Petrographic investigations of seafloor sediments from the Kita-Bayonnaise submarine caldera, Shichito-Iwojima Ridge, Izu-Ogasawara Arc, northwestern Pacific

Marine Geology, 112 (1993) 271-290 Elsevier Science Publishers B.V., Amsterdam

271

Petrographic investigations of seafloor sediments from the Kita-Bayonnaise submarine caldera, Shichito-Iwojima Ridge, Izu-Ogasawara Arc, northwestern Pacific Kokichi Iizasa Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, Ibaraki 305, Japan (Received June 29, 1992; revision accepted December 15, 1992)

ABSTRACT Iizasa, K., 1993. Petrographic investigations of seafloor sediments from the Kita-Bayonnaise submarine caldera, Shichito-Iwojima Ridge, Izu-Ogasawara Arc, northwestern Pacific. Mar. Geol., 112: 271-290. Seafloor sediment samples were recovered by box and gravity corers from the Kita-Bayonnaise submarine caldera, Shichito-Iwojima Ridge, Izu-Ogasawara Arc, northwestern Pacific. The samples were separated into heavy, light, and claysized fractions for assessment of the hydrothermal contribution to the sediments of the submarine caldera, and have been determined by microscopy, SEM-EDX, and XRD. Heavy fractions are composed of predominant magnetic minerals, dominant silicates and common sulfides and sulfate, minor sulfosalt, rutile, phosphates, carbonates and Mn-oxyhydroxides. Primary hydrothermal phases are composed of barite, pyrite, sphalerite, chalcopyrite, galena, fahlore, marcasite and futile. Secondary mineral associations are usually present around galena and chalcopyrite grains. The assemblages of barite-pellet type pyrite-chalcopyrite-sphalerite and sphalerite-chalcopyrite-pyrite with galena or fahlore suggest that hydrothermal mineralization took place in the caldera. The assemblages of epidote-carbonate or chlorite in heavy transparent fractions and of sericite-quartz-feldspar in light fractions are indicative of hydrothermal alteration origin because of only dacite sampled from the caldera. Some of the smectite, chlorite, sericite and kutnahorite in clay-sized fractions could also support the existence of hydrothermal activity. Other minerals consisting of hypersthene, augite, hornblende, quartz, feldspar and biotite are observed in heavy and light fractions as detritus of dacite in the caldera. Significant amounts of sulfides and barite mixtures (ranging from about 1.0 to 3.5 wt.% in treated fractions, 0.063-0.25 mm) in gravity core samples suggest that hydrothermal mineralization took place several times in the caldera. Many lines of evidence indicate the presence of hydrothermal mineralizations associated with Ag and Th in the Kita-Bayonnaise submarine caldera. The mineralogical analyses are powerful for exploring signs of past or present hydrothermal mineralization in submarine calderas.

Introduction Sediments related to submarine h y d r o t h e r m a l activities on volcanic island arcs are often enriched in metals precipitated as mainly a m o r p h o u s Fe hydroxides scavenging m i n o r elements, goethite and M n oxides (Ferguson and Lambert, 1972; E x o n and C r o n a n , 1983; Smith and C r o n a n , 1983; Varnavas and C r o n a n , 1991). The K i t a - B a y o n n a i s e submarine caldera is located 400 k m south o f T o k y o , Japan, on the S h i c h i t o - I w o j i m a Ridge (a volcanic front), 0025-3227/93/$06.00

Izu-Ogasawara Arc, northwestern Pacific ( M u r a k a m i and Ishihara, 1985). As the caldera floor is enclosed by walls, the distribution pattern and a b u n d a n c e o f each mineral can reflect its source rocks. In particular the analysis o f heavy minerals is often used for establishing their provenance (e.g. Rittenhouse, 1943; Imbrie and Van Andel, 1964; Valentine and C o m m e a u , 1990). H e a v y minerals in sandy sediments empirically tend to concentrate finer-grained sand fractions [#60 mesh (0.25 mm) to #230 mesh (0.063 mm)]. In this study the 6 0 - 2 3 0 mesh subsamples were

© 1993 - - Elsevier Science Publishers B.V. All rights reserved.

272

K. IIZASA

tentatively taken for heavy mineral analysis, because seafloor sediments sampled from the Kita-Bayonnaise submarine caldera are mainly composed of sand-sized grains which are convenient to identify heavy mineral species. This study demonstrates the usefulness of heavy and light mineral analyses including identification of clay-sized fraction in caldera sediments concerning the assessment of hydrothermal mineralization in both active and fossil hydrothermal fields in a submarine caldera and discusses the possibility of modern hydrothermal activity in the KitaBayonnaise submarine caldera.

Geologiealsetting The Kita-Bayonnaise submarine caldera lies in the northern part of the Shichito-Iwojima Ridge, Izu-Ogasawara Arc which is separated by the Sofugan Tectonic Line (Yuasa, 1985). It is one of eight Quaternary submarine calderas in the northern part of the arc (Yuasa et al., 1991; Fig. 1). The arc extending from north to south on the east edge of the Phillipine sea plate, is cornposed of two major ridges, from west to east, the Nishi-shichito and the Shichito-Iwojima Ridges. Several back-arc rifts are present in between the

Tokyo

,

14 ~

J

\

ql,.~

.;

Pacific

~I"

~'~ • Kita-Bayonnaise

Shikoku

,,.,

~

.--..

¢ /I ev---rI I \ ~"'x~ ,~/ ,Central ConeJ / ? ~ X MyojinKnoll ~

_l~

4~

,"~

,'-~x

l

~

-

) ~

..¢~

it

I

06"

3Km

139°50"E

53"

(RC),

Fig. 1. Topography and sample location in the Kita-Bayonnaise submarine caldera: grabs (G), gravity cores and dredge (D) samples. Abbreviations of inset map, h, s and t indicate back-arc riffs, the Hachijo, Sumisu and Torishima, respectively, and is the Sofugan Tectonic Line. The bathymetric map is modified from E. Saito and F. Murakami (pers. commun., 1990). Depth contours in meters.

S.T.L.

