Geological mapping of the area near the Yaguki mine indicates that the Paleozoic formation has a synclinal structure with its axis plunging N 14°-19°W 22°. Intrusion of granitic rocks formed some local domes. The ore bodies are localized on the trough of the syncline and the top of the domal structures. This criteria would be very valuable in prospecting for mineral deposits in this area by delineating the local dome-structures.

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Fluid inclusions in quartz phenocrysts of quartz porphyry, quartz veinlets cutting quartz porphyry, and quartz and sphalerite in ores, were studied by the heating-stage and freezing-stage methods. Polyphase fluid inclusions carrying transparent cubic crystals were especially found in samples from deeper places. Liquid inclusions were found in most samples. Filling temperature of liquid inclusions was in the range of 200°C and 300°C, and the salinity varied from 20 weight percent to 1 percent. Gaseous inclusions were mainly found in shallower places with some polyphase inclusions suggesting the "boiling" of ore-forming fluids. Generally speaking, the temperature and salinity of ore-forming fluids at the Toyoha mine, Hokkaido, decreased from earlier to later stages and from deeper to shallower places.

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Millerite has been found as a micro-volume mineral in the copper sulphide ores from the Nippo No. 3 orebody of the Kamaishi mine, Iwate Prefecture, Japan. The copper sulphide ores containing the millerite are characterized by the bornite-chalcopyrite assemblage and essentially differ from other types of those from the Nippo ore deposit, which usually have the chalcopyritepyrrhotites-cubanite assemblage.
The present millerite usually occurs as narrow "veinlet", cluster and irregularly-shaped patch in chalcopyrite which is commonly intergrown graphically with bornite, and the aggregate ranges from extremely fine-to comparatively coarse-grained, accompanying minute grains of bornite. But on rare occasions, it is also contained in bornite. In addition, it occurs as radial aggregate intergrown intimately with chalcopyrite, surrounding the spherical or oval body which consists of the particular intergrowth of reticulated network of siegenite with chalcopyrite. The body is rimmed by the skin of siegenite, and the core of the body is often occupied by the irregularly-shaped "remnant" of cobalt or cobaltian pentlandite, showing relict fabrics. Under the ore microscope, it is white with faint cream overtone against chalopyrite, and its reflection pleochroism in air is weak, but perceptible at grain boundaries and is enhanced in oil. Between crossed nicols, it is moderately anisotropic in air and enhanced in oil, but colour effects become less striking.
The mineralogical properties of the present millerite may be summarized as follows. The reactions for standard etch reagents: HgCL₂, stains brown or pinkish-brown colour; H₂O₂, stains brown colour; HNO₃, KOH, KCN, FeCl₃ and aqua regia, all negative. The Vickers microhardness number (VHN) : 215-237 kg/mm² for a 50g load. The average chemical composition as determined by electron microprobe method: Cu 0.9, Ni 63.9, Co 0.0, Fe 0.8, S 35.3, Total 100.9 (in weight percent) and the corresponding structural formula on the basis of S = 1: (Cu_0.01 Fe_0.01 Ni_0.99)_1.01 S₁, Z = 9 (hexagonal structure cell); Z = 3 (true rhombohedral structure cell). The X-ray powder diffraction data for the present material agree well with those on millerite from Quebec, Canada given by BERRY and THOMPSON (1962) and are in harmony with space group R3m, with a₀ = 9.62Å, c₀ = 3.15Å; a_rh = 5.65Å, r = 116°40'. The calculation of the density based upon the average chemical composition and the unit-cell volume (252.2ų for hexagonal structure cell) as determined by X-ray diffractometry led to a result of 5.41 g/cm³.
Although the physico-chemical environments under which the copper sulphide ores in question were formed are not yet fully known, the mineral genesis of the millerite is considered on the basis of both previous and present works. As a result, it is suggested that the mineral might not be supergene but hypogene and the formation of the mineral might be contemporaneous approximately with that of siegenite resulting from the expense of cobalt or cobaltian pentlandite by the action of hydrothermal solution under higher sulphur fugacity during the later stage of mineralization.

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