Eleven gabbroids and two granitoids were analyzed by NAA method. Magnetite-series gabbroids have generally higher values (range, 0.4-5.3 ppb Au; average, 1.8 ppb Au) than the ilmenite-series gabbroids (range 0.2-0.6 ppb Au; average, 0.4 ppb Au); the result consistent with that observed on granitoids (ISHIHARA, KIMURA, et al., 1985). The gold contents depend upon the bulk composition being low in the gabbroids and high in felsic granitoids: the average contents are 1.1 ppb Au in gabbro, 2.4 ppb Au in diorite-tonalite, 3.0 ppb Au in granodiorite and 3.8 ppb Au in granite. Gold deposits tend to occur in the magnetite-series plutonic terranes, indicating a genetic relationship with the magmatism.

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Ag/Au total production ratio and Au-Ag minerals from three types of Japanese deposits, which have produced both gold and silver, are summarized. These deposits include, Au-Ag vein (Type 1-A), base metal (Cu, Pb, Zn, Mn, (Sn), (W), (Bi), (Mo), (Sb)) vein (Type 1-B), and Au rich disseminated-type ones (Type 2).
Generally, base metal rich deposits (Type 1-B) are characterized by that, (1) the Ag/Au total production ratio is high, (2) dominant Ag minerals are Se poor argentite and sometimes Ag sulfosalts (pyrargyrite, polybasite, argentian tetrahedrite), (3) Se and Te-bearing Au-Ag minerals are absent, and (4) Sn, W, and Bi minerals are sometimes found. In contrast, Au-Ag rich deposits (Type 1-A and Type 2) have the following features; (1) the Ag/Au total production ratio is low, (2) dominant Ag minerals in Type 1-A are Ag-Se-S minerals (naumannite, aguilarite, and Sebearing argentite), and Ag sulfosalts (polybasite, pyrargyrite, argentian tetrahedrite), (3) Hg, As, Sb, and Te minerals are occasionally found, (4) in Type 2, Ag minerals except Au-Ag series minerals are rare.
The above differences in the mineralogical characteristics of the Au-Ag rich and base metal rich deposits are considered to be resulted from the differences in chemical nature of ore fluids (sulfur and oxygen activities, CO₂ concentration, pH and temperature), depositional mechanism and depth of formation. The relationship between Ag/ Au total production ratio and kinds of metals concentrated to the deposits found in the Japanese deposits is also observed in epithermal Au-Ag vein-type deposits in western U.S.A. and active geothermal systems accompanied by precipitations of base metal and precious metals.

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The silver abundances were determined for common sulfide minerals (374 samples)-pyrite, chalcopyrite, sphalerite, galena, arsenopyrite, pyrrhotite, bornite and tennantite-and manganese ore minerals (162 samples)-rhodochrosite, rhodonite, pyroxmangite, tephroite, spessartine, braunite and hausmannite-from varieties of ore deposits, with the atomic absorption analyzer and the electron microprobe X-ray analyzer. The following conclusions are reached.
(i) Tennantite from the kuroko deposits of the Shakanai mine and the Sangkaropi area, is highly argentiferous. Silver is concentrated in galena in the sedimentary deposits of the Broken Hill district, in the skarn deposits of the Kamioka Mine and in the Toyoha deposits of the vein type. On the other hand, silver is much abundant in chalcopyrite in the porphyry copper deposits of the Mamut Mine and in the Ohe deposits of the vein type. Bornite from the submarine exhalative-sedimentary deposits of the Mount Lyell mine is rich in silver, as compared with coexisting chalcopyrite and pyrite.
(ii) In sulfide ores, silver generally has an affinity for tennantite≥;galena≥;chalcopyrite≥sphalerite, pyrite> pyrrhotite, arsenopyrite.
(iii) Silver is below the limit of detection in most of the manganese ore minerals, though present in small amounts in some of rhodochrosite from the vein type deposits of Neogene Tertiary. Manganese ore minerals from the bedded manganese ore deposits of Pretertiary are extremely poor in silver.

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The classification of 76 vein-type Au-Ag deposits in Japan, based on the facts whether adularia and/or ruby silver are reported or not, revealed following facts: (1) ruby silver occurs frequently in the deposits where adularia also occurs; and (2) adularia is always formed prior to ruby silver. These imply that the environment where adularia is formed becomes frequently favorable to the formation of ruby silver, as time passes.