273

PETROGRAPHY OF SEDIMENTS FROM THE IZU-OGASAWARA ARC

ridges (Karig and Moore, 1975; Tamaki et al., 1981). The caldera has a central cone and a summit named Myojin Knoll with the shallowest depth of 364 m (Yuasa et al., 1991). Its floor is 3 x 4 km wide and is located at a maximum water depth of 1405 m. The caldera rim is nearly circular and about 600-700 m deep. The wall of the caldera consists of pumice layers in the upper part and of dacitic lava flows in the lower part (M. Yuasa, pers. commun., 1991). The caldera floor is mainly deposited by ash and pumice ranging from granule to cobble with lesser amounts of foraminiferans and radiolarians (A. Nishimura, pers. commun., 1988). Mn-coated dacitic rocks dredged from the central cone consist of feldspar, clino- and orthopyroxenes, hornblende and quartz. A large amount of dacitic pumice was also dredged from the eastern wall of the caldera,

Samples and methods Sediment samples were collected from the caldera floor during the 1986-1989 R/V Hakureimaru cruises (Fig. 1). The top 15 cm sediments of the four box cores (G) are brownish silty sand to cobble-sized pumice fragments and volcanic glass associated with lesser amounts of foraminiferans and radiolarians, The three gravity cores (RC) up to 1 m length have generally the same components as the box cores (Fig. 2). The color of the sediments from RC575 is dark reddish-brown at the surface for the sandy layer and is light to olive gray for the sandy to silty layer toward the downcore. The color of the sediments from RC613 is dark brown for the surface sandy layer and turns from white to light brown in the middle part to brownish moss green in the lower part. The sediment is intercalated by eight Mn-disseminated layers with varing thickness in the upper and middle parts, Sediments from RC615 are of brownish white tint in the upper cobble-rich layer and light brown in the lower pebble-rich granule layer, Sediments were dried until constant weight at a room temperature and 10-230 g were taken for mineral analysis. Clay-sized fractions were prepared as oriented mounts on glass for determina-

tion by X-ray diffractometry (XRD) by hydraulic elutriation after rinsing several times with deionized water. Clay-sized fractions were prepared approximately 40 mg at dry weight which is convenient to obtain relative abundance from X-ray peak intensities (Sudo et al., 1961). The rinsed sediments were dried at ll0°C and disaggregated, followed by sieving with different mesh sizes, i.e. #18 (1 mm), #60 (0.25 mm) and #230 (0.063 mm). The separated samples were weighed. The finer sand fractions (0.25-0.063 mm) were subsequently separated by tetrabromoethane (sp. gr., about 2.96) for heavy mineral analysis. Magnetic minerals were removed from heavy mineral fractions by a bar magnet and weighed. Other heavy residues were separated using an isodynamic separator into transparent and opaque minerals, and then weighed. Since barite, phosphates and rutile are almost included in the opaque fractions using the isodynamic separation, barite, phosphates and rutile are dealt with the components of opaque fractions. All opaque and several magnetic minerals were mounted in polysynthetic resin for polishing, followed by microscopic identification. Modal analyses were done by counting with grids on the polished sections of each sample under an ore microscope. Representative polished sections were semiquantitatively analyzed by scanning electron microscope (SEM) JEOL JSM-6400 equipped with an energy dispersive spectrometer (EDS). Thin sections of all transparent heavy and several light fractions were prepared for petrographic observation.

Results

Mineralogy Heavy fractions The SEM examination shows that opaque and translucent minerals in heavy residues consist of barite in tabular and foliated forms, euhedral pyrite, chalcopyrite and sphalerite, and some chalcopyrite and sphalerite with pitted surfaces (Fig. 3). Many grains have several assemblages of sulfides and sulfate. They display well-preservedmorphologies except for chemically weathered chalcopyrite and sphalerite, implying less transport.

274

K. IIZASA RC575

RC613

RC615

cm 0 15

10

1

14

2

13

3

1

4

2 20

12

5 6

11 30

7 8

3

9

c2 40

10 10 11 12 13

5 0 - - 9

14

is

cl

r .........................

Legend

16 17 l8

60

silt

| ]~19

very fine to fine sand

70

medium sand 6

coarse to very coarse sand 20

very coarse sand to granule

80

granule to pebble 5 4

3

9

0

-

cobble 21

2

manganese-bearing ..........

gradual boundary

~

grading

22 1 23 Fig. 2. Schematic description o f sediment columns sampled from the Kita-Bayonnaise submarine caldera floor. Small numbers indicate order of samples used for mineral analyses and are the same number as used hereafter.

According to microscopic examinations on polished sections the grains consist of sphalerite, galena, chalcopyrite, pyrite, marcasite, fahlore, barite, bornite, digenite, covellite, pyromorphite, cerussite, apatite, rutile, magnetite and ilmenite, which are detailed as follows, Barite occurs as tabular- and lath-shaped single

grains (20-400 lam) in most cases. Two modes of occurrence are present as relatively large tabular and lath, and fibrous aggregates (Fig. 4A and B). The former is accompanied with fine grains of pellet-type pyrite, chalcopyrite, sphalerite and galena and the latter is found with fine chalcopyrite.

PETROGRAPHYOF SEDIMENTSFROMTHE IZU-OGASAWARAARC

275

Fig. 3. SEM photomicrographs of heavies showing (A) barite rose (G3597), (B) foliated aggregate of barite (G3341), (C) sphalerite aggregate (RC575-6), (D) chalcopyrite aggregate (RC613-17), (E) aggregate of cubic pyrite (RC613-17), and (F) chalcopyrite aggregate subject to chemical weathering (RC575-6).

Pyrite is usually present as a single euhedral grain (5-800 ~tm). In some cases euhedral pyrite is associated with sphalerite, chalcopyrite and idiomorphic quartz encrusting cavities of lithic fragments (Fig. 4C). Subhedral pyrite is next in

abundance. Veinlets and fine scattered grains of pyrite occur in a lithic fragment (Fig. 4D). Pellettype and euhedral pyrite (several micrometers to 150 lain) sometimes occurs in aggregates of tabular and lath shape barite. Pyrite is often associated

276

K. IIZASA

Fig. 4. Back-scattered electron images of heavy minerals on polished sections showing (A) tabular- and lath-shaped barite (white;

ba) and pellet-type pyrite (light grey; py) (RC575-6), (B) fibrous and some lath barite (RC575-6), (C) pyrite, sphalerite (sp) and euhedral quartz (dark grey; q) crystals encrusting cavities (RC575-6), (D) scattered pyrite in silicates (RC575-6), (E) assemblage of sphalerite, fahlore (fa) and chalcopyrite (cp) (RC575-6), (F) assemblage of galena (gn), sphalerite and pyrite as primary, and

PETROGRAPHYOF SEDIMENTSFROMTHE IZU-OGASAWARAARC

277

pyromorphite (pyrn), digenite (dg) and covellite (cv) as secondary (RC575-6), (G) pyromorphite, Ag-bearing-digenite and -covellite around galena grain (RC613-18), (H) Ag- and Th-bearing coveUite (cvl), Ag-bearing covellite (cv2), and rhythmic bands of cerussite (cs) and pyromorphite around a galena grain (RC613-5), (I) colloform covellite (RC575-6), (J) barite and chalcopyrite intimately associated with many prismatic rutile (ru) crystals, and (K) acicular and prismatic rutile crystals associated with pyrite and quartz (q), (L) photomicrograph in reflected light showing assemblage of chalcopyrite, sphalerite and pyrite as primary, and bornite (bo) digenite and covellite as secondary (RC575-6).

