Role of basement in epithermal deposits: The Kushikino and Hishikari gold deposits,southwestern Japan |
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| Institution: | 1. Geological Survey of Japan, AIST, Central 7, Tsukuba 305-8567, Japan;2. Research Institute for Humanity and Nature, Kamigamo, Kyoto 603-8047, Japan;1. Geological Survey of Japan, AIST, Central 7, Tsukuba 305-8567, Japan;2. Research Institute for Humanity and Nature, Kamigamo, Kyoto 603-8047, Japan;1. Economic Geology Unit, Department of Geology, University of Buea, P.O. Box 63, Buea, South West Region, Cameroon;2. Higher Institute of Science, Engineering and Technology, Cameroon Christian University, P.O. Box 5, Bali, North West Region, Cameroon;3. Mineral Resources, Technische Universität Clausthal, Clausthal-Zellerfeld, Germany;4. Department of Earth and Environmental Sciences, Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana;1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, China;2. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;3. ARC Center of Excellence for Core to Crust Fluid Systems (CCFS) and The Institute for Geoscience Research, Department of Applied Geology, Curtin University of Technology, Perth, WA 6845, Australia;4. School of Geosciences, The University of Sydney, NSW 2006, Australia;1. State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China;2. School of Environment, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand;3. Solid Earth Studies Laboratory, Department of Geology, University of Regina, Regina, Saskatchewan S4S 0A2, Canada;4. Zinjin Mining Group Company Limited, Xiamen, Fujian 361006, China;5. State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, China;1. Department of Earth and Environmental Sciences, Yamagata University, 1-4-12 Kojirakawa, Yamagata 990-8560, Japan;2. Neotectonics Research Group, Tono Geoscience Center, Japan Atomic Energy Agency, 959-31, Jorinji, Izumi-cho, Toki, Gifu 509-5102, Japan;3. Kyoto Fission-Track Co., Ltd., 44-4, Minamitajiri, Omiya, Kita-ku, Kyoto 603-8832, Japan;4. Mizunami Underground Research Laboratory, Geoscientific Research Department, Tono Geoscience Center, Sector of Decommissioning and Radioactive Waste Management, Japan Atomic Energy Agency, 1-64, Yamanouchi, Akiyo, Mizunami, Gifu 509-6132, Japan;5. Geochemical Research Center, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan;6. Department of Earth and Environmental Sciences, School of Science, Graduate School of Science and Technology, Kumamoto University, 2-39-1, Kurokami, Chuo-ku, Kumamoto 860-8555, Japan |
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| Abstract: | The magma–ore deposit relationship of most low-sulfidation epithermal ore deposits is still unclear, partly because many stable isotopic studies of such deposits have indicated the predominance of meteoric waters within hydrothermal fluids. However, it is certainly true that hydrothermal systems are ultimately driven by magmatic intrusions, and epithermal gold deposits might therefore be produced by magmatic activity even in deposits having has no obvious links to a magma. We re-examine the genesis of two typical low-sulfidation epithermal gold deposits, the Kushikino and Hishikari deposits, using structural simulations and isotope data.Many epithermal gold deposits including the Kushikino and Hishikari deposits have been discovered in Kyushu, southwestern Japan. The Kushikino deposit comprises fissure-filling veins within Neogene andesitic volcanics that overlie unconformably Cretaceous sedimentary basement. The veins consist of gold- and silver-bearing quartz and calcite with minor amounts of adularia, sericite and sulfides. Although carbon and oxygen isotopic data for the veins indicate a meteoric origin of the ore fluid, finite element simulations suggest that the vein system might have formed in direct response to magma intrusion. In particular, geophysical data suggest that intruding magma has uplifted the basement rocks, thereby producing fractures and veins and a positive Bouguer anomaly, and providing the heat necessary to drive an ore-forming hydrothermal system.The second component of this study has been to investigate the nature and evolution of the Kushikino and Hishikari epithermal systems. Isotope data document the geochemical evolution of the hydrothermal fluids. We conclude that the existence of sedimentary basement rocks at depth might have affected the strontium and carbon isotopic ratios of the Kushikino and Hishikari ore fluids. The 87Sr/86Sr ratios and δ13C–δ18O trend reveal that major ore veins in the Hishikari deposit can be distinguished from shallow barren veins. It was suggested isotopically that fluids responsible for the barren veins in nearby shallow and barren circulation systems were only controlled by the shallow host rocks. Such multi-isotope systematics provide a powerful tool with which to determine the center of hydrothermal activity and thereby document the evolution of hydrothermal fluids. |
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