| Co-Investigator(Kenkyū-buntansha) |
ISHIYAMA Daizo Akita University, Fuculty of Engineering and Resource Science, Associate Professor, 工学資源学部, 助教授 (30193361)
SAIKI Kazuto Osaka University, Graduate School of Science, Associate Professor, 大学院・理学研究科, 助教授 (50292363)
SHIKAZONO Naotatsu Keio University, Faculty of Science and Technology, Professor, 理工学部, 教授 (10011751)
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| Research Abstract |
High-performance ArF laser abration equipment for induced coupled plasma mass spectrometer (ICP-MS) were introduced in 1999 during this research period for analyses of covering various kinds of geological samples.
1)First topics of this study is the mineralogy and textures of active Vienna Wood seafloor hydrothermal deposit, Manus Basin, Papua New Guinea. Three hydrothermal sites are distributed in the Manus Back-arc basin : Pacmanus, Desmos and Vienna Wood sites. The mineralogy and formation environments for the massive sulphide ores from the Vienna Wood hydrothermal site were examined in this study.
The Vienna Wood hydrothermal site is hosted by MORB-type basalt in graben morphology. Three types of ore samples were collected from the Vienna Wood site : (1)top of 15m tall active chimneys (type IA), (2)small active chimneys 3 meters away from the big chimneys (type IB) and (3)dead chimneys sampled 20 meters away from the active chimney (type II). The type IA ores consist of massive anhyd
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rite with disseminated dendritic pyrite, sphalerite and chalcopyrite. Type IB ores consist of minor anhydrite, Fe-oxide, dendritic pyrite, subhedral pyrite, sphalerite and chalcopyrite. Type II ores are dominated by framboidal to idiomorphic pyrite, dendritic pyrite, subhedral pyrite, chalcopyrite and sphalerite with minor anhydrite and baryte.
Bulk chemical composition of the Vienna Wood ores show high Zn (16.5-40.0 wt.%), Cu (1.0-2.0 wt.%) and Pb (0.01-1.7 wt.%) contents in type IB than type IA (Zn = 0.3-2 wt.%, Cu = <0.1 wt.%, Pb =< 0.01 wt.%) or type II (Zn = 10-17 wt.%, Cu = <1.0wt.%, Pb =<0.4 wt.%). The mineralogy and formation environment for the Vienna Wood site are quite different from actively forming pollymetallic sulphide deposits associated with felsic volcanic rocks in the Pacmanus and Desmos sites.
2)Second target is the geochemical features of Cyprus-type ore deposits. The Ghuzayn Cu-Fe massive sulfide ore deposit is one of the typical Cyprus-type deposits hosted in Oman ophiolite which providing a uniqu opportunity to study geochemical conditions of paleo-oceanic crust. Vertical distribution of Cu, Zn, Co and Ni in sheeted-dike complex, altered pillow basalts and massive sulfide ores were examined to clarify the chemical mobility in oceanic crust because of hydrothermal circulation and precipitation of sulfide minerals.
Cu and Co are deposited as chalcopyrite and trace element in pyrite from hydrothermal solution. Among some circulating elements, preferential fixation of Zn and N to chlorite in altered rocks is responsible to the reason that these metals are not precipitated into massive ores.
3)Geochemical characteristics of sedimentary rocks of pre-and post-Kuroko mineralization, Akita, is the third topics. The Hokuroku Basin is the most famous Kuroko district for seafloor hydrothermal mineralization around 15 m.y.ago. To clarify the environmental changes of Middle Miocene age, trace elements in sedimentary rocks of pre-and post-Kuroko ages are analyzed by ICP-MS. Two Akita areas are chosen for the geochemical research on sedimentary rocks. First is the Wagaomono area located in the western flank of mountain range of NE Japanese Island-Arc. The Dainihon-Yoshino Kuroko deposit is distributed in the upper Nishikurosawa Formation of this region. Second is the Iwaki area located in "Akita Oil Field" of the coastal district of Japan Sea.
(LA)-ICP-MS for 34 elements in the sedimentary rocks and O and C isotope spectrometry for carbonates are employed. Systematics of SiO2-Fe2O3 reveals that the Wagaomono mudstones are likely to be the sediments of the mixed sources between biogenic and acidic tuffs origin. Mn/U ratios from the Nishikurosawa to the Onnagawa stages decrease dramatically from 1,200 to 24. These compositional changes show that sedimentary environments change clearly from oxidation state to reducing condition. Chondrite-normalized REE patterns of sedimentary rocks from all analyzed specimens show clear Eu negative anomalies. These Eu anomalies of sedimentary rocks from the Nishikurosawa Stage are larger than those the Onnagawa Stage in Wagaomono area. Mn crusts of carbonate nodules in the Iwaki area show the distinctive negative anomalies of cerium. Those data could indicate that the sedimentary conditions and their diagenetic processes of the Onnagawa age are much reducing than those of the Nishikurosawa age, in spite of almost the same abundance of Fe and Mn concentration. It is obvious that geochemical information of minor elements in fine-grained sedimentary rocks by rapid LA-ICP-MS analysis is quite effective to discuss the sedimentary environments. Less
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