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Assessment of the hydrothermal contribution to seafloor sediments in the Myojinsho submarine caldera, Shichito-Iwojima ridge, Izu-Ogasawara arc, Japan
Marine Geology, 114 (1993) 119-132
119
Elsevier Science Publishers B.V., Amsterdam
Assessment of the hydrothermal contribution to seafloor sediments in the Myojinsho submarine caldera, Shichito-Iwojima ridge, Izu-Ogasawara arc, Japan Kokichi Iizasa
Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305, Japan (Received October 20, 1992; revision accepted May 21, 1993)
ABSTRACT Iizasa, K., 1993. Assessment of the hydrothermal contribution to seafloor sediments in the Myojinsho submarine caldera, Shichito-Iwojima ridge, Izu-Ogasawara arc, Japan. Mar. Geol., 114:119-132. To assess the hydrothermal contribution to sediments of submarine calderas on a volcanic front, seafloor sediments from the Myojinsho submarine caldera, Shichito-Iwojima ridge, Izu-Ogasawara arc, northwestern Pacific were chemically and mineralogically analysed. The caldera (ca. 5 × 6 km wide and 1114 m water depth) has a fiat-topped central cone (ca. 330 m deep) and is covered with muddy to sandy sediments dominated by fragments of volcanic ash with lesser amounts of foraminifera, radiolaria and diatoms. Microscopic observations of the heavy mineral fraction indicate that the sediments include sulfides, barite, fahlore and epidote associated with chlorite, suggesting the presence of hydrothermal activity. Ag is present in fahlore in polished sections according to SEM supplemented with EDX. The sediments are relatively enriched in Au, Zn, Pb, Cu, Co, As, Sr, Ca, Mg, Fe, Mn and Ba compared to fresh dacitic rocks sampled from the caldera. Factor analysis of the compositional data produces three major factors--detrital, hydrogenous and hydrothermal. One site nearest to a probable hydrothermal source was evaluated by the characteristic assemblage and abundance of hydrothermal minerals, and factor analysis of the sediment samples.
Introduction The I z u - O g a s a w a r a arc is one o f the a r c - t r e n c h systems in the western Pacific and a site o f hydrothermal activity. This has resulted in h y d r o t h e r m a l M n oxides on submarine volcanoes (Yuasa and Yokota, 1982; Usui et al., 1986; Usui and Nishimura, 1992), a h y d r o t h e r m a l l y altered rock with m a j o r pyrite in a submarine caldera (Urabe et al., 1987), barite-bearing silica chimneys in a back-arc rift (Urabe and K u s a k a b e , 1990), b a r i t e sulfide fragments and rocks with disseminated sulfides in a submarine caldera (Iizasa et al., 1992), and sulfide chimneys in a submarine volcano (S. K a s u g a and Y. K a t o , pers. c o m m u n . , 1992). The Geological Survey o f J a p a n (GSJ) conducted a research project to evaluate potential ore resources associated with the h y d r o t h e r m a l activity 0025-3227/93/$06.00
o f the Myojinsho submarine caldera in the I z u - O g a s a w a r a arc f r o m 1984 to 1989. The caldera (approximately 5 x 6 k m wide and 1114 m maxim u m water depth) is located a b o u t 420 k m south o f Tokyo, on the S h i c h i t o - I w o j i m a ridge (a volcanic front) associated with m a n y Q u a t e r n a r y volcanoes (Yuasa et al., 1991) (Fig. 1). The caldera has a flat-topped central cone (ca. 330 m deep), and its s o m m a s are the Bayonnaise R o c k s to the west and the Myojin Reef to the northeast (Takada and Yuasa, 1990; E. Saito and F. M u r a k a m i , pers. c o m m u n . , 1990). D u r i n g the G H (Geological Survey Ship Hakurei-Maru) 88-3 and 89-1 cruises to the caldera, fragmental massive barite-sulfide samples and m a n y rocks with disseminated sulfides were found in the caldera (Iizasa et al., 1992). Before the finding o f the barite-sulfide samples and rocks associated with sulfides, m e t h o d s for
© 1993 - - Elsevier Science Publishers B.V. All rights reserved.
120
K. IIZASA
57'
140°00'E
03'
Fig. 1. Locations of samples in the Myojinsho submarine caldera. Inset map indicates the northern part of the Izu-Ogasawara Arc and backarc rifts, Hachijo (h), Sumisu (s) and Torishima (t) (modified after Karig and Moore, 1975; Tamaki et al., 1981; Taylor et al., 1984; Honza and Tamaki, 1985). Dotted areas are the seafloor of the caldera. Depth contours in meters.
evaluating potential mineral resources in the submarine caldera were carried out. This study describes the mineralogical and geochemical methods applied to seafloor sediments of the caldera and assesses the hydrothermal contribution to the sediments.
Sample description and methods Four gravity core (RC) samples up to about 1 m length were recovered at the caldera floor and consist mainly of dark grey volcanic ash layers (clay- to coarse-sized grains) and pumice with lesser amounts of foraminifera, radiolaria and diatoms. One core sample (RC574) was disturbed by coring. One sample (RC612) included two brownish thin layers consisting of amorphous Fe hydroxides. The uppermost 15 cm of sediments were also obtained at the caldera floor by six box cores (G) and two rock samplers (RS) and consist of almost the same components as RC samples. Dredge (D) samples were recovered at three sites west of the central cone. These samples are abundant dacite and lesser amounts of (lapilli) tuff and
tuffaceous sandstone. The surfaces of tuff and dacite exposed to seawater are partly coated by Mn oxide films. The dacite is composed of clinoand ortho-pyroxene, hornblende, feldspar, quartz and opaque minerals (Iizasa et al., 1992). The analysis of heavy minerals is often used for establishing their provenance (e.g., Rittenhouse, 1943; Imbrie and Van Andel, 1964; Valentine and Commeau, 1990; Iizasa, 1993). According to Rittenhouse (1943) heavy minerals between #60 mesh (0.25 mm) and #230 mesh (0.063 mm) in river deposits generally make up about 50 to 70 wt.% of whole heavy minerals included in the bulk sediments, while remaining heavy minerals tend to exist in the finer fractions less than 0.063 mm. In this study the 60-230 mesh subsamples were tentatively taken for heavy mineral analysis although hydraulic conditions in the submarine caldera do not necessarily coincide with those in rivers. Sediments were dried at 100°C and 10 to 230 g were taken for heavy mineral analysis. Then they were disaggregated followed by sieving with mesh sizes, #18 ( l m m ) , #60 (0.25mm) and #230 (0.063 mm). The separated samples were weighed.
HYDROTHERMAL CONTRIBUTION TO SEAFLOOR SEDIMENTS
The finer sand fractions (0.25-0.063 mm) were subsequently separated by tetrabromoethane (sp. gr., about 2.96) for the 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 included in the opaque fraction during the isodynamic separation, they are dealt with as components of the opaque fraction. 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 the ore microscope. Representative polished sections were semiquantitatively analyzed by scanning electron microscope (SEM) JEOL JSM-6400 equipped with energy dispersive X-ray spectrometer (EDX). Thin sections of all transparent heavy and several light fractions were prepared for petrographic observation. Multivariate factor analysis for searching end members composing a large chemical dataset was effectively employed to marine sediments (Dymond, 1981; Leinen and Pisias, 1984; McMurtry et al., 1991; Murphy et al., 1991) and stream sediments (Kelley and Kelley, 1992). In this study the factor analysis is applied to element concentrations of caldera floor sediments. The bulk chemical composition of all of the sediments was determined for Au, Sb, As, Co, Cr, Th and U using instrumental neutron activation analysis (INNA), Ag, Zn, Pb, Cu, Ni, Cd and V using direct current plasma (DCP), and Ba, Rb and Sr using X-ray fluorescence spectrometry (XRF), and that of volcanic rocks for whole-rock analyses was carried out for major elements and Rb using XRF, Sr, Ba, Pb, Ni, Co, Cr, V, Cu, Zn, Ag and Cd using inductively coupled plasma emission spectrometry (ICP), and Au, Sb, As, Th and U using INNA at X-ray Assay Laboratories, Canada. Results and discussion
Mineral compositions in seafloor sediments Nonmagnetic heavy fractions after isodynamic separation consist of chalcopyrite, sphalerite,
121
galena, pyrite, marcasite, covellite, digenite, bornite, barite, fahlore, ruffle and apatite in opaque fractions, and clino- and ortho-pyroxene, hornblende, epidote with chlorite and carbonate (probably dolomite) in transparent fractions while magnetic ones are composed of magnetite, ilmenite and hematite. Some representative thin sections of light mineral fractions indicate an assemblage of sericite-quartz-feldspar, quartz, feldspar and calcite (foraminifera). Mineral associations in opaque fractions are described below. A SEM photomicrograph shows an aggregation of euhedral sphalerite, pyrite and barite, and materials among the minerals are C u - F e - S substance, probably chalcopyrite (Fig. 2A). An assemblage is composed of subhedral sphalerite and pyrite, and minute anhedral chalcopyrite in reflected light (Fig. 2B). Chalcopyrite occurs partly on sphalerite as films. A characteristic assemblage consists of subhedral fahlore and chalcopyrite, sphalerite along the chalcopyrite, covellite after the chalcopyrite and barite (Fig. 2C). The fahlore contains Ag according to EDX analysis. Relatively large subhedral grains of fahlore are associated with minute chalcopyrite (Fig. 2D). A radiation of chalcopyrite is embedded in a pyrite grain (Fig. 2E). A detrital grain of epidote radiation is associated with feldspar in transmitted light (Fig. 2F). Pyrite and marcasite grains are in general euhedral to subhedral forms and relatively weathered to hydroxides around their rims. Sphalerite, chatcopyrite, galena and fahlore exist in euhedral to anhedral forms, while barite and ruffle occur in euhedral shape as a whole. Digenite and covellite are usually associated with chalcopyrite as secondary minerals. Ruffle occurs mainly as aggregates of tetragonal and acicular shapes, and often as associations with pyrite and barite. These minerals show various associations of several grains, indicating hydrothermally-derived minerals because there are no observations of the various mineral associations in abundant volcanic rocks of the caldera (Table 1). Magnetite, ilmenite, hematite, clino- and orthopyroxene, hornblende, quartz and feldspar are components of volcanic rocks of the caldera. Apatite is usually associated with volcanic glass which represents detritus of volcanics composing the caldera. Calcite occurs as fragments of fora-
122
K. IIZASA
Fig. 2. Mineral associations in heavy fractions. (A) SEM photomicrograph of sphalerite (sp), pyrite (py), chalcopyrite (cp) and barite (ba) aggregate, (B) photomicrograph in reflected light showing sphalerite, pyrite and chalcopyrite aggregate, (C) barite, sphalerite, chalcopyrite, fahlore (fa) and covellite (cv) aggregate in reflected light, (D) relatively large fahlore and minute chalcopyrite grains in reflected light, (E) chalcopyrite radiation embedded in a pyrite grain in reflected light, and (F) epidote (ep) radiation with feldspar (re/) in transmitted light.
