• Journal of the Mineralogical Society of Korea
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• v.25 no.4
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• pp.233-247
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• 2012

We have researched the distribution and characteristics of seafloor hydrothermal deposits for the development of economic mineral deposits in the Lau Basin, Tonga since 2009. In this study, we interpreted hydrothermal alteration around TA 26 seamounts of the Tofua volcanic arc using X-ray diffraction analysis for bulk sample and preferred-oriented specimen of clay fraction. We used 2 core samples and several surface samples. Plagioclase and quartz are dominant mineral in the basement rock, whereas kaolin mineral and smectite are superior in marine surface sediments. Especially sulfate and sulfide minerals such as gypsum, barite, sphalerite, and pyrite are predominant in the vent sediments. When we compare the mineral composition between basement rock and sea surface sediments, argillic alteration zone composed of kaolin mineral and smectite could be produced by hydrothermal fluids. Based on the downcore variation of mineral assemblages, most portion of MC08H-06 core could be interpreted as argillic alteration zone composed of kaolin mineral and smectite except top 2 cm area. Various sulfate or sulfide minerals and argillic alteration zone suggest a high probability of massive sulfide deposits in the seafloor of the TA 26 seamount.

• Journal of the Mineralogical Society of Korea
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• v.27 no.4
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• pp.169-181
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• 2014

Korea government has consistently investigated the development of economic mineral deposits in the Tofua volcanic arc, Tonga since 2008 for the secure of sea floor mineral resources. We studied the composition and distribution of minerals formed by hydrothermal activity around TA 25 seamounts of the Tofua volcanic arc, Lau Basin, Tonga, using X-ray diffraction analysis, scanning electron microscopy, X-ray fluorescence spectrometry, and inductively coupled plasma atomic emission spectrometry. We used 7 core samples and 9 surface sediment samples. Barite, sphalerite, and clinoclase are present in the most volcanic vent area. Gypsum, smectite, and kaolin mineral are distributed in vent A area, chalcopyrite, pyrite, smectite, and kaolin mineral are in vent B and C area, and gypsum, chalcopyrite, pyrite, and goethite are in vent D area. From the study of clay fraction, smectite and few kaolinite are detected in the most studied area except inner part of caldera, which suggest that argillic alteration are dominant in the volcanic vent areas. Various sulfide or arsenide minerals were found in the hydrothermal vent B, C, and D. The mineralogy and geochemistry suggest higher hydrothermal activities in volcanic vent B, C, and D compared to vent A and inner caldera area. Therefore higher probabilities of massive sulfide deposits may occur in hydrothermal vent B, C, and D.

• Economic and Environmental Geology
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• v.48 no.4
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• pp.273-285
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• 2015

The extensive hydrothermal deposits have been found, for the first time, on the western TA25 seamount caldera in the Tonga arc. The seafloor hydrothermal vents are active and immature, emitting the transparent fluids of which temperatures range from $150^{\circ}C$ to $242^{\circ}C$ (average=$203^{\circ}C$). The recovered hydrothermal sulfides are mainly composed of sphalerite, pyrite, marcasite, galena, chalcopyrite, covellite, tennantite, enargite and sulfates such as barite, gypsum/anhydrite. Predominant sphalerite categorize it into Zn-rich hydrothermal ore body. Zn-rich sulfide ores have minor enargite, indicating that mineralization occurred in high sulfidation environment. The proportion and FeS content of sphalerite increase from outside to inside of the hydrothermal ores, respectively. In particular, sphalerite has a great silver content (up to ~10 wt.%). Chalcopyrite is more frequently observed in mound than in the chimney, implying mineralization temperature in the mound is higher than in the chimney. Homogenization temperatures and salinities from fluid inclusions in barite at the mound range from $148^{\circ}C$ to $341^{\circ}C$ (average=$213^{\circ}C$) and 0.4 to 3.6 equiv. wt.% NaCl, respectively. Homogenization temperatures suggest that sulfides in the mound mineralized at a higher temperature (${\geq}200^{\circ}C$) than in the chimney.

• Economic and Environmental Geology
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• v.46 no.3
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• pp.207-214
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• 2013

To identify rock-magnetic properties of volcanogenic hydrothermal sulfide deposits, chimneys were obtained from the Tofua Arc in Southwest Pacific, using a remotely operated vehicle (ROV) and Grab with AV cameras (GTVs). Three different types of chimneys used in this study are a high-temperature chimney with venting fluid-temperature of about $200^{\circ}C$ (ROV01), a low-temperature chimney of about $80^{\circ}C$ (GTV01), and an inactive chimney (ROV02). Magnetic properties of ROV01 are dominated by pyrrhotite, except for the outermost that experienced severe oxidation. Concentration and grain-size of ROV01 pyrrhotite are relatively low and fine. For GTV01, both magnetic concentration and grain-size increase from interior to margin. Pyrrhotite, dominant in the core, becomes mixed with hematite in the rim of the chimney due to secondary oxidation. High concentration and large grain-size of magnetic minerals characterize the ROV02. Dominant magnetic phases are pyrrhotite, hematite and goethite. In particular, the outermost rim shows a presence of magnetite produced by magnetotactic bacterial activity. Such distinctive contrast in magnetic concentration, grain-size and mineralogy among three different types of chimney enables the rock-magnetic study to characterize an evolution of hydrothermal deposits.