpyrite. It has a conical shape of 24 cm diameter(Fig. 3B). Chimney 354-3 was collected from the simi-lar place to Chimney 354-2. Sphalerite rich chimney isporous especially inside (Fig. 3C). Around Chimneys

35°N

30°

25°

20°

15°

10°135° 140° 145° 150°E

SOUTH

MIDDLEDive356

GUAMDive353, 354

Figure 1: Mariana Backarc Basin and location of dives by "Shinkai 6500" in 1996.Dives 353 and 354 at the Forecast Vent Field in the South Mariana Trough.Dive 356 at the Alice Spring Field in the Middle Mariana Trough.

C. Iwaida and H. Ueno

JAMSTEC Rep. Res. Dev., Volume 1, March 2005, 1–12 3

(a) Dive353, 354 (b) Dive35613° 24' 00" 13° 13' 30"

13° 13' 00"

13° 12' 30"

144° 43' 00"144° 42' 00"

13° 23' 30"

143° 55' 00" 143° 55' 30"

Figure 2: Tracks of "Shinkai 6500" dives and collected chimney samples.(a) Dives 353 and 354 at the depth of 1,450 m in the South Mariana Trough. (b) Dive 356 at the depth of 3,600 m inthe Middle Mariana Trough.

354-2 and 354-3, the measured temperature is about200°C.

Chimney 356-5 is collected at the depth of 3,602 m(18°12.843'N, 144°42.420'E) in the Middle MarianaTrough. The sampling site is the center of an activevent. The chimney of 57 cm in height is a part of upperrim of a large chimney of 3.5 m in height. Chalcopyrite,sphalerite and barite are visible. As shown in Figure 4,there are many conduits as fluid passes. Around thelarge chimney, a hydrothermal vent organism communi-ty of white sea anemone, hairly gastropous andgalatheid crab were found. The maximum temperaturemeasured here was 267°C.

Since the discovery of the Alice Spring Field in theMiddle Mariana Trough by Alvin in 1987, the hydro-thermal vent field has been surveyed many times men-tioned above. During this survey three dives (Dives 355,356 and 357) were done in the Middle Mariana Trough.Track records and video images of these dives show thatthe active hydrothermal region is extended 50 m wide.In the northern area connecting this active region, thereexists the large area composed of too many big deadchimneys up to 10 m in height and their mounds. It isvery easy to distinguish sulfide deposits from basaltlavas. The exposed area of these sulfide deposits is over200 m south to north and over 130 m west to east. Bythe analogy of Kuroko deposits such as theTsunokakezawa orebody of the Fukazawa Kuroko

deposits (Tanimura et al., 1974), the thickness isassumed as 8 m. Supposing the density of 1.9 g/cm3, theorebody in the Middle Mariana Trough reaches over 0.4million metric tons.

2.2 Ore mineralsMain ore minerals in the Mariana Trough are spha-

lerite (ZnS), wurtzite (ZnS), chalcopyrite (CuFeS2),marcasite (FeS2), pyrite (FeS2), tetrahedrite-tennantite(Cu12Sb4S13–Cu12As4S13) and galena (PbS) with minoramount of enargite (Cu3AsS4).

Sphalerite is a dominant ore mineral together withchalcopyrite. Sphalerite has close relation to chalcopy-rite such as dot-like texture (Figs. 6A, 6G and 6H) anddendritic structure (Figs. 7C and 7D) of chalcopyrite inhost sphalerite. Chemical compositions of sphalerite areanalyzed by a wavelength-dispersive type EPMA usingnatural or synthetic sulfide and pure metal standards,and some of them are shown in Table 1. As seen inFigure 8 (South-B and Middle), FeS contents of spha-lerite without some exception (South-A) range fromnear zero to 2.5 mol %, and similar to those of otherseafloor hydrothermal deposits and also Kurokodeposits (Kitazono and Ueno, 2003). High FeS contentdata of 7 to 10 mol % from anhydrite and gypsum richChimney 354-2 (South-A) are may be resulted fromspecial circumstances.

Wurtzite is small amounts in chimney and mound



samples from the South Mariana Trough, while it occursin plenty from the Middle Mariana Trough. Wurtzite isdistinguishable from sphalerite by the optical propertyexhibiting bireflectance in thin section. Crystals ofwurtzite are often subhedral.