278

either with sphalerite, chalcopyrite, fahlore or galena. Rims of pyrite grains are occasionally altered to Fe-rich material with lesser amounts of Si, Ca, Pb, AI, K, P and C1 by EDS analyses, probably Fe-oxyhydroxides. Sphalerite is usually present as a single grain (20-300 Ixm) in euhedral to anhedral forms and is associated with fahlore, chalcopyrite and pyrite, and rarely with galena (Fig. 4E and F). Many pitted holes are sometimes observed in sphalerite grains. According to EDS, sphalerite consists of almost pure end member with transparency to Cdand Fe-bearing varieties with brownish gray tint. Sb- and Cu-bearing sphalerite tend to occur with fahlore and chalcopyrite. These trace elements suggest the presence of submicroscopic inclusions of chalcopyrite and/or fahlore in sphalerite grains although no inclusions were observed under the ore microscope in this case. Chalcopyrite occurs as euhedral to anhedral forms and often displays many pitted holes on the crystal surface. Chalcopyrite occurs as a single grain (5-500 ktm) and is accompanied by pyrite, sphalerite, galena and fahlore. Chalcopyrite grains are partly altered to bornite, covellite and digenite. Some chalcopyrites observed in sphalerite display dust-like appearances which delineate original crystal forms probably replaced by sphalerite. Minute chalcopyrite (< 10 txm) occurs in aggregates of fibrous barite. No trace elements were detected in chalcopyrite by EDS analysis, Fahlore is the mineral similar to tennantite under the ore microscope. This mineral occurs as euhedral to anhedral forms (10-200 ~tm) and is usually associated with sphalerite and chalcopyrite or pyrite (Fig. 4E). The EDS analysis reveals that fahlore usually includes varying amounts of Sb, Fe and Zn other than Cu, As and S. Sb is not detected in some grains, implying an end member close to tennantite. Ag was not detected in all grains studied. Galena usually occurs as a single grain and associated with sphalerite, chalcopyrite, pyrite and fahlore and rarely with barite (Fig. 4F). Most galena now turns to anhedral grains (5-200 ~ t m ) which remain traces of original euhedral forms subject to remarkable alteration to the association of pyromorphite and cerussite and Ag-bearing-

K. IIZASA

digenite and Ag- and Th-bearing covellite (Fig. 4G and H). No inclusions in galena were found under the microscope and no trace elements were detected by EDS. Pyromorphite and cerussite are present as rhythmic bands, intermingled phases, and irregular patches encrusting galena (Fig. 4F-H). Ca is included in pyromorphite according to EDS. Digenite and covellite commonly occur with pyromorphite and cerussite around galena (Fig. 4F-H). These minerals are usually in anhedral form. Covellite is sometimes observed in colloform texture (Fig. 41). Digenite and covellite in contact with galena concentrate Ag and/or Th according to EDS, while those with chalcopyrite contain no Ag and Th. These results suggest the presence of Ag and Th in galena although no Agand Th-bearing minerals are included in galena following ore-microscopic and EDS examinations. Rutile usually occurs as aggregates (40-100 Ixm) of acicular, prismatic and tetragonal crystals (around 5-50 Ixm). The mineral is sometimes associated with barite-chalcopyrite, pyrite, marcasite, sphalerite or quartz (Fig. 4J and K). Bornite is present as a secondary mineral altered from chalcopyrite and is associated with digenite, covellite, sphalerite and pyrite (Fig. 4L). Marcasite occurs in euhedral to subhedral shape associated with pyrite and chalcopyrite or as single grain (40-100 ~tm). Radial aggregates of lath to spindle form marcasite are present in small amounts. Hydrothermal Mn-oxyhydroxides sometimes coat pumice fragments and contain major Mn and minor Mg, Ca and K were detected by EDS analyses. Apatite (50-200 ~tm) occurs as an euhedral prismatic form with volcanic glass. Trace elements Sr, A1 and S were detected in some apatite by EDS analyses. Magnetic minerals consist of major magnetite (40-120 ~tm) and minor ilmenite (50-100 ~tm) in euhedral form. Magnetite often includes a few blebs of pyrrhotite. Most magnetite grains include varying amounts of Ti, Mn and A1 according to EDS analyses. Ilmenite contains lesser amounts of Mn and Mg following EDS analyses. Heavy transparent minerals are composed of

PETROGRAPHY OF SEDIMENTSFROM THE IZU-OGASAWARAARC

major hypersthene, common augite and minor hornblende with trace amounts of epidote and carbonate. Each grain of hypersthene, augite and hornblende commonly occurs as a single fresh, prismatic forms, implying less transport. These silicates are considered to be derived from dacitic rocks composing the submarine caldera. Epidote occurs as radiations of prismatic forms and is present in a single angular to subangular grain and associations with chlorite, carbonate, quartz and feldspar (Fig. 5A). Carbonate usually appears as a single grain and sometimes in association with epidote and opaque minerals.

Light fractions Light minerals in selected thin sections are composed of large amounts of feldspar and quartz, common volcanic glass, small amounts of sericite, chlorite and biotite. Some grains consist of sericite-quartz-feldspar assemblages implying derivatives from hydrothermal alteration halos because the absence of sericite in the dacitic rocks from the caldera (Fig. 5B). Quartz and feldspar in this fraction commonly occur as a single grain in angular to subangular shape implying less transport, and most of them seem to be of dacitic origin in the caldera,

Clay-sizedfractions The clay-sized fraction consists mainly of predominant calcite, dominant sericite, chlorite and

279

gypsum, and minor smectite, kutnahorite, quartz and feldspar as listed in Table 1. Varying amounts of gypsum occurs in most of the samples. The distribution and abundance of gypsum appear to have no relation to those of sulfides, barite and other heavy minerals. However, gypsum in this case is probably an artifact which forms during sample preparation, such as oxidation of sulfides. The X R D reflections of smectite expand from about 15 to 17/~ after ethylene glycol treatment. The smectite is dominant in a subsample of core RC575-6 which includes significant amounts of sulfides and barite. Minor amount of smectite occurs also sporadically in other sample splits in cores RC575, RC613 and RC615, and surface sediments. The sporadic distributions of smectite seem to be in accordance with those of relatively dominant sulfides and barite. Since the chlorite X-ray reflection (001) of this study usually overlaps with smectite, the intensities of chlorite reflections (001, 002 and 003) were measured after ethylene glycolation. Chlorite falls into Mg and Fe varieties based on the triangular diagram of basal reflection intensity ratios presented by Oinuma et al. (1972). Mg-Fe chlorite is almost ubiquitously distributed in all the samples. Kutnahorite occurs closely with Mn-bearing layers of upper and middle sequences in RC613. The distributions of kutnahorite are not neccessar-

Fig. 5. Photomicrographs in transmitted light showing (A) carbonate (c) in an epidote (ep) grain (G3341), and (B) sericite (sc) occurring in a feldspar (fel) aggregate (G3341).