Hydrogenous
Detrital
chalcopyrite (cp) sphalerite (sp) covellite (cv) epidote (ep)
Hydrothermal
fahlore (fa)
sericite (ser)
pyrite (py) digenite (dg) rutile (ru)
Mn-oxyhydroxides
cp-ma in mt with gl
magnetite (mt) clinopyroxene calcite
ilmenite orthopyroxene
py-ru-ba, ep-ch-q-fel, ser-q-fel hematite quartz (q) volcanic glass (gl) feldspar (fel) apatite
cp-py-sp-fa-gn, py-ma-sp, ba-cp-sp-py, ba-py-ma-gn, sp-cp-bo-cv, cv-dg,
marcasite (ma) galena (gn) bornite (bo) chlorite (ch)
Mineral composition and association
Source
Mineral composition and association of seafloor sediments from the Myojinsho submarine caldera
TABLE 1
carbonate
barite (ba)
coatings on sediment particles and volcanic rocks
derivatives from volcanic rocks derivatives from volcanic rocks fragments of forams
primary hydrothermal minerals primary hydrothermal minerals secondary minerals after chalcopyrite minerals from hydrothermal alteration halos
Remarks
124
K. IIZASA
TABLE 2 Abundances of heavy and light mineral fractions of seafloor sediments from the Myojinsho submarine caldera Sample
S/WH (wt.%)
S/GF
HT/GF
M/GF
L/GF
M/WH
GF/T
rutile/S
G3340 (surface) G3598 (surface) G3599 (surface) G3600 (surface) G3601 (surface) G3602 (surface) RS92 (surface) RS93 (surface) RC574-I (top) RC574-2 RC574-3 RC574-4 RC574-5 RC574-6 (bottom) RC61 I-1 (top) RC611-2 RC611-3 RC611-4 RC611-5 (bottom) RC612-1 (top) RC612-2 RC612-3 RC612-4 RC612-5 RC612-6 (bottom) RC617-1 (top) RC617-2 RC617-3 RC617-4 RC617-5 RC617-6 RC617-7 RC617-8 RC617-9 RC617-10 RC617-11 RC617-12 RC617-13 (bottom)
0.28 6.37 0.95 0.91 0.23 0.23 1.21 1.26 1.16 0.56 0.92 0.67 0.85 0.63 1.12 5.67 7.32 1.33 4.35 3.55 6.64 6.68 2.08 2.58 6.18 0.29 1.13 1.30 1.97 1.60 5.17 3.09 2.23 1.15 1.16 1.00 1.50 1.04
0.01 0.18 0.01 0.03 0.01 0.01 0.04 0.03 0.03 0.02 0.03 0.00 0.02 0.05 0.02 0.08 0.09 0.08 0.06 0.10 0.31 0.04 0.07 0.13 0.02 0.02 0.06 0.09 0.12 0.08 0.27 0.02 0.07 0.11 0.09 0.10 0.11 0.09
1.01 0.88 0.43 1.09 1.22 1.79 1.05 0.61 1.05 1.25 0.91 0.11 0.80 2.65 0.35 0.25 0.18 1.44 0.34 0.65 0.79 0.08 0.91 1.02 0.02 1.72 1.26 1.74 1.12 1.29 1.11 0.12 0.75 2.11 1.48 2.09 1.58 1.53
2.34 1.75 0.92 1.68 3.10 4.42 1.84 1.71 1.86 2.00 2.01 0.31 1.34 5.56 1.26 1.04 0.96 4.56 0.87 2.15 3.60 0.45 2.20 3.77 0.21 6.70 4.28 5.15 4.85 3.74 3.86 0.62 2.31 7.65 5.83 7.82 5.60 7.23
96.65 97.19 98.64 97.20 95.66 93.78 97.08 97.65 97.05 96.73 97.06 99.58 97.84 91.75 98.37 98.64 98.77 93.91 98.74 97.09 95.30 99.43 96.82 95.09 99.75 91.55 94.39 93.02 93.90 94.90 94.76 99.24 96.86 90.13 92.61 89.99 92.70 91.16
69.57 62.40 67.23 60.01 71.59 71.04 62.90 72.63 63.16 61.11 69.23 72.22 61.54 67.31 77.44 76.23 77.91 74.96 69.07 74.00 76.66 79.84 69.20 76.72 85.78 79.34 76.35 73.78 79.61 73.18 73.65 81.69 73.74 77.50 78.88 78.13 76.81 81.69
40.70 45.75 9.46 30.23 52.08 77.87 23.80 27.75 25.09 23.03 3.04 39.90 6.91 57.77 25.79 17.73 15.26 59.85 13.58 34.16 8.28 8.84 41.43 25.85 12.67 69.52 59.31 59.49 46.81 60.66 50.94 10.22 51.23 44.12 65.60 65.22 65.07 62.77
6.10 1.51 5.76 3.12 13.28 11.63 1.21 2.82 0.80 1.64 0.60 0.52 1.44 0.70 1.99 0.60 0.68 1.78 0.68 1.92 2.34 2.41 0.68 2.13 0.68 12.10 2.38 0.84 0.85 1.12 0.95 0.43 1.10 0.76 0.93 0.68 0.34 0.93
Weight of; GF = grain-size fraction dealt with heavy mineral separation, S = sulfides, barite, fahlore, apatite and rutile in GF, WH = whole heavy minerals in GF, HT = heavy transparent minerals in GF, M = magnetic minerals in GF, L =light fractions in GF, and T = total amounts of sediments treated. miniferans. commonly
Amorphous
Mn
precipitates
occur
p e r c e n t a g e s o f sulfides, b a r i t e , f a h l o r e , a p a t i t e a n d
on sediment particles and rock frag-
r u t i l e (S) b y w h o l e h e a v y m i n e r a l s ( W H ) i n g r a i n -
m e n t s as films.
size f r a c t i o n ( G F ) d e a l t w i t h h e a v y m i n e r a l a n a l y s i s
Mineral abundances and distribution The abundances seafloor sediments
o f h e a v y m i n e r a l f r a c t i o n s in a r e l i s t e d i n T a b l e 2. W e i g h t
r a n g e f r o m 0.23 t o 7.32. W e i g h t p e r c e n t a g e s o f S / G F a r e f r o m o n l y 0.01 t o 0.31. W e i g h t p e r c e n t ages of heavy transparent minerals (HT) by GF a r e f r o m 0 . 0 2 t o 2.65 a n d t h o s e o f m a g n e t i c m i n e r a l s ( M ) b y G F r a n g e f r o m 0.21 t o 7.82.
H Y D R O T H E R M A L C O N T R I B U T I O N TO S E A F L O O R S E D I M E N T S
125
wt.%
wt.%
0
0
1 2 3 4 5 6
20
40
f
i
60
80
i
I
G3602i RC617-1
~
north
G3340~ RS92<
ba
RS93
py
G3598< . gn ~ c
RC574-1 - ~ fa
de,a
RC612-1 RC611-1 '( G3599!
. / - -.><2-
G3600'c G3601 !
Fig. 3. Approximate abundance of individual ore minerals in S of the surface sediment samples.
3
• S/GF A HT/GF
A r= 0 . 8 7 / ~
2-
A A~" "'A
v
-
0 0
2
n
..*"
J,,*'-#. 4
A
6
8
Wt. % (M/GF) Fig. 4. Diagram of M/GF versus S/GF and HT/GF in the caldera sediments.
Mineral abundances of heavy fractions in each sediment sample are, in decreasing order, M/GF, HT/GF and S/GF. Mineral abundances in S of each sediment sample generally tend to decrease as follows: pyrite, barite or marcasite, sphalerie or chalcopyrite, fahlore and galena (Fig. 3). The abundance of rutile in S seems to be similar to that of sphalerite and chalcopyrite except for three locations (G3601, G3602 and RC617-1) with about
12wt.%. Apatite occurs in trace amounts. Relationships between HT/GF and M/GF correlate well (r=0.87), suggesting similar hydraulic behaviors (Fig. 4). The abundance of barite negatively correlates with those of pyrite ( r = - 0 . 7 9 ) and marcasite ( r = - 0 . 7 0 ) . Fe-oxides consist of abundant magnetite and trace ilmenite and hematite. Silicates are abundant feldspar and quartz, common clino- and ortho-pyroxene, and trace hornblende, epidote, chlorite and sericite. Surficial distribution of sphalerite, chalcopyrite, galena and fahlore concentrate in the western sediment samples (G3598, RC574-1 and 612-1), and the total abundance of them in S amounts to about 10 wt.%. Concerning vertical distribution of S in each RC sample, relatively high abundances of S/GF seem to be distributed in subsurface layers of the RC samples but the mineral associations of sphalerite, chalcopyrite, galena, fahlore and probably barite in S generally tend to concentrate in surface sediments. The vertical distribution of marcasite in RC samples is likely to increase toward the deeper parts of the samples (RC574, 611 and 617). In contrast, barite decreases toward the deeper parts of the RC samples. Pyrite is ubiquitously distributed in both the surface and subsurface sediments of the caldera floor.
TABLE 3 Trace element composition of the seafloor sediments from the Myojinsho submarine caldera
Sample G3340 (surface) G3598 (surface) G3599 (surface) G3600 (surface) G3601 (surface) G3602 (surface) RC5741 (top) RC5742 RC5743 RC5744 RC5745 RC5746 (bottom) RC611-1 (top) RC611-2 RC611-3 RC611-4 RC611-5 (bottom) RC612-1 (top) RC612-2 RC612-3 RC612-4 RC612-5 RC612-6 (bottom) RC617-1 (top) RC617-2 RC617-3 RC617-4 RC617-5 RC617-6 RC617-7 RC617-8 RC617-9 RC617-10 RC617-11 RC617-12 RC617-13 (bottom) RS92 (surface) RS93 (surface) a :
Au a
Zn b
p.p.b,
p.p.m.
<5 6 5 <5 <5 <5 6 16 <5 <5 5 <5 <5 <5 <5 <5 5 9 <5 5 <5 10 6 <5 <5 <5 <5 <5 9 <5 5 <5 <5 <5 <5 <5 <5 <5
81.6 170.0 92.9 85.6 84.7 79.5 95.2 113.0 81.0 77.5 77.8 76.5 88.1 79.7 81.8 82.6 82.3 114.0 80.2 90.8 80.6 78.5 90.4 79.5 72.2 77.9 78.8 76.9 79.5 68.8 75.8 80.0 75.3 78.2 102.0 81.6 79.5 110
INNA; b = DCP; and c = XRF.
