Abstract There are three main gold deposits, Kasuga, Iwato, and Akeshi, in the Nansatsu district of southern Kyushu, Japan ( Fig. 1 ). These three deposits have similar geologic characteristics and are the type deposits for the so-called Nansatsu- style of mineralization ( Urashima, 1975 ), more commonly referred to these days as high-sulfidation epithermal Au mineralization ( Hedenquist, 1987 ). This style of mineralization is believed to be a product of high-temperature acid fluids genetically related to magmatic activity at shallow depths ( Matsuhisa et al., 1991 ; Hedenquist et al., 1994 ). More than 27 tons (t) of gold have been produced from these three deposits. The ore from these deposits, which is hosted by highly silicified rock, is sold to copper smelters in Japan as flux.

Abstract The Nansatsu district of southern Kyushu has been the site of calc-alkaline volcanism for the last 10 m.y., shifting eastward with time. Associated hydrothermal activity followed deposition of the volcanic host rocks by about 0.5 m.y. and was characterized by interaction of magmatic fluids with meteoric water under epithermal conditions, resulting in the formation of high sulfidation Cu-Au deposits at Kasuga, Iwato, and Akeshi. The orebodies consist of >95 wt percent SiO 2 and result from leaching of the original andesite lava and pyroclastic flows by acid chloride-sulfate waters. These are inferred to have formed when magmatic vapors containing HCl and SO 2 condensed into meteoric water. The residual silica (now quartz) orebodies are best developed where the host was initially permeable. The margins of the quartz bodies are abrupt, with narrow (1-2 m) halos representing the reaction front of acid waters isochemically dissolving the host rock. The halo comprises alunite (strongly zoned in Na and K, with P-rich cores), dickite, and/or kaolinite ± pyrophyllite, grading out into illite and interlayered illite-smectite clays, and finally, propylitic alteration. This pattern is characteristic of deposits of this type throughout the world, for example, at Summitville, Colorado, and Lepanto, Philippines. Mineralization occurred after initial leaching by the vapor condensates, with metals transported by a dense magmatic fluid. Mixing with meteoric water and the subsequent temperature decrease caused the general decrease in grade toward the margin of the quartz bodies; ore grades are restricted to the quartz bodies. Gold is most closely associated with enargite and pyrite; later minerals include covellite and then sulfur. The last stage of activity was steam-heated, with descending waters oxidizing sulfides to goethite and locally remobilizing Au into fractures (this varies in degree between deposits). Erosion exposed the orebodies to supergene weathering, continuing the sulfide oxidation and Au remobilization. Stable isotope results indicate that hypogene alunite formed from a mixture of magmatic fluid (δ 18 O = 7 ± 2‰, δD = −25 ± 5‰, similar to nearby active volcanic discharges) with local meteoric water. In contrast, the clays in the marginal halo have isotopic compositions indicating a δ 18 O shift of 6 to 8 per mil from local meteoric water values, probably due to water-rock interaction, and the δ 18 O values of residual silica quartz may also be due to meteoric water domination. Fluid inclusion study of postmineralization quartz crystals indicates that the fluids had a salinity of about 1 wt percent NaCl equiv during late quartz growth, though there is evidence in one sample for higher salinity fluid having been present, up to 30 wt percent NaCI equiv (some inclusions contain daughter minerals of halite and sulfur). The T h values of over 1,000 measurements on late quartz from the ore zones indicate that the mean temperature during that stage ranged from <200°C at Akeshi to about 200°C at Iwato and 230°C at Kasuga. The presence of vapor-rich inclusions, some with T hv similar to T hl , indicate the presence at times of a two-phase fluid in the center of the ore zones, with depths of about 150 to 300 m below the paleowater table. The mineralizing fluid was relatively oxidized (sulfide/ sulfate ratio about 3:1), close to pyrite-alunite coexistence. Under these redox conditions, a pH of 3 and over a temperature range of 200° to 300°C, AuCl 2 − complexing may dominate over HAu(SH) 2 at salinities above about 2 wt percent NaCl. Several conditions are conducive for high sulfidation mineralization to occur: a crystallizing magma exsolves a fluid, with lower pressure conditions favoring metal fractionation from melt to fluid, the exsolved fluid separates into vapor and saline liquid phases due to immiscibility, with the latter being metal rich, the gas-rich (HCl and SO 2 + H 2 S) vapor ascends to the surface, with at least a portion condensed into meteoric water, forming an acid fluid which leaches the host rock to create permeable zones for later mixing, and the dense, metal-bearing fluid also ascends into this leached zone and precipitates Cu sulfosalts, sulfides, and Au upon mixing with meteoric water . If the saline liquid is not released from its source adjacent to the magma, due to lack of fracturing, or if there is a strong hydraulic gradient caused by high relief, only the vapor-related stage may occur. This will leave leached, barren rock which is characteristic of many eroded volcanic terranes.