280

K. IIZASA TABLE

1

Relative abundance

of clay-sized fraction of seafloor sediments from the Kita-Bayonnaise Sericite

Chlorite

Smectite

Gypsum

G3341 (surface)

xx

xx

t

G3595 (surface)

xx

xx

t

G3596 (surface)

xx

xx

G3597 (surface)

xx

RC575-15

Calcite

Quartz

Plagioclase

t

xxx

x

x

x

xxx

x

x

t

t

xxx

x

x

xx

x

t

xxx

x

x

t

xx

x

t

xxx

x

x

x

t

xx

x

RC575-13

x

x

t

xxx

x

RC575-12

x

x

x

xxx

x

RCS75-11

x

xx

x

xx

x

RCS75-I0

x

x

t

x

xx

t

RC575-9

xx

x

t

x

xxx

x

RC575-8

x

x

x

xx

x

t

RC575-7

x

x

t

x

xxx

x

t

RC575-6

xx

xx

xx

xx

xx

x

t

RC575-5

x

xx

x

xx

xx

t

t

RCS75-4

xx

xx

x

xx

xx

x

t

RC575-3

x

x

x

xxx

x

x

RCS75-2

xx

xx

xx

xx

x

x

RC575-1

t

xx

xxx

x

(top)

t

xx

xx

x

x

xxx

x

x

RC613-2 RC613-3

xx xx

xx xx

x x

x x

t

xxx xxx

x x

x x

RC613-4 RC613-S

xx xx

xx xx

x x

x x

x t

xxx xxx

x x

x x

RC613-6

xx

xx

x

xx

xxx

x

x

RC613-7

xx

xx

x

xx

xxx

x

x

RC613-8

xx

xx

x

xx

xxx

x

x

RC613-9

xx

x

x

xxx

x

x

RC613-10

xx

xx

x

xx

t

xxx

x

x

RC613-11

xx

x

t

xx

xx

xxx

x

x

RC613-12

xx

xx

t

xx

xx

xxx

x

x

RC613-13

xx

xx

x

x

xx

xxx

x

x

RC613-14

xx

xx

t

x

x

xx

x

x

RC613-IS

xx

xx

t

x

x

xxx

x

x

RC613-16

xx

xx

x

x

x

xxx

x

x

RC613-17

xx

xx

x

x

t

xx

x

x

RC613-18

xx

xx

t

xx

t

xxx

x

x

R C 6 1 3 - 19

xx

xx

x

xxx

xx

x

x

RC613-20

xx

xx

t

xx

xxx

x

x

RC613-21

x

x

x

xx

x

x

RC613-22

xx

x

x

xxx

x

x

RC615-2

xx

xx

x

x

x

RC615-3

x

xx

x

x

RC615-c2

x

x

t

x

RC615-cl

t

t

X-ray examinations

t

ily in accordance barite

and

kutnahorite surface

other

x x

x

x

a r e c a r r i e d o u t u n d e r t h e s a m e c o n d i t i o n s w i t h r e s p e c t t o all t h e s a m p l e s . F u l l s c a l e o f X - r a y r e f l e c t i o n

i n t e n s i t y is 100. x x x = m a j o r ( 1 0 0 - 5 0 ) , x x = c o m m o n

and

caldera

RC575-14

RC613-1

(top)

Kutnahorite

submarine

with heavy

occurs sediments,

in

those

of

abundant

minerals cores

RC575

in

sulfides,

RC613. and

(50-I0), x =minor

( 1 0 - 5 ) a n d t = t r a c e ( < 5).

Abundancesof heavyfractions

No

RC615,

Approximate nent

in all the

abundances samples

of each

recovered

heavy

generally

compodecrease

PETROGRAPHY OF SEDIMENTS FROM THE IZU-OGASAWARA ARC

with the following order: magnetic minerals (magnetite and ilmenite), silicates, barite, sulfides, rutile, phosphates, fahlore and Mn oxyhydroxides (Table 2). Abundances of whole heavy minerals in each site range from about 1 to 13 wt.% (Fig. 6). Total weights of mixtures of mainly sulfides and barite in the treated fraction of RC575 range from about 0.01 to 3.45 wt.%, those of Fe-oxides from 0.70 to 5.76 wt.%, and those of transparent minerals from 0.66 to 2.92 wt.%. The sediment column of RC575 includes two particular layers (RC5756 and -8) significantly enriched in the mixtures (3.45 and 1.01 wt.%) consisting of major pyrite and barite, common chalcopyrite and sphalerite, minor marcasite, and trace galena and fahlore (Figs. 7 and 8). Fe-oxides are abundantly distributed in several layers but their distribution patterns do not necessarily accord with sulfides and barite, Abundances of Fe-oxides indicate a good correlation with those of transparent heavy minerals, In core RC613 total weights of sulfides and barite range from about 0.05 to 0.88 wt.%, those of Fe-oxides from 0.54 to 6.46 wt.%, and those of transparent minerals from 0.24 to 3.12 wt.%. Five layers (RC613-6 to 8, l0 and 17) are relatively enriched in the mixtures of major amount of barite, common pyrite, minor sphalerite, chalcopyrite and marcasite, and trace galena and fahlore (Figs. 7 and 8). Fe-oxides show two abundant distributions in the middle and lower parts of the column but their distributions in the column are not in accordance with those of sulfides and barite but rather show a good correlation with transparent heavy minerals, In core RC615 total weights of sulfides and barite range from about 0.01 to 0.04 wt.%, those of Fe-oxides from 4.18 to 9.26 wt.%, and those of transparent minerals from 1.47 to 3.95 wt.%. No significant abundance of sulfides and barite is obtained but Fe-oxides in an RC615-c2 layer indicare the most abundant fraction in all the samples in the caldera, Abundances of heavy minerals in surface sediments range from about 0.09 to 0.31 wt.% mixtures of sulfides and barite, 1.78 to 8.32 wt.% Feoxides, and 0.72 to 4.57 wt.% transparent minerals. Two samples (G3341 and G3597) contain relatively abundant Fe-oxides.