pb b
Cu b
Co a
Ni b
Ba c
Sb a
As a
Ag b
Cd b
Vb
Rb c
Sr c
Cr a
Th a
Ua
7 24 15 7 6 4 14 37 3 2 4 <2 7 5 4 3 5 19 2 4 <2 7 9 3 3 2 3 2 6 4 2 3 2 <2 <2 <2 7 19
71.6 86.2 76.8 76.2 62.0 54.2 82.6 95.6 93.1 61.5 92.5 59.3 78.6 77.6 89.7 77.7 80.5 92.3 88.5 103.0 63.9 81.7 112.0 54.3 75.7 68.6 89.6 67.9 73.1 98.7 73.1 68.9 64.0 69.9 67.9 68.7 100.0 96.3
18 15 16 16 18 18 16 17 16 18 16 19 16 14 17 18 14 17 17 16 19 14 15 17 18 19 17 19 17 15 19 20 19 19 26 20 19 17
12 25 16 8 23 20 13 10 6 9 8 9 13 8 9 9 7 11 9 8 8 11 8 24 8 21 12 17 9 6 8 20 24 20 30 23 12 12
157 387 186 182 131 138 295 371 240 152 231 187 181 285 213 210 262 243 213 250 196 265 272 134 221 149 229 188 214 246 226 146 211 181 160 145 308 381
0.2 1.3 0.6 0.3 0.2 0.3 0.5 0.8 0.2 0.2 0.2 0.2 0.4 0.4 0.2 0.3 0.3 0.7 0.3 0.4 <2 0.4 0.5 0.2 0.3 <.2 0.3 <.2 0.2 0.2 0.2 <0.2 0.2 <0.2 0.2 0.2 0.5 0.8
9 19 13 12 7 5 9 13 8 4 7 3 9 10 10 8 11 14 6 11 4 9 12 4 8 4 6 3 7 7 5 3 3 3 5 4 10 13
<0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 < 0.5 <0.5 < 0.5 <0.5 <0.5 < 0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5
< < < < < < <
189 184 157 175 192 190 168 167 181 163 158 163 164 141 139 157 150 182 131 163 118 153 157 156 161 167 155 166 177 189 201 161 153 164 188 199 191 163
41 39 40 42 24 40 54 33 40 38 31 27 36 37 18 31 30 35 30 33 34 23 31 32 13 30 34 47 34 26 27 26 40 35 33 24 20 22
259 315 297 242 231 265 245 244 206 225 223 235 228 226 226 225 199 252 225 220 229 219 215 336 239 250 217 239 229 222 249 224 233 232 207 231 259 344
20 37 15 5 37 28 18 7 5 7 <2 3 15 4 3 <2 3 8 3 3 4 4 3 36 3 27 6 22 2 2 5 28 29 25 45 35 7 12
0.7 0.8 0.8 0.8 0.7 0.6 0.6 0.6 0.7 0.5 0.8 < 0.5 0.8 0.6 0.7 0.7 0.6 0.9 0.7 0.7 0.5 1.0 0.7 <0.5 0.6 <0.5 0.6 0.5 0.7 0.8 0.6 0.5 0.7 0.8 0.5 0.6 <0.5 1.2
<0.5 <0.5 0.6 <0.5 <0.5 <0.5 <0.5 0.5 <0.5 <0.5 0.5 < 0.5 <0.5 0.5 <0.5 0.5 0.5 <0.5 <0.5 <0.5 <0.5 0.6 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5
1 1 1 1 1 1 1 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 <1 < 1 <1 < 1 < 1 < 1 < 1 < 1 < 1 1 <1 <1 <1 1 1 < 1 <1 <1 1 <1 I < 1 < 1
N >
127
HYDROTHERMALCONTRIBUTIONTO SEAFLOORSEDIMENTS TABLE 4 Major element composition of the seafloor sediments from the Myojinsho submarine caldera analyzed by XRF SiO2 A1203 (wt.%)
CaO
MgO
Na20
K20
Fe203
MnO
TiO 2
P205
LOI
SUM
Sample G3340 (surface) G3598 (surface) G3599 (surface) G3600 (surface) G3601 (surface) G3602 (surface) RC574-I (top) RC574-2 RC574-3 RC574-4 RC574-5 RC574-6 (bottom) RC611-1 (top) RC611-2 RC611-3 RC611-4 RC611-5 (bottom) RC612-I (top) RC612-2 RC612-3 RC612-4 RC612-5 RC612-6 (bottom) RC617-1 (top) RC617-2 RC617-3 RC617-4 RC617-5 RC617-6 RC617-7 RC617-8 RC617-9 RC617-10 RC617-11 RC617-12 RC617-13 (bottom) RS92 (surface) RS93 (surface)
60.4 58.6 58.8 61.8 60.7 60.0 61.0 60.7 61.4 60.9 61.7 59.9 62.4 62.6 61.7 61.5 62.3 60.7 61.0 61.2 61.8 63.0 61.6 58.9 61.0 60.1 61.1 59.8 60.4 61.6 60.1 59.9 61.4 60.9 58.5 59.6 59.4 57.2
6.61 7.09 6.95 5.99 6.84 7.18 6.30 6.11 5.70 6.67 5.34 6.73 5.81 5.55 5.77 5.88 5.06 6.29 6.16 5.25 6.28 5.01 4.97 8.26 6.09 6.78 6.10 6.86 6.55 5.59 6.58 6.89 6.46 6.66 6.71 6.79 6.57 7.88
2.31 2.32 2.18 2.12 2.53 2.58 2.31 2.36 2.31 2.75 2.16 2.78 2.04 2.05 2.22 2.36 1.95 2.25 2.49 2.14 2.52 1.96 1.97 2.64 2.38 2.77 2.39 2.75 2.55 2.30 2.69 2.92 2.60 2.72 3.40 2.90 2.80 2.60
4.02 4.06 4.24 4.30 4.11 3.90 4.03 4.05 4.06 3.68 3.95 3.90 4.30 4.22 4.23 4.18 4.32 4.14 4.05 4.15 4.06 4.25 3.95 3.96 4.16 3.99 4.17 4.05 4.00 4.21 3.98 4.03 3.99 4.02 3.54 3.90 3.70 3.80
1.02 1.00 1.08 1.00 0.84 0.86 0.90 0.83 0.91 0.71 0.95 0.67 1.02 0.96 0.86 0.80 0.99 1.07 0.77 0.89 0.78 0.86 0.94 0.90 0.75 0.82 0.86 0.80 0.82 0.88 0.72 0.71 0.84 0.77 0.73 0.70 0.86 1.00
6.60 6.42 6.24 6.23 6.75 6.60 6.71 7.12 6.78 7.46 6.81 7.39 6.18 6.11 6.74 6.83 6.16 6.54 6.97 6.84 6.69 6.07 6.70 6.37 6.89 7.25 6.88 7.08 7.24 6.51 7.29 7.56 6.88 7.04 10.00 7.71 7.38 6.92
0.16 0.24 0.28 0.16 0.16 0.16 0.16 0.14 0.13 0.15 0.13 0.15 0.14 0.13 0.13 0.14 0.13 0.16 0.14 0.13 0.15 0.13 0.15 0.15 0.14 0.15 0.14 0.15 0.14 0.13 0.15 0.15 0.15 0.14 0.18 0.17 0.15 0.15
0.571 0.556 0.554 0.566 0.581 0.578 0.575 0.604 0.606 0.648 0.609 0.637 0.555 0.551 0.590 0.602 0.566 0.573 0.602 0.584 0.599 0.556 0.590 0.538 0.595 0.630 0.614 0.609 0.621 0.603 0.613 0.649 0.605 0.602 0.880 0.687 0.607 0.586
0.12 0.12 0.13 0.13 0.11 0.10 0.12 0.15 0.12 0.11 0.12 0.10 0.11 0.11 0.12 0.12 0.11 0.13 0.11 0.12 0.09 0.13 0.12 0.09 0.12 0.09 0.11 0.10 0.13 0.11 0.10 0.10 0.09 0.09 0.10 0.09 0.13 0.16
3.69 5.23 6.00 3.38 2.62 3.08 3.54 3.38 3.23 1.38 3.69 2.15 3.08 3.54 3.23 2.46 3.85 4.00 2.31 4.15 2.15 3.38 4.92 3.54 3.23 2.00 2.92 2.31 2.77 3.23 2.31 1.54 1.54 1.38 1.23 1.69 2.85 5.39
100.2 99.8 100.3 100.1 100.3 100.3 100.6 100.4 100.2 100.4 100.2 100.1 100.2 100.6 100.5 100.0 100.0 100.3 100.0 100.1 100.6 99.9 100.5 100.5 100.3 100.2 100.4 100.3 100.5 100.2 100.3 100.3 100.2 100.2 100.3 100.2 99.73 100.19
14.6 14.1 13,8 14,4 15.0 15,2 14.9 14.9 14.9 15,9 14.7 15.6 14,5 14,7 14.8 15.1 14.5 14.4 15,3 14,6 15,4 14,5 14.5 15.1 14,9 15,6 15,0 15,7 15,2 15,0 15.7 15.8 15.6 15.8 15.0 15.9 15.2 14.4
Chemical composition of sediments and rocks Bulk chemical compositions of downcore and surface sediments from the Myojinsho submarine caldera are shown in Tables 3 and 4 and those of v o l c a n i c r o c k s d r e d g e d in t h e c a l d e r a a r e a l s o i n d i c a t e d i n T a b l e 5. Main sources to the caldera floor sediments seem to be limited to the rock components of the caldera wall and central cone because of the high
r e l i e f o f t h e c a l d e r a wall. T h e r e f o r e , t h e c h e m i c a l composition of caldera floor sediments should reflect b a s i c a l l y t h a t o f s o u r c e r o c k s ( m a i n l y v o l canic
rocks)
which
were
chosen
as b a c k g o u n d
v a l u e s i n o r d e r t o e x t r a c t t h e h i g h a n o m a l y in t h e caldera sediments. Elements with anomalous concentrations in the sediment samples were selected on the criteria that the ranges (mean values with one standard deviation) of the samples are higher than those of the volcanic rocks or slightly overlap.
128
K. IIZASA
TABLE 5 Major and trace element compositions of the dacitic rocks from the Myojinsho submarine caldera Sample
D1075 D1076-2-2 D1076-2-3 DI076-2-5 D1077-2
D1075 D1076-2-2 D1076-2-3 D1076-2-5 D1077-2
D1075 D1076-2-2 D1076-2-3 D1076-2-5 D1077-2
SiO2 a wt.%
TiO2 ~ wt.%
A1203¢ wt.%
Fe203 a wt.%
MnO a wt.%
MgO a wt.%
CaO a wt.%
Na20a wt.%
K20 a wt.%
P205 a
wt.%
LOI wt.%
Total wt.%
66.4 69.7 69.8 69.7 69.9
0.62 0.58 0.58 0.64 0.64
15.0 14.2 13.9 14.0 13.9
5.37 4.52 4.36 4.34 4.48
0.14 0.12 0.12 0.14 0.14
1.72 1.36 1.20 1.03 1.00
5.30 4.59 4.38 4.37 4.26
3.99 3.95 4.03 4.37 4.32
0.79 0.85 0.96 0.86 0.93
0.11 0.09 0.09 0. I 1 0.12
0.62 0.62 0.77 0.46 0.77
100.1 100.7 100.3 100.1 100.6
Rb a p.p.m,
Srb p.p.m,
Bab p.p.m,
Pb b p.p.m,
Ni b p.p.m,
Co b p.p.m,
Crb p.p.m,
Vb p.p.m,
Cu b p.p.m,
Zn b p.p.m,
As c p.p.m,
Sb p.p.m.
25 25 50 22 45
186 160 148 161 162
195 239 235 224 232
<2 <2 <2 <2 <2
4 43 34 52 48
10 8 8 6 S
15 109 75 110 102
99 81 74 35 35
37.0 20.7 17.7 9.1 7.9
65.3 66.5 64.7 74.4 79.1
3 3 4 4 4
0.3 0.2 0.2 0.4 0.3
Ag b p.p.m,
Cd b p.p.m,
Thc p.p.m,
U¢ p.p.m,
Au c p.p.b.
Remarks
0.8 <0.1 <0.1 <0.1 <0.1
<1 <1 <1 <1 < 1
<0.5 0.7 0.8 0.5 0.6
0.6 0.7 <0.5 <0.5 <0.5
<5 <5 <5 <5 <5
Pumice Gray, massive, pbcpx > opx Glassy, massive, pl > cpx > opx White thin layer in matrix, pbcp > opx Brownish black, layering, pbcpx > opx
a = XRF; b = ICP; and c = INNA. pl = plagioclase, cpx = clinopyroxene, opx = orthopyroxene.