Pyrite takes euhedral or subhedral structure. Dendritictextures of pyrite are recognized in Chimney 354-2(Fig. 6F). The chemical compositions of pyrite are stoi-chiometric (Table 1).

Marcasite is also a common primary mineral in exam-ined chimney and mound samples. Strong unisotropicfeature is characteristic. Marcasite often takes colloformtexture (Figs. 6B and 6C).

Chalcopyrite occurs dominantly in chimney andmound samples. As seen in Figure 6D, it takes unhedraltexture. Chalcopyrite shows often dendritic texturesresulted from the rapid cooling at the seafloor (Figs. 7C,7D and 7F). Chemical compositions of chalcopyrite areshown in Table 1. Some of them have 0.5 atom % ofZn.

Tetrahedrite-tnnantite is found in every sample, andassociated with sphalerite and/or galena (Figs. 7E and7G). Its chemical compositions of limited numbers fallin tetrahedrite side, Sb rich. Ag contents are 3 to 7 wt %.

Other ore minerals are galena and enargite. Chemicalcompositions of galena are listed in Table 1. One of

A

C D

B

Figure 3: Chimney samples from the Mariana Trough.A) Chimney 353-1, B) Chimney 354-2, and C) Chimney 354-3 from the South MarianaTrough. D) Chimney 356-5 from the Middle Mariana Trough.Each stripe of black or white is 1 cm.



them shows slightly high Ag contents. Enargite is foundonly in Chimney 354-3. A few of pinkish brown subhe-dral grains (Ramdohr, 1980) appear with chalcopyriteand sphalerite in anhydrite (Fig. 7A). The chemicalcomposition by EPMA is concordant to that of standardone.

2.3 Gangue mineralsGangue minerals are three minerals belong sulfate

class; anhydrite (CaSO4), gypsum (CaSO4·2H2O) andbarite (BaSO4). Anhydrite and gypsum are common asconstituent minerals. Anhydrite and gypsum occur aselongated rectangle and fibrous shapes, respectively(Fig. 6E). Barite which appears in all samples is a maingangue mineral. Characteristic rectangle shapesobserved commonly (Fig. 7E). Although the collectedvent fluid has very high silica concentration, no crystal-lized silica mineral is recognized. However, the existingof amorphous silica mineral is suggested because of theX-ray broadening peak of 4 Å.

2.4 Mineralogy and geochemistryOre and gangue minerals except for enargite are con-

firmed by X-ray methods using Norelco-type diffrac-tometer or Gandolfi camera.

UPPER

CONDUIT

CONDUIT Coating Tennantite– Tetrahedrite Cavity

Coating Barite Cavity

Coating Barite Cavity

Coating Chalcopyrite Cavity

0 10 (cm)

FINE–VESICULAR ZONE

Figure 4: Sketch of a cut plane of Chimney 356-5.

gyps

um

anhy

drite

barit

e

chal

copy

rite

pyrit

e

mar

casi

te

spha

lerit

e

wur

tzite

tenn

antit

e-te

trah

edrit

e

gale

na

enar

gite

353–1354–2

354–3356–5

0

10

20

30

40

50

60

356–5

354–3354–2353–1

MIDDLE

MIDDLE

SOUTH

SOUTH

Figure 5: Ore and gangue minerals and their quantity from the Mariana Trough.



A

C

E

G

B

D

F

H

0 40

0 40

0 80 0 40

0 50 0 100

0 40

0 100

sp

sp

sp

spsp

sp

sp

mc

mc

mc

mc

py

py

py

cp

cp

cp

cp

cpcp

cp

cp

cp

mc

mc

mc

mc

mc

mc

anh

mc

Figure 6: Photomicrographs in reflected light of chimneys from the South Mariana Trough.[Chimney 353-1] A) Chalcopyrite in sphlerite. B) Collform marcasite. C) Chalocopyrite and marcasite in sphalerite. D)

Marcasite and unhedral chalcopyrite. [Chimney 354-2] E) Collform marcasite and anhydrite. F) Dendritic pyrite.[Chimney 354-3] G) Chalcopyrite, marcasite and Sphalerite. H) chalcopyrite and sphalerite. sp: sphalerite cp: chalcopyrite mc: marcasite py: pyrite anh: anhydrite



A

C

E

G

B

D

F

H

0 200

0 200

0 50 0 40

0 400 50

0 40

0 50

mc

mc

mc

mc

en

gn

gn

sp

sp

sp

sp

sp

sp

sp

sp

cp gn

cp

sp

gn

sp

sp

cp

cp

gn

gn

py

py

brt

ten–tet

ten–tet

ten–tet

sp

cp

cp

cp

cp

Figure 7: Photomicrographs in reflected light of chimneys from the South Mariana Trough (A, B) and Middle Mariana Trough(C–H).[Chimney 354-3] A) Enargite. B) Collform marcasite. [Chimney 356-5] C) & D) Dendritic chalcopyrite with sphalerite.

