The Kamaishi mine, located in northern Honshu, is composed of skarn-type iron and copper deposits. The Carboniferous Nagaiwa-Onimaru formation of largely limestone has been strongly skarnized at its contact part with the Ganidake igneous complex of early Cretaceous intrusion. At the neighborhood of Shinyama, the sedimentary rocks are folded and thrusted, thus exhibiting four crests of anticline. This structure is preserved from Daito to Nippo, i.e., not disturbed by intrusion of the Ganidake igneous complex. Diorite porphyrite intruding along the thrusts is good host for the skarnization.
Distribution of the iron orebodies intercrosses the general trend of the Paleozoic formation. Shape of the orebodies is controlled by that of the nearest igneous rocks which are, free from skarnization. The iron ore occurs mostly in garnet skarn zone which is considered comtemporaneous with the iron mineralization. Copper mineralization is generally later than the iron mineralization. Zonal arrangement is distinct; the iron orebodies occur at the igneous rock side whereas the copper orebodies at the sedimentary rock side, especially of limestone. The pair is observed from north to south in the orebodies such as the Ohmine-Nippo, the Sahinai-8D and the Shinyama-2-4D.
Copper minerals are condensed in hedenbergite-rich skarn zone which predominantly occur at the contact with limestone. As observed in the orebodies of 2-5D, 8D and Nippo D ₄-D ₅, copper deposits are present at both sides of the limestone. apex. The ore deposits of the eastern wing where thrust is supposed to exist are always larger than those of the western wing. Copper concentration is also observed in some parts of the iron orebodies near limestone. The copper grade is inversely proportional to the distance from the limestone. Another type of structural control of the copper concentration is seen in the Nippo D ₃ orebody where the ores occur to follow NW-SE fractured zone.

View full abstract

Massive Cu-Pb-Zn deposits in the Lachlan Fold Belt of Australia are compared with the Kuroko ores in the Green Tuff Belt of Japan. Whilst the deposits of both these important provinces formed in submarine felsic volcanic-sedimentary environments, the Australian deposits are relatively rarely associated with felsic lava domes, and they characteristically have lower Cu/ (Pb+Zn) ratios, better developed depositional banding and little graded or breccia ore. It appears that most of the major Australian deposits formed in basins peripheral to volcanic centres, in contrast with the proximal deposition of the main Japanese ores.

View full abstract

The Iwami deposit of the Kuroko-type is situated in the Green Tuff region of the Inner Zone of Southwest Japan. The ore deposit is composed of stratiform Kuroko, gypsum and stockwork ore bodies in descending order. The stockwork ore body is economically more important than the stratiform ore body which consists of clayey ores and fragments of stockwork ores. The clayey ores are composed of fine grained sulfide minerals showing colloform texture and aggregates of sphalerite crystals up to 20 mm in diameter. Sphalerite suitable for inclusion studies in the stratiform ores is found in the fragments of the stockwork ores and the aggregates of sphalerite crystals. Sulfide minerals in the stratiform and stockwork ores are sphalerite, galena, pyrite and chalcopyrite, with minor luzonite and tetrahedrite. The wall rocks of the stockwork ores are composed of dacitic pyroclastics and small rhyolitic intrusives of Miocene age.
Samples were collected from five levels (-30m to -105m). The filling temperatures of 395 fluid inclusions in sphalerite and quartz from 89 localities were determined with the microscope heating stage. All the inclusions were two-phase and most ranged from 20 to 50 μ in maximum dimension. Salinities of inclusion fluids in sphalerite were studied with the freezing stage. Many small inclusions were observed in gypsum from the gypsum ore body, calcite from the veins of the stockwork ore body and barite from the stratiform ore body. Most of them were monophase (liquid) inclusions and some were extremely liquid-dominant two-phase inclusions.
Filling temperatures range generally from 200°C to 310°C for sphalerite and from 230°C to 295°C for quartz. The temperature values for quartz fall in approximately the same range as that for sphalerite. Salinities of inclusions in sphalerite indicate approximately 1 to 5 wt% NaCl equivalent. It is clearly seen that the temperatures for sphalerite increase downword in the stockwork ore body. In the stratiform ore body the temperatures for sphalerite from the fragments of stockwork ores range from 230° to 265°C and correspond to values for the stockwork ores of -30 m and -55 m levels. The temperatures for sphalerite from the aggregates of sphalerite crystals range from 250° to 270°C.
The ore forming succession in the Iwami deposit is considered as follows. At first, gypsum precipitated on the bottom of a small basin from sea water at the biginning of mineralization at low temperature. The stockwork ore body was formed, probably at a range of about 200° to 300°C, in the "breccia pipe". At the same time the stratiform fine grained clayey ores were formed on the bottom of the basin of at least 540 m depth. The fragments of the stockwork ores were mixed with clayey stratiform ores due to collapse of the upper part of the stockwork ore body. After this collapse mineralization continued and coase sphalerite aggregates were formed from fine grained sphalerite in the clayey sediments. Calcite veins were formed below 100°C in the waning stage of the mineralization.

View full abstract

Chemical compositions of tetrahedrite group minerals from some Japanese ore deposits are summarized. It is clarified that the chemical compositions (Cu, Ag, Zn, Fe, Sb, As) are different in different types of oredeposits. For example, zinc content of the minerals increases in the order, skarn and epithermal Cu-Pb-Zn <epithermal Au-Ag<kuroko.
Summarizing analytical data on the tetrahedrite group minerals, chemical formula for the minerals is regarded as (Cu, Ag) ₁₀ (Fe, Zn) ₂ (As, Sb) ₄S ₁₃ Using this chemical formula, the relationship between zinc and iron contents and physicochemical variables (fo ₂, pH, ΣS etc.) is derived. It is considered that Zn/Fe ratio in the minerals is largely dependent on fo ₂ as well as the other variables and has maximum value at the boundary between the predominance field of oxidized sulfur species and that of reduced sulfur species at constant other variables. This consideration and zinc and iron content of the minerals from kuroko type deposits suggest that kuroko type deposition has occurred near at the boundary between the predominance field of reduced sulfur species and that of oxidized sulfur species. The fo ₂ ranges at the time of formations of the other types ore depositions are also estimated. These estimations are in good agreement with those previously made by the other geochemical data such as mineral assemblages and iron content of sphalerite.

View full abstract

The ore deposits of the Kaneuchi mine are wolframite and scheelite bearing quartz veins occurring in slate, sandstone and chert of Permian period. Filling temperature and salinity of fluid inclusions in quartz and some scheelite were measured with the heating-cooling stage and compressing-heating stage investigated by HAYAKAWA and NAMBU (1973, 1978). Leakage of fluid from the inclusions in the case of brittle samples were prevented by the use of the compressing-heating stage.
Filling temperature of primary inclusions of quartz is similar to that of scheelite. Thus quartz can be useful to estimate temperature and salinity condition of the mineralization. The filling temperature of quartz varies from 200 to 300°C and is positively correlated with the salinity (Fig.5). CO₂-bearing inclusions are not uncommon. Both filling temperature and salinity of the primary inclusions increase from west to east in the vein swarm indicating the ore solution flowed out from east, which supports the hidden granitic cusp proposed by IMAI et al. (1972) in that area.

View full abstract