Chlorine Geochemistry of Dacites in the Hishikari Gold Mining Area, Kyushu, Japan
The volcanism in the Hishikari Au mining area, southern Kyushu, Japan, is characterized by continued bimodal mode (andesite and dacite). The mineralization stage overlaps with the dacite eruptions. The magma erupted these dacites (especially Kurozonsan Dacites) are characterized by high fO2, high Cl content, rather low crystallization temperature and emplacement at shallow depth. The features are similar to those of I type granitic rocks associated with mineralization. In such high fO2 and Cl rich magma, S should form an oxidized species such as SO2, and Cl should bind with Au to form AuCl2. In the country rock, the accretionary Shimanto Supergroup, the AuCl2 discharged from magma becomes unstable with decreasing fO2 and temperature. As the Shimanto Supergroup has abundant organic matter, the oxidized S species from magma is converted to H2S and/or HS-. The Au decomposed from AuCl2 would react with HSto form Au(HS)2. The Au(HS)2 would be transported upward through the thick Shimanto Supergroup. Near the surface, the mixing with oxic groundwater, as well as cooling and boiling, decomposes the Au(HS)2. Hence, the conversion from chloro-complex to bisulfide-complex might play an important role in forming the large reserve, the high grade and the high precious/base metal ratio of the Hishikari epithermal gold deposits. 2011年 8月 29日受付,同年 9月 26日受理 * IHI検査計測、研究開発事業部 (Research Development Division, IHI Inspection and Instrumentation Co., Ltd.) **鹿児島大学フロンテイアサイエンス研究推進センター (Frontier Science Research Center, Kagoshima University) Keyword: Hishikari epithermal gold deposits, Dacite, Chloro-complex, Bi-sulfide complex, Reduction and oxidation of ore fluid