281

In general sediments from the eastern seafloor tend to be enriched in chalcopyrite and pyrite, while the western seafloor sediments are enriched in sphalerite, barite and probably galena. South seafloor sediments are characterized by negligible amounts of sulfides and barite. Discussion

Redistribution of elements in some sulfides Galena grains are significantly altered resulting in pyromorphite, cerussite and Ag-bearing-digenite and Ag- and Th-bearing-covellite. Ag appears to be derived from the host galena inferred from the mode of occurrence of the Ag-bearing minerals and absence of Ag-bearing minerals in other heavy minerals (Fig. 9). For the same reason, some Agbearing covellites include Th, probably derived from host galena. On the other hand, Cu seems to be supplied mainly by chalcopyrite. Ca and P in pyromorphite and C in cerussite are possibly supplied during the decomposition of volcanic glass and organic compounds in ambient sediments through interstitial water. Chalcopyrite is often associated with covellite, digenite and bornite in which Cu is secondarily provided by host chalcopyrite. Pyrite rims consist of Fe oxyhydroxides associated with minor elements, Pb, Ca, P, Si, A1 and K. Fe is mainly supplied from host pyrite, Pb from galena and the remaining minor elements are possibly from breakdown materials of volcanic glass in ambient sediments through interstitial water.

Possibility of modern hydrothermalactivity andsite of mineralization Deposition of sulfide particulates in finer sand fractions probably occurs within several hundred meters around hydrothermal vents at an oceanic ridge (Feely et al., 1987). The source of heavy minerals in the caldera floor sediments seems to be most probably limited within the caldera because of well-preserved crystal surfaces and high relief about 700 to 900 m of the caldera. The mode of occurrence of pyrite, chalcopyrite, sphalerite, galena, fahlore, marcasite and barite in

282

K. IIZASA

TABLE 2 Approximate abundance of heavy and light fractions (63-250 ~tm) in seafloor sediments from the Kita-Bayonnaise submarine caldera Mineral/S Mineral: sphalerite galena

chalcopyrite

pyrite

fahlore

marcasite

barite

digenite covellite

bornite

Sample

wt.%

G3341 (surface) G3595 (surface) G3596 (surface) G3597 (surface) RC575-15 (top) RC575-14 RC575-13 RC575-12 RC575-11 RC575-10 RC575-9 RC575-8 RC575-7 RC575-6 RC575-5 RC575-4 RC575-3 RC575-2 RC575-1 (bottom) RC613-1 (top) RC613-2 RC613-3 RC613-4 RC613-5 RC613-6 RC613-7 RC613-8 RC613-9 RC613-10 RC613-11 RC613-12 RC613-13 RC613-14 RC613-15 RC613-16 RC613-17 RC613-18 RC613-19 RC613-20 RC613-21 RC613-22 (bottom) RC615-2 (under the top) RC615-3 RC615-c2 RC615-cl (bottom)

1.043 0.347 0.173 0.259 1.184 0.430 0.173 4.408 2.617 2.120 2.996 0.928 2.793 7.321 6.163 3.963 6.279 7.050 4.398 1.040 1.555 ~ 1.127 0.260 2.220 6.539 11.166 11.392 10.635 14.791 3.959 1.299 1.715 1.891 1.042 2.643 6.329 4.697 3.005 3.667 1.271 0.763 2.172

0.000 0.000 0.000 0.327 0.802 0.000 0.000 0.322 0.000 0.322 0.487 0.160 0.161 0.312 0.160 0.000 0.805 0.796 0.000 0.000 0.000 0.000 0.000 0.324 0.483 0,963 0.807 0.484 1.903 1.438 0.000 0.488 0.489 0,000 0,162 0,480 0.324 0.000 0.000 0.161 0.000 0.000

0.183 0.911 0.182 0.181 0.444 1.265 0.818 4.010 1.065 3.120 5.309 14.193 10.144 11.502 11.804 5.434 4.193 6.529 8.624 0.182 0.363 0.364 0.546 2.154 2.768 4.446 3.218 3.131 3.426 0.708 0.273 1.172 1.536 0.822 2.599 4.081 4.310 2.076 0.986 0.534 0.357 0.000

0.895 3.011 2.340 2.109 11.102 10.952 4.786 30.878 29.561 25.652 14.102 37.146 31.703 59.083 40.097 24.948 26.979 36.189 36.251 3.681 9.340 3.235 4.904 17.802 23.612 27.765 27.905 29.241 27.751 7.047 3.009 5.186 3.429 1.900 22.498 29.869 19,347 12.600 18.988 17.771 18.338 1.118

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.200 0.000 0.000 0.000 0.100 0.000 0.291 0.697 0.000 0.000 0.396 0.000 0.000 0.000 0.000 0.000 0,000 0.000 0.000 0.000 0.100 0.000 0.199 0.000 0.000 0.000 0.000 0.000 0.299 0.000 0.000 0,000 0.000 0.000 0.000

0.218 0.000 0.217 0.000 0.530 0.539 0.000 2.019 5.928 2.977 0.322 0.106 0.637 1.857 1.694 1.188 1.170 1.158 4.136 0.000 0.433 0.000 0.000 0.321 0.639 0.742 0.533 1.173 0.838 0.211 0.000 0.752 0,323 0.000 0.962 0.635 1.178 0.323 2.887 6.266 5.635 0.000

94.867 95.361 96.902 94.834 81.274 83.498 92,786 53.990 57.576 62,136 70.217 45.273 47.268 15.566 33.508 59.935 54,636 42,608 40,986 93,983 83,423 94.994 92.317 74.373 64.576 52.706 54.443 53.405 46.956 82.514 95.419 88.332 89.745 95.678 70.341 55.790 68.781 81.046 73.199 73.299 74.657 94.847

0.000 0.000 0.000 0.000 0.607 0.000 0.000 0.243 0.242 0.365 1.966 0.727 2.795 1.654 2.182 0.000 0.609 1.205 1.943 0.000 0.496 0.000 0.000 0.245 0.122 0.243 0.122 0.367 0.240 0.605 0,000 0.246 0.370 0.000 0.122 0.848 0.245 0.370 0.000 0.122 0.000 0.000

0.000 0.000 0.000 0.000 0.499 0,000 0.000 0.200 0.199 0,300 1.615 0.597 2.296 1.358 1.792 0.000 0.500 0.990 1.596 0.000 0.408 0.000 0.000 0.201 0.100 0.200 0.100 0,301 0.197 0.497 0.000 0.202 0.304 0.000 0.101 0.697 0.201 0.304 0.000 0.100 0.000 0.000

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0,000 0.107 0.000 0.000 0.000 0.109 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0,000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0,000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.436 1,477 1.930

0.000 0.000 0.000

0.000 0.000 0.000

0.561 0.671 1.242

0.000 0.000 0.000

0.546 0.000 0.000

95.628 96.871 92.421

0.000 0.000 0.000

0.000 0.000 0,000

0.000 0.000 0.000

PETROGRAPHY OF SEDIMENTS FROM THE

TABLE 2

1ZU-OGASAWARAARC

283

(continued) Mineral/S

Mineral:

pyromorphite cerussite apatite

Sample

(wt.%)

