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.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2005)
