Abstract The Yamagano deposit represents the third-largest known gold deposit in the Hokusatsu region of southern Kyushu. Its total historical gold production is 28.4 tonne (t) (Society of Resources and Material, 1989), which places it sixth in terms of gold-producing mines in Japan. Gold mineralization in Japan is dominated by low-sulfidation epithermal deposits of which the Yamagano deposit is typical, and like all other deposits of this style (with the exception of Hishikari) it has ceased production.

Abstract The Hishikari high-grade low-sulfidation epithermal gold deposit is located in the Hokusatsu district, about 30 km north of the Kagoshima International Airport, Kagoshima Prefecture ( Fig. 1 ). In early 1981, a narrow (15 cm) but high-grade (290 g/t Au) vein was discovered 200 m below the surface by the Metal Mining Agency of Japan ( MITI, 1982 ), and subsequent development has proved Hishikari to have substantial reserves of high-grade ore. Production from July 1985 to March 2000 totaled 105 tonnes (t) of gold and 58 t of silver from 1.87 Mt of ore. The production plus total reserves are estimated to be 5.5 Mt, including 3.5 Mt at an average grade of 60 to 70 g/t Au in the Honko-Sanjin ore zone and 2 Mt at 20 to 25 g/t Au in the Yamada ore zone, for a total of approximately 260 t of contained gold. The Hishikari deposit, especially the Honko-Sanjin zone, is characterized by high gold contents in almost all veins and by a very high aluminum content in the veins due to abundant adularia and common smectite. This article presents a summary of the gold deposit at Hishikari and the detailed mineralogical description of a high-grade vein, the Hosen no. 1 vein, hosted by basement sedimentary rocks.

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 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.

Abstract The Iriki district was chosen by the Metal Mining Agency of Japan (MMAJ) as a target for epithermal gold exploration on the basis of the following criteria: the presence of steam-heated acid alteration, a gravity anomaly high, geochemical characteristics of hot spring water similar to those of water from the Hishikari gold deposit, and old mining activity. In 1994 and 1995 the MMAJ carried out exploration that included alteration mapping, geochemical and geophysical surveys, and diamond drilling. of note, several drill holes intersected gold mineralization returning approximately 6 g/t Au at more than 4 m beneath the steam-heated acid alteration. On the basis of the sulfide assemblage, mineralization in the Iriki district is not typically high sulfida- tion, as alteration assemblages and the presence of arsenopyrite may suggest, but is more akin to the intermediate-sulfidation style described by Hedenquist et al. (2000 and 2001). The characteristics of the mineralization in the Iriki district are concordant with the magmatic characteristics of hot springs from the Iriki and with the presence of young host volcanic rocks in the Iriki district.

Abstract The Kushikino gold mine (Gold Park) is located 30 km northwest of Kagoshima city (Fig. 1) and historically has been the fourth largest gold mine in Japan. The mine was opened in 1660 by a lord of the Satsuma Province (a province in southern Kyushu during the Edo era) and development took place mainly on the Serigano vein group (Fig. 2) in the south central part of the area (Miyahisa and Wakabayashi, 1972). In the late Meiji era, 1905, Mitsui Mining and Smelting Co., Ltd. acquired the central part of the Kushikino area including the Kushikino no. 1 vein. In 1913, an all-slime cyanidation plant, the first of its kind in Japan at that time, began treating ore at a rate of 150 (t) per day. Since 1660 the mine has produced 55 t of gold and 497 t of silver, of which more than 80 percent was produced from the Kushikino no. 1 vein. Approximate average Au and Ag grades have been reported as 6.7 g/t and 61 g/t, respectively (MITI, 1979). In 1988, the Kushikino Gold Park was opened adjacent to the operating gold mine. In the park, the western part of the champion vein (Kushikino no. 1 vein) around the “New” inclined shaft (Shin-shako) can be seen in the adit (mine level 2L, 60 m above sea level), though the mining operation is no longer active. This article compiles previous studies and revises our understanding of the epithermal gold mineralization at the Kushikino deposit.