G3341 (surface) 0.000 G3595 (surface) 0.000 G3596 (surface) 0.000 G3597 (surface) 0.311 RC575-15 (top) 0.763 RC575-14 0.000 RC575-13 0.000 RC575-12 0.306 RC575-11 0,000 RC575-10 0.306 RC575-9 0.464 RC575-8 0,152 RC575-7 0,153 RC575-6 0.297 RC575-5 0.152 RC575-4 0.000 RC575-3 0.766 RC575-2 0.757 RC575-1 (bottom) 0,000 RC613-1 (top) 0,000 RC613-2 0.000 RC613-3 0.000 RC613-4 0.000 RC613-5 0.308 RC613-6 0.460 RC613-7 0.916 RC613-8 0.767 RC613-9 0.461 RC613-10 1.810 RC613-11 1.368 RC613-12 .0.000 RC613-13 0.464 RC613-14 0.465 RC613-15 0.000 RC613-16 0.154 RC613-17 0.457 RC613-18 0.308 RC613-19 0.000 RC613-20 0.000 RC613-210.153 0.142 RC613-22 (bottom) 0.000 RC615-2 0.000 (under the top) RC615-3 0.000 RC615-c2 0.000 RC615-cl (bottom) 0.000

S/G

M/G

T/G

L/G

G/W

SC/W

rutile

0.000 0.000 .000 0.290 0.710 0.000 0.000 0.285 0.000 0.285 0.431 0.142 0.142 0.276 0.142 0.000 0.713 0,705 0.000 0.000 0.000 0.000 0.000 0.287 0.428 0.852 0.714 0.429 1.684 1.273 0.000 0.432 0.433 0.000 0.143 0.425 0.287 0.000 0.000 0.000 0.000 0.000

0.000 0.000 0.000 0,209 0.000 0.000 0.140 0.959 1.091 1.507 0.899 0,204 0.820 0.199 0.614 1.948 1.439 0.339 0.888 0.000 0.279 0.280 0.210 0.759 0.000 0.000 0.000 0.000 0.135 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.138 0.000 0.000 0.182 0.068 0.000

2,795 0,371 0.186 1.479 2.084 3.316 1,297 2.180 1.719 0.909 1.193 0.271 1.089 0.176 0.995 2,585 1.910 1.169 1.178 l.ll4 3.703 0.000 1.763 1.007 0.273 0.000 0.000 0.274 0.269 0.181 0.000 1.011 1.013 0.558 0.274 0.090 0.183 0.276 0.274 0.346 0.182 1.863

0.196 0.182 0.093 0.309 0.037 0.067 0.032 0.010 0,065 0.148 0,072 1.012 0.195 3.455 0.038 0,094 0.042 0.080 0.042 0.054 0.050 0.059 0.061 0.100 0.576 0.672 0.785 0.114 0.813 0.165 0.089 0.121 0.136 0.300 0.326 0.880 0.314 0.142 0.194 5.333 0.145 0.012

7.005 2.140 1,781 8.321 1.382 2.222 1.439 0,966 3.497 5.764 1,572 3.683 1.373 4.321 1.176 3,419 1.282 1.024 0.697 0.836 1.300 1.468 1.840 1.209 2.860 3.569 4.707 0.543 3.508 1.777 1.311 1.496 1.881 2.050 1.104 2.022 1.887 4.296 6.458 2.930 1.340 4.184

4.573 0.875 0.722 4,103 1.346 0.824 1.047 0.957 1.799 1.606 1.186 2.489 1.178 2.924 0.904 2.185 0.985 0.944 0.655 0,407 0,596 0.617 0.817 0.559 1.362 1.883 1.992 0.242 1.764 0.834 0.671 0.609 0.592 0.860 0.491 0.823 0.887 1.973 3.118 91.391 1.091 1.474

88,226 96,803 97.404 87.267 97.235 96.888 97.482 98.068 94.639 92.481 97.170 92.817 97.253 89,300 97,882 94.303 97.693 97.952 98.605 98.704 98.055 97.856 97.282 98.132 95.202 93.877 92.517 99.100 93.915 97.223 97.929 97.774 97.391 96.790 98.079 96.275 96.912 93.589 90.231 6.24 97.424 94.330

23.62 15.91 33.08 49.99 34.89 21.63 35.38 24.61 34.95 12.09 38.15 36.94 39.91 15.84 35.41 19.72 32.93 46.24 7.07 42.92 47.02 41.64 33.02 43.79 39.94 22.23 39.84 44.29 28.35 35.56 45.24 41.11 34.11 38.68 42.54 42.05 39.59 14.10 9.89 5.94 37.51 20.78

15.34 18,58 25.52 25.16 24.28 24.47 43.11 71.56 13.64 9.21 35.33 26.07 42.75 13.89 48,57 15.38 41.90 20.26 90.79 44.51 22.11 27.51 28.86 38.39 11.84 12.74 23.51 44.16 12.66 29.77 35.30 32.63 27.27 22.09 46.65 24.35 28.45 8.52 8.51

0.000 0.000 0.000

0.774 0.421 2.338

2.055 0.559 2.069

0.042 0.044 0.009

5.233 9.263 6.469

2.363 3.951 2.040

92.362 86.741 91.483

16.41 12.27 11.35

7.97 6.58 11.17

21.20 19.75

Weights of S = consisting mainly of sulfides + barite, M = magnetic minerals, T = transparent heavy minerals, L = light fractions, G = fractions 63-250 lam dealt with mineral separation, SC = fractions < 63 lam and W = whole sediment treated.

284

K. IIZASA A

wt. % top

0

1 ,

I

,

2

3

I

I

i

4

5

!

