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11.
Association of Electrum and Calcite and Its Significance to the Genesis of the Hishikari Gold Deposits, Southern Kyushu, Japan 总被引:1,自引:1,他引:1
Abstract. Mineral assemblage, precipitation sequence and textures of the gold‐bearing veins from the Hishikari epithermal vein‐type deposits, southern Kyushu, Japan, were examined. In addition, fluid inclusion microthermometry and carbon and oxygen isotopic compositions of calcite were determined. Calcite, and that replaced by quartz, were commonly observed throughout the precipitation sequence of the veins. Thus, calcite must be a more common gangue constituent initially than observed presently. Association of calcite and electrum is observed immediately subsequent to columnar adularia in some vein samples. In addition, close association of electrum with pseudo‐acicular quartz, and electrum with truscottite were observed. The initial coprecipitation of electrum and calcite might be a common phenomenon in the gold‐bearing veins at the Hishikari deposits. The Th (homogenization temperature) data from the Honko‐Sanjin deposits are generally higher than those from the Yamada deposit. Samples that show association of calcite and electrum yielded higher Th (206–217°C, average) than the Th data from calcite associated with low‐grade Au ore or barren (180–204°C, average). The measured Tm (temperature of last melting point of ice) range from ‐0.4 to 0.0°C. The result suggests that the salinity of the hydrothermal solution was low during the precipitation both of calcite associated with Au mineralization and of barren calcite. Fluid inclusion evidence suggestive of boiling of hydrothermal solution for the precipitation of calcite was not recognized in the present work. The δ13C and δ18O values of calcite range from ‐10.8 to —4.7 % and from +3.2 to +15.2 %, respectively. The δ13C value of H2CO3 and the δ18O value of H2O in the hydrothermal fluids calculated assuming isotopic equilibrium with calcite using the temperature obtained by fluid inclusion microthermometry, range from ‐14.4 to ‐9.1 %, and from ‐6.2 to +5.5 %, respectively. Thus, the calculated δ18O values of H2O for calcite further confirm the presence of the 18O‐enriched ore fluids during the mineralization at the Hishikari deposits. The hydrothermal solution isotopically equilibrated with the sedimentary basement rocks was responsible for the gold mineralization associated with calcite. 相似文献
12.
Genji Saito Kozo Uto Kohei Kazahaya Hiroshi Shinohara Yoshihisa Kawanabe Hisao Satoh 《Bulletin of Volcanology》2005,67(3):268-280
Among the series of eruptions at Miyakejima volcano in 2000, the largest summit explosion occurred on 18 August 2000. During this explosion, vesiculated bombs and lapilli having cauliflower-like shapes were ejected as essential products. Petrological observation and chemical analyses of the essential ejecta and melt inclusions were carried out in order to investigate magma ascent and eruption processes. SEM images indicate that the essential bombs and lapilli have similar textures, which have many tiny bubbles, crystal-rich and glass-poor groundmass and microphenocrysts of plagioclase, augite and olivine. Black ash particles, which compose 40% of the air-fall ash from the explosion, also have similar textures to the essential bombs. Whole-rock analyses show that the chemical composition of all essential ejecta is basaltic (SiO2=51–52 wt%). Chemical analyses of melt inclusions in plagioclase and olivine phenocrysts indicate that melt in the magma had 0.9–1.9 wt% H2O, <0.011 wt% CO2, 0.04–0.17 wt% S and 0.06–0.1 wt% Cl. The variation in volatile content suggests degassing of the magma during ascent up to a depth of about 1 km. The ratio of H2O and S content of melt inclusions is similar to that of volcanic gas, which has been intensely and continuously emitted from the summit since the end of August 2000, indicating that the 18 August magma is the source of the gas emission. Based on the volatile content of the melt inclusions and the volcanic gas composition, the initial bulk volatile content of the magma was estimated to be 1.6–1.9 wt% H2O, 0.08–0.1 wt% CO2, 0.11–0.17 wt% S and 0.06–0.07 wt% Cl. The basaltic magma ascended from a deeper chamber (10 km) due to decrease in magma density caused by volatile exsolution with pressure decrease. The highly vesiculated magma, which had at least 30 vol% bubbles, may have come into contact with ground water at sea level causing the large explosion of 18 August 2000.Editorial responsibility: S. Nakada, T. DuittAn erratum to this article can be found at 相似文献