To compare the chemical composition of sediment samples with volcanic rocks, seven representative sediments were rinsed several times by deionized water until no seasalt was detected as AgC1 in the rinsed water. The results are that Zn, Pb, Cu, Co, Ba, As, V, Sr, A1, Ca, Mg, Fe, Mn and LOI have positve anomaly (Fig. 5). Au concentrations (maximum 16 p.p.b.) are relatively enriched in several samples in comparison with those of the volcanic rocks but the values are eliminated from multivariate factor analysis because most of them are below the detection limit. Distribution o f some trace elements in sediments
The distribution of Zn, Cu, Ba, Pb, As, V, Sr and Co in surface sediments of the caldera floor is shown in Fig. 6. The former three elements tend to be enriched in the western floor sediments (G3598, RC574-1, RC612-1, RS92 and 93). As and Pb are relatively abundant in a sample of G3598. Sr is abundant in samples of G3598,
RC617-1 and RS93. V and Co do not significantly fluctuate in comparison with other elements. The vertical distributions of Zn and Cu in RC samples except for RC611 moderately fluctuate and those in RC574 and 612 samples seem to be relatively enriched in comparison with the other two RC samples. Ba in all RC samples varies significantly and that of an RC574-2 sample indicates the highest values in the four RC samples. V and Sr in RC574 and 611 samples are relatively constant values, but those in RC612 and 617 samples vary significantly. As and Co do not show significant variations in each RC sample, but As appears to be slightly enriched in RC574 and 612 samples of the four RC samples. Relationship between mineral abundance and some trace element concentration
If trace elements such as Zn, Cu, Pb, Ba and As can be considered as constituents represented by sphalerite, chalcopyrite (and fahlore), galena,
HYDROTHERMAL CONTRIBUTION TO SEAFLOOR SEDIMENTS
barite and fahlore, respectively, there may be meaningful relations between abundances of the minerals and the element concentrations. Concerning the seafloor sediments of the caldera, the relative abundances of the above-mentioned minerals appear to correlate well with the varying concentrations of each element represented by the minerals. This suggests that the mineral abundances in heavy fractions between 0.25 and 0.063 mm in this study are useful for grasping the tendancy of trace element concentrations.
A 400. 350 30o
~ Q-.
250 20o 150 lO~
Zn
Cu
Ba
V
Sr
Cr
SiO2
B 50
o~ .d
40 3(3
Q_
129
212
t
JL
10 0 Pb
Co
Ni
As
Rb
A120 3
Q
D
[]
D
!
e~
Sb
Th CaO MgO
K20 MnO P205 LOI N a 2 0 Fe203 TiO 2
Fig. 5. Diagrams of comparison of chemical composition of sediments (solid circle with bars) with that of volcanic rocks (open box). Each value indicates the range of a mean value with one standard deviation. (A) p.p.m, exceptfor SiO2 (wt.%), (B) p.p.m, except for A1203 (wt.%) and (C)wt.% except for Sb (p.p.m.) and Th (p.p.m.).
Factor loadings R-mode multivariate factor analysis with varimax rotation was employed to chemical data with anomalies which were selected by the comparison of background values of volcanic rocks from the caldera. The analysis revealed three major factors (Fig. 7). Factor 1 that accounts for 45% of the total data variance shows large positive loadings on Zn, Pb, Cu, Ba and As. This factor implies hydrothermal sources which can be interpreted as sphalerite, galena, chalcopyrite, barite and fahlore identified in the sediments of the caldera. Factor 2 accounts for 42% of the total variance and has signifincant positive loadings on Fe, Co, Mg and A1, and a negative one on LOI. This factor can be interpreted as anhydrous Fe-bearing materials, pyrite and marcasite as hydrothermal sulfides, and magnetite and ilmenite as detrital oxides; all of which were identified in the caldera sediments. The mode of occurrence and abundances of the sulfides are, however, distinctly different from those of oxides derived from volcanic rocks of the caldera. The factor 2 contains the mixture of hydrothermal and detrital materials. The reason why these hydrothermal Fe-sulfides do not appear in the element associations in factor 1 also suggests that the majority of the Fe-sulfides is different in origin from sulfides of factor 1. Therefore, the Fe-sulfides seem to be of detrital origin from pyritized rocks in another stage of hydrothermal activity. Factor 3 represents 13% of the total data variance and has high positive loadings on Sr, Ca and Mn, and a negative loading on Cu. This factor is
130
K. IIZASA
p.p.m. 100
0 n
G3602"
??
I
200 n
?v
I
p.p.m. 300
t
I
,
400
10
I
I
20
30 n
I
+,sr Co
RC617-1
north
G3340 RS92 RS93" westem part of the caldera
G3598 RC574-1 RC612-1 RC611-1 G3599
south to east
G3600G3601"
Fig. 6. Variations of some trace elements with anomaly in the surface sediments of the caldera.
related to hydrogenous origin as amorphous manganese hydroxide due to the presence of manganese hydroxide coatings on volcanic rocks and may possibly also be related to biogenic origin as foraminifera. Factor scores
High positive scores for factor 1 are mainly distributed in the western floor sediments which include a variety of ore minerals, sphalerite, chalcopyrite, galena, fahlore and barite. Varying positive scores also occur in the subsurface sediments of the northwestern and the southwestern floors. These distributions closely coincide with sites where various associations of the ore minerals are relatively abundant. Positive scores for factor 2 are distributed in the subsurface layers of western and northwestern floor and occur in two surface sediments west of the central cone. The effect of factor 2 of northwestern samples (RC617) is stronger than that of factor 1. Positive scores for factor 3 mainly occur in the surface floor sediment samples of the caldera, suggesting that Mn deposition widely took place on sediment surfaces.
Conclusions
(1) Hydrothermally-derived minerals occur in the seafloor sediments: chalcopyrite, sphalerite, galena, fahlore, pyrite, marcasite, barite, rutile, covellite, bornite, digenite, epidote, chlorite, carbonate (probably dolomite) and sericite. The associations of the hydrothermal minerals are chalcopyrite- pyrite- sphalerite- fahlore(- galena), pyrite-marcasite-sphalerite, barite-pyritemarcasite, barite-galena, barite-chalcopyritesphalerite-pyrite, sphalerite-chalcopyrite-bornitecovellite, covellite-digenite, pyrite-rutile(-barite), epidote-chlorite-feldspar-quartz, and sericitequartz-feldspar. Fahlore contains Ag. (2) The relative abundances of each ore mineral in sediment samples tend to decrease as follows: pyrite, barite or marcasite, sphalerite or chalcopyrite, fahlore and galena. The distribution and abundance of chalcopyrite, sphalerite, fahlore and galena seem to concentrate significantly in the surface sediment samples of the western floor of the caldera while those of pyrite are ubiquitously distributed in the surface and subsurface floor samples. The abundance of barite negatively correlates with that of pyrite and marcasite. (3) Detrital minerals consist of major magnetite,
131
HYDROTHERMALCONTRIBUTIONTO SEAFLOORSEDIMENTS Factor 1 (45 %) Hydrothermal source (sulfides, barite and fahlore)
-1 Zn Pb Cu Co Ba As V
Sr AI Ca Mg Fe Mn LOI
Factor 2 (42 %) Mixed sources (pyrite and marcasite from hydrothermally altered rocks, and magnetite from volcanic rocks)
0
-1 Zn Pb Cu Co Ba As V
Sr AI Ca Mg Fe Mn LOI
Factor 3 (13 %) Hydrogenous source (Mn hydroxides)
Acknowledgements
O"
-1 Zn Pb Cu Co Ba As V
Pb, Cu, Ba and As which represent a hydrothermal end member consisting of sphalerite, galena, chalcopyrite, barite and fahlore. Factor 2 has high positive loadings on Co, A1, Mg and Fe which signify mixed sources of detrital pyrite and marcasite from hydrothermally-altered rocks, and magnetite and ilmenite from volcanics of the caldera. Factor 3 represents large positive loadings on Sr, Ca and Mn, suggesting a hydrogenous source as Mn hydroxides. (6) The distribution of positive scores for factor 1 concentrates significantly in the western floor of the caldera. Positive scores for factor 2 are distributed in the western and northwestern floor. Positve scores for factor 3 are ubiquitous throughout the caldera floor. High factor scores for factor 1 together with mineralogical data indicate that Agand probably Au-bearing sulfide and barite mineralizations took place in the western floor of the caldera.
Sr AI Ca Mg Fe MnLOI
The author wish to thank officers and crews of R/V Hakurei-maru, and on-board scientists of GSJ for their help in the collection of the samples. He is indebted to Dr. P. Jarvis, GSJ, for his critical reading of the manuscript and Mr. M. Kojima, University of Tokyo, for his support on sample preparations. This study was supported by the fund of AIST, MITI.
Fig. 7. Factor loadings on element associations with anomaly in the caldera floor sediment samples.
References
and trace hematite and ilmenite, and are also clinoand ortho-pyroxene, hornblende, quartz, feldspar and apatite derived from volcanic rocks of the caldera. (4) Geochemically anomalous elements were selected as Zn, Pb, Cu, Co, Ba, As, V, Sr, A1, Ca, Mg, Fe, Mn and LOI. Au is partly enriched in several seafloor sediment samples as compared with volcanic rocks composing the caldora. (5) R-mode multivariate factor analysis for the chemical data with anomalies revealed that three major sources contributed to the seafloor sediments. Factor 1 shows positive loadings on Zn,
D y m o n d , J., 1981. Geochemistry of Nazca plate surface sediments: An evaluation of hydrothermal, biogenic, detrital, and hydrogenous sources. Geol. Soc. A m . Mem., 154: 133-173. Honza, E. and Tamaki, K., 1985. The Bonin arc. In: A.E.M. Nairn, F.G. Stehli and S. Uyeda (Editors), The Pacific Ocean. (The Ocean Basins and Margins, 7A.) Plenum, New York, pp. 459-502. Iizasa, K., 1993. Petrographic investigations of seafloor sediments from the K i t a - B a y o n n a i s e submarine caldera, Shichito-Iwojima Ridge, I z u - O g a s a w a r a Arc, northwestern Pacific. Mar. Geol., 112: 271-290. Iizasa, K., Yuasa, M. and Yokota, S., 1992. Mineralogy and geochemistry of volcanogenic sulfides from the Myojinsho submarine caldera, the Shichito-Iwojima Ridge, Izu+Ogasawara Arc, Northwestern Pacific. Mar. Geol., 108: 39-58.