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The aeromagnetic and gravity data were recently collected and integrated with the existing data to create nationwide maps. Using these data, magnetic and gravity analyses were performed in the Hokusatsu district, and the relation between the geophysical models and epithermal gold-silver mineralization is discussed.
The gold-silver deposits frequently occur on the edge of magnetic bodies which are inferred to lie to the northwest of the Sakurajima volcano and to the northwest of the Kirishima volcano. Also, they occur around the small closures of gravity high. A poor correspondence between the observed magnetic anomaly, and the magnetic anomaly due to the topographic model and surface geology leads us to conclude that magnetic anomalies are not resulted from outcropping bodies of the Pliocene to early Pleistocene volcanics but concealed intrusive rocks. Small closures of gravity high are interpreted due to concealed bodies having stock-like geometries.
Intrusion magmas would accelerate development of fracture systems and the hydrothermal convection systems. Accordingly, the localization of the gold-silver deposits on the margin of the inferred magnetic bodies is genetic. We estimate that the inferred igneous intrusions are closely related to the volcanisms from Pliocene to early Pleistocene time, taking account of those intrusive rocks being below the Curie point temperature, the present distribution of hot springs in and around the intrusive rocks, and the Curie point depth map in southern Kyushu.

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The Hokusatsu district of southern Kyushu has been known as one of the most mineralized districts of gold in japan, and many epithermal gold-silver quartz vein deposits occur especially in the Hokusatsu district. Recently special attention has been paid to this district by exploration geologists because of extremely gold-rich veins discovered at Hishikari by Metal Mining Agency of Japan (MMAJ) in 1981.
In this paper, regional characteristics of physiography and tectonic elements were analyzed with special reference to the gold-silver mineralization in the Hokusatsu district, using topgraphic maps, Landsat Imagery and gravity data, as well as geologic information. The results are as follows.
Many collapsed basins were observed and it was found that major gold-silver deposits tend to occur at outer fringes of those basins. The following three stages were destinguished for the development history of those volcanismrelated basins.
1st. stage (Miocene): Relatively small scaled collapsed basins were formed, and many steep inclined fractures were associated, providing the most suitable environment for the vein-type mineralization.
2nd. stage (Pliocene-Early Pleistocene): The Hokusatsu-type collapsed basins were formed in association with a series of violent volcanism mainly in the Hokusatsu district. The gold-sivler mineralization occured at the fringe of the collapsed basins formed by the volcanism of this stage.
3rd. stage (Late Pleistocene-Holocene): The center of main volcanic activity moved eastward along Kagoshima Bay. The Kagoshima Bay-type collapsed basins known as a so-called Quaternary caldera were formed along Kagoshima Bay. Volcanism of this stage is still active and some geothermal systems are associated. Gold mineralization is also expected to have formed in relation to the volcanism of this stage.

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A computer simulation analysis was carried out on the gold-quartz vein system of the Kushikino ore deposits in order to understand the formation mechanism. The results indicate that uplifting of the basement at eastsoutheast part of the arbitrary set-up tectonic system gives rise to the observed vein system. This uplifting portion coincides with high gravimetric anomaly. Thus, the anomaly would have been caused by uplifting of the basement, which may have been possibly due to intrusion of igneous body.
Most of gold-quartz vein deposits in the northern Kagoshima region (the Hokusatsu region) occur around the high gravimetric anomaly. This fact implies that the pushing-up force here emphasized is indispensable for the formation of the ore deposits. This kind of analysis would provide practical information for fractured areas around the anomaly, thus useful for gold exploration.

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Oxygen and carbon isotope effects due to boiling and mixing of fluids were examined for epithermal vein systems. The magnitude of oxygen isotope change of fluid which could be produced by boiling is not greater than 2‰ at temperatures higher than 100°C. In case of carbon isotopes, boiling could produce an increase in δ ¹³ C values of precipitating calcite (up to 2 to 3‰, ) due to increasing pH of the fluid. Mixing of a deep hydrothermal fluid with a low-temperature groundwater could provide a wide range of δ ¹⁸O values in a relatively small temperature interval, depending upon variable initial compositions of the deep hydrothermal fluid. The data for quartz and calcite from the Kushikino gold-bearing epithermal veins suggest either boiling or mixing with a low-temperature groundwater of a slightly ¹⁸O-enriched (by 3 to 4‰) deep hydrothermal fluid.

