The Nurukawa kuroko deposit was discovered in 1984 and commenced its production in 1987. Current monthly production is 5, 000 tons of ore grading 6.8g/t Au, 123g/t Ag, 0.74%Cu, 3.86%Pb, and 7.92%Zn. The deposit consists of five discrete ore bodies; Nos 5 and 3 ore bodies are under mining operation and the rests are in exploration stage. Among them, No. 5 ore body is the largest in size and highest in ore grade. It is characterized by gold and silver rich black ore (massive sulfide) and gold-bearing siliceous ore which develops in footwall tuff breccias. The latter ore is subdivided into two types, namely, stratiform and stockwork types, on the basis of its mode of occurrence and distribution. The stratiform type siliceous ore is composed of fragments of gold-bearing network ore of quartz-chalcopyrite-pyrite and interstitial siliceous matrix rich in lead and zinc. On the contrary, the stockwork type siliceous ore is rich in chalcopyrite and pyrite and is overprinted by lead and zinc mineralization.
No.3 ore body consists of bedded clayey black ore which is considered to have replaced tuff breccia, fine-grained massive black ore contaminated with muddy material, and underlying gypsum ore. No. 2 ore body is composed of massive sulfide ore which is zoned vertically into black and yellow ores, and ore composed of fragmented and trans-ported massive sulfides. Those are underlain by gypsum and siliceous ores.
The wall-rock alteration zoning around the Nurukawa deposit is; sericite-chlorite, mixed-layer clay, montmorillonite, and zeolite zones, in ascending order. Kaolinite and kaolinite-montmorillonite mixed-layer clay mineral are observed above the black ore of the Nos. 3 and 5 ore bodies. The mineralization age determined by K-Ar method on sericite samples has a range between 10.7 and 12.5Ma with an average of 11.7Ma. Homogenization temperatures of fluid inclusions in quartz cluster around 265-281°C. It is suggested that no boiling had occurred during the formation of the deposit. The δ³⁴S data on mineral separates and bulk sulfide ores range from +4.8 to+6.8% except one sample.
All of these geochemical data coincide with "ordinary" values of the kuroko deposits of the Hokuroku area. Therefore, it is not easy to explain the reason why the Nurukawa deposit has extremely high gold content compared to other kuroko deposits. However, positive relationship between salinity and filling temperature of the fluid inclusions and geologic reconstruction of the area both suggest that the formation of the deposit appears to be happened near shore where ore-forming solution was diluted by meteoric water. This mixing might effectively cause the precipitation of gold.

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Sulfur isotopic compositions of sulfide minerals (sphalerite, chalcopyrite, pyrite, pyrrhotite, etc.) from plutonic and subvolcanic vein deposits of the Kinki district in Inner Zone of Southwest Japan are examined in relation to regional metallogeny with special reference to sulfur isotopic variations among mineralization stages of individual vein deposits. The δ³⁴S values of sulfide minerals from plutonic vein deposits are in the range from -10.5 to +1.8‰. Plutonic vein deposits genetically related to Cretaceous granitoids in San'yo belt are characterized by larger negative δ³⁴S values (-10.5 to -5.7‰), while those related to Palaeogene granitoids in San'in belt are characterized by higher values (-0.4 to +1.8‰). On the other hand, δ³⁴S values of subvolcanic vein deposits are in the range from -6.2 to +5.0‰.
The δ³⁴S values in plutonic W-Sn-Cu vein deposits at the Otani mine are characterized by relatively larger negative values, but sulfide minerals formed in the latest stage tend to show higher δ³⁴S values than earlier stage ones.
In representative subvolcanic polymetallic (Cu, Zn, Pb, Sn, W) vein deposits at Akenobe mine, three major mineralization stages, called Pb-Zn stage, Cu-Zn stage and Sn-W stage, are distinguished from area to latest by major tectonic breaks. It is inferred that δ³⁴S values of sulfide minerals decrease from Pb-Zn stage (-1.8 to -1.40‰) to Cu-Zn stage (-3.5 to -0.6‰) and increase from Cu-Zn stage to Sn-W stage (-2.4 to +1.0‰).
In general, δ³⁴S values may have increased from early to later stage of mineralization in both plutonic and sub-volcanic vein deposits in the district.
The δ³⁴S value of polymetallic vein deposit transecting Kieslager deposit at the Akenobe-minamidani deposit is -4.1‰ which is similar to those of Cu-Zn stage at the Akenobe mine.