E) Tetrahedrite-tennanatite, shalerite, galena and pyrite with barite. F) Chalcopyrite and sphalerite. G) Sphalerite, tetra-hedrite-tennantite and galena. H) Euhedral galena. en: enargite mc: marcasite sp: sphalerite cp: chalcopyrite gn: galena ten-tet: tetrahedrite-tennantite py: pyrite brt: barite



Sphalerite

Wt%ZnFeMnCuCdSTotalatm%ZnFeMnCuCdSTotal

FeSmol%

61.084.260.120.000.02

34.67100.15

44.613.640.100.000.01

51.63100.00

7.53

SOUTHAa2–1

60.315.820.120.160.04

33.3399.78

44.545.030.110.120.02

50.19100.00

10.10

Ca2–2

64.390.290.020.890.17

34.1499.9

47.560.250.020.680.07

51.42100.00

0.52

MIDDLEHa5–1

65.8 0.70 0.05 0.16 0.50

32.92 100.13

49.01

0.61 0.04 0.12 0.22

50.00 100.00

1.22

Ca3–8

64.95 1.26 0.01 0.82 0.42

32.59 100.05

48.48

1.10 0.01 0.63 0.18

49.60 100.00

2.18

Aa5–6

64.42 1.30 0.05 0.83 0.38

33.24 100.22

47.77

1.13 0.04 0.63 0.16

50.26 100.00

2.27

Aa5–8

66.430.520.040.110.24

32.6199.95

49.640.450.040.080.10

49.68100.00

0.90

Ja5–1Sample No.

Chalcopyrite

Wt%Cu ZnFeMnCdSTotalatm%Cu ZnFeMnCdSTotal

33.520.81

29.690.000.00

35.9299.94

24.070.57

24.260.000.00

51.11100.00

MIDDLEAa4–1

33.940.76

30.230.040.02

34.9999.98

24.510.53

24.840.030.01

50.07100.00

Aa4–5

34.270.00

30.790.000.00

34.9299.98

24.740.00

25.300.000.00

49.96100.00

Aa5–1

34.11 0.00

30.50 0.00 0.03

35.43 100.07

24.53

0.00 24.96

0.00 0.01

50.49 100.00

Aa5–3Sample No.

Pyrite

Wt%FeCuZnCdSNiCoTotalatm%FeCuZnCdNiCoSTotal

46.190.220.000.00

53.100.000.05

99.56

33.250.140.000.000.000.03

66.57100.00

SOUTHAa2–2

46.710.050.000.00

53.430.000.04

100.23

33.400.030.000.000.000.03

66.54100.00

Ba3–2

46.231.170.350.01

52.440.000.06

100.26

33.270.740.220.000.000.04

65.73100.00

MIDDLEAa5–2Sample No.

Marcasite

Wt% Fe Cu Zn Cd S Ni Co Total atm% Fe Cu Zn Cd S Ni Co Total

47.030.010.080.00

52.820.010.00

99.95

33.810.010.050.00

66.130.010.00

100.00

SOUTHCa2–5Sample No.

Galena

Wt% Pb Cu Ag Sb S Total atm% Pb Cu Ag Sb S Total

86.35

0.09 0.32 0.25

13.07 100.08

50.17

0.17 0.36 0.25

49.06 100.00

MIDDLEHa5–1

86.040.140.000.00

13.5399.71

49.470.260.000.00

50.26100.00

Aa4–2

85.930.060.060.08

14.10100.23

48.420.110.060.08

51.33100.00

Ja5–1Sample No.