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Recent discovery of the high-grade gold deposit at Hishikari not in Cenozoic volcanics but in the underlying basement sediments leads us to examine role of the basement for the gold mineralization. Type of the basements for late Cenozoic gold deposits was briefly reviewed, and the basement rocks, mostly of sedimentary, were analyzed for trace amount of gold in selected areas of southwestern Japan. The results indicate that Ag/Au ratio and the size of gold deposits are independent of the type of the basement but dependent upon the position in the magmatic arcs, and the gold contents are as low as 1 ppb even in the basement of rich gold fields including Hishikari, Kushikino and Taio deposits in Kyushu Island.
Sulfur isotopic ratio of pyrites from altered zones and ore veins of the Hishikari deposit is averaged as δ³⁴SCDT 0‰, while the sulfur in unaltered rocks of the Shimanto Supergroup has an average of -12‰, which is unlikely to be the source for the 0‰-ore sulfur. Thus, a magmatic source is proposed for sulfur, and possibly gold, of the Hishikari deposit. In referring to the recent geological and geophysical works at the Hishikari mine, the ore deposit is considered being related to a hidden magnetite-series felsic porphyry plug which intruded at western margin of the kakuto caldera. Rising magmatic ore solutions through a normal fault zone in the uplifted basement block met the meteoric water and cooled down near the level of unconformity plane about 500 meters below the surface existing ca. 1 Ma ago, and precipitated native gold. Thus, the Hishikari deposit is of a unconformity-controlled type. The unusually high-grade ores of the ore deposit can be explained by strong concentration of gold in the primary magmatic ore solution, proximity to the hidden intrusion and the suitable condition for the gold precipitation.

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The gold-silver deposit of the Nebazawa mine, Gunma Prefecture, is of a Neogene epithermal vein-type, and is hosted in the Katashina-gawa rhyolitic rocks probably of Paleogene age. Paragenetic and compositional variations of Au-Ag minerals have been investigated for the ginguro ores from the Manzai No. 2 and No. 3 veins and the Hon-pi vein, which are the major veins in this deposit.
Microscopic observation reveals that there is a regularity in sequence of mineral deposition throughout the ginguro ores studied. The suggested general sequence of mineral deposition in the ginguro ores is as follows: (1) pyrite, marcasite and arsenopyrite, (2) chalcopyrite, sphalerite and galena, (3) Au-Ag minerals, and (4) covellite. Despite the similarity of the general sequence of mineral deposition, the observed Au-Ag mineral assemblage significantly varies with time and space, and can be classified into the following four types: (1) Au-Ag-Sb-As-S type (electrum, pyrargyrite, proustite, minerals of polybasite-aresenopolybasite series, miargyrite and freibergite), (2) Ag-S-Se type (native silver, argentite and selenian argentite) and (3) Au-Ag type (electrum and native silver) in the Manzai No. 2 and No. 3 veins, and (4) Au-Ag-Sb-S-Se type (electrum, native silver, pyrargyrite, polybasite and freibergite) in the Hon-pi vein. Electron microprobe studies for the Ag sulfosalt minerals also reveal that there is a significant difference in their Sb/(Sb + As) and Se/(S + Se) ratios between the Au-Ag-Sb-As-S type and the Au-Ag-Sb-S-Se type. A possibility is discussed that the observed paragenetic and compositional variations resulted from local and/or temporal changes in some physicochemical parameters (especially in oxidation potential and concentrations of CO2 and Cl) during the mineralizations.

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Sphalerite from the epithermal vein-type gold-silver deposit of the Nebazawa mine contains 1.0-20.4 mol% FeS and 0.1-12.6 mol% MnS. The Fes and MnS conterts are in general positively correlated, but their variation patterns are different corresponding to the ore types. The different variation patterns may be ascribable basically to changes in partition coefficients of Fe and Mn between sphalerite and aqueous solution, which are probably due to local and/or temporal changes in physicochemical conditions during the mineralizations. Although further details remain in question, the fact of the occurrence of manganoan sphalerite throughout this deposit would solely provide an important key to the origin and evolution of ore fluid responsible for the formation of this deposit. Thermodynamic consideration on the partition of Zn, Fe and Mn between sphalerite and aqueous solution suggests that the ore fluid responsible for the deposition of manganoan sphalerite should be unusually high in Mn ²⁺/Zn ²⁺and Mn ²⁺/Fe ²⁺ concentration ratios. Basement rocks in the mining area are probably the Paleozoic to Mesozoic formations of the Ashio belt, in which a number of bedded manganese ore deposits are known. It thus seems that the ore fluid responsible for the formation of the Nebazawa deposit was evolved by the interaction with pre-existing bedded mangenese ore deposits in the basement rocks. This provides a possibility that the basement rocks played an important role as a potensial source for at least some of the metallic constituents of this deposit.