Abstract The bonanza-grade, low-sulfidation epithermal Hishikari gold deposit is located in the Plio-Pleistocene volcanic area of southern Kyushu, Japan. The concealed veins were discovered in 1981 and the mine has since produced 5.462 million metric tons (Mt) of ore averaging 44.3 g/t Au (242 t Au) from 1985 to the end of 2018, at which time reserves were 7.98 Mt at 20.9 g/t Au. The Hishikari deposit consists of the Honko, Sanjin, and Yamada ore zones, which occur in a NE-trending area 2.8 km long and 1.0 km wide. The veins are hosted by basement sedimentary rocks of the Cretaceous Shimanto Supergroup and by overlying Hishikari Lower Andesites of Pleistocene age. Sinter occurs about 100 m above the Yamada ore zone. Temperature-controlled hydrothermal alteration zones occupy an area of >5 km long and 2 km wide. The Honko and Sanjin veins occur within a chlorite-illite alteration zone (paleotemperature >230°C), whereas the Yamada veins occur within an interstratified clay mineral zone (150°–230°C). The marginal alteration comprises quartz-smectite (100°–150°C) and cristobalite-smectite (<100°C) zones. Ore-grade veins are located between –60- and 120-m elev, with the paleowater table over the Honko-Sanjim veins at ~300-m elev. Overall, the Ag/Au wt ratio is about 0.6. Vein-forming minerals consist of quartz, adularia, and clay minerals plus truscottite, with electrum and minor pyrite, chalcopyrite, naumannite, galena, and sphalerite. The major veins formed from repeated episodes of boiling and strong fluid flow inferred from bands of quartz, adularia, and smectite with bladed quartz, columnar adularia, and truscottite.

ABSTRACT The genesis of the forearc magmatism in southwest Japan at 14 Ma was studied using geologic and seismic observations. Before the magmatism, the Shimanto accretionary complexes were uplifted by 1000–3000 m between 21 and 17 Ma during the opening of the Japan Sea and the Shikoku Basin. Opening of the Japan Sea and the Shikoku Basin terminated at 15 Ma, when the Kinan Seamount Chain on the Shikoku Basin activated. The magmatic products are distributed at segment boundaries and in aseismic areas of the subducting Philippine Sea plate. The segment boundaries are located on syncline and anticline axes of the subducting slab at Kyushu. The magmatic products at Shikoku are distributed at places where olistostromes uplifted between 21 and 17 Ma. Beneath the Kumano volcanic rocks at Kii, a significant discontinuity in the locations of deep earthquakes is observed. These observations indicate that rupturing of the subducting slab by the load of the overriding plate occurred at around 14 Ma. The slab rupturing would have enabled subslab asthenosphere and/or magma to be injected into the plate interface through the tear and cause the forearc magmatism. Since the oceanic plate has a number of preexisting weaknesses, such as fracture zones, slab rupturing could occur more commonly than previously considered. The forearc magmatism caused by slab rupture is an important process associated with the growth of continental crust in subduction zones.

Abstract The Kyushu-Ryukyu Arc extends for over 1000 km south from the large island of Kyushu down the Ryukyu Island chain towards Taiwan. It has been produced by subduction of oceanic plates beneath the Eurasian continent and exposes a wide range of Palaeozoic, Mesozoic and Cenozoic rocks formed in an active continental margin setting (Maruyama et al. 1997; Isozaki et al. 2010). In this chapter we describe the geology of the arc from the oldest (Palaeozoic and Triassic) to youngest (Neogene and Quaternary) rocks, and discuss the geodynamic evolution of the area.

