Pyrite, chalcopyrite and sphalerite are the main constituent sulfides of the ordinary ores of the Besshi-type deposits in the Sambagawa belt. Pyrrhotite and bornite occur in significant amounts in some deposits. The amounts of other sulfides and sulfosalts are extremely small. Minerals uncommon in ordinary massive ores, such as galena, tetrahedritess, etc., are found more frequently in trace amounts in copper-rich ores occurring as offshoots, tongues and veins in and around the main stratified orebodies. These minerals may have been formed during the metamorphic deformation of the deposits and the recrvstallization of constituent common sulfides.
The notable geochemical characteristics of the Besshi-type ores are their high cobalt, low lead and low barium contents. Although most of the available sulfur isotope data of sulfides are only from the Besshi deposit, the spread of sulfur isotope ratios of the Besshi-type deposits is very narrow, ranging in δ34S from + 0.4 to +3.9‰ with a mean of +2.1‰. The Se/S ratios of pyrite also display a narrow distribution around its mean of 1.32×10-4 in six examined deposits.
These mineral and geochemical characteristics of the Besshi-type ores can possibly be ascribed to the chemistry of related basic volcanics. Modern copper-rich sea-floor sulfide deposits are quite similar to the Besshi-type deposits in their chemistry. These two deposits is somewhat different from each other in mineralogy, but sea-floor alteration and hydrothermal replacement in the mound interiors should change the primary mineral assemblages of the sea-floor sulfide deposits toward those of the Besshi-type deposits.

View full abstract

The Shimokawa volcanogenic massive sulfide ore deposits were formed 126 Ma by submarine exhalative activity on the mid-oceanic spreading axis between the Kula-Izanagi and the Farallon plates. A hydrothermal system resulted from the convective circulation of seawater near the spreading axis, as evidenced by the sheeted-dike com-plex, was responsible for the sub-sea-floor. metamorphism and subsequent ore deposition. During mineral deposition, massive sulfide ores were separated by a double diffusive phenomenon from quartz-chlorite and sulfide laminated ores. Ore formation appears to have occurred during a quiescent stage of volcanism; the ore deposits were possibly covered by later siliceous exhalites and/or lavas and then these covers were removed during a subsequent tectonic phase. As trench approached, the oceanic crust containing the ore deposits was overlain by distal and proximal turbidites; these turbidites originated from the Eurasian continental margin volcanic belt in Sikhote-Allin, prior to the Cenozoic opening of the Sea of Japan. These Hidaka Supergroup epiclastic sediments, underlain by oceanic crust and the massive sulfide deposits, suffered oblique slip subduction that caused scaling-off a part of the oceanic crust, particularly the sheeted-dike complex. This part of accretionary prism is known as the Shimokawa tectono-stratigraphic unit, and contains a chaotic mixture of deformed epiclastic sediments and melange units of oceanic crust, in which semi-continuous massive ore deposits occur more or less parallel to the general tectonic trend; eventually the opening of the Sea of Japan caused this belt to be displaced from the continental margin to their present position in the Hidaka zone of Hokkaido.
Other volcanogenic massive sulfide deposits in the Cretaceous Shimanto accretion complex, such as Makimine in eastern Kyushu and Asakawa in eastern Shikoku, appear to have similar allochthonous origin.

View full abstract

Ores of the Shimokawa massive sulfide deposits are composed mainly of pyrite, pyrrhotite, chalcopyrite, sphalerite, and magnetite. Minor amounts of cobalt pentlandite, cobaltian mackinawite, cubanite, cobaltite, and galena are observed under the microscope. Gangue minerals are quartz, calcite, siderite, sericite, chlorite, amphibole, apatite, and carbonaceous materials. The assemblage of pyrite-pyrrhotite-magnetite-siderite is considered to have been formed under a condition close to 250°C, 10^1.7 atm f_co2, 10^-38 atm fo2, and 10^-14 atm f_s2. The pyrr-hotite-pyrite-cubanite-chalcopyrite assemblage has been modified by a small change in f_s2 during the retrograde process from iss-pyrite-pyrrhotite assemblage formed above 300 or 350°C. The cobalt-iron partition temperatures between pyrrhotite and pyrite are 200-300°C for monoclinic pyrrhotite-pyrite and 270-350°C for hexagonal pyr-rhotite-pyrite. The gangue mineral composition containing siderite and calcite as the main constituents is consistent with the estimated submarine environment rich in carbonaceous materials where an adequate supply of elastics from adjacent continental areas was available.

View full abstract

In eastern Chugoku and western Kinki regions there are a number of stratiform sulfide ore deposits in Carboniferous and Permian submarine volcanic rocks and their metamorphic equivalents. They can be classified into three groups: (1) ore deposits in Carboniferous basic schist of the lower formation of the Maniwa Group of the Sangun metamorphic zone (Besshi-type stratiform sulfide ore deposits); (2) ore deposits in Permian basic schist and basic volcanic rocks of the lower formation of the Maizuru Group (Besshi-type stratiform sulfide ore deposits); (3) ore deposits in Permian acidic volcanic rocks of the middle formation of the Maizuru Group (Yanahara-type stratiform sulfide ore deposits). All ore deposits were folded three times: Late Paleozoic to Early Triassic (Sangun metamorphism), Middle Triassic and Jurassic ages. The Yanahara and Fukuzawa ore deposits were also influenced by contact metamorphism caused by Cretaceous granitic rocks. These ore deposits show a remarkable uniformity in sulfur isotope ratio, ranging from +0.6 to +2.99‰, despite the differences in age and acidity among the host volcanic rocks.
Both the Besshi-and Yanahara-type ore deposits are volcanic exhalative in origin. The Besshi-type ore deposits are genetically related to basic submarine volcanism, whereas the Yanahara-type ore deposits are related to acidic submarine volcanism.

