3:30 PM - 3:45 PM
[SCG44-07] Formation of bipyramidal quartz within the hydrothermal vents: inights from Kuroko samples and hydrothermal experiments
Keywords:submarine hydrothermal vents, bipyramidal quartz, chimney, silica, kuroko, submarine hydrothermal deposits
Mineral precipitation from the hydrothermal vent fluids has been thought to occur in response to rapid cooling of hydrothermal fluids by mixing with cold seawater, but the mechanism of mineral precipitation still poorly understood. Kuroko is one of the typical massive sulfide deposits distributed along the western side of the central mountains of NE Japan, and formed at seafloor associated with rhyolitic magmatism. The Kuroko sample with the concentric structures represents the chimneys in submarine hydrothermal deposits (Shimazaki and Horikoshi, 1990). In addition to sulfides, bipyramidal quartz grains has been reported in such chimney-like structure, and is thought to be the key to understanding the physico-chemical conditions within chimneys. In this study, we analyzed the quartz-bearing Kuroko samples and conducted the hydrothermal flow-through experiments on silica precipitation. Comparing natural and experimental silica minerals, we discuss the formation and transport processes of quartz particles within the submarine hydrothermal vents.
We analyzed two Kuroko samples containing bipyramidal quartz grains collected from the Hanaoka mine (Doyashiki and Matsumine deposits) in the Hokuroku district, NE Japan. The samples show banded structure mainly composed of barite, galena, sphalerite, chalcopyrite, pyrite, and quartz. The quartz grains show euhedral bipyramidal shape with a long axial length of 0.07-0.45 mm and aspect ratio of 3-5. The bipyramidal quartz contains pyrite. The quartz contained fluid inclusions with size less than 5 micrometers, and showed homogenization temperature of 281-309℃. These features suggest that quartz crystals were formed from the high temperature fluids exceeding 300℃.
The silica precipitation experiments were conducted using the flow-through apparatus with vertical flow-path at constant pressure of 25 MPa with using high-silica aqueous solution (~300 mg/kgH2O) by dissolution of granite. Within the cylindrical vessel, temperature increased from 350℃ to 430℃ along the flow-path, and silica minerals precipitation occur at around 390℃ within the inner tube in response to the drop of solubility of silica minerals. The silica precipitates were caught by 23 stainless steel nets placed at 10 mm intervals. We conducted two runs with flow rate of 0.1 ml/min and the different durations 1 h and 11 h, respectively. The precipitation of various silica minerals was observed along the flow-path. In the 1h run, amorphous silica occurred at 9-10 cm from the inlet of the alumina tube, cristobalite at 10-14 cm. In contrast, the 11h run, the precipitation of spherical amorphous silica were observed at 7 cm, and quartz particles at 8-10 cm. The quartz grained showed euhedral bipyramidal shape with size of 5.9-103 micrometers. These occurrences suggests that the silica nucleated as amorphous silica, in response to its low interfacial energy with water, and the silica particle grew and aggregated, and then transformed into more stable phases, such as cristobalite or quartz via heterogeneous nucleation on metastable silica phases (Okamoto et al., 2015). The Stokes' equation predict that the silica particles smaller than ~14 micrometers can move upward in ascending flow velocity of 0.01 m/s in the experiments, which is consistent with the particle size observed within the reaction vessel, suggesting that the bipyramidal quartz formation occurred in suspension. The bipyramidal quartz grains in the Kuroko samples were similar to that produced in the hydrothermal experiments. Because of the larger grain size of quartz grains, the fluid ascending velocity within the chimney was estimated to be 0.04-1.6 m/s, which is consistent with that inferred for the active submarine hydrothermal vents (Nozaki et al. 2016).
References Shimazaki, H., Horikoshi, E., 1990. Mining Geology, 40, 313-321. Okamoto, A., Kuwatani, T., et al. 2015. Physical Review E, 92, 042130. Nozaki T., Ishibashi, J.I., Shimada, K., et al. 2016. Scientific Reports, 6, 22163.
We analyzed two Kuroko samples containing bipyramidal quartz grains collected from the Hanaoka mine (Doyashiki and Matsumine deposits) in the Hokuroku district, NE Japan. The samples show banded structure mainly composed of barite, galena, sphalerite, chalcopyrite, pyrite, and quartz. The quartz grains show euhedral bipyramidal shape with a long axial length of 0.07-0.45 mm and aspect ratio of 3-5. The bipyramidal quartz contains pyrite. The quartz contained fluid inclusions with size less than 5 micrometers, and showed homogenization temperature of 281-309℃. These features suggest that quartz crystals were formed from the high temperature fluids exceeding 300℃.
The silica precipitation experiments were conducted using the flow-through apparatus with vertical flow-path at constant pressure of 25 MPa with using high-silica aqueous solution (~300 mg/kgH2O) by dissolution of granite. Within the cylindrical vessel, temperature increased from 350℃ to 430℃ along the flow-path, and silica minerals precipitation occur at around 390℃ within the inner tube in response to the drop of solubility of silica minerals. The silica precipitates were caught by 23 stainless steel nets placed at 10 mm intervals. We conducted two runs with flow rate of 0.1 ml/min and the different durations 1 h and 11 h, respectively. The precipitation of various silica minerals was observed along the flow-path. In the 1h run, amorphous silica occurred at 9-10 cm from the inlet of the alumina tube, cristobalite at 10-14 cm. In contrast, the 11h run, the precipitation of spherical amorphous silica were observed at 7 cm, and quartz particles at 8-10 cm. The quartz grained showed euhedral bipyramidal shape with size of 5.9-103 micrometers. These occurrences suggests that the silica nucleated as amorphous silica, in response to its low interfacial energy with water, and the silica particle grew and aggregated, and then transformed into more stable phases, such as cristobalite or quartz via heterogeneous nucleation on metastable silica phases (Okamoto et al., 2015). The Stokes' equation predict that the silica particles smaller than ~14 micrometers can move upward in ascending flow velocity of 0.01 m/s in the experiments, which is consistent with the particle size observed within the reaction vessel, suggesting that the bipyramidal quartz formation occurred in suspension. The bipyramidal quartz grains in the Kuroko samples were similar to that produced in the hydrothermal experiments. Because of the larger grain size of quartz grains, the fluid ascending velocity within the chimney was estimated to be 0.04-1.6 m/s, which is consistent with that inferred for the active submarine hydrothermal vents (Nozaki et al. 2016).
References Shimazaki, H., Horikoshi, E., 1990. Mining Geology, 40, 313-321. Okamoto, A., Kuwatani, T., et al. 2015. Physical Review E, 92, 042130. Nozaki T., Ishibashi, J.I., Shimada, K., et al. 2016. Scientific Reports, 6, 22163.