Abstract The Kasuga mine is located 4 km west of Makurazaki city on the southern tip of Satsuma peninsula, Kagoshima Prefecture. Among several silica bodies scattered in the Kasuga area, Kasuga Honkotai (the main orebody) is the major gold producer; it has a cumulative gold production to date of approximately 9.3 tons (t) of gold. The silica body of the main orebody extends 500 m east- west and 100 m north-south with a thickness of about 80 m (Figs. 1 and 2). The average gold content of the silica body ranges from 2 to 3 ppm, and relatively high-grade ore grades 5 to 10 ppm along fissures or brecciated zones (Urashima et al., 1987).

Abstract Epithermal gold deposits are the principal source of gold in Japan, and mesothermal vein gold or by-product gold from skarn deposits, VMS (the Kuroko type and the Besshi type), and polymetallic veins contribute historically only 10 percent of domestic gold production. Gold production from epithermal gold deposits of Kyushu amounts to 284 tonnes and comprises about 40 percent of total Japanese gold production, 576 tonnes, from the Meiji Restoration (1868) to present (1999). The silver/gold ratio of ores in Kyushu is less than three and differs from the value that exceeds 10 in other areas ( Fig. 1 ). The Yamagano mine in the Edo era and later the Taio and Kushikino mines were the largest gold-silver mines in Kyushu before the discovery of the Hishikari deposit. At present the Hishikari underground operation and three opencut mines of the Nansatsu-type gold deposits at Kasuga, Iwato, and Akeshi are producing gold. The major gold deposits in Kyushu, typically of the low- sulfidation vein type and locally the high-sulfidation Nansatsu type, occur in extinct or waning geothermal systems of the Pliocene and Pleistocene volcanic areas. The young formation ages relate to the well-preserved shallow bonanza zones and overlying thick argillic alteration zones, and in places surface expression of hydrothermal activity, such as sinters and nearby acid alteration related to steam- heated acid hot springs. This contribution aims to present supplementary data to a previous review paper ( Izawa and Urashima, 1989 ), which described the relationship

Abstract The main part of this chapter describes a geological journey westwards from central to SW Japan and is specically aimed at the overseas visitor new to Japanese held geology. This is followed by a brief overview of the Japanese geopark system and museums with geological exhibits. There are currently over 30 geoparks in Japan, providing one of the most dense networks in the world of sites of special geological interest.

Abstract The late Cenozoic volcanic activity in Kyushu is characterized by large-scale volcano-tectonic depressions. The sites of these depressions together with associated volcanism and gold mineralization migrated southeastward in northern Kyushu and eastward in southern Kyushu. Thus, Quaternary gold deposits in Kyushu occur within 30 km west from the present volcanic front; the Pliocene gold deposits occur farther away from the active volcanoes. Typical Quaternary gold mineralization in Kyushu is the quartz vein type with associated adularia and minor calcite. Although host rocks are predominantly andesitic volcanic rocks and sometimes rhyolite and lacustrine sediments, the major portion of high-grade quartz veins of the Hishikari deposit discussed here is hosted in basement sedimentary rocks close to the unconformity between the basement and overlying Quaternary andesites. Five distinct alteration types are recognizable on the basis of mineral assemblages for Quaternary gold deposits. Two are the deeper propylitic alteration and the shallower smectite-zeolite alteration of the widespread and temperature-controlled type; the rest are mica-chlorite alteration, argillic alteration, and silicification of the fracture-controlled type. Most Quaternary and some Pliocene gold deposits in Kyushu are located near small Bouguer anomaly highs in areas of regional gravity anomaly lows. In the case of the youngest deposits the gravity anomaly highs are overlapped by low electrical resistivities. The small gravity anomaly highs have been ascribed to underlying uplifted blocks of basement. The low resistivity anomalies are caused by the presence of argillic alteration of the shallow portion of the mineralized systems.