132
Imbrie, J. and Van Andel, T.H., 1964. Vector analysis of heavymineral data. Geol. Soc. Am. Bull., 75: 1131-1156. Karig, D.E. and Moore, G.F., 1975. Tectonic complexities in the Bonin arc system. Tectonophysics, 27: 97-118. Kelley, K.D. and Kelley, D.L., 1992. Reconnaissance exploration geochemistry in the central Brooks Range, northern Alaska: implications for exploration of sediment-hosted zinc-lead-silver deposits. J. Geochem. Explor., 42: 273-300. Leinen, M. and Pisias, N., 1984. An objective technique for determining end-member compositions and for partitioning sediments according to their sources. Geochim. Cosmochim. Acta., 48: 47-62. McMurtry, G.M., De Carlo, E.H. and Kim, K.H., 1991. Accumulation rates, chemical partitioning, and Q-mode factor analysis of metalliferous sediments from the North Fiji Basin. Mar. Geol., 98: 271-295. Murphy, E., McMurtry, G.M., Kim, K.H. and DeCarlo, E.H., 1991. Geochemistry and geochronology of a hydrothermal ferromanganese deposit from the North Fiji Basin. Mar. Geol., 98: 297-312. Rittenhouse, G., 1943. Transportation and deposition of heavy minerals. Bull. Geol. Soc. Am., 54: 1725-1780. Takada, A. and Yuasa, M., 1990. Geological map, Hachijo Jima 1:200,000. Geol. Surv. Jpn. Tamaki, K., Tanahashi, M., Okuda, Y. and Honza, E., 1981. Seismic reflection profiling in the Ogasawara (Bonin) Arc and the northern Mariana Arc. In: E. Honza, E. Inoue and T. Ishihara (Editors), Geological Investigation of the Ogasawara (Bonin) and Northern Mariana Arcs. Geol. Surv. Jpn., Cruise Rep., 14:83 91.
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Taylor, B., Hussong, D. and Fryer, P., 1984. Rifting of the Bonin Arc. EOS, Trans. Am. Geophys. Union, 65: 1006. Urabe, T., Yuasa, M. and Nakao, S. and On-board Scientists, 1987. Hyrothermal sulfides from a submarine caldera in the Shichito-Iwojima Ridge, Northwestern Pacific. Mar. Geol., 74: 295-299. Urabe, T. and Kusakabe, M., 1990. Barite silica chimneys from the Sumisu Rift, Izu-Bonin Arc: possible analog to hematitic chert associated with Kuroko deposits. Earth Planet. Sci. Lett., 100: 283-290. Usui, A. and Nishimura, A., 1992. Submersible observations of hydrothermal manganese deposits on the Kaikata Seamount, Izu-Ogasawara (Bonin) Arc. Mar. Geol., 106: 203-216. Usui, A., Yuasa, M., Yokota, S., Nohara, M., Nishimura, A. and Murakami, F., 1986. Submarine hydrothermal manganese deposits from the Ogasawara (Bonin) arc, off the Japan islands. Mar. Geol., 73:311 322. Valentine, P.C. and Commeau, J.A., 1990. Fine-grained rutile in the Gulf of Maine--diagenetic origin, source rocks, and sedimentary environment of deposition. Econ. Geol., 85: 862-876. Yuasa, M. and Yokota, S., 1982. Hydrothermal manganese and ferromanganese concretions from seafloor of the Ogasawara arc-trench region, Northwestern Pacific. CCOP Tech. Bull., 15: 51-64. Yuasa, M., Murakami, F., Saito, E. and Watanabe, K., 1991. Submarine topography of seamounts on the volcanic front of the Izu-Ogasawara (Bonin) arc. Bull. Geol. Surv. Jpn., 42: 703-743.
119
Elsevier Science Publishers B.V., Amsterdam
Assessment of the hydrothermal contribution to seafloor sediments in the Myojinsho submarine caldera, Shichito-Iwojima ridge, Izu-Ogasawara arc, Japan Kokichi Iizasa
Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305, Japan (Received October 20, 1992; revision accepted May 21, 1993)
ABSTRACT Iizasa, K., 1993. Assessment of the hydrothermal contribution to seafloor sediments in the Myojinsho submarine caldera, Shichito-Iwojima ridge, Izu-Ogasawara arc, Japan. Mar. Geol., 114:119-132. To assess the hydrothermal contribution to sediments of submarine calderas on a volcanic front, seafloor sediments from the Myojinsho submarine caldera, Shichito-Iwojima ridge, Izu-Ogasawara arc, northwestern Pacific were chemically and mineralogically analysed. The caldera (ca. 5 × 6 km wide and 1114 m water depth) has a fiat-topped central cone (ca. 330 m deep) and is covered with muddy to sandy sediments dominated by fragments of volcanic ash with lesser amounts of foraminifera, radiolaria and diatoms. Microscopic observations of the heavy mineral fraction indicate that the sediments include sulfides, barite, fahlore and epidote associated with chlorite, suggesting the presence of hydrothermal activity. Ag is present in fahlore in polished sections according to SEM supplemented with EDX. The sediments are relatively enriched in Au, Zn, Pb, Cu, Co, As, Sr, Ca, Mg, Fe, Mn and Ba compared to fresh dacitic rocks sampled from the caldera. Factor analysis of the compositional data produces three major factors--detrital, hydrogenous and hydrothermal. One site nearest to a probable hydrothermal source was evaluated by the characteristic assemblage and abundance of hydrothermal minerals, and factor analysis of the sediment samples.
Introduction The I z u - O g a s a w a r a arc is one o f the a r c - t r e n c h systems in the western Pacific and a site o f hydrothermal activity. This has resulted in h y d r o t h e r m a l M n oxides on submarine volcanoes (Yuasa and Yokota, 1982; Usui et al., 1986; Usui and Nishimura, 1992), a h y d r o t h e r m a l l y altered rock with m a j o r pyrite in a submarine caldera (Urabe et al., 1987), barite-bearing silica chimneys in a back-arc rift (Urabe and K u s a k a b e , 1990), b a r i t e sulfide fragments and rocks with disseminated sulfides in a submarine caldera (Iizasa et al., 1992), and sulfide chimneys in a submarine volcano (S. K a s u g a and Y. K a t o , pers. c o m m u n . , 1992). The Geological Survey o f J a p a n (GSJ) conducted a research project to evaluate potential ore resources associated with the h y d r o t h e r m a l activity 0025-3227/93/$06.00
o f the Myojinsho submarine caldera in the I z u - O g a s a w a r a arc f r o m 1984 to 1989. The caldera (approximately 5 x 6 k m wide and 1114 m maxim u m water depth) is located a b o u t 420 k m south o f Tokyo, on the S h i c h i t o - I w o j i m a ridge (a volcanic front) associated with m a n y Q u a t e r n a r y volcanoes (Yuasa et al., 1991) (Fig. 1). The caldera has a flat-topped central cone (ca. 330 m deep), and its s o m m a s are the Bayonnaise R o c k s to the west and the Myojin Reef to the northeast (Takada and Yuasa, 1990; E. Saito and F. M u r a k a m i , pers. c o m m u n . , 1990). D u r i n g the G H (Geological Survey Ship Hakurei-Maru) 88-3 and 89-1 cruises to the caldera, fragmental massive barite-sulfide samples and m a n y rocks with disseminated sulfides were found in the caldera (Iizasa et al., 1992). Before the finding o f the barite-sulfide samples and rocks associated with sulfides, m e t h o d s for
© 1993 - - Elsevier Science Publishers B.V. All rights reserved.
120
K. IIZASA
57'
140°00'E
03'
Fig. 1. Locations of samples in the Myojinsho submarine caldera. Inset map indicates the northern part of the Izu-Ogasawara Arc and backarc rifts, Hachijo (h), Sumisu (s) and Torishima (t) (modified after Karig and Moore, 1975; Tamaki et al., 1981; Taylor et al., 1984; Honza and Tamaki, 1985). Dotted areas are the seafloor of the caldera. Depth contours in meters.
evaluating potential mineral resources in the submarine caldera were carried out. This study describes the mineralogical and geochemical methods applied to seafloor sediments of the caldera and assesses the hydrothermal contribution to the sediments.
Sample description and methods Four gravity core (RC) samples up to about 1 m length were recovered at the caldera floor and consist mainly of dark grey volcanic ash layers (clay- to coarse-sized grains) and pumice with lesser amounts of foraminifera, radiolaria and diatoms. One core sample (RC574) was disturbed by coring. One sample (RC612) included two brownish thin layers consisting of amorphous Fe hydroxides. The uppermost 15 cm of sediments were also obtained at the caldera floor by six box cores (G) and two rock samplers (RS) and consist of almost the same components as RC samples. Dredge (D) samples were recovered at three sites west of the central cone. These samples are abundant dacite and lesser amounts of (lapilli) tuff and
tuffaceous sandstone. The surfaces of tuff and dacite exposed to seawater are partly coated by Mn oxide films. The dacite is composed of clinoand ortho-pyroxene, hornblende, feldspar, quartz and opaque minerals (Iizasa et al., 1992). The analysis of heavy minerals is often used for establishing their provenance (e.g., Rittenhouse, 1943; Imbrie and Van Andel, 1964; Valentine and Commeau, 1990; Iizasa, 1993). According to Rittenhouse (1943) heavy minerals between #60 mesh (0.25 mm) and #230 mesh (0.063 mm) in river deposits generally make up about 50 to 70 wt.% of whole heavy minerals included in the bulk sediments, while remaining heavy minerals tend to exist in the finer fractions less than 0.063 mm. In this study the 60-230 mesh subsamples were tentatively taken for heavy mineral analysis although hydraulic conditions in the submarine caldera do not necessarily coincide with those in rivers. Sediments were dried at 100°C and 10 to 230 g were taken for heavy mineral analysis. Then they were disaggregated followed by sieving with mesh sizes, #18 ( l m m ) , #60 (0.25mm) and #230 (0.063 mm). The separated samples were weighed.
HYDROTHERMAL CONTRIBUTION TO SEAFLOOR SEDIMENTS
The finer sand fractions (0.25-0.063 mm) were subsequently separated by tetrabromoethane (sp. gr., about 2.96) for the 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 included in the opaque fraction during the isodynamic separation, they are dealt with as components of the opaque fraction. 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 the ore microscope. Representative polished sections were semiquantitatively analyzed by scanning electron microscope (SEM) JEOL JSM-6400 equipped with energy dispersive X-ray spectrometer (EDX). Thin sections of all transparent heavy and several light fractions were prepared for petrographic observation. Multivariate factor analysis for searching end members composing a large chemical dataset was effectively employed to marine sediments (Dymond, 1981; Leinen and Pisias, 1984; McMurtry et al., 1991; Murphy et al., 1991) and stream sediments (Kelley and Kelley, 1992). In this study the factor analysis is applied to element concentrations of caldera floor sediments. The bulk chemical composition of all of the sediments was determined for Au, Sb, As, Co, Cr, Th and U using instrumental neutron activation analysis (INNA), Ag, Zn, Pb, Cu, Ni, Cd and V using direct current plasma (DCP), and Ba, Rb and Sr using X-ray fluorescence spectrometry (XRF), and that of volcanic rocks for whole-rock analyses was carried out for major elements and Rb using XRF, Sr, Ba, Pb, Ni, Co, Cr, V, Cu, Zn, Ag and Cd using inductively coupled plasma emission spectrometry (ICP), and Au, Sb, As, Th and U using INNA at X-ray Assay Laboratories, Canada. Results and discussion
Mineral compositions in seafloor sediments Nonmagnetic heavy fractions after isodynamic separation consist of chalcopyrite, sphalerite,
121
galena, pyrite, marcasite, covellite, digenite, bornite, barite, fahlore, ruffle and apatite in opaque fractions, and clino- and ortho-pyroxene, hornblende, epidote with chlorite and carbonate (probably dolomite) in transparent fractions while magnetic ones are composed of magnetite, ilmenite and hematite. Some representative thin sections of light mineral fractions indicate an assemblage of sericite-quartz-feldspar, quartz, feldspar and calcite (foraminifera). Mineral associations in opaque fractions are described below. A SEM photomicrograph shows an aggregation of euhedral sphalerite, pyrite and barite, and materials among the minerals are C u - F e - S substance, probably chalcopyrite (Fig. 2A). An assemblage is composed of subhedral sphalerite and pyrite, and minute anhedral chalcopyrite in reflected light (Fig. 2B). Chalcopyrite occurs partly on sphalerite as films. A characteristic assemblage consists of subhedral fahlore and chalcopyrite, sphalerite along the chalcopyrite, covellite after the chalcopyrite and barite (Fig. 2C). The fahlore contains Ag according to EDX analysis. Relatively large subhedral grains of fahlore are associated with minute chalcopyrite (Fig. 2D). A radiation of chalcopyrite is embedded in a pyrite grain (Fig. 2E). A detrital grain of epidote radiation is associated with feldspar in transmitted light (Fig. 2F). Pyrite and marcasite grains are in general euhedral to subhedral forms and relatively weathered to hydroxides around their rims. Sphalerite, chatcopyrite, galena and fahlore exist in euhedral to anhedral forms, while barite and ruffle occur in euhedral shape as a whole. Digenite and covellite are usually associated with chalcopyrite as secondary minerals. Ruffle occurs mainly as aggregates of tetragonal and acicular shapes, and often as associations with pyrite and barite. These minerals show various associations of several grains, indicating hydrothermally-derived minerals because there are no observations of the various mineral associations in abundant volcanic rocks of the caldera (Table 1). Magnetite, ilmenite, hematite, clino- and orthopyroxene, hornblende, quartz and feldspar are components of volcanic rocks of the caldera. Apatite is usually associated with volcanic glass which represents detritus of volcanics composing the caldera. Calcite occurs as fragments of fora-
122
K. IIZASA
Fig. 2. Mineral associations in heavy fractions. (A) SEM photomicrograph of sphalerite (sp), pyrite (py), chalcopyrite (cp) and barite (ba) aggregate, (B) photomicrograph in reflected light showing sphalerite, pyrite and chalcopyrite aggregate, (C) barite, sphalerite, chalcopyrite, fahlore (fa) and covellite (cv) aggregate in reflected light, (D) relatively large fahlore and minute chalcopyrite grains in reflected light, (E) chalcopyrite radiation embedded in a pyrite grain in reflected light, and (F) epidote (ep) radiation with feldspar (re/) in transmitted light.