I

6 ,

l

RC575-15 RC575-14

~

l

RC575-13

I

RC575-12

~

RC575-11

[]

T/G

•

M/G

•

S/G

10 20 30

I

RC575-10

I

RC575-9

40

g

50

v

60

t~

I

RC575-8 RC575-7

I

70

RC575-6

80

RC575-5 ~ RC575-4 RC575-3

90

RC575-2 RC575-I

B

wt, % top

,

RC613-1 RC613-2 RC613-3

==,,,1,,=---I 1

RC613-4 RC613-5

~

RC613-6 RC613-7 RC613-8 RC613-9

1

2

I

I

,

3

4

I

I

5 ,

6

I

i

I

7 ,

I

-

10

-- 20 I t

3O

i ~

40

RC613-10 RC613-11 RC613-12 RC613-13

t ~ _--"""-'-7

RC613-14 RC613-15

~ ~

RC613-16 RC613-17

~ ~

RC613-18 RC613-19

~ _

RC613-20 RC613-21 RC613-22

n

t

[]

T/G

•

M/G

•

S/G

50 t~

t 60 ] 70 80

t

90 I t

Fig. 6. Downcore variation of abundance of whole heavy fractions, transparent (T), magnetic (M), and sulfides and sulfate (S) in a treated fraction (G) ranging from 0.063 to 0.25 mm. (A) sediment column from the east caldera floor, (B) sediment column from the west caldera floor. Depth (in centimeters) of the sediment columns of (A) and (B) shows approximate scale.

the caldera suggests that there are two types of mineralization in the caldera. First, the existence of pyrite, chalcopyrite and sphalerite in cavities encrusted by quartz and scattered pyrite in lithic fragments may imply the presence o f disseminated mineralization in the caldera. Secondly, the presence of barite aggregates with pellet-type pyrite,

fine-grained chalcopyrite and sphalerite indicates hydrothermal activities responsible for baritesulfide mineralization. The texture and assemblage are similar to those of hydrothermal massive barite-sulfide samples on sediment surface from the Myojinsho submarine caldera south of the Kita-Bayonnaise submarine caldera (Iizasa et al.,

PETROGRAPHY OF SEDIMENTS FROM THE IZU-QGASAWARA ARC

285

wt. %

A

top

0

20

40

I

RC575-15 RC575-14 RC575-13"

i

~ ..~O •:::~ ......................................

RC575-12"

60

I

i

.

.

.

"l~..

~ .................................

•".'~iii. . . . . ......../h I~.':::::" .......................................................... ~;':.:::•

RC575-5-

".......................D : ~ •

RC575-4 -

I

..IIII.'..'2.-E]

".................................0

RC575-7" RC575-6"

,

I-li.'.....................................................

•':':2;:ii "

RC575-8"

I

f"l..... ........III'£~'2:"1Z]

....•" .

RC575-9-

,

• "

RC575-10

100

80

I

....... • ...... pyrite

......................

'"-"t~ ....

barite

• . ..................................................... .tS]

RC575-3 -

• . ...............•

RC575-2" RC575-1

~ ....................."1~.......

t~

B

wt. % top 0 RC613-1- • . .

20 '

'

'

RC613-2" "(I:ID RC613-3" 0. . . . . . . . RC613-4• ........ RC613-5".............• . . . RC613-6"'•....• RC613-7" ,

40

60

80

t

~

I

100 i......1~

J

D:.i":~" . .

. .

.

. .

.

.

.

.

.

"D O .....

.........D" ......... El.. . . . . . . . . . . . . . . . . .

RC613-8Q "~ RC613-9 ~ ~ ...I~ RC613-10 •..........O 1~":""................. RC613-11 " .• ............................................................ RC613-12" ~ " . . . . . . . . . . . RC613-13~ RC613-14RC613-15 O ........... RC613-16 ......................• . . . . . . . . . RC 613-17 "~];::.0 r-l:::;::~i..... RC613-18 ...• . . . . . . . . . . . . . . . . . . . . . . . . RC613-19e(:" RC613-20 ' " .g RC613-21 # RC613-22 •

• .... ......Q - "

pyrite barite

"D ....... .

i:1~ U]"....

1~".................................... 1"3....... [~2~.~2~:'3D I~. D

Fig. 7. Downcore variation of abundance of barite and pyrite in the mixtures of sulfides and sulfate (S) in Fig. 6.

1992), and also to those of Kuroko-type deposits (Eldridge et al., 1983). These occurrence together with primary associations of sphalerite, chalcopyrite, galena, fahlore, marcasite and pyrite suggest that hydrothermal mineralizations took place in the caldera. Epidote-carbonate or chlorite assemblages in heavy fractions and sericite-quartz-feld-

spar assemblages in light fractions also suggest that they were derived from hydrothermally altered halos in the caldera (Fig. 10). Distribution and abundance of sulfides, fahlore and barite indicate that two probable sources of those minerals are present in the east and west parts of the caldera. In the east side of the caldera,

286

K. IIZASA A

wt. %

top

0

RC575-15 RC575-14 RC575-13 RC575-12

- ~,...O - g?| C .~.~y~,"

2

RC575-10- ~

4

8

~

......i " -

.~2q[:l~...'.~".'?~ ...........

14

.

. ............. . " ....................................................................................... . . . .

"X, tg

RC575-4RC575-3

~

RC575-2

> .............

chalcopyrite

.-"'-l~r.... marcasite

~-.. ....... "10............

r:c575-5-

12

10

.... O .... sphalerite

RCSVs-8RCS75"1~9 O.. . i-""iiiii~"'":~~, RC575-7 RC575-6"

6

: .'4

.:~? .......................

....."W ........................ m

"..............................~::;;O

..,
RC575-1

.................~

...................

4

6

wt. % top

0

2

I

,

n

.

I

•

l

8 .

I

10 .

I

12 .

I

14 .

I

•

RC613-11~1~ (3~ RC613-2 "[:1~t' ~3 RC613-3 ~ . ~ RC613-4

RC613-6 "] RC613-7 " RC613-8" RC613-9"

~

" i

..... ".................. "O"..................... ";:'.211 ................. 'Q .~" /(3 I~ O ~-............... RC613-1O" ¢,i~.._. ~-....41 ................. ZI~27:~:z="O RC613-11- ¢)i" .......... ........ O ................................................................ RC613d2 C ~ . . . Q ~ .......

~ ~ ~>

RC613-13 " ./l~Ix_~ RC613 14" ~' _ ! ~ " .'~ RC613~17 ~ RC613-18" RC613-19RC613-20" RC613-21 RC613-22

~. ':=:':2"2"i.............'.'.'2.::0 ~ ...... : : ~ .... ~:'...-...~Z....R"- ~.'.''~" ~ ...... ".::.~-~,'~ ........ IJR fly .................. I~Cf 1~

..... O""

sphalerile

....II .... chalcopyrite .

......ta .... marcasae

Fig, 8. Downcore variation of abundance of chalcopyrite, sphalerite and marcasite in the mixtures of sulfides and sulfate (S) in Fig. 6.

major hydrothermal activities resulting in the mineralization enriched in chalcopyrite and pyrite took place at least twice judging from the abundances of heavy mineral associations in RC575. On the other hand, in the west major mineralization associated mainly with sphalerite, galena and barite took place several times according to the heavy mineral associations in RC613.

