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[R2-P-2] (entry) Bipyramidal quartz formed within chimneys of Kuroko deposits
Keywords:submarine hydrothermal vents, bipyramidal quartz, chimney, silica, kuroko
Mineral precipitation from the hydrothermal vent fluids has been thought to occur in response to quenching of hydrothermal fluids by mixing with cold seawater, but the mechanism of mineral precipitation still poorly understood. Kuroko deposit is one of the typical massive sulfide deposits. 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 have been reported in such chimney-like structure, and is thought to be the key to understanding the physico-chemical conditions within chimneys.
We analyzed two Kuroko-type ore samples containing quartz grains collected from the Hanaoka mine (Doyashiki and Matsumine deposits) in the Hokuroku district, NE Japan. The samples mainly composed of barite, galena, sphalerite, chalcopyrite, pyrite and quartz. The quartz grains show euhedral bipyramidal shape with a long axial length of 0.01-0.52 mm and aspect ratio of ~3, and contained fluid inclusions. We investigate the homogenization temperature, and they 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 ppm). Within the cylindrical vessel, temperature increased from 350℃ to 430℃ along the flow path. We conducted two runs with flow rate of 0.1 ml/min and the different durations 1 h and 11 h, respectively. In the 1 h run, amorphous silica and cristobalite were observed. In the 11 h run, amorphous silica and quartz were observed. The quartz grains showed euhedral bipyramidal shape with size of 5.9-103 micrometers. 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 in experiments, 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.
References: Shimazaki, H., Horikoshi, E., 1990. Mining Geology, 40, 313-321.
We analyzed two Kuroko-type ore samples containing quartz grains collected from the Hanaoka mine (Doyashiki and Matsumine deposits) in the Hokuroku district, NE Japan. The samples mainly composed of barite, galena, sphalerite, chalcopyrite, pyrite and quartz. The quartz grains show euhedral bipyramidal shape with a long axial length of 0.01-0.52 mm and aspect ratio of ~3, and contained fluid inclusions. We investigate the homogenization temperature, and they 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 ppm). Within the cylindrical vessel, temperature increased from 350℃ to 430℃ along the flow path. We conducted two runs with flow rate of 0.1 ml/min and the different durations 1 h and 11 h, respectively. In the 1 h run, amorphous silica and cristobalite were observed. In the 11 h run, amorphous silica and quartz were observed. The quartz grains showed euhedral bipyramidal shape with size of 5.9-103 micrometers. 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 in experiments, 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.
References: Shimazaki, H., Horikoshi, E., 1990. Mining Geology, 40, 313-321.