Hydrogenous
Detrital
chalcopyrite (cp) sphalerite (sp) covellite (cv) epidote (ep)
Hydrothermal
fahlore (fa)
sericite (ser)
pyrite (py) digenite (dg) rutile (ru)
Mn-oxyhydroxides
cp-ma in mt with gl
magnetite (mt) clinopyroxene calcite
ilmenite orthopyroxene
py-ru-ba, ep-ch-q-fel, ser-q-fel hematite quartz (q) volcanic glass (gl) feldspar (fel) apatite
cp-py-sp-fa-gn, py-ma-sp, ba-cp-sp-py, ba-py-ma-gn, sp-cp-bo-cv, cv-dg,
marcasite (ma) galena (gn) bornite (bo) chlorite (ch)
Mineral composition and association
Source
Mineral composition and association of seafloor sediments from the Myojinsho submarine caldera
TABLE 1
carbonate
barite (ba)
coatings on sediment particles and volcanic rocks
derivatives from volcanic rocks derivatives from volcanic rocks fragments of forams
primary hydrothermal minerals primary hydrothermal minerals secondary minerals after chalcopyrite minerals from hydrothermal alteration halos
Remarks
124
K. IIZASA
TABLE 2 Abundances of heavy and light mineral fractions of seafloor sediments from the Myojinsho submarine caldera Sample
S/WH (wt.%)
S/GF
HT/GF
M/GF
L/GF
M/WH
GF/T
rutile/S
G3340 (surface) G3598 (surface) G3599 (surface) G3600 (surface) G3601 (surface) G3602 (surface) RS92 (surface) RS93 (surface) RC574-I (top) RC574-2 RC574-3 RC574-4 RC574-5 RC574-6 (bottom) RC61 I-1 (top) RC611-2 RC611-3 RC611-4 RC611-5 (bottom) RC612-1 (top) RC612-2 RC612-3 RC612-4 RC612-5 RC612-6 (bottom) RC617-1 (top) RC617-2 RC617-3 RC617-4 RC617-5 RC617-6 RC617-7 RC617-8 RC617-9 RC617-10 RC617-11 RC617-12 RC617-13 (bottom)
0.28 6.37 0.95 0.91 0.23 0.23 1.21 1.26 1.16 0.56 0.92 0.67 0.85 0.63 1.12 5.67 7.32 1.33 4.35 3.55 6.64 6.68 2.08 2.58 6.18 0.29 1.13 1.30 1.97 1.60 5.17 3.09 2.23 1.15 1.16 1.00 1.50 1.04
0.01 0.18 0.01 0.03 0.01 0.01 0.04 0.03 0.03 0.02 0.03 0.00 0.02 0.05 0.02 0.08 0.09 0.08 0.06 0.10 0.31 0.04 0.07 0.13 0.02 0.02 0.06 0.09 0.12 0.08 0.27 0.02 0.07 0.11 0.09 0.10 0.11 0.09
1.01 0.88 0.43 1.09 1.22 1.79 1.05 0.61 1.05 1.25 0.91 0.11 0.80 2.65 0.35 0.25 0.18 1.44 0.34 0.65 0.79 0.08 0.91 1.02 0.02 1.72 1.26 1.74 1.12 1.29 1.11 0.12 0.75 2.11 1.48 2.09 1.58 1.53
2.34 1.75 0.92 1.68 3.10 4.42 1.84 1.71 1.86 2.00 2.01 0.31 1.34 5.56 1.26 1.04 0.96 4.56 0.87 2.15 3.60 0.45 2.20 3.77 0.21 6.70 4.28 5.15 4.85 3.74 3.86 0.62 2.31 7.65 5.83 7.82 5.60 7.23
96.65 97.19 98.64 97.20 95.66 93.78 97.08 97.65 97.05 96.73 97.06 99.58 97.84 91.75 98.37 98.64 98.77 93.91 98.74 97.09 95.30 99.43 96.82 95.09 99.75 91.55 94.39 93.02 93.90 94.90 94.76 99.24 96.86 90.13 92.61 89.99 92.70 91.16
69.57 62.40 67.23 60.01 71.59 71.04 62.90 72.63 63.16 61.11 69.23 72.22 61.54 67.31 77.44 76.23 77.91 74.96 69.07 74.00 76.66 79.84 69.20 76.72 85.78 79.34 76.35 73.78 79.61 73.18 73.65 81.69 73.74 77.50 78.88 78.13 76.81 81.69
40.70 45.75 9.46 30.23 52.08 77.87 23.80 27.75 25.09 23.03 3.04 39.90 6.91 57.77 25.79 17.73 15.26 59.85 13.58 34.16 8.28 8.84 41.43 25.85 12.67 69.52 59.31 59.49 46.81 60.66 50.94 10.22 51.23 44.12 65.60 65.22 65.07 62.77
6.10 1.51 5.76 3.12 13.28 11.63 1.21 2.82 0.80 1.64 0.60 0.52 1.44 0.70 1.99 0.60 0.68 1.78 0.68 1.92 2.34 2.41 0.68 2.13 0.68 12.10 2.38 0.84 0.85 1.12 0.95 0.43 1.10 0.76 0.93 0.68 0.34 0.93
Weight of; GF = grain-size fraction dealt with heavy mineral separation, S = sulfides, barite, fahlore, apatite and rutile in GF, WH = whole heavy minerals in GF, HT = heavy transparent minerals in GF, M = magnetic minerals in GF, L =light fractions in GF, and T = total amounts of sediments treated. miniferans. commonly
Amorphous
Mn
precipitates
occur
p e r c e n t a g e s o f sulfides, b a r i t e , f a h l o r e , a p a t i t e a n d
on sediment particles and rock frag-
r u t i l e (S) b y w h o l e h e a v y m i n e r a l s ( W H ) i n g r a i n -
m e n t s as films.
size f r a c t i o n ( G F ) d e a l t w i t h h e a v y m i n e r a l a n a l y s i s
Mineral abundances and distribution The abundances seafloor sediments
o f h e a v y m i n e r a l f r a c t i o n s in a r e l i s t e d i n T a b l e 2. W e i g h t
r a n g e f r o m 0.23 t o 7.32. W e i g h t p e r c e n t a g e s o f S / G F a r e f r o m o n l y 0.01 t o 0.31. W e i g h t p e r c e n t ages of heavy transparent minerals (HT) by GF a r e f r o m 0 . 0 2 t o 2.65 a n d t h o s e o f m a g n e t i c m i n e r a l s ( M ) b y G F r a n g e f r o m 0.21 t o 7.82.
H Y D R O T H E R M A L C O N T R I B U T I O N TO S E A F L O O R S E D I M E N T S
125
wt.%
wt.%
0
0
1 2 3 4 5 6
20
40
f
i
60
80
i
I
G3602i RC617-1
~
north
G3340~ RS92<
ba
RS93
py
G3598< . gn ~ c
RC574-1 - ~ fa
de,a
RC612-1 RC611-1 '( G3599!
. / - -.><2-
G3600'c G3601 !
Fig. 3. Approximate abundance of individual ore minerals in S of the surface sediment samples.
3
• S/GF A HT/GF
A r= 0 . 8 7 / ~
2-
A A~" "'A
v
-
0 0
2
n
..*"
J,,*'-#. 4
A
6
8
Wt. % (M/GF) Fig. 4. Diagram of M/GF versus S/GF and HT/GF in the caldera sediments.
Mineral abundances of heavy fractions in each sediment sample are, in decreasing order, M/GF, HT/GF and S/GF. Mineral abundances in S of each sediment sample generally tend to decrease as follows: pyrite, barite or marcasite, sphalerie or chalcopyrite, fahlore and galena (Fig. 3). The abundance of rutile in S seems to be similar to that of sphalerite and chalcopyrite except for three locations (G3601, G3602 and RC617-1) with about
12wt.%. Apatite occurs in trace amounts. Relationships between HT/GF and M/GF correlate well (r=0.87), suggesting similar hydraulic behaviors (Fig. 4). The abundance of barite negatively correlates with those of pyrite ( r = - 0 . 7 9 ) and marcasite ( r = - 0 . 7 0 ) . Fe-oxides consist of abundant magnetite and trace ilmenite and hematite. Silicates are abundant feldspar and quartz, common clino- and ortho-pyroxene, and trace hornblende, epidote, chlorite and sericite. Surficial distribution of sphalerite, chalcopyrite, galena and fahlore concentrate in the western sediment samples (G3598, RC574-1 and 612-1), and the total abundance of them in S amounts to about 10 wt.%. Concerning vertical distribution of S in each RC sample, relatively high abundances of S/GF seem to be distributed in subsurface layers of the RC samples but the mineral associations of sphalerite, chalcopyrite, galena, fahlore and probably barite in S generally tend to concentrate in surface sediments. The vertical distribution of marcasite in RC samples is likely to increase toward the deeper parts of the samples (RC574, 611 and 617). In contrast, barite decreases toward the deeper parts of the RC samples. Pyrite is ubiquitously distributed in both the surface and subsurface sediments of the caldera floor.
