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Kasuga |
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Kyushu, Japan |
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Au SiO2
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Super Porphyry Cu and Au
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Kasuga is one of a group of three main deposits (the other two being, Iwato and Akeshi), which are found in the Nansatsu district of southern Kyushu, Japan and have together produced more than 27 tonnes of gold. All three have similar geologic characteristics, known as the "Nansatsu-style", more commonly referred to as high-sulphidation epithermal Au mineralisation.
The basement rocks in the Nansatsu district are dominated by Cretaceous sandstone rich trench turbidites with minor submarine basalt and chert, overlain by Tertiary-Quaternary calc-alkalic volcanic rocks which are associated with caldera development and the outflow of 5.9 to 6.4 Ma pyroclastics emplaced shortly before the mineralising event. Across the district, there was an east-ward shift with time in the locus of calc-alkaline volcanism, which includes pyroxene-hornblende, hornblende- and pyroxene-andesite flows and their pyroclastic equivalents, and tuffaceous siltstones. Hydrothermal activity displays a similar eastward younging pattern to the volcanism.
The gold mineralisation is entirely restricted to the intensely silicified rocks which form the core of these deposits. The silica core of each system is enveloped by argillic alteration that is zoned from an inner alunite-kaolinite, through to an outer smectite and/or interstratified illite/smectite interval. This argillic zone is usually less than 1 to 10 m thick and grades outward into propylitic alteration and finally into fresh country rock. The silica zone is underlain by deep alteration characterised by abundant pyrite and typically Na rich alunite, while dickite dominates over kaolinite. At depth the alunite dickite grades into a sericite-chlorite assemblage.
The principal ore minerals are enargite and electrum, with rare luzonite in the upper sections and ubiquitous and dominant pyrite throughout. Gold grades, which are intimately associated with goethite as native gold in the upper parts of the orebody, are roughly concentric, decreasing outwards. The main high grade zone contains 5 g/t Au in the western part of the pit where the fracture density is highest.
The mine produces around 10 000 tonnes per month of silica flux which contains approximately 4 g/t Au retrieved during the copper smelting process for which the flux is utilised. The reserves at the end of WW2 were approximately 1.6 Mt of ore, with around 0.8 Mt now remaining. Larger tonnages of lower grade (around 1 g/t Au) is known nearby. Historic production from Kasuga between 1929 and 1994 amounteds to some 8.8 tonnes of Au and 5.0 tonnes of Ag. While small, the deposit is a good example to illustrate many of the characteristics of high sulphidation Au-Ag mineralisation in an intensively studied district.
The most recent source geological information used to prepare this decription was dated: 1999.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Hayashi T 2001 - Geology and gold exploration in the Nansatsu district, southern Kyushu, Japan: in Feebrey CA, Hayashi T, Taguchi S, 1994 Epithermal Gold Mineralization and Modern Analogues, Kyushu, Japan Society of Economic Geologists, Guidebook Series v. 34 pp 127-132
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Hedenquist J W, Matsuhisa Y, Izawa E, White N C, Giggenbach W F, Aoki M 1994 - Geology, geochemistry, and origin of high sulfidation Cu-Au mineralization in the Nansatsu District, Japan: in Econ. Geol. v 89 pp 1 - 30
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Izawa E 1991 - Hydrothermal alteration associated with Nansatsu-type gold mineraliastion in the Kasuga area, Kagoshima Prefecture, Japan: in Geological Survey of Japan Report No. 277 pp 49-52
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Izawa E, Taguchi S, Kobayashi T, Watanabe K 1992 - Gold Mineralization in Volcano-Geothermal Areas of Kyushu: in Urabe T, Aoki M (Eds.), Mineral Deposits of Japan and the Philippines 29th IGC Field Trip C33 Guide Book, The Society of Resource Geology, Tokyo, Japan v6 pp 143-172
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Watanabe Y 2005 - Late Cenozoic evolution of epithermal gold metallogenic provinces in Kyushu, Japan: in Mineralium Deposita v40 pp 307-323
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Yukihiro Matsuhisa 1992 - Epithermal gold mineralization in Kyushu, Japan: in Epithermal Gold in Asia and the Pacific, Mineral Concentrations and Hydrocarbon Accumulations in the ESCAP Region UN Econ & Social Comm to Asia & the Pacific v6 pp 23-27
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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