Possible sources of clay-sized fractions Smectite plays an important role as an indicator of hydrothermal alteration halos. Smectite, for example, as a low temperature product, occurs in hydrothermal fields present in oceanic ridges (Bischoff, 1972; Haymon and Kastner, 1981; McMurtry et al., 1983), back-arc basins

PETROGRAPHYOF SEDIMENTSFROMTHEIZU-0GASAWARAARC

287

sphalerite,galena, chalcopyrite,fahlore,pyrite,, marcasite,barite, epidote,carbonate,n l t i l e

Hydrothermal

pyrornorphite,cerussite, digenite,covellite,bornite ~russite ) y ~ C~

'~ i-~ - ~

~

:~!/

hypersth...... gite, ~ hornblende,magnetite, ilmenite, apatite

Light mine

bvi°titecav ~c~ias~s(f°raminifera)'~

b

Clay-sizedfraction [ smectite,sericite,chlorite

4

Fig. 9, Model of elementmigrationduring chemicalweathering after sulfide deposition and secondary minerals around the sulfides. (McMurtry et al., 1991), and the Loihi submarine volcano (Malahoff et al., 1982; De Carlo et al., 1983). Since smectite is also ubiquitously present as eolian, detrital and weathered materials in ocean floor sediments (e.g. Arrhenius, 1963; Griffin et al., 1968; Aoki et al., 1974; Hein et al., 1979), these origins cannot be ruled out in the studied area. Smectite distribution in the samples, however, is not uniform but sporadic. In particular dominant smectite occurs in accordance with predominant mixtures of sulfides, sulfate and sulfosalt (RC5756). These results suggest that some smectite studied could be of hydrothermal origin, M g - F e chlorite and sericite are also useful as indicators of hydrothermal alteration halos accompanied with Kuroko deposits (e.g. Shirozu, 1974; Date et al., 1983; Urabe et al., 1983) and with epithermal vein deposits on land (e.g. Nagasawa et al., 1976; Nagsawa, 1981; Takeuchi, 1984; Izawa, 1986). While Griffin et al. (1968) and Windom (1976) presented that M g - F e chlorite and illite (referred to fine-grained micas)are widely distrib-

Fig. 10. Summaryof end-membersof eachmineralin sediments. Although secondary associations of pyromorphite, cerussite, digeniteand bornite are secondaryproducts, these mineralsa r e includedin hydrothermalderivativesbecause their host minerals are galena and chalcopyrite. uted in ocean floor sediments as major continental sources. Local sources of chlorite are also present in the greenstone of the Mid-Atlantic ridge (Copeland et al. 1971). M g - F e chlorite and sericite are commonly distributed in the Kita-Bayonnaise submarine caldera and show no particular abundance as compared to smectite and kutnahorite. However, the presence of sericite-quartz-feldspar, and epidote-chlorite assemblages in light and heavy fractions, respectively, is indicative that some of these clay-sized minerals derived from hydrothermal alteration halos. Clay-sized kutnahorite is closely distributed in Mn-rich layers in the upper and middle part of the sediment core (RC613). Mn material is probably of hydrothermal origin, based on the limited occurrence of hydrothermal Mn-oxyhydroxides on pumice in the caldera floor sediments and Mn-

288 coated dacitic rocks on the central cone of the caldera. The coincident appearance with the Mn material is suggestive of a hydrothermal product during hydrothermal activities. The distribution is also indicative of kutnahorite formation taking place several times in the west part of the caldera floor in the past. Although most of quartz and feldspar in the light fractions appear to be of detrital origin from dacite in the caldera according to the angular shapes of quartz and feldspar, a possibilility of eolian origin can not be ruled out regarding some clay-sized quartz and feldspar. Calcite must be detritus derived from foraminiferal material in the caldera because of the existence of foraminiferal calcite fragments in the light fractions.

Fine-grained futile Rutile commonly occurs in hydrothermally altered igneous rocks (Force, 1980), in porphyry copper deposits (Force, 1976), in wallrocks of metamorphosed massive sulfide deposits (Nesbitt and Kelly, 1978), and in sedimentary rocks as diagenetic origin (Valentine and Commeau, 1990). Characteristic assemblages of rutile with baritechalcopyrite, pyrite, marcasite and sphalerite in this study suggest that the rutile studied derived from hydrothermal alteration zones.

Conclusions Sediments grabbed and cored from the Kita-Bayonnaise submarine caldera floor were divided into heavy, light and clay-sized fractions. Primary hydrothermal minerals of predominant pyrite and barite, dominant sphalerite and chalcopyrite, minor galena, fahlore, marcasite and rutile were identified in the heavy fractions. These minerals occur as a single grain and as the assemblages of barite-pellet type pyrite-chalcopyrite-sphalerite, chalcopyrite-pyrite-sphalerite with galena or fahlore, and rutile associated with barite-chalcopyrite, pyrite, marcasite or sphalerite, pyrite and sphalerite with quartz crystals in vugs, and scattered pyrite in lithic fragments. Secondary minerals in trace amounts, Ag-bearing-digenite, Ag- and

K. IIZASA Th-bearing-covellite, pyromorphite and cerussite, commonly occur around galena. Other secondary phases, bornite, Ag-absent-digenite and -covellite, are mainly accompanied with chalcopyrite. Significant amounts of magnetite and minor ilmenite and apatite are present as detrital origin. Epidote-carbonate or chlorite in the heavy fractons, sericite-quartz-feldspar in the light fractions, and some of smectite, Mg-Fe chlorite and sericite, and kutnahorite in the clay-sized fractions are indicative of hydrothermal alteration origin because of no metamorphosed rocks in the caldera. Hypersthene, augite and hornblende in transparent heavy minerals, and feldspar, quartz, volcanic glass and biotite in the light fractions are of dacitic origin, and calcite is of foraminiferal origin. Kutnahorite occurs closely with Mn-bearing layers. Layers enriched in sulfides and barite mixtures in gravity core samples (RC575 and RC613) suggest that hydrothermal mineralization took place more than twice in the caldera. In particular the east-side samples of the caldera tend to be enriched in chalcopyrite and pyrite in comparison with the west-side samples which are enriched with sphalerite, galena and barite. Many hydrothermally derived minerals indicate that hydrothermal activities responsible for sulfides and sulfate mineralizations associated with Ag and Th took place both in the east and west side of the caldera. The petrographic analyses are useful methods for evaluating both active and fossil hydrothermal phenomena in submarine calderas.

Acknowledgements This work has been supported by the help of on-board staffs during the 1984 to 1989 R/V Hakurei-maru cruises and the cooperation of collegues in the Geological Survey of Japan. I thank Dr. A. Usui for helpful and constructive reviews, Dr. M. Arita for invaluable suggestions, and Dr. P. Jarvis for critical reading of the manuscript. I particularly wish to acknowledge the aid of Mr. M. Kojima, University of Tokyo, for samplepreparations. This research was funded by the Agency of Industrial Science and Technology.

PETROGRAPHY OF SEDIMENTS FROM THE IZU-OGASAWARA ARC

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