Hydrothermal sulfides from a submarine caldera in the Shichito-Iwojima Ridge, northwestern Pacific

Marine Geology, 74 (1987) 295-299

295

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

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HYDROTHERMAL SULFIDES FROM A SUBMARINE CALDERA IN THE SHICHITO-IWOJIMA RIDGE, NORTHWESTERN PACIFIC T. URABE 1, M. YUASA 1, S. NAKAO 1 and on-board scientists* 1Geological Survey o/Japan, 1-1-3 Yatabe-Higashi, Tsukuba 305 (Japan) (Received June 25, 1986; accepted for publication October 27, 1986)

Abstract Urabe, T., Yuasa, M., Nakao, S. and on-board scientists, 1987. Hydrothermal sulfides from a submarine caldera in the Shichito-Iwojima Ridge, northwestern Pacific. Mar. Geol., 74: 295-299. A large mineralized block of two-pyroxene andesite was dredged from a 913 m deep wall of a submarine caldera of the Kaikata Seamount, about 180 km north of Iwojima island. The Kaikata caldera (26 ° 42'N, 141 ° 05'E) is a seamount in active volcanic front, Schichito-Iwojima Ridge, to the west of Ogasawara (Bonin) Trench. Sulfide minerals observed in the sample are dominantly pyrite with minor amount of sphalerite and Cu-Fe-sulfide (chalcopyrite?). They occur in 5 mm thick veinlets together with euhedral quartz and chlorite. The high filling temperatures ( ~ 290 ° C) of fluid inclusions in the quartz crystals and enrichment of gold (max. 142 ppb) in vein materials both indicate a hydrothermal origin of the sulfides. This is, to our knowledge, the first discovery of hydrothermal precious and basemetal mineralizations associated with front volcanism in the marine environment.

Introduction The Geological Survey of Japan conducted geological and geophysical investigations off Izu-Ogasawara (Bonin) islands with R/V Hakurei-Maru in April and May, 1985. During cruise GH85-1, a large angular block (56 cm across) of mineralized andesite was dredged together with fresh two-pyroxene andesite and dolerite. The sampling site was the bottom of the submarine caldera of the Kaikata Seamount in the volcanic front, Shichito-Iwojima *T. Ishihara, M. Joshima, Y. Kinoshita, K. Kisimoto, T. Miyazaki, F. Murakami, A. Nishimura, M. Nohara, Y. Okamura, E. Saito, A. Usui, K. Watanabe and T. Yamazaki.

0025-3227/87/$03.50

Ridge, to the west of Ogasawara Trench. In May 1986, we returned to the sampling site and dredged more than 30 kg of altered andesite, all of which are intensely disseminated with sulfides. This is a preliminary report of the discovery of hydrothermal sulfide deposits, which is the first one of this kind from a submarine volcano in volcanic front.

Geologic setting The Izu-Ogasawara area is typical of much of the Mariana-type arc-trench system (Uyeda and Kanamori, 1979). It consists of several tectonic elements (Fig. 1 ), namely, from east to

© 1987 Elsevier Science Publishers B.V.

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west, Ogasawara (Bonin) Trench ( subduction zone), Ogasawara Ridge (non-volcanic fore-arc rift), Ogasawara Trough (fore-arc basin), Schichito-Iwojima Ridge (active volcanic island arc), Sumisu Depression (back-arc depression), and Nishi-shichito Ridge (volcanic ridge on arc crust) (Honza et al., 1981; Tamaki, 1985; Honza and Tamaki, 1985).

The island arc is divided by a major fault called the Sofugan Tectonic Line into a northern and a southern segment (Yuasa and Murakami, 1985; Yuasa, 1986). Fore-arc rift and back-arc depressions are lacking in the northern and southern segment, respectively. The Sumisu Depression is one of the several back-arc depressions which are named, from north to south, Hachijo, Sumisu, and Torishima. Previous workers (Karig and Moore, 1975; Tamaki, 1985) have pointed out that back-arc spreading is currently active along the axes of these depressions. The caldera of Kaikata Seamount is about 3 km across, and its wall is about 400 m high (Fig. 2 ). The temperature of the seawater inside the caldera (7.8 °C) was found to be a few degrees higher than that of the surrounding water (Fig. 3), but no signs of fluid venting were observed through deep-tow camera and video surveys. The sulfide-bearing rock was dredged from the foot of the southern wall of the caldera at a depth of 913 m (26°41.75'N, 141°04.96'E). Hydrothermal manganese crusts were found to cover a widely area on the northern and western flanks of the highest peak of the Kaikata Seamount (Usui et al., 1986). No manganese

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crust except a few small pieces were collected from the caldera bottom or other parts of the seamount.