Table 1: Typical chemical compositions of ore minerals from the Mariana Trough



The quantity of ore and gangue minerals is indicatedin Figure 5. Sphalerite is more dominant than wurtzitein the South Mariana Trough, whereas wurtzite is dis-tinctly rich in the Middle Mariana Trough. As iron sul-fide, marcasite is fairly abundant in the South MarianaTrough, but it is rare in the Middle Mariana Trough.The quantity of gypsum is larger than that of anhydritein the South Mariana Trough. In the Middle MarianaTrough, only anhydrite is recognized and no gypsum isfound. Barite is poor in the South Mariana Trough,whereas it is rich in the Middle Mariana Trough.

The order of crystallization of the chimney is compli-cate, but following events can be pointed out. The mar-casite and pyrite have two stages. Sphalerite and chal-copyrite follow the first stage marcasite or pyrite. Baritealso mineralizes almost similar time. Anhydrite isslightly late.

The bulk chemical composition of chimneys isimportant by stand points of genesis of ore deposits andocean mining (Scott, 2001). We are trying to get manyreliable bulk chemical compositions by the atomic

Fre

eque

ncy

SOUTH–A

SOUTH–BMIDDLE

FeSmol%

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10

Figure 8: FeS contents of sphalerite from the MarianaTrough.Details in the text.

(a) SOUTH

(b) MIDDLE

n = 48

Anhydrite

sphalerite

wurtzite

Fre

eque

ncy

Fre

eque

ncy

wurtzite

n = 42

T (°C)

T (°C)

5

10

15

100 200 300

5

10

15

100 200 300100 200 300

Figure 9: Histograms of the pressure-corrected trapping temperature of fluid.(a) South Mariana Trough (b) Middle Mariana Trough



absorption method. In this step, a few data are obtained.Chimneys have high gold contents of 10 to 30 ppm andhigh silver ones. Details of the bulk chemical composi-tion will appear soon in another paper.

3. Fluid InclusionFluid inclusion in wurtzite and anhydrite was exam-

ined. The Roedder's (1984) criteria to distinguishbetween primary and secondary inclusions are principal-ly used. All primary inclusions in this study are com-posed of two phases, liquid and gas. Fluid inclusions inwurtzite are generally small as several µm or less indiameter, and not much abundant. Each inclusion iselongated ellipsoidal. Those in anhydrite are also smalland irregular in shape.

The analytical procedure of fluid inclusion for heatingis as follows. Homogenization temperatures were meas-ured using Linkham TH-600 heating stage with a preci-sion of ±2°C. The measured homogenization tempera-ture must be corrected depend on pressure to obtain atrue trapping temperature of fluid. These correction val-ues are calculated using the data of Potter (1977). At thedepth of 1,450 m (14.5 MPa), correction values at thehomogenization temperature range of 220°C to 270°Care about 12°C. At the depth of 3,600 m (36.0 MPa),those of 230°C, 280°C and 330°C are 29°C, 32°C and38°C. The pressure-corrected trapping temperatureswere plotted on histograms (Fig. 9). The trapping tem-peratures of the South Mariana Trough range widelyfrom 180°C to 340°C with the mean of 240°C (Fig. 9a),and those of the Middle Mariana Trough range from220°C to 360°C with the mean of 277°C.

These temperatures and pressures are clearly belowthe boiling curve (Bischoff and Rosenbauer, 1984:Bischoff and Pitzer, 1985), and it is said that the boilingof fluid has not occurred at both hydrothermal ventfields in the Mariana Trough. This is supported byobservation of the constant gas-liquid ratio in fluidinclusion.

The salinity by the final melting point depression offluid inclusion was examined. The final melting point ismeasured by the cooling stage with a precision ±0.1°Ccorrespond to 0.1 wt% NaCl equivalent. The salinityobtained from fluid inclusion in anhydrite fromChimney 364-2 in the South Mariana Trough is 2.5 and2.8 wt% NaCl equivalent derived from –1.5 and –1.7°Cdepression, respectively. The salinity from fluid inclu-sion in wurtzite from Chimney 365-5 in the MiddleMariana Trough is 3.5 wt% NaCl equivalent derivedfrom –2.1°C depression.