TABLE 3 Trace element composition of the seafloor sediments from the Myojinsho submarine caldera
Sample G3340 (surface) G3598 (surface) G3599 (surface) G3600 (surface) G3601 (surface) G3602 (surface) RC5741 (top) RC5742 RC5743 RC5744 RC5745 RC5746 (bottom) RC611-1 (top) RC611-2 RC611-3 RC611-4 RC611-5 (bottom) RC612-1 (top) RC612-2 RC612-3 RC612-4 RC612-5 RC612-6 (bottom) RC617-1 (top) RC617-2 RC617-3 RC617-4 RC617-5 RC617-6 RC617-7 RC617-8 RC617-9 RC617-10 RC617-11 RC617-12 RC617-13 (bottom) RS92 (surface) RS93 (surface) a :
Au a
Zn b
p.p.b,
p.p.m.
<5 6 5 <5 <5 <5 6 16 <5 <5 5 <5 <5 <5 <5 <5 5 9 <5 5 <5 10 6 <5 <5 <5 <5 <5 9 <5 5 <5 <5 <5 <5 <5 <5 <5
81.6 170.0 92.9 85.6 84.7 79.5 95.2 113.0 81.0 77.5 77.8 76.5 88.1 79.7 81.8 82.6 82.3 114.0 80.2 90.8 80.6 78.5 90.4 79.5 72.2 77.9 78.8 76.9 79.5 68.8 75.8 80.0 75.3 78.2 102.0 81.6 79.5 110
INNA; b = DCP; and c = XRF.
pb b
Cu b
Co a
Ni b
Ba c
Sb a
As a
Ag b
Cd b
Vb
Rb c
Sr c
Cr a
Th a
Ua
7 24 15 7 6 4 14 37 3 2 4 <2 7 5 4 3 5 19 2 4 <2 7 9 3 3 2 3 2 6 4 2 3 2 <2 <2 <2 7 19
71.6 86.2 76.8 76.2 62.0 54.2 82.6 95.6 93.1 61.5 92.5 59.3 78.6 77.6 89.7 77.7 80.5 92.3 88.5 103.0 63.9 81.7 112.0 54.3 75.7 68.6 89.6 67.9 73.1 98.7 73.1 68.9 64.0 69.9 67.9 68.7 100.0 96.3
18 15 16 16 18 18 16 17 16 18 16 19 16 14 17 18 14 17 17 16 19 14 15 17 18 19 17 19 17 15 19 20 19 19 26 20 19 17
12 25 16 8 23 20 13 10 6 9 8 9 13 8 9 9 7 11 9 8 8 11 8 24 8 21 12 17 9 6 8 20 24 20 30 23 12 12
157 387 186 182 131 138 295 371 240 152 231 187 181 285 213 210 262 243 213 250 196 265 272 134 221 149 229 188 214 246 226 146 211 181 160 145 308 381
0.2 1.3 0.6 0.3 0.2 0.3 0.5 0.8 0.2 0.2 0.2 0.2 0.4 0.4 0.2 0.3 0.3 0.7 0.3 0.4 <2 0.4 0.5 0.2 0.3 <.2 0.3 <.2 0.2 0.2 0.2 <0.2 0.2 <0.2 0.2 0.2 0.5 0.8
9 19 13 12 7 5 9 13 8 4 7 3 9 10 10 8 11 14 6 11 4 9 12 4 8 4 6 3 7 7 5 3 3 3 5 4 10 13
<0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 < 0.5 <0.5 < 0.5 <0.5 <0.5 < 0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5
< < < < < < <
189 184 157 175 192 190 168 167 181 163 158 163 164 141 139 157 150 182 131 163 118 153 157 156 161 167 155 166 177 189 201 161 153 164 188 199 191 163
41 39 40 42 24 40 54 33 40 38 31 27 36 37 18 31 30 35 30 33 34 23 31 32 13 30 34 47 34 26 27 26 40 35 33 24 20 22
259 315 297 242 231 265 245 244 206 225 223 235 228 226 226 225 199 252 225 220 229 219 215 336 239 250 217 239 229 222 249 224 233 232 207 231 259 344
20 37 15 5 37 28 18 7 5 7 <2 3 15 4 3 <2 3 8 3 3 4 4 3 36 3 27 6 22 2 2 5 28 29 25 45 35 7 12
0.7 0.8 0.8 0.8 0.7 0.6 0.6 0.6 0.7 0.5 0.8 < 0.5 0.8 0.6 0.7 0.7 0.6 0.9 0.7 0.7 0.5 1.0 0.7 <0.5 0.6 <0.5 0.6 0.5 0.7 0.8 0.6 0.5 0.7 0.8 0.5 0.6 <0.5 1.2
<0.5 <0.5 0.6 <0.5 <0.5 <0.5 <0.5 0.5 <0.5 <0.5 0.5 < 0.5 <0.5 0.5 <0.5 0.5 0.5 <0.5 <0.5 <0.5 <0.5 0.6 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5
1 1 1 1 1 1 1 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 <1 < 1 <1 < 1 < 1 < 1 < 1 < 1 < 1 1 <1 <1 <1 1 1 < 1 <1 <1 1 <1 I < 1 < 1
N >
127
HYDROTHERMALCONTRIBUTIONTO SEAFLOORSEDIMENTS TABLE 4 Major element composition of the seafloor sediments from the Myojinsho submarine caldera analyzed by XRF SiO2 A1203 (wt.%)
CaO
MgO
Na20
K20
Fe203
MnO
TiO 2
P205
LOI
SUM
Sample G3340 (surface) G3598 (surface) G3599 (surface) G3600 (surface) G3601 (surface) G3602 (surface) RC574-I (top) RC574-2 RC574-3 RC574-4 RC574-5 RC574-6 (bottom) RC611-1 (top) RC611-2 RC611-3 RC611-4 RC611-5 (bottom) RC612-I (top) RC612-2 RC612-3 RC612-4 RC612-5 RC612-6 (bottom) RC617-1 (top) RC617-2 RC617-3 RC617-4 RC617-5 RC617-6 RC617-7 RC617-8 RC617-9 RC617-10 RC617-11 RC617-12 RC617-13 (bottom) RS92 (surface) RS93 (surface)
60.4 58.6 58.8 61.8 60.7 60.0 61.0 60.7 61.4 60.9 61.7 59.9 62.4 62.6 61.7 61.5 62.3 60.7 61.0 61.2 61.8 63.0 61.6 58.9 61.0 60.1 61.1 59.8 60.4 61.6 60.1 59.9 61.4 60.9 58.5 59.6 59.4 57.2
6.61 7.09 6.95 5.99 6.84 7.18 6.30 6.11 5.70 6.67 5.34 6.73 5.81 5.55 5.77 5.88 5.06 6.29 6.16 5.25 6.28 5.01 4.97 8.26 6.09 6.78 6.10 6.86 6.55 5.59 6.58 6.89 6.46 6.66 6.71 6.79 6.57 7.88
2.31 2.32 2.18 2.12 2.53 2.58 2.31 2.36 2.31 2.75 2.16 2.78 2.04 2.05 2.22 2.36 1.95 2.25 2.49 2.14 2.52 1.96 1.97 2.64 2.38 2.77 2.39 2.75 2.55 2.30 2.69 2.92 2.60 2.72 3.40 2.90 2.80 2.60
4.02 4.06 4.24 4.30 4.11 3.90 4.03 4.05 4.06 3.68 3.95 3.90 4.30 4.22 4.23 4.18 4.32 4.14 4.05 4.15 4.06 4.25 3.95 3.96 4.16 3.99 4.17 4.05 4.00 4.21 3.98 4.03 3.99 4.02 3.54 3.90 3.70 3.80
1.02 1.00 1.08 1.00 0.84 0.86 0.90 0.83 0.91 0.71 0.95 0.67 1.02 0.96 0.86 0.80 0.99 1.07 0.77 0.89 0.78 0.86 0.94 0.90 0.75 0.82 0.86 0.80 0.82 0.88 0.72 0.71 0.84 0.77 0.73 0.70 0.86 1.00
6.60 6.42 6.24 6.23 6.75 6.60 6.71 7.12 6.78 7.46 6.81 7.39 6.18 6.11 6.74 6.83 6.16 6.54 6.97 6.84 6.69 6.07 6.70 6.37 6.89 7.25 6.88 7.08 7.24 6.51 7.29 7.56 6.88 7.04 10.00 7.71 7.38 6.92
0.16 0.24 0.28 0.16 0.16 0.16 0.16 0.14 0.13 0.15 0.13 0.15 0.14 0.13 0.13 0.14 0.13 0.16 0.14 0.13 0.15 0.13 0.15 0.15 0.14 0.15 0.14 0.15 0.14 0.13 0.15 0.15 0.15 0.14 0.18 0.17 0.15 0.15
0.571 0.556 0.554 0.566 0.581 0.578 0.575 0.604 0.606 0.648 0.609 0.637 0.555 0.551 0.590 0.602 0.566 0.573 0.602 0.584 0.599 0.556 0.590 0.538 0.595 0.630 0.614 0.609 0.621 0.603 0.613 0.649 0.605 0.602 0.880 0.687 0.607 0.586
0.12 0.12 0.13 0.13 0.11 0.10 0.12 0.15 0.12 0.11 0.12 0.10 0.11 0.11 0.12 0.12 0.11 0.13 0.11 0.12 0.09 0.13 0.12 0.09 0.12 0.09 0.11 0.10 0.13 0.11 0.10 0.10 0.09 0.09 0.10 0.09 0.13 0.16
3.69 5.23 6.00 3.38 2.62 3.08 3.54 3.38 3.23 1.38 3.69 2.15 3.08 3.54 3.23 2.46 3.85 4.00 2.31 4.15 2.15 3.38 4.92 3.54 3.23 2.00 2.92 2.31 2.77 3.23 2.31 1.54 1.54 1.38 1.23 1.69 2.85 5.39
100.2 99.8 100.3 100.1 100.3 100.3 100.6 100.4 100.2 100.4 100.2 100.1 100.2 100.6 100.5 100.0 100.0 100.3 100.0 100.1 100.6 99.9 100.5 100.5 100.3 100.2 100.4 100.3 100.5 100.2 100.3 100.3 100.2 100.2 100.3 100.2 99.73 100.19
14.6 14.1 13,8 14,4 15.0 15,2 14.9 14.9 14.9 15,9 14.7 15.6 14,5 14,7 14.8 15.1 14.5 14.4 15,3 14,6 15,4 14,5 14.5 15.1 14,9 15,6 15,0 15,7 15,2 15,0 15.7 15.8 15.6 15.8 15.0 15.9 15.2 14.4
Chemical composition of sediments and rocks Bulk chemical compositions of downcore and surface sediments from the Myojinsho submarine caldera are shown in Tables 3 and 4 and those of v o l c a n i c r o c k s d r e d g e d in t h e c a l d e r a a r e a l s o i n d i c a t e d i n T a b l e 5. Main sources to the caldera floor sediments seem to be limited to the rock components of the caldera wall and central cone because of the high
r e l i e f o f t h e c a l d e r a wall. T h e r e f o r e , t h e c h e m i c a l composition of caldera floor sediments should reflect b a s i c a l l y t h a t o f s o u r c e r o c k s ( m a i n l y v o l canic
rocks)
which
were
chosen
as b a c k g o u n d
v a l u e s i n o r d e r t o e x t r a c t t h e h i g h a n o m a l y in t h e caldera sediments. Elements with anomalous concentrations in the sediment samples were selected on the criteria that the ranges (mean values with one standard deviation) of the samples are higher than those of the volcanic rocks or slightly overlap.