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 Epithermal Au and Ag deposits of both vein and bulk-tonnage styles may be broadly grouped into high-, intermediate-, and low-sulfidation types based on the sulfidation states of their hypogene sulfide assemblages. The high- and low-sulfidation types may be subdivided using additional parameters, particularly related igneous rock types and metal content. Most high-sulfidation deposits are generated in calc-alkaline andesitic-dacitic arcs characterized by near-neutral stress states or mild extension, although a few major deposits also occur in compressive arcs characterized by the suppression of volcanic activity. Rhyolitic rocks generally lack appreciable high-sulfidation deposits. Highly acidic fluids produced the advanced argillic lithocaps that presage high-sulfidation mineralization, which itself is due to higher pH, moderate- to low-salinity fluids. Similar lithocaps in the Bolivian Sn-Ag belt, some mineralized with Ag and Sn, accompany reduced, ilmenite series magmatism. Intermediate-sulfidation epithermal deposits occur in a broadly similar spectrum of andesitic-dacitic arcs but commonly do not show such a close connection with porphyry Cu deposits as do many of the high-sulfidation deposits. However, igneous rocks as silicic as rhyolite are related to a few intermediate-sulfidation deposits. These deposits form from fluids spanning broadly the same salinity range as those responsible for the high-sulfidation type, although Au-Ag, Ag-Au, and base metal-rich Ag-(Au) subtypes reveal progressively higher ore-fluid salinities. Most low-sulfidation deposits, including nearly 60 percent of the world's bonanza veins, are associated with bimodal (basalt-rhyolite) volcanic suites in a broad spectrum of extensional tectonic settings, including intra-, near-, and back-arc, as well as postcollisional rifts. Some low-sulfidation deposits, however, accompany extension-related alkaline magmatism, which, unlike the bimodal suites, is capable of generating porphyry Cu deposits. Extensional arcs characterized by active andesitic-dacitic volcanism do, however, host a few low-sulfidation deposits. Low-sulfidation deposits genetically linked to bimodal volcanism are formed from extremely dilute fluids, whereas modestly saline contributions account for the low-sulfidation deposits in alkaline centers. Early lithocap-forming and high-sulfidation fluids, as well as low-sulfidation fluids in deposits associated with alkaline igneous rocks, display clear evidence for a close genetic relationship to magmatism and, although the linkage is less intimate, late high-sulfidation fluids responsible for much of the Au introduction along with similar intermediate-sulfidation fluids also seem to owe much to their magmatic parentage. Where ascending intermediate-sulfidation fluids enter lithocaps, they evolve to high-sulfidation fluids. Eventual neutralization and lowering of sulfidation state by wall-rock interaction can convert high- back to inter-mediate-sulfidation fluids, as confirmed by both spatial and paragenetic transitions from high- to interme-diate-sulfidation mineralization. In contrast, low-sulfidation fluids other than those of alkaline affiliation lack such clear-cut connections to magmatism, although Giggenbach's work on the geothermal fluids associated with the Taupo Volcanic Zone in New Zealand suggests that a deep magmatic source different from that of fluids in andesitic arc terranes is probable. In addition, at least in some regions, there appears to be a correlation between the reduced sulfide assemblages of low-sulfidation deposits and the reduced nature of the volcanic rocks with which they are associated. Therefore, it may be argued that the defining characteristics of epithermal deposits are related directly to their magmatic roots, notwithstanding the existence of important unanswered questions, especially regarding the source of low-sulfidation fluids. This review puts forward several exploration guidelines for epithermal precious metal deposits. Exploration activity in andesitic-dacitic arcs should be restricted to high- and potentially related intermediate-sulfidation deposits containing Au and/or Ag, whereas a variety of rift-related bimodal suites and convergent-margin alkaline rocks offer the prime environments for Ag-deficient, low-sulfidation Au deposits (Ag/Au <∼15). Bonanza Au veins are more likely to be of the low-sulfidation type and to be discovered at relatively shallow paleodepths in bimodal rift settings, where rhyolitic and/or basaltic rocks may be proximal to Au ore. Even tholeiitic basalts in emergent mid-ocean ridge or hot-spot settings might possess underappreciated epithermal Au potential. Subaerial extensions to some volcanic-hosted massive sulfide (VMS) belts may possess low-sulfidation Au potential because of the broadly similar volcanotectonic settings for both deposit types. The reduced, ilmenite series volcanic rocks of the Bolivian Sn-Ag belt are unfavorable for epithermal Au. Deficiency of volcanic rocks in epithermal provinces is typical of highly compressive arcs (high- and intermediate-sulfidation deposits) and some rifts swamped by fluviolacustrine sedimentation with silica sinter occurrences (low-sulfidation deposits). In contrast to high- and intermediate-sulfidation deposits, exploration for low-sulfidation Au deposits, even where exposed, may be hampered by the visually subtle nature of many outcropping examples.