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Hosokura Pb-Zn ore deposits consisting of fissure-filling veins of epithermal to mesothermal type, occur in regionally altered andesitic lava and pyroclastics of the Hosokura Formation of Miocene age in northeastern Japan. Superimposed on the reginal alteration, the ore-related alteration is seen along veins, and this alteration was studied in the Fuji-Honpi vein area.
Based on the mineral assemblages, the wall rock alteration around this vein is divided into three zones, A, B and C, from the vein side outwards. Characteristic mineals are K-feldspar in the A zone, K-feldspar and albitized plagioclase (An 10-20%; whereas, An 40% in the regionally altered rock) in the B zone and albitized plagioclase (An 30-40%) in the C zone. Sericite and chlorite are seen in all these zones, but 001 reflection (2θ) of the sericite shift to the higher angle side and the chlorite becomes Fe₂+ rich towards the vein. Both A and B zones are characterized by addition of K₂O, and leaching of Na₂O and CaO; thus an alteration index, K₂O/(K₂O+Na₂O+CaO), is proposed. The index is inversely correlated with magnetic susceptibility (r=-0.98) and positively (r=+0.88) with "migration degree" of the chemicals which was obtained from loss and gain calculation.
The alteration zones are extensively developed in brecciated rocks of two generations: one along the ore vein which was originally fault breccia, while the other along the top of andesitic lava which was flow-top breccia at the paleo-surface. Width of the A and B zones is generally larger in the upper horizons wherever the permeable rocks exist.
It is considered that upward migrated heated meteoric and magmatic fluids along fault zone in andestic lava met a large amount of cool meteoric water contained in porous pyroclastic rocks (Fig. 11). Temperature of the fluid was dropped off, then high-grade ores were formed in the andesitic lava. The bonanza is also shown by the amounts of normative quartz in the altered rocks (Fig. 12). Application of this study for mineral exploration is given in the flow chert of Fig. 13.

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Two methods for estimating modal composition of alteration minerals in host rocks of the Hosokura Pb-Zn vein deposits are developed.
(i) First one is to calculate Altered Rock Normative Mineral (A.R.N.M.) based on bulk chemistry data. This enables us to estimate normative composition of alteration minerals; that is, quartz, plagioclase (albite+arnorthite), K-feldspar, chlorite (Fe- and Mg-chlorite), montmorillonite, and calcite (Table 1).
Calculated results coincide very well with observed relative amounts of constituent minerals which were determined by combining X-ray powder diffraction and microscopic observation. Furthermore, normative compositions of plagioclase (albite and anorthite) and chlorite (Fe- and Mg-end members) show good agreement with actual chemical composition of these solid solutions. Therefore, the proposed A.R.N.M. method appears to be useful for the present purpose.
(ii) Second method is to estimate mineral mode on the basis of X-ray diffraction data. At first, we measure the intensity (area) of a characteristic peak of each alteration mineral. Then normalized X-ray diffraction intensity of the mineral was calculated as a ratio of the area to the total areas of characteristic peaks of quartz, K-feldspar, plagioclase, sericite, chlorite, montmorillonite, and calcite. The normalized intensity shows strong positive correlation with normative percent of the mineral. This proportional relation was used to construct calibration curve to compute weight percent of the mineral from the X-ray peak intensity. Estimated chemical composition of altered rocks based on this method coincides very well with actual analytical results. Therefore, it is concluded that the second method is also valid for the purpose.
Furthermore, these two methods are also applicable not only to the intermediate type alteration (UTADA, 1977) but also to the acidic or alkaline alteration after partial modification.

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K-Ar ages were determined for sericite-bearing ores collected from pyrophyllite ("Roseki") deposits distributed in the Chugoku district; Yanoshokozan, Sankin, Kumano and Hohro (Hiroshima Prefecture), Ohguni (Shimane Prefecture) and Naburi (Yamaguchi Prefecture). The results show that the formation ages of these ore deposits are ranging from 62 to 87 Ma. The Hohro and Naburi deposits are dated to be 84.0 Ma and 87.1 Ma, respectively, which correspond both to the ages of the host rock volcanism (Takada rhyolitic rocks). However, the Ohguni, Yanoshokozan, Sankin and Kumano deposits are dated to be 61.4 Ma, 66.6-66.8 Ma, 72.7 and 77.5 Ma, respectively. It is noted that the formation ages of these ore deposits are younger than those of the host Takada and Kisa volcanics. Moreover, the determined ages are continuously distributed in the range between Late Cretaceous to Early Palaeogene periods. Thus, the inferred intimate geological association of the "Roseki" deposit with the host volcanics established in the previous literatures should be re-examined.

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