128
K. IIZASA
TABLE 5 Major and trace element compositions of the dacitic rocks from the Myojinsho submarine caldera Sample
D1075 D1076-2-2 D1076-2-3 DI076-2-5 D1077-2
D1075 D1076-2-2 D1076-2-3 D1076-2-5 D1077-2
D1075 D1076-2-2 D1076-2-3 D1076-2-5 D1077-2
SiO2 a wt.%
TiO2 ~ wt.%
A1203¢ wt.%
Fe203 a wt.%
MnO a wt.%
MgO a wt.%
CaO a wt.%
Na20a wt.%
K20 a wt.%
P205 a
wt.%
LOI wt.%
Total wt.%
66.4 69.7 69.8 69.7 69.9
0.62 0.58 0.58 0.64 0.64
15.0 14.2 13.9 14.0 13.9
5.37 4.52 4.36 4.34 4.48
0.14 0.12 0.12 0.14 0.14
1.72 1.36 1.20 1.03 1.00
5.30 4.59 4.38 4.37 4.26
3.99 3.95 4.03 4.37 4.32
0.79 0.85 0.96 0.86 0.93
0.11 0.09 0.09 0. I 1 0.12
0.62 0.62 0.77 0.46 0.77
100.1 100.7 100.3 100.1 100.6
Rb a p.p.m,
Srb p.p.m,
Bab p.p.m,
Pb b p.p.m,
Ni b p.p.m,
Co b p.p.m,
Crb p.p.m,
Vb p.p.m,
Cu b p.p.m,
Zn b p.p.m,
As c p.p.m,
Sb p.p.m.
25 25 50 22 45
186 160 148 161 162
195 239 235 224 232
<2 <2 <2 <2 <2
4 43 34 52 48
10 8 8 6 S
15 109 75 110 102
99 81 74 35 35
37.0 20.7 17.7 9.1 7.9
65.3 66.5 64.7 74.4 79.1
3 3 4 4 4
0.3 0.2 0.2 0.4 0.3
Ag b p.p.m,
Cd b p.p.m,
Thc p.p.m,
U¢ p.p.m,
Au c p.p.b.
Remarks
0.8 <0.1 <0.1 <0.1 <0.1
<1 <1 <1 <1 < 1
<0.5 0.7 0.8 0.5 0.6
0.6 0.7 <0.5 <0.5 <0.5
<5 <5 <5 <5 <5
Pumice Gray, massive, pbcpx > opx Glassy, massive, pl > cpx > opx White thin layer in matrix, pbcp > opx Brownish black, layering, pbcpx > opx
a = XRF; b = ICP; and c = INNA. pl = plagioclase, cpx = clinopyroxene, opx = orthopyroxene.
To compare the chemical composition of sediment samples with volcanic rocks, seven representative sediments were rinsed several times by deionized water until no seasalt was detected as AgC1 in the rinsed water. The results are that Zn, Pb, Cu, Co, Ba, As, V, Sr, A1, Ca, Mg, Fe, Mn and LOI have positve anomaly (Fig. 5). Au concentrations (maximum 16 p.p.b.) are relatively enriched in several samples in comparison with those of the volcanic rocks but the values are eliminated from multivariate factor analysis because most of them are below the detection limit. Distribution o f some trace elements in sediments
The distribution of Zn, Cu, Ba, Pb, As, V, Sr and Co in surface sediments of the caldera floor is shown in Fig. 6. The former three elements tend to be enriched in the western floor sediments (G3598, RC574-1, RC612-1, RS92 and 93). As and Pb are relatively abundant in a sample of G3598. Sr is abundant in samples of G3598,
RC617-1 and RS93. V and Co do not significantly fluctuate in comparison with other elements. The vertical distributions of Zn and Cu in RC samples except for RC611 moderately fluctuate and those in RC574 and 612 samples seem to be relatively enriched in comparison with the other two RC samples. Ba in all RC samples varies significantly and that of an RC574-2 sample indicates the highest values in the four RC samples. V and Sr in RC574 and 611 samples are relatively constant values, but those in RC612 and 617 samples vary significantly. As and Co do not show significant variations in each RC sample, but As appears to be slightly enriched in RC574 and 612 samples of the four RC samples. Relationship between mineral abundance and some trace element concentration
If trace elements such as Zn, Cu, Pb, Ba and As can be considered as constituents represented by sphalerite, chalcopyrite (and fahlore), galena,
HYDROTHERMAL CONTRIBUTION TO SEAFLOOR SEDIMENTS
barite and fahlore, respectively, there may be meaningful relations between abundances of the minerals and the element concentrations. Concerning the seafloor sediments of the caldera, the relative abundances of the above-mentioned minerals appear to correlate well with the varying concentrations of each element represented by the minerals. This suggests that the mineral abundances in heavy fractions between 0.25 and 0.063 mm in this study are useful for grasping the tendancy of trace element concentrations.
A 400. 350 30o
~ Q-.
250 20o 150 lO~
Zn
Cu
Ba
V
Sr
Cr
SiO2
B 50
o~ .d
40 3(3
Q_
129
212
t
JL
10 0 Pb
Co
Ni
As
Rb
A120 3
Q
D
[]
D
!
e~
Sb
Th CaO MgO
K20 MnO P205 LOI N a 2 0 Fe203 TiO 2
Fig. 5. Diagrams of comparison of chemical composition of sediments (solid circle with bars) with that of volcanic rocks (open box). Each value indicates the range of a mean value with one standard deviation. (A) p.p.m, exceptfor SiO2 (wt.%), (B) p.p.m, except for A1203 (wt.%) and (C)wt.% except for Sb (p.p.m.) and Th (p.p.m.).
Factor loadings R-mode multivariate factor analysis with varimax rotation was employed to chemical data with anomalies which were selected by the comparison of background values of volcanic rocks from the caldera. The analysis revealed three major factors (Fig. 7). Factor 1 that accounts for 45% of the total data variance shows large positive loadings on Zn, Pb, Cu, Ba and As. This factor implies hydrothermal sources which can be interpreted as sphalerite, galena, chalcopyrite, barite and fahlore identified in the sediments of the caldera. Factor 2 accounts for 42% of the total variance and has signifincant positive loadings on Fe, Co, Mg and A1, and a negative one on LOI. This factor can be interpreted as anhydrous Fe-bearing materials, pyrite and marcasite as hydrothermal sulfides, and magnetite and ilmenite as detrital oxides; all of which were identified in the caldera sediments. The mode of occurrence and abundances of the sulfides are, however, distinctly different from those of oxides derived from volcanic rocks of the caldera. The factor 2 contains the mixture of hydrothermal and detrital materials. The reason why these hydrothermal Fe-sulfides do not appear in the element associations in factor 1 also suggests that the majority of the Fe-sulfides is different in origin from sulfides of factor 1. Therefore, the Fe-sulfides seem to be of detrital origin from pyritized rocks in another stage of hydrothermal activity. Factor 3 represents 13% of the total data variance and has high positive loadings on Sr, Ca and Mn, and a negative loading on Cu. This factor is
130
K. IIZASA
p.p.m. 100
0 n
G3602"
??
I
200 n
?v
I
p.p.m. 300
t
I
,
400
10
I
I
20
30 n
I
+,sr Co
RC617-1
north
G3340 RS92 RS93" westem part of the caldera
G3598 RC574-1 RC612-1 RC611-1 G3599
south to east
G3600G3601"
Fig. 6. Variations of some trace elements with anomaly in the surface sediments of the caldera.
related to hydrogenous origin as amorphous manganese hydroxide due to the presence of manganese hydroxide coatings on volcanic rocks and may possibly also be related to biogenic origin as foraminifera. Factor scores
High positive scores for factor 1 are mainly distributed in the western floor sediments which include a variety of ore minerals, sphalerite, chalcopyrite, galena, fahlore and barite. Varying positive scores also occur in the subsurface sediments of the northwestern and the southwestern floors. These distributions closely coincide with sites where various associations of the ore minerals are relatively abundant. Positive scores for factor 2 are distributed in the subsurface layers of western and northwestern floor and occur in two surface sediments west of the central cone. The effect of factor 2 of northwestern samples (RC617) is stronger than that of factor 1. Positive scores for factor 3 mainly occur in the surface floor sediment samples of the caldera, suggesting that Mn deposition widely took place on sediment surfaces.
Conclusions
(1) Hydrothermally-derived minerals occur in the seafloor sediments: chalcopyrite, sphalerite, galena, fahlore, pyrite, marcasite, barite, rutile, covellite, bornite, digenite, epidote, chlorite, carbonate (probably dolomite) and sericite. The associations of the hydrothermal minerals are chalcopyrite- pyrite- sphalerite- fahlore(- galena), pyrite-marcasite-sphalerite, barite-pyritemarcasite, barite-galena, barite-chalcopyritesphalerite-pyrite, sphalerite-chalcopyrite-bornitecovellite, covellite-digenite, pyrite-rutile(-barite), epidote-chlorite-feldspar-quartz, and sericitequartz-feldspar. Fahlore contains Ag. (2) The relative abundances of each ore mineral in sediment samples tend to decrease as follows: pyrite, barite or marcasite, sphalerite or chalcopyrite, fahlore and galena. The distribution and abundance of chalcopyrite, sphalerite, fahlore and galena seem to concentrate significantly in the surface sediment samples of the western floor of the caldera while those of pyrite are ubiquitously distributed in the surface and subsurface floor samples. The abundance of barite negatively correlates with that of pyrite and marcasite. (3) Detrital minerals consist of major magnetite,
131
HYDROTHERMALCONTRIBUTIONTO SEAFLOORSEDIMENTS Factor 1 (45 %) Hydrothermal source (sulfides, barite and fahlore)
-1 Zn Pb Cu Co Ba As V
Sr AI Ca Mg Fe Mn LOI
Factor 2 (42 %) Mixed sources (pyrite and marcasite from hydrothermally altered rocks, and magnetite from volcanic rocks)
0
-1 Zn Pb Cu Co Ba As V
Sr AI Ca Mg Fe Mn LOI
Factor 3 (13 %) Hydrogenous source (Mn hydroxides)
Acknowledgements
O"
-1 Zn Pb Cu Co Ba As V
Pb, Cu, Ba and As which represent a hydrothermal end member consisting of sphalerite, galena, chalcopyrite, barite and fahlore. Factor 2 has high positive loadings on Co, A1, Mg and Fe which signify mixed sources of detrital pyrite and marcasite from hydrothermally-altered rocks, and magnetite and ilmenite from volcanics of the caldera. Factor 3 represents large positive loadings on Sr, Ca and Mn, suggesting a hydrogenous source as Mn hydroxides. (6) The distribution of positive scores for factor 1 concentrates significantly in the western floor of the caldera. Positive scores for factor 2 are distributed in the western and northwestern floor. Positve scores for factor 3 are ubiquitous throughout the caldera floor. High factor scores for factor 1 together with mineralogical data indicate that Agand probably Au-bearing sulfide and barite mineralizations took place in the western floor of the caldera.
Sr AI Ca Mg Fe MnLOI
The author wish to thank officers and crews of R/V Hakurei-maru, and on-board scientists of GSJ for their help in the collection of the samples. He is indebted to Dr. P. Jarvis, GSJ, for his critical reading of the manuscript and Mr. M. Kojima, University of Tokyo, for his support on sample preparations. This study was supported by the fund of AIST, MITI.
Fig. 7. Factor loadings on element associations with anomaly in the caldera floor sediment samples.
References
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