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Silver deposits of the Karuizawa mine are network and vein types developed in fracture zone of Miocene rhyolite lava dome altered hydrothermally. The network ore body consists of veinlets along fracture and spots of ore filled up cavity after spherulite in rhyolite. Its ore is principally composed of barite, sphalerite and galena in association with small amounts of quartz, pyrite, marcasite, chalcopyrite, polybasite, pyrargyrite and argentian tetrahedrite (12 to 18 wt% Ag). On the other hand, the vein along fissure in silicified rhyolite consists mainly of galena, sphalerite, pyrite, marcasite and barite associated with quartz, chalcopyrite and small amounts of bournonite, argentian tetrahedrite, polybasite and pyrargyrite. These mineral assemblages in ores are similar to those of kuroko ore. In high silver ore, fine grained silver minerals such as polybasite, pyrargyrite and argentian tetrahedrite are usually found as inclusion in galena, and often show a pseudo-micrographic texture with galena. These silver minerals were crystallized at late stage of the mineralization as shown in Fig.9. Homogenization temperature and salinity of fluid inclusion in barite are 230°to 320°C and 3.6 to 5.0 wt% NaCl equivalent, respectively. Hydrothermal alterations of silicification and adularization are conspicuously observed in or around the ore deposite. K-Ar age for adularia of the alteration product is 12.8±0.6 Ma which is approximately same as that of alteration rock of the kuroko deposits. The ore deposits of the Karuizawa mine are thought to have been produced by mineralization in the relation to formation of the kuroko deposits.

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Chemical analytical study on Au-Ag series mineral (i.e. native gold and electrum) from some gold deposits in the Korean Peninsula revealed that (1) Ag atomic % of Au-Ag series mineral varies very widely, ranging from 2.1 to 68.1, (2) Ag content of Au-Ag series mineral is different for different type of deposits; 4.0-68.1 Ag atomic % for Korean-type, 2.1-7.6 for alaskite-type deposits (the Ugu and Geumjeong deposits) and 58.4-65.1 for epithermal-type deposits (the Tongyeong deposit), and (3) Ag content for epithermal gold-silver vein deposits in Japan is higher than that for Korean-type and alaskite-type, while it is similar to that for epithermal-type in the Korean Peninsula.
It is theoretically derived that temperature, Cl- and H2S concentrations and pH affect Ag/Au ratio of Au-Ag series mineral. Negative correlation between Ag content of Au-Ag series mineral and homogenization temperature of fluid inclusions suggests that temperature is one of the most important variable in controlling Ag/Au ratio of Au-Ag series mineral. Korean-type and alaskite-type deposits are intimately associated with granitic rocks and occur mostly in Precambrian rocks such as gneiss, while epithermal vein deposits are not associated with granitic rocks and occur in young volcanic rocks. This geological contrast appears to be consistent with the compositional characteristics of Au-Ag series mineral and temperature of formation estimated from fluid inclusion study; Koreantype and alaskite-type deposits have been formed at deeper positions and at higher temperature condition that epithermal vein deposits.

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Gold and silver bearing quartz veins of the Geumwang mine in South Korea occur in Cretaceous granite altered hydrothermally. Ores from the mine are exceptionally silver rich as the Korean type gold deposit. The ore minerals occur in two or three sulfide bands formed at early to middle stages and a sulfosalt band at the late stage of mineralization. Electrum and silver minerals of polybasite, pyrargyrite and argentian tetrahedrite appears in sulfide band or stringer consisting of pyrite, arsenopyrite, galena, sphalerite, chalcopyrite and quartz in the vein. Also silver-antimony sulfosalt minerals such as pyrargyrite, polybasite, miargyrite, diaphorite and argentian tetrahedrite with native silver occur as a band in central portion of the quartz vein in association with some amounts of pyrite, sphalerite, arsenopyrite and quartz. Composition of electrum is 48.4 to 52.7 wt% Ag (63.0 to 67.4 at% Ag). Meanwhile sphalerite has 2.2 to 4.2 mole% Fes. Homogenization temperatures of fluid inclusion (two phases) in quartz of the sulfide band are from 183°to 310°C(240°C in average) with a peak at 240°C, commonly 220°to 270°C. The ore mineralogy suggests that temperature (T) and sulfur fugacity (fs2) of the formation of the sulfide and sulfosalt bands are estimated as T: 190°to 240°C, fs ₂: 10 ^-15.5 to 10 ^-13.5 atm, and T: 130°to 170°C, fs2: 10 ^-19.5 to 10 ^-17.0 atm, respectively. Such low temperature and low sulfur fugacity mineralization crystallized silver-antimony sulfosalts are considered.to be singular as the Korean type gold deposit.

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