View full abstract

Chemical compositions and characteristics of zoning patterns in sphalerite from 16 Besshi-type, skarn, vein, and Kuroko deposits were examined by electron microprobe. Compositional zoning of FeS in sphalerite varies according to the deposit type. Ore textures and compositional zoning of minerals are generally complex. The remarkable zoning heterogeneity in sphalerite which has not been metamorphosed suggests these features are primary. Sphalerite from metamorphosed Besshi-type deposits is characterized by a distinct lack of compositional zoning.
If a deposit has been thermally metamorphosed, originally zoned sphalerite may have been compositionally homogenized by the diffusional migration of atoms. The average grain size of sphalerite from strongly metamorphosed deposits is less than 1 mm.If the mean displacement of cations in sphalerite is assumed to be 1 mm at annealing temperatures of 300° and 250°C, calculated times of homogenization based on experimentally determined diffusivities are 23 and 470 million years, respectively. Sanbagawa metamorphic rocks range in age from 60 to 100 Ma; therefore, Besshi-type deposits could have been subjected to temperatures over 300°C for several tens of million years.
In skarn deposits, it is feasible that post-depositional diffusion in the cooling stage of plutonic intrusion related to mineralization generates gently curved zoning patterns of iron and manganese in the margins of sphalerite crystals. However, the compositional inhomogeneity of sphalerites from some other types of deposits appears to be precluded because of weak post-depositional annealing activity.

View full abstract

The K-Ar ages of sericites collected from sericite and kaolin deposits in the Chugoku district were determined and their formation stages were discussed.
Sericites from sericite deposits are dated as follows; Ohgin (31.1 Ma), Iwaya (36.4 Ma), Nabeyama (45.6 Ma), Yoshida (48.5 Ma), Unnan (50.3 Ma), Igi (50.6 Ma), Hida (56.9 Ma), Ohguni (61.4 Ma) and Kumano (77.5 Ma).
Sericites from Komaki, Kohtachi and Toyosaka kaolin deposits are dated at 55.6 Ma, 76.5 Ma and 79.5 Ma, respectively.
The results indicate that the formation ages of sericite and kaolin deposits are comparable to or slightly younger than those of respective hosts or nearby granitic rocks whose age data are available at the present. Therefore, the mineralization stages of these clay deposits appear to be concluded to correspond with the hydrothermal stages of granitic activities in the Chugoku district during the period from Late Cretaceous to Palaeogene.

View full abstract

Sulfide scales were collected from a test well of the Oku-Aizu geothermal field. Veinlets composed of similar sulfide minerals in the country rocks were also obtained from the drilled cores. Both were deposited from the compressed geothermal hot water in this area. These minerals are listed in Table 1. The analyses of the geothermal water are shown in Table 4. From the analytical data, the pH of the geothermal fluid is calculated to be 4.47 at 250°C in the case of activity coefficient equals to unity.
Two kinds of rock alteration are recognized in this area. The one is alteration caused by the neutral to alkaline hot water. The other is due to the acid hot water. The minerals of both alterations are shown in Table 3. The rocks along fissures containing the present compressed geothermal fluid are subjected to neutral to alkaline alteration. From these facts, the relation log fo2 pH-log f _s2 of the environment of the geothermal fluid is shown in Fig.9. The acid alteration occurs sporadically in this area. The acid alteration would be referred to the vapor-dominated system, separated by boiling from the compressed hot geothermal fluid. It might condense to acid hot water during cooling.
In Japan, the epithermal gold deposits are classified into two types. The one is gold (silver) -quartz (adularia) vein, surrounded by the rocks suffered neutral to alkaline alteration; adulariazation, sericitization and/or chloritization. The other is the massive silicified rock containing gold, which is surrounded by the rocks subjected to acid alteration; kaolinization and alunitization. The both types of gold deposits might be attributed to the same phenomena as the two kinds of geothermal fluids stated above.

View full abstract

This paper describes a simple method of calculating pH for geothermal fluids at deep reservoir. A complete recovery of both vapor and aqueous phases at the orifice of drilled well is prerequisite for the calculation. The first step in the calculation procedure is to combine H₂O in the vapor with the aqueous phase. The charge balance of the resulting solution is expressed by Eq. (2). Then dissolve CO₂ and H₂S remaining in the vapor phase into the solution and write again an expression for charge balance, which is given by Eq. (3). A quantitative relationship between H ⁺and related species, as formulated by Eq. (4), is obtained by subtracting Eq. (2) from Eq. (3). The terms in Eq. (4) can be calculated by using dissociation constants for pH sensitive species at the presumed reservoir temperature. Solving Eq. (4) for (H ⁺), we get the pH of the geothermal fluid. This calculation procedure has been applied to the geothermal fluid from the Oku-Aizu geothermal field, Nishiyama, Fukushima Prefecture.

View full abstract