Mineralogy The surface of the rock is not entirely coated with manganese oxide film. This suggests that the sample was derived from an outcrop on the caldera wall. Hydrothermal alteration (chloritization) and silicification are observed along the rims of network veinlets. The veinlets are about 5 mm thick and composed mostly of euhedral pyrite having edges of 0.1-0.5 mm and prismatic quartz 1 to 3 mm in length. Tiny sphalerite grains occur as solid inclusions in pyrite crystals. A Cu-Fe-S mineral, possibly chalcopyrite, is attached to the surface of pyrite (Fig. 4). Neutron activation analysis on the mixture of pyrite and quartz within the veinlet reveals its maximum gold and silver contents to be 142 ppb and 1.8 ppm, respectively (Table 1 ). These values are not within the economic grade but are significant anomalies from the geochemical

Fig. 4. S E M image of an C U - F e - S mineral (chalcopyrite?) on the surface of a pyrite crystal.Bar: 5 ILm.

point of view. The average gold and silver contents in altered host rocks ( n = 4) also show quite high values namely 45 ppb and 0.7 ppm, respectively. Hydrothermal manganese oxide (RS 16-1) from the western flank of the Kaikata Seamount (point Mn in Fig. 2) has gold and silver contents of 23 ppb and 0.1 ppm, respectively, which are much higher than the values in precious metal-bearing ferromanganese crusts recently reported from South Rennel Trough, southwestern Pacific (Bolton et al., 1986). TABLE1

Gold and silvercontents in vein constituents,host rocks, and hydrothermal manganese crustfrom the Kaikata Seamount (analyticalmethod: NAA) Sample No.

Au (ppb)

Ag (ppm)

Description

D695-1-21 D695-1-22 D695-1-23 D695-1-31 D695-1-32 D695-1-41 D695-1-42 RS16-1

43 142 57 40 41 20 23

0.8 1.2 1.8 0.4 0.1 1.4 0.1 0.1

Pyritized and altered andesite Mixture of vein pyrite and quartz Mixture of vein pyrite and quartz Pyritized and altered andesite Oxydized pyrite in vein Altered andesite Vein quartz (no sulfide) Hydrothermal manganese oxide

298 TABLE 2 Filling temperatures of fluid inclusions in quartz from the Kaikata caldera Inclusion No.

Fillingtemperature ( °C)

1 2 3 4

297 303 288 284

av.

298 302 289 284 293

Equipment used: LINKAM TH600.

Rare fluid inclusions of up to 20 pm in length are found in quartz crystals. The measured filling temperatures of these fluid inclusions ( n-~ 4 ) fall in a narrow range between 284 and 303 ° C with the average of 293 ° C (Table 2 ). If we assume that the salinity of the fluid is equal to that of seawater, the obtained temperature is calculated to be nearly equal to the boiling temperature of the fluid at a depth of 900 m, based on the data of Haas (1976) and Sourirajan and Kennedy (1962).

Discussion and conclusions Venting fluid from the active black smokers at 21 °N in the East Pacific Rise (RISE group, 1980) is not boiling but is following an adiabat below the liquid-vapor curve (Bischoff, 1980). On the other hand, there is an indirect evidence of boiling in the submarine hydrothermal system at the Mid-Atlantic Ridge (Delaney and Cosens, 1982 ). It is noteworthy that the precipitation of metals is known to be controlled by boiling in continental hydrothermal systems. For example, a zone of boiling at Broadland and Waiotapu geothermal systems, New Zealand, separates a lower zone of base metal precipitation from an upper precious-metal-rich zone (Ewers and Keays, 1977; Hedenquist and Henley, 1985). It is envisaged, therefore, that also in the marine environment, a deposit associated with a shallower submarine volcanichydrothermal system is more effectively

enriched in gold and silver than deposits formed in the deeper ocean, because of the greater possibility of boiling. This study gives no positive evidence of boiling because the ratios of gas bubbles to liquid in several fluid inclusions of quartz from the Kaikata Seamount do not vary significantly. Nevertheless, the remarkable enrichment of precious metals implies that near-boiling submarine hydrothermal activity at shallow depths can be effective in concentrating gold and silver. This idea is supported by the evidence that the northern end of the Shichito-Iwojima Ridge, Izu Peninsula, is known for its cluster of epithermal gold-quartz veins with ages between 1.36 and 3.7 Ma. (Metal Mining Agency of Japan, unpubl, data). It is, therefore, not improbable to find present-day mineralization of precious and base metals in the volcanic front of the Ogasawara Arc. There was a growing conviction that the "Black Smokers" may found in back-arc basin (Rona, 1973; Fujioka, 1983), inasmuch as the spreading process and volcanic activity are similar to those at the mid-oceanic ridge. Recent discovery of "dead" vent chimneys of sulfides from the Lau Basin back-arc spreading center (Hawkins, 1986) and possible sulfide mineralization at the "Mound area" in the Mariana Trough (Lonsdale and Hawkins, 1985) both justify the hypothesis. An island arc is far more important as a site ofvolcanogenic massive sulfide deposits than a mid-oceanic ridge, judging from these deposits now on land (e.g. Sawkins, 1976). For example, Kuroko-type mineralization is characteristic to the Mariana-type subduction to which the Izu-Ogasawara Arc belongs (Uyeda and Nishiwaki, 1980). The present discovery implies that not only the back-arc spreading center but also the volcanic front is important for the site of mineralization in an arc-trench system.

Acknowledgements We are indebted to Dr. Y. Shimazaki for his critical reading of the manuscript and to the

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