4. Sulfur IsotopeSulfur isotope values of sulfide and sulfate minerals

were measured by a conventional technique. Sulfide andsulfate chips to examine sulfur isotope were separatedby hand picking using a binocular microscope. 32S/34Sratios are expressed relative to the Canon Diablo mete-orite, and are given δ34S‰. The δ34S values obtainedfrom the Mariana Trough are shown in Table 2. Theδ34S values of pyrite are +7.3 and +7.7‰. The δ34S val-ues of sphalerite are +2.7 and +3.3‰, and identical withthe reported data of sphalerite and galena (average+2.7‰) from the Middle Mariana Trough (Kusakabe etal., 1990). The value of chalcopyrite is +3.0‰. Presentsulfide δ34S values from the Mariana Trough are smallerthan those of the North Knoll, Iheya Ridge of theOkinawa Trough (Ueno et al., 2003). The δ34S values ofbarite as a sulfate mineral from the South MarianaTrough are +19.8 and +20.9‰, and are similar to previ-ous data of barite from the Middle Mariana Trough(Kusakabe et al., 1990) and those of other seafloorhydrothermal deposits (Scott, 1997).

5. Discussion and ConclusionChimney and mound samples from two active

hydrothermal vent fields in the Mariana Trough wereexamined mineralogically and geochemically. The twohydrothermal vent fields are situated at the differentdepths; 1,450 m in the South Mariana Trough, and3,600 m in the Middle Mariana Trough. All chimneyand mound samples occurring from both hydrothermalvent fields are sulfide rich, gray to black in color andporous. Main ore minerals are sphalerite, wurtzite, chal-copyrite, marcasite, pyrite, tetrahedrite-tennantitite andgalena with minor enargite. Gangue minerals are anhy-drite, gypsum and barite. No silica mineral is found, butthe existence of amorphous silica is assumed by the 4 Åbroadening peak of X-ray pattern. The large abundanceof anhydrite compared to gypsum and of wurtzite com-pared to sphalerite occurs in the Middle Mariana

Table 2: Sulfur isotope of sulfide and sulfate minerals fromthe Mariana Trough

Sample No. Mineral δ34S (‰)

Sulfide 353-01 Py1 353-01 Py2 356-05 Sp1 356-05 Sp2 356-05 CpSulfate 354-02 Ba1 354-02 Ba2

PyritePyriteSphaleriteSphaleriteChalcopyrite

BariteBarite

+ 7.7+ 7.3+ 3.3+ 2.7+ 3.0

+19.8+20.8



Trough, whereas gypsum and sphalerite are rich in theSouth Mariana Trough. This phenomenon may beexplained by the retrogressive change of minerals aftercrystallization.

The pressure-corrected trapping temperature of fluidobtained from homogenization temperature of fluidinclusion in the South Mariana Trough averages 240°C.The measured maximum temperature of vent fluid in theSouth Mariana Trough is 210°C. The pressure-correctedtrapping temperature in the Middle Mariana Troughaverages 278°C. In the Middle Mariana Trough, themeasured maximum temperature during this survey is267°C, and its records of 1987 and 1992 are 287°C and280°C, respectively. Both hydrothermal vent fields wereobserved as active ones. However, the hydrothermalactivity in the South Mariana Trough is seemed to beslightly declined because the measured maximum tem-perature of present fluid (210°C) is lower than the trap-ping temperature (240°C). The salinity derived from thefinal melting point depression of fluid inclusion rangesfrom 2.5 to 3.5 wt% NaCl equivalent.

FeS contents of sphalerite are low, and similar toother seafloor hydrothermal deposits and Kurokodeposits. The low FeS content whose contour lines fallin the pyrite stable region on the f-T diagram implies thecircumstances of the high sulfur fugacity in thesedeposits (Barton and Toulmin, 1966).

Recently, high gold seafloor hydrothermal deposits inthe Izu-Bonin Arc have been reported (Watanabe andKajimura, 1994: Iizasa et al., 1999). It is noteworthythat the existence of high gold and silver ore of 10 to 30ppm Au is preliminarily approved in the MarianaTrough which is south extension of the Izu-Bornin Arc.

AcknowledgementsThanks are due to Dr. Katsunori Fujikura of JAM-

STEC, Chief Scientist of the Mariana Leg in 1996. Wealso thank Observers; Drs. Takashi Okutani of NihonUniversity and Kazunori Hasegawa of National ScienceMuseum. We are indebted to Commander Masahiko Idaand operation team of “Shinkai 6500”, and CaptainFusao Saito and his crew of R/V Yokosuka.

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(Received December 24, 2004)

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