14:15 〜 14:30
[SCG53-03] 東青ヶ島海丘カルデラ熱水サイトの鉱物学的記載および鉛同位体比組成
キーワード：海底鉱物資源、海底熱水鉱床、東青ヶ島海丘カルデラ熱水サイト、Pb同位体、伊豆－小笠原弧、金鉱化作用
The Higashi Aogashima Knoll Caldera (HAKC) hydrothermal site is located 12 km east offshore from Aogashima Island at Izu-Bonin Arc. Four seafloor hydrothermal sites have been discovered in HAKC; Central Cone, Southeast, East and South of East Sites. All sulfide-rich rocks at these sites have high Au concentration whose average values exceeding 10 ppm. Among four sites, Central Cone and Southeast Sites located in the southern part of HAKC exhibit high Au concentrations ca. twice as much as that of East and South of East Sites in the northern part of HAKC. To understand the gold mineralization system at HAKC, with special focus on the origin of metal elements and concentration differences of Au among the four sites, we conducted mineralogical observations and geochemical analyses in this study; (1) microscopic observations, (2) measurement of galena composition using an electron probe micro analyzer (EPMA), (3) mineral identification using a X-ray diffractometry (XRD) and (4) in situ analysis of Pb isotopic composition of galena, bulk ore and volcanic rock using femto-second laser ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS) and MC-ICP-MS.
In chimney samples from active vents at East and South of East Sites, four types of galena were observed under the microscope; Type (1): Galena (<100 μm in size) occurring at the outside of sulfide mineral-rich part (KMROV#229R07), Type (2) : Galena (100–300 μm in size) at the outside of sulfide mineral-rich part (KMROV#229R09, R10 and R12), Type (3): Galena (200–500 μm in size) at the outside of sulfide mineral-rich part structures (KMROV#230R05) and Type (4): Galena (<100 μm in size) at the outside of dendritic pyrite-rich part (KMROV#229R08). These galena grains were analyzed with EPMA for 17 elements including Ag and Sb. Substantial substitution of Pb by Ag and Sb was observed in all samples. The Pb (mol%) vs. Ag + Sb (mol%) diagram shows higher degree in substitution of the Pb site by Ag and Sb in Types (1) and (4) than that of Types (2) and (3), with the maximum Ag + Sb concentrations in the Pb site of 12.8 mol% in Type (1).
Pb isotopic compositions of galena and bulk ore samples showed a systematic variation. The Pb isotopic compositions become gradually more radiogenic in the order of Central Cone, Southeast, South of East and East Sites. There are two plausible mechanisms to produce such an isotopic variation; (1) variable Pb isotopic compositions of volcanic rocks around Aogashima Island and HAKC and/or (2) higher contribution of sediment (eolian dust; yellow sand) at South of East and East Sites. The Pb isotopic compositions of South of East and East Sites are more radiogenic than those of other two sites, suggesting the higher contribution of an end-member having radiogenic Pb isotopic composition such as sediment. If the radiogenic Pb isotopic compositions of South of East and East Sites were indeed due to the contribution of sedimentary materials (scenario 2), hydrothermal fluid which deposited the sulfides at these sites could have substantially reacted with sediments before discharge, thermally decomposing the organic matters in the sediment. This can reduce the hydrothermal fluid and decrease the gold solubility, which might have precipitated gold in the subseafloor sediment before fluid discharge, potentially resulting in lower gold grade of these chimneys. Sulfur fugacity (fS2) estimated by previously reported sphalerite chemistry indicated that the fS2 of East and South of East Sites are equivalent or lower than that of Central Cone and Southeast Sites, partially supporting the above hypothesis.
In chimney samples from active vents at East and South of East Sites, four types of galena were observed under the microscope; Type (1): Galena (<100 μm in size) occurring at the outside of sulfide mineral-rich part (KMROV#229R07), Type (2) : Galena (100–300 μm in size) at the outside of sulfide mineral-rich part (KMROV#229R09, R10 and R12), Type (3): Galena (200–500 μm in size) at the outside of sulfide mineral-rich part structures (KMROV#230R05) and Type (4): Galena (<100 μm in size) at the outside of dendritic pyrite-rich part (KMROV#229R08). These galena grains were analyzed with EPMA for 17 elements including Ag and Sb. Substantial substitution of Pb by Ag and Sb was observed in all samples. The Pb (mol%) vs. Ag + Sb (mol%) diagram shows higher degree in substitution of the Pb site by Ag and Sb in Types (1) and (4) than that of Types (2) and (3), with the maximum Ag + Sb concentrations in the Pb site of 12.8 mol% in Type (1).
Pb isotopic compositions of galena and bulk ore samples showed a systematic variation. The Pb isotopic compositions become gradually more radiogenic in the order of Central Cone, Southeast, South of East and East Sites. There are two plausible mechanisms to produce such an isotopic variation; (1) variable Pb isotopic compositions of volcanic rocks around Aogashima Island and HAKC and/or (2) higher contribution of sediment (eolian dust; yellow sand) at South of East and East Sites. The Pb isotopic compositions of South of East and East Sites are more radiogenic than those of other two sites, suggesting the higher contribution of an end-member having radiogenic Pb isotopic composition such as sediment. If the radiogenic Pb isotopic compositions of South of East and East Sites were indeed due to the contribution of sedimentary materials (scenario 2), hydrothermal fluid which deposited the sulfides at these sites could have substantially reacted with sediments before discharge, thermally decomposing the organic matters in the sediment. This can reduce the hydrothermal fluid and decrease the gold solubility, which might have precipitated gold in the subseafloor sediment before fluid discharge, potentially resulting in lower gold grade of these chimneys. Sulfur fugacity (fS2) estimated by previously reported sphalerite chemistry indicated that the fS2 of East and South of East Sites are equivalent or lower than that of Central Cone and Southeast Sites, partially supporting the above hypothesis.