[HRE13-06] SIMS分析により解明された沖縄トラフ海底熱水鉱床の極端な硫黄同位体分別
キーワード：硫黄同位体、二次イオン質量分析装置、沖縄トラフ、バクテリア硫酸還元、伊是名海穴、伊平屋北海丘
Seafloor hydrothermal deposit, one of the types of seafloor mineral resources, has long been paid attention as a future producer of Cu-Pb-Zn±Au±Ag resources. This type of sulfide deposit is mainly composed of pyrite, sphalerite, galena and chalcopyrite with other sulfide/sulfate minerals. Sulfur in the main constituent sulfide minerals has been generally derived from mixture of magmatic and seawater sulfur [1,2]. Here, we report an extreme sulfur isotope fractionation in pyrite grains due to the bacterial sulfate reduction from the two modern seafloor hydrothermal deposits of the Iheya-North Knoll and Izena Hole in the middle Okinawa Trough revealed by the SIMS analysis.
We used drilling core samples of the Iheya-North Knoll and Izena Hole obtained through the IODP Exp. 331 and cruise CK16-05 (Exp. 909). Based on the visual core descriptions and microscopic observations, the drill core sample in the subseafloor sulfide layer beneath a sediment of the Izena Hole has a pyrite texture of (1) framboid (including recrystallized one), (2) colloform (including marcasite) and (3) euhedral (including pyrrhotite pseudomorph) along with maturation processes/hydrothermal overprinting. Sulfur isotopes (d34S) of the framboidal, colloform and euhedral pyrites have ranges from -38.91 to -2.84‰ (-17.28 ± 10.21‰; n = 47, average ± 1SD), -13.63 to -2.96‰ (-7.36 ± 2.47‰; n = 29) and -13.43 to -3.80‰ (-6.78 ± 2.69‰; n = 19), respectively. The framboidal pyrite in the pumice layer above the subseafloor sulfide layer exhibits the especially narrow and light d34S from -34.31 to -37.13‰ (n = 6). Moreover, the subseafloor sulfide layer is considered to be formed by the replacement/mineralization of porous pumice layer based on the microscopic observations [3]. Similar sulfur isotope fractionation was also observed at the flank of the mound in the Iheya-North Knoll whose framboidal and euhedral pyrite grains have d34S ranging from -38.03 to -10.35‰ (-28.25 ± 9.84‰; n = 10) and +0.36 to +3.86‰ (+2.85 ± 1.11‰; n = 15), respectively. Combined with the recently reported extreme sulfur isotope fractionation in the Spanish and American volcanogenic massive sulfide (VMS) deposits [4,5], a replacement mineralization process beneath a seafloor using framboidal pyrite grains derived from bacterial sulfate reduction plays a key role to form a large-scale seafloor hydrothermal deposit.
[1] Ohmoto (1996) Ore Geol. Rev., 10, 135-177. [2] Shanks (2001) Rev. Mineral. Geochem., 43, 469-525. [3] Nozaki et al. (2018) AGU Fall Meeting Abstr. [4] Velasco-Acebes et al. (2019) Mineral. Deposita, 54, 913-934. [5] Slack et al. (2019) Chem. Geol., 513, 226-238.
We used drilling core samples of the Iheya-North Knoll and Izena Hole obtained through the IODP Exp. 331 and cruise CK16-05 (Exp. 909). Based on the visual core descriptions and microscopic observations, the drill core sample in the subseafloor sulfide layer beneath a sediment of the Izena Hole has a pyrite texture of (1) framboid (including recrystallized one), (2) colloform (including marcasite) and (3) euhedral (including pyrrhotite pseudomorph) along with maturation processes/hydrothermal overprinting. Sulfur isotopes (d34S) of the framboidal, colloform and euhedral pyrites have ranges from -38.91 to -2.84‰ (-17.28 ± 10.21‰; n = 47, average ± 1SD), -13.63 to -2.96‰ (-7.36 ± 2.47‰; n = 29) and -13.43 to -3.80‰ (-6.78 ± 2.69‰; n = 19), respectively. The framboidal pyrite in the pumice layer above the subseafloor sulfide layer exhibits the especially narrow and light d34S from -34.31 to -37.13‰ (n = 6). Moreover, the subseafloor sulfide layer is considered to be formed by the replacement/mineralization of porous pumice layer based on the microscopic observations [3]. Similar sulfur isotope fractionation was also observed at the flank of the mound in the Iheya-North Knoll whose framboidal and euhedral pyrite grains have d34S ranging from -38.03 to -10.35‰ (-28.25 ± 9.84‰; n = 10) and +0.36 to +3.86‰ (+2.85 ± 1.11‰; n = 15), respectively. Combined with the recently reported extreme sulfur isotope fractionation in the Spanish and American volcanogenic massive sulfide (VMS) deposits [4,5], a replacement mineralization process beneath a seafloor using framboidal pyrite grains derived from bacterial sulfate reduction plays a key role to form a large-scale seafloor hydrothermal deposit.
[1] Ohmoto (1996) Ore Geol. Rev., 10, 135-177. [2] Shanks (2001) Rev. Mineral. Geochem., 43, 469-525. [3] Nozaki et al. (2018) AGU Fall Meeting Abstr. [4] Velasco-Acebes et al. (2019) Mineral. Deposita, 54, 913-934. [5] Slack et al. (2019) Chem. Geol., 513, 226-238.