In order to determine the phase relations in the iron-rich portion of the Cu-Fe-S system at low temperatures below 300°C, phase relations of cubanite must be brought to light. Therefore, heating experiments of cubanite have been carried out by the hydrothermal and rigid fused silica tube method.
As the results, it is recognized that cubanite decomposed to two phases of very fine texture, as BORCHERT (1934) reported. The temperature of decomposition decreases with the increase of PH₂O pressure under the hydrothermal conditions, that is, at 1500 atm, 500 atm and 50 atm, the temperature is 185°C, 200°C and 210°C, respectively. In the experiments using evacuated rigid silica-glass tubes, the decomposition temperature is 215°C.
The one phase which forms the host of the texture is optically very similar to chalcopyrrhotite which was reported by RAMDOHR (1969), while the X-ray powder diffraction of the phase agrees with the reported patterns of so-called "high-cubanite".
The other phase, which forms very fine lamellae in the host, corresponds to chalcopyrite in the chemical composition, optical properties and X-ray powder diffraction pattern.
In this report, the former is named x-phase in spite of the similarity to chalcopyrrhotite optically, because the data of the X-ray diffraction pattern of chalcopyrrhotite have not been reported hitherto, and the latter is presumed to be chalcopyrite.
It has been reported that the occurrence of cubanite shows the high formation temperature of deposits, but to the contrary, cubanite is stable only at low temperatures below about 200°C.

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From 1966 to 1974, a number of drillings were carried by Metallic Mining Agency of Japan for exploration of "Kuroko-type" ore deposits in Aizu mineralization area, northeast Japan. As the results, a promising ore body (lower Ohshio Ore Body) was discovered. Futhermore, a large amount of important information on Neogene stratigraphy, geological structure, mineralogy of ore minerals, and hydrothermal alterations relating to the mineralization had been obtained.
Though "Kuroko-type" ore deposits in this are area embedded in various stratigraphic horizons ranging from middle to upper Miocene, characteristic authigenic minerals are distributed zonally around them. They are grouped into several zones by a definite assemblage of silicates. A typical arrangement of the zones is as follows: from the nearest to the ore body to the margin, K-feldspar zone→chlorite-sericite zone→mixed layer mineral zone→montmorillonite zone→analcime zone→mordenite zone→clinoptilolite-mordenite zone.
The shapes of alteration zones are as like as mashroom in usual, which suggest horizontal extending of hydrothermal solutions in porous sediments. Chemically, pbtasium and magnesium are concentrated in the K-feldspar zone and the chlorite-sericite zone in the central part, while the zeolite zones at the margin are characterized by concentration of sodium and calcium. Thus, the zoning suggests chemical redistributions by hydrothermal solutions which may have moved within each semi-closed space around ore deposits. The temperatures of hydrothermal solution at the center of alteration zones are estimated as 200°-250°C from fluid inclusion data of ore minerals, while those at the margin are as 40°-80°C from diagenetic condition data of clinoptilolite and mordenite. Then, horizontal thermal gradients around two ore deposits are tentatively estimated as 4.0-5.2 and 3.0-5.2°C/100m.

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Several zoned skarns are developed in the Shinyama Number 3 orebody of the Kamaishi mine at the contact between Paleozoic limestone and diorite porphyry intrusives of Cretaceous age. The textural relations and mineralogy of each skarn are strongly influenced by the primary host rock in which the skarn formed. Hedenbergite skarn is developed in areas in which the primary host rock was limestone. In this skarn the pyroxenes exhibit a distinctive granoblastic texture and twinning of individual grains is rare. A sequential arrangement of metasomatic alteration zones enveloping garnet veins is observed in the zoned skarn which formed in the diorite porphyry.
The succession of skarns observed with increasing distance from the garnet veins is: (1) garnet veins and/or garnet skarn, (2) ferrosalite skarn, (3) epidote pyroxene skarn, and (4) epidote amphibole skarn. The primary igneous texture of the diorite porphyry is generally quite well preserved throughout the entire endoskarn. There appears to be no relation, however, between the degree of preservation of the primary igneous texture in the successive skarns and the distance of the skarns from the garnet veins. The garnet veins are considered to have formed in the channelways through which the skarn-forming fluids flowed.
Petrographic examination of the mineralogy and textural relations of the different zoned skarns reveals that the first mineral to break down in the diorite porphyry is plagioclase. It is altered to epidote, clinopyroxene, garnet and calcite. Plagioclase phenocrysts appear to have been more reactive to the metasomatic fluid than plagioclase occurring in the groundmass. In the case of clinopyroxene the reverse relation is observed: clinopyroxene in the groundmass is readily altered to amphibole, whereas clinopyroxene phenocrysts appear resistant to mineralogical change in the zoned skarns.
The chemical compositions of clinopyroxene and epidote from the different skarns and from the fresh diorite porphyry were determined using electron microprobe. Clinopyroxenes display a gradual increase in their iron and manganese contents away from the diorite porphyry towards the ferrosalite skarn. Within the epidote pyroxene skarn a slight increase in the iron content and decrease in the aluminum content of epidote is observed from the epidote amphibole skarn towards the epidote pyroxene skarn.

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The variations of pH and Eh with time in the system of colloidal silica-HAuCl₄-H₂O were measured for about 3 weeks at 25°±1°C. Gold sols appear together with gold metal and silica minerals during the run. Those results are considered to be reflected in the generation, increase and decrease of some ions and/or molecular species in the system.
On the simplified model based on above effects, the relation of pH and Eh may be approximately given in the following equation, that is,
Eh≅0.338+0.009log a²AuCl₈/a²Cl-⋅a ³H₂